Showing 1–50 of 57 results for author: He, P

Local Nonlinear Transforms effectively Reveal Primordial Information in Large-Scale Structure

Authors: Yun Wang, Hao-Ran Yu, Yu Yu, Ping He

Abstract: To eliminate gravitational non-Gaussianity, we introduce the $\mathcal{Z}$-$κ$ transform, a simple local nonlinear transform of the matter density field that emulates the inverse of nonlinear gravitational evolution. Using $N$-body simulations, we show that the $\mathcal{Z}$-$κ$ transform with $κ=6$ or $κ\to\infty$ (i.e., log) substantially Gaussianizes the density distribution, and recovers the l… ▽ More To eliminate gravitational non-Gaussianity, we introduce the $\mathcal{Z}$-$κ$ transform, a simple local nonlinear transform of the matter density field that emulates the inverse of nonlinear gravitational evolution. Using $N$-body simulations, we show that the $\mathcal{Z}$-$κ$ transform with $κ=6$ or $κ\to\infty$ (i.e., log) substantially Gaussianizes the density distribution, and recovers the linear power spectrum. In an extended parameter space including primordial non-Gaussianity, summed neutrino mass, and $Λ$CDM parameters, Fisher analysis demonstrates that power spectra of transformed fields provide strong complementary constraints. A central result is that these power spectra can directly capture the local primordial non-Gaussianity imprinted in large-scale structure. This opens a new avenue for probing the physics of the early Universe with Stage-IV surveys using two-point statistics. △ Less

Submitted 13 December, 2025; originally announced December 2025.

Comments: 5 pages, 3 figures, 1 table, comments welcomed

CWTHF: Subhalo Identification with Continuous Wavelet Transform

Authors: Minxing Li, Yun Wang, Ping He

Abstract: With advances in cosmology and computer science, cosmological simulations now resolve structures in increasingly fine detail. As key tracers of hierarchical structure formation, subhalos are among the most important objects within these simulations. In our previous work, we established that the continuous wavelet transform (CWT) can effectively extract clustering information and serve as a robust… ▽ More With advances in cosmology and computer science, cosmological simulations now resolve structures in increasingly fine detail. As key tracers of hierarchical structure formation, subhalos are among the most important objects within these simulations. In our previous work, we established that the continuous wavelet transform (CWT) can effectively extract clustering information and serve as a robust halo finder. Here, we extend the CWT framework to subhalo identification by adapting the CWTHF (Continuous Wavelet Transform Halo Finder) code. This extension extends the unbinding procedure, which enables the reliable identification of gravitationally bound substructures. The algorithm identifies density peaks within known halos or subhalos and segments the surrounding volume accordingly. Once a new subhalo is registered, its position is recorded to prevent duplicate detection. We validate our approach using the TNG50-2 and TNG100-1 simulations, as well as a single Friends-of-Friends (FOF) halo, by comparing the resulting CWT catalog against the reference SUBFIND catalog. Because the method inherits the original computational framework, our subhalo finder maintains a favorable linear time complexity of $\mathcal{O}(N)$. △ Less

Submitted 10 December, 2025; originally announced December 2025.

Comments: 16pages, 12 figures

A quantum information method for early universe with non-trivial sound speed

Authors: Shi-Cheng Liu, Lei-Hua Liu, Bichu Li, Hai-Qing Zhang, Peng-Zhang He

Abstract: Many quantum gravitational frameworks, such as DBI inflation, k-essence, and effective field theories obtained by integrating out heavy modes, can lead to a non-trivial sound speed. Meanwhile, our universe can be described as an open system. Under the non-trivial sound speed, we employ the method of open quantum systems combined with Arnoldi iterations to study the Krylov complexity throughout the… ▽ More Many quantum gravitational frameworks, such as DBI inflation, k-essence, and effective field theories obtained by integrating out heavy modes, can lead to a non-trivial sound speed. Meanwhile, our universe can be described as an open system. Under the non-trivial sound speed, we employ the method of open quantum systems combined with Arnoldi iterations to study the Krylov complexity throughout the early universe, including the inflationary, radiation-dominated, and matter-dominated epochs. A key ingredient in our analysis is the open two-mode squeezed state formalism and the generalized Lanczos algorithm. To numerically compute the Krylov complexity, we are the first time to derive the evolution equations for the parameters $r_k$ and $φ_k$ within an open two-mode squeezed state. Our results indicate that the Krylov complexity exhibits a similar trend in both the standard case and the case with non-trivial sound speed. To distinguish between these two scenarios, we also investigate the Krylov entropy for completeness. The evolution of the Krylov entropy shows a clear difference between the standard case and the non-trivial sound speed case. Furthermore, based on the behavior of the Lanczos coefficients, we find that the case of non-trivial sound speed behaves as a maximally chaotic system. However, our numerical results suggest that the Krylov complexity does not saturate to a constant value due to the huge expansion of spacetime background. This study offers a new perspective for exploring the early universe through the quantum information. △ Less

Submitted 4 October, 2025; originally announced October 2025.

Comments: 32 pages, 8 figures

Precision calculation of the EFT likelihood with primordial non-Gaussianities

Authors: Ji-Yuan Ke, Yun Wang, Ping He

Abstract: We perform a precision calculation of the effective field theory (EFT) conditional likelihood for large-scale structure (LSS) using the saddle-point expansion method in the presence of primordial non-Gaussianities (PNG). The precision is manifested at two levels: one corresponding to the consideration of higher-order noise terms, and the other to the inclusion of contributions around the saddle po… ▽ More We perform a precision calculation of the effective field theory (EFT) conditional likelihood for large-scale structure (LSS) using the saddle-point expansion method in the presence of primordial non-Gaussianities (PNG). The precision is manifested at two levels: one corresponding to the consideration of higher-order noise terms, and the other to the inclusion of contributions around the saddle points. In computing the latter, we encounter the same issue of the negative modes as in the context of false vacuum decay, which necessitates deforming the original integration contour into a combination of the steepest descent contours to ensure a convergent and real result. We demonstrate through detailed calculations that, upon incorporating leading-order PNG, both types of extensions introduce irreducible field-dependent contributions to the conditional likelihood. This insight motivates the systematic inclusion of additional effective terms within the forward modeling framework. Our work facilitates Bayesian forward modeling under non-Gaussian initial conditions, thereby enabling more stringent constraints on the parameters describing PNG. △ Less

Submitted 8 August, 2025; v1 submitted 16 May, 2025; originally announced May 2025.

Comments: 28 pages, 1 figure; v2: major revision, add many discussions, accepted by JCAP

Journal ref: JCAP 09 (2025) 029

The dynamical and thermodynamic effects of turbulence for the cosmic baryonic fluid

Authors: Yun Wang, Minxing Li, Ping He

Abstract: Both simulations and observations indicate that the so-called missing baryons reside in the intergalactic medium (IGM) known as the warm-hot intergalactic medium (WHIM). In this paper, we employ the IllustrisTNG50-1 simulation to demonstrate that turbulence in the cosmic baryonic fluid is crucial for correctly understanding both the spatial distribution and the physical origins of the missing bary… ▽ More Both simulations and observations indicate that the so-called missing baryons reside in the intergalactic medium (IGM) known as the warm-hot intergalactic medium (WHIM). In this paper, we employ the IllustrisTNG50-1 simulation to demonstrate that turbulence in the cosmic baryonic fluid is crucial for correctly understanding both the spatial distribution and the physical origins of the missing baryons in the universe. First, we find that dynamical effects cause the gas to be detained in low-density and intermediate-density regions, resulting in high baryon fractions, while prevent the convergence of the gas in high-density regions, leading to low baryon fractions. Second, turbulent energy is converted into thermal energy, and the injection and dissipation of turbulent energy have essentially reached a balance from $z=1$ to $0$. This indicates that the cosmic fluid is in a steady state within this redshift range. Due to turbulent heating, as redshift decreases, an increasing amount of warm gas is heated and transitions into the WHIM, and some even into hot gas. We find that, compared with turbulence in the cosmic fluid, shocks are unimportant in intermediate-density regions and even negligible in high-density regions, both dynamically and thermodynamically. This finding not only provides the origin of WHIM in terms of both dynamics and thermodynamics, but also questions the traditional view of the shock heating and highlights the importance of turbulence in shaping the large-scale structure of the universe, particularly in the evolution of galaxies and galaxy clusters. In addition to TNG50-1, we validated our key findings with TNG50-2, TNG100-1, WIGEON, and EAGLE simulations, demonstrating that resolution, box size, and subgrid-physics variations do not affect our main conclusions. △ Less

Submitted 28 November, 2025; v1 submitted 9 March, 2025; originally announced March 2025.

Comments: 14 pages, 13 figures

Bilateral constraints on proton Lorentz violation effects

Authors: Ping He, Bo-Qiang Ma

Abstract: Since the early reports of events beyond the Greisen-Zatsepin-Kuzmin (GZK) cutoff, the investigation of ultrahigh-energy cosmic rays has emerged as a fundamental method for testing Lorentz Invariance violation (LV) effects. Recent advances in observational capabilities have resulted in more stringent constraints on LV parameters. This study delves into the potentially anomalous phenomena arising f… ▽ More Since the early reports of events beyond the Greisen-Zatsepin-Kuzmin (GZK) cutoff, the investigation of ultrahigh-energy cosmic rays has emerged as a fundamental method for testing Lorentz Invariance violation (LV) effects. Recent advances in observational capabilities have resulted in more stringent constraints on LV parameters. This study delves into the potentially anomalous phenomena arising from subluminal and superluminal LV effects, encompassing aspects such as proton decay and atypical threshold behavior of photo-pion production from protons within the GZK region. High-energy proton observations in cosmic rays have imposed stringent constraints on superluminal proton LV effects, while the confirmation of the GZK cutoff has established a robust boundary for the anomalous phenomena associated with subluminal proton LV effects. The research provides rigorously bilateral constraints on proton LV effects from both subluminal and superluminal standpoints. △ Less

Submitted 4 March, 2025; originally announced March 2025.

Comments: 16 pages, 4 figures, final version

Journal ref: Astrophys.J.981 (2025) 45

Calculating the EFT likelihood via saddle-point expansion

Authors: Ji-Yuan Ke, Yun Wang, Ping He

Abstract: In this paper, we extend the functional approach for calculating the EFT likelihood by applying the saddle-point expansion. We demonstrate that, after suitable reformulation, the likelihood expression is consistent with the path integral required to be computed in the theory of false vacuum decay. In contrast to the saddle-point approximation, the application of the saddle-point expansion necessit… ▽ More In this paper, we extend the functional approach for calculating the EFT likelihood by applying the saddle-point expansion. We demonstrate that, after suitable reformulation, the likelihood expression is consistent with the path integral required to be computed in the theory of false vacuum decay. In contrast to the saddle-point approximation, the application of the saddle-point expansion necessitates more nuanced considerations, particularly concerning the treatment of the negative eigenvalues of the second derivative of the action at the saddle point. We illustrate that a similar issue arises in the likelihood calculation, which requires approximating the original integral contour through the combination of the steepest descent contours in the field space. As a concrete example, we focus on calculating the EFT likelihood under a Gaussian distribution and propose a general procedure for computing the likelihood using the saddle-point expansion method for arbitrary partition functions. Precise computation of the likelihood will benefit Bayesian forward modeling, thereby enabling more reliable theoretical predictions. △ Less

Submitted 25 March, 2025; v1 submitted 22 February, 2025; originally announced February 2025.

Comments: 21 pages plus appendix, 2 figures, Accepted for publication in JCAP

Journal ref: JCAP 04 (2025) 064

Comparative Analysis of EMCEE, Gaussian Process, and Masked Autoregressive Flow in Constraining the Hubble Constant Using Cosmic Chronometers Dataset

Authors: Jing Niu, Jie-Feng Chen, Peng He, Tong-Jie Zhang

Abstract: The Hubble constant ($\mathrm{H}_0$) is essential for understanding the universe's evolution. Different methods, such as Affine Invariant Markov chain Monte Carlo Ensemble sampler (EMCEE), Gaussian Process (GP), and Masked Autoregressive Flow (MAF), are used to constrain $\mathrm{H}_0$ using $H(z)$ data. However, these methods produce varying $\mathrm{H}_0$ values when applied to the same dataset.… ▽ More The Hubble constant ($\mathrm{H}_0$) is essential for understanding the universe's evolution. Different methods, such as Affine Invariant Markov chain Monte Carlo Ensemble sampler (EMCEE), Gaussian Process (GP), and Masked Autoregressive Flow (MAF), are used to constrain $\mathrm{H}_0$ using $H(z)$ data. However, these methods produce varying $\mathrm{H}_0$ values when applied to the same dataset. To investigate these differences, we compare the methods based on their sensitivity to individual data points and their accuracy in constraining $\mathrm{H}_0$. We introduce Multiple Random Sampling Analysis (MRSA) to assess their sensitivity to individual data points. Our findings reveal that GP is more sensitive to individual data points than both MAF and EMCEE, with MAF being more sensitive than EMCEE. Sensitivity also depends on redshift: EMCEE and GP are more sensitive to $H(z)$ at higher redshifts, while MAF is more sensitive at lower redshifts. For accuracy assessment, we simulate $H_{\mathrm{sim}}(z_{\mathrm{sim}})$ datasets with a prior $\mathrm{H}_{\mathrm{0prior}}$. Comparing the constrained $\mathrm{H_{0sim}}$ values with $\mathrm{H}_{\mathrm{0prior}}$ shows that EMCEE is the most accurate, followed by MAF, with GP being the least accurate, regardless of the simulation method. △ Less

Submitted 17 February, 2025; originally announced February 2025.

Comments: 13 pages, 10 figures, 5 tables

Constraining the Hubble Constant with a Simulated Full Covariance Matrix Using Neural Networks

Authors: Jing Niu, Peng He, Tong-Jie Zhang

Abstract: The Hubble parameter, $H(z)$, plays a crucial role in understanding the expansion history of the universe and constraining the Hubble constant, $\mathrm{H}_0$. The Cosmic Chronometers (CC) method provides an independent approach to measuring $H(z)$, but existing studies either neglect off-diagonal elements in the covariance matrix or use an incomplete covariance matrix, limiting the accuracy of… ▽ More The Hubble parameter, $H(z)$, plays a crucial role in understanding the expansion history of the universe and constraining the Hubble constant, $\mathrm{H}_0$. The Cosmic Chronometers (CC) method provides an independent approach to measuring $H(z)$, but existing studies either neglect off-diagonal elements in the covariance matrix or use an incomplete covariance matrix, limiting the accuracy of $\mathrm{H}_0$ constraints. To address this, we use a Positive-Definite Covariance Network (PD-CovNet) to simulate the full $33 \times 33$ covariance matrix based on a previously published $15 \times 15$ covariance matrix. Hyperparameters are chosen via leave-one-z-out validation, and performance is benchmarked against a Gaussian-process (GP) baseline. Under identical five-fold cross-validation over redshift groups, we prove that PD-CovNet is a reliable generator of the full covariance compared to the GP baseline. Using this full PD-CovNet-simulated covariance alongside three comparators with different covariance specifications, we constrain $\mathrm{H}_0$ with two independent methods (EMCEE and GP). Across all covariance specifications and both constraint methods, standardized differences and two-sided p-values show no statistically meaningful shift in the central value of the constrained $\mathrm{H}_0$. However, the precision of the constrained $\mathrm{H}_0$ depends on both covariance and method: EMCEE is uniformly more precise than GP once covariance is modeled; within a fixed method, incorporating more covariance reduces precision; and PD-CovNet hyperparameters have a modest effect on uncertainty. These results indicate the importance of accurate covariance modeling in CC-based $\mathrm{H}_0$ constraints. △ Less

Submitted 22 November, 2025; v1 submitted 17 February, 2025; originally announced February 2025.

Comments: 11 pages, 3 figures, and 6 tables. Accepted for publication in MNRAS

Journal ref: Monthly Notices of the Royal Astronomical Society, 545 (2026), staf2051

Redshift drift effect through the observation of HI 21cm signal with SKA

Authors: Jiangang Kang, Tong-Jie Zhang, Peng He, Ming Zhu

Abstract: This study presents the findings of using the Square Kilometre Array (SKA) telescope to measure redshift drift via the HI 21cm signal, employing semi-annual observational interval within redshift around z $\sim$ 1 with main goal is to directly gauge the universe's expansion acceleration rate with millimeter-per-second (mm/s) precision. The SKA can detect over a billion HI 21cm emissions from indiv… ▽ More This study presents the findings of using the Square Kilometre Array (SKA) telescope to measure redshift drift via the HI 21cm signal, employing semi-annual observational interval within redshift around z $\sim$ 1 with main goal is to directly gauge the universe's expansion acceleration rate with millimeter-per-second (mm/s) precision. The SKA can detect over a billion HI 21cm emissions from individual galaxies to the redshift z $\sim$ 2 and thousands of absorption lines from Damped Lyman-alpha (DLA) systems against bright quasars to the redshift z $\sim$ 13, with the sensitivity limit of 100 mJy. By utilizing SKA's high spectral resolution settings (0.001, 0.002, 0.005, 0.01 Hz) to detect redshift drift, particularly focusing on the 0.001 and 0.002 Hz configuration, one aims to achieve the necessary mm/s in precision measurement by the 0.5-year observation period. The velocity drift rate, crucially determined by the two operational regimes within 0.01 to 0.21 mm/s and 0.031 to 0.17 mm/s, respectively, exceeds the theoretical accuracy limit of 1.28 mm/s. The analysis thoroughly restricts cosmological parameters related to dark energy using the Sandage-Loeb (SL) signal from the HI 21cm emission and absorption lines. It estimates $\rm H_0$ of about 70 km/s/Mpc, $\rm Ω_m$ near 0.3, with w close to -1, $\rm w_0$ around -1, and $\rm w_a$ approaching -0.1. These results strongly endorse the SL effect as an effective method for confirming cosmic acceleration and exploring the dark sector in real-time cosmology with the SKA. △ Less

Submitted 25 January, 2025; originally announced January 2025.

Comments: 13 pages,13 figures,1 table. Accepted for publication on ApJ

Journal ref: The Astrophysical Journal, Volume 982, Issue 2, id.177,10 pp. (2025)

CWTHF: Identifying Dark Matter Halos with Continuous Wavelet Transform

Authors: Minxing Li, Yun Wang, Ping He

Abstract: Cosmological simulations are an important method for investigating the evolution of the Universe. In order to gain further insight into the processes of structure formation, it is necessary to identify isolated bound objects within the simulations, namely, the dark matter halos. The continuous wavelet transform (CWT) is an effective tool used as a halo finder due to its ability to extract clusteri… ▽ More Cosmological simulations are an important method for investigating the evolution of the Universe. In order to gain further insight into the processes of structure formation, it is necessary to identify isolated bound objects within the simulations, namely, the dark matter halos. The continuous wavelet transform (CWT) is an effective tool used as a halo finder due to its ability to extract clustering information from the input data. In this study, we introduce CWTHF (Continuous Wavelet Transform Halo Finder), the first wavelet-based, MPI-parallelized halo finder, marking a novel approach in the field of cosmology. We calculate the CWT from the cloud-in-cell (CIC) grid and segment the grid based on the local CWT maxima. We then investigate the effects of the parameters that influence our program and identify the default settings. A comparison with the conventional friends-of-friends (FOF) method demonstrates the viability of CWT for halo finding. Although the actual performance is not faster than FOF, the linear time complexity of $\mathcal{O}(N)$ of our identification scheme indicates its significant potential for future optimization and application. △ Less

Submitted 20 March, 2025; v1 submitted 17 January, 2025; originally announced January 2025.

Comments: 18 pages, 15 figures, 1 table, Accepted by ApJ

Journal ref: ApJ (2025), 984, 68

Theoretical Radio Signals from Radio-Band Gravitational Waves Converted from the Neutron Star Magnetic Field

Authors: Wei Hong, Zhen-Zhao Tao, Peng He, Tong-Jie Zhang

Abstract: Gravitational waves (GWs) can convert into electromagnetic waves in the presence of a magnetic field via the Gertsenshtein-Zeldovich (GZ) effect. The characteristics of the magnetic field substantially affect this conversion probability. This paper confirms that strong magnetic fields in neutron stars significantly enhance the conversion probability, facilitating detectable radio signatures of ver… ▽ More Gravitational waves (GWs) can convert into electromagnetic waves in the presence of a magnetic field via the Gertsenshtein-Zeldovich (GZ) effect. The characteristics of the magnetic field substantially affect this conversion probability. This paper confirms that strong magnetic fields in neutron stars significantly enhance the conversion probability, facilitating detectable radio signatures of very high-frequency (VHF, $\left(10^6-10^{11}\mathrm{~Hz}\right)$) gravitational waves. We theoretically identify two distinct signatures using single-dish telescopes (FAST, TMRT, QTT, GBT) and interferometers (SKA1/2-MID): transient signals from burst-like gravitational wave sources and persistent signals from cosmological background gravitational wave sources. These signatures are mapped to graviton spectral lines derived from quantum field theory by incorporating spin-2 and mass constraints, resulting in smooth, featureless profiles that are critical for distinguishing gravitational wave signals from astrophysical foregrounds. FAST attains a characteristic strain bound of $h_c<10^{-23}$, approaching $10^{-24}$ in the frequency range of $1-3\mathrm{~GHz}$ with a 6-hour observation period. This performance exceeds the $5 σ$ detection thresholds for GWs originating from primordial black holes (PBHs) and nears the limits set by Big Bang nucleosynthesis. Additionally, projections for SKA2-MID indicate even greater sensitivity. Detecting such gravitational waves would improve our comprehension of cosmological models, refine the parameter spaces for primordial black holes, and function as a test for quantum field theory. This approach addresses significant deficiencies in VHF GW research, improving detection sensitivity and facilitating the advancement of next-generation radio telescopes such as FASTA and SKA, which feature larger fields of view and enhanced gain. △ Less

Submitted 4 September, 2025; v1 submitted 5 December, 2024; originally announced December 2024.

Comments: 40 pages, 19 figures, 4 tables. Accepted for publication in ApJ. We infer two novel types of the converted radio signals: transient and persistent signals. Considering the mass and spin of the graviton, the expected spectral line shape of the graviton is derived. FAST is the most sensitive telescope to detect VHFGWs in a single-dish telescope. In addition, SKA2-MID has greater detection potential

Journal ref: ApJ, 990: 156 (29pp), 2025

Estimating Cosmological Parameters and Reconstructing Hubble Constant with Artificial Neural Networks: A Test with covariance matrix and mock H(z)

Authors: Jie-feng Chen, Tong-Jie Zhang, Peng He, Tingting Zhang, Jie Zhang

Abstract: In this work, we reconstruct the H(z) based on observational Hubble data with Artificial Neural Network, then estimate the cosmological parameters and the Hubble constant. The training data we used are covariance matrix and mock H(z), which are generated based on the real OHD data and Gaussian Process(GP). The use of the covariance matrix propagates the correlated uncertainties and improves traini… ▽ More In this work, we reconstruct the H(z) based on observational Hubble data with Artificial Neural Network, then estimate the cosmological parameters and the Hubble constant. The training data we used are covariance matrix and mock H(z), which are generated based on the real OHD data and Gaussian Process(GP). The use of the covariance matrix propagates the correlated uncertainties and improves training efficiency. Using the reconstructed H(z) data, we first determine the Hubble constant and compare it with CMB-based measurements. To constrain cosmological parameters, we sample on the reconstructed data and calculate the corresponding posterior distributions with Markov Chain Monte Carlo (MCMC). Through comprehensive statistical comparisons, we demonstrate that the parameter estimation using reconstructed samples achieves comparable statistical accuracy to the result derived from real OHD data. △ Less

Submitted 3 September, 2025; v1 submitted 10 October, 2024; originally announced October 2024.

Comments: 21 pages, 11 figures. Accepted for publication in the European Physical Journal C

Journal ref: Eur. Phys.J.C (2025)85:1005

Capturing primordial non-Gaussian signatures in the late Universe by multi-scale extrema of the cosmic log-density field

Authors: Yun Wang, Ping He

Abstract: We construct two new summary statistics, the scale-dependent peak height function (scale-PKHF) and the scale-dependent valley depth function (scale-VLYDF) of matter density, and forecast their constraining power on primordial non-Gaussianity and cosmological parameters based on \textsc{Quijote} and \textsc{Quijote-PNG} simulations at $z=0$. With the Fisher analysis, we demonstrate that these stati… ▽ More We construct two new summary statistics, the scale-dependent peak height function (scale-PKHF) and the scale-dependent valley depth function (scale-VLYDF) of matter density, and forecast their constraining power on primordial non-Gaussianity and cosmological parameters based on \textsc{Quijote} and \textsc{Quijote-PNG} simulations at $z=0$. With the Fisher analysis, we demonstrate that these statistics outperform the power spectrum and bispectrum. Key findings include: (1) the constraint on the scalar spectral index $n_s$ obtained from the scale-VLYDF/scale-PKHF is 1.59/1.10 times tighter than that from the joint analysis of power spectrum and bispectrum; (2) the combination of the two statistics yields a slight improvement in constraining $\{f_\mathrm{NL}^\mathrm{local}, f_\mathrm{NL}^\mathrm{equil}\}$ over the power spectrum-bispectrum combination, and provides a 1.39-fold improvement in the constraint on $f_\mathrm{NL}^\mathrm{ortho}$; (3) after incorporating the power spectrum with our new statistics, parameter constraints surpass those from power spectrum-bispectrum combination by factors up to 2.93. This work offers an effective scheme for extracting primordial signals from the late Universe, paving the way for further breakthroughs in precision cosmology. △ Less

Submitted 21 December, 2024; v1 submitted 25 August, 2024; originally announced August 2024.

Comments: 8 pages, 5 figures, 1 table. Accepted for Publication in PRD

Journal ref: Phys.Rew. D (2025), 111, L041302

Turbulence, Thermal Pressure, and Their Dynamical Effects on Cosmic Baryonic Fluid

Authors: Yun Wang, Ping He

Abstract: We employ the IllustrisTNG simulation data to investigate the turbulent and thermal motions of the cosmic baryonic fluid. With continuous wavelet transform techniques, we define the pressure spectra, or density-weighted velocity power spectra, as well as the spectral ratios, for both turbulent and thermal motions. We find that the magnitude of the turbulent pressure spectrum grows slightly from… ▽ More We employ the IllustrisTNG simulation data to investigate the turbulent and thermal motions of the cosmic baryonic fluid. With continuous wavelet transform techniques, we define the pressure spectra, or density-weighted velocity power spectra, as well as the spectral ratios, for both turbulent and thermal motions. We find that the magnitude of the turbulent pressure spectrum grows slightly from $z=4$ to $2$ and increases significantly from $z=2$ to $1$ at large scales, suggesting progressive turbulence injection into the cosmic fluid, whereas from $z=1$ to $0$, the spectrum remains nearly constant, indicating that turbulence may be balanced by energy transfer and dissipation. The magnitude of the turbulent pressure spectra also increases with environmental density, with the highest density regions showing a turbulent pressure up to six times that of thermal pressure. We also explore the dynamical effects of turbulence and thermal motions, discovering that while thermal pressure provides support against structure collapse, turbulent pressure almost counteracts this support, challenging the common belief that turbulent pressure supports gas against overcooling. △ Less

Submitted 17 July, 2024; originally announced July 2024.

Comments: 7 pages, 4 figures, 1 table, submitted to the MNRAS Letters. arXiv admin note: text overlap with arXiv:2404.11255

Journal ref: MNRAS Letters (2024), 534, L14

Identifying Halos in Cosmological Simulations with Continuous Wavelet Analysis: The 2D Case

Authors: Minxing Li, Yun Wang, Ping He

Abstract: Continuous wavelet analysis is gaining popularity in science and engineering for its ability to analyze data across spatial and scale domains simultaneously. In this study, we introduce a wavelet-based method to identify halos and assess its feasibility in two-dimensional (2D) scenarios. We begin with the generation of four pseudo-2D datasets from the SIMBA dark matter simulation by compressing th… ▽ More Continuous wavelet analysis is gaining popularity in science and engineering for its ability to analyze data across spatial and scale domains simultaneously. In this study, we introduce a wavelet-based method to identify halos and assess its feasibility in two-dimensional (2D) scenarios. We begin with the generation of four pseudo-2D datasets from the SIMBA dark matter simulation by compressing thin slices of three-dimensional (3D) data into 2D. We then calculate the continuous wavelet transform (CWT) directly from the particle distributions, identify local maxima that represent actual halos, and segment the CWT to delineate halo boundaries. A comparison with the traditional friends-of-friends (FOF) method shows that our CWT-identified halos, while contain slightly fewer particles, have smoother boundaries and are more compact in dense regions. In contrast, the CWT method can link particles over greater distances to form halos in sparse regions due to its spatial segmentation scheme. The spatial distribution and halo power spectrum of both CWT and FOF halos demonstrate substantial consistency, validating the 2D applicability of CWT for halo detection. Our identification scheme operates with a linear time complexity of $\mathcal{O}(N)$, suggesting its suitability for analyzing significantly larger datasets in the future. △ Less

Submitted 19 August, 2024; v1 submitted 1 May, 2024; originally announced May 2024.

Comments: 19 pages, 13 figures, 2 table, comments welcome, accepted by ApJ

Journal ref: ApJ (2024), 973, 39

Abnormal threshold behaviors of photo-pion production off the proton in the GZK region

Authors: Ping He, Bo-Qiang Ma

Abstract: The confirmation of the existence of GZK cut-off was tortuous, leading to activities to explore new physics, such as the cosmic-ray new components, unidentified cosmic-ray origins, unknown propagation mechanism, and the modification of fundamental physics concepts like the tiny Lorentz invariance violation (LV). The confirmation of the GZK cut-off provides an opportunity to constrain the LV effect… ▽ More The confirmation of the existence of GZK cut-off was tortuous, leading to activities to explore new physics, such as the cosmic-ray new components, unidentified cosmic-ray origins, unknown propagation mechanism, and the modification of fundamental physics concepts like the tiny Lorentz invariance violation (LV). The confirmation of the GZK cut-off provides an opportunity to constrain the LV effect. We use a phenomenological framework to restudy the GZK mechanism under the Planck scale deformation of the proton and pion dispersion relations. Restudying the photon induced pion production of the proton $\mathrm{p}+γ\to\mathrm{p}+π^0$, we predict abnormal threshold behaviors of this reaction under different LV modifications. Therefore, we can study the LV effects not only from the conventional GZK cut-off, but also from potentially threshold anomalies of the pion production process. We divide the LV parameter space into three regions, and analyze the constraints from current observations in each region. The current observations have set strict constraints on a certain LV region. However, for others LV regions, further experimental observations and theoretical researches are still needed, and we also find survival space for some theoretical explorations that permit specific LV effects. △ Less

Submitted 24 April, 2024; originally announced April 2024.

Comments: 9 latex pages, 3 figures, final version

Journal ref: Euro.Phys.J. C 84 (2024) 401

Turbulence revealed by wavelet transform: power spectrum and intermittency for the velocity field of the cosmic baryonic fluid

Authors: Yun Wang, Ping He

Abstract: We use continuous wavelet transform techniques to construct the global and environment-dependent wavelet statistics, such as energy spectrum and kurtosis, to study the fluctuation and intermittency of the turbulent motion in the cosmic fluid velocity field with the IllustrisTNG simulation data. We find that the peak scale of the energy spectrum define a characteristic scale, which can be regarded… ▽ More We use continuous wavelet transform techniques to construct the global and environment-dependent wavelet statistics, such as energy spectrum and kurtosis, to study the fluctuation and intermittency of the turbulent motion in the cosmic fluid velocity field with the IllustrisTNG simulation data. We find that the peak scale of the energy spectrum define a characteristic scale, which can be regarded as the integral scale of turbulence, and the Nyquist wavenumber can be regarded as the dissipation scale. With these two characteristic scales, the energy spectrum can be divided into the energy-containing range, the inertial range and the dissipation range of turbulence. The wavelet kurtosis is an increasing function of the wavenumber $k$, first grows rapidly then slowly with $k$, indicating that the cosmic fluid becomes increasingly intermittent with $k$. In the energy-containing range, the energy spectrum increases significantly from $z = 2$ to $1$, but remains almost unchanged from $z = 1$ to $0$. We find that both the environment-dependent spectrum and kurtosis are similar to the global ones, and the magnitude of the spectrum is smallest in the lowest-density and largest in the highest-density environment, suggesting that the cosmic fluid is more turbulent in a high-density than in a low-density environment. In the inertial range, the energy spectrum's exponent is steeper than both the Kolmogorov and Burgers exponents, indicating more efficient energy transfer compared to Kolmogorov or Burgers turbulence. △ Less

Submitted 9 August, 2024; v1 submitted 17 April, 2024; originally announced April 2024.

Comments: 19 pages, 11 figures, 2 tables, accepted by ApJ

Journal ref: ApJ (2024), 974,107

Merging history of massive galaxies at 3<z<6

Authors: Kemeng Li, Zhen Jiang, Ping He, Qi Guo, Jie Wang

Abstract: The observational data of high redshift galaxies become increasingly abundant, especially since the operation of the James Webb Space Telescope (JWST), which allows us to verify and optimize the galaxy formation model at high redshifts. In this work, we investigate the merging history of massive galaxies at $3 < z < 6$ using a well-developed semi-analytic galaxy formation catalogue. We find that t… ▽ More The observational data of high redshift galaxies become increasingly abundant, especially since the operation of the James Webb Space Telescope (JWST), which allows us to verify and optimize the galaxy formation model at high redshifts. In this work, we investigate the merging history of massive galaxies at $3 < z < 6$ using a well-developed semi-analytic galaxy formation catalogue. We find that the major merger rate increases with redshift up to 3 and then flattens. The fraction of wet mergers, during which the sum of the cold gas mass is higher than the sum of the stellar mass in two merging galaxies, also increases from $\sim$ 34\% at $z = 0$ to 96\% at $z = 3$. Interestingly, almost all major mergers are wet at $z > 3$ . This can be attributed to the high fraction ($> 50\%$) of cold gas at $z > 3$. In addition, we study some special systems of massive merging galaxies at $3 < z < 6$, including the massive gas-rich major merging systems and extreme dense proto-clusters, and investigate the supermassive black hole-dark matter halo mass relation and dual AGNs. We find that the galaxy formation model reproduces the incidence of those observed massive galaxies, but fails to reproduce the relation between the supermassive black hole mass and the dark matter halo mass at $z \sim 6$. The latter requires more careful estimates of the supermassive black hole masses observationally. Otherwise, it could suggest modifications of the modeling of the supermassive black hole growth at high redshifts. △ Less

Submitted 6 December, 2023; originally announced December 2023.

Comments: 9 pages,8figures

Journal ref: 2023,Research in Astronomy and Astrophysics, 23(1), 015010

White Paper and Roadmap for Quantum Gravity Phenomenology in the Multi-Messenger Era

Authors: R. Alves Batista, G. Amelino-Camelia, D. Boncioli, J. M. Carmona, A. di Matteo, G. Gubitosi, I. Lobo, N. E. Mavromatos, C. Pfeifer, D. Rubiera-Garcia, E. N. Saridakis, T. Terzić, E. C. Vagenas, P. Vargas Moniz, H. Abdalla, M. Adamo, A. Addazi, F. K. Anagnostopoulos, V. Antonelli, M. Asorey, A. Ballesteros, S. Basilakos, D. Benisty, M. Boettcher, J. Bolmont , et al. (79 additional authors not shown)

Abstract: The unification of quantum mechanics and general relativity has long been elusive. Only recently have empirical predictions of various possible theories of quantum gravity been put to test, where a clear signal of quantum properties of gravity is still missing. The dawn of multi-messenger high-energy astrophysics has been tremendously beneficial, as it allows us to study particles with much higher… ▽ More The unification of quantum mechanics and general relativity has long been elusive. Only recently have empirical predictions of various possible theories of quantum gravity been put to test, where a clear signal of quantum properties of gravity is still missing. The dawn of multi-messenger high-energy astrophysics has been tremendously beneficial, as it allows us to study particles with much higher energies and travelling much longer distances than possible in terrestrial experiments, but more progress is needed on several fronts. A thorough appraisal of current strategies and experimental frameworks, regarding quantum gravity phenomenology, is provided here. Our aim is twofold: a description of tentative multimessenger explorations, plus a focus on future detection experiments. As the outlook of the network of researchers that formed through the COST Action CA18108 ``Quantum gravity phenomenology in the multi-messenger approach (QG-MM)'', in this work we give an overview of the desiderata that future theoretical frameworks, observational facilities, and data-sharing policies should satisfy in order to advance the cause of quantum gravity phenomenology. △ Less

Submitted 17 January, 2025; v1 submitted 1 December, 2023; originally announced December 2023.

Journal ref: Class. Quantum Grav. 42 032001,2025

How do baryonic effects on the cosmic matter distribution vary with scale and local density environment?

Authors: Yun Wang, Ping He

Abstract: In this study, we investigate how the baryonic effects vary with scale and local density environment mainly by utilizing a novel statistic, the environment-dependent wavelet power spectrum (env-WPS). With four state-of-the-art cosmological simulation suites, EAGLE, SIMBA, Illustris, and IllustrisTNG, we compare the env-WPS of the total matter density field between the hydrodynamic and dark matter-… ▽ More In this study, we investigate how the baryonic effects vary with scale and local density environment mainly by utilizing a novel statistic, the environment-dependent wavelet power spectrum (env-WPS). With four state-of-the-art cosmological simulation suites, EAGLE, SIMBA, Illustris, and IllustrisTNG, we compare the env-WPS of the total matter density field between the hydrodynamic and dark matter-only (DMO) runs at $z=0$. We find that the clustering is most strongly suppressed in the emptiest environment of $ρ_\mathrm{m}/\barρ_\mathrm{m}<0.1$ with maximum amplitudes $\sim67-89$ per cent on scales $\sim1.86-10.96\ h\mathrm{Mpc}^{-1}$, and less suppressed in higher density environments on small scales (except Illustris). In the environments of $ρ_\mathrm{m}/\barρ_\mathrm{m}\geqslant0.316$ ($\geqslant10$ in EAGLE), the feedbacks also lead to enhancement features at intermediate and large scales, which is most pronounced in the densest environment of $ρ_\mathrm{m}/\barρ_\mathrm{m}\geqslant100$ and reaches a maximum $\sim 7-15$ per cent on scales $\sim0.87-2.62\ h\mathrm{Mpc}^{-1}$ (except Illustris). The baryon fraction of the local environment decreases with increasing density, denoting the feedback strength, and potentially explaining some differences between simulations. We also measure the volume and mass fractions of local environments, which are affected by $\gtrsim 1$ per cent due to baryon physics. In conclusion, our results show that the baryonic processes can strongly modify the overall cosmic structure on the scales of $k>0.1\ h\mathrm{Mpc}^{-1}$, which encourages further research in this direction. △ Less

Submitted 21 January, 2024; v1 submitted 31 October, 2023; originally announced October 2023.

Comments: 12 pages, 12 figures, and 3 tables; accepted by MNRAS

Journal ref: MNRAS (2024), 528, 3797-3808

Comprehensive analysis on photon-electron Lorentz-violation parameter plane

Authors: Ping He, Bo-Qiang Ma

Abstract: Large High Altitude Air Shower Observatory~(LHAASO) opens the window of ultra-high-energy~(UHE) photon detection, broadens the path of testing basic physical concept such as Lorentz symmetry, and brings possibility of potential high-energy physical phenomenon research such as photon decay and electron decay. Currently, the UHE photons from LHAASO observation set strict constraints on photon and el… ▽ More Large High Altitude Air Shower Observatory~(LHAASO) opens the window of ultra-high-energy~(UHE) photon detection, broadens the path of testing basic physical concept such as Lorentz symmetry, and brings possibility of potential high-energy physical phenomenon research such as photon decay and electron decay. Currently, the UHE photons from LHAASO observation set strict constraints on photon and electron Lorentz symmetry violation~(LV) effects. To obtain a global impression of the photon-electron LV parameter plane, we make a detailed analysis for photon decay and electron decay. Our discussion gives the corresponding decay thresholds and energy-momentum distributions in different LV parameter configurations. We get corresponding constraints on photon LV parameter, electron LV parameter and the photon-electron LV parameter plane from LHAASO observation. For the space allowed for LV effect, that is beyond relativity, we also provide corresponding boundaries from LHAASO observation. △ Less

Submitted 6 September, 2023; v1 submitted 3 August, 2023; originally announced August 2023.

Comments: 10 pages,3 figures, final version for publication

Journal ref: Phys.Rev.D 108 (2023) 063006

Subhalo abundance and satellite spatial distribution in Milky Way-Andromeda-like paired haloes

Authors: Kemeng Li, Shi Shao, Ping He, Qing Gu, Jie Wang

Abstract: We study the subhalo and satellite populations in haloes similar to the Milky Way (MW)-Andromeda paired configuration in the Millennium II and P-Millennium simulations. We find subhaloes are $5\%-15\%$ more abundant in paired haloes than their isolated counterparts that have the same halo mass and large-scale environmental density. Paired haloes tend to reside in a more isotropic environment than… ▽ More We study the subhalo and satellite populations in haloes similar to the Milky Way (MW)-Andromeda paired configuration in the Millennium II and P-Millennium simulations. We find subhaloes are $5\%-15\%$ more abundant in paired haloes than their isolated counterparts that have the same halo mass and large-scale environmental density. Paired haloes tend to reside in a more isotropic environment than isolated haloes, the shear tensor of their large-scale tidal field is possibly responsible for this difference. We also study the thickness of the spatial distribution of the top 11 most massive satellite galaxies obtained in the semi-analytic galaxy sample constructed from the Millennium II simulation. Moreover, satellites that have lost their host subhaloes due to the resolution limit of the simulation have been taken into account. As a result, we find that the difference in the distribution of the satellite thickness between isolated and paired haloes is indistinguishable, which suggests that the paired configuration is not responsible for the observed plane of satellites in the Milky Way. The results in this study indicate the paired configuration could bring some nonnegligible effect on the subhalo abundance in the investigation of the Milky Way's satellite problems. △ Less

Submitted 12 June, 2023; originally announced June 2023.

Comments: 10 pages, 9 figures

Journal ref: 2022,Research in Astronomy and Astrophysics, 22(12), 125020

Reconstruction of the dark energy scalar field potential by Gaussian process

Authors: Jing Niu, Kang Jiao, Peng He, Tong-Jie Zhang

Abstract: Dark energy is believed to be responsible for the acceleration of the universe. In this paper, we reconstruct the dark energy scalar field potential $V(φ)$ using the Hubble parameter $H(z)$ through Gaussian Process analysis. Our goal is to investigate dark energy using various $H(z)$ datasets and priors. We find that the selection of prior and the $H(z)$ dataset significantly affects the reconstru… ▽ More Dark energy is believed to be responsible for the acceleration of the universe. In this paper, we reconstruct the dark energy scalar field potential $V(φ)$ using the Hubble parameter $H(z)$ through Gaussian Process analysis. Our goal is to investigate dark energy using various $H(z)$ datasets and priors. We find that the selection of prior and the $H(z)$ dataset significantly affects the reconstructed $V(φ)$. And we compare two models, Power Law and Free Field, to the reconstructed $V(φ)$ by computing the reduced chi-square. The results suggest that the models are generally in agreement with the reconstructed potential within a $3σ$ confidence interval, except in the case of Observational $H(z)$ data (OHD) with the Planck 18 (P18) prior. Additionally, we simulate $H(z)$ data to measure the effect of increasing the number of data points on the accuracy of reconstructed $V(φ)$. We find that doubling the number of $H(z)$ data points can improve the accuracy rate of reconstructed $V(φ)$ by 5$\%$ to 30$\%$. △ Less

Submitted 6 July, 2024; v1 submitted 8 May, 2023; originally announced May 2023.

Comments: 17 pages, 12 figures, 4 tables, accepted by APJ on 05 July 2024

Journal ref: The Astrophysical Journal, Volume 972, Issue 1, 2024

Comparisons between fast algorithms for the continuous wavelet transform and applications in cosmology: the 1D case

Authors: Yun Wang, Ping He

Abstract: The continuous wavelet transform (CWT) is very useful for processing signals with intricate and irregular structures in astrophysics and cosmology. It is crucial to propose precise and fast algorithms for the CWT. In this work, we review and compare four different fast CWT algorithms for the 1D signals, including the FFTCWT, the V97CWT, the M02CWT, and the A19CWT. The FFTCWT algorithm implements t… ▽ More The continuous wavelet transform (CWT) is very useful for processing signals with intricate and irregular structures in astrophysics and cosmology. It is crucial to propose precise and fast algorithms for the CWT. In this work, we review and compare four different fast CWT algorithms for the 1D signals, including the FFTCWT, the V97CWT, the M02CWT, and the A19CWT. The FFTCWT algorithm implements the CWT using the Fast Fourier Transform (FFT) with a computational complexity of $\mathcal{O}(N\log_2N)$ per scale. The rest algorithms achieve the complexity of $\mathcal{O}(N)$ per scale by simplifying the CWT into some smaller convolutions. We illustrate explicitly how to set the parameters as well as the boundary conditions for them. To examine the actual performance of these algorithms, we use them to perform the CWT of signals with different wavelets. From the aspect of accuracy, we find that the FFTCWT is the most accurate algorithm, though its accuracy degrades a lot when processing the non-periodic signal with zero boundaries. The accuracy of $\mathcal{O}(N)$ algorithms is robust to signals with different boundaries, and the M02CWT is more accurate than the V97CWT and A19CWT. From the aspect of speed, the $\mathcal{O}(N)$ algorithms do not show an overall speed superiority over the FFTCWT at sampling numbers of $N\lesssim10^6$, which is due to their large leading constants. Only the speed of the V97CWT with real wavelets is comparable to that of the FFTCWT. However, both the FFTCWT and V97CWT are substantially less efficient in processing the non-periodic signal because of zero padding. Finally, we conduct wavelet analysis of the 1D density fields, which demonstrate the convenience and power of techniques based on the CWT. We publicly release our CWT codes as resources for the community. △ Less

Submitted 7 October, 2023; v1 submitted 8 February, 2023; originally announced February 2023.

Comments: 17 pages, 17 figures, 3 tables, accepted for publication in RAS techniques and instruments

Journal ref: 2023, RAS Techniques and Instruments, Volume 2, Issue 1, 307 - 323

Joint photon-electron Lorentz violation parameter plane from LHAASO data

Authors: Ping He, Bo-Qiang Ma

Abstract: The Large High Altitude Air Shower Observatory~(LHAASO) is one of the most sensitive gamma-ray detector arrays, whose ultrahigh-energy~(UHE) work bands not only help to study the origin and acceleration mechanism of UHE cosmic rays, but also provide the opportunity to test fundamental physics concepts such as Lorentz symmetry. LHAASO directly observes the $1.42~\mathrm{PeV}$ highest-energy photon.… ▽ More The Large High Altitude Air Shower Observatory~(LHAASO) is one of the most sensitive gamma-ray detector arrays, whose ultrahigh-energy~(UHE) work bands not only help to study the origin and acceleration mechanism of UHE cosmic rays, but also provide the opportunity to test fundamental physics concepts such as Lorentz symmetry. LHAASO directly observes the $1.42~\mathrm{PeV}$ highest-energy photon. By adopting the synchrotion self-Compton model, LHAASO also suggests that the $1.12~\mathrm{PeV}$ high-energy photon from Crab Nebula corresponds to a $2.3~\mathrm{PeV}$ high-energy electron. We study the $1.42~\mathrm{PeV}$ photon decay and the $2.3~\mathrm{PeV}$ electron decay to perform a joint analysis on photon and electron two-dimensional Lorentz violation~(LV) parameter plane. Our analysis is systematic and comprehensive, and we naturally get the strictest constraints from merely considering photon LV effect in photon decay and electron LV effect in electron decay. Our result also permits the parameter space for new physics beyond relativity. △ Less

Submitted 7 November, 2022; v1 submitted 26 October, 2022; originally announced October 2022.

Comments: 8 latex pages, 3 figures, final version for journal publication

Journal ref: Physics Letters B 835 (2022) 137536

Lorentz Symmetry Violation of Cosmic Photons

Authors: Ping He, Bo-Qiang Ma

Abstract: As a basic symmetry of space-time, Lorentz symmetry has played important roles in various fields of physics, and it is a glamorous question whether Lorentz symmetry breaks. Since Einstein proposed special relativity, Lorentz symmetry has withstood very strict tests, but there are still motivations for Lorentz symmetry violation (LV) research from both theoretical consideration and experimental fea… ▽ More As a basic symmetry of space-time, Lorentz symmetry has played important roles in various fields of physics, and it is a glamorous question whether Lorentz symmetry breaks. Since Einstein proposed special relativity, Lorentz symmetry has withstood very strict tests, but there are still motivations for Lorentz symmetry violation (LV) research from both theoretical consideration and experimental feasibility, that attract physicists to work on LV theories, phenomena and experimental tests with enthusiasm. There are many theoretical models including LV effects, and different theoretical models predict different LV phenomena, from which we can verify or constrain LV effects. Here, we introduce three types of LV theories: quantum gravity theory, space-time structure theory and effective field theory with extra-terms. Limited by the energy of particles, the experimental tests of LV are very difficult; however, due to the high energy and long propagation distance, high-energy particles from astronomical sources can be used for LV phenomenological researches. Especially with cosmic photons, various astronomical observations provide rich data from which one can obtain various constraints for LV researches. Here, we review four common astronomical phenomena which are ideal for LV studies, together with current constraints on LV effects of photons. △ Less

Submitted 16 June, 2022; originally announced June 2022.

Comments: 34 latex pages, no figure, final version published in journal

Journal ref: Universe 8 (2022) 323

Simultaneous Dependence of Matter Clustering on Scale and Environment

Authors: Yun Wang, Ping He

Abstract: In this work, we propose new statistical tools that are capable of characterizing the simultaneous dependence of dark matter and gas clustering on the scale and the density environment, and these are the environment-dependent wavelet power spectrum (env-WPS), the environment-dependent bias function (env-bias), and the environment-dependent wavelet cross-correlation function (env-WCC). These statis… ▽ More In this work, we propose new statistical tools that are capable of characterizing the simultaneous dependence of dark matter and gas clustering on the scale and the density environment, and these are the environment-dependent wavelet power spectrum (env-WPS), the environment-dependent bias function (env-bias), and the environment-dependent wavelet cross-correlation function (env-WCC). These statistics are applied to the dark matter and baryonic gas density fields of the \texttt{TNG100-1} simulation at redshifts of $z=3.0$-$0.0$, and to \texttt{Illustris-1} and \texttt{SIMBA} at $z=0$. The measurements of the env-WPSs suggest that the clustering strengths of both the dark matter and the gas increase with increasing density, while that of a Gaussian field shows no density dependence. By measuring the env-bias and env-WCC, we find that they vary significantly with the environment, scale, and redshift. A noteworthy feature is that at $z=0.0$, the gas is less biased in denser environments of $Δ\gtrsim 10$ around $3 \ h\mathrm{Mpc}^{-1}$, due to the gas reaccretion caused by the decreased AGN feedback strength at lower redshifts. We also find that the gas correlates more tightly with the dark matter in both the most dense and underdense environments than in other environments at all epochs. Even at $z=0$, the env-WCC is greater than $0.9$ in $Δ\gtrsim 200$ and $Δ\lesssim 0.1$ at scales of $k \lesssim 10 \ h\mathrm{Mpc}^{-1}$. In summary, our results support the local density environment having a non-negligible impact on the deviations between dark matter and gas distributions up to large scales. △ Less

Submitted 22 July, 2022; v1 submitted 24 February, 2022; originally announced February 2022.

Comments: 16 pages, 11 figures, and 1 table, accepted by APJ

Journal ref: 2022, ApJ, 934, 112

Continuous wavelet analysis of matter clustering using the Gaussian-derived wavelet

Authors: Yun Wang, Hua-Yu Yang, Ping He

Abstract: Continuous wavelet analysis has been increasingly employed in various fields of science and engineering due to its remarkable ability to maintain optimal resolution in both space and scale. Here, we introduce wavelet-based statistics, including the wavelet power spectrum, wavelet cross-correlation and wavelet bicoherence, to analyze the large-scale clustering of matter. For this purpose, we perfor… ▽ More Continuous wavelet analysis has been increasingly employed in various fields of science and engineering due to its remarkable ability to maintain optimal resolution in both space and scale. Here, we introduce wavelet-based statistics, including the wavelet power spectrum, wavelet cross-correlation and wavelet bicoherence, to analyze the large-scale clustering of matter. For this purpose, we perform wavelet transforms on the density distribution obtained from the one-dimensional Zel'dovich approximation and then measure the wavelet power spectra and wavelet bicoherences of this density distribution. Our results suggest that the wavelet power spectrum and wavelet bicoherence can identify the effects of local environments on the clustering at different scales. Moreover, we apply the statistics based on the three-dimensional isotropic wavelet to the IllustrisTNG simulation at z = 0, and investigate the environmental dependence of the matter clustering. We find that the clustering strength of the total matter increases with increasing local density except on the largest scales. Besides, we notice that the gas traces the dark matter better than stars on large scales in all environments. On small scales, the cross-correlation between the dark matter and gas first decreases and then increases with increasing density. This is related to the impacts of the AGN feedback on the matter distribution, which also varies with the density environment in a similar trend to the cross-correlation between the dark matter and gas. Our findings are qualitatively consistent with previous studies about the matter clustering. △ Less

Submitted 8 May, 2022; v1 submitted 11 December, 2021; originally announced December 2021.

Comments: 22 pages, 22 figures, 2 tables

Journal ref: 2022, ApJ, 934, 77

The spatial distribution deviation and the power suppression of baryons from dark matter

Authors: Hua-Yu Yang, Yun Wang, Ping He, Weishan Zhu, Long-Long Feng

Abstract: The spatial distribution between dark matter and baryonic matter of the Universe is biased or deviates from each other. In this work, by comparing the results derived from IllustrisTNG and WIGEON simulations, we find that many results obtained from TNG are similar to those from WIGEON data, but differences between the two simulations do exist. For the ratio of density power spectrum between dark m… ▽ More The spatial distribution between dark matter and baryonic matter of the Universe is biased or deviates from each other. In this work, by comparing the results derived from IllustrisTNG and WIGEON simulations, we find that many results obtained from TNG are similar to those from WIGEON data, but differences between the two simulations do exist. For the ratio of density power spectrum between dark matter and baryonic matter, as scales become smaller and smaller, the power spectra for baryons are increasingly suppressed for WIGEON simulations; while for TNG simulations, the suppression stops at $k=15-20h{\rm Mpc}^{-1}$, and the power spectrum ratios increase when $k>20h{\rm Mpc}^{-1}$. The suppression of power ratio for WIGEON is also redshift-dependent. From $z=1$ to $z=0$, the power ratio decreases from about 70% to less than 50% at $k=8h{\rm Mpc}^{-1}$. For TNG simulation, the suppression of power ratio is enhanced with decreasing redshifts in the scale range $k>4h{\rm Mpc}^{-1}$, but is nearly unchanged with redshifts in $k<4h{\rm Mpc}^{-1}$ These results indicate that turbulent heating can also have the consequence to suppress the power ratio between baryons and dark matter. Regarding the power suppression for TNG simulations as the norm, the power suppression by turbulence for WIGEON simulations is roughly estimated to be 45% at $k=2h{\rm Mpc}^{-1}$, and gradually increases to 69% at $k=8h{\rm Mpc}^{-1}$, indicating the impact of turbulence on the cosmic baryons are more significant on small scales. △ Less

Submitted 9 November, 2021; originally announced November 2021.

Journal ref: 2022, MNRAS, 509, 1036

Parametric instabilities and particles heating of circularly polarized Alfvén waves with an incoherent spectrum: two-dimensional hybrid simulations

Authors: Peng He

Abstract: Plasma ions heating (especially minor heavy ions preferential heating) in fast solar wind and solar corona is an open question in space physics. However, Alfvén waves have been always considered as a candidate of energy source for corona heating. In this paper, by using a two-dimensional (2-D) hybrid simulation model in a low beta electron-proton-alpha plasma system, we have investigated the relat… ▽ More Plasma ions heating (especially minor heavy ions preferential heating) in fast solar wind and solar corona is an open question in space physics. However, Alfvén waves have been always considered as a candidate of energy source for corona heating. In this paper, by using a two-dimensional (2-D) hybrid simulation model in a low beta electron-proton-alpha plasma system, we have investigated the relationships between plasma ions heating and power spectra evolution of density and magnetic field fluctuations excited from the parametric instabilities of initial pump Alfvén waves with an incoherent spectrum at different propagation angles theta_k0B0 (an oblique angle between the initial pump wave vector k0 and the background magnetic field B0). It is found that, the wave-wave coupling as well as wave-particle interaction play key roles in ions heating, and an Alfvén spectrum with small propagation angle (e.g. theta_k0B0=15degree) can most effectively heat alpha particles in perpendicular direction as well as in parallel direction for both proton and alpha particle than the case of a monochromatic Alfvén wave or an Alfvén spectrum with larger propagation angle. △ Less

Submitted 16 August, 2021; originally announced August 2021.

Comments: 36 pages, 16 figures (in the end of the paper)

Data-Based Optimal Bandwidth for Kernel Density Estimation of Statistical Samples

Authors: Zhen-Wei Li, Ping He

Abstract: It is a common practice to evaluate probability density function or matter spatial density function from statistical samples. Kernel density estimation is a frequently used method, but to select an optimal bandwidth of kernel estimation, which is completely based on data samples, is a long-term issue that has not been well settled so far. There exist analytic formulae of optimal kernel bandwidth,… ▽ More It is a common practice to evaluate probability density function or matter spatial density function from statistical samples. Kernel density estimation is a frequently used method, but to select an optimal bandwidth of kernel estimation, which is completely based on data samples, is a long-term issue that has not been well settled so far. There exist analytic formulae of optimal kernel bandwidth, but they cannot be applied directly to data samples, since they depend on the unknown underlying density functions from which the samples are drawn. In this work, we devise an approach to pick out the totally data-based optimal bandwidth. First, we derive correction formulae for the analytic formulae of optimal bandwidth to compute the roughness of the sample's density function. Then substitute the correction formulae into the analytic formulae for optimal bandwidth, and through iteration, we obtain the sample's optimal bandwidth. Compared with analytic formulae, our approach gives very good results, with relative differences from the analytic formulae being only 2%-3% for a sample size larger than 10^4. This approach can also be generalized easily to cases of variable kernel estimations. △ Less

Submitted 25 April, 2021; originally announced April 2021.

Comments: 7 pages, 8 figures

Journal ref: Commun. Theor. Phys. 70 (2018) 728-734

The continuous wavelet derived by smoothing function and its application in cosmology

Authors: Yun Wang, Ping He

Abstract: The wavelet analysis technique is a powerful tool and is widely used in broad disciplines of engineering, technology, and sciences. In this work, we present a novel scheme of constructing continuous wavelet functions, in which the wavelet functions are obtained by taking the first derivative of smoothing functions with respect to the scale parameter. Due to this wavelet constructing scheme, the in… ▽ More The wavelet analysis technique is a powerful tool and is widely used in broad disciplines of engineering, technology, and sciences. In this work, we present a novel scheme of constructing continuous wavelet functions, in which the wavelet functions are obtained by taking the first derivative of smoothing functions with respect to the scale parameter. Due to this wavelet constructing scheme, the inverse transforms are only one-dimensional integrations with respect to the scale parameter, and hence the continuous wavelet transforms constructed in this way are more ready to use than the usual scheme. We then apply the Gaussian-derived wavelet constructed by our scheme to computations of the density power spectrum for dark matter, the velocity power spectrum and the kinetic energy spectrum for baryonic fluid. These computations exhibit the convenience and strength of the continuous wavelet transforms. The transforms are very easy to perform, and we believe that the simplicity of our wavelet scheme will make continuous wavelet transforms very useful in practice. △ Less

Submitted 5 August, 2021; v1 submitted 19 April, 2021; originally announced April 2021.

Comments: 9 Pages, 4 Figures, comments welcome

Journal ref: Commun. Theor. Phys., 73 (2021), 095402

Turbulence-induced deviation between baryonic field and dark matter field in the spatial distribution of the Universe

Authors: Hua-Yu Yang, Ping He, Weishan Zhu, Long-Long Feng

Abstract: The cosmic baryonic fluid at low redshifts is similar to a fully developed turbulence. In this work, we use simulation samples produced by the hybrid cosmological hydrodynamical/N-body code, to investigate on what scale the deviation of spatial distributions between baryons and dark matter is caused by turbulence. For this purpose, we do not include the physical processes such as star formation, s… ▽ More The cosmic baryonic fluid at low redshifts is similar to a fully developed turbulence. In this work, we use simulation samples produced by the hybrid cosmological hydrodynamical/N-body code, to investigate on what scale the deviation of spatial distributions between baryons and dark matter is caused by turbulence. For this purpose, we do not include the physical processes such as star formation, supernovae (SNe) and active galactic nucleus (AGN) feedback into our code, so that the effect of turbulence heating for IGM can be exhibited to the most extent. By computing cross-correlation functions $r_m(k)$ for the density field and $r_v(k)$ for the velocity field of both baryons and dark matter, we find that deviations between the two matter components for both density field and velocity field, as expected, are scale-dependent. That is, the deviations are the most significant at small scales and gradually diminish on larger and larger scales. Also, the deviations are time-dependent, i.e. they become larger and larger with increasing cosmic time. The most emphasized result is that the spatial deviations between baryons and dark matter revealed by velocity field are more significant than that by density field. At z = 0, at the 1% level of deviation, the deviation scale is about 3.7 $h^{-1}$Mpc for density field, while as large as 23 $h^{-1}$Mpc for velocity field, a scale that falls within the weakly non-linear regime for the structure formation paradigm. Our results indicate that the effect of turbulence heating is indeed comparable to that of these processes such as SN and AGN feedback. △ Less

Submitted 19 September, 2020; originally announced September 2020.

Comments: 13 pages, 9 figures

Journal ref: 2020, MNRAS, 498, 4411

On the Gravitational Instabilities of Protoplanetary Disks

Authors: Ning Sui, Ping He, Min Li

Abstract: The gravitational instabilities are important to the evolution of the disks and the planet formation in the disks. We calculate the evolution of the disks which form from the collapse of the molecular cloud cores. By changing the properties of the cloud cores and the hydrodynamical viscosity parameters, we explore their effects on the properties of the gravitational instabilities. We find that the… ▽ More The gravitational instabilities are important to the evolution of the disks and the planet formation in the disks. We calculate the evolution of the disks which form from the collapse of the molecular cloud cores. By changing the properties of the cloud cores and the hydrodynamical viscosity parameters, we explore their effects on the properties of the gravitational instabilities. We find that the disk is unstable when the angular velocity of the molecular cloud core is larger than a critical value. The time duration of the instability increases as the angular velocity of the core increases. The increase of the hydrodynamical viscosity parameter hardly affects the stability of the disk, but decreases the time duration of the critical state of the gravitational instability in the disk. The instability of the disks can happen at very early time of evolution of the disk, which is consistent with the observations. △ Less

Submitted 6 December, 2018; originally announced December 2018.

Comments: 10 pages, 9 figures, accepted by PASP

Constraining the optical depth of galaxies and velocity bias with cross-correlation between kinetic Sunyaev-Zeldovich effect and peculiar velocity field

Authors: Yin-Zhe Ma, Guo-Dong Gong, Ning Sui, Ping He

Abstract: We calculate the cross-correlation function $\langle (ΔT/T)(\mathbf{v}\cdot \mathbf{n}/σ_{v}) \rangle$ between the kinetic Sunyaev-Zeldovich (kSZ) effect and the reconstructed peculiar velocity field using linear perturbation theory, to constrain the optical depth $τ$ and peculiar velocity bias of central galaxies with Planck data. We vary the optical depth $τ$ and the velocity bias function… ▽ More We calculate the cross-correlation function $\langle (ΔT/T)(\mathbf{v}\cdot \mathbf{n}/σ_{v}) \rangle$ between the kinetic Sunyaev-Zeldovich (kSZ) effect and the reconstructed peculiar velocity field using linear perturbation theory, to constrain the optical depth $τ$ and peculiar velocity bias of central galaxies with Planck data. We vary the optical depth $τ$ and the velocity bias function $b_{v}(k)=1+b(k/k_{0})^{n}$, and fit the model to the data, with and without varying the calibration parameter $y_{0}$ that controls the vertical shift of the correlation function. By constructing a likelihood function and constraining $τ$, $b$ and $n$ parameters, we find that the quadratic power-law model of velocity bias $b_{v}(k)=1+b(k/k_{0})^{2}$ provides the best-fit to the data. The best-fit values are $τ=(1.18 \pm 0.24) \times 10^{-4}$, $b=-0.84^{+0.16}_{-0.20}$ and $y_{0}=(12.39^{+3.65}_{-3.66})\times 10^{-9}$ ($68\%$ confidence level). The probability of $b>0$ is only $3.12 \times 10^{-8}$ for the parameter $b$, which clearly suggests a detection of scale-dependent velocity bias. The fitting results indicate that the large-scale ($k \leq 0.1\,h\,{\rm Mpc}^{-1}$) velocity bias is unity, while on small scales the bias tends to become negative. The value of $τ$ is consistent with the stellar mass--halo mass and optical depth relation proposed in the previous literatures, and the negative velocity bias on small scales is consistent with the peak background-split theory. Our method provides a direct tool to study the gaseous and kinematic properties of galaxies. △ Less

Submitted 12 December, 2017; v1 submitted 23 November, 2017; originally announced November 2017.

Comments: 13 pages, 13 figures, 3 tables

Journal ref: Monthly Notices of Royal Astronomical Society, 475, 379--390 (2018)

Constraining cosmology with pairwise velocity estimator

Authors: Yin-Zhe Ma, Min Li, Ping He

Abstract: In this paper, we develop a full statistical method for the pairwise velocity estimator previously proposed, and apply Cosmicflows-2 catalogue to this method to constrain cosmology. We first calculate the covariance matrix for line-of-sight velocities for a given catalogue, and then simulate the mock full-sky surveys from it, and then calculate the variance for the pairwise velocity field. By appl… ▽ More In this paper, we develop a full statistical method for the pairwise velocity estimator previously proposed, and apply Cosmicflows-2 catalogue to this method to constrain cosmology. We first calculate the covariance matrix for line-of-sight velocities for a given catalogue, and then simulate the mock full-sky surveys from it, and then calculate the variance for the pairwise velocity field. By applying the $8315$ independent galaxy samples and compressed $5224$ group samples from Cosmicflows-2 catalogue to this statistical method, we find that the joint constraint on $Ω^{0.6}_{\rm m}h$ and $σ_{8}$ is completely consistent with the WMAP 9-year and Planck 2015 best-fitting cosmology. Currently, there is no evidence for the modified gravity models or any dynamic dark energy models from this practice, and the error-bars need to be reduced in order to provide any concrete evidence against/to support $Λ$CDM cosmology. △ Less

Submitted 27 October, 2015; v1 submitted 21 September, 2015; originally announced September 2015.

Comments: 7 pages, 6 figures

Journal ref: A&A 583, A52 (2015)

Pressure of Degenerate and Relativistic electrons in a superhigh magnetic field

Authors: Zhi Fu Gao, Wang Na, Peng Qiu He, Du Yuan Jie

Abstract: Based on our previous work, we deduce a general formula for pressure of degenerate and relativistic electrons,Pe, which is suitable for superhigh magnetic fields, discuss the quantization of Landau levels of electrons, and consider the quantum electrodynam-ic(QED) effects on the equations of states (EOSs) for different matter systems. The main conclusions are as follows:Pe is related to the magnet… ▽ More Based on our previous work, we deduce a general formula for pressure of degenerate and relativistic electrons,Pe, which is suitable for superhigh magnetic fields, discuss the quantization of Landau levels of electrons, and consider the quantum electrodynam-ic(QED) effects on the equations of states (EOSs) for different matter systems. The main conclusions are as follows:Pe is related to the magnetic field B, matter density ?, and electron fraction Ye ; the stronger the magnetic field, the higher the electron pressure becomes; the high electron pressure could be caused by high Fermi energy of electrons in a superhigh magnetic field; compared with a common radio pulsar, a magnetar could be a more compact oblate spheroid-like deformed neutron star due to the anisotropic total pressure; and an increase in the maximum mass of a magnetar is expected because of the positive contribution of the magnetic field energy to the EOS of the star. △ Less

Submitted 10 December, 2013; v1 submitted 5 December, 2013; originally announced December 2013.

Comments: 26 Papers, 5 Figures, Published in Modern Physics Letters A ., Vol.28, No.36, p.1350138 (2013)

Fluid-like entropy and equilibrium statistical mechanics of self-gravitating systems

Authors: Dong-Biao Kang, Ping He

Abstract: The statistical mechanics of self-gravitating systems has not been well understood, and still remains an open question so far. In a previous study by Kang & He, we showed that the fluid approximation may give a clue to further investigate this problem. In fact, there are indeed many dynamical similarities between self-gravitating and fluid systems. Based on a fluid-like entropy, that work explaine… ▽ More The statistical mechanics of self-gravitating systems has not been well understood, and still remains an open question so far. In a previous study by Kang & He, we showed that the fluid approximation may give a clue to further investigate this problem. In fact, there are indeed many dynamical similarities between self-gravitating and fluid systems. Based on a fluid-like entropy, that work explained successfully the outer density profiles of dark matter halos, but there left some drawbacks with the calculation concerning extremizing process of the entropy. In the current paper, with the improved extremizing calculation -- including an additional differential constraint of dynamical equilibrium and without any other assumptions, we confirm that statistical-mechanical methods can give a density profile with finite mass and finite energy. Moreover, this density profile is also consistent with the observational surface brightness of the elliptical galaxy NGC 3379. In our methods, the density profile is derived from the equation of state, which is obtained from entropy principle but does not correspond to the maximum entropy of the system. Finally, we suggest an alternative entropy form, a hybrid of Boltzmann-Gibbs and Tsallis entropy, whose global maximum may give rise to this equation of state. △ Less

Submitted 23 April, 2011; v1 submitted 20 April, 2011; originally announced April 2011.

Comments: 6 pages, 3 figures, MNRAS, in the press

Journal ref: MNRAS (2011), 416, 32

Second-order solutions of the equilibrium statistical mechanics for self-gravitating systems

Authors: Ping He

Abstract: In a previous study, we formulated a framework of the entropy-based equilibrium statistical mechanics for self-gravitating systems. This theory is based on the Boltzmann-Gibbs entropy and includes the generalized virial equations as additional constraints. With the truncated distribution function to the lowest order, we derived a set of second-order equations for the equilibrium states of the syst… ▽ More In a previous study, we formulated a framework of the entropy-based equilibrium statistical mechanics for self-gravitating systems. This theory is based on the Boltzmann-Gibbs entropy and includes the generalized virial equations as additional constraints. With the truncated distribution function to the lowest order, we derived a set of second-order equations for the equilibrium states of the system. In this work, the numerical solutions of these equations are investigated. It is found that there are three types of solutions for these equations. Both the isothermal and divergent solutions are thermally unstable and have unconfined density profiles with infinite mass, energy and spatial extent. The convergent solutions, however, seem to be reasonable. Although the results cannot match the simulation data well, because of the truncations of the distribution function and its moment equations, these lowest-order convergent solutions show that the density profiles of the system are confined, the velocity dispersions are variable functions of the radius, and the velocity distributions are also anisotropic in different directions. The convergent solutions also indicate that the statistical equilibrium of self-gravitating systems is by no means the thermodynamic equilibrium. These solutions are just the lowest-order approximation, but they have already manifested the qualitative success of our theory. We expect that higher-order solutions of our statistical-mechanical theory will give much better agreement with the simulation results concerning dark matter haloes. △ Less

Submitted 11 February, 2012; v1 submitted 4 April, 2011; originally announced April 2011.

Comments: 7 pages, 5 figures, MNRAS, in the press

Journal ref: MNRAS (2012), 421, 2088 - 2093

Saddle-point entropy states of equilibrated self-gravitating systems

Authors: Ping He, Dong-Biao Kang

Abstract: In this Letter, we investigate the stability of the statistical equilibrium of spherically symmetric collisionless self-gravitating systems. By calculating the second variation of the entropy, we find that perturbations of the relevant physical quantities should be classified as long- and short-range perturbations, which correspond to the long- and short-range relaxation mechanisms, respectively.… ▽ More In this Letter, we investigate the stability of the statistical equilibrium of spherically symmetric collisionless self-gravitating systems. By calculating the second variation of the entropy, we find that perturbations of the relevant physical quantities should be classified as long- and short-range perturbations, which correspond to the long- and short-range relaxation mechanisms, respectively. We show that the statistical equilibrium states of self-gravitating systems are neither maximum nor minimum, but complex saddle-point entropy states, and hence differ greatly from the case of ideal gas. Violent relaxation should be divided into two phases. The first phase is the entropy-production phase, while the second phase is the entropy-decreasing phase. We speculate that the second-phase violent relaxation may just be the long-wave Landau damping, which would work together with short-range relaxations to keep the system equilibrated around the saddle-point entropy states. △ Less

Submitted 31 March, 2011; originally announced March 2011.

Comments: 5 pages, 1 figure, MNRAS Letter, in the press

Journal ref: MNRAS Letters (2011), 414, L21

Equilibrium statistical mechanics for self-gravitating systems: local ergodicity and extended Boltzmann-Gibbs/White-Narayan statistics

Authors: Ping He

Abstract: The long-standing puzzle surrounding the statistical mechanics of self-gravitating systems has not yet been solved successfully. We formulate a systematic theoretical framework of entropy-based statistical mechanics for spherically symmetric collisionless self-gravitating systems. We use an approach that is very different from that of the conventional statistical mechanics of short-range interacti… ▽ More The long-standing puzzle surrounding the statistical mechanics of self-gravitating systems has not yet been solved successfully. We formulate a systematic theoretical framework of entropy-based statistical mechanics for spherically symmetric collisionless self-gravitating systems. We use an approach that is very different from that of the conventional statistical mechanics of short-range interaction systems. We demonstrate that the equilibrium states of self-gravitating systems consist of both mechanical and statistical equilibria, with the former characterized by a series of velocity-moment equations and the latter by statistical equilibrium equations, which should be derived from the entropy principle. The velocity-moment equations of all orders are derived from the steady-state collisionless Boltzmann equation. We point out that the ergodicity is invalid for the whole self-gravitating systems, but it can be re-established locally. Based on the local ergodicity, using Fermi-Dirac-like statistics, with the nondegenerate condition and the spatial independence of the local microstates, we rederive the Boltzmann-Gibbs entropy. This is consistent with the validity of the collisionless Boltzmann equation, and should be the correct entropy form for collisionless self-gravitating systems. Apart from the usual constraints of mass and energy conservation, we demonstrate that the series of moment or virialization equations must be included as additional constraints on the entropy functional when performing the variational calculus; this is an extension to the original prescription by White & Narayan. Any possible velocity distribution can be produced by the statistical-mechanical approach that we have developed with the extended Boltzmann-Gibbs/White-Narayan statistics. Finally, we discuss the questions of negative specific heat and ensemble inequivalence for self-gravitating systems. △ Less

Submitted 27 October, 2011; v1 submitted 29 March, 2011; originally announced March 2011.

Comments: 15 pages, 6 figures, MNRAS, in the press

Journal ref: MNRAS (2012), 419, 1667

A statistical-mechanical explanation of dark matter halo properties

Authors: Dong-Biao Kang, Ping He

Abstract: Cosmological N-body simulations have revealed many empirical relationships of dark matter halos, yet the physical origin of these halo properties still remains unclear. On the other hand, the attempts to establish the statistical mechanics for self-gravitating systems have encountered many formal difficulties, and little progress has been made for about fifty years. The aim of this work is to stre… ▽ More Cosmological N-body simulations have revealed many empirical relationships of dark matter halos, yet the physical origin of these halo properties still remains unclear. On the other hand, the attempts to establish the statistical mechanics for self-gravitating systems have encountered many formal difficulties, and little progress has been made for about fifty years. The aim of this work is to strengthen the validity of the statistical-mechanical approach we have proposed previously to explain the dark matter halo properties. By introducing an effective pressure instead of the radial pressure to construct the specific entropy, we use the entropy principle and proceed in a similar way as previously to obtain an entropy stationary equation. An equation of state for equilibrated dark halos is derived from this entropy stationary equation, by which the dark halo density profiles with finite mass can be obtained. We also derive the anisotropy parameter and pseudo-phase-space density profile. All these predictions agree well with numerical simulations in the outer regions of dark halos. Our work provides further support to the idea that statistical mechanics for self-gravitating systems is a viable tool for investigation. △ Less

Submitted 30 March, 2011; v1 submitted 5 December, 2010; originally announced December 2010.

Comments: 5 pages, 4 figures, Accepted by A&A

Journal ref: A&A (2011), 526, A147

Entropy principle and complementary second law of thermodynamics for self-gravitating systems

Authors: Ping He, Dong-Biao Kang

Abstract: (abbreviated) The statistical mechanics of self-gravitating systems is a long-held puzzle. In this work, we employ a phenomenological entropy form of ideal gas, first proposed by White & Narayan, to revisit this issue. By calculating the first-order variation of the entropy, subject to the mass- and energy-conservation constraints, we obtain an entropy stationary equation. Incorporated with the Je… ▽ More (abbreviated) The statistical mechanics of self-gravitating systems is a long-held puzzle. In this work, we employ a phenomenological entropy form of ideal gas, first proposed by White & Narayan, to revisit this issue. By calculating the first-order variation of the entropy, subject to the mass- and energy-conservation constraints, we obtain an entropy stationary equation. Incorporated with the Jeans equation, and by specifying some functional form for the anisotropy parameter beta, we numerically solve the two equations, and demonstrate that the velocity anisotropy parameter plays an important role to attain a density profile that is finite in mass, energy, and spatial extent. If incorporated again with some empirical density profile from simulations, our theoretical predictions of the anisotropy parameter, and the radial pseudo-phase-space density in the outer non-gravitationally degenerate region of the dark matter halo, agree rather well with the simulation data, and the predictions are also acceptable in the middle weak-degenerate region of the dark halo. The second-order variational calculus reveals the seemingly paradoxical but actually complementary consequence that the equilibrium state of self-gravitating systems is the global minimum entropy state for the whole system under long-range violent relaxation, but simultaneously the local maximum entropy state for every and any small part of the system under short-range two-body relaxation and Landau damping. This minimum-maximum entropy duality means that the standard second law of thermodynamics needs to be re-expressed or generalized for self-gravitating systems. We believe that our findings, especially the complementary second law of thermodynamics, may provide crucial clues to the development of the statistical physics of self-gravitating systems as well as other long-range interaction systems. △ Less

Submitted 14 May, 2010; v1 submitted 12 March, 2009; originally announced March 2009.

Comments: 11 pages, 6 eps figures, to be published in MNRAS, final version

Report number: CAS-KITPC/ITP-089

Journal ref: Mon.Not.Roy.Astron.Soc.406:2678-2688,2010

Distribution Function in Center of Dark Matter Halo

Authors: Ding Ma, Ping He

Abstract: N-body simulations of dark matter halos show that the density profiles of halos behave as $ρ(r)\propto r^{-α(r)}$, where the density logarithmic slope $α\simeq 1\sim1.5$ in the center and $α\simeq 3\sim 4$ in the outer parts of halos. However, some observations are not in agreement with simulations in the very central region of halos. The simulations also show that velocity dispersion anisotropy… ▽ More N-body simulations of dark matter halos show that the density profiles of halos behave as $ρ(r)\propto r^{-α(r)}$, where the density logarithmic slope $α\simeq 1\sim1.5$ in the center and $α\simeq 3\sim 4$ in the outer parts of halos. However, some observations are not in agreement with simulations in the very central region of halos. The simulations also show that velocity dispersion anisotropy parameter $β\approx 0$ in the inner part of the halo and the so called "pseudo phase-space density" $ρ/σ^3$ behaves as a power-law in radius $r$. With these results in mind, we study the distribution function and the pseudo phase-space density $ρ/σ^3$ of the center of dark matter halos and find that they are closely-related. △ Less

Submitted 1 August, 2008; v1 submitted 2 June, 2008; originally announced June 2008.

Comments: 8 pages, accepted by Int. J. Mod. Phys. D, add a quotation and remove the appendix

Journal ref: Int. J. Mod. Phys. D, 18 (2009) 477

Distribution Function of Dark Matter with Constant Anisotropy

Authors: Ding Ma, Ping He

Abstract: N-body simulations of dark matter halos show that the density is cusped near the center of the halo. The density profile behaves as $r^{-γ}$ in the inner parts, where $γ\simeq 1$ for the NFW model and $γ\simeq 1.5$ for the Moore's model, but in the outer parts, both models agree with each other in the asymptotic behavior of the density profile. The simulations also show the information about ani… ▽ More N-body simulations of dark matter halos show that the density is cusped near the center of the halo. The density profile behaves as $r^{-γ}$ in the inner parts, where $γ\simeq 1$ for the NFW model and $γ\simeq 1.5$ for the Moore's model, but in the outer parts, both models agree with each other in the asymptotic behavior of the density profile. The simulations also show the information about anisotropy parameter $β(r)$ of velocity distribution. $β\approx 0$ in the inner part and $β\approx 0.5$ (radially anisotropic) in the outer part of the halo. We provide some distribution functions $F(E,L)$ with the constant anisotropy parameter $β$ for the two spherical models of dark matter halos: a new generalized NFW model and a generalized Moore model. There are two parameters $α$ and $ε$ for those two generalized models to determine the asymptotic behavior of the density profile. In this paper, we concentrate on the situation of $β(r)=1/2$ from the viewpoint of the simulation. △ Less

Submitted 1 August, 2008; v1 submitted 10 March, 2008; originally announced March 2008.

Comments: 11 pages, accepted for publication in Int. J. Mod. Phys. D, add a quotation

Journal ref: Int.J.Mod.Phys.D17:1283-1294,2008

Low-Redshift Cosmic Baryon Fluid on Large Scales and She-Leveque Universal Scaling

Authors: Ping He, Jiren Liu, Long-Long Feng, Chi-Wang Shu, Li-Zhi Fang

Abstract: We investigate the statistical properties of cosmic baryon fluid in the nonlinear regime, which is crucial for understanding the large-scale structure formation of the universe. With the hydrodynamic simulation sample of the Universe in the cold dark matter model with a cosmological constant, we show that the intermittency of the velocity field of cosmic baryon fluid at redshift z=0 in the scale… ▽ More We investigate the statistical properties of cosmic baryon fluid in the nonlinear regime, which is crucial for understanding the large-scale structure formation of the universe. With the hydrodynamic simulation sample of the Universe in the cold dark matter model with a cosmological constant, we show that the intermittency of the velocity field of cosmic baryon fluid at redshift z=0 in the scale range from the Jeans length to about 16 Mpc/h can be extremely well described by She-Leveque's universal scaling formula. The baryon fluid also possesses the following features: (1) for volume weight statistics, the dissipative structures are dominated by sheets, and (2) the relation between the intensities of fluctuations is hierarchical. These results imply that the evolution of highly evolved cosmic baryon fluid is similar to a fully developed turbulence. △ Less

Submitted 9 February, 2006; v1 submitted 10 January, 2006; originally announced January 2006.

Comments: 4 pages, 3 figures

Journal ref: Phys.Rev.Lett. 96 (2006) 051302

X-ray Emission of Baryonic Gas in the Universe: Luminosity-Temperature Relationship and Soft-Band Background

Authors: Tong-Jie Zhang, Jiren Liu, Long-long Feng, Ping He, Li-Zhi Fang

Abstract: We study the X-ray emission of baryon fluid in the universe using the WIGEON cosmological hydrodynamic simulations. It has been revealed that cosmic baryon fluid in the nonlinear regime behaves like Burgers turbulence, i.e. the fluid field consists of shocks. Like turbulence in incompressible fluid, the Burgers turbulence plays an important role in converting the kinetic energy of the fluid to t… ▽ More We study the X-ray emission of baryon fluid in the universe using the WIGEON cosmological hydrodynamic simulations. It has been revealed that cosmic baryon fluid in the nonlinear regime behaves like Burgers turbulence, i.e. the fluid field consists of shocks. Like turbulence in incompressible fluid, the Burgers turbulence plays an important role in converting the kinetic energy of the fluid to thermal energy and heats the gas. We show that the simulation sample of the $Λ$CDM model without adding extra heating sources can fit well the observed distributions of X-ray luminosity versus temperature ($L_{\rm x}$ vs. $T$) of galaxy groups and is also consistent with the distributions of X-ray luminosity versus velocity dispersion ($L_{\rm x}$ vs. $σ$). Because the baryonic gas is multiphase, the $L_{\rm x}-T$ and $L_{\rm x}-σ$ distributions are significantly scattered. If we describe the relationships by power laws $L_{\rm x}\propto T^{α_{LT}}$ and $L_{\rm x}\propto σ^{α_{LV}}$, we find $α_{LT}>2.5$ and $α_{LV}>2.1$. The X-ray background in the soft $0.5-2$ keV band emitted by the baryonic gas in the temperature range $10^5<T<10^7$ K has also been calculated. We show that of the total background, (1) no more than 2% comes from the region with temperature less than $10^{6.5}$ K, and (2) no more than 7% is from the region of dark matter with mass density $ρ_{\rm dm}<50 \barρ_{\rm dm}$. The region of $ρ_{\rm dm}>50\barρ_{\rm dm}$ is generally clustered and discretely distributed. Therefore, almost all of the soft X-ray background comes from clustered sources, and the contribution from truly diffuse gas is probably negligible. This point agrees with current X-ray observations. △ Less

Submitted 9 May, 2006; v1 submitted 9 January, 2006; originally announced January 2006.

Comments: 32 pages including 14 figures and 2 tables. Final version for publication in ApJ

Journal ref: Astrophys.J. 642 (2006) 625-635

A Parameter-free Statistical Measurement of Halos with Power Spectra

Authors: Ping He, Long-Long Feng, Li-Zhi Fang

Abstract: We show that, in the halo model of large-scale structure formation, the difference between the Fourier and the DWT (discrete wavelet transform) power spectra provides a statistical measurement of the halos. This statistical quantity is free from parameters related to the shape of the mass profile and the identification scheme of halos. That is, the statistical measurement is invariant in the sen… ▽ More We show that, in the halo model of large-scale structure formation, the difference between the Fourier and the DWT (discrete wavelet transform) power spectra provides a statistical measurement of the halos. This statistical quantity is free from parameters related to the shape of the mass profile and the identification scheme of halos. That is, the statistical measurement is invariant in the sense that models with reasonably defined and selected parameters of the halo models should yield the same difference of the Fourier and DWT spectra. This feature is useful to extract ensemble averaged properties of halos, which cannot be obtained with the identification of individual halo. To demonstrate this point, we show with WIGEON hydrodynamical simulation samples that the spectrum difference provides a quantitative measurement of the discrepancy of the distribution of baryonic gas from that of the underlying dark matter field within halos. We also show that the mass density profile of halos in physical space can be reconstructed with this statistical measurement. This profile essentially is the average over an ensemble of halos, including well virialized halos as well as halos with significant internal substructures. Moreover, this reconstruction is sensitive to the tail of the mass density profile. We showed that the profile with $1/r^3$ tail gives very different result from that of $1/r^2$. Other possible applications of this method are discussed as well. △ Less

Submitted 31 May, 2005; v1 submitted 4 April, 2005; originally announced April 2005.

Comments: To appear in ApJ, final version

Journal ref: Astrophys.J. 628 (2005) 14-20

The velocity field of baryonic gas in the universe

Authors: Bryan Kim, Ping He, Jesús Pando, Long-Long Feng, Li-Zhi Fang

Abstract: The dynamic evolution of the baryonic intergalactic medium (IGM) caused by the underlying dark matter gravity is governed by the Navier-Stokes equations in which many cooling and heating processes are involved. However, it has long been recognized that the growth mode dynamics of cosmic matter clustering can be sketched by a random force driven Burgers' equation if cooling and heating are ignore… ▽ More The dynamic evolution of the baryonic intergalactic medium (IGM) caused by the underlying dark matter gravity is governed by the Navier-Stokes equations in which many cooling and heating processes are involved. However, it has long been recognized that the growth mode dynamics of cosmic matter clustering can be sketched by a random force driven Burgers' equation if cooling and heating are ignored. Just how well the dynamics of the IGM can be described as a Burgers fluid has not been fully investigated probably because cooling and heating are essential for a detailed understanding of the IGM. Using IGM samples produced by a cosmological hydrodynamic simulation in which heating and cooling processes are properly accounted for, we show that the IGM velocity field in the nonlinear regime shows the features of a Burgers fluid, that is, when the Reynolds number is high, the velocity field consists of an ensemble of shocks. Consequently, (1) the IGM velocity $v$ is generally smaller than that of dark matter; (2) for the smoothed field, the IGM velocity shows tight correlation with dark matter given by $v \simeq s v_{dm}$, with $s<1$, such that the lower the redshift, the smaller $s$; (3) the velocity PDFs are asymmetric between acceleration and deceleration events; (4) the PDF of velocity difference $Δv=v(x+r)-v(x)$ satisfies the scaling relation for a Burgers fluid, i.e., $P(Δv)=(1 r^y)F(Δv/r^y)$. We find the scaling function and parameters for the IGM which are applicable to the entire scale range of the samples (0.26 - 8 h$^{-1}$ Mpc). These properties show that the similarity mapping between the IGM and dark matter is violated on scales much larger than the Jeans length of the IGM. △ Less

Submitted 4 October, 2005; v1 submitted 8 February, 2005; originally announced February 2005.

Comments: 14 pages, 10 jpg-figures, accepted for publication in the Astrophysical Journal. References added

Journal ref: Astrophys.J.625:599-612,2005; Addendum-ibid.639:571,2006