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Hunting Dark Matter with the Einstein Telescope
Authors:
A. J. Iovino,
M. Maggiore,
N. Muttoni,
A. Riotto
Abstract:
Too light primordial black holes evaporate and are therefore strongly constrained by various bounds, e.g. Cosmic Microwave Background distortion. However, if they are formed strongly clustered, the corresponding haloes may collapse in heavier black holes which may form the entirety of the dark matter of the universe. The indirect signal of such scenario is the production of a flat stochastic backg…
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Too light primordial black holes evaporate and are therefore strongly constrained by various bounds, e.g. Cosmic Microwave Background distortion. However, if they are formed strongly clustered, the corresponding haloes may collapse in heavier black holes which may form the entirety of the dark matter of the universe. The indirect signal of such scenario is the production of a flat stochastic background of gravitational waves which is detectable by the Einstein Telescope.
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Submitted 7 April, 2026;
originally announced April 2026.
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Black Hole Mergers as the Fastest Photon Ring Scramblers
Authors:
D. Giataganas,
G. F. Giudice,
A. Ianniccari,
A. J. Iovino,
A. Kehagias,
F. Quevedo,
D. Perrone,
A. Riotto
Abstract:
Black holes are the most efficient scramblers in nature. By mapping the instantaneous mass and angular momentum of two spinless black holes in a quasi-circular binary onto those of an effective Kerr black hole, we demonstrate that the final state of the merger remnant corresponds with remarkable accuracy to the configuration that renders null geodesics unstable at the highest possible rate. This s…
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Black holes are the most efficient scramblers in nature. By mapping the instantaneous mass and angular momentum of two spinless black holes in a quasi-circular binary onto those of an effective Kerr black hole, we demonstrate that the final state of the merger remnant corresponds with remarkable accuracy to the configuration that renders null geodesics unstable at the highest possible rate. This suggests a deep connection between the properties of black holes resulting from binary mergers and their unstable null orbits.
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Submitted 9 March, 2026;
originally announced March 2026.
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Dark Matter from Eternity
Authors:
G. Franciolini,
M. Peloso,
A. Riotto
Abstract:
We propose that the totality of dark matter in the universe might ascribe its origin to one of the key properties of cosmological inflation, that it may be eternal: regions that at the end of the primordial accelerated expansion of the universe never reheated, but keep eternally inflating, manifest themselves as primordial black holes in our observable universe. This mechanism can provide a primor…
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We propose that the totality of dark matter in the universe might ascribe its origin to one of the key properties of cosmological inflation, that it may be eternal: regions that at the end of the primordial accelerated expansion of the universe never reheated, but keep eternally inflating, manifest themselves as primordial black holes in our observable universe. This mechanism can provide a primordial black hole abundance which is larger than the standard one due to the gravitational collapse of sizeable overdensities in the radiation phase. It also predicts a broad spectrum for the curvature perturbation and a flat stochastic gravitational wave background at a level of $Ω_\text{GW} h^2 \simeq 10^{-10}$ up to the mHz.
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Submitted 9 February, 2026;
originally announced February 2026.
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Standard Model Higgs Peaks: a Note on the Vacuum Instability during Inflation
Authors:
G. Franciolini,
A. Kehagias,
A. Riotto
Abstract:
In the Standard Model, the Higgs potential develops an instability at high field values when the quartic self-coupling runs negative. Large quantum fluctuations during cosmic inflation could drive the Higgs field beyond the potential barrier, creating regions that would be catastrophic for our observable universe. We point out that the extreme-value statistics describing the peaks (maxima) of the…
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In the Standard Model, the Higgs potential develops an instability at high field values when the quartic self-coupling runs negative. Large quantum fluctuations during cosmic inflation could drive the Higgs field beyond the potential barrier, creating regions that would be catastrophic for our observable universe. We point out that the extreme-value statistics describing the peaks (maxima) of the Higgs values is the correct statistics to infer the condition to avoid vacuum instability. Even if this statistics delivers a bound on the Hubble rate during inflation which is only a factor $\sqrt{2}$ stronger than the one commonly adopted in the literature, it is qualitatively distinct and we believe worthwhile communicating it.
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Submitted 6 January, 2026;
originally announced January 2026.
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Are Primordial Black Holes Truly Fine-Tuned?
Authors:
A. J. Iovino,
A. Riotto
Abstract:
Single-field inflationary models which generate primordial black holes through the enhancement of the curvature primordial power at small scales are commonly criticized and frequently dismissed because they require a large amount of fine-tuning in the parameters setting the ultra slow-roll phase. However, the standarly adopted definition of fine-tuning has a clear drawback: the more the primordial…
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Single-field inflationary models which generate primordial black holes through the enhancement of the curvature primordial power at small scales are commonly criticized and frequently dismissed because they require a large amount of fine-tuning in the parameters setting the ultra slow-roll phase. However, the standarly adopted definition of fine-tuning has a clear drawback: the more the primordial black hole abundance is small and cosmologically harmless, the larger the parameter space is fine-tuned. A reliable measure of fine-tuning should deliver a large value when the primordial black hole abundance is fine-tuned and at the same time reduce to something close to unity when it encounters typical sensitivity. Motivated by such arguments, we use the (modified version of) Wilson's naturalness criterion for quantifying the fine-tuning and naturalness and we show that the primordial black hole models are not technically unnatural.
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Submitted 22 December, 2025;
originally announced December 2025.
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Uncovering the population of compact binary mergers and their formation pathways with gravitational waves through the Einstein Telescope
Authors:
M. Arca-Sedda,
I. Dvorkin,
G. Franciolini,
M. C. Artale,
M. Branchesi,
E. Bortolas,
M. Colpi,
V. De Luca,
A. Ghosh,
M. Maggiore,
M. Mapelli,
B. Mestichelli,
M. Mezcua,
S. Nissanke,
L. Paiella,
A. Riotto,
F. Santoliquido,
N. Tamanini,
R. Schneider,
C. Ugolini,
M. P. Vaccaro,
K. Yakut
Abstract:
Ground-based gravitational-wave (GW) observatories have transformed our view of compact-object mergers, yet their reach still limits a comprehensive reconstruction of the processes that generate these systems. Only next-generation observatories, with order-of-magnitude improvements in sensitivity and access to lower frequencies, will be capable of radically extending this detection horizon. GW obs…
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Ground-based gravitational-wave (GW) observatories have transformed our view of compact-object mergers, yet their reach still limits a comprehensive reconstruction of the processes that generate these systems. Only next-generation observatories, with order-of-magnitude improvements in sensitivity and access to lower frequencies, will be capable of radically extending this detection horizon. GW observations will make it possible to detect the complete population of binary black hole (BBH) mergers out to redshifts of $z \simeq 100$. This capability will deliver an unprecedented map of merger events across cosmic time and enable precise reconstruction of their mass and spin distributions, while for several thousand events the signal-to-noise ratio will surpass 100, enabling precision physics of BHs and neutron stars (NSs). The access to lower frequencies will also open the intermediate-mass window, detecting systems of order $\sim 10^3 M_\odot$, potentially in coordination with multi-band observations from LISA. At higher redshifts, where Population III stars have so far remained beyond reach - even for the James Webb Space Telescope - GW observations by next-generation detectors will routinely provide observations of BH mergers thought to originate from these primordial stellar populations. Such measurements are expected to play a central role in clarifying the early assembly of supermassive black holes. A single detection of a binary BH system at $z \gtrsim 30$, or of a compact object with sub-solar mass and no tidal deformability, would constitute strong evidence for the existence of primordial black holes. Such a discovery would have profound consequences for our understanding of dark matter and the early Universe. Ultimately, the GW observations will become revolutionary for identifying the physical channels responsible for compact binary formation.
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Submitted 19 December, 2025;
originally announced December 2025.
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Understanding the Nature of Scalar-Induced Gravitational Waves
Authors:
A. J. Iovino,
G. Perna,
D. Perrone,
D. Racco,
A. Riotto
Abstract:
We offer a physical interpretation of the origin of the scalar-induced gravitational wave background, showing that it is mainly produced around the peaks of the scalar perturbations. We also provide a compact expression to estimate the amount of scalar-induced gravitational waves generated by peaks.
We offer a physical interpretation of the origin of the scalar-induced gravitational wave background, showing that it is mainly produced around the peaks of the scalar perturbations. We also provide a compact expression to estimate the amount of scalar-induced gravitational waves generated by peaks.
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Submitted 29 September, 2025;
originally announced September 2025.
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Nonlinearities of Schwarzschild Black Hole Head-on Collisions
Authors:
Alex Kehagias,
Davide Perrone,
Antonio Riotto
Abstract:
We derive analytically the amplitude of the quadratic quasi-normal mode generated in the ringdown stage of the gravitational waveform produced by the ultra-relativistic head-on collision of two non-spinning Schwarzschild black holes. Although being a highly nonlinear event, second-order perturbation theory suffices and that nonlinearities may be derived by a simple bootstrapping procedure.
We derive analytically the amplitude of the quadratic quasi-normal mode generated in the ringdown stage of the gravitational waveform produced by the ultra-relativistic head-on collision of two non-spinning Schwarzschild black holes. Although being a highly nonlinear event, second-order perturbation theory suffices and that nonlinearities may be derived by a simple bootstrapping procedure.
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Submitted 25 August, 2025;
originally announced August 2025.
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GW231123: a Possible Primordial Black Hole Origin
Authors:
Valerio De Luca,
Gabriele Franciolini,
Antonio Riotto
Abstract:
GW231123, the heaviest binary black hole merger detected by the LIGO-Virgo-KAGRA collaboration to date, lies in the pair-instability mass gap and exhibits unusually high component spins. In this letter, we show that both merging black holes may have a primordial origin with smaller initial masses. The observed masses and, crucially, the spins of GW231123 are naturally accommodated within the most…
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GW231123, the heaviest binary black hole merger detected by the LIGO-Virgo-KAGRA collaboration to date, lies in the pair-instability mass gap and exhibits unusually high component spins. In this letter, we show that both merging black holes may have a primordial origin with smaller initial masses. The observed masses and, crucially, the spins of GW231123 are naturally accommodated within the most vanilla primordial black hole framework, once cosmological accretion is taken into account. Interestingly, the parameter space needed to explain the inferred GW231123 rate is at the edge of the exclusion region from Xray and CMB observations, suggesting that this interpretation can be either confirmed or ruled out. The upcoming O5 observing run by the collaboration should detect ${\cal O}(20)$ similar events, testing their mass-spin correlation, while next-generation detectors would be capable of observing high redshift events, as predicted in this scenario.
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Submitted 13 August, 2025;
originally announced August 2025.
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Primordial black hole ringdown: The irreducible stochastic gravitational wave background
Authors:
Valerio De Luca,
Antonio J. Iovino,
Antonio Riotto
Abstract:
Independently from the formation mechanism of primordial black holes in the early Universe, their generation is accompanied by a ringdown phase during which they relax to a stationary configuration and gravitational waves under the form of quasinormal modes are emitted. Such gravitational waves generate an irreducible and unavoidable stochastic background which is testable by current and future ex…
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Independently from the formation mechanism of primordial black holes in the early Universe, their generation is accompanied by a ringdown phase during which they relax to a stationary configuration and gravitational waves under the form of quasinormal modes are emitted. Such gravitational waves generate an irreducible and unavoidable stochastic background which is testable by current and future experiments. In particular, for primordial black holes with masses exceeding $10^{14}\,M_{\odot}$, the associated stochastic background lies within the frequency range accessible to current and upcoming cosmic microwave background experiments, thereby providing a direct observational way to probe the existence of such extremely massive objects.
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Submitted 20 February, 2026; v1 submitted 5 July, 2025;
originally announced July 2025.
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Nonlinearities in Kerr Black Hole Ringdown from the Penrose Limit
Authors:
D. Perrone,
A. Kehagias,
A. Riotto
Abstract:
We provide a fully analytical approach to calculate the nonlinearities of the gravitational waves in the ringdown of a Kerr black hole in the eikonal limit. The corresponding quasi-normal modes are associated to the orbits of a closed circular null geodesic and the problem can be analyzed by taking the Penrose limit around it. We calculate analytically the amplitude and the phase of the quadratic…
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We provide a fully analytical approach to calculate the nonlinearities of the gravitational waves in the ringdown of a Kerr black hole in the eikonal limit. The corresponding quasi-normal modes are associated to the orbits of a closed circular null geodesic and the problem can be analyzed by taking the Penrose limit around it. We calculate analytically the amplitude and the phase of the quadratic quasi-normal modes as well as its dependence on the black hole spin.
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Submitted 2 July, 2025;
originally announced July 2025.
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The AdS Perspective on the Nonlinear Tails in Black Hole Ringdown
Authors:
Alex Kehagias,
Antonio Riotto
Abstract:
Black holes gradually settle into their static configuration by emitting gravitational waves, whose amplitude diminish over time according to a power-law decay at fixed spatial locations. We show that the nonlinear tails in the presence of a quadratic source, which have been recently found to potentially dominate over the linear ones, can be simply derived from the AdS$_2$$\times$S$^2$ spacetime p…
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Black holes gradually settle into their static configuration by emitting gravitational waves, whose amplitude diminish over time according to a power-law decay at fixed spatial locations. We show that the nonlinear tails in the presence of a quadratic source, which have been recently found to potentially dominate over the linear ones, can be simply derived from the AdS$_2$$\times$S$^2$ spacetime perspective with their amplitudes being related to the Aretakis constants.
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Submitted 17 June, 2025;
originally announced June 2025.
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Black hole spectroscopy: from theory to experiment
Authors:
Emanuele Berti,
Vitor Cardoso,
Gregorio Carullo,
Jahed Abedi,
Niayesh Afshordi,
Simone Albanesi,
Vishal Baibhav,
Swetha Bhagwat,
José Luis Blázquez-Salcedo,
Béatrice Bonga,
Bruno Bucciotti,
Giada Caneva Santoro,
Pablo A. Cano,
Collin Capano,
Mark Ho-Yeuk Cheung,
Cecilia Chirenti,
Gregory B. Cook,
Adrian Ka-Wai Chung,
Marina De Amicis,
Kyriakos Destounis,
Oscar J. C. Dias,
Walter Del Pozzo,
Francisco Duque,
Will M. Farr,
Eliot Finch
, et al. (43 additional authors not shown)
Abstract:
The "ringdown" radiation emitted by oscillating black holes has great scientific potential. By carefully predicting the frequencies and amplitudes of black hole quasinormal modes and comparing them with gravitational-wave data from compact binary mergers we can advance our understanding of the two-body problem in general relativity, verify the predictions of the theory in the regime of strong and…
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The "ringdown" radiation emitted by oscillating black holes has great scientific potential. By carefully predicting the frequencies and amplitudes of black hole quasinormal modes and comparing them with gravitational-wave data from compact binary mergers we can advance our understanding of the two-body problem in general relativity, verify the predictions of the theory in the regime of strong and dynamical gravitational fields, and search for physics beyond the Standard Model or new gravitational degrees of freedom. We summarize the state of the art in our understanding of black hole quasinormal modes in general relativity and modified gravity, their excitation, and the modeling of ringdown waveforms. We also review the status of LIGO-Virgo-KAGRA ringdown observations, data analysis techniques, and the bright prospects of the field in the era of LISA and next-generation ground-based gravitational-wave detectors.
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Submitted 24 August, 2025; v1 submitted 29 May, 2025;
originally announced May 2025.
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Nonlinear Tails of Gravitational Waves in Schwarzschild Black Hole Ringdown
Authors:
Alex Kehagias,
Antonio Riotto
Abstract:
Schwarzschild black holes evolve toward their static configuration by emitting gravitational waves, which decay over time following a power law at fixed spatial positions. We derive this power law analytically for the second-order even gravitational perturbations, demonstrating that it is determined by the fact that the second-order source decays as the inverse square of the distance. Quadratic gr…
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Schwarzschild black holes evolve toward their static configuration by emitting gravitational waves, which decay over time following a power law at fixed spatial positions. We derive this power law analytically for the second-order even gravitational perturbations, demonstrating that it is determined by the fact that the second-order source decays as the inverse square of the distance. Quadratic gravitational modes with multipole $\ell$ decay according to a law $\sim t^{-2\ell-1}$, in contrast to the linear Price law scaling $\sim t^{-2\ell-3}$. Consequently, nonlinear tails may persist longer than their linear counterparts.
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Submitted 8 April, 2025;
originally announced April 2025.
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The Science of the Einstein Telescope
Authors:
Adrian Abac,
Raul Abramo,
Simone Albanesi,
Angelica Albertini,
Alessandro Agapito,
Michalis Agathos,
Conrado Albertus,
Nils Andersson,
Tomas Andrade,
Igor Andreoni,
Federico Angeloni,
Marco Antonelli,
John Antoniadis,
Fabio Antonini,
Manuel Arca Sedda,
M. Celeste Artale,
Stefano Ascenzi,
Pierre Auclair,
Matteo Bachetti,
Charles Badger,
Biswajit Banerjee,
David Barba-Gonzalez,
Daniel Barta,
Nicola Bartolo,
Andreas Bauswein
, et al. (463 additional authors not shown)
Abstract:
Einstein Telescope (ET) is the European project for a gravitational-wave (GW) observatory of third-generation. In this paper we present a comprehensive discussion of its science objectives, providing state-of-the-art predictions for the capabilities of ET in both geometries currently under consideration, a single-site triangular configuration or two L-shaped detectors. We discuss the impact that E…
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Einstein Telescope (ET) is the European project for a gravitational-wave (GW) observatory of third-generation. In this paper we present a comprehensive discussion of its science objectives, providing state-of-the-art predictions for the capabilities of ET in both geometries currently under consideration, a single-site triangular configuration or two L-shaped detectors. We discuss the impact that ET will have on domains as broad and diverse as fundamental physics, cosmology, early Universe, astrophysics of compact objects, physics of matter in extreme conditions, and dynamics of stellar collapse. We discuss how the study of extreme astrophysical events will be enhanced by multi-messenger observations. We highlight the ET synergies with ground-based and space-borne GW observatories, including multi-band investigations of the same sources, improved parameter estimation, and complementary information on astrophysical or cosmological mechanisms obtained combining observations from different frequency bands. We present advancements in waveform modeling dedicated to third-generation observatories, along with open tools developed within the ET Collaboration for assessing the scientific potentials of different detector configurations. We finally discuss the data analysis challenges posed by third-generation observatories, which will enable access to large populations of sources and provide unprecedented precision.
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Submitted 29 August, 2025; v1 submitted 15 March, 2025;
originally announced March 2025.
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Non-linear Quasi-Normal Modes of the Schwarzschild Black Hole from the Penrose Limit
Authors:
Alex Kehagias,
Davide Perrone,
Antonio Riotto
Abstract:
The Penrose limit connects a plane wave geometry to the photon ring of a black hole, where the quasi-normal modes are located in the eikonal limit. Utilizing this simplification, we analytically extract the quadratic-level non-linearities in the quasi-normal modes of a Schwarzschild black hole for the $(\ell\times\ell)\to 2\ell$ channel. We demonstrate that this result is independent of $\ell$ and…
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The Penrose limit connects a plane wave geometry to the photon ring of a black hole, where the quasi-normal modes are located in the eikonal limit. Utilizing this simplification, we analytically extract the quadratic-level non-linearities in the quasi-normal modes of a Schwarzschild black hole for the $(\ell\times\ell)\to 2\ell$ channel. We demonstrate that this result is independent of $\ell$ and further confirm it through symmetry arguments.
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Submitted 12 March, 2025;
originally announced March 2025.
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The Irrelevance of Primordial Black Hole Clustering in the LVK mass range
Authors:
F. Crescimbeni,
V. Desjacques,
G. Franciolini,
A. Ianniccari,
A. J. Iovino,
G. Perna,
D. Perrone,
A. Riotto,
H. Veermäe
Abstract:
We show that in realistic models where primordial black holes are formed due to the collapse of sizeable inflationary perturbations, their initial spatial clustering beyond Poisson distribution does not play any role in the binary mergers, including sub-solar primordial black holes, responsible for the gravitational waves detectable by LIGO-Virgo-KAGRA. This is a consequence of the existing FIRAS…
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We show that in realistic models where primordial black holes are formed due to the collapse of sizeable inflationary perturbations, their initial spatial clustering beyond Poisson distribution does not play any role in the binary mergers, including sub-solar primordial black holes, responsible for the gravitational waves detectable by LIGO-Virgo-KAGRA. This is a consequence of the existing FIRAS CMB distortion constraints on the relevant scales. This conclusion might not hold for lighter masses potentially accessible by future gravitational wave observations.
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Submitted 2 May, 2025; v1 submitted 3 February, 2025;
originally announced February 2025.
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How Well Do We Know the Scalar-Induced Gravitational Waves?
Authors:
A. J. Iovino,
S. Matarrese,
G. Perna,
A. Ricciardone,
A. Riotto
Abstract:
Gravitational waves sourced by amplified scalar perturbations are a common prediction across a wide range of cosmological models. These scalar curvature fluctuations are inherently nonlinear and typically non-Gaussian. We argue that the effects of non-Gaussianity may not always be adequately captured by an expansion around a Gaussian field, expressed through nonlinear parameters such as…
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Gravitational waves sourced by amplified scalar perturbations are a common prediction across a wide range of cosmological models. These scalar curvature fluctuations are inherently nonlinear and typically non-Gaussian. We argue that the effects of non-Gaussianity may not always be adequately captured by an expansion around a Gaussian field, expressed through nonlinear parameters such as $f_{\rm{NL}}$. As a consequence, the resulting amplitude of the stochastic gravitational wave background may differ significantly from predictions based on the standard quadratic source model routinely used in the literature.
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Submitted 9 December, 2024;
originally announced December 2024.
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Nonlinear Effects in Black Hole Ringdown Made Simple: Quasi-Normal Modes as Adiabatic Modes
Authors:
A. Kehagias,
A. Riotto
Abstract:
The nonlinear nature of general relativity manifests prominently throughout the merger of two black holes, from the inspiral phase to the final ringdown. Notably, the quasi-normal modes generated during the ringdown phase display significant nonlinearities. We show that these nonlinear effects can be effectively captured by zooming in on the photon ring through the Penrose limit. Specifically, we…
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The nonlinear nature of general relativity manifests prominently throughout the merger of two black holes, from the inspiral phase to the final ringdown. Notably, the quasi-normal modes generated during the ringdown phase display significant nonlinearities. We show that these nonlinear effects can be effectively captured by zooming in on the photon ring through the Penrose limit. Specifically, we model the quasi-normal modes as null particles trapped in unstable circular orbits around the black holes and show that they can be interpreted as adiabatic modes, perturbations that are arbitrarily close to large diffeomorphisms. This enables the derivation of a simple analytical expression for the QNM nonlinearities for Schwarzschild and Kerr black holes which reproduces well the existing numerical results.
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Submitted 12 November, 2024;
originally announced November 2024.
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Flea on the elephant: Tidal Love numbers in subsolar primordial black hole searches
Authors:
Valerio De Luca,
Gabriele Franciolini,
Antonio Riotto
Abstract:
Detecting subsolar objects in black hole binary mergers is considered a smoking gun signature of primordial black holes. Their supposedly vanishing tidal Love number is generically thought to help distinguish them from other subsolar and more deformable compact objects, such as neutron stars. We show that a large and detectable Love number of primordial black holes can be generated in the presence…
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Detecting subsolar objects in black hole binary mergers is considered a smoking gun signature of primordial black holes. Their supposedly vanishing tidal Love number is generically thought to help distinguish them from other subsolar and more deformable compact objects, such as neutron stars. We show that a large and detectable Love number of primordial black holes can be generated in the presence of even small disturbances of the system, thus potentially jeopardizing their discovery. However, such small perturbations are not tightly bound and are therefore disrupted before the mergers. We show that they leave a characteristic signature in the gravitational waveform that could be observed with current and future gravitational wave detectors. Thus, they may still hint towards the primordial nature of the black holes in the merger. Finally, we demonstrate that disregarding possible environmental effects in the matched-filter search for subsolar gravitational wave events can lead to a decreased sensitivity in the detectors.
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Submitted 8 November, 2024; v1 submitted 26 August, 2024;
originally announced August 2024.
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Deciphering the Instability of the Black Hole Ringdown Quasinormal Spectrum
Authors:
A. Ianniccari,
A. J. Iovino,
A. Kehagias,
P. Pani,
G. Perna,
D. Perrone,
A. Riotto
Abstract:
The spectrum of the quasinormal modes of the gravitational waves emitted during the ringdown phase following the merger of two black holes is of primary importance in gravitational astronomy. However, the spectrum is extremely sensitive to small disturbances of the system, thus potentially jeopardizing the predictions of the gravitational wave observables. We offer an analytical and intuitive expl…
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The spectrum of the quasinormal modes of the gravitational waves emitted during the ringdown phase following the merger of two black holes is of primary importance in gravitational astronomy. However, the spectrum is extremely sensitive to small disturbances of the system, thus potentially jeopardizing the predictions of the gravitational wave observables. We offer an analytical and intuitive explanation of such an instability and its properties based on the transfer matrix approach of quantum mechanics. We also give a simple interpretation of the fact that the prompt ringdown response in the time domain and the black hole greybody factor receive parametrically small corrections, thus being robust observables.
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Submitted 29 July, 2024;
originally announced July 2024.
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Curbing PBHs with PTAs
Authors:
A. J. Iovino,
G. Perna,
A. Riotto,
H. Veermäe
Abstract:
Sizeable primordial curvature perturbations needed to seed a population of primordial black holes (PBHs) will be accompanied by a scalar-induced gravitational wave signal that can be detectable by pulsar timing arrays (PTA). We derive conservative bounds on the amplitude of the scalar power spectrum at the PTA frequencies and estimate the implied constraints on the PBH abundance. We show that only…
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Sizeable primordial curvature perturbations needed to seed a population of primordial black holes (PBHs) will be accompanied by a scalar-induced gravitational wave signal that can be detectable by pulsar timing arrays (PTA). We derive conservative bounds on the amplitude of the scalar power spectrum at the PTA frequencies and estimate the implied constraints on the PBH abundance. We show that only a small fraction of dark matter can consist of stellar mass PBHs when the abundance is calculated using threshold statistics. The strength and the shape of the constraint depend on the shape of the power spectrum and the nature of the non-Gaussianities. We find that constraints on the PBH abundance arise in the mass range $0.1-10^3\, M_{\odot}$, with the sub-solar mass range being constrained only for narrow curvature power spectra. These constraints are softened when positive non-Gaussianity is introduced and can be eliminated when $f_{\rm NL} \gtrsim 5$. On the other hand, if the PBH abundance is computed via the theory of peaks, the PTA constraints on PBHs are significantly relaxed, signalling once more the theoretical uncertainties in assessing the PBH abundance. We further discuss how strong positive non-Gaussianites can allow for heavy PBHs to potentially seed supermassive BHs.
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Submitted 16 October, 2024; v1 submitted 28 June, 2024;
originally announced June 2024.
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A Short Note on the Love Number of Extremal Reissner-Nordstrom and Kerr-Newman Black Holes
Authors:
Alex Kehagias,
Davide Perrone,
Antonio Riotto
Abstract:
We provide a simple proof of why the Love number vanishes for extremal Reissner-Nordstrom and Kerr-Newman black holes. The argument is based on a conformal inversion isometry of the spacetime connecting the horizon with large distances.
We provide a simple proof of why the Love number vanishes for extremal Reissner-Nordstrom and Kerr-Newman black holes. The argument is based on a conformal inversion isometry of the spacetime connecting the horizon with large distances.
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Submitted 27 June, 2024;
originally announced June 2024.
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Why the Universal Threshold for Primordial Black Hole Formation is Universal
Authors:
Alex Kehagias,
Davide Perrone,
Antonio Riotto
Abstract:
We show why the threshold for primordial black hole formation is universal (independent from the shape of the perturbation) when expressed in terms of the volume averaged compaction function. The proof is rooted in the self-similarity of the gravitational collapse phenomenon at criticality.
We show why the threshold for primordial black hole formation is universal (independent from the shape of the perturbation) when expressed in terms of the volume averaged compaction function. The proof is rooted in the self-similarity of the gravitational collapse phenomenon at criticality.
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Submitted 8 May, 2024;
originally announced May 2024.
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The Black Hole Formation -- Null Geodesic Correspondence
Authors:
Andrea Ianniccari,
Antonio J. Iovino,
Alex Kehagias,
Davide Perrone,
Antonio Riotto
Abstract:
We provide evidence for a correspondence between the formation of black holes and the stability of circular null geodesics around the collapsing perturbation. We first show that the critical threshold of the compaction function to form a black hole in radiation is well approximated by the critical threshold for the appearance of the first unstable circular orbit in a spherically symmetric backgrou…
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We provide evidence for a correspondence between the formation of black holes and the stability of circular null geodesics around the collapsing perturbation. We first show that the critical threshold of the compaction function to form a black hole in radiation is well approximated by the critical threshold for the appearance of the first unstable circular orbit in a spherically symmetric background. We also show that the critical exponent in the scaling law of the primordial black hole mass close to the threshold is set by the inverse of the Lyapunov coefficient of the unstable orbits when a self-similar stage is developed close to criticality.
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Submitted 26 July, 2024; v1 submitted 3 April, 2024;
originally announced April 2024.
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Quasinormal Modes and Universality of the Penrose Limit of Black Hole Photon Rings
Authors:
D. Giataganas,
A. Kehagias,
A. Riotto
Abstract:
We study the physics of photon rings in a wide range of axisymmetric black holes admitting a separable Hamilton-Jacobi equation for the geodesics. Utilizing the Killing-Yano tensor, we derive the Penrose limit of the black holes, which describes the physics near the photon ring. The obtained plane wave geometry is directly linked to the frequency matrix of the massless wave equation, as well as th…
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We study the physics of photon rings in a wide range of axisymmetric black holes admitting a separable Hamilton-Jacobi equation for the geodesics. Utilizing the Killing-Yano tensor, we derive the Penrose limit of the black holes, which describes the physics near the photon ring. The obtained plane wave geometry is directly linked to the frequency matrix of the massless wave equation, as well as the instabilities and Lyapunov exponents of the null geodesics. Consequently, the Lyapunov exponents and frequencies of the photon geodesics, along with the quasinormal modes, can be all extracted from a Hamiltonian in the Penrose limit plane wave metric. Additionally, we explore potential bounds on the Lyapunov exponent, the orbital and precession frequencies, in connection with the corresponding inverted harmonic oscillators and we discuss the possibility of photon rings serving as holographic horizons in a holographic duality framework for astrophysical black holes. Our formalism is applicable to spacetimes encompassing various types of black holes, including stationary ones like Kerr, Kerr-Newman, as well as static black holes such as Schwarzschild, Reissner-Nordström, among others.
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Submitted 21 January, 2025; v1 submitted 15 March, 2024;
originally announced March 2024.
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The Future of Primordial Black Holes: Open Questions and Roadmap
Authors:
Antonio Riotto,
Joe Silk
Abstract:
We discuss some of the the open questions and the roadmap in the physics of primordial black holes. Black holes are the only dark matter candidate that is known to actually exit. Their conjectured primordial role is admittedly based on hypothesis rather than fact, most straightforwardly as a simple extension to the standard models of inflation, or even, in homage to quantum physics, more controver…
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We discuss some of the the open questions and the roadmap in the physics of primordial black holes. Black holes are the only dark matter candidate that is known to actually exit. Their conjectured primordial role is admittedly based on hypothesis rather than fact, most straightforwardly as a simple extension to the standard models of inflation, or even, in homage to quantum physics, more controversially via a slowing-down of Hawking evaporation. Regardless of one's stance on the theoretical basis for their existence, the possibility of primordial black holes playing a novel role in dark matter physics and gravitational wave astronomy opens up a rich astrophysical phenomenology that we lay out in this brief overview.
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Submitted 5 March, 2024;
originally announced March 2024.
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Can we identify primordial black holes? Tidal tests for subsolar-mass gravitational-wave observations
Authors:
F. Crescimbeni,
G. Franciolini,
P. Pani,
A. Riotto
Abstract:
The detection of a subsolar object in a compact binary merger is regarded as one of the smoking gun signatures of a population of primordial black holes~(PBHs). We critically assess whether these systems could be distinguished from stellar binaries, for example composed of white dwarfs or neutron stars, which could also populate the subsolar mass range. At variance with PBHs, the gravitational-wav…
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The detection of a subsolar object in a compact binary merger is regarded as one of the smoking gun signatures of a population of primordial black holes~(PBHs). We critically assess whether these systems could be distinguished from stellar binaries, for example composed of white dwarfs or neutron stars, which could also populate the subsolar mass range. At variance with PBHs, the gravitational-wave signal from stellar binaries is affected by tidal effects, which dramatically grow for moderately compact stars as those expected in the subsolar range. We forecast the capability of constraining tidal effects of putative subsolar neutron star binaries with current and future LIGO-Virgo-KAGRA (LVK) sensitivities as well as next-generation experiments. We show that, should LVK O4 run observe subsolar neutron-star mergers, it could measure the (large) tidal effects with high significance. In particular, for subsolar neutron-star binaries, O4 and O5 projected sensitivities would allow measuring the effect of tidal disruption on the waveform in a large portion of the parameter space, also constraining the tidal deformability at ${\cal O}(10\%)$ level, thus excluding a primordial origin of the binary. Vice versa, for subsolar PBH binaries, model-agnostic upper bounds on the tidal deformability can rule out neutron stars or more exotic competitors. Assuming events similar to the subthreshold candidate SSM200308 reported in LVK O3b data are PBH binaries, O4 projected sensitivity would allow ruling out the presence of neutron-star tidal effects at $\approx 3 σ$ C.L., thus strengthening the PBH hypothesis. Future experiments would lead to even stronger ($>5σ$) conclusions on potential discoveries of this kind.
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Submitted 8 July, 2024; v1 submitted 28 February, 2024;
originally announced February 2024.
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The Primordial Black Hole Abundance: The Broader, the Better
Authors:
A. Ianniccari,
A. J. Iovino,
A. Kehagias,
D. Perrone,
A. Riotto
Abstract:
We show that the abundance of primordial black holes, if formed through the collapse of large fluctuations generated during inflation and unless the power spectrum of the curvature perturbation is very peaked, is always dominated by the broadest profile of the compaction function, even though statistically it is not the most frequent. The corresponding threshold is therefore 2/5. This result exace…
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We show that the abundance of primordial black holes, if formed through the collapse of large fluctuations generated during inflation and unless the power spectrum of the curvature perturbation is very peaked, is always dominated by the broadest profile of the compaction function, even though statistically it is not the most frequent. The corresponding threshold is therefore 2/5. This result exacerbates the tension when combining the primordial black hole abundance with the signal seen by pulsar timing arrays and originated from gravitational waves induced by the same large primordial perturbations.
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Submitted 28 June, 2024; v1 submitted 16 February, 2024;
originally announced February 2024.
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The PTA Hellings and Downs Correlation Unmasked by Symmetries
Authors:
Alex Kehagias,
Antonio Riotto
Abstract:
The Hellings and Downs correlation curve describes the correlation of the timing residuals from pairs of pulsars as a function of their angular separation on the sky and is a smoking-gun signature for the detection of an isotropic stochastic background of gravitational waves. We show that it can be easily obtained from realizing that Lorentz transformations are conformal transformations on the cel…
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The Hellings and Downs correlation curve describes the correlation of the timing residuals from pairs of pulsars as a function of their angular separation on the sky and is a smoking-gun signature for the detection of an isotropic stochastic background of gravitational waves. We show that it can be easily obtained from realizing that Lorentz transformations are conformal transformations on the celestial sphere and from the conformal properties of the two-point correlation of the timing residuals. This result allows several generalizations, e.g. the calculation of the three-point correlator of the time residuals and the inclusion of additional polarization modes (vector and/or scalar) arising in alternative theories of gravity.
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Submitted 19 January, 2024;
originally announced January 2024.
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Renormalized Primordial Black Holes
Authors:
Gabriele Franciolini,
Andrea Ianniccari,
Alex Kehagias,
Davide Perrone,
Antonio Riotto
Abstract:
The formation of primordial black holes in the early universe may happen through the collapse of large curvature perturbations generated during a non-attractor phase of inflation or through a curvaton-like dynamics after inflation. The fact that such small-scale curvature perturbation is typically non-Gaussian leads to the renormalization of composite operators built up from the smoothed density c…
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The formation of primordial black holes in the early universe may happen through the collapse of large curvature perturbations generated during a non-attractor phase of inflation or through a curvaton-like dynamics after inflation. The fact that such small-scale curvature perturbation is typically non-Gaussian leads to the renormalization of composite operators built up from the smoothed density contrast and entering in the calculation of the primordial black abundance. Such renormalization causes the phenomenon of operator mixing and the appearance of an infinite tower of local, non-local and higher-derivative operators as well as to a sizable shift in the threshold for primordial black hole formation. This hints that the calculation of the primordial black hole abundance is more involved than what generally assumed.
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Submitted 6 November, 2023;
originally announced November 2023.
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Primordial black holes and their gravitational-wave signatures
Authors:
Eleni Bagui,
Sebastien Clesse,
Valerio De Luca,
Jose María Ezquiaga,
Gabriele Franciolini,
Juan García-Bellido,
Cristian Joana,
Rajeev Kumar Jain,
Sachiko Kuroyanagi,
Ilia Musco,
Theodoros Papanikolaou,
Alvise Raccanelli,
Sébastien Renaux-Petel,
Antonio Riotto,
Ester Ruiz Morales,
Marco Scalisi,
Olga Sergijenko,
Caner Unal,
Vincent Vennin,
David Wands
Abstract:
In the recent years, primordial black holes (PBHs) have emerged as one of the most interesting and hotly debated topics in cosmology. Among other possibilities, PBHs could explain both some of the signals from binary black hole mergers observed in gravitational wave detectors and an important component of the dark matter in the Universe. Significant progress has been achieved both on the theory si…
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In the recent years, primordial black holes (PBHs) have emerged as one of the most interesting and hotly debated topics in cosmology. Among other possibilities, PBHs could explain both some of the signals from binary black hole mergers observed in gravitational wave detectors and an important component of the dark matter in the Universe. Significant progress has been achieved both on the theory side and from the point of view of observations, including new models and more accurate calculations of PBH formation, evolution, clustering, merger rates, as well as new astrophysical and cosmological probes. In this work, we review, analyse and combine the latest developments in order to perform end-to-end calculations of the various gravitational wave signatures of PBHs. Different ways to distinguish PBHs from stellar black holes are emphasized. Finally, we discuss their detectability with LISA, the first planned gravitational-wave observatory in space.
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Submitted 30 October, 2023;
originally announced October 2023.
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The Sign of non-Gaussianity and the Primordial Black Holes Abundance
Authors:
Hassan Firouzjahi,
Antonio Riotto
Abstract:
The abundance of primordial black holes changes in the presence of local non-Gaussianity. A positive non-linear parameter $f_{NL}$ increases the abundance while a negative one reduces it. We show that in non-attractor single-field models of inflation which enhance the curvature power spectrum and may give rise to primordial black holes, $f_{NL}$ is always positive, when computed in correspondence…
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The abundance of primordial black holes changes in the presence of local non-Gaussianity. A positive non-linear parameter $f_{NL}$ increases the abundance while a negative one reduces it. We show that in non-attractor single-field models of inflation which enhance the curvature power spectrum and may give rise to primordial black holes, $f_{NL}$ is always positive, when computed in correspondence of the peak of the curvature power spectrum where the primordial black hole abundance has its maximum. This implies that the interpretation of the recent pulsar timing arrays data from scalar-induced gravitational waves generated at primordial black hole formation may not be supported by invoking non-Gaussianity within non-attractor single-field models.
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Submitted 19 September, 2023;
originally announced September 2023.
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Non-linear Black Hole Ringdowns: an Analytical Approach
Authors:
Davide Perrone,
Thomas Barreira,
Alex Kehagias,
Antonio Riotto
Abstract:
Due to the nature of gravity, non-linear effects are left imprinted in the quasi-normal modes generated in the ringdown phase of the merger of two black holes. We offer an analytical treatment of the quasi-normal modes at second-order in black hole perturbation theory which takes advantage from the fact that the non-linear sources are peaked around the light ring. As a byproduct, we describe why t…
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Due to the nature of gravity, non-linear effects are left imprinted in the quasi-normal modes generated in the ringdown phase of the merger of two black holes. We offer an analytical treatment of the quasi-normal modes at second-order in black hole perturbation theory which takes advantage from the fact that the non-linear sources are peaked around the light ring. As a byproduct, we describe why the amplitude of the second-order mode relative to the square of the first-order amplitude depends only weakly on the initial condition of the problem.
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Submitted 8 April, 2024; v1 submitted 30 August, 2023;
originally announced August 2023.
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How well do we know the primordial black hole abundance? The crucial role of nonlinearities when approaching the horizon
Authors:
Valerio De Luca,
Alex Kehagias,
Antonio Riotto
Abstract:
We discuss the non-linear corrections entering in the calculation of the primordial black hole abundance from the non-linear radiation transfer function and the determination of the true physical horizon crossing. We show that the current standard techniques to calculate the abundance of primordial black holes suffer from uncertainties and argue that the primordial black hole abundance may be much…
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We discuss the non-linear corrections entering in the calculation of the primordial black hole abundance from the non-linear radiation transfer function and the determination of the true physical horizon crossing. We show that the current standard techniques to calculate the abundance of primordial black holes suffer from uncertainties and argue that the primordial black hole abundance may be much smaller than what routinely considered. This would imply, among other consequences, that the interpretation of the recent pulsar timing arrays data from scalar-induced gravitational waves may not be ruled out because of an overproduction of primordial black holes.
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Submitted 7 September, 2023; v1 submitted 25 July, 2023;
originally announced July 2023.
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Primordial Black Holes and Loops in Single-Field Inflation
Authors:
Hassan Firouzjahi,
Antonio Riotto
Abstract:
Using the $δN$ formalism we calculate the one-loop correction to the large-scale power spectrum of the curvature perturbation in the standard scenario where primordial black holes are formed in the early universe thanks to a phase of ultra-slow-roll in single-field inflation. We explicitly show that one-loop corrections are negligible when the transition from the ultra-slow-roll to the slow-roll p…
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Using the $δN$ formalism we calculate the one-loop correction to the large-scale power spectrum of the curvature perturbation in the standard scenario where primordial black holes are formed in the early universe thanks to a phase of ultra-slow-roll in single-field inflation. We explicitly show that one-loop corrections are negligible when the transition from the ultra-slow-roll to the slow-roll phase is smooth. We conclude that the PBH formation scenario through a ultra-slow-roll phase is viable.
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Submitted 16 April, 2023;
originally announced April 2023.
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Searching for Primordial Black Holes with the Einstein Telescope: impact of design and systematics
Authors:
G. Franciolini,
F. Iacovelli,
M. Mancarella,
M. Maggiore,
P. Pani,
A. Riotto
Abstract:
Primordial Black Holes (PBHs) have recently attracted much attention as they may explain some of the LIGO/Virgo/KAGRA observations and significantly contribute to the dark matter in our universe. The next generation of Gravitational Wave (GW) detectors will have the unique opportunity to set stringent bounds on this putative population of objects. Focusing on the Einstein Telescope (ET), in this p…
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Primordial Black Holes (PBHs) have recently attracted much attention as they may explain some of the LIGO/Virgo/KAGRA observations and significantly contribute to the dark matter in our universe. The next generation of Gravitational Wave (GW) detectors will have the unique opportunity to set stringent bounds on this putative population of objects. Focusing on the Einstein Telescope (ET), in this paper we analyse in detail the impact of systematics and different detector designs on our future capability of observing key quantities that would allow us to discover and/or constrain a population of PBH mergers. We also perform a population analysis, with a mass and redshift distribution compatible with the current observational bounds. Our results indicate that ET alone can reach an exquisite level of accuracy on the key observables considered, as well as detect up to tens of thousands of PBH binaries per year, but for some key signatures (in particular high--redshift sources) the cryogenic instrument optimised for low frequencies turns out to be crucial, both for the number of observations and the error on the parameters reconstruction. As far as the detector geometry is concerned, we find that a network consisting of two separated L--shaped interferometers of 15 (20)~km arm length, oriented at $45^{\circ}$ with respect to each other performs better than a single triangular shaped instrument of 10 (15)~km arm length, for all the metrics considered.
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Submitted 6 April, 2023;
originally announced April 2023.
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Science with the Einstein Telescope: a comparison of different designs
Authors:
Marica Branchesi,
Michele Maggiore,
David Alonso,
Charles Badger,
Biswajit Banerjee,
Freija Beirnaert,
Enis Belgacem,
Swetha Bhagwat,
Guillaume Boileau,
Ssohrab Borhanian,
Daniel David Brown,
Man Leong Chan,
Giulia Cusin,
Stefan L. Danilishin,
Jerome Degallaix,
Valerio De Luca,
Arnab Dhani,
Tim Dietrich,
Ulyana Dupletsa,
Stefano Foffa,
Gabriele Franciolini,
Andreas Freise,
Gianluca Gemme,
Boris Goncharov,
Archisman Ghosh
, et al. (51 additional authors not shown)
Abstract:
The Einstein Telescope (ET), the European project for a third-generation gravitational-wave detector, has a reference configuration based on a triangular shape consisting of three nested detectors with 10 km arms, where in each arm there is a `xylophone' configuration made of an interferometer tuned toward high frequencies, and an interferometer tuned toward low frequencies and working at cryogeni…
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The Einstein Telescope (ET), the European project for a third-generation gravitational-wave detector, has a reference configuration based on a triangular shape consisting of three nested detectors with 10 km arms, where in each arm there is a `xylophone' configuration made of an interferometer tuned toward high frequencies, and an interferometer tuned toward low frequencies and working at cryogenic temperature. Here, we examine the scientific perspectives under possible variations of this reference design. We perform a detailed evaluation of the science case for a single triangular geometry observatory, and we compare it with the results obtained for a network of two L-shaped detectors (either parallel or misaligned) located in Europe, considering different choices of arm-length for both the triangle and the 2L geometries. We also study how the science output changes in the absence of the low-frequency instrument, both for the triangle and the 2L configurations. We examine a broad class of simple `metrics' that quantify the science output, related to compact binary coalescences, multi-messenger astronomy and stochastic backgrounds, and we then examine the impact of different detector designs on a more specific set of scientific objectives.
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Submitted 17 June, 2023; v1 submitted 28 March, 2023;
originally announced March 2023.
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The Primordial Black Hole Formation from Single-Field Inflation is Still Not Ruled Out
Authors:
A. Riotto
Abstract:
In response to a recent criticism, appeared in arXiv:2303.00341, we argue that the standard scenario to form primordial black holes in the early universe based on a phase of ultra-slow-roll in single-field inflation is not ruled out.
In response to a recent criticism, appeared in arXiv:2303.00341, we argue that the standard scenario to form primordial black holes in the early universe based on a phase of ultra-slow-roll in single-field inflation is not ruled out.
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Submitted 3 March, 2023;
originally announced March 2023.
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Explaining Nonlinearities in Black Hole Ringdowns from Symmetries
Authors:
Alex Kehagias,
Davide Perrone,
Antonio Riotto,
Francesco Riva
Abstract:
It has been recently pointed out that nonlinear effects are necessary to model the ringdown stage of the gravitational waveform produced by the merger of two black holes giving rise to a remnant Kerr black hole. We show that this nonlinear behaviour is explained, both on the qualitative and quantitative level, by near-horizon symmetries of the Kerr black hole within the Kerr/CFT correspondence.
It has been recently pointed out that nonlinear effects are necessary to model the ringdown stage of the gravitational waveform produced by the merger of two black holes giving rise to a remnant Kerr black hole. We show that this nonlinear behaviour is explained, both on the qualitative and quantitative level, by near-horizon symmetries of the Kerr black hole within the Kerr/CFT correspondence.
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Submitted 23 January, 2023;
originally announced January 2023.
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The Primordial Black Hole Formation from Single-Field Inflation is Not Ruled Out
Authors:
Antonio Riotto
Abstract:
A standard scenario to form primordial black holes in the early universe is based on a phase of ultra-slow-roll in single-field inflation when the amplitude of the short scale modes is enhanced compared to the CMB plateau. Based on general arguments, we show that the loop corrections to the large-scale linear power spectrum from the short modes are small and conclude that the scenario is not ruled…
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A standard scenario to form primordial black holes in the early universe is based on a phase of ultra-slow-roll in single-field inflation when the amplitude of the short scale modes is enhanced compared to the CMB plateau. Based on general arguments, we show that the loop corrections to the large-scale linear power spectrum from the short modes are small and conclude that the scenario is not ruled out.
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Submitted 2 January, 2023;
originally announced January 2023.
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Heavy primordial black holes from strongly clustered light black holes
Authors:
Valerio De Luca,
Gabriele Franciolini,
Antonio Riotto
Abstract:
We show that heavy primordial black holes may originate from much lighter ones if the latter are strongly clustered at the time of their formation. While this population is subject to the usual constraints from late-time universe observations, its relation to the initial conditions is different from the standard scenario and provides a new mechanism to generate massive primordial black holes even…
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We show that heavy primordial black holes may originate from much lighter ones if the latter are strongly clustered at the time of their formation. While this population is subject to the usual constraints from late-time universe observations, its relation to the initial conditions is different from the standard scenario and provides a new mechanism to generate massive primordial black holes even in the absence of efficient accretion, opening new scenarios, e.g. for the generation of supermassive black holes.
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Submitted 15 April, 2023; v1 submitted 25 October, 2022;
originally announced October 2022.
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High-redshift JWST Observations and Primordial Non-Gaussianity
Authors:
M. Biagetti,
G. Franciolini,
A. Riotto
Abstract:
Several bright and massive galaxy candidates at high redshifts have been recently observed by the James Webb Space Telescope. Such early massive galaxies seem difficult to reconcile with standard $Λ$ Cold Dark Matter model predictions. We discuss under which circumstances such observed massive galaxy candidates can be explained by introducing primordial non-Gaussianity in the initial conditions of…
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Several bright and massive galaxy candidates at high redshifts have been recently observed by the James Webb Space Telescope. Such early massive galaxies seem difficult to reconcile with standard $Λ$ Cold Dark Matter model predictions. We discuss under which circumstances such observed massive galaxy candidates can be explained by introducing primordial non-Gaussianity in the initial conditions of the cosmological perturbations.
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Submitted 16 February, 2023; v1 submitted 10 October, 2022;
originally announced October 2022.
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Measuring properties of primordial black hole mergers at cosmological distances: effect of higher order modes in gravitational waves
Authors:
Ken K. Y. Ng,
Boris Goncharov,
Shiqi Chen,
Ssohrab Borhanian,
Ulyana Dupletsa,
Gabriele Franciolini,
Marica Branchesi,
Jan Harms,
Michele Maggiore,
Antonio Riotto,
B. S. Sathyaprakash,
Salvatore Vitale
Abstract:
Primordial black holes (PBHs) may form from the collapse of matter overdensities shortly after the Big Bang. One may identify their existence by observing gravitational wave (GW) emissions from merging PBH binaries at high redshifts $z\gtrsim 30$, where astrophysical binary black holes (BBHs) are unlikely to merge. The next-generation ground-based GW detectors, Cosmic Explorer and Einstein Telesco…
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Primordial black holes (PBHs) may form from the collapse of matter overdensities shortly after the Big Bang. One may identify their existence by observing gravitational wave (GW) emissions from merging PBH binaries at high redshifts $z\gtrsim 30$, where astrophysical binary black holes (BBHs) are unlikely to merge. The next-generation ground-based GW detectors, Cosmic Explorer and Einstein Telescope, will be able to observe BBHs with total masses of $\mathcal{O}(10-100)~M_{\odot}$ at such redshifts. This paper serves as a companion paper of arXiv:2108.07276, focusing on the effect of higher-order modes (HoMs) in the waveform modeling, which may be detectable for these high redshift BBHs, on the estimation of source parameters. We perform Bayesian parameter estimation to obtain the measurement uncertainties with and without HoM modeling in the waveform for sources with different total masses, mass ratios, orbital inclinations and redshifts observed by a network of next-generation GW detectors. We show that including HoMs in the waveform model reduces the uncertainties of redshifts and masses by up to a factor of two, depending on the exact source parameters. We then discuss the implications for identifying PBHs with the improved single-event measurements, and expand the investigation of the model dependence of the relative abundance between the BBH mergers originating from the first stars and the primordial BBH mergers as shown in arXiv:2108.07276.
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Submitted 6 October, 2022;
originally announced October 2022.
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Ruling out Initially Clustered Primordial Black Holes as Dark Matter
Authors:
V. De Luca,
G. Franciolini,
A. Riotto,
H. Veermäe
Abstract:
Combining constraints from microlensing and Lyman-$α$ forest, we provide a simple argument to show that large spatial clustering of stellar-mass primordial black holes at the time of formation, such as the one induced by the presence of large non-Gaussianities, is ruled out. Therefore, it is not possible to evade existing constraints preventing stellar-mass primordial black holes to be a dominant…
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Combining constraints from microlensing and Lyman-$α$ forest, we provide a simple argument to show that large spatial clustering of stellar-mass primordial black holes at the time of formation, such as the one induced by the presence of large non-Gaussianities, is ruled out. Therefore, it is not possible to evade existing constraints preventing stellar-mass primordial black holes to be a dominant constituent of the dark matter by boosting their initial clustering.
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Submitted 17 October, 2022; v1 submitted 2 August, 2022;
originally announced August 2022.
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On the Cosmological Stability of the Higgs Instability
Authors:
Valerio De Luca,
Alex Kehagias,
Antonio Riotto
Abstract:
The Standard Model Higgs potential becomes unstable at large Higgs field values where its quartic coupling becomes negative. While the tunneling lifetime of our current electroweak vacuum is comfortably longer than the age of the universe, quantum fluctuations during inflation might push the Higgs over the barrier, forming patches which might be lethal for our universe. We study the cosmological e…
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The Standard Model Higgs potential becomes unstable at large Higgs field values where its quartic coupling becomes negative. While the tunneling lifetime of our current electroweak vacuum is comfortably longer than the age of the universe, quantum fluctuations during inflation might push the Higgs over the barrier, forming patches which might be lethal for our universe. We study the cosmological evolution of such regions and find that, at least in the thin wall approximation, they may be harmless as they collapse due to the backreaction of the Higgs itself. The presence of the Standard Model Higgs instability can provide a novel mechanism to end inflation and to reheat the universe through the evaporation of the black holes left over by the collapse of the Higgs bubbles. The bound on the Hubble rate during inflation may be therefore relaxed.
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Submitted 11 September, 2022; v1 submitted 20 May, 2022;
originally announced May 2022.
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Constraining high-redshift stellar-mass primordial black holes with next-generation ground-based gravitational-wave detectors
Authors:
Ken K. Y. Ng,
Gabriele Franciolini,
Emanuele Berti,
Paolo Pani,
Antonio Riotto,
Salvatore Vitale
Abstract:
The possible existence of primordial black holes in the stellar mass window has received considerable attention because their mergers may contribute to current and future gravitational-wave detections. Primordial black hole mergers, together with mergers of black holes originating from Population~III stars, are expected to dominate at high redshifts ($z\gtrsim 10$). However the primordial black ho…
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The possible existence of primordial black holes in the stellar mass window has received considerable attention because their mergers may contribute to current and future gravitational-wave detections. Primordial black hole mergers, together with mergers of black holes originating from Population~III stars, are expected to dominate at high redshifts ($z\gtrsim 10$). However the primordial black hole merger rate density is expected to rise monotonically with redshift, while Population~III mergers can only occur after the birth of the first stars. Next-generation gravitational-wave detectors such as Cosmic Explorer~(CE) and Einstein Telescope~(ET) can access this distinctive feature in the merger rates as functions of redshift, allowing for a direct measurement of the abundance of the two populations, and hence for robust constraints on the abundance of primordial black holes. We simulate four-months worth of data observed by a CE-ET detector network and perform hierarchical Bayesian analysis to recover the merger rate densities. We find that if the Universe has no primordial black holes with masses of $\mathcal{O}(10M_{\odot})$, the projected upper limit on their abundance $f_{\rm PBH}$ as a fraction of dark matter energy density may be as low as $f_{\rm PBH}\sim \mathcal{O}({10^{-5}})$, about two orders of magnitude lower than current upper limits in this mass range. If instead $f_{\rm PBH}\gtrsim 10^{-4}$, future gravitational wave observations would exclude $f_{\rm PBH}=0$ at the 95\% credible interval.
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Submitted 25 April, 2022;
originally announced April 2022.
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Cosmology with the Laser Interferometer Space Antenna
Authors:
Pierre Auclair,
David Bacon,
Tessa Baker,
Tiago Barreiro,
Nicola Bartolo,
Enis Belgacem,
Nicola Bellomo,
Ido Ben-Dayan,
Daniele Bertacca,
Marc Besancon,
Jose J. Blanco-Pillado,
Diego Blas,
Guillaume Boileau,
Gianluca Calcagni,
Robert Caldwell,
Chiara Caprini,
Carmelita Carbone,
Chia-Feng Chang,
Hsin-Yu Chen,
Nelson Christensen,
Sebastien Clesse,
Denis Comelli,
Giuseppe Congedo,
Carlo Contaldi,
Marco Crisostomi
, et al. (155 additional authors not shown)
Abstract:
The Laser Interferometer Space Antenna (LISA) has two scientific objectives of cosmological focus: to probe the expansion rate of the universe, and to understand stochastic gravitational-wave backgrounds and their implications for early universe and particle physics, from the MeV to the Planck scale. However, the range of potential cosmological applications of gravitational wave observations exten…
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The Laser Interferometer Space Antenna (LISA) has two scientific objectives of cosmological focus: to probe the expansion rate of the universe, and to understand stochastic gravitational-wave backgrounds and their implications for early universe and particle physics, from the MeV to the Planck scale. However, the range of potential cosmological applications of gravitational wave observations extends well beyond these two objectives. This publication presents a summary of the state of the art in LISA cosmology, theory and methods, and identifies new opportunities to use gravitational wave observations by LISA to probe the universe.
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Submitted 11 April, 2022;
originally announced April 2022.
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A Note on the Abundance of Primordial Black Holes: Use and Misuse of the Metric Curvature Perturbation
Authors:
V. De Luca,
A. Riotto
Abstract:
The formation of Primordial Black Holes (PBHs) through the collapse of large fluctuations in the early universe is a rare event. This manifests itself, for instance, through the non-Gaussian tail of the formation probability. To compute such probability and the abundance of PBHs, the curvature perturbation is frequently adopted. In this note we emphasize that its use does not provide the correct P…
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The formation of Primordial Black Holes (PBHs) through the collapse of large fluctuations in the early universe is a rare event. This manifests itself, for instance, through the non-Gaussian tail of the formation probability. To compute such probability and the abundance of PBHs, the curvature perturbation is frequently adopted. In this note we emphasize that its use does not provide the correct PBH formation probability. Through a path-integral approach we show that the exact calculation of the PBH abundance demands the knowledge of multivariate joint probabilities of the curvature perturbation or, equivalently, of all the corresponding connected correlators.
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Submitted 1 April, 2022; v1 submitted 22 January, 2022;
originally announced January 2022.
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Probing Anisotropies of the Stochastic Gravitational Wave Background with LISA
Authors:
Nicola Bartolo,
Daniele Bertacca,
Robert Caldwell,
Carlo R. Contaldi,
Giulia Cusin,
Valerio De Luca,
Emanuela Dimastrogiovanni,
Matteo Fasiello,
Daniel G. Figueroa,
Gabriele Franciolini,
Alexander C. Jenkins,
Marco Peloso,
Mauro Pieroni,
Arianna Renzini,
Angelo Ricciardone,
Antonio Riotto,
Mairi Sakellariadou,
Lorenzo Sorbo,
Gianmassimo Tasinato,
Jesus Torrado,
Sebastien Clesse,
Sachiko Kuroyanagi
Abstract:
We investigate the sensitivity of the Laser Interferometer Space Antenna (LISA) to the anisotropies of the Stochastic Gravitational Wave Background (SGWB). We first discuss the main astrophysical and cosmological sources of SGWB which are characterized by anisotropies in the GW energy density, and we build a Signal-to-Noise estimator to quantify the sensitivity of LISA to different multipoles. We…
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We investigate the sensitivity of the Laser Interferometer Space Antenna (LISA) to the anisotropies of the Stochastic Gravitational Wave Background (SGWB). We first discuss the main astrophysical and cosmological sources of SGWB which are characterized by anisotropies in the GW energy density, and we build a Signal-to-Noise estimator to quantify the sensitivity of LISA to different multipoles. We then perform a Fisher matrix analysis of the prospects of detectability of anisotropic features with LISA for individual multipoles, focusing on a SGWB with a power-law frequency profile. We compute the noise angular spectrum taking into account the specific scan strategy of the LISA detector. We analyze the case of the kinematic dipole and quadrupole generated by Doppler boosting an isotropic SGWB. We find that $β\, Ω_{\rm GW}\sim 2\times 10^{-11}$ is required to observe a dipolar signal with LISA. The detector response to the quadrupole has a factor $\sim 10^3 \,β$ relative to that of the dipole. The characterization of the anisotropies, both from a theoretical perspective and from a map-making point of view, allows us to extract information that can be used to understand the origin of the SGWB, and to discriminate among distinct superimposed SGWB sources.
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Submitted 21 January, 2022;
originally announced January 2022.