Showing 1–46 of 46 results for author: Tamvakis, K

Inflationary assessment of $F(\mathcal{R},\tilde{\mathcal{R}})$ Einstein-Cartan models

Authors: Theodoros Katsoulas, Kyriakos Tamvakis

Abstract: In the framework of $F(\mathcal{R},\tilde{\mathcal{R}})$ Einstein-Cartan gravity with an action depending both of the Ricci scalar and the so-called Holst-invariant curvature we consider models that include cubic terms of the latter in the action and study their inflationary behavior. These terms can have a considerable effect either positive or negative in relation to the agreement with present o… ▽ More In the framework of $F(\mathcal{R},\tilde{\mathcal{R}})$ Einstein-Cartan gravity with an action depending both of the Ricci scalar and the so-called Holst-invariant curvature we consider models that include cubic terms of the latter in the action and study their inflationary behavior. These terms can have a considerable effect either positive or negative in relation to the agreement with present observational data, depending on parameters. In parameter regions where the quadratic models fail to produce results consistent with observational data, the presence of these additional cubic terms can lead to compatible predictions. △ Less

Submitted 2 December, 2025; originally announced December 2025.

Comments: 10 pages, 6 figures

Toward a gravitational network: Bridging metric-affine gravity and no-scale supergravity

Authors: Ioannis D. Gialamas, Theodoros Katsoulas, Kyriakos Tamvakis

Abstract: We consider a class of models in the framework of metric-affine gravity and establish their correspondence to the bosonic sector of a class of no-scale supergravity models. The excellent inflationary behavior of the gravitational models considered is carried over to the corresponding supergravity ones, thus, enriching the landscape of inflation-compatible models. We consider a class of models in the framework of metric-affine gravity and establish their correspondence to the bosonic sector of a class of no-scale supergravity models. The excellent inflationary behavior of the gravitational models considered is carried over to the corresponding supergravity ones, thus, enriching the landscape of inflation-compatible models. △ Less

Submitted 7 November, 2025; v1 submitted 2 June, 2025; originally announced June 2025.

Comments: 7 pages, 1 figure, matches published version

Journal ref: Phys. Rev. D 112 (2025) 9, 096011

Keeping the relation between the Starobinsky model and no-scale supergravity ACTive

Authors: Ioannis D. Gialamas, Theodoros Katsoulas, Kyriakos Tamvakis

Abstract: We introduce a modification of the Starobinsky model in the form of an additional cubic Ricci scalar curvature term $\sim αR^3$, scaled by a dimensionless parameter $α$, with the resulting inflaton potential being the standard Starobinsky potential modified to first parametric order by an additive term. The resulting potential is identical to the potential obtained by a modification of the superpo… ▽ More We introduce a modification of the Starobinsky model in the form of an additional cubic Ricci scalar curvature term $\sim αR^3$, scaled by a dimensionless parameter $α$, with the resulting inflaton potential being the standard Starobinsky potential modified to first parametric order by an additive term. The resulting potential is identical to the potential obtained by a modification of the superpotential employed in the construction of the Starobinsky model in the framework of no-scale supergravity, thus, extending the correspondence between a class of no-scale supergravity models and modifications of the Starobinsky model. We analyze the inflationary predictions of the model and find that for $-4.2 \times 10^{-5} \lesssim α\lesssim -1.9 \times 10^{-5}$, the modified Starobinsky model is in full agreement with the recent observational data from the Atacama Cosmology Telescope, for a range of $e$-folds, $N_\star = 50-60$. △ Less

Submitted 19 September, 2025; v1 submitted 6 May, 2025; originally announced May 2025.

Comments: 14 pages, 3 figures

Journal ref: JCAP 09 (2025) 060

Dynamically induced spin-2 mass in a Weyl-invariant framework

Authors: Ioannis D. Gialamas, Kyriakos Tamvakis

Abstract: We combine the ghost-free bimetric theory of gravity with the concept of local Weyl invariance, realized in the framework of Einstein-Cartan gravity. The gravitational sector, characterized by two independent metrics and two independent connections, is coupled to a scalar field that can in principle develop a non-vanishing expectation value through radiative corrections. The spectrum of the model,… ▽ More We combine the ghost-free bimetric theory of gravity with the concept of local Weyl invariance, realized in the framework of Einstein-Cartan gravity. The gravitational sector, characterized by two independent metrics and two independent connections, is coupled to a scalar field that can in principle develop a non-vanishing expectation value through radiative corrections. The spectrum of the model, apart from the massless standard graviton and a pair of axion-like pseudoscalars, associated with the presence of the Holst invariants in the action, includes an additional spin-$2$ state of a non-vanishing Fierz-Pauli mass proportional to the scalar field vacuum expectation value. We analyze the phenomenology of the model and specify the conditions under which the massive spin-$2$ state could be a primary dark matter candidate. △ Less

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

Comments: 16 pages, 1 figure, matches published version

Journal ref: Phys. Rev. D 112, 084051 (2025)

General Einstein-Cartan quadratic gravity with derivative couplings

Authors: Theodoros Katsoulas, Kyriakos Tamvakis

Abstract: Within the framework of Einstein-Cartan gravity we consider an action, containing up to quadratic terms of the Ricci scalar and the Holst invariant, coupled non-minimally to a scalar field, including couplings of its derivatives to curvature. We derive the equivalent metric theory, featuring an extra dynamical pseudoscalar degree of freedom associated with the presence of the Holst term in the act… ▽ More Within the framework of Einstein-Cartan gravity we consider an action, containing up to quadratic terms of the Ricci scalar and the Holst invariant, coupled non-minimally to a scalar field, including couplings of its derivatives to curvature. We derive the equivalent metric theory, featuring an extra dynamical pseudoscalar degree of freedom associated with the presence of the Holst term in the action. We study the evolution of the resulting two-field system in a FRW background and show that it evolves rapidly into an effective single-field inflationary model. We find that the model is consistent with the latest observational data for a wide range of its parameters, determining the necessary upper limits on derivative coupling parameters. △ Less

Submitted 17 June, 2025; v1 submitted 24 February, 2025; originally announced February 2025.

Journal ref: JCAP 06 (2025) 022

Cosmic inflation in metric-affine gravity

Authors: Ioannis D. Gialamas, Kyriakos Tamvakis

Abstract: In the context of metric-affine gravity theories, where the metric and connection are independent, we examine actions involving quadratic terms in the Ricci scalar curvature and the Holst invariant. These actions are non-minimally coupled to a scalar field. We explore the behavior of the corresponding effective metric theory, which includes an extra dynamic pseudoscalar degree of freedom. Detailed… ▽ More In the context of metric-affine gravity theories, where the metric and connection are independent, we examine actions involving quadratic terms in the Ricci scalar curvature and the Holst invariant. These actions are non-minimally coupled to a scalar field. We explore the behavior of the corresponding effective metric theory, which includes an extra dynamic pseudoscalar degree of freedom. Detailed analysis of inflationary predictions reveals compliance with recent observations across various parameters, potentially allowing a higher tensor-to-scalar ratio. The spectral index's direction of change varies based on parameter positioning. △ Less

Submitted 29 October, 2024; originally announced October 2024.

Comments: 11 pages, 2 figures, Contribution to Proceedings of HEP 2023, Ioannina, Greece

Inflation in Weyl-invariant Einstein-Cartan gravity

Authors: Ioannis D. Gialamas, Kyriakos Tamvakis

Abstract: We consider Weyl-invariant quadratic Einstein-Cartan gravity coupled to a scalar field and study the inflationary behaviour of the coupled system of the scalar field and the pseudoscalar associated with the Holst invariant. We find that the model is characterized by effective single-field inflation occurring at small field values and analyze its predictions which are in comfortable agreement with… ▽ More We consider Weyl-invariant quadratic Einstein-Cartan gravity coupled to a scalar field and study the inflationary behaviour of the coupled system of the scalar field and the pseudoscalar associated with the Holst invariant. We find that the model is characterized by effective single-field inflation occurring at small field values and analyze its predictions which are in comfortable agreement with existing observations for a range of parameter values. △ Less

Submitted 19 January, 2025; v1 submitted 21 October, 2024; originally announced October 2024.

Comments: 9 pages, 2 figures, matches published version

Journal ref: Phys. Rev. D 111, 044007 (2025)

Inflation and reheating in quadratic metric-affine gravity with derivative couplings

Authors: Ioannis D. Gialamas, Theodoros Katsoulas, Kyriakos Tamvakis

Abstract: Within the framework of metric-affine theories of gravity, where both the metric and connection are treated as independent variables, we consider actions quadratic in the Ricci scalar curvature coupled non-minimally to a scalar field through derivative couplings. Our analysis delves into the inflationary predictions, revealing their consistency with the latest observational constraints across a wi… ▽ More Within the framework of metric-affine theories of gravity, where both the metric and connection are treated as independent variables, we consider actions quadratic in the Ricci scalar curvature coupled non-minimally to a scalar field through derivative couplings. Our analysis delves into the inflationary predictions, revealing their consistency with the latest observational constraints across a wide range of parameters. This compatibility permits adjustments such as an increase in the spectral index and a reduction in the tensor-to-scalar ratio. While we do not propose a specific reheating mechanism, our analysis demonstrates that within the quadratic model of inflation, the maximum reheating temperature can reach $\sim 3\times10^{15}\, {\rm GeV}$. △ Less

Submitted 6 June, 2024; v1 submitted 13 March, 2024; originally announced March 2024.

Comments: 21 pages, 9 figures, matches published version

Journal ref: JCAP 06, 005 (2024)

On the absence of ghosts in quadratic bigravity

Authors: Ioannis D. Gialamas, Kyriakos Tamvakis

Abstract: An extension of the bimetric theory of gravity is considered that includes quadratic Ricci curvature terms associated with each metric. The issue of the Boulware-Deser ghost is analyzed. The Hamiltonian constraint is derived and the existence of a secondary constraint is shown, proving that the theory is ghost-free. An extension of the bimetric theory of gravity is considered that includes quadratic Ricci curvature terms associated with each metric. The issue of the Boulware-Deser ghost is analyzed. The Hamiltonian constraint is derived and the existence of a secondary constraint is shown, proving that the theory is ghost-free. △ Less

Submitted 6 March, 2024; v1 submitted 24 November, 2023; originally announced November 2023.

Comments: 13 pages, matches published version

Journal ref: JCAP 03, 016 (2024)

Bimetric Starobinsky model

Authors: Ioannis D. Gialamas, Kyriakos Tamvakis

Abstract: The bimetric theory of gravity is an extension of general relativity that describes a massive spin-$2$ particle in addition to the standard massless graviton. The theory is based on two dynamical metric tensors with their interactions constrained by requiring the absence of the so-called Boulware-Deser ghost. It has been realized that the quantum interactions of matter fields with gravity are boun… ▽ More The bimetric theory of gravity is an extension of general relativity that describes a massive spin-$2$ particle in addition to the standard massless graviton. The theory is based on two dynamical metric tensors with their interactions constrained by requiring the absence of the so-called Boulware-Deser ghost. It has been realized that the quantum interactions of matter fields with gravity are bound to generate modifications to the standard Einstein-Hilbert action such as quadratic curvature terms. Such a quadratic Ricci scalar term is present in the so-called Starobinsky model which has been proven to be rather robust in its inflationary predictions. In the present article we study a generalization of the Starobinsky model within the bimetric theory and find that its inflationary behavior stays intact while keeping all consistency requirements of the bimetric framework. The interpretation of the massive spin-2 particle as dark matter remains a viable scenario, as in standard bigravity. △ Less

Submitted 29 September, 2023; v1 submitted 11 July, 2023; originally announced July 2023.

Comments: 7 pages, 3 figures, title slightly edited, matches published version

Journal ref: Phys. Rev. D 108, 104023 (2023)

Bimetric-Affine Quadratic Gravity

Authors: Ioannis D. Gialamas, Kyriakos Tamvakis

Abstract: Bimetric gravity, is a theory of gravity that posits the existence of two interacting and dynamical metric tensors. The spectrum of bimetric gravity consists of a massless and a massive spin-2 particle. The form of the interactions between the two metrics $g_{μν}$ and $f_{μν}$ is constrained by requiring absence of the so called Boulware-Deser ghost. In this work we extend the original bimetric th… ▽ More Bimetric gravity, is a theory of gravity that posits the existence of two interacting and dynamical metric tensors. The spectrum of bimetric gravity consists of a massless and a massive spin-2 particle. The form of the interactions between the two metrics $g_{μν}$ and $f_{μν}$ is constrained by requiring absence of the so called Boulware-Deser ghost. In this work we extend the original bimetric theory to its bimetric-affine counterpart, in which the two connections, associated with the Ricci scalars, are treated as independent variables. We examine in detail the case of an additional quadratic in the Ricci scalar curvature term $\mathcal{R}^2(g,Γ)$ and we find that this theory is free of ghosts for a wide range of the interaction parameters, not excluding the possibility of a Dark Matter interpretation of the massive spin-2 particle. △ Less

Submitted 2 May, 2023; v1 submitted 20 March, 2023; originally announced March 2023.

Comments: matches version to be published in PRD

Journal ref: Phys. Rev. D 107, 104012 (2023)

Inflation in Metric-Affine Quadratic Gravity

Authors: Ioannis D. Gialamas, Kyriakos Tamvakis

Abstract: In the general framework of Metric-Affine theories of gravity, where the metric and the connection are independent variables, we consider actions quadratic in the Ricci scalar curvature and the Holst invariant (the contraction of the Riemann curvature with the Levi-Civita antisymmetric tensor) coupled non-minimally to a scalar field. We study the profile of the equivalent effective metric theory,… ▽ More In the general framework of Metric-Affine theories of gravity, where the metric and the connection are independent variables, we consider actions quadratic in the Ricci scalar curvature and the Holst invariant (the contraction of the Riemann curvature with the Levi-Civita antisymmetric tensor) coupled non-minimally to a scalar field. We study the profile of the equivalent effective metric theory, featuring an extra dynamical pseudoscalar degree of freedom, and show that it reduces to an effective single-field inflationary model. We analyze in detail the inflationary predictions and find that they fall within the latest observational bounds for a wide range of parameters, allowing for an increase in the tensor-to-scalar ratio. The spectral index can either decrease or increase depending on the position in parameter space. △ Less

Submitted 15 March, 2023; v1 submitted 19 December, 2022; originally announced December 2022.

Comments: 21 pages, 7 figures, 3 tables, JCAP accepted version

Journal ref: JCAP 03 (2023) 042

Late time acceleration in Palatini gravity

Authors: Ignatios Antoniadis, Anthony Guillen, Kyriakos Tamvakis

Abstract: We investigate the effect of the quadratic correction $αR^2$ and non-minimal coupling $ξφ^2 R$ on a quintessence model with an exponential potential $V(φ) = M^4\exp(-λφ)$ in the Palatini formulation of gravity. We use dynamical system techniques to analyze the attractor structure of the model and uncover the possible trajectories of the system. We find that the quadratic correction cannot play a r… ▽ More We investigate the effect of the quadratic correction $αR^2$ and non-minimal coupling $ξφ^2 R$ on a quintessence model with an exponential potential $V(φ) = M^4\exp(-λφ)$ in the Palatini formulation of gravity. We use dynamical system techniques to analyze the attractor structure of the model and uncover the possible trajectories of the system. We find that the quadratic correction cannot play a role in the late time dynamics, except for unacceptably large values of the parameter $α$; although it can play a role at early times. We find viable evolutions, from a matter-dominated phase to an accelerated expansion phase, with the dynamics driven by the non-minimal coupling. These evolutions correspond to trajectories where the field ends up frozen, thus acting as a cosmological constant. △ Less

Submitted 7 November, 2022; v1 submitted 27 July, 2022; originally announced July 2022.

Comments: 25 pages, 3 figures, some paragraphs added

Addendum to "Ultraviolet behaviour of Higgs inflation models"

Authors: Ignatios Antoniadis, Anthony Guillen, Kyriakos Tamvakis

Abstract: This article is an addendum to [1]. We extend our computation of the on-shell scattering amplitudes in an arbitrary inflaton background $\barφ_1$. Although the effective Einstein frame cutoff for $\barφ_1>>M_P/\sqrtξ$ turns out to be $\barφ_1$ or $ξ\barφ_1$ for the $U(1)$ model, this is not the case for the realistic doublet Higgs model where the effective Einstein frame cutoff turns out to be the… ▽ More This article is an addendum to [1]. We extend our computation of the on-shell scattering amplitudes in an arbitrary inflaton background $\barφ_1$. Although the effective Einstein frame cutoff for $\barφ_1>>M_P/\sqrtξ$ turns out to be $\barφ_1$ or $ξ\barφ_1$ for the $U(1)$ model, this is not the case for the realistic doublet Higgs model where the effective Einstein frame cutoff turns out to be the standard $M_P/\sqrtξ$ for both the Palatini and metric formulations. Then, as it has been pointed out in [1] the background $\barφ_1$ is the effective Jordan frame cutoff for both the Palatini and metric formulations. △ Less

Submitted 11 April, 2022; v1 submitted 18 March, 2022; originally announced March 2022.

Comments: 8 pages, 0 figures added preprint number

Ultraviolet behaviour of Higgs inflation models

Authors: Ignatios Antoniadis, Anthony Guillen, Kyriakos Tamvakis

Abstract: We study the ultraviolet behaviour of Higgs inflation models above the apparent unitarity violation scale arising from the large non minimal coupling to gravity, by computing on-shell 4-point scattering amplitudes in the presence of a large inflaton background, away from the electroweak vacuum. We find that all tree-level amplitudes are well behaved at high energies below the inflaton background t… ▽ More We study the ultraviolet behaviour of Higgs inflation models above the apparent unitarity violation scale arising from the large non minimal coupling to gravity, by computing on-shell 4-point scattering amplitudes in the presence of a large inflaton background, away from the electroweak vacuum. We find that all tree-level amplitudes are well behaved at high energies below the inflaton background that can thus take values up to the Planck scale. This result holds in both the metric and Palatini formulation, and is independent of the frame (Jordan or Einstein) as expected. The same result also holds if an $R^2$ term is added to the action. △ Less

Submitted 17 June, 2021; originally announced June 2021.

Comments: 21 pages, 0 figures

Extended interactions in the Palatini-$R^2$ inflation

Authors: Angelos Lykkas, Kyriakos Tamvakis

Abstract: The Palatini formulation of the Starobinsky model does not yield a propagating scalaron that can assume the role of the inflaton field as in the conventional metric formulation. In the so-called Palatini-${R}^2$ models this role is assumed by a fundamental scalar field nonminimally coupled to gravity. In this article we consider an extension of the interactions of this field by introducing a field… ▽ More The Palatini formulation of the Starobinsky model does not yield a propagating scalaron that can assume the role of the inflaton field as in the conventional metric formulation. In the so-called Palatini-${R}^2$ models this role is assumed by a fundamental scalar field nonminimally coupled to gravity. In this article we consider an extension of the interactions of this field by introducing a field dependence to the $R^2$ coefficient in the form of logarithmic corrections. We examine the resulting predictions of the inflationary observables in the framework of slow-roll approximation and investigate the possible implications of the reheating process on these predictions. We find that the model predictions lie in the favoured region of the observations and that the assumed scalar field dependence of the $R^2$ coupling tends to decrease the value of the spectral index $n_s$ and potentially enhance the tensor-to-scalar ratio $r$, circumventing the highly suppressed values commonly cited for models considered in the same framework. △ Less

Submitted 19 August, 2021; v1 submitted 18 March, 2021; originally announced March 2021.

Comments: 20 pages, 6 figures, 2 tables, JCAP accepted version

Constant-roll in the Palatini-$R^2$ models

Authors: Ignatios Antoniadis, Angelos Lykkas, Kyriakos Tamvakis

Abstract: We consider models of a scalar field coupled to quadratic $R\!+\!R^2$ gravity in the framework of the Palatini formulation. The resulting Einstein-frame generalized $k$-inflation effective theory is analyzed assuming that the constant-roll condition holds. We focus on a quartic self-interaction potential, a case of particular appeal modelling Higgs inflation, considering the cases of minimal and n… ▽ More We consider models of a scalar field coupled to quadratic $R\!+\!R^2$ gravity in the framework of the Palatini formulation. The resulting Einstein-frame generalized $k$-inflation effective theory is analyzed assuming that the constant-roll condition holds. We focus on a quartic self-interaction potential, a case of particular appeal modelling Higgs inflation, considering the cases of minimal and non-minimal coupling of the inflaton to gravity. For an appropriate range of the model parameters in the large field domain the obtained values for the inflationary observables are found in agreement with current observations. △ Less

Submitted 21 April, 2020; v1 submitted 28 February, 2020; originally announced February 2020.

Comments: 17 pages, 1 figure, 2 tables, JCAP accepted version

Single-field inflation in models with an $R^2$ term

Authors: Ignatios Antoniadis, Alexandros Karam, Angelos Lykkas, Thomas Pappas, Kyriakos Tamvakis

Abstract: We present two cases where the addition of the $R^2$ term to an inflationary model leads to single-field inflation instead of two-field inflation as is usually the case. In both cases we find that the effect of the $R^2$ term is to reduce the value of the tensor-to-scalar ratio $r$. We present two cases where the addition of the $R^2$ term to an inflationary model leads to single-field inflation instead of two-field inflation as is usually the case. In both cases we find that the effect of the $R^2$ term is to reduce the value of the tensor-to-scalar ratio $r$. △ Less

Submitted 29 December, 2019; originally announced December 2019.

Comments: 18 pages, 7 figures, invited talk given at Corfu Summer Institute 2019 "School and Workshops on Elementary Particle Physics and Gravity", 31 August - 25 September 2019, Corfu, Greece

Frame-dependence of inflationary observables in scalar-tensor gravity

Authors: Alexandros Karam, Thomas Pappas, Kyriakos Tamvakis

Abstract: By means of the Green's function method, we computed the spectral indices up to third order in the slow-roll approximation for a general scalar-tensor theory in both the Einstein and Jordan frames. Using quantities which are invariant under the conformal rescaling of the metric and transform as scalar functions under the reparametrization of the scalar field, we showed that the frames are equivale… ▽ More By means of the Green's function method, we computed the spectral indices up to third order in the slow-roll approximation for a general scalar-tensor theory in both the Einstein and Jordan frames. Using quantities which are invariant under the conformal rescaling of the metric and transform as scalar functions under the reparametrization of the scalar field, we showed that the frames are equivalent up to this order due to the underlying assumptions. Nevertheless, care must be taken when defining the number of $e$-folds. △ Less

Submitted 1 April, 2019; v1 submitted 8 March, 2019; originally announced March 2019.

Comments: 15 pages, 2 figures, talk (based on arXiv:1707.00984) given at Corfu Summer Institute 2018 "School and Workshops on Elementary Particle Physics and Gravity", September 2018

Rescuing Quartic and Natural Inflation in the Palatini Formalism

Authors: I. Antoniadis, A. Karam, A. Lykkas, T. Pappas, K. Tamvakis

Abstract: When considered in the Palatini formalism, the Starobinsky model does not provide us with a mechanism for inflation due to the absence of a propagating scalar degree of freedom. By (non)--minimally coupling scalar fields to the Starobinsky model in the Palatini formalism we can in principle describe the inflationary epoch. In this article, we focus on the minimally coupled quartic and natural infl… ▽ More When considered in the Palatini formalism, the Starobinsky model does not provide us with a mechanism for inflation due to the absence of a propagating scalar degree of freedom. By (non)--minimally coupling scalar fields to the Starobinsky model in the Palatini formalism we can in principle describe the inflationary epoch. In this article, we focus on the minimally coupled quartic and natural inflation models. Both theories are excluded in their simplest realization since they predict values for the inflationary observables that are outside the limits set by the Planck data. However, with the addition of the $R^2$ term and the use of the Palatini formalism, we show that these models can be rendered viable. △ Less

Submitted 26 February, 2019; v1 submitted 3 December, 2018; originally announced December 2018.

Comments: JCAP accepted version, 16 pages, 7 figures

Nonminimal Coleman--Weinberg Inflation with an $R^2$ term

Authors: Alexandros Karam, Thomas Pappas, Kyriakos Tamvakis

Abstract: We extend the Coleman--Weinberg inflationary model where a scalar field $φ$ is non-minimally coupled to gravity with the addition of the $R^2$ term. We express the theory in terms of two scalar fields and going to the Einstein frame we employ the Gildener--Weinberg formalism, compute the one-loop effective potential and essentially reduce the problem to the case of single-field inflation. It turns… ▽ More We extend the Coleman--Weinberg inflationary model where a scalar field $φ$ is non-minimally coupled to gravity with the addition of the $R^2$ term. We express the theory in terms of two scalar fields and going to the Einstein frame we employ the Gildener--Weinberg formalism, compute the one-loop effective potential and essentially reduce the problem to the case of single-field inflation. It turns out that there is only one free parameter, namely, the mixing angle between the scalars. For a wide range of this angle, we compute the inflationary observables which are in agreement with the latest experimental bounds. The effect of the $R^2$ term is that it lowers the value of the tensor-to-scalar ratio $r$. △ Less

Submitted 24 February, 2019; v1 submitted 30 October, 2018; originally announced October 2018.

Comments: typos fixed, equation (3.16) and discussion around it added, updated references

Palatini inflation in models with an $R^2$ term

Authors: I. Antoniadis, A. Karam, A. Lykkas, K. Tamvakis

Abstract: The Starobinsky model, considered in the framework of the Palatini formalism, in contrast to the metric formulation, does not provide us with a model for inflation, due to the absence of a propagating scalar degree of freedom that can play the role of the inflaton. In the present article we study the Palatini formulation of the Starobinsky model coupled, in general nonminimally, to scalar fields a… ▽ More The Starobinsky model, considered in the framework of the Palatini formalism, in contrast to the metric formulation, does not provide us with a model for inflation, due to the absence of a propagating scalar degree of freedom that can play the role of the inflaton. In the present article we study the Palatini formulation of the Starobinsky model coupled, in general nonminimally, to scalar fields and analyze its inflationary behavior. We consider scalars, minimally or nonminimally coupled to the Starobinsky model, such as a quadratic model, the induced gravity model or the standard Higgs-like inflation model and analyze the corresponding modifications favorable to inflation. In addition we examine the case of a classically scale-invariant model driven by the Coleman-Weinberg mechanism. In the slow-roll approximation, we analyze the inflationary predictions of these models and compare them to the latest constraints from the Planck collaboration. In all cases, we find that the effect of the $R^2$ term is to lower the value of the tensor-to-scalar ratio. △ Less

Submitted 20 November, 2018; v1 submitted 24 October, 2018; originally announced October 2018.

Comments: 22 pages, 8 figures, JCAP accepted version

Frame-invariant approach to higher-dimensional scalar-tensor gravity

Authors: Alexandros Karam, Angelos Lykkas, Kyriakos Tamvakis

Abstract: The recent interest in modified theories of gravity, involving some type of non-minimal coupling to the Ricci scalar, and the calculation of cosmological observables in the Einstein or the Jordan frame, motivate the formulation of these theories in terms of quantities that are invariant under frame transformations. Furthermore, in view of the description of gravity and its geometry motivated by st… ▽ More The recent interest in modified theories of gravity, involving some type of non-minimal coupling to the Ricci scalar, and the calculation of cosmological observables in the Einstein or the Jordan frame, motivate the formulation of these theories in terms of quantities that are invariant under frame transformations. Furthermore, in view of the description of gravity and its geometry motivated by string theory, such a formulation could be extended to include theories of extra spatial dimensions. In the present article, we generalize the construction of frame-invariant quantities, concerning a general, $D$-dimensional scalar-tensor theory. Then, we limit our scope to the 5D braneworld scenario, where we study thick brane solutions that are localized on the 3-brane and extend the invariant formulation to the case of multiple scalar fields (non-)minimally coupled to gravity. △ Less

Submitted 14 June, 2018; v1 submitted 13 March, 2018; originally announced March 2018.

Comments: PRD accepted version, 16 pp, no figures

Journal ref: Phys. Rev. D 97, 124036 (2018)

Constant-Roll (Quasi-)Linear Inflation

Authors: Alexandros Karam, Luca Marzola, Thomas Pappas, Antonio Racioppi, Kyriakos Tamvakis

Abstract: In constant-roll inflation, the scalar field that drives the accelerated expansion of the Universe is rolling down its potential at a constant rate. Within this framework, we highlight the relations between the Hubble slow-roll parameters and the potential ones, studying in detail the case of a single-field Coleman-Weinberg model characterised by a non-minimal coupling of the inflaton to gravity.… ▽ More In constant-roll inflation, the scalar field that drives the accelerated expansion of the Universe is rolling down its potential at a constant rate. Within this framework, we highlight the relations between the Hubble slow-roll parameters and the potential ones, studying in detail the case of a single-field Coleman-Weinberg model characterised by a non-minimal coupling of the inflaton to gravity. With respect to the exact constant-roll predictions, we find that assuming an approximate slow-roll behaviour yields a difference of $Δr = 0.001$ in the tensor-to-scalar ratio prediction. Such a discrepancy is in principle testable by future satellite missions. As for the scalar spectral index $n_s$, we find that the existing 2-$σ$ bound constrains the value of the non-minimal coupling to $ξ_φ\sim 0.29-0.31$ in the model under consideration. △ Less

Submitted 5 May, 2018; v1 submitted 27 November, 2017; originally announced November 2017.

Comments: 2 figures and a few comments added, matches version published in JCAP

Frame-dependence of higher-order inflationary observables in scalar-tensor theories

Authors: Alexandros Karam, Thomas Pappas, Kyriakos Tamvakis

Abstract: In the context of scalar-tensor theories of gravity we compute the third-order corrected spectral indices in the slow-roll approximation. The calculation is carried out by employing the Green's function method for scalar and tensor perturbations in both the Einstein and Jordan frames. Then, using the interrelations between the Hubble slow-roll parameters in the two frames we find that the frames a… ▽ More In the context of scalar-tensor theories of gravity we compute the third-order corrected spectral indices in the slow-roll approximation. The calculation is carried out by employing the Green's function method for scalar and tensor perturbations in both the Einstein and Jordan frames. Then, using the interrelations between the Hubble slow-roll parameters in the two frames we find that the frames are equivalent up to third order. Since the Hubble slow-roll parameters are related to the potential slow-roll parameters, we express the observables in terms of the latter which are manifestly invariant. Nevertheless, the same inflaton excursion leads to different predictions in the two frames since the definition of the number of e-folds differs. To illustrate this effect we consider a nonminimal inflationary model and find that the difference in the predictions grows with the nonminimal coupling and it can actually be larger than the difference between the first and third order results for the observables. Finally, we demonstrate the effect of various end-of-inflation conditions on the observables. These effects will become important for the analyses of inflationary models in view of the improved sensitivity of future experiments. △ Less

Submitted 19 September, 2017; v1 submitted 2 July, 2017; originally announced July 2017.

Comments: Title changed per journal request, typos fixed, a few comments and references added, to appear in PRD

Journal ref: Phys. Rev. D 96, 064036 (2017)

Towards a formulation of $f(R)$ supergravity

Authors: G. A. Diamandis, A. B. Lahanas, K. Tamvakis

Abstract: The supersymmetric completion of $R+R^2$ gravity is known to be equivalent to standard supergravity coupled to two chiral supermultiples with a Kähler potential of the no-scale type. The generalization of this programme to $f(R)$ gravity can be carried out in an analogous framework in terms of four chiral multiplets with appropriately chosen superpotential and Kähler potential. Although the constr… ▽ More The supersymmetric completion of $R+R^2$ gravity is known to be equivalent to standard supergravity coupled to two chiral supermultiples with a Kähler potential of the no-scale type. The generalization of this programme to $f(R)$ gravity can be carried out in an analogous framework in terms of four chiral multiplets with appropriately chosen superpotential and Kähler potential. Although the construction strategy has been known for sometime, we carry out this programme further by setting these theories in a closed form. The general framework considered can accommodate supergravity actions local in the scalar curvature, dual to ordinary $N = 1$ supergravities. Although these are in general plagued by ghosts, the framework studied in this work offers a possibility that these can decouple as can be seen in specific examples. △ Less

Submitted 20 November, 2015; v1 submitted 3 September, 2015; originally announced September 2015.

Comments: 21 pages, references added, typos corrected

Journal ref: Phys. Rev. D 92, 105023 (2015)

Inflation in no-scale supergravity

Authors: A. B. Lahanas, K. Tamvakis

Abstract: $R+R^2$ Supergravity is known to be equivalent to standard Supergravity coupled to two chiral supermultiples with a no-scale Kähler potential. Within this framework, that can accomodate vanishing vacuum energy and spontaneous supersymmetry breaking, we consider modifications of the associated superpotential and study the resulting models, which, viewed as generalizations of the Starobinsky model,… ▽ More $R+R^2$ Supergravity is known to be equivalent to standard Supergravity coupled to two chiral supermultiples with a no-scale Kähler potential. Within this framework, that can accomodate vanishing vacuum energy and spontaneous supersymmetry breaking, we consider modifications of the associated superpotential and study the resulting models, which, viewed as generalizations of the Starobinsky model, for a range of the superpotential parameters, describe viable single-field slow-roll inflation. In all models studied in this work the tensor to scalar ratio is found to be small, well below the upper bound established by the very recent PLANCK and BICEP2 data. △ Less

Submitted 15 March, 2015; v1 submitted 26 January, 2015; originally announced January 2015.

Comments: 15 pages, 6 figures, references and comments added

Journal ref: Phys. Rev. D 91, 085001 (2015)

Inflationary behavior of $R^2$ gravity in a conformal framework

Authors: A. B. Lahanas, K. Tamvakis

Abstract: Models of inflation are tightly constrained by the PLANCK satellite data. Among them, Starobinsky's model with an exponential type potential seems to be challenged by the recent BICEP2 results. The model is based on the existence of $\,~ R^2$ terms in the Einstein-Hilbert action, which have their origin in the conformal-anomaly. Conformal (or Weyl) gravitational theories are relevant when matter f… ▽ More Models of inflation are tightly constrained by the PLANCK satellite data. Among them, Starobinsky's model with an exponential type potential seems to be challenged by the recent BICEP2 results. The model is based on the existence of $\,~ R^2$ terms in the Einstein-Hilbert action, which have their origin in the conformal-anomaly. Conformal (or Weyl) gravitational theories are relevant when matter fields become effectively massless; i.e. their masses are negligible in comparison with the spacetime curvature. These theories may include other, additional scalar fields. We show that their presence under general conditions does not destabilize the inflationary behaviour encountered in the Starobinsky model, although the issue of the exact quantitative agreement with existing data, like the tensor to scalar ratio, rests on the choice of parameters. △ Less

Submitted 11 December, 2014; v1 submitted 5 May, 2014; originally announced May 2014.

Comments: 17 pages, 8 figures, revised version, references and text added, typos corrected

Journal ref: Phys. Rev. D 90, 123530 (2014)

Scalar Emission in the Bulk in a Rotating Black Hole Background

Authors: S. Creek, O. Efthimiou, P. Kanti, K. Tamvakis

Abstract: We study the emission of scalars into the bulk from a higher-dimensional rotating black hole. We obtain an analytic solution to the field equation by employing matching techniques on expressions valid in the near-horizon and far-field regimes. Both analytic and numerical results for the absorption probability, in the low-energy and low-angular momentum limit, are derived and found to be in excel… ▽ More We study the emission of scalars into the bulk from a higher-dimensional rotating black hole. We obtain an analytic solution to the field equation by employing matching techniques on expressions valid in the near-horizon and far-field regimes. Both analytic and numerical results for the absorption probability, in the low-energy and low-angular momentum limit, are derived and found to be in excellent agreement. We also compute the energy emission rate, and show that the brane-to-bulk ratio of the energy emission rates for scalar fields remains always larger than unity in the aforementioned regime. △ Less

Submitted 25 October, 2007; v1 submitted 3 September, 2007; originally announced September 2007.

Comments: 14 pages, Latex file, 5 figures, references added

Journal ref: Phys.Lett.B656:102-111,2007

Synergistic Gravity and the Role of Resonances in GRS-Inspired Braneworlds

Authors: C. Bogdanos, A. Dimitriadis, K. Tamvakis

Abstract: We consider 5D braneworld models of quasi-localized gravity in which 4D gravity is reproduced at intermediate scales while the extra dimension opens up at both the very short and the very long distances, where the geometry is flat. Our main interest is the interplay between the zero mode of these models, whenever a normalizable zero mode exists, and the effects of zero energy graviton resonant m… ▽ More We consider 5D braneworld models of quasi-localized gravity in which 4D gravity is reproduced at intermediate scales while the extra dimension opens up at both the very short and the very long distances, where the geometry is flat. Our main interest is the interplay between the zero mode of these models, whenever a normalizable zero mode exists, and the effects of zero energy graviton resonant modes coming from the contributions of massive KK modes. We first consider a compactified version of the GRS model and find that quasi-localized gravity is characterized by a scale for which both the resonance and the zero mode have significant contribution to 4D gravity. Above this scale, gravity is primarily mediated by the zero mode, while the resonance gives only minor corrections. Next, we consider an asymmetric version of the standard non-compact GRS model, characterized by different cosmological constants on each AdS side. We show that a resonance is present but the asymmetry, through the form of the localizing potential, can weaken it, resulting in a shorter lifetime and, thus, in a shorter distance scale for 4D gravity. As a third model exhibiting quasi-localization, we consider a version of the GRS model in which the central positive tension brane has been replaced by a configuration of a scalar field propagating in the bulk. △ Less

Submitted 16 March, 2009; v1 submitted 7 June, 2007; originally announced June 2007.

Comments: 18 pages, 3 figures, added 1 figure, revised version as published in Class. Quant. Grav

Journal ref: Class.Quant.Grav.25:045008,2008

Greybody Factors for Brane Scalar Fields in a Rotating Black-Hole Background

Authors: S. Creek, O. Efthimiou, P. Kanti, K. Tamvakis

Abstract: We study the evaporation of (4+n)-dimensional rotating black holes into scalar degrees of freedom on the brane. We calculate the corresponding absorption probabilities and cross-sections obtaining analytic solutions in the low-energy regime, and compare the derived analytic expressions to numerical results, with very good agreement. We then consider the high-energy regime, construct an analytic… ▽ More We study the evaporation of (4+n)-dimensional rotating black holes into scalar degrees of freedom on the brane. We calculate the corresponding absorption probabilities and cross-sections obtaining analytic solutions in the low-energy regime, and compare the derived analytic expressions to numerical results, with very good agreement. We then consider the high-energy regime, construct an analytic high-energy solution to the scalar-field equation by employing a new method, and calculate the absorption probability and cross-section for this energy regime, finding again a very good agreement with the exact numerical results. We also determine the high-energy asymptotic value of the total cross-section, and compare it to the analytic results derived from the application of the geometrical optics limit. △ Less

Submitted 3 May, 2007; v1 submitted 31 January, 2007; originally announced January 2007.

Comments: Latex file, 30 pages, 5 figures, typos corrected, version published in Phys. Rev. D

Journal ref: Phys.Rev.D75:084043,2007

Brane Cosmological Evolution With Bulk Matter

Authors: C. Bogdanos, K. Tamvakis

Abstract: We consider the cosmological evolution of a brane for general bulk matter content. In our setup the bulk pressure and the energy exchange densities are comparable to the brane energy density. Adopting a phenomenological fluid ansatz and generalizations of it, we derive a set of exact solutions of the Friedmann equation that exhibit accelerated expansion. We find that the effective equation of st… ▽ More We consider the cosmological evolution of a brane for general bulk matter content. In our setup the bulk pressure and the energy exchange densities are comparable to the brane energy density. Adopting a phenomenological fluid ansatz and generalizations of it, we derive a set of exact solutions of the Friedmann equation that exhibit accelerated expansion. We find that the effective equation of state parameter for the dark energy can exhibit w=-1 crossing without the presence of exotic matter. △ Less

Submitted 14 September, 2006; originally announced September 2006.

Comments: 11 Pages

Journal ref: Phys.Lett.B646:39-46,2007

Graviton Emission in the Bulk from a Higher-Dimensional Schwarzschild Black Hole

Authors: S. Creek, O. Efthimiou, P. Kanti, K. Tamvakis

Abstract: We consider the evaporation of (4+n)-dimensional non-rotating black holes into gravitons. We calculate the energy emission rate for gravitons in the bulk obtaining analytical solutions of the master equation satisfied by all three types (S,V,T) of gravitational perturbations. Our results, valid in the low-energy regime, show a vector radiation dominance for every value of n, while the relative m… ▽ More We consider the evaporation of (4+n)-dimensional non-rotating black holes into gravitons. We calculate the energy emission rate for gravitons in the bulk obtaining analytical solutions of the master equation satisfied by all three types (S,V,T) of gravitational perturbations. Our results, valid in the low-energy regime, show a vector radiation dominance for every value of n, while the relative magnitude of the energy emission rate of the subdominant scalar and tensor radiation depends on n. The low-energy emission rate in the bulk for gravitons is well below that for a scalar field, due to the absence of the dominant l=0,1 modes from the gravitational spectrum. Higher partial waves though may modify this behaviour at higher energies. The calculated low-energy emission rate, for all types of degrees of freedom decreases with n, although the full energy emission rate, integrated over all frequencies, is expected to increase with n, as in the previously studied case of a bulk scalar field. △ Less

Submitted 22 February, 2006; v1 submitted 18 January, 2006; originally announced January 2006.

Comments: 17 pages, 2 figures, minor corrections, accepted by Phys. Lett. B

Journal ref: Phys.Lett.B635:39-49,2006

Quest for Localized 4-D Black Holes in Brane Worlds. II : Removing the bulk singularities

Authors: P. Kanti, I. Olasagasti, K. Tamvakis

Abstract: We analyze further the possibility of obtaining localized black hole solutions in the framework of Randall-Sundrum-type brane-world models. We consider black hole line-elements analytic at the horizon, namely, generalizations of the Painleve and Vaidya metrics, which are taken to have a decaying dependence of the horizon on the extra dimension. These backgrounds have no other singularities apart… ▽ More We analyze further the possibility of obtaining localized black hole solutions in the framework of Randall-Sundrum-type brane-world models. We consider black hole line-elements analytic at the horizon, namely, generalizations of the Painleve and Vaidya metrics, which are taken to have a decaying dependence of the horizon on the extra dimension. These backgrounds have no other singularities apart from the standard black hole singularity which is localized in the direction of the fifth dimension. Both line-elements can be sustained by a regular, shell-like distribution of bulk matter of a non-standard form. Of the two, the Vaidya line-element is shown to provide the most attractive, natural choice: despite the scaling of the horizon, the 5D spacetime has the same topological structure as the one of a RS-Schwarzschild spacetime and demands a minimal bulk energy-momentum tensor. △ Less

Submitted 20 November, 2003; v1 submitted 21 July, 2003; originally announced July 2003.

Comments: 18 pages, Latex file, 4 eps figures, typos corrected, references added, version to appear in Phys. Rev. D

Report number: CERN-TH/2003-140

Journal ref: Phys.Rev. D68 (2003) 124001

Gravity in higher codimension de Sitter Brane Worlds

Authors: I. Olasagasti, K. Tamvakis

Abstract: We study solutions of Einstein's equations corresponding codimension n>2 global topological defects with de Sitter slices. We analyze a class of solutions that are cylindrically symmetric and admit positive, negative or zero bulk cosmological constant. We derive the relevant graviton equations. For an extended brane, the properties of the solution depend on apropriate boundary conditions that th… ▽ More We study solutions of Einstein's equations corresponding codimension n>2 global topological defects with de Sitter slices. We analyze a class of solutions that are cylindrically symmetric and admit positive, negative or zero bulk cosmological constant. We derive the relevant graviton equations. For an extended brane, the properties of the solution depend on apropriate boundary conditions that the exterior solutions have to satisfy near the core. As an alternative we consider matching copies of the exterior solution related by symmetry. We show that we can get localization only when the bulk cosmological constant is negative. We obtain a condition on the global defect symmetry breaking scale which ultimately controls the size of the n-1 internal dimensions at the position of the brane. The induced metric on the brane, in the case of mirror spacetimes, is a direct product of a de Sitter space and an (n-1)-sphere, while the metric of the embedding spacetime is a warped product and the actual size of the (n-1)-sphere changes as we move along the radial direction. The solutions possess naked singularities, which nevertheless satisfy no-flow conditions. △ Less

Submitted 11 March, 2003; originally announced March 2003.

Comments: 15 pages, 2 figures

Journal ref: Phys.Rev. D68 (2003) 064016

Challenges and Obstacles for a Bouncing Universe in Brane Models

Authors: Panagiota Kanti, Kyriakos Tamvakis

Abstract: A Brane evolving in the background of a charged AdS black-hole displays in general a bouncing behaviour with a smooth transition from a contracting to an expanding phase. We examine in detail the conditions and consequences of this behaviour in various cases. For a cosmological-constant-dominated Brane, we obtain a singularity-free, inflationary era which is shown to be compatible only with an i… ▽ More A Brane evolving in the background of a charged AdS black-hole displays in general a bouncing behaviour with a smooth transition from a contracting to an expanding phase. We examine in detail the conditions and consequences of this behaviour in various cases. For a cosmological-constant-dominated Brane, we obtain a singularity-free, inflationary era which is shown to be compatible only with an intermediate-scale fundamental Planck mass. For a radiation-dominated Brane, the bouncing behaviour can occur only for background-charge values exceeding those allowed for non-extremal black holes. For a matter-dominated Brane, the black-hole mass affects the proper volume or the expansion rate of the Brane. We also consider the Brane evolving in an asymmetric background of two distinct charged AdS black hole spacetimes being bounded by the Brane and find that, in the case of an empty critical Brane, bouncing behaviour occurs only if the black-hole mass difference is smaller than a certain value. The effects of a Brane curvature term on the bounce at early and late times are also investigated. △ Less

Submitted 16 May, 2003; v1 submitted 8 March, 2003; originally announced March 2003.

Comments: 23 pages, Latex file, comments and references added, version to appear in Phys. Rev. D

Report number: CERN-TH/2003-055

Journal ref: Phys.Rev. D68 (2003) 024014

Schwarzschild Black Branes and Strings in higher-dimensional Brane Worlds

Authors: P. Kanti, I. Olasagasti, K. Tamvakis

Abstract: We consider branes embedded in spacetimes of codimension one and two, with a warped metric tensor for the subspace parallel to the brane. We study a variety of brane-world solutions arising by introducing a Schwarzschild-like black hole metric on the brane and we investigate the properties of the corresponding higher-dimensional spacetime. We demonstrate that normalizable bulk modes lead to a va… ▽ More We consider branes embedded in spacetimes of codimension one and two, with a warped metric tensor for the subspace parallel to the brane. We study a variety of brane-world solutions arising by introducing a Schwarzschild-like black hole metric on the brane and we investigate the properties of the corresponding higher-dimensional spacetime. We demonstrate that normalizable bulk modes lead to a vanishing flow of energy through the naked singularities. From this point of view, these singularities are harmless. △ Less

Submitted 31 July, 2002; originally announced July 2002.

Comments: 22 pages, Latex file

Report number: CERN-TH/2002-162

Journal ref: Phys.Rev. D66 (2002) 104026

Quest for Localized 4-D Black Holes in Brane Worlds

Authors: Panagiota Kanti, Kyriakos Tamvakis

Abstract: We investigate the possibility of obtaining localized black hole solutions in brane worlds by introducing a dependence of the four-dimensional line--element on the extra dimension. An analysis, performed for the cases of an empty bulk and of a bulk containing either a scalar or a gauge field, reveals that no conventional type of matter can support such a dependence. Considering a particular ansa… ▽ More We investigate the possibility of obtaining localized black hole solutions in brane worlds by introducing a dependence of the four-dimensional line--element on the extra dimension. An analysis, performed for the cases of an empty bulk and of a bulk containing either a scalar or a gauge field, reveals that no conventional type of matter can support such a dependence. Considering a particular ansatz for the five-dimensional line--element that corresponds to a black hole solution with a ``decaying'' horizon, we determine the bulk energy--momentum tensor capable of sustaining such a behaviour. It turns out that an exotic, shell-like distribution of matter is required. For such solutions, the black hole singularity is indeed localized near the brane and the spacetime is well defined near the AdS horizon, in contrast to the behaviour found in black string type solutions. △ Less

Submitted 2 February, 2002; v1 submitted 31 October, 2001; originally announced October 2001.

Comments: 17 pages, RevTex, 3 figures, version to appear in Physical Review D, comments and references added, typos corrected

Journal ref: Phys.Rev.D65:084010,2002

Singularity-free cosmological solutions in quadratic gravity

Authors: P. Kanti, J. Rizos, K. Tamvakis

Abstract: We study a general field theory of a scalar field coupled to gravity through a quadratic Gauss-Bonnet term $ξ(φ) R^2_{GB}$. The coupling function has the form $ξ(φ)=φ^n$, where $n$ is a positive integer. In the absence of the Gauss-Bonnet term, the cosmological solutions for an empty universe and a universe dominated by the energy-momentum tensor of a scalar field are always characterized by the… ▽ More We study a general field theory of a scalar field coupled to gravity through a quadratic Gauss-Bonnet term $ξ(φ) R^2_{GB}$. The coupling function has the form $ξ(φ)=φ^n$, where $n$ is a positive integer. In the absence of the Gauss-Bonnet term, the cosmological solutions for an empty universe and a universe dominated by the energy-momentum tensor of a scalar field are always characterized by the occurrence of a true cosmological singularity. By employing analytical and numerical methods, we show that, in the presence of the quadratic Gauss-Bonnet term, for the dual case of even $n$, the set of solutions of the classical equations of motion in a curved FRW background includes singularity-free cosmological solutions. The singular solutions are shown to be confined in a part of the phase space of the theory allowing the non-singular solutions to fill the rest of the space. We conjecture that the same theory with a general coupling function that satisfies certain criteria may lead to non-singular cosmological solutions. △ Less

Submitted 22 November, 1998; v1 submitted 21 June, 1998; originally announced June 1998.

Comments: Latex, 25 pages, 6 figures, some explanatory sentences and Comments added, version to appear in Physical Review D

Journal ref: Phys. Rev. D 59, 083512 (1999)

Dilatonic Black Holes in Higher-Curvature String Gravity II: Linear Stability

Authors: P. Kanti, N. E. Mavromatos, J. Rizos, K. Tamvakis, E. Winstanley

Abstract: We demonstrate linear stability of the dilatonic Black Holes appearing in a string-inspired higher-derivative gravity theory with a Gauss- Bonnet curvature-squared term. The proof is accomplished by mapping the system to a one-dimensional Schrodinger problem which admits no bound states. This result is important in that it constitutes a linearly stable example of a black hole that bypasses the `… ▽ More We demonstrate linear stability of the dilatonic Black Holes appearing in a string-inspired higher-derivative gravity theory with a Gauss- Bonnet curvature-squared term. The proof is accomplished by mapping the system to a one-dimensional Schrodinger problem which admits no bound states. This result is important in that it constitutes a linearly stable example of a black hole that bypasses the `no-hair conjecture'. However, the dilaton hair is `secondary'in the sense that it is not accompanied by any new quantum number for the black hole solution. △ Less

Submitted 26 March, 1997; originally announced March 1997.

Comments: 25 pages LaTeX file, 5 LaTeX figures (included)

Report number: Oxford OUTP-12-P

Journal ref: Phys.Rev. D57 (1998) 6255-6264

Coloured Black Holes in Higher Curvature String Gravity

Authors: P. Kanti, K. Tamvakis

Abstract: We consider the combined Yang Mills-Dilaton-Gravity system in the presence of a Gauss-Bonnet term as it appears in the $4D$ Effective Superstring Action. We give analytical arguments and demonstrate numerically the existence of black hole solutions with non-trivial dilaton and Yang Mills hair for the particular case of SU(2) gauge fields. The thermodynamical properties of the solutions are also… ▽ More We consider the combined Yang Mills-Dilaton-Gravity system in the presence of a Gauss-Bonnet term as it appears in the $4D$ Effective Superstring Action. We give analytical arguments and demonstrate numerically the existence of black hole solutions with non-trivial dilaton and Yang Mills hair for the particular case of SU(2) gauge fields. The thermodynamical properties of the solutions are also discussed. △ Less

Submitted 1 September, 1996; originally announced September 1996.

Comments: 15 pages, 3 LATEX figures already included

Report number: CERN-TH/96-233

Journal ref: Phys.Lett.B392:30-38,1997

Dilatonic Black Holes in Higher Curvature String Gravity

Authors: P. Kanti, N. E. Mavromatos, J. Rizos, K. Tamvakis, E. Winstanley

Abstract: We give analytical arguments and demonstrate numerically the existence of black hole solutions of the $4D$ Effective Superstring Action in the presence of Gauss-Bonnet quadratic curvature terms. The solutions possess non-trivial dilaton hair. The hair, however, is of ``secondary" type", in the sense that the dilaton charge is expressed in terms of the black hole mass. Our solutions are not cover… ▽ More We give analytical arguments and demonstrate numerically the existence of black hole solutions of the $4D$ Effective Superstring Action in the presence of Gauss-Bonnet quadratic curvature terms. The solutions possess non-trivial dilaton hair. The hair, however, is of ``secondary" type", in the sense that the dilaton charge is expressed in terms of the black hole mass. Our solutions are not covered by the assumptions of existing proofs of the ``no-hair" theorem. We also find some alternative solutions with singular metric behaviour, but finite energy. The absence of naked singularities in this system is pointed out. △ Less

Submitted 10 November, 1995; originally announced November 1995.

Comments: 22 pages, Latex file, 7 Latex figures already included

Report number: CERN-TH/95-297, OUTP-95-43P

Journal ref: Phys.Rev.D54:5049-5058,1996

Classical moduli $O(α')$ hair

Authors: P. Kanti, K. Tamvakis

Abstract: We extend existing treatments of black hole solutions in String Gravity to include moduli fields. We compute the external moduli and dilaton hair, as well as of their associated axions,to $O(α')$ in the framework of the loop corrected superstring effective action for a Kerr-Newman black hole background. We extend existing treatments of black hole solutions in String Gravity to include moduli fields. We compute the external moduli and dilaton hair, as well as of their associated axions,to $O(α')$ in the framework of the loop corrected superstring effective action for a Kerr-Newman black hole background. △ Less

Submitted 6 April, 1995; originally announced April 1995.

Comments: 11 pages,LaTex file,no figures

Report number: IOA-317/95

Journal ref: Phys.Rev. D52 (1995) 3506-3511

Classical moduli hair for Kerr black holes in String Gravity

Authors: P. Kanti, K. Tamvakis

Abstract: We compute the external moduli and dilaton hair to $O(α')$ in the framework of the one-loop corrected superstring effective action for a rotating black hole background. We compute the external moduli and dilaton hair to $O(α')$ in the framework of the one-loop corrected superstring effective action for a rotating black hole background. △ Less

Submitted 5 March, 1995; v1 submitted 15 February, 1995; originally announced February 1995.

Comments: 8 pages,LaTex file,no figures(one reference corrected,some terms in Einstein's equation changed,no effect on our results).

Report number: IOA-316/95

On the existence of singularity-free solutions in quartic gravity

Authors: J. Rizos, K. Tamvakis

Abstract: We study a general field theory of a scalar field coupled to gravitation through a quadratic Gauss-Bonnet term $ξ(φ)$$R_{GB}^2$. We show that, under mild assumptions about the function $ξ(φ)$, the classical solutions in a spatially flat FRW background include singularity - free solutions. We study a general field theory of a scalar field coupled to gravitation through a quadratic Gauss-Bonnet term $ξ(φ)$$R_{GB}^2$. We show that, under mild assumptions about the function $ξ(φ)$, the classical solutions in a spatially flat FRW background include singularity - free solutions. △ Less

Submitted 20 January, 1994; originally announced January 1994.

Comments: 9 pages, LATEX, uses epsf.tex macro, (1 figure included in uuencode+compress EPSF form), IOA-296

Journal ref: Phys.Lett. B326 (1994) 57-61

Singularity-free cosmological solutions of the superstring effective action

Authors: I. Antoniadis, J. Rizos, K. Tamvakis

Abstract: We study the cosmological solutions of the one loop corrected superstring effective action, in a Friedmann-Robertson-Walker background, and in the presence of the dilaton and modulus fields. A particularly interesting class of solutions is found which avoid the initial singularity and are consistent with the perturbative treatment of the effective action. We study the cosmological solutions of the one loop corrected superstring effective action, in a Friedmann-Robertson-Walker background, and in the presence of the dilaton and modulus fields. A particularly interesting class of solutions is found which avoid the initial singularity and are consistent with the perturbative treatment of the effective action. △ Less

Submitted 7 May, 1993; originally announced May 1993.

Comments: 20 pages, plain tex, 7 figures available in postscript files upon request, CPTH-A239.0593

Journal ref: Nucl.Phys. B415 (1994) 497-514