arXiv subjects

S. D. Odintsov

Publications and source records attributed to S. D. Odintsov.

At least 37 records · Page 2Linked to original sources

arXiv2025

Effective Equation of State Oscillations at Matter-Radiation Equality and Primordial Gravitational Waves

The theory controlling the Universe's evolution in the classical regime has to be motivated by particle physics reasoning and should also generate inflation and dark energy eras in a unified way. One such framework is $F(R)$ gravity. In this work we examine a class of exponential deformations of $R^2$ gravity motivated by fundamental physics of scalaron evolution in a de Sitter background. As we show this class of models describe both inflation and the dark energy era in a viable way compatible with the Planck constraints on inflation and the cosmological parameters. Regarding the inflationary era, the exponentially deformed $R^2$ model also yields a rescaled Einstein-Hilbert term which remarkably does not affect the dynamics and the inflationary evolution is identical to that of an $R^2$ model. The dark energy era is also found to be viable and mimics the $Λ$-Cold-Dark-Matter model. More importantly, this class of $F(R)$ gravity exponential $R^2$ deformations also has an important characteristic, and specifically it yields total equation of state oscillations deeply in the matter domination era, for redshifts $z\sim 3400$, so near the matter-radiation equality. These total equation of state deformations at such a large redshift may directly affect the energy spectrum of the primordial gravitational waves. Indeed as we show, the effect is measurable and it leads to an enhancement of the tensor perturbations energy spectrum for low frequencies probed by the future LiteBIRD mission. This enhancement might have a measurable effect on the $B$-modes of the Cosmic Microwave Background radiation and thus may be detectable by the LiteBIRD mission. Only a handful of theoretical frameworks can generate the gravitational wave pattern generated by the class of exponentially deformed $R^2$ models we presented.

gr-qc↗

arXiv2025

A Power-law Inflation Tail for the Standard $R^2$-Inflation and the Trans-Planckian Censorship Conjecture

The conceptual problems of the standard slow-roll inflationary scenario include the Trans-Planckian Censorship Conjecture issue, which severely restricts the tensor-to-scalar ratio in the standard minimally coupled scalar field inflation. Motivated by the fact that a scalar field in its vacuum configuration can be minimally coupled to gravity, or conformally coupled, and also that the first quantum corrections of the scalar field action include $R^2$ corrections, in this work we assume that $R^2$ gravity in the presence of a scalar field with constant equation of state parameter co-exist and control the early Universe. Constant equation of state parameter scalar field result from exponential scalar potentials. In our approach, the standard slow-roll era is controlled by the $R^2$ gravity and is followed by a power-law inflationary tail governed by a minimally coupled scalar field with an quintessential equation of state parameter, stemming from an exponential scalar potential. The fact that the total equation of state parameter after the end of the slow-roll era is equal to the value determined by the scalar field, has an effect on the duration of the $R^2$ governed slow-roll era, and it actually shortens the duration of the slow-roll era, by an extent which depends on the reheating temperature too, after all the inflationary patches have ended. The power-law inflationary tail to the standard $R^2$ inflation, solves the Trans-Planckian Censorship Conjecture issues, and also the Swampland conjecture can be amended in this context. We also perform a dynamical system study to confirm numerically our findings.

gr-qc↗

arXiv2025

Einstein-Gauss-Bonnet Cosmology Confronted with Observations

Several models within the framework of Einstein-Gauss-Bonnet gravities are considered with regard their late-time phenomenological viability. The models contain a non-minimally coupled scalar field and satisfy a constraint on the scalar field Gauss-Bonnet coupling, that guarantees that the speed of the tensor perturbations is equal to the speed of light. The late-time cosmological evolution of these Einstein-Gauss-Bonnet models is confronted with the observational data including the Pantheon plus Type Ia supernovae catalogue, the Hubble parameter measurements (cosmic chronometers), data from cosmic microwave background radiation (CMB) and baryon acoustic oscillations (BAO) including the latest measurements from Dark Energy Spectroscopic Instrument (DESI). Among the considered class of models some of them do not fit the CMB and BAO data. However, there exists some models that generate a viable Einstein-Gauss-Bonnet scenario with well-behaved late-time cosmological evolution that fits the observational data essentially better in comparison to the standard $Λ$-Cold-Dark-Matter model.

gr-qc↗

arXiv2025

Non-metricity approach to Jackiw-Teitelboim gravity

We construct 2d Jackiw-Teitelboim (JT) gravity in the framework of symmetric teleparallel gravity based on a non-metricity tensor. In symmetric teleparallel gravity, we often use the scalar quantity $Q$ composed of the bilinear terms of the non-metricity tensor but $Q$ is not a unique scalar quantity from the viewpoint of covariance. However, other scalar quantities composed of the non-metricity tensor generate ghosts in general. In two dimensions, because a propagating gravitational mode does not exist, if we consider general scalar quantities, there might appear any propagating modes although these modes are often ghosts. We consider a general combination of the terms given by the bi-linear of the non-metricity tensors. After finding the condition that the conformal form of the metric is compatible with the coincident gauge, where connections vanish, we determine the combination so that a solution with constant curvature exists. The obtained model corresponds to 2d JT gravity in the framework of the symmetric teleparallel gravity. The conformal mode propagates as a scalar in this theory and we also consider the condition that the scalar mode is not a ghost.

hep-th↗

arXiv2024

Black holes and their shadows in $F(R)$ gravity

We investigate the radii of the photon sphere and the black hole shadow in the framework of $F(R)$ gravity. For this purpose, we derive the field equation for the corresponding theory when the general spherically symmetric and static configuration is considered. This equation is the third-order differential equation with respect to $F_R(r)\equiv \left. \frac{dF(R)}{dR}\right|_{R=R(r)}$, where $r$ is the radial coordinate. Solving the equation, we find $F(R)$ as a function of $r$, $F_R=F_R(r)$. By using the assumed and obtained geometry, one can calculate the scalar curvature $R$ as a function of $r$, $R=R(r)$, which could be solved with respect to $r$ as $r=r(R)$. Then one finds the functional form of $F_R$ as a function of the scalar curvature $R$, $F_R=F_R(R)=F_R\left( r=r\left(R\right)\right)$.

gr-qc↗

arXiv2024

Rips and regular future scenario with Holographic Dark Energy: A comprehensive look

Interest on the possible future scenarios the universe could have has grew substantially with breakthroughs on late-time acceleration. Holographic dark energy (HDE) presents a very interesting approach towards addressing late-time acceleration, presenting an intriguing interface of ideas from quantum gravity and cosmology. In this work we present an extensive discussion of possible late-time scenarios, focusing on rips and similar events, in a universe with holographic dark energy. We discuss these events in the realm of the generalized Nojiri-Odintsov cutoff and also for the more primitive holographic cutoffs like Hubble, particle and event horizon cutoffs. We also discuss the validity of the generalized second law of thermodynamics and various energy conditions in these regimes. Our work points towards the idea that it is not possible to have alternatives of the big rip consistently in the simpler HDE cutoffs, and shows the flexibility of the generalized HDE cutoff as well.

gr-qc↗

arXiv2024

Black holes, photon sphere, and cosmology in ghost-free $f(\mathcal{G})$ gravity

The improved version of ghost-free $f(\mathcal{G})$ gravity introduced in Phys.~Lett.~B \textbf{631} (2005), 1-6 is proposed and investigated. It is demonstrated that improved ghost-free $f(\mathcal{G})$ gravity may be consistently applied to describe the expanding universe (with horizon) as well as the static spacetime like black holes. Interestingly, such a theory looks like a close avatar of scalar-Einstein-Gauss-Bonnet gravity with the only difference that the scalar field is not a dynamical one so that many predictions of ghost-free $f(\mathcal{G})$ gravity are very similar to that of sEGB gravity. We discuss the gravitational wave in the improved model with the condition that the propagating speed of the gravitational wave is identical to that of the propagation speed of light. A model that describes inflation in the early universe and could satisfy the observational constraints is also constructed. The solution of ghost-free $f(\mathcal{G})$ gravity realising the given static spacetime with spherical symmetry is proposed. Some of such solutions realise explicitly the Reissner-Nordström black hole or the Hayward black hole, which is a regular black hole without curvature singularity, We also construct a black hole in our theory which contains the Arnowitt-Deser-Misner (ADM) mass, the horizon radius, and the radius of the photon sphere as independent parameters. The radius of the black hole shadow in this model as well as photon sphere radius are estimated. It is shown that there exists a parameter region which satisfies the constraints coming from Event Horizon Telescope observations. Furthermore, we construct model where the radius of the black hole shadow or photon orbit is smaller than the horizon radius supposed from the ADM mass.

gr-qc↗

arXiv2024

$F(Q)$ gravity with Gauss-Bonnet corrections: from early-time inflation to late-time acceleration

We show that in the $f(Q)$ gravity with a non-metricity scalar $Q$, the curvatures in Einstein's gravity, that is, the Riemann curvature constructed from the standard Levi-Civita connection, could not be excluded or naturally appear. The first observation is that even in $f(Q)$ gravity, the conservation of the matter energy-momentum tensor is not described by the covariant derivatives in the non-metricity gravity but that is given by the Levi-Civita connection. The commutator of the covariant derivatives in Einstein's gravity inevitably induces the Riemann curvature. There is no symmetry nor principle which prohibits the Riemann curvature in non-metricity gravity. Based on this observation, we propose and investigate $f\left(Q, \mathcal{G} \right)$ gravity with the Gauss-Bonnet invariant $\mathcal{G}$ and its generalisations. We show how $f\left(Q, \mathcal{G} \right)$ models realising any given the Friedmann-Lemaître-Robertson-Walker (FLRW) spacetime can be reconstructed. We apply the reconstruction formalism to cosmology. Explicitly, the gravity models which realise slow roll or constant roll inflation, dark energy epoch as well as the unification of the inflation and dark energy are found. The dynamical autonomous system and the gravitational wave in the theory under investigation are discussed. It is found the condition that the de Sitter spacetime becomes the (stable) fixed point of the system.

gr-qc↗

arXiv2024

The Necessity of Multi-Band Observations of the Stochastic Gravitational Wave Background

In this work we highlight an important perspective for the complete understanding of the stochastic gravitational background structure. The stochastic gravitational wave background is perhaps the most important current and future tool towards pinpointing the early Universe phenomenology related with the inflationary era and the subsequent reheating era. Many mysteries are inherent to the stochastic spectrum so in this work we highlight the fact that the complete understanding of early Universe physics and of astrophysical processes requires data from many distinct frequency band ranges. The combination of these data will provide a deeper and better understanding of the physics that forms the stochastic gravitational wave background, in both cases that it is of cosmological or astrophysical origin. We also discuss how the reheating temperature may be determined by combining multi-band frequency data from gravitational wave experiments and we also discuss how the shape of the gravitational wave energy spectrum can help us better understand the physical processes that formed it.

gr-qc↗

arXiv2024

Well-defined $f(Q)$ Gravity, Reconstruction of FLRW Spacetime and Unification of Inflation with Dark Energy Epoch

We formulate the convenient and mimetic $f(Q)$ gravities in terms of the metric and four scalar fields with the $Q$ being the non-metricity scalar. As a result, it is shown that the obtained field equations are well-defined. By using the field equations, we show how the $f(Q)$ models which realise any given FLRW spacetime may be reconstructed. We construct convenient $f(Q)$ gravity and mimetic $f(Q)$ gravity, which describe the inflation and dark energy epochs and even unify the inflation and dark energy. Moreover, a radiation-dominated epoch and early dark energy may be easily included in the above theories as is explicitly shown. It is shown that the inflationary spectral index $n_s$ and the tensor-to-scalar ratio $r$ are consistent with Planck data for some models of $f(Q)$ gravity. Corresponding asymptotic forms of $f(Q)$ and mimetic $f(Q)$ are given. Finally, some remarks on possible reconstruction for spherically symmetric spacetime are given.

gr-qc↗

arXiv2024

Wormholes inside stars and black holes

We construct models of two exotic objects: (i) a wormhole whose throat is hidden by a stellar object like a neutron star; and (ii) a wormhole inside a black hole. We work within Einstein's gravity coupled to two scalar fields with a specific choice of the scalar field Lagrangian. In general, the model contains ghosts, but they are eliminated using the constraints given by the Lagrange multiplier fields. The constraints are a generalization of the mimetic constraint, where non-dynamical dark matter effectively appears. As a result, in our model, instead of the non-dynamical dark matter, non-dynamical exotic matter like a phantom effectively arises. For the mixed wormhole-plus-star system, we find the corresponding mass-radius relations and show that it is possible to get characteristics comparable to those of ordinary neutron stars. For the wormhole inside the black hole, we find an extremal limit where the radius of the throat coincides with the radius of the event horizon and demonstrate that the Hawking temperature vanishes in this limit.

gr-qc↗

arXiv2024

Propagation of Gravitational Waves in Einstein--Gauss-Bonnet Gravity for Cosmological and Spherically Symmetric Spacetimes

In this work, we examine the propagation of gravitational waves in cosmological and astrophysical spacetimes in the context of Einstein--Gauss-Bonnet gravity, in view of the GW170817 event. The perspective we approach the problem is to obtain a theory which can produce a gravitational wave speed that is equal to that of light in the vacuum, or at least the speed can be compatible with the constraints imposed by the GW170817 event. As we show, in the context of Einstein--Gauss-Bonnet gravity, the propagation speed of gravity waves in cosmological spacetimes can be compatible with the GW170817 event, and we reconstruct some viable models. However, the propagation of gravity waves in spherically symmetric spacetimes violates the GW170817 constraints, thus it is impossible for the gravitational wave that propagates in a spherically symmetric spacetime to have a propagating speed which is equal to that of light in the vacuum. The same conclusion applies to the Einstein--Gauss-Bonnet theory with two scalars. We discuss the possible implications of our results on spherically symmetric spacetimes.

gr-qc↗

arXiv2023

Propagation of Gravitational Waves in a Dynamical Wormhole Background for Two-scalar Einstein-Gauss-Bonnet Theory

In this work, we propose a model of Einstein--Gauss-Bonnet gravity coupled with two scalar fields. The two scalar fields are considered to be ``frozen'' or they become non-dynamical by employing appropriate constraints in terms of Lagrange multiplier fields. We show that, even in the case that the arbitrary spherically symmetric spacetime is dynamical, we can construct a model where the wormhole spacetime is a stable solution in this framework. We especially concentrate on the model reproducing the dynamical wormhole, where the wormhole appears in a finite-time interval. We investigate the propagation of the gravitational wave in the wormhole spacetime background and we show that the propagation speed is different from that of light $\to$ light in general, and there is a difference in the speeds between the incoming propagating wave and the outgoing propagating gravitational wave.

gr-qc↗

arXiv2023

Finite-time Cosmological Singularities and the Possible Fate of the Universe

Singularities in any physical theory are either remarkable indicators of the unknown underlying fundamental theory, or indicate a change in the description of the physical reality. In General Relativity there are three fundamental kinds of singularities that might occur, firstly the black hole spacelike crushing singularities, e.g. in the Schwarzschild case and two cosmological spacelike singularities appearing in finite-time, namely, the Big Bang singularity and the Big Rip singularity. In the case of black hole and Big Bang singularity, the singularity indicates that the physics is no longer described by the classical gravity theory but some quantum version of gravity is probably needed. The Big Rip is a future singularity which appears in the context of General Relativity due to a phantom scalar field needed to describe the dark energy era. Apart from the Big Rip singularity, a variety of finite-time future singularities, such as, sudden singularity, Big Freeze singularity, generalized sudden singularity, $w$-singularity and so on, are allowed in various class of cosmological models irrespective of their origin. The occurrence of these finite-time singularities has been intensively investigated in the context of a variety of dark energy, modified gravity, and other alternative cosmological theories. These singularities suggest that the current cosmological scenario is probably an approximate version of a fundamental theory yet to be discovered. In this review we provide a concrete overview of the cosmological theories constructed in the context of Einstein's General Relativity and modified gravity theories that may lead to finite-time cosmological singularities. We also discuss various approaches suggested in the literature that could potentially prevent or mitigate finite-time singularities within the cosmological scenarios.

gr-qc↗

arXiv2023

Recent Advances on Inflation

We review recent trends on inflationary dynamics in the context of viable modified gravity theories. After providing a general overview of the inflationary paradigm emphasizing on what problems of hot Big Bang theory inflation solves, and a somewhat introductory presentation of single field inflationary theories with minimal and non-minimal couplings, we review how inflation can be realized in terms of several string motivated models of inflation, which involve Gauss-Bonnet couplings of the scalar field, higher order derivatives of the scalar field, and some subclasses of viable Horndeski theories. We also present and analyze inflation in the context of Chern-Simons theories of gravity, including various subcases and generalizations of string corrected modified gravities which also contain Chern-Simons correction terms, with the scalar field being identified with the invisible axion, which is the most viable to date dark matter candidate. We also provide a detailed account of vacuum $f(R)$ gravity inflation, and also inflation in $f(R,ϕ)$ and kinetic-corrected $f(R,ϕ)$ theories of gravity. In the end of the review we discuss the technique for calculating the overall effect of modified gravity on the waveform of the standard general relativistic gravitational wave form.

gr-qc↗

arXiv2023

Inflationary Attractors Predictions for Static Neutron Stars in the Mass-Gap Region

In this work we study static neutron stars in the context of several inflationary models which are popular in cosmology. These inflationary models are non-minimally coupled scalar theories which yield a viable inflationary phenomenology in both Jordan and Einstein frames. By considering the constraints from inflationary theories, which basically determine the values of the potential strength, usually considered as a free parameter in astrophysical neutron star works, we construct and solve the Tolman-Oppenheimer-Volkoff equations using a solid python-3 LSODA integrator. For our study we consider several popular inflationary models, such as the universal attractors, the $R^p$ attractors (three distinct model values), the induced inflation, the quadratic inflation, the Higgs inflation and the $a$-attractors (two distinct model values) and for the following popular equations of state the WFF1, the SLy, the APR, the MS1, the AP3, the AP4, the ENG, the MPA1 and the MS1b. We construct the $M-R$ diagram and we confront the resulting theory with theoretical and observational constraints. As we demonstrate, remarkably, all the neutron stars produced by all the inflationary models we considered are compatible with all the constraints for the MPA1 equation of state. It is notable that for this particular equation of state, the maximum masses of the neutron stars are in the mass-gap region with $M>2.5M_{\odot}$, but lower than the 3 solar masses causal limit. We also make the observation that as the NICER constraints are pushed towards larger radii, as for example in the case of the black widow pulsar PSR J0952-0607, it seems that equations of state that produce neutron stars with maximum masses in the mass gap region, with $M>2.5M_{\odot}$, but lower than the 3 solar masses causal limit, are favored and are compatible with the modified NICER constraints.

gr-qc↗

arXiv2023

Inflationary Dynamics and Swampland Criteria for Modified Gauss-Bonnet Gravity Compatible with GW170817

In this article we present an alternative formalism for the inflationary phenomenology of rescaled Einstein-Gauss-Bonnet models which are in agreement with the GW170817 event. By constraining the propagation velocity of primordial tensor perturbations, an approximate form for the time derivative of the scalar field coupled to the Gauss-Bonnet density is extracted. In turn, the overall degrees of freedom decrease and similar to the case of the canonical scalar field, only one scalar function needs to be designated, while the other is extracted from the continuity equation of the scalar field. We showcase explicitly that the slow-roll indices can be written in a closed form as functions of three dimensionless parameters, namely $x=\frac{1}{2α}\bigg(\frac{κξ'}{ξ''}\bigg)^2$, $β=8H^2ξ''$ and $γ=\frac{ξ'ξ'''}{ξ''^2}$ and in turn, we prove that the Einstein-Gauss-Bonnet model can in fact produce a blue-tilted tensor spectral index if the condition $β\geq1$ is satisfied, which is possible only for Einstein-Gauss-Bonnet models with $ξ''(ϕ_k)>0$. Afterwards, a brief comment on the running of the spectral indices is made where it is shown that $a_{\mathcal{S}}(k_*)$ and $a_{\mathcal{T}}(k_*)$ in the constrained case are approximately of the order $\mathcal{O}(10^{-3})$, if not smaller. Last but not least, we examine the conditions under which the Swampland criteria are satisfied. We connect the tracking condition related to scalar field theories with the present models, and we highlight the important feature of the models we propose that the tracking condition can be satisfied only if the Swampland criteria are simultaneously satisfied, however the cases with $ξ\sim1/V$ and $ξ\sim V$ are excluded, as they cannot describe the inflationary era properly.

gr-qc↗

arXiv2022

Formalizing Anisotropic Inflation in Modified Gravity

Motivated by the fact that the pre-inflationary era may evolve in an exotic way, in this work we formalize anisotropic evolution in the context of modified gravity, focusing on pre-inflationary and near the vicinity of the inflationary epochs. We specialize on specific metrics like Bianchi and Taub and we formalize the inflationary theory in vacuum $F(R)$ gravity, in $F(R)$ gravity with an extra scalar field and in Gauss-Bonnet gravity. We discuss the qualitative effects of the anisotropies on the evolution of the Universe and also we consider several specific solutions, like the de Sitter solution, in both the isotropic and anisotropic contexts. Furthermore, several exotic modified gravity cosmological solutions, like the ones which contain finite time singularities, are also discussed in brief.

gr-qc↗