arXiv subjects

V. K. Oikonomou

Publications and source records attributed to V. K. Oikonomou.

At least 91 records · Page 5Linked to original sources

arXiv2022

$f(R)$-Gravity Generated Post-inflationary Eras and their Effect on Primordial Gravitational Waves

In this work we shall consider the effects of a geometrically generated post-inflationary era on the energy spectrum of the primordial gravitational waves. Specifically, we shall consider a post-inflationary constant equation of state era, generated by the synergistic effect of $f(R)$ gravity and of radiation and matter perfect fluids. Two cases of interest shall be studied, one with equation of state parameter $w=-1/3$, in which case the Universe neither accelerates nor decelerates, and one with $w=0$ so an early matter domination era. For the evaluation of the inflationary observational indices which is relevant for the calculation of the gravitational waves energy spectrum, we also took into account the effects of the constant equation of state parameter era, on the $e$-foldings number. In both the $w=-1/3$ and $w=0$ cases, the energy spectrum of the primordial gravitational waves is amplified, but for the $w=0$ case, the effect is stronger.

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arXiv2022

Integral $F(R)$ Gravity and Saddle Point Condition as a Remedy for the $H_0$-tension

In this work, we shall provide an $F(R)$ gravity theoretical framework for solving the $H_0$-tension. Specifically, by exploiting the $F(R)$ gravity correspondence with a scalar-tensor theory, we shall provide a condition in which when it is satisfied, the $H_0$-tension is alleviated. The condition that remedies the $H_0$-tension restricts the corresponding $F(R)$ gravity, and we present in brief the theoretical features of the constrained $F(R)$ gravity theory in both the Jordan and Einstein frames. The condition that may remedy the $H_0$-tension is based on the existence of a metastable de Sitter point that occurs for redshifts near the recombination. This metastable de Sitter vacuum restricts the functional form of the $F(R)$ gravity in the Jordan frame. We also show that by appropriately choosing the $F(R)$ gravity, along with the theoretical solution offered for the $H_0$-tension problem, one may also provide a unified description of the inflationary era with the late-time accelerating era, in terms of two extra de Sitter vacua. We propose a new approach to $F(R)$ gravity by introducing a new class of integral $F(R)$ gravity functions, which may be wider than the usual class expressed in terms of elementary $F(R)$ gravity functions. Finally, the Einstein frame inflationary dynamics formalism is briefly discussed.

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arXiv2022

A Panorama of Viable $F(R)$ Gravity Dark Energy Models

In this work we shall study the late-time dynamics of several $F(R)$ gravity models. By appropriately expressing the field equations in terms of the redshift and of a statefinder function, we shall solve numerically the field equations using appropriate physical motivated initial conditions. We consider models which, by construction, are described by a nearly $R^2$-model at early epochs and we fine tune the parameters to achieve viability and compatibility with the latest Planck constraints at late times. Such models provide a unified description of inflation and dark energy era and notably a common feature of all the models is the presence of dark energy oscillations. Furthermore, we show that, in contrast to general relativistic fluids and scalar field descriptions, a large spectrum of different dark energy physics is generated by simple $F(R)$ gravity models, varying from phantom, to nearly de Sitter and to quintessential dark energy eras.

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arXiv2022

Chirality of Gravitational Waves in Chern-Simons $f(R)$ Gravity Cosmology

In this paper we shall consider an axionic Chern-Simons corrected $f(R)$ gravity theoretical framework, and we shall study the chirality of the generated primordial gravitational waves. Particularly, we shall consider two main axion models, the canonical misalignment axion model and the kinetic axion model, both of which provide an interesting particle phenomenology, in the presence of $R^2$ terms in the inflationary Lagrangian. The axion does not affect significantly the background evolution during the inflationary era, which is solely controlled by $R^2$ gravity. However, the due to the presence of the Chern-Simons term, the tensor perturbations are directly affected, and our aim is to reveal the extent of effects of the Chern-Simons term on the gravitational waves modes, for both the axion models. We aim to produce analytical descriptions of the primordial tensor modes behavior, and thus we solve analytically the evolution equations of the tensor modes, for a nearly de Sitter primordial evolution controlled by the $R^2$ gravity. We focus the analytical study on superhorizon and subhorizon modes. For the misalignment model, we were able to produce analytic solutions for both the subhorizon and superhorizon modes, in which case we found the behavior of the circular polarization function. Our results indicate that the produced tensor spectrum is strongly chiral. For the kinetic axion model though, analytic solutions are obtained only for the superhorizon modes. In order to have a grasp of the behavior of the chirality of the tensor modes, we studied the chirality of the superhorizon modes, however a more complete study is needed, which is impossible to do analytically though.

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arXiv2022

Primordial Gravitational Waves Predictions for GW170817-compatible Einstein-Gauss-Bonnet Theory

In this work we shall calculate in detail the effect of an GW170817-compatible Einstein-Gauss-Bonnet theory on the energy spectrum of the primordial gravitational waves. The spectrum is affected by two characteristics, the overall amplification/damping factor caused by the GW170817-compatible Einstein-Gauss-Bonnet theory and by the tensor spectral index and the tensor-to-scalar ratio. We shall present the formalism for studying the inflationary dynamics and post-inflationary dynamics of GW170817-compatible Einstein-Gauss-Bonnet theories for all redshifts starting from the radiation era up to the dark energy era. We exemplify our formalism by using two characteristic models, which produce viable inflationary and dark energy eras. As we demonstrate, remarkably the overall damping/amplification factor is of the order of unity, thus the GW170817-compatible Einstein-Gauss-Bonnet models affect the primordial gravitational waves energy spectrum only via their tensor spectral index and the tensor-to-scalar ratio. Both models have a blue tilted tensor spectrum, and therefore the predicted energy spectrum of the primordial gravity waves can be detectable by most of the future gravitational waves experiments, for various reheating temperatures.

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arXiv2022

$\mathcal{R}^2$ Quantum Corrected Scalar Field Inflation

String theory enjoys an elevated role among quantum gravity theories, since it seems to be the most consistent UV completion of general relativity and the Standard Model. However, it is hard to verify the existence of this underlying theory on terrestrial accelerators. One way to probe string theory is to study its imprints on the low-energy effective inflationary Lagrangian, which are quantified in terms of high energy correction terms. It is highly likely, thus, to find higher order curvature terms combined with string moduli, that is scalar fields, since both these types of interactions and matter fields appear in string theory. In this work we aim to stress the probability that the inflationary dynamics are controlled by the synergy of scalar fields and higher order curvature terms. Specifically, we shall consider a well motivated quantum corrected canonical scalar field theory, with the quantum corrections being of the $\mathcal{R}^2$ type. The reason for choosing minimally coupled scalar theory is basically because if scalar fields are evaluated in their vacuum configuration, they will either be minimally coupled or conformally coupled. Here we choose the former case, and the whole study shall be performed in the string frame (Jordan frame), in contrast to similar studies in the literature where the Einstein frame two scalar theory is considered. We derive the field equations of the quantum-corrected theory at leading order and we present the form the slow-roll indices obtain for the quantum corrected theory. We exemplify our theoretical framework by using the quadratic inflation model, and as we show, the $\mathcal{R}^2$ quantum corrected quadratic inflation model produces a viable inflationary phenomenology, in contrast with the simple quadratic inflation model.

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arXiv2022

Spectrum of Primordial Gravitational Waves in Modified Gravities: A Short Overview

In this work we shall exhaustively study the effects of modified gravity on the energy spectrum of the primordial gravitational waves background. S. Weinberg has also produced significant works related to the primordial gravitational waves with the most important one being the effects of neutrinos on primordial gravitational waves. With this sort review, our main aim is to gather all the necessary information for studying the effects of modified gravity on primordial gravitational waves in a concrete and quantitative way and in a single paper. After reviewing all the necessary techniques for extracting the general relativistic energy spectrum, and how to obtain in a WKB way the modified gravity damping or amplifying factor, we concentrate on specific forms of modified gravity of interest. The most important parameter involved for the calculation of the effects of modified gravity on the energy spectrum is the parameter $a_M$ which we calculate for the cases of $f(R,ϕ)$ gravity, Chern-Simons-corrected $f(R,ϕ)$ gravity, Einstein-Gauss-Bonnet-corrected $f(R,ϕ)$ gravity, and higher derivative extended Einstein-Gauss-Bonnet-corrected $f(R,ϕ)$ gravity. The exact forms of $a_M$ is presented explicitly for the first time in the literature. With regard to Einstein-Gauss-Bonnet-corrected $f(R,ϕ)$ gravity, and higher derivative extended Einstein-Gauss-Bonnet-corrected $f(R,ϕ)$ gravity theories, we focus on the case that the gravitational wave propagating speed is equal to that of light's in vacuum. We provide expressions for $a_M$ expressed in terms of the cosmic time and in terms of the redshift, which can be used directly for the numerical calculation of the effect of modified gravity on the primordial gravitational wave energy spectrum.

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arXiv2022

Bottom-Up Reconstruction of Viable GW170817 Compatible Einstein-Gauss-Bonnet Theories

In this work we shall use a bottom-up approach for obtaining viable inflationary Einstein-Gauss-Bonnet models which are also compatible with the GW170817 event. Specifically, we shall use a recently developed theoretical framework in which we shall specify only the tensor-to-scalar ratio, in terms of the $e$-foldings number. Starting from the tensor-to-scalar ratio, we shall reconstruct from it the Einstein-Gauss-Bonnet theory which can yield such a tensor-to-scalar ratio, finding the scalar potential and the Gauss-Bonnet coupling scalar function as functions of the $e$-foldings number. Accordingly, the calculation of the spectral index of the primordial scalar perturbations, and of the tensor spectral index easily is greatly simplified and these observational indices can easily be found. After presenting the general formalism for the bottom-up reconstruction, we exemplify our findings by presenting several Einstein-Gauss-Bonnet models of interest which yield a viable inflationary phenomenology. These models have also an interesting common characteristic, which is a blue tilted tensor spectral index. We also investigate the predicted energy spectrum of the primordial gravitational waves for these Einstein-Gauss-Bonnet models, and as we show, all the models yield a detectable primordial wave energy power spectrum.

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arXiv2022

Amplification of Primordial Gravitational Waves by a Geometrically Driven non-canonical Reheating Era

In order to describe inflation in general relativity, scalar fields must inevitably be used, with all the setbacks of that description. On the other hand, $f(R)$ gravity and other modified gravity theories seem to provide a unified description of early and late-time dynamics without resorting to scalar or phantom theories. The question is, can modified gravity affect directly the mysterious radiation domination era? Addressing this question is the focus in this work, and we shall consider the case for which in the early stages of the radiation domination era, namely during the reheating era, the background equation of state parameter is different from $w=1/3$. As we show, in the context of $f(R)$ gravity, an abnormal reheating era can affect the primordial gravitational wave energy spectrum today. Since future interferometers will exactly probe this era, which consists of subhorizon modes that reentered the horizon during the early stages of the radiation domination era, the focus in this work is how a short abnormal reheating era that deviates from the standard perfect fluid pattern with $w\neq 1/3$, and generated by higher order curvature terms, can affect the primordial gravitational wave energy spectrum. Using a WKB approach, we calculate the effect of an $f(R)$ gravity generated abnormal reheating era, and as we show the primordial gravitational wave spectrum is significantly amplified, a result which is in contrast to the general relativistic case, where the effect is minor.

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arXiv2022

Neutron Stars in Scalar-tensor Gravity with Quartic Order Scalar Potential

In this work we investigate the effects of a non-minimally coupled quartic order scalar model on static neutron stars, with the non-minimal coupling in the Jordan frame being of the form $f(ϕ) = 1 + ξϕ^2$. Particularly we derive the Einstein frame Tolman-Oppenheimer-Volkoff equations, and by numerically integrating them for both the interior and the exterior of the neutron star, using a double shooting python 3 based numerical code, we extract the masses and radii of the neutron stars evaluated finally in the Jordan frame, along with several other related physical quantities of interest. With regard to the equation of state for the neutron star, we use a piecewise polytropic equation of state with the central part being Skyrme-Lyon (SLy), Akmal-Pandharipande-Ravenhall (APR) or the Wiringa-Fiks-Fabrocini (WFF1) equations of state. The resulting $M-R$ graphs are compatible with the observational bounds imposed by the GW170817 event which require the radius of a static $M\sim 1.6 M_{\odot}$ neutron star to be larger than $R=10.68^{+15}_{-0.04}$km and the radius of a static neutron star corresponding to the maximum mass of the star to be larger than $R=9.6^{+0.14}_{-0.03}$km. Moreover, the WFF1 EoS, which was excluded for static neutron stars in the context of general relativity, for the a quartic order scalar model neutron star model provides realistic results compatible with the GW170817 event.

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arXiv2022

Anisotropic compact stars in $D\rightarrow 4$ limit of Gauss-Bonnet gravity

In the frame of Gauss-Bonnet gravity and in the limit of $D\to 4$, based on the fact that spherically symmetric solution derived using any of regularization schemes will be the same form as the original theory \cite{Banerjee:2020yhu,Hennigar:2020lsl,Casalino:2020kbt,Aoki:2020lig}, we derive a new interior spherically symmetric solution assuming specific forms of the metric potentials that have two constants . Using the junction condition we determine these two constants. By using the data of the star EXO $1785-248 $, whose mass is $ { M=1.3\pm0.2 \,{\textrm M}_\odot}$ and radius $ { l=8.849\pm0.4\,{\textrm km}}$, we calculate the numerical values of these constants, in terms of the dimensionful coupling parameter of the Gauss-Bonnet term, and eventually, we get real values for these constants. In this regard, we show that the components of the energy-momentum tensor has a finite value at the center of the star as well as a smaller value to the surface of the star. Moreover, we show that the equations of the state behave in a non-linear way due to the impact of the Gauss-Bonnet term. Using the Tolman-Oppenheimer-Volkoff equation, the adiabatic index, and stability in the static state we show that the model under consideration is always stable. Finally, the solution of this study is matched with observational data of other pulsars showing satisfactory results

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arXiv2022

Did the Universe Experienced a Pressure non-Crushing Type Cosmological Singularity in the Recent Past?

In the recent literature it has been shown that the $H_0$ tension may be eliminated if an abrupt physics transition changed the Cepheid parameters in the near past of the Universe, nearly $70-150\,$Myrs ago. In this letter we stress the possibility that this abrupt transition was caused by the smooth passage of our Universe through a pressure finite-time cosmological singularity. Being a non-crushing type singularity the pressure singularity can leave its imprints in the Universe, since it occurs globally and literally everywhere. We discuss how this scenario could easily be realized by $F(R)$ gravity, with the strong energy conditions being satisfied without the need for a scalar field or specific matter fluids. We also stress the fact that the pressure singularity can affect the effective gravitational constant of $F(R)$ gravity. Moreover, we stress the fact that pressure singularities can disrupt the trajectories of bound objects in the Universe, which is also pointed out in the literature, even in the context of general relativity. We also show numerically in a general relativistic framework that elliptic trajectories are distorted and changed to different elliptic trajectories when the Universe passes through the pressure singularity. Such a disruption of the trajectories could have tidal effects on the surface of the earth, for example on sea waters and oceans, regarding the distortion of moon's elliptic trajectory. Accordingly, the distortion of earth's trajectory around the sun could have affected climatologically earth $70-150\,$Myrs ago.

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arXiv2022

Quantitative Predictions for $f(R)$ Gravity Primordial Gravitational Waves

In this work we shall develop a quantitative approach for extracting predictions on the primordial gravitational waves energy spectrum for $f(R)$ gravity. We shall consider two distinct models which yield different phenomenology, one pure $f(R)$ gravity model and one Chern-Simons corrected potential-less $k$-essence $f(R)$ gravity model in the presence of radiation and non-relativistic perfect matter fluids. The two $f(R)$ gravity models were carefully chosen in order for them to describe in a unified way inflation and the dark energy era, in both cases viable and compatible with the latest Planck data. Also both models mimic the $Λ$-Cold-Dark-Matter model and specifically the pure $f(R)$ model only at late times, but the Chern-Simons $k$-essence model during the whole evolution of the model up to the radiation domination era. In addition they guarantee a smooth transition from the inflationary era to the radiation, matter domination and subsequently to the dark energy era. Using a WKB approach introduced in the relevant literature by Nishizawa, we derive formulas depending on the redshift that yield the modified gravity effect, quantified by a multiplicative factor, a ``damping'' in front of the General Relativistic waveform. In order to calculate the effect of the modified gravity, which is the ``damping'' factor, we solve numerically the Friedmann equations using appropriate initial conditions and by introducing specific statefinder quantities. As we show, the pure $f(R)$ gravity gravitational wave energy spectrum is slightly enhanced, but it remains well below the sensitivity curves of future gravitational waves experiments. In contrast, the Chern-Simons $k$-essence $f(R)$ gravity model gravitational wave energy spectrum is significantly enhanced and two signals are predicted which can be verified by future gravitational wave experiments.

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arXiv2021

Uniqueness of the Inflationary Higgs Scalar for Neutron Stars and Failure of non-inflationary Approximations

Neutron stars are perfect candidates to investigate the effects of a modified gravity theory, since the curvature effects are significant and more importantly, potentially testable. In most cases studied in the literature in the context of massive scalar-tensor theories, inflationary models were examined. The most important of scalar-tensor models is the Higgs model, which, depending on the values of the scalar field, can be approximated by different scalar potentials, one of which is the inflationary. Since it is not certain how large the values of the scalar field will be at the near vicinity and inside a neutron star, in this work we will answer the question, which potential form of the Higgs model is more appropriate in order for it to describe consistently a static neutron star. As we will show numerically, the non-inflationary Higgs potential, which is valid for certain values of the scalar field in the Jordan frame, leads to extremely large maximum neutron star masses, however the model is not self-consistent, because the scalar field approximation used for the derivation of the potential, is violated both at the center and at the surface of the star. These results shows the uniqueness of the inflationary Higgs potential, since it is the only approximation for the Higgs model, that provides self-consistent results.

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arXiv2021

$R^2$ Inflation Revisited and Dark Energy Corrections

The vacuum $R^2$ model is known to generate a quasi-de Sitter evolution for inflation, using solely the slow-roll assumptions. Using standard reconstruction techniques, we demonstrate that the $f(R)$ gravity which actually realizes the quasi-de Sitter evolution is not simply the $R^2$ model but a deformed $R^2$ model which contains extra terms in addition to the $R^2$ model. We analyze in detail the inflationary dynamics of the deformed $R^2$ model and we demonstrate that the predictions are quite close to the ones of the pure $R^2$ model, regardless the values of the free parameters. Basically the deformed $R^2$ model is also a single parameter inflationary model, exactly like the ordinary $R^2$ model. In contrast to the early-time era, where the deformed $R^2$ model is quite similar to the $R^2$ model, at late times, the phenomenological picture is different. The deformed $R^2$ model describes stronger gravity compared to the ordinary $R^2$ with an effectively smaller effective Planck mass. We propose the addition of an early dark energy term which does not affect at all the inflationary era, but takes over the control of the late-time dynamics at late times. We study in some detail the predicted dark energy era evolution, and we demonstrate that the dark energy corrected deformed $R^2$ model can describe a viable dark energy era, compatible with the Planck constraints on cosmological parameters. Furthermore the model is distinct from the $Λ$-Cold-Dark-Matter model, but shows a qualitatively similar behavior.

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arXiv2021

Pre-Inflationary Bounce Effects on Primordial Gravitational Waves of $f(R)$ Gravity

In this work we shall study a possible pre-inflationary scenario for our Universe and how this might be realized by $f(R)$ gravity. Specifically, we shall introduce a scenario in which the Universe in the pre-inflationary era contracts until it reaches a minimum magnitude, and subsequently expands, slowly entering a slow-roll quasi-de Sitter inflationary era. This pre-inflationary bounce avoids the cosmic singularity, and for the eras before and after the quasi-de Sitter inflationary stage, approximately satisfies the string theory motivated scale factor duality $a(t)=a^{-1}(-t)$. We investigate which approximate forms of $f(R)$ can realize such a non-singular pre-inflationary scenario, the quasi-de Sitter patch of which is described by an $R^2$ gravity, thus the exit from inflation is guaranteed. Furthermore, since in string theory pre-Big Bang scenarios lead to an overall amplification of the gravitational wave energy spectrum, we examine in detail this perspective for the $f(R)$ gravity generating this pre-inflationary non-singular bounce. As we show, in the $f(R)$ gravity case, the energy spectrum of the primordial gravitational waves background is also amplified, however the drawback is that the amplification is too small to be detected by future high frequency interferometers. Thus we conclude that, as in the case of single scalar field theories, $f(R)$ gravity cannot produce detectable stochastic gravitational waves and a synergistic theory of scalars and higher order curvature terms might be needed.

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arXiv2021

Maximum Baryon Masses for Static Neutron Stars in $f(R)$ Gravity

We investigate the upper mass limit predictions of the baryonic mass for static neutron stars in the context of $f(R)$ gravity. We use the most popular $f(R)$ gravity model, namely the $R^2$ gravity, and calculate the maximum baryon mass of static neutron stars adopting several realistic equations of state and one ideal equation of state, namely that of causal limit. Our motivation is based on the fact that neutron stars with baryon masses larger than the maximum mass for static neutron star configurations inevitably collapse to black holes. Thus with our analysis, we want further to enlighten the predictions for the maximum baryon masses of static neutron stars in $R^2$ gravity, which, in turn, further strengthens our understanding of the mysterious mass-gap region. As we show, the baryon masses of most of the equations of states studied in this paper, lie in the lower limits of the mass-gap region $M\sim 2.5-5 M_{\odot}$, but intriguingly enough, the highest value of the maximum baryon masses we found is of the order of $M\sim 3 M_{\odot}$. This upper mass limit also appears as a maximum static neutron star gravitational mass limit in other contexts. Combining the two results which refer to baryon and gravitational masses, we point out that the gravitational mass of static neutron stars cannot be larger than three solar masses, while based on maximum baryon masses results of the present work, we can conspicuously state that it is highly likely the lower mass limits of astrophysical black holes in the range of $M\sim 2.5-3 M_{\odot}$. This, in turn, implies that maximum neutron star masses in the context of $R^2$ gravity are likely to be in the lower limits of the range of $M\sim 2.4-3 M_{\odot}$. Hence our work further supports the General Relativity claim that neutron stars cannot have gravitational masses larger than $3$$M_{\odot}$.

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arXiv2021

Ghost-free $F\left( R,\mathcal{G} \right)$ Gravity

In this work we shall address the ghost issue of $F\left( R,\mathcal{G} \right)$ gravity, which is known to be plagued with ghost degrees of freedom. These ghosts occur due to the presence of higher than two derivatives in the field equations, and can arise even when considering cosmological perturbations, where superluminal modes may arise in the theory. If we consider the quantum theory, the ghosts generate the negative norm states, which give the negative probabilities, and therefore the ghosts are physically inconsistent. Motivated by the importance of $F\left( R,\mathcal{G} \right)$ gravity for providing viable inflationary and dark energy phenomenologies, in this work we shall provide a technique that can render $F\left( R,\mathcal{G} \right)$ gravity theories free from ghost degrees of freedom. This will be done by introducing two auxiliary scalar fields, and by employing the Lagrange multiplier technique, the theory is ghost free in the Einstein frame. Also the framework can be viewed as a reconstruction technique and can be used as a method in order to realize several cosmological evolutions of interest. We demonstrate how we can realize several cosmologically interesting phenomenologies by using the reconstruction technique.

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