V. K. Oikonomou

• PhD
• Academic Staff at Aristotle University of Thessaloniki

We introduce a new solution concept, called periodicity, for selecting optimal strategies in strategic form games. This periodicity solution concept yields new insight into nontrivial games. In mixed strategy strategic form games, periodic solutions yield values for the utility function of each player that are equal to the Nash equilibrium ones. In...

We systematically review some standard issues and also the latest developments of modified gravity in cosmology, emphasizing on inflation, bouncing cosmology and late-time acceleration era. Particularly, we present the formalism of standard modified gravity theory representatives, like $F(R)$, $F(\mathcal{G})$ and $F(T)$ gravity theories, but also...

At a mixed Nash equilibrium, the payoff of a player does not depend on her own action, as long as her opponent sticks to his. In a periodic strategy, a concept developed in a previous paper (arXiv:1307.2035v4), in contrast, the own payoff does not depend on the opponent's action. Here, we generalize this to multi-player simultaneous perfect informa...

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 finit...

We investigate the effects of short axion kination eras on the energy spectrum of the primordial gravitational waves corresponding to modes that re-enter the Hubble horizon at the post-electroweak symmetry breaking epoch, well within the radiation domination era. Our main assumption is the existence of an extremely weakly coupled hidden sector betw...

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 c...

In this work we construct a formalism that can reveal the general characteristics of classes of viable $F(R)$ inflationary theories. The assumptions we make is that the slow-roll era occurs, and that the de Sitter scalaron mass $m^2(R)$ of the $F(R)$ gravity is positive or zero, for both the inflationary and late-time quasi de Sitter eras, a necess...

In this work we shall consider the effects of a non-trivial topology on the effective potential of the Standard Model. Specifically we shall assume that the spacetime topology is $S^1\times R^3$ and we shall calculate the Standard Model effective potential for such a topological spacetime. As we demonstrate, for small values of the compact dimensio...

We study the inflationary phenomenology of a rescaled Einstein–Gauss–Bonnet (EGB) gravity. In this framework, the gravitational constant of the Einstein–Hilbert term is rescaled due to effective terms active in the high curvature era. Basically, the total theory is an F(R,G,ϕ) theory with the Gauss–Bonnet part contributing only a non-minimal coupli...

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...

We study the inflationary phenomenology of a rescaled Einstein-Gauss-Bonnet gravity. In this framework, the gravitational constant of the Einstein-Hilbert term is rescaled due to effective terms active in the high curvature era. Basically, the total theory is an $F(R,G,\phi)$ theory with the Gauss-Bonnet part contributing only a non-minimal couplin...

In this work we investigate the phenomenological implications of several nontrivial Higgs-boson- axion couplings, which cover most of the possible nonperturbative scenarios. Specifically we consider the combination of having higher-order nonrenormalizable Higgs-boson-axion couplings originating from a weakly coupled effective theory combined with n...

What if the dark matter Sector is truly dark, self-interacting and unreachable by terrestrial experiments? How could we find hints of such dark sector if it is experimentally unreachable by any terrestrial experiment? In this work we study a low-mass-scale mirror Standard Model which can act as a model for dark matter, which interacts only gravitat...

In this work we investigate the phenomenological implications of several non-trivial axion-Higgs couplings, which cover most of the possible non-perturbative scenarios. Specifically we consider the combination of having higher order non-renormalizable Higgs-axion couplings originating from a weakly coupled effective theory combined with non-perturb...

We consider the implications of a scalar field interacting with the dark matter fluid on the energy spectrum of primordial gravitational waves. We choose an interaction type which, before a critical matter density ρmc, during the reheating era or early radiation domination era, the scalar field loses energy, transferring it to the dark matter fluid...

In this work we study the inflationary framework of Einstein-Gauss-Bonnet theories which produce a primordial gravitational wave speed that respects the constraint imposed by the GW170817 event, namely $\left| c_T^2 - 1 \right| < 6 \times 10^{-15}$ in natural units. For these theories in general, the scalar Gauss-Bonnet coupling function $\xi (\phi...

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...

We consider the implications of a scalar field interacting with the dark matter fluid on the energy spectrum of the primordial gravitational waves. We choose an interaction type which before a critical matter density $\rho_m^c$ during the reheating era or early radiation domination era, the scalar field loses energy transferring it to the dark matt...

The Laser Interferometer Space Antenna (LISA) mission, scheduled for launch in the mid-2030s, is a gravitational wave space observatory designed to detect sources emitting in the millihertz band. LISA is an European Space Agency flagship mission, currently entering the Phase-B development phase. It is expected to help us improve our understanding o...

In this work we consider an axionic scalar-tensor theory of gravity and its effects on static neutron stars. The axionic theory is considered in the regime in which the axion oscillates around its potential minimum, which cosmologically occurs post-inflationary, when the Hubble rate is of the same order as the axion mass. We construct the TOV equatio...

The significance of the electroweak phase transition is undeniable, and although initially it was believed that it was second order, it is now believed that it is a first-order transition. However, it is not a strong first-order phase transition in the context of the standard model, and the remedy to this issue is to use the Higgs portal and direct...

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 with is to obtain a theory which can produce a gravitational wave speed that is equal to that of light in the vacuum, or...

General relativity has been the dominant paradigm theory of gravity for the last century, it manages to explain a plethora of astronomical and cosmological observations with high accuracy. A major challenge faced by general relativity is, that it predicts a singularity as the beginning of our universe. In this paper, we shall explore a well-known i...

We examine the effects of a stiff pre-recombination era on the present day's energy spectrum of the primordial gravitational waves. If the background total equation of state parameter at the pre-recombination era is described by a kination era one, this directly affects the modes with characteristic wavenumbers which reenter the Hubble horizon duri...

The Laser Interferometer Space Antenna (LISA) has two scientific objectives of cosmological focus: to probe the expansion rate of the universe, and to understand stochastic gravitational-wave backgrounds and their implications for early universe and particle physics, from the MeV to the Planck scale. However, the range of potential cosmological app...

In this work we present several characteristic examples of theories of gravity and particle physics scenarios that may yield an observable energy spectrum of stochastic primordial gravitational waves, compatible with the 2023 NANOGrav observations. The resulting theories yield a flat or a peaklike energy spectrum, and we further seek the conditions...

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 couplin...

In this work we present several characteristic examples of theories of gravity and particle physics scenarios that may yield an observable energy spectrum of stochastic primordial gravitational waves, compatible with the 2023 NANOGrav observations. The resulting theories yield a flat or a peak-like energy spectrum, and we further seek the condition...

We study a power-law $F(R)$ gravity with an early dark energy term, that can describe both the early-time and the late-time acceleration of the Universe. We confront this scenario with recent observational data including the Pantheon Type Ia supernovae, measurements of the Hubble parameter $H(z)$ (Cosmic Chronometers), data from Baryon Acoustic Osc...

In this work we study static neutron stars in the context of several inflationary models which are popular in cosmology. These inflationary models are nonminimally coupled scalar theories which yield a viable inflationary phenomenology in both Jordan and Einstein frames. By considering the constraints from inflationary theories, which basically det...

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 de...

We study a power-law F(R) gravity with an early dark energy term, that can describe both the early-time and the late-time acceleration of the Universe. We confront this scenario with recent observational data including the Pantheon Type Ia supernovae, measurements of the Hubble parameter H(z) (Cosmic Chronometers), data from Baryon Acoustic Oscilla...

In this paper, we worked in the framework of an inflationary f(R,T) theory in the presence of a canonical scalar field. More specifically, the f(R,T) = γR + 2καT gravity. The values of the dimensionless parameters α and γ are taken to be α ≥ 0 and 0 < γ ≤ 1. The motivation for this study was the striking similarities between the slow-roll parameter...

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 densi...

We investigate the effects of short axion kination eras on the energy spectrum of the primordial gravitational waves corresponding to modes that reenter the Hubble horizon at the postelectroweak symmetry breaking epoch well within the radiation domination era. Our main assumption is the existence of an extremely weakly coupled hidden sector between...

In this paper, we worked in the framework of an inflationary $f(R,T)$ theory, in the presence of a canonical scalar field. More specifically, the $f(R,T)=\gamma R+2\kappa\alpha T$ gravity. The values of the dimensionless parameters $\alpha$ and $\gamma$ are taken to be $\alpha \geq 0$ and $0 < \gamma \leq 1$. The motivation for that study was the s...

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 densi...

This study focuses on the static neutron star perspective for two types of cosmological inflationary attractor theories, namely the induced inflationary attractors and the quadratic inflationary attractors. The two cosmological models can be discriminated cosmologically, since one of the two does not provide a viable inflationary phenomenology, thu...

This study focuses on the static neutron star perspective for two types of cosmological inflationary attractor theories, namely the induced inflationary attractors and the quadratic inflationary attractors. The two cosmological models can be discriminated cosmologically, since one of the two does not provide a viable inflationary phenomenology, thu...

Under the main assumption that the axion scalar field mainly composes the dark matter in the Universe, in this paper we shall extend the formalism of kinetic axion $R^2$ gravity to include Gauss-Bonnet terms non-minimally coupled to the axion field. As we demonstrate, this non-trivial Gauss-Bonnet term has dramatic effects on the inflationary pheno...

In this work we study the neutron star phenomenology of Rp attractor theories in the Einstein frame. The Einstein frame Rp attractor theories have the attractor property that they originate from a large class of Jordan frame scalar theories with arbitrary non-minimal coupling. These theories in the Einstein frame provide a viable class of inflation...

In this work we study the neutron star phenomenology of $R^p$ attractor theories in the Einstein frame. The Einstein frame $R^p$ attractor theories have the attractor property that they originate from a large class of Jordan frame scalar theories with arbitrary non-minimal coupling. These theories in the Einstein frame provide a viable class of inf...

We examine the Chandrasekhar limit for white dwarfs in f(R) gravity, with a simple polytropic equation of state describing stellar matter. We use the most popular f(R) gravity model, namely the f(R)=R+αR2 gravity, and calculate the parameters of the stellar configurations with polytropic equation of state of the form p=Kρ1+1/n for various values of...

In this work, we study a new class of attractor models which we shall call generalized Rp-attractor models. This class of models is based on a generalization of the Einstein frame potential of Rp f(R) gravity models in the Jordan frame. We present the attractor properties of the corresponding nonminimally coupled Jordan frame theory, and we calcula...

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...

In this work we shall study a new class of attractor models which we shall call generalized $R^p$-attractor models. This class of models is based on a generalization of the Einstein frame potential of $R^p$ $f(R)$ gravity models in the Jordan frame. We present the attractor properties of the corresponding non-minimally coupled Jordan frame theory,...

In this work we examine the effects of a pre-inflationary de Sitter bounce on the energy spectrum of the primordial gravitational waves. Specifically we assume that the Universe is described by several evolution patches, starting with a de Sitter pre-inflationary bounce which is followed by an quasi-de Sitter slow-roll inflationary era, followed by...

In this work we examine the effects of a pre-inflationary de Sitter bounce on the energy spectrum of the primordial gravitational waves. Specifically we assume that the Universe is described by several evolution patches, starting with a de Sitter pre-inflationary bounce which is followed by an quasi-de Sitter slow-roll inflationary era, followed by...

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...

In this work we consider the effect of an $R^2$ term on the kinetic misalignment axion theory. By using the slow-roll assumptions during inflation and the field equations, we construct an autonomous dynamical system for the kinetic axion, including the effects of the $R^2$ term and we solve numerically the dynamical system. As we demonstrate, the p...

In this work we consider a combined theoretical framework comprised by $F(R)$ gravity and a kinetic scalar field. The kinetic energy of the scalar field dominates over its potential for all cosmic times, and the kinetic scalar potential is chosen to be small and non-trivial. In this case, we show that the primordial gravitational wave energy spectr...

In this work we consider the effect of an $R^2$ term on the kinetic misalignment axion theory. By using the slow-roll assumptions during inflation and the field equations, we construct an autonomous dynamical system for the kinetic axion, including the effects of the $R^2$ term and we solve numerically the dynamical system. As we demonstrate, the p...

In this short review, we discuss how Earth’s climatological and geological history and also how the shadows of galactic black holes might reveal our Universe’s past evolution. Specifically we point out that a pressure singularity that occurred in our Universe’s past might have left its imprint on Earth’s geological and climatological history and on...

In this short review, we discuss how Earth's climatological and geological history and also how the shadows of galactic black holes might reveal our Universe's past evolution. Specifically we point out that a pressure singularity that occurred in our Universe's past might have left its imprint on Earth's geological and climatological history and on...

In this work we consider a combined theoretical framework comprised by F(R) gravity and a kinetic scalar field. The kinetic energy of the scalar field dominates over its potential for all cosmic times, and the kinetic scalar potential is chosen to be small and non-trivial. In this case, we show that the primordial gravitational wave energy spectrum...

The black hole observations obtained so far indicate one thing: similar “donuts” exist in the sky. But what if some of the observed black hole shadows that will obtained in the future are different from the others? In this work the aim is to show that a difference in the shadow of some observed black holes in the future, might explain the H0-tension...

In this work we investigate the quantitative effects of the misalignment kinetic axion on R2 inflation. Due to the fact that the kinetic axion possesses a large kinetic energy that dominates its potential energy, during inflation its energy density redshifts as stiff matter fluid and evolves in a constant-roll way, making the second slow-roll index...

The black hole observations obtained so far indicate one thing: similar "donuts" exist in the sky. But what if some of the observed black hole shadows that will obtained in the future are different from the others? In this work the aim is to show that a difference in the shadow of some observed black holes in the future, might explain the $H_0$-ten...

In this work we investigate the quantitative effects of the misalignment kinetic axion on $R^2$ inflation. Due to the fact that the kinetic axion possesses a large kinetic energy which dominates its potential energy, during inflation its energy density redshifts as stiff matter fluid and evolves in a constant-roll way, making the second slow-roll i...

In this work we shall provide a model-independent general calculation of the running of the spectral index for vacuum F(R) gravities. We shall exploit the functional form of the spectral index and of the tensor-to-scalar ratio in order to present a general ns−r relation for vacuum F(R) gravity theories. As we show, viable F(R) gravity theories can...

In this work we shall provide a model-independent general calculation of the running of the spectral index for vacuum $F(R)$ gravities. We shall exploit the functional form of the spectral index and of the tensor-to-scalar ratio in order to present a general $n_s-r$ relation for vacuum $F(R)$ gravity theories. As we show, viable $F(R)$ gravity theo...

In this work, we investigate the effects of a geometrically generated early dark energy era on the energy spectrum of the primordial gravitational waves. The early dark energy era, which we choose to have a constant equation of state parameter w, is synergistically generated by an appropriate f(R) gravity in the presence of matter and radiation per...

In this work we investigate the effects of a geometrically generated early dark energy era on the energy spectrum of the primordial gravitational waves. The early dark energy era, which we choose it to have a constant equation of state parameter $w$, is synergistically generated by an appropriate $f(R)$ gravity in the presence of matter and radiati...

In this work the effects of a geometrically generated post‐inflationary era on the energy spectrum of the primordial gravitational waves are considered. Specifically, a post‐inflationary constant equation of state era, generated by the synergistic effect of f(R)$f(R)$ gravity and of radiation and matter perfect fluids shall be considered. Two cases...

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...

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 b...

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 partic...

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 corr...

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...

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 part...

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...

In this work, we shall provide an F(R) gravity theoretical framework for solving the H0-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 H0-tension is alleviated. The condition that remedies the H0-tension restricts the corresponding F(R...

In the recent literature it has been shown that the H0 tension may be eliminated if an abrupt physics transition changed the Cepheid parameters in the near past of the Universe, nearly 70 -150Myrs 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...

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...

The Laser Interferometer Space Antenna (LISA) has two scientific objectives of cosmological focus: to probe the expansion rate of the universe, and to understand stochastic gravitational-wave backgrounds and their implications for early universe and particle physics, from the MeV to the Planck scale. However, the range of potential cosmological app...

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 effect...

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 shor...

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 r...

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 effect...

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...

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 modi...

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 tenso...

In the frame of Gauss–Bonnet gravity and in the limit of D→4, based on the fact that spherically symmetric solution derived using any of regularization schemes will be the same form as the original theory, we derive a new interior spherically symmetric solution assuming specific forms of the metric potentials that have two constants. Using the junc...

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 R2gravity, 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 ca...

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 spe...

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+ξϕ2. Particularly we derive the Einstein frame Tolman–Oppenheimer–Volkoff equations, and by numerically integrating them for both the interior and the exter...

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)$f(R)$ 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...

In this work, we generalize the constant-roll condition for minimally coupled canonical scalar field inflation. Particularly, we shall assume that the scalar field satisfies the condition [Formula: see text], and we derive the field equations under this assumption. We call the framework extended constant-roll framework. Accordingly, we calculate th...