arXiv subjects

V. K. Oikonomou

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

At least 55 records · Page 3Linked to original sources

arXiv2025

Rescaled Einstein-Gauss-Bonnet Gravity Inflation

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 $F(R,G,ϕ)$ theory with the Gauss-Bonnet part contributing only a non-minimal coupling to the scalar field, so it is a theory with string theory origins and with a non-trivial $F(R)$ gravity part. The $F(R)$ gravity part in the high curvature regime contributes only a rescaled Einstein-Hilbert term and thus the resulting theory is effectively a rescaled version of a standard Einstein-Gauss-Bonnet theory. We develop the formalism of rescaled Einstein-Gauss-Bonnet gravity, taking in account the GW170817 constraints on the gravitational wave speed. We show explicitly how the rescaled theory affects directly the primordial scalar and tensor perturbations, and how the slow-roll and observational indices of inflation are affected by the rescaling of the theory. We perform a thorough phenomenological analysis of several models of interest and we show that is it possible to obtain viable inflationary theories compatible with the latest Planck data. Also among the studied models there are cases that yield a relatively large blue tilted tensor spectral index and we demonstrate that these models can lead to detectable primordial gravitational waves in the future gravitational wave experiments. Some of the scenarios examined, for specific values of the reheating temperature may be detectable by SKA, LISA, BBO, DECIGO and the Einstein Telescope.

gr-qc↗

arXiv2024

Theoretical Probes of Higgs-Axion non-perturbative Couplings

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-perturbative couplings of the form $\sim εΛ_c^2|H|^2\cos (\fracϕ{f_a})$. Since we consider the misalignment axion, the non-perturbative couplings can be expanded in the form of a perturbation expansion in powers of $ϕ/f_a$, thus after the electroweak symmetry breaking, the effective potential of the axion is drastically affected by these terms. We investigate the phenomenological implications of these terms for various values of the mass scale $Λ_c$, and some scenarios are theoretically disfavored, while other scenarios with non-perturbative Higgs-axion couplings of the form $\sim εΛ_c^2|H|^2\cos (\fracϕ{f_a})$ with $Λ_c\sim 10^{-10}\times m_a$ and $m_a\sim 10^{-10}\,$eV, lead to a characteristic pattern in the stochastic gravitational wave background via the deformation of the background equation of state parameter occurring at frequencies probed by the Einstein Telescope. We also consider loop effects from the Higgs sector caused by the term $\sim εΛ_c^2|H|^2\cos (\fracϕ{f_a})$ if we close the Higgs in one loop.

hep-ph↗

arXiv2024

Revisiting Einstein-Gauss-Bonnet Theories after GW170817

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 $ξ(ϕ)$ is arbitrary and unrelated to the scalar potential. We develop the inflationary formalism of this theory, and present analytic forms for the slow-roll indices, the observational indices and the $e$-foldings number, assuming solely a slow-roll era and also that the constraints imposed by the GW170817 event hold true. We present in detail an interesting class of models that produces a viable inflationary era. Finally, we investigate the behavior of the Einstein-Gauss-Bonnet coupling function during the reheating era, in which case the evolution of the Hubble rate satisfies a constant-roll-like relation $\dot{H}=δH^2$. As we show, the behavior of the scalar Gauss-Bonnet coupling during the reheating is different from that of the inflationary era. This is due to the fact that the evolution governing the reheating is different compared to the inflationary era.

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

Implications of a Scalar Field Interacting with the Dark Matter Fluid on the Primordial Gravitational Waves

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 $ρ_m^c$ during the reheating era or early radiation domination era, the scalar field loses energy transferring it to the dark matter fluid, while after the critical matter density $ρ_m^c$ the dark matter fluid loses energy transferring it to the scalar field. The scalar field is assumed to have an exponential potential and at the critical matter density with $ρ_m\sim ρ_m^c$, at which point, the interaction between the scalar and the dark matter fluid is switched off, we demand that the effective equation of state of scalar field is described by a matter dominated era. This is crucial since it affects the behavior of the trajectories and the fixed points of the two dimensional dynamical system composed by the dark matter fluid and the scalar field. Specifically the phase space contains two stable dark matter dominated final attractors and two unstable stiff era dominated fixed points. Thus, there exists the remarkable possibility that the Universe might feel the passing of the scalar field through the unstable kinetic dominated fixed points, during the reheating era, with the total equation of state parameter of the Universe being deformed to be larger than $w=1/3$. This deformation of the total equation of state parameter during the reheating era can potentially have significant effects on the energy spectrum of the primordial gravitational waves. Also the model we use also contains an $F(R)$ gravity which controls in a dominant way inflation and the late-time acceleration, in a phenomenologically viable way.

gr-qc↗

arXiv2024

Electroweak Phase Transition in Singlet Extensions of The Standard Model with Dimension-Six Operators

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 directly couple the Higgs to a hidden scalar sector. This can result in a strong electroweak phase transition, while the couplings to a hidden scalar are constrained by several phenomenological constraints, such as the sphaleron rate criterion and the branching ratio of the Higgs to invisible channels. In this work, we consider the standard singlet extensions of the Standard Model, including dimension-six non-renormalizable operators that couple a real singlet scalar field with the Higgs doublet. As a result, we examine the effects of those Higgs-singlet couplings on the electroweak phase transition. The effective theory, where the non-renormalizable couplings originate from, is considered to be active beyond 15$\,$TeV. As we show, the Universe experiences a two-step electroweak phase transition, a primary phase transition in the singlet sector at a high temperature, and then a subsequent first-order phase transition from the singlet vacuum to the electroweak vacuum. The singlet's phase transition can either be second-order or first-order, depending on the singlet mass and its couplings to the Higgs. In particular, we show that the dimension-six operator assists in generating a strong electroweak phase transition in regions of the parameter space that were excluded in the previous singlet extensions of the Standard Model. This is further apparent for low singlet masses $m_S < m_H/2$ which are rarely taken into account in the literature due to the invisible branching ratio of the Higgs boson.

hep-ph↗

arXiv2024

Phenomenology of Axionic Static Neutron Stars with Masses in the Mass-Gap Region

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 equations for this axionic theory and for a spherically symmetric static spacetime and we solve these numerically using a quite robust double shooting LSODA based python integration method. Regarding the equations of state, we used nine mainstream and quite popular ones, namely, the WFF1, the SLy, the APR, the MS1, the AP3, the AP4, the ENG, the MPA1 and the MS1b, using the piecewise polytropic description for each. From the extracted data we calculate the Jordan frame masses and radii, and we confront the resulting phenomenology with five well-known neutron star constraints. As we demonstrate, the AP3, the ENG and the MPA1 equations of state yield phenomenologically viable results which are compatible with the constraints, with the MPA1 equation of state enjoying an elevated role among the three. The reason is that the MPA1 fits well the phenomenological constraints. A mentionable feature is the fact that all the viable phenomenologically equations of state produce maximum masses which are in the mass-gap region with $M_{max}>2.5M_{\odot}$, but lower that the causal 3 solar masses limit. We also compare the neutron star phenomenology produced by the axionic scalar-tensor theory with the phenomenology produced by inflationary attractors scalar-tensor theories.

gr-qc↗

arXiv2024

Red or Blue Tensor Spectrum from GW170817-compatible Einstein-Gauss-Bonnet Theory: A Detailed Analysis

In this work we shall prove that the tensor spectral index of the primordial tensor perturbations for GW170817-compatible Einstein-Gauss-Bonnet theories, takes the approximate simplified form $n_{\mathcal{T}}\simeq 2\left(-1+\frac{1}{λ(ϕ)} \right)ε_1$ at leading order, with $λ(ϕ)$ being a function of the scalar field which depends on the scalar field potential and the second derivative of the scalar-Gauss-Bonnet coupling $ξ''(ϕ)$. With our analysis we aim to provide a definitive criterion for selecting Einstein-Gauss-Bonnet models that can provide a blue-tilted inflationary phenomenology, by simply looking at the scalar potential and the scalar-Gauss-Bonnet coupling. We shall prove this using two distinct approaches and as we show the tilt of the tensor spectral index is determined by the values of the potential $V(ϕ)$ and of scalar-Gauss-Bonnet coupling at first horizon crossing. Specifically the blue-tilted tensor spectral index can occur when $ξ''(ϕ_*)V(ϕ_*)>0$ at first horizon crossing.

gr-qc↗

arXiv2024

Non-minimal Derivative Coupling Theories Compatible with GW170817

In this work we aim to revive the interest for non-minimal derivative coupling theories of gravity, in light of the GW170817 event. These theories include a string motivated non-minimal kinetic term for the scalar field of the form $\sim ξ(ϕ) G^{μν}\partial_μϕ\partial_νϕ$ and predict that the primordial tensor perturbations have a speed that it is distinct from the speed of light. Due to the fact that the Universe is classical during and in the post-inflationary epoch, there is no fundamental reason for the graviton to change its mass, the GW170817 event severely constrained these theories. We analyze and formalize the inflationary phenomenology of these theories, using both the latest Planck data and the GW170817 event data to constrain these theories. Due to the fact that there is no constraint in the choice of the scalar potential and the non-minimal coupling function, we provide several classes of viable models, using convenient forms of the minimal coupling function in terms of the scalar potential, aiming for analyticity, and we discuss the advantages and disadvantages of each viable model.

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↗

arXiv2024

Einstein-Gauss-Bonnet Cosmological Theories at Reheating and at the End of the Inflationary Era

In this work we study the GW170817-compatible Einstein-Gauss-Bonnet theories during the reheating and the end of inflationary era. Given the scalar field potential $V(ϕ)$ which can have some intrinsic importance for the theory, determining the scalar coupling function $ξ(ϕ)$ can be cumbersome due to lack of analyticity. The GW170817 observation constrains the scalar coupling function and the scalar field potential to have some interdependence, thus during the slow-roll era one can calculate the scalar coupling function. However, when the slow-roll era ends, it is expected that the scalar coupling function should have a different form and the same applies for the reheating era, assuming that the scalar potential of the theory does not change. In this work we exactly aim to highlight this feature of Einstein-Gauss-Bonnet theories, as the Universe evolves through distinct sequential evolution eras, and we focus on how to determine the scalar coupling function during the various evolutionary eras, from inflation to the reheating era. Regarding both the end of the inflationary era and the reheating era, it is found that the Hubble rate obeys a constant-roll-like condition of the form $\dot{H}=δH^2$, thus the determination of the scalar Gauss-Bonnet function $ξ(ϕ)$ is reduced to solving a differential equation. A mentionable feature of the era exactly at the end of inflation is that the Klein-Gordon equation is decoupled from the field equations, because the Gauss-Bonnet invariant is zero. We provide several examples of interest to support our arguments.

gr-qc↗

arXiv2023

Primordial Cosmology of an Emergent-like Universe from Modified Gravity: Reconstruction and Phenomenology Optimization with a Genetic Algorithm

In this work we consider the realization of a variant emergent Universe scenario in the context of $f(R)$ gravity. We use well-known reconstruction techniques existing in the literature, and we find the approximate form of the vacuum $f(R)$ that reproduces the specific variant emergent Universe scale factor in the large curvature approximation. As we show, in the variant emergent Universe scenario, the Hubble horizon shrinks primordially and the Universe undergoes in an accelerated expansion era. In a perturbation theory approach, the scalar and tensor curvature perturbations can be expressed in terms of the phenomenological indices $ε_i , i=1,3,4$, usually used for inflationary phenomenology, and we extract the spectral indices for the scalar and tensor perturbations, along with the tensor-to-scalar ratio expressed in terms of the perturbation indices. Using a powerful genetic algorithm we investigate in depth the parameter space of the model, quantified by several free parameters, in order to study the viability of the model when this is compared with the Planck 2018 data. We also investigate the implications of the vacuum $f(R)$ gravity we found on the Big Bang nucleosynthesis.

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

A Stiff Pre-CMB Era with a Mildly Blue-tilted Tensor Inflationary Era can Explain the 2023 NANOGrav Signal

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 during this stiff era. The stiff era causes a broken-power-law effect on the energy spectrum of the gravitational waves. We use two approaches, one model agnostic and a specific model that can realize this scenario. In all cases, the inflationary era can be realized either by some theory leading to a standard red-tilted tensor spectral index or by some theory which has a mild tensor spectral index $n_{\mathcal{T}}=0.17-0.37$ like an Einstein-Gauss-Bonnet theory. For the model agnostic scenario case, the NANOGrav signal can be explained by the stiff pre-recombination era combined with an inflationary era with a mild blue-tilted tensor spectral index $n_{\mathcal{T}}=0.37$ and a low-reheating temperature $T_R\sim 0.1$GeV. In the same case, the red-tilted inflationary theory signal can be detectable by the future LISA, BBO and DECIGO experiments. The model dependent approach is based on a Higgs-axion model which can yield multiple deformations of the background total equation of state parameter, causing multiple broken-power-law behaviors occurring in various eras before and after the recombination era. In this case, the NANOGrav signal is explained by this model in conjunction with an inflationary era with a really mild blue-tilted tensor spectral index $n_{\mathcal{T}}=0.17$ and a low-reheating temperature $T_R\sim 20\,$GeV. In this case, the signal can be detectable by the future Litebird experiment, which is a very characteristic pattern in the tail of the primordial gravitational wave energy spectrum.

astro-ph.CO↗

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

Axion-Neutrino Couplings, Late-time Phase Transitions and the Far Infrared Physics

The far infrared physics is a fascinating topic for theoretical physics, since the foundation of quantum field theory and neutrinos seem to be strongly related with the far infrared physics of our Universe. In this work we shall explore the possibility of a late-time thermal phase transition caused by the axion-neutrino interactions. The axion is assumed to be the misalignment axion which is coupled primordially to a chiral symmetric neutrino. The chiral symmetry is supposed to be broken either spontaneously or explicitly, and two distinct phenomenological models of axion-neutrinos are constructed. The axion behaves as cold dark matter during all its evolution eras, however if we assume that the axion and the neutrino fields interact coherently in a classical way as fields, or as ensembles, then we consider thermal effects in the axion sector, due to the values of operators $ϕ$ for the axion and $\barνν$ due to the neutrinos. The thermal equilibrium between the two has no effect to the axion effective potential for a wide temperature range. As we show, contrary to the existing literature, the axion never becomes destabilized due to the finite temperature effects, however if axion-Higgs higher order non-renormalizable operators are present in the Lagrangian, the axion potential is destabilized in the temperature range $T\sim 0.1\,$MeV down to $T\sim 0.01\,$eV and a first order phase transition takes place. The initial axion vacuum decays to the energetically more favorable axion vacuum, and the latter decays to the Higgs vacuum which is more preferable. This late-time phase transition might take place in the redshift range $z\sim 385-37$ and thus it may cause density fluctuations in the post-recombination era.

hep-ph↗

arXiv2023

Flat Energy Spectrum of Primordial Gravitational Waves vs Peaks and the NANOGrav 2023 Observation

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 conditions which if hold true, the energy spectrum can be compatible with the recent NANOGrav stochastic gravitational wave detection. As we show, in most cases a blue tilted spectrum combined with a relatively low reheating temperature is needed, the scale of which is determined by whether the radiation domination era is ordinary or it is an abnormal radiation domination era. One intriguing Higgs-axion model, which predicts short slow-roll eras for the axion field at the post-electroweak breaking epoch, which eventually change the total equation of state parameter at the reheating era, can explain the NANOGrav signal, if a blue tilted tensor spectral index inflationary era precedes the reheating era, and a reheating temperature of the order $\mathcal{O}(400)\,$GeV. This specific model produces an energy spectrum of primordial gravitational waves with a characteristic peak that is detectable from both the NANOGrav and future LISA experiment, but not from the future Einstein telescope.

astro-ph.CO↗

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↗