arXiv subjects

F. P. Fronimos

Publications and source records attributed to F. P. Fronimos.

At least 19 recordsLinked to original sources

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 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↗

arXiv2023

Kinetic Axion Dark Matter in String Corrected $f(R)$ Gravity

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 phenomenology and on the kinetic axion scenario. Specifically, in the context of our formalism, the kinetic axion ceases to be kinetically dominated at the end of the inflationary era, since the condition $\dotϕ\simeq 0$ naturally emerges in the theory. Thus, unlike the case of kinetic axion $R^2$ gravity, the Gauss-Bonnet corrected kinetic axion $R^2$ gravity leads to an inflationary era which is not further extended and the reheating era commences right after the inflationary era, driven by the $R^2$ fluctuations.

gr-qc↗

arXiv2023

Inflationary phenomenology of non-minimally coupled Einstein-Chern-Simons gravity

In this work we investigate the inflationary era in the presence of a canonical scalar field and Chern-Simons parity violating corrections. It was also assumed that a non minimal coupling between curvature and the scalar field is present. For the shake of completeness, the slow-roll and the constant-roll scenarios were examined separately. In the context of this scalar-tensor theory, inflation can be viable for both scenarios since the observational indices take acceptable values according to the most recent Planck data. Furthermore, the involvement of the Chern-Simons term has no effect on the background equations, in contrast to the scalar function which couples with the Ricci scalar and participates in the equations of motion. However, the Chern-Simons term ensures the chirality of stochastic gravitational waves. A blue-tilted tensor spectral index of primordial curvature perturbations can be manifested since, tensor modes are strongly affected by the Chern-Simons term. Lastly, the Swampland criteria and the Lyth-bound were examined in order to distinguish the effective field theories towards the path of a consistent M-theory.

gr-qc↗

arXiv2023

Swampland criteria for rescaled Einstein-Hilbert gravity with string corrections

In this work we examine the Swampland criteria for a specific class of rescaled $f(R)$ gravitational models, that are capable of unifying the primordial era of the Universe with the late-time era with the inclusion of string corrections. In particular, we develop separately the theoretical framework of Gauss-Bonnet and Chern-Simons theories considering that, the rescale parameter is constrained to reside in the area $0<α<1$. As showcased, in the context of the aforementioned theories, a viable inflationary phenomenology consistent with the latest Planck data can be obtained for both cases for a wide variety of values. The Swampland criteria which where examined are satisfied, not necessarily simultaneously, for small values of the rescale parameter, which is in agreement with the case of a canonical scalar field with absent string corrective terms. The Gauss-Bonnet model is also further constrained, in order to obtain a propagation velocity of tensor perturbations which coincides with that of light, according to the recent observations from the GW170817. As a result of this assumption the degrees of freedom of the theory are reduced. An interesting feature which arises from the overall phenomenology is that, due to the inclusion of string corrections the tensor spectral index of primordial perturbations is now capable of obtaining a positive value which is not possible in the case of the canonical scalar field. Last but not least, the power-law model which is known to be incompatible with observations is now rendered viable by including a parity violating term and as showcased, it satisfies the Swampland criteria as well.

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arXiv2022

Reheating era in Gauss-Bonnet theories of gravity compatible with the GW170817 event

In the present article we showcase how the reheating era can be described properly in the context of Einstein-Gauss-Bonnet gravity assuming that the primordial gravitational waves propagate with the velocity of light. The equations of the duration of reheating along with the reheating temperature are derived and as demonstrated, their expressions are quite similar to the case of a canonical scalar field where now the second derivative of the Gauss-Bonnet scalar coupling function appears and effectively alters the numerical value of the scalar potential. The appearance of such term is reminiscing of a $λR$ model of gravity where $λ$ is now dynamical. We consider two viable inflationary models of interest, the former involves an error function as scalar Gauss-Bonnet coupling function and the latter a Woods-Saxon scalar potential. It is shown that for both models the aforementioned quantities can be in agreement with theoretical expectations. The only constraint that is needed is the assumption that the second time derivative of the Gauss-Bonnet scalar coupling function is actually lesser than the Planck mass squared, that is $\ddotξ<\frac{M_{Pl}^2}{8}$ in order to obtain a viable description. We find that a free parameter of the theory and specifically the potential amplitude for the Woods-Saxon model during the inflationary era, dictates the effective equations of state and therefore the reheating epoch can be described either by a type of stiff matter with EoS parameter equal to unity or by an EoS parameter close to that of the radiation.

gr-qc↗

arXiv2022

Quasiclassical representation of the Volkov propagator and the tadpole diagram in a plane wave

The solution of the Dirac equation in the presence of an arbitrary plane wave, corresponding to the so-called Volkov states, has provided an enormous insight in strong-field QED. In [Phys. Rev. A \textbf{103}, 076011 (2021)] a new "fully quasiclassical" representation of the Volkov states has been found, which is equivalent to the one known in the literature but which more transparently shows the quasiclassical nature of the quantum dynamics of an electron in a plane-wave field. Here, we derive the corresponding expression of the propagator by constructing it using the fully quasiclassical form of the Volkov states. The found expression allows one, together with the fully quasiclassical expression of the Volkov states, to compute probabilities in strong-field QED in an intense plane wave by manipulating only 2-by-2 rather than 4-by-4 Dirac matrices as in the usual approach. Moreover, apart from the exponential functions featuring the classical action of an electron in a plane wave, the fully quasiclassical Volkov propagator depends only on the electron kinetic four-momentum in the plane wave, which is a gauge-invariant quantity. Finally, we also compute the one-loop tadpole diagram in a plane wave starting from the Volkov propagator and we find that after renormalization it identically vanishes.

hep-ph↗

arXiv2022

Late-time Cosmology of scalar field assisted $f(\mathcal{G})$ gravity

In this work we present the late-time behaviour of the Universe in the context of Einstein-Gauss-Bonnet gravitational theory. The theory involves a scalar field, which represents low-effective quantum corrections, assisted by a function $f(\mathcal{G})$ solely depending from the Gauss-Bonnet topological invariant $\mathcal{G}$. It is considered that the dark energy serves as the impact of all geometric terms, which are included in the gravitational action and the density of dark energy acts as a time dependent cosmological constant evolving with an infinitesimal rate and driving the Universe into an accelerating expansion. We examine two cosmological models of interest. The first involves a canonical scalar field in the presence of a scalar potential while the second, involves a scalar field which belongs to a generalized class of theories $f(ϕ,X)$ namely the k-essence scalar field in the absence of scalar potential. As it is proved, the aforementioned models are in consistency with the latest Planck data and in relatively good agreement with the $Λ$CDM standard cosmological model. The absence of dark energy oscillations at the early stages of matter dominated era, which appear in alternative scenarios of cosmological dynamics in the context of modified $f(R)$ gravitational theories, indicates an advantage of the theory for the interpretation of late-time phenomenology.

gr-qc↗

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.

gr-qc↗

arXiv2021

Inflation with exotic kinetic terms in Einstein-Chern-Simons gravity

An alternative scenario about the phenomenology of primordial Universe is k-inflation. According to this concept, inflation can be achieved by nonstandard kinetic term of scalar field, namely the inflaton. In this project we focus on k-essence models in the presence of a higher order and a linear kinetic term. Furthermore, the inflationary phenomenology with a Dirac-Born-Infeld scalar field is briefly examined, which arises from quantum theories of gravity such as superstring theory. Our approach about the inflationary era is that it can be described in the context of Einstein's gravity involving quantum corrections such as the Chern-Simons string inspired parity violating gravitational term. The equations of motion namely, the Friedmann equation, the Raychadhuri equation and the Klein-Gordon equation for an expanding background are extracted from the gravitational action utilizing the variational principle. The consequential system of differential equations with respect to Hubble's parameter and the inflaton field was quite perplexed in order to be solved with an analytic way. Therefore, the slow-roll conditions during inflationary era were imposed and terms with minor numerically contribution were neglected. From the overall phenomenological analysis it is proved that, models with exotic kinetic terms can generate viable results in consistency with the latest Planck data. Finally, the presence of Chern-Simons quantum corrections shifts the primordial spectral tensor index to blue. Even though blue gravitational waves have yet to be observed, if detected, compatibility with the aforementioned theory can be achieved.

gr-qc↗

arXiv2021

Generalized Horndeski-Like Einstein Gauss-Bonnet Inflation with Massless Primordial Gravitons

In this work we shall introduce a theoretical framework comprised by a non-minimal coupled canonical scalar field, a non-minimal coupling to the Gauss-Bonnet invariant and a non-minimal kinetic coupling. This theoretical framework is basically a non-minimally coupled Einstein-Gauss-Bonnet theory with extra corrections of the form of a non-minimal kinetic coupling. In order to comply with the GW170817 event, we shall impose a constraint on the propagation speed of the primordial tensor perturbations that it is equal to that of light's in vacuum, and this constraint basically specifies the way that the scalar potential and the non-minimal couplings of the theory can be chosen. The whole theoretical framework, which belongs to the larger class of Horndeski theories, cannot yield viable results, due to the fact that the primordial gravitational wave speed is not equal to that of light's. Thus we study this theory by also imposing the constraint of having gravity wave speed equal to the light speed. We directly examine the inflationary phenomenology of our theoretical framework and by assuming the slow-roll conditions, we derived the equations of motion in such a way so that analytical results may be extracted. By using several well motivated models we demonstrate the framework leads to a viable phenomenology.

gr-qc↗

arXiv2021

Inflation with Gauss-Bonnet and Chern-Simons higher-curvature-corrections in the view of GW170817

Inflationary era of our Universe can be characterized as semi-classical because it can be described in the context of four-dimensional Einsteins's gravity involving quantum corrections. These string motivated corrections originate from quantum theories of gravity such as superstring theories and include higher gravitational terms as, Gauss-Bonnet and Chern-Simons terms. In this paper we investigated inflationary phenomenology coming from a scalar field, with quadratic curvature terms in the view of GW170817. Firstly, we derived the equations of motion, directly from the gravitational action. As a result, formed a system of differential equations with respect to Hubble's parameter and the inflaton field which was very complicated and cannot be solved analytically, even in the minimal coupling case. Based on the observations from GW170817, which have shown that the speed of the primordial gravitational wave is equal to the speed of light, our equations of motion where simplified after applying this constraint, the slow-roll approximations and neglecting the string corrections. We described the dynamics of inflationary phenomenology and proved that theories with Gauss-Bonnet term can be compatible with recent observations. Also, the Chern-Simons term leads to asymmetric generation and evolution of the two circular polarization states of gravitational wave. Finally, viable inflationary models are presented, consistent with the observational constraints. The possibility of a blue tilted tensor spectral index is briefly investigated.

gr-qc↗

arXiv2021

Logarithmic corrected Einstein-Gauss-Bonnet inflation compatible with GW170817

In this paper we investigate the inflationary phenomenology of an Einstein-Gauss-Bonnet theory with the extension of a logarithmic modified $f(R)$ gravity, compatible with the GW170817 event. The main idea of our work is to study different results for an almost linear Ricci scalar through logarithmic corrections and examine whether such model is viable. First of all, the theoretical framework under slow-roll evolution of the scalar field is presented and also developed the formalism of the constant-roll evolution making predictions for the non- Gaussianities of the models is developed , since the constant-roll evolution is known to enhance non-Gaussianities. As shown, the non-Gaussianities are of the order $\mathcal{O}\sim(10^{-1})$. Furthermore, the slow-roll indices and the observational indices of inflation, are calculated for several models of interest. As demonstrated, the phenomenological viability of the models at hand is achieved for a wide range of the free parameters and the logarithmic term has a minor contribution to numerical calculations, as expected.

gr-qc↗

arXiv2021

Late-Time Cosmology of Scalar-Coupled $f(R, \mathcal{G})$ Gravity

In this work by using a numerical analysis, we investigate in a quantitative way the late-time dynamics of scalar coupled $f(R,\mathcal{G})$ gravity. Particularly, we consider a Gauss-Bonnet term coupled to the scalar field coupling function $ξ(ϕ)$, and we study three types of models, one with $f(R)$ terms that are known to provide a viable late-time phenomenology, and two Einstein-Gauss-Bonnet types of models. Our aim is to write the Friedmann equation in terms of appropriate statefinder quantities frequently used in the literature, and we numerically solve it by using physically motivated initial conditions. In the case that $f(R)$ gravity terms are present, the contribution of the Gauss-Bonnet related terms is minor, as we actually expected. This result is robust against changes in the initial conditions of the scalar field, and the reason is the dominating parts of the $f(R)$ gravity sector at late times. In the Einstein-Gauss-Bonnet type of models, we examine two distinct scenarios, firstly by choosing freely the scalar potential and the scalar Gauss-Bonnet coupling $ξ(ϕ)$, in which case the resulting phenomenology is compatible with the latest Planck data and mimics the $Λ$-Cold-Dark-Matter model. In the second case, since there is no fundamental particle physics reason for the graviton to change its mass, we assume that primordially the tensor perturbations propagate with the speed equal to that of light's, and thus this constraint restricts the functional form of the scalar coupling function $ξ(ϕ)$, which must satisfy the differential equation $\ddotξ=H\dotξ$.

gr-qc↗

arXiv2021

$k$-Inflation-corrected Einstein-Gauss-Bonnet Gravity with Massless Primordial Gravitons

In the present paper, we study the inflationary phenomenology of a $k$-inflation corrected Einstein-Gauss-Bonnet theory. Non-canonical kinetic terms are known for producing Jean instabilities or superluminal sound wave velocities in the aforementioned era, but we demonstrate in this work that by adding Gauss-Bonnet string corrections and assuming that the non-canonical kinetic term $ωX^γ$ is in quadratic, one can obtain a ghost free description. Demanding compatibility with the recent GW170817 event forces one to accept that the relation $\ddotξ=H\dotξ$ for the scalar coupling function $ξ(ϕ)$. As a result, the scalar functions of the theory are revealed to be interconnected and by assuming a specific form for one of them, specifies immediately the other. Here, we shall assume that the scalar potential is directly derivable from the equations of motion, once the Gauss-Bonnet coupling is appropriately chosen, but obviously the opposite is feasible as well. As a result, each term entering the equations of motion, can be written in terms of the scalar field and a relatively tractable phenomenology is produced. For quadratic kinetic terms, the resulting scalar potential is quite elegant functionally. Different exponents, which lead to either a more perplexed solution for the scalar potential, are still a possibility which was not further studied. We also discuss in brief the non-Gaussianities issue under the slow-roll and constant-roll conditions holding true, and we demonstrate that the predicted amount of non-Gaussianities is significantly enhanced in comparison to the $k$-inflation free Einstein-Gauss-Bonnet theory.

gr-qc↗

arXiv2020

Reviving non-Minimal Horndeski-like Theories after GW170817: Kinetic Coupling Corrected Einstein-Gauss-Bonnet Inflation

After the recent GW170817 event of the two neutron stars merging, many string corrected cosmological theories confronted the non-viability peril. This was due to the fact that most of these theories produce massive gravitons primordially. Among these theories were the ones containing a non-minimal kinetic coupling correction term in the Lagrangian, which belong to a subclass of Horndeski theories. In this work we demonstrate how these theories may be revived and we show how these theories can produce primordial gravitational waves with speed $c_T^2=1$ in natural units, thus complying with the GW170817 event. As we show, if the gravitational action of an Einstein-Gauss-Bonnet theory also contains a kinetic coupling of the form $\sim ξ(ϕ) G^{μν}\partial_μϕ\partial_νϕ$, the condition of having primordial massless gravitons, or equivalently primordial gravitational waves with speed $c_T^2=1$ in natural units, results to certain conditions on the scalar field dependent coupling function of the Gauss-Bonnet term, which is also the non-minimal coupling of the kinetic coupling. We extensively study the phenomenological implications of such a theory focusing on the inflationary era, by only assuming slow-roll dynamics for the scalar field. Accordingly, we briefly study the case that the scalar field evolves in a constant-roll way. By using some illustrative examples, we demonstrate that the viability of the theoretical framework at hand may easily be achieved. Also, theories containing terms of the form $\sim ξ(ϕ)\Boxϕg^{μν}\partial_μϕ\partial_νϕ$ and $\sim ξ(ϕ)\left(g^{μν}\partial_μϕ\partial_νϕ\right)^2$ also lead to the same gravitational wave speed as the theory we shall study in this paper, so this covers a larger class of Horndeski theories.

gr-qc↗

arXiv2020

Canonical Scalar Field Inflation with String and $R^2$-Corrections

Assuming that a scalar field controls the inflationary era, we examine the combined effects of string and $f(R)$ gravity corrections on the inflationary dynamics of canonical scalar field inflation, imposing the constraint that the speed of the primordial gravitational waves is equal to that of light's. Particularly, we study the inflationary dynamics of an Einstein-Gauss-Bonnet gravity in the presence of $αR^2$ corrections, where $α$ is a free coupling parameter. As it was the case in the pure Einstein-Gauss-Bonnet gravity, the realization that the gravitational waves propagate through spacetime with the velocity of light, imposes the constraint that the Gauss-Bonnet coupling function $ξ(ϕ)$ obeys the differential equation $\ddotξ=H\dotξ$, where $H$ is the Hubble rate. Subsequently, a relation for the time derivative of the scalar field is extracted which implies that the scalar functions of the model, which are the Gauss-Bonnet coupling and the scalar potential, are interconnected and simply designating one of them specifies the other immediately. In this framework, it is useful to freely designate $ξ(ϕ)$ and extract the corresponding scalar potential from the equations of motion but the opposite is still feasible. We demonstrate that the model can produce a viable inflationary phenomenology and for a wide range of the free parameters. Also, a mentionable issue is that when the coupling parameter $α$ of the $R^2$ correction term is $α<10^{-3}$ in Planck Units, the $R^2$ term is practically negligible and one obtains the same equations of motion as in the pure Einstein-Gauss-Bonnet theory, however the dynamics still change, since now the time derivative of $\frac{\partial f}{\partial R}$ is nonzero.

gr-qc↗

arXiv2020

Non-minimally Coupled Einstein-Gauss-Bonnet Gravity with Massless Gravitons: The Constant-roll Case

In this letter we study the behavior of non-minimally coupled Einstein-Gauss-Bonnet gravity theories with the constant-roll condition. Recalling the results of the striking GW170817 event, we demand that the velocity of the gravitational waves is equated to unity in natural units, meaning that $c_T^2=1$. This is a powerful restriction since it leads to a decrease in the degrees of freedom and subsequently reveals a connection between the scalar functions of the theory which presumably have different origins. In this framework, we shall assume that a scalar potential is present and can be extracted easily from the equations of motion by simply designating the scalar coupling functions. Obviously, a different approach is feasible but such choice will prove to be extremely convenient. Afterwards, we impose certain approximations in order to facilitate our study. Each assumption is capable of producing different phenomenology so a summary of all the possible configurations for Hubble's parameter, its derivative and the scalar potential along with the corresponding assumptions are present at the end of the paper. We show that compatibility under the constant-roll assumption can be achieved for a variety of model functions and different approaches, although one must always be aware of the imposed approximations since the possibility of a model producing viable results while simultaneously violating even a single approximation exists. Finally, in the end, a new formalism which leads to a plethora of convenient coupling functions according to the readers choice is presented. Utilizing such formalism may lead to new cases for coupling functions which have not been used yet, but are in fact able of producing viable phenomenology for the inflationary era.

gr-qc↗