arXiv subjects

S. D. Odintsov

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

At least 19 recordsLinked to original sources

arXiv2026

Adiabatic Perturbations in GW170817-Compatible Einstein-Gauss-Bonnet Inflation

We study the adiabaticity of the cosmological perturbations in the context of inflationary Einstein-Gauss-Bonnet theories. We focus on viable inflationary Einstein-Gauss-Bonnet theories which are compatible with the current Cosmic Microwave Background radiation experiments and also are compatible with the GW170817 observations. We derive the effects of the adiabaticity requirement on the Einstein-Gauss-Bonnet physical parameters and we show that the sound speed of the scalar perturbations and the propagation speed of the tensor perturbations are constrained. We consider two classes of inflationary viable and GW170817-compatible theories, and in the first class the adiabaticity is not violated during inflation, while in the second class the adiabaticity is violated only at the end of inflation. We discuss the effects of the adiabaticity violation in the second class of models. From our analysis, it seems that only one class of viable EGB inflationary theories, which is also compatible with the GW170817 event, is free from adiabaticity pathologies.

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arXiv2026

Dark Energy in Ghost-free non-local Gravity

Ghost-free non-local gravity is investigated with regards to its late-time dynamics. Viable solutions in this model are confronted with the observational data including the Pantheon+ catalogue of Type Ia supernovae, the Dark Energy Spectroscopic Instrument, the measurements of baryon acoustic oscillations and the Hubble parameter estimations $H(z)$. The ghost-free non-local gravity is found to be successful in these tests in comparison to the $Λ$CDM model and can be also comparable with the generalized exponential $F(R)$ gravity scenario. However the model encounters difficulties when the data from the above observations and the cosmic microwave background radiation data are combined. In tests with the whole set of Pantheon+, DESI, $H(z)$ and CMB data, the generalized exponential $F(R)$ model is essentially more successful. This success is related with the dynamical behavior of its effective dark energy equation of state evolving from a phantom to a quintessence phase during the late-time epoch, whereas the ghost-free non-local model demonstrates only a quintessence behavior. Hence the ghost-free non-local gravity scenario is successful only when the Pantheon+, DESI and $H(z)$ data are considered. The generalized exponential $F(R)$ model satisfies the viability conditions and in tests with all observational data including CMB surpasses the $Λ$CDM model in $χ^2$ statistics and also with information criteria.

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arXiv2026

ACT Data and Positive Running of the Spectral Index for Scalar Theory and Modified Gravity

In this work we address the possibility of having a positive running of the spectral index in inflationary theories. The recent ACT data indicate mildly that the running of the spectral index might be positive, and several other physical indications point out this possibility. If the running of the spectral index is confirmed to be positive by future cosmic microwave background experiments, this can rule out quite popular inflationary scenarios. We investigate how it is possible to obtain a positive running of the spectral index in the context of minimally coupled scalar field gravity and modified gravity. For the modified gravity we choose two mainstream and of string origin candidate theories, $F(R)$ gravity and Einstein-Gauss-Bonnet gravity. In the case of scalar field inflation and $F(R)$ gravity inflation, we demonstrate the difficulties for obtaining a positive running of the spectral index for a viable inflationary regime, so scalar theories and $F(R)$ gravity are mostly compatible with the Planck data. But nuanced scalar field scenarios can be compatible with the ACT data and produce a positive running of the spectral index. In the context of Einstein-Gauss-Bonnet theories which are compatible with the GW170817 event, the running of the spectral index can easily be positive while in parallel having a viable inflationary era.

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arXiv2026

String-inspired Gauss-Bonnet Gravity Inflation and ACT

In this article we present a systematic observational verification of the ghost-free string-inspired $f(R,\mathcal{G})$ model, where the Gauss-Bonnet invariant is non-minimally coupled to an auxiliary scalar field $\chi$ through the coupling function $h(\chi)$. Previous studies confirmed the theoretical viability of this framework using phenomenological parameter choices. In this work, for the first time, a systematic comparison with observational data from Planck 2018 and the Atacama Comsology Telescope is carried out via a Bayesian MCMC analysis using the Cobaya code. We explore an extended set of sixteen models constructed from four types of the Hubble parameter combined with power-law, exponential, hybrid, and inverse logarithmic coupling functions $h(\chi)$. The hybrid coupling $h(\chi) = \gamma e^{b_1\chi}\chi^{b_2}$, introduced in this context, allows for interpolation between the power-law and exponential forms, providing additional flexibility in controlling the Gauss-Bonnet contribution at different stages of inflation. All sixteen models reproduce the red spectral tilt of scalar perturbations consistent with CMB observations, yielding $n_s \approx 0.97$ at $N = 60$ e-folds. We find that the preference for the dataset is systematically determined by the choice of Hubble parametrization rather than by the coupling function. The parameter $\mu\approx0.1$ remains stable in all configurations, suggesting its fundamental role within the ghost-free formalism.

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arXiv2026

$\mathcal{R}^2$-corrected Tachyon Scalar Field Inflation, the ACT Data, and Phantom Transition

Phantom divide line transitions are not possible in the context of single scalar field scalar-tensor theories. In this article we study a combined framework of a tachyonic minimally coupled single scalar field theory in the presence of an $\mathcal{R}^2$ correction term and with a rescaled Einstein-Hilbert term of the form $\sim λ\frac{\mathcal{R}}{16πG}$. Such terms can be part of an $f(\mathcal{R})$ gravity which in the large curvature regime yields such correction terms effectively. Alternatively, such terms can simply be quantum corrections to the scalar field action. We aim to answer two questions, firstly if this framework can lead to phantom divide line transitions and secondly whether the resulting model can be compatible with the ACT data. The model we studied is an inverse square power-law model, well known from tachyon inflation models. As we show, the field equations can be cast in terms of the scalar field solely, however the resulting theory is distinct from a single scalar field theory, because the phantom divide line is crossed during inflation. Thus initially the tachyonic nature of the scalar field generates a phantom equation of state parameter, and during inflation the phantom divide line is crossed, with the effective equation of state parameter at the end of inflation being $w=-1/3$ which corresponds to the non-accelerating state of the Universe. The model is proved to be compatible with the ACT data, only when the gravity during inflation is stronger than Einstein-Hilbert gravity, with the effective gravitational constant during inflation being $\frac{G}λ$. The effective theory is valid only during inflation, thus Big-Bang nucleosynthesis is not affected by the rescaling of the Einstein-Hilbert gravity. The feature of a phantom crossing in $f(\mathcal{R},ϕ)$ frameworks is new in the literature.

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arXiv2026

Ghost-free non-local $F(R)$ Gravity Compatible with ACT

We confront the ghost-free non-local $F(R)$ gravity theories with the latest Atacama Cosmology Telescope (ACT) constraints on the spectral index of the scalar perturbations and the updated constraints of Planck/BICEP on the tensor-to-scalar ratio. After reviewing how the ghost-free non-local version of $F(R)$ gravity can be obtained, we show that the de Sitter solution can be obtained in this framework. Also, we show that the resulting theory can be cast in terms of an $F(R,ϕ)$ theory of gravity. We analyze two models of non-local $F(R)$ gravity, one power-law and the $R^2$ model, and we show that both models can be compatible with the ACT and updated Planck/BICEP constraints.

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arXiv2026

May Negative Mass Objects exist in the sky?

We conjecture the possibility of negative mass objects (NMOs) existing in the sky. It is shown that they may not be so exotic as usually expected. We show that NMOs appear as solutions of standard gravitational equations if we consider the system of a compact positive mass object, cosmological fluid and negative cosmological constant. We also construct models which generate such NMOs as solutions within the two-scalar theory and scalar-Einstein-Gauss-Bonnet gravity inspired by string theory. The orbits of the photon and massive particles are investigated in the background, where there is a negative mass object which realises a kind of effective anti-gravity. It is explicitly found that the bound system consisting of a positive mass object and a negative mass object can be formed in spite that a positive mass object suffers the repulsive force from the NMO. The possibility that such exotic objects might be observed is discussed. A simple conjecture about their possible masses is made, too. As an even more exotic object, we consider a non-trivial object with vanishing mass and investigate its properties.

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arXiv2026

Viable f(R) Scenarios Unifying Inflation with Realistic Dynamical Dark Energy

Two $F(R)$ gravity models are tested on the basis of their viability during all stages of cosmological evolution. It is shown that these models can describe both the early-time inflationary epoch and the dark energy epoch. The models are confronted with the latest observational data, including the Pantheon+ catalogue with Type Ia supernovae, the Dark Energy Spectroscopic Instrument measurements of baryon acoustic oscillations, the Hubble parameter estimations and data from cosmic microwave background radiation. Investigation of the viability conditions for these models, in particular, the condition $\frac{dF}{dR}>0$ required a deep analysis. Both models appeared to be viable during the early-time era, but for the late-time evolution the viability conditions are not fulfilled in definite domains in the parameter spaces of these models. However the best fitted parameters, determined in confrontation with the mentioned observational data, lie far from the forbidden domains for both models. These $F(R)$ gravity models describe the observations with the large advantage over the $Λ$-Cold-Dark-Matter model, not only in $χ^2$ statistics, but also with Akaike and Bayesian information criteria. This success of the two $F(R)$ gravity scenarios is connected with their capability to mimic dynamical dark energy, similarly to models with variable equation of state, that is necessary for describing the latest Pantheon+ and DESI observational data.

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arXiv2026

Is Phantom Divide Crossing in General Relativity Completely Impossible? Shortcomings in canonical and minimally coupled scalar field and Possible Solutions in $k$-essence Models

General relativity has its successes at the local astrophysical level, however, it seems to be insufficient in describing the Universe at large scales. In this work we investigate how the most general field theories in the context of general relativity can accomodate a phantom-to-quintessence transition which may be essential element of realistic Dark Energy scenarios in the late Universe. As we demonstrate in a very detailed manner, this is impossible for a canonical and minimally coupled single scalar field theory, but it may be possible for ghost condensate theories like $k$-essence theories. We point out how the ghost instabilities may be eliminated, and we analyze the quantitative features of a $k$-essence theory that may realize a phantom-to-quintessence transition in the late Universe. We also qualitatively compare the difficulties and fine-tunings required for $k$-essence theories to realize a phantom-to-quintessence transition, and how such a transition is naturally realized in modified gravity, without unnecessary fine-tunings and ghost eliminations.

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arXiv2026

A Remedy of the Trans-Planckian Censorship Problem with Smooth Slow-roll to Power-law Inflation Transitions in Scalar Field Theory

It is known that if the standard slow-roll inflation is followed by a power-law inflationary regime, then the trans-Planckian modes may be safely be contained in the Hubble horizon and never exit it during inflation. In this work we investigate how to realize a smooth transition between a slow-roll and a power-law inflationary regime in the context of single scalar field inflation. As we show it is possible to realize such a smooth transition by generalizing the kinetic energy of single scalar field in the form $\dotϕ^2=β(ϕ)V(ϕ)$, where $β(ϕ)$ is some appropriate function of the scalar field. Using two distinct approaches we show that it is possible to realize a smooth transition from a slow-roll to a power-law inflationary regime, and the two approaches produce identical results regarding the slow-roll regime. Also we show that the slow-roll regime is quite short, about $N\sim 30$ $e$-foldings, with the flatness and horizon problems being solved with the synergistic effect of the two inflationary patches. The slow-roll era is found to be compatible with the Atacama Cosmology Telescope data.

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arXiv2025

Phantom Crossing and Oscillating Dark Energy with $F(R)$ Gravity

In this work, we shall consider how a dynamical oscillating and phantom crossing dark energy era can be realized in the context of $F(R)$ gravity. We approach the topic from a theoretical standpoint considering all the conditions that may lead to a consistent phantom crossing behavior and separately how the $F(R)$ gravity context may realize oscillating dark energy era. Apart from our qualitative considerations, we study in a quantitative way two $F(R)$ gravity dark energy models which are viable cosmologically and also exhibit simultaneously phantom crossing behavior and also oscillating dark energy. We consider these models by solving numerically the field equations using appropriate statefinder parameters engineered for dark energy studies. As we show, $F(R)$ provides a natural extension of Einstein's general relativity which can naturally realize a transition from a phantom era to a quintessential era, a feature supported by recent observational data, without resorting to phantom scalar fields to realize the phantom evolution.

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arXiv2025

Dynamical Dark Energy from $F(R)$ Gravity Models Unifying Inflation with Dark Energy: Confronting the Latest Observational Data

A class of viable $F(R)$ gravity models which can provide a unified description of inflation with the dark energy era is confronted with the latest observational data on the dark energy era. These models have the unique characteristic that the de Sitter scalaron mass in the Einstein frame counterpart theory is a monotonic function of the curvature, which renders them viable descriptions for both the inflationary and the late-time acceleration eras. We also compare these models with other well-known viable $F(R)$ gravity models and with the $Λ$-Cold-Dark-Matter model. As we show, the most phenomenologically successful models are those which deviate significantly from the $Λ$-Cold-Dark-Matter model. Also some of the models presented, provide a statistically favorable description of the dark energy eras, compared with the exponential $F(R)$ gravity model and of course compared with the $Λ$-Cold-Dark-Matter model. All the models we present in this article are confronted with the observational data from the Planck collaboration, the Pantheon plus data from Type Ia supernovae, the two rounds of observations of the Dark Energy Spectroscopic Instrument, data from baryon acoustic oscillations and the Hubble constant measurements by SH0ES group. As we show, two of the models are statistically favorable by the data.

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arXiv2025

Confronting Rainbow-deformed $f(R)$ Gravity with the ACT Data

In this work we examine a theoretical scenario which combines two fundamental theoretical proposals for the early Universe and a possible evolution for the early post-inflationary epoch. Specifically, we assume that the early Universe contains gravity's rainbow effects on the spacetime and the inflationary Lagrangian contains $R^2$ corrections or $f(R)$ gravity corrections in general. In addition we assume that the era beyond the end of inflation until the reheating temperature is reached, is a kination era. Both theories, $R^2$ and gravity's rainbow emerge from a quantum context so their effects should be first checked at the theory which connects the quantum with the classical, hence inflation. Spacetime is four dimensional and the effects of the quantum theory could possibly be imprinted in the inflationary Lagrangian and of course on the spacetime itself. For the gravity's rainbow deformed Starobinsky model, both Lagrangian quantum effects and spacetime quantum effects are combined. As we show the resulting theory is compatible with the ACT data. We also consider power-law $f(R)$ gravity deformations and we show that in this case, the model is viable without the need of extending the slow-roll era of inflation.

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arXiv2025

Power-law $F(R)$ Gravity as Deformations to Starobinsky Inflation in View of ACT

In this work we aim to provide a new parametrization of power-law $F(R)$ gravity inflation framework in the Jordan frame. It is known in the literature that the power-law $F(R)$ gravity inflation of the form $F(R)=R+βR^n$ is non-viable and produces a power-law evolution. We demonstrate that the standard approach in power-law $F(R)$ gravity inflation has some parametrization issues that may lead to inconsistencies and we introduce a new parametrization which elevates the role of power-law $F(R)$ gravity deformations of the Starobinsky inflation, making it viable and compatible with both the Planck and ACT data. In our approach the power-law $F(R)$ gravity inflation is disentangled from a power-law evolution.

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arXiv2025

Solving the Trans-Planckian Censorship Problem with a Power-law Tail in $R^2$ Inflation: A Dynamical System Approach

In this work we elaborate on solving the trans-Planckian censorship problem of standard slow-roll inflation by using a power-law inflationary tail generated by a scalar field with an exponential potential. We use a quantitative approach by studying in detail the phase space of a combined $F(R,ϕ)$ cosmological system, focusing on the de Sitter and power-law subspaces of the total phase space. As we show, the de Sitter subspace of the $F(R,ϕ)$ system shares the same fixed points as the vacuum $F(R)$ gravity system and the trajectories in the phase space tend to these fixed points. However, the power-law subspace is not stable and cannot be realized by the combined $F(R,ϕ)$ system. To this end, we propose a well-motivated phenomenological $F(R)$ gravity model for which the $R^2$ term is switched off below a critical curvature near the end of the $R^2$ slow-roll inflationary era, and below that critical curvature, only the Einstein-Hilbert gravity term and the scalar field remain in the effective inflationary Lagrangian. The remaining system can successfully realize a power-law tail of the $R^2$ slow-roll era.

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arXiv2025

GW170817 Viable Einstein-Gauss-Bonnet Inflation Compatible with the Atacama Cosmology Telescope Data

In this work we investigate several Einstein-Gauss-Bonnet models that are compatible with the GW170817 event, the Atacama Cosmology Telescope data and the BICEP/Keck updated Planck constraints on the tensor-to-scalar ratio. We consider two distinct classes of Einstein-Gauss-Bonnet theories, which are equally successful for GW170817-compatible model building and we examine their viability against the Atacama Cosmology Telescope data and the updated Planck constraints on the tensor-to-scalar ratio. The two models are distinct since the first class relates directly the non-minimal Gauss-Bonnet scalar coupling function with the scalar potential and yields $c_T^2=1$ while in the second class, the non-minimal Gauss-Bonnet scalar coupling function and the scalar potential are freely but conveniently chosen and the class of models respects the constraint $\left| c_T^2 - 1 \right| < 6 \times 10^{-15}$. We provide several examples of models belonging to both the two classes of GW170817-compatible Einstein-Gauss-Bonnet theories and we demonstrate that Einstein-Gauss-Bonnet theories provide a promising theoretical framework for inflationary dynamics.

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arXiv2025

Interacting phantom dark energy: new accelerating scaling attractors

We perform a detailed investigation of interacting phantom cosmology, by applying the powerful method of dynamical system analysis. We consider two well-studied interaction forms, namely one global and one local one, while the novel ingredient of our work is the examination of new potentials for the phantom field. Our analysis shows the existence of saddle matter-dominated points, stable dark-energy dominated points, and scaling accelerating solutions, that can attract the Universe at late times. As we show, some of the stable accelerating scaling attractors, in which dark matter and dark energy can co-exist, alleviating the cosmic coincidence problem, are totally new, even for the previously studied interaction rates, and arise purely from the novel potential forms.

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arXiv2025

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

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

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