arXiv subjects

V. K. Oikonomou

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

At least 19 recordsLinked to original sources

arXiv2026

ACT-compatible Inflation in Scalar Field Coupled $f\left(Q, \tilde R\right)$ Gravity

In this work, we construct models compatible with ACT constraints on inflation in the framework of $f\left(Q, \tilde R\right)$ gravity, with $f(R)$ gravity coupled to a scalar field. The $f\left(Q, \tilde R\right)$ theory is equivalent to $f(Q, B)$ gravity models or $f(Q, C)$ gravity where $B$ or $C$ is the difference between $Q$ and the scalar curvature $\tilde R$ in Einstein's gravity, so, $B=Q-\tilde R$ or $C=\tilde R - Q$. Using appropriate reconstruction techniques compatible with $f\left(Q, \tilde R\right)$ theory, we propose two functional behaviors of the Hubble rate $H$ that satisfy the constraints, and we construct models that realize these behaviors of the Hubble rate $H$ using $f\left(Q, \tilde R\right)$ gravity and $f(R)$ gravity coupled with a scalar field. We also discuss the reheating stage after inflation. Although the scenario of $f(R)$ gravity coupled with a scalar field is well-known, since the model can be rewritten in the form of Einstein's gravity coupled with two scalar fields, the non-trivial problem occurs in the $f\left(Q, \tilde R\right)$ gravity case, because the number of dynamical degrees of freedom in $f(Q)$ gravity is not settled.

gr-qc↗

arXiv2026

Is MOND an Emergent Effective Law of a Complex Dark Matter Sector?

In this work we aim to present a new perspective for MOND theory, namely that MOND theory is not an answer for the missing matter at galactic scales, but it provides a phenomenological description of the dynamics that DM should produce at galactic scales. We also aim to point out that MOND dynamics at galactic scales is achieved by scale-dependent self-interacting dark matter, which can behave as collisionless and collisional, depending on the scales and the physical processes. To this end, we first aim to highlight the successes of the $Λ$-Cold-Dark-Matter model and how it overwhelms over the MOND paradigm. Using textbook physics, we present the successes of the $Λ$-Cold-Dark-Matter at cosmological and cluster scales, and how these phenomena cannot be described consistently by MOND theories. Thus, by excluding MOND as being a viable description of nature at all scales, we conclude that MOND may not be the answer behind missing matter in Newtonian galactic dynamics, but it is just showing how the dynamics of dark matter should behave at galactic scales.

gr-qc↗

arXiv2026

Singlet Extended Mirror Standard Model World as Dark Matter and Gravitational Waves Imprints of a High Scale Mirror Phase Transitions

We study a mirror Standard Model world which contains a high scale duplicate of the Standard Model, and includes a singlet mirror scalar extension coupled to the mirror Higgs particle. Among the interactions of the mirror Higgs to the mirror singlet scalar, we include dimension-six non-renormalizable operators. We examine the electroweak phase transition of this mirror singlet extended world and we show that the phase transition is a strong first order phase transition, the bubble collision of which can be detectable by LISA, the BBO and the DECIGO gravitational wave experiments. We also provide a rough estimate of the abundance of the mirror singlet and we show that the whole dark matter in the Universe may be comprised by mirror particles and atoms.

hep-ph↗

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.

gr-qc↗

arXiv2026

From Quantum Correlations to Inflationary Tracking Scalar Field Evolution

The tracking condition $\dotϕ^2=γH^{-m}$ for single scalar field theory leads to analytic inflationary solutions, which are compatible with the current cosmic microwave background radiation experiments. To our knowledge this is the only analytic solution of inflation which is compatible with the data, to date. In this work we seek for some theoretical basis that can lead to the tracking condition $\dotϕ^2=γH^{-m}$. As we show, if the Universe is seen pre-inflationary as a quantum statistical system, the scalar field may emerge as a collective condensate of the quantum degrees of freedom. In the quantum-to-classical transition of the Universe, the scalar field two point function yields the susceptibility of the theory with an inherent correlation length. Using information theoretic motivation and Wilsonian quantum field theoretic arguments, we demonstrate that the tracking conditions $\dotϕ^2=γH^{-m}$ emerge from this framework.

gr-qc↗

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.

gr-qc↗

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.

gr-qc↗

arXiv2026

High-scale Mirror Standard Model Dark Matter, Dark Phase Transitions and Gravitational Waves Implications

We consider a scenario for dark matter in the Universe, according to which the dark matter sector is comprised by a dark Standard Model sector which interacts only gravitationally with the ordinary Standard Model sector. This dark Standard Model sector is assumed to have the same symmetries as the ordinary Standard Model, with the couplings and the scale of the mirror Standard Model sector being different than the ordinary Standard Model sector. Specifically, the scale of the mirror Standard Model sector will be assumed to be quite higher compared to the ordinary Standard Model. Also the Yukawa couplings among the mirror Higgs and the mirror fermions are assumed to be different from those of the Standard Model and we examine the effects of the different scale and of the different Yukawas on the evolution of the Universe. As we show, a mirror world phase transition occurs at high temperatures of the baryonic Universe, which can be first order or second order, depending on the scale of the Universe and the Yukawa couplings. These are dark phase transitions which occur quite earlier than the real world Standard Model electroweak phase transition. The case of a second order phase transition is quite interesting phenomenologically, since it can potentially have a direct imprint on the spectrum of stochastic gravitational waves for frequencies probed by the future gravitational wave detectors. Also we examine whether this mirror dark matter world can form atoms and as we show in some scenario the high scale mirror dark matter can have both atomic and subatomic particle components. We also give an approximation of the total equation of state of high scale mirror DM and we discuss how high scale mirror DM can reconcile contradicting observations like the Bullet cluster and the Abell 520 cluster.

hep-ph↗

arXiv2026

Reconstructing ACT-compatible and GW170817-compatible Einstein-Gauss-Bonnet Inflation from the Observational Indices

In this work we use an inverse reconstruction technique for constructing ACT-compatible and GW170817-compatible Einstein-Gauss-Bonnet inflationary theories. From a given tensor-to-scalar ratio using the reconstruction technique, we find which scalar Gauss-Bonnet coupling function and which scalar potential can yield the given tensor-to-scalar ratio. We present the formalism and the viable theories pass a series of observational tests, including the amplitude of the scalar perturbations, which is non-trivial for Einstein-Gauss-Bonnet theories. We present four viable models of inflation that pass all the observational tests.

gr-qc↗

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.

gr-qc↗

arXiv2026

Recombination Thickness as an Uncertainty in Inflationary Observables

Standard CMB analysis assumes a direct deterministic mapping between the multipole probed by the CMB $\ell$ and the primordial wavenumber $k$. Since the recombination era has a finite duration, this mapping is probabilistic by construction. We elevate the power spectrum of the primordial perturbations to a probability distribution caused by the finite duration of the recombination era. We show that a finite recombination width introduces a Gaussian smoothing scale in $\ln k$ with $σ_{\ln k} \sim σ_η/ D_*$, leading to a probabilistic mapping from multipoles to inflationary e-folds. This effect is zero in standard power-law inflationary scenarios, but it may become relevant for scenarios with exotic oscillating features of the primordial power spectrum, which will be probed by the future CMB experiments. The observed effective power spectrum is the true primordial spectrum blurred by the uncertainty in scale reconstruction, which is mathematically identical to a Bayesian marginalization over a latent variable, and thus there is a propagation of the measurement error in the independent variable, which is another more formal way to view the smoothing effect. Our results indicate that the smoothing has quantifiable effects on the spectral index and its running, but more importantly the difference between the TT and EE inferred spectral indices, $n_s^{TT}-n_s^{EE}$, is non-trivial, in contrast to standard inflation without smoothing, and might become observable by future cosmic microwave background experiments. Any tension in $n_s^{TT}-n_s^{EE}$ could indicate oscillations in the primordial spectrum and the effects of the power spectrum smoothing. Finally, a minimal Fisher matrix analysis is performed to investigate the observability prospects of the smoothing effect.

astro-ph.CO↗

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 $χ$ through the coupling function $h(χ)$. 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 $h(χ)$. The hybrid coupling $h(χ) = γe^{b_1χ}χ^{b_2}$, 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 $μ\approx0.1$ remains stable in all configurations, suggesting its fundamental role within the ghost-free formalism.

gr-qc↗

arXiv2026

String Corrected Scalar Field Inflation Compatible with the ACT Data

We consider the impact of the first string corrections of minimally coupled single scalar field theory on inflationary dynamics. Specifically we consider separately the string corrections $\sim α'ξ(ϕ)c_2\,\left( \partial_μϕ\partial^μϕ\right)^2$ and $\sim α'c ξ(ϕ)\square ϕ\partial_μϕ\partial^μϕ$, where $α'$ is the square of the string scale. Our aim is to develop a theory which is self consistent in the sense that the field equations reproduce themselves in the slow-roll approximation. Such a requirement for the theory with $\sim α'ξ(ϕ) c_2\left( \partial_μϕ\partial^μϕ\right)^2$ resulted to a trivial non-minimal coupling function $ξ(ϕ)$, however a self-consistent framework emerged from the theory with correction term $\sim α' c ξ(ϕ)\square ϕ\partial_μϕ\partial^μϕ$. The resulting theory can easily be worked out analytically and we obtained an inflationary theory that can easily be fitted with the Atacama Cosmology Telescope constraints on the scalar spectral index and the updated Planck constraints on the tensor-to-scalar ratio.

gr-qc↗

arXiv2026

Analytic Singular Slow-roll Inflation

We study a class of minimally coupled scalar field theories which leads to analytic solutions for the Hubble rate and the scalar field, where the scalar field obeys a generalized tracking law $\dotϕ^2\sim H^{-m}$. The inflationary phenomenology for this class of models can be studied fully analytically. The resulting phenomenology is compatible with the ACT data and for limiting cases, the spectral index is bluer than the ACT constraints and tends to the value $n_{\mathcal{S}}=0.98$, while in the limiting case, the tensor-to-scalar ratio takes very small values, nearly zero. In addition, we prove analytically that the phenomenology is a one-parameter model, and the inflationary observables encode the scaling exponent $m$ of the generalized kinetic attractor $\dotϕ^2\sim H^{-m}$. Furthermore, the tensor-to-scalar ratio and the spectral index have a simple linear and $m$-dependent relation. More importantly, the resulting cosmology describes a Universe that has a finite scale factor at $t=0$, thus non-singular, evolves and expands realizing a slow-roll inflationary era and after that it reaches classically a pressure singularity. Classically, the Universe can pass through this singularity, and a turnaround cosmology is realized with the Universe contracting after the turnaround point. However, before the singularity is realized classically, the quantum phenomena dominate the evolution, avoiding the singularity. Specifically we consider the Nojiri-Odintsov conformal anomaly mechanism and we qualitatively show that the conformal anomaly erases the classical singular evolution and at the same time it enhances particle creation, which eventually reheats the Universe. Thus in this model the scalar field oscillations and the numerous couplings of the inflaton to the Standard Model particles are not required for reheating.

gr-qc↗

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.

gr-qc↗

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.

gr-qc↗

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.

gr-qc↗

arXiv2026

General Scalar Field Inflation ACT Attractors: Utilizing the $n_s(r)$ relation

The ACT data have severely constrained the single scalar field models. Known models of inflation, like the Starobinsky model, the Higgs model and the $a$-attractors are at least $2σ$ off the ACT data. In this work we aim to provide a top-to-bottom approach in single scalar field inflationary cosmology compatible with the ACT data. Specifically, inspired by the fact that the Starobinsky model, the Higgs model and the $a$-attractors, all being plateau potentials, result to the same attractor relation between the spectral index of scalar perturbations and the tensor-to-scalar ratio, which is of the form $n_s(r)=1-αr^{1/2}$, in this work we seek for attractors of the form $n_s(r)=f(r)$ that may lead to ACT-compatible inflation. Specifically, we fix the function $f(r)$ to have a specific desirable form and then solve the differential equation $n_s(r)=f(r)$ to find the potential which results to the relation $n_s(r)=f(r)$. We discovered analytically three classes of potentials which are variants of the general form $n_s(r)=γ\pm βr \pm r^{1/2}$ and all these models are found to be compatible with the ACT data.

gr-qc↗