arXiv subjects

Yuri Shtanov

Publications and source records attributed to Yuri Shtanov.

At least 19 recordsLinked to original sources

arXiv2026

Effective scalaron--photon interaction in $f(R)$ gravity

We revisit the effective coupling of the scalaron to gauge fields in $f(R)$ gravity minimally coupled to the Standard Model, focusing on the scalaron decay into two photons. Treating the scalaron as an intrinsic component of the Jordan-frame metric, we analyse quantum effects arising from its coupling to the energy--momentum tensor. In this framework, the trace anomaly contributes to the scalaron--gauge boson interaction. Using Fujikawa's approach, we obtain the trace-anomaly contribution, associated with the transformation of matter fields between the Jordan and Einstein frames, first in QED and then in the full Standard Model. The resulting effective scalaron--photon interaction agrees with direct perturbative calculations that include the trace anomaly and differs from the result obtained when only the classical expression for the trace of the energy--momentum tensor is taken into account in the scalaron interaction. In the limit where the scalaron mass is much smaller than the masses of particles circulating in the loop, the diagrammatic contribution from the classical expression of the trace of the energy--momentum tensor cancels the anomaly-induced term, leading to a vanishing effective coupling and a strong suppression of the scalaron decay rate into photons. These results clarify the origin of discrepancies in the literature concerning the effective scalaron coupling to massless gauge fields. They stem from inequivalent prescriptions for incorporating quantum loop effects in $f (R)$ gravity, leading to different scalaron--gauge field interactions with direct implications for scalaron dark-matter phenomenology.

hep-th↗

arXiv2025

Braneworld Dark Energy in light of DESI DR2

Recent observational results from the DESI collaboration reveal tensions with the standard $Λ$CDM model and favour a scenario in which dark energy (DE) decays over time. The DESI DR2 data also suggest that the DE equation of state (EoS) may have been phantom-like ($w < - 1$) in the past, evolving to $w > - 1$ at present, implying a recent crossing of the phantom divide at $w = - 1$. Scalar field models of DE naturally emerge in ultraviolet-complete theories such as string theory, which is typically formulated in higher dimensions. In this work, we investigate a broad class of $thawing~scalar~field~models$, including the simple quadratic, quartic, exponential, symmetry-breaking and axion potentials, propagating on a (4+1)-dimensional ghost-free phantom braneworld, and demonstrate that their effective EoS exhibits a phantom-divide crossing. Alongside the Hubble parameter and EoS of DE, we also analyse the evolution of the $Om$ diagnostic, and demonstrate that the time dependence of these quantities is in excellent agreement with the DESI DR2 observations. Furthermore, we perform a comprehensive parameter estimation using Markov Chain Monte Carlo sampling, and find that the $χ^2$ values for all our models are remarkably close to that of the widely used CPL parametrisation, indicating that our models fit the data very well.

astro-ph.CO↗

arXiv2025

Interactions of the scalaron dark matter in $f (R)$ gravity

In $f(R)$ gravity, the scalaron -- a scalar degree of freedom arising from modification of General Relativity -- could account for all dark matter in the universe if its mass lies in the meV--MeV range. In this work, we revisit the scalaron's interactions with Standard Model particles, assuming their minimal coupling to gravity. In particular, we provide a detailed calculation of the scalaron's decay rate into two photons -- a one-loop process of significant interest that has been the subject of discrepancies in the literature. We demonstrate that a direct evaluation of loop diagrams with appropriate regularisation eliminates the ambiguities inherent in methods relying on Jacobians from field redefinitions. Assuming the scalaron constitutes all of dark matter, we calculate the average cosmological background radiation produced by its decays into photons. We also estimate the contribution of primordial scalarons emitted in the hot early universe to the present dark matter density and find it to be negligible. Our results support all key aspects of the original scenario, in which scalaron dark matter behaves as a coherently oscillating field.

astro-ph.CO↗

arXiv2024

Scalaron dark matter and the thermal history of the universe

In metric $f(R)$ gravity minimally coupled to the Standard Model, the scalaron field can act as a dark-matter candidate if its mass lies in the range $\text{meV} \lesssim m \lesssim \text{MeV}$. The evolution of the scalaron is influenced by the trace of the stress-energy tensor, whose behaviour, as shown in our previous work, becomes non-adiabatic during the electroweak crossover, potentially triggering scalaron oscillations. While we previously approximated this crossover as a second-order phase transition at the one-loop level, the transition is actually smoother. In this paper, we refine our analysis to account for this smooth crossover and show that scalaron oscillations are still excited in a qualitatively similar manner, driven by the rapid dynamics of the electroweak crossover observed in numerical lattice simulations, provided the scalaron mass is sufficiently small. We also investigate the time-dependent contribution to the stress-energy trace due to the trace anomaly of quantum chromodynamics. Our results indicate that, while the trace anomaly shifts the scalaron's equilibrium value, this shift evolves adiabatically compared to the fast oscillations of the scalaron, meaning that the trace anomaly does not significantly affect the potential cosmological scenarios for scalaron evolution.

hep-ph↗

arXiv2024

Discreteness effects in $N$-body simulations of warm dark matter

In cosmological $N$-body simulations of warm dark matter, thermal velocities of dark-matter particles are sometimes taken into account by adding random initial velocities to the particles of simulation. However, a particle in the $N$-body system represents a huge collection of dark-matter particles, whose average thermal velocity is very close to zero. We consider justification of the procedure of adding thermal velocities in $N$-body simulations and build a simple model of their influence on the power spectrum. Our model captures the physical effect of suppression of the power spectrum at small wave numbers and also explains its artificial enhancement at large wave numbers, observed in numerical simulations with added thermal velocities. The cause of this enhancement is the disturbance of the growth rate of the density profile introduced when adding random initial thermal velocities. Specifically, the model predicts a turnover in the behavior of the simulated power spectrum at a certain wave number $k_*$, beyond which it grows as $P (k) \propto k^2$. Our treatment is generalized to a system consisting of several matter components with different thermal velocity dispersion. We also estimate the effects of discreteness related to the bulk velocity field and establish the conditions under which these effects dominate over those of thermal velocities.

astro-ph.CO↗

arXiv2024

Electroweak symmetry breaking by gravity

We consider a simple scale-invariant action coupling the Higgs field to the metric scalar curvature $R$ and containing an $R^2$ term that exhibits spontaneous breaking of scale invariance and electroweak symmetry. The coefficient of the $R^2$ term in this case determines the self-coupling of the Higgs boson in the Einstein frame, and the scalaron becomes a dilaton weakly coupled to the Higgs boson. Majorana mass terms for right-handed neutrinos can be generated in a scale-invariant manner by using the Higgs-field invariant; in this case, the existing experimental limits on the Higgs-boson total width rule out Majorana mass values in a certain range. The model inherits the naturalness issues of general relativity connected with the smallness of the gravitational and cosmological constants.

hep-ph↗

arXiv2023

Tabletop potentials for inflation from $f(R)$ gravity

We show that a large class of modified gravity theories (MOG) with the Jordan-frame Lagrangian $f(R)$ translate into scalar-field (scalaron) models with hilltop potentials in the Einstein frame. (A rare exception to this rule is provided by the Starobinsky model for which the corresponding scalaron potential is plateau-like for $ϕ> 0$.) We find that MOG models featuring two distinct mass scales lead to scalaron potentials that have a flattened hilltop, or tabletop. Inflationary evolution in tabletop models agrees very well with CMB observations. Tabletop potentials therefore provide a new and compelling class of MOG-based inflationary models. By contrast, MOG models with a single mass scale generally correspond to steep hilltop potentials and fail to reproduce the CMB power spectrum. Inflationary evolution in hilltop/tabletop models can proceed in two alternative directions: towards the stable point at small $R$ describing the observable universe, or towards the asymptotic region at large $R$. The MOG models which we examine have several new properties including the fact that gravity can become asymptotically vanishing, with $G_{\rm eff} \to 0$, at infinite or large finite values of the scalar curvature $R$. A universe evolving towards the asymptotically vanishing gravity region at large $R$ will either run into a 'Big-Rip' singularity, or inflate eternally.

gr-qc↗

arXiv2022

Initial conditions for the scalaron dark matter

The scalaron of the metric $f(R)$ gravity can constitute dark matter if its mass is in the range $4\,\text{meV} \lesssim m \lesssim 1\,\text{MeV}$. We give an overview of such $f (R)$ gravity theory minimally coupled to the Standard Model. Similarly to other dark-matter models based on scalar fields, this model has the issue of initial conditions. Firstly, the initial conditions for the scalaron are to be tuned in order to produce the observed amount of dark matter. Secondly, the primordial spatial inhomogeneities in the field are to be sufficiently small because they generate entropy (or isocurvature) perturbations, which are constrained by observations. We consider these issues in the present paper. The initial conditions for the scalaron presumably emerge at the inflationary stage. We point out that the homogeneous part of the scalaron initial value is largely unpredictable because of quantum diffusion during inflation. Thus, to account for the observed amount of dark matter, one has to resort to anthropic considerations. Observational constraints on the primordial spatial inhomogeneity of the scalaron are translated into upper bounds on the energy scale of inflation, which happen to be low but not too restrictive.

astro-ph.CO↗

arXiv2022

On the Conformal Frames in $f(R)$ Gravity

We discuss gravitational physics in the Jordan and Einstein frames of $f (R)$ gravity coupled to the Standard Model. We elucidate the way in which the observed gravitational coupling arises in the Einstein frame for generic $f (R)$. We point out that the effect of "running units" in the Einstein frame is related to the fact that the explicit and implicit quantum parameters of the Standard Model, such as the Higgs vacuum expectation value and the parameter $Λ_\text{QCD}$, are modified by the conformal transformation of the metric and matter fields and become scalaron-dependent. Considering the scalaron of $f (R)$ gravity describing dark matter, we show that the effect of running units in this case is extremely weak, making two frames practically equivalent.

gr-qc↗

arXiv2021

Phantom braneworld and the Hubble tension

Braneworld models with induced gravity exhibit phantom-like behaviour of the effective equation of state of dark energy. They can, therefore, naturally accommodate higher values of $H_0$, preferred by recent local measurements, while satisfying the CMB constraints. We test the background evolution in such phantom braneworld scenarios with the current observational datasets. We find that the phantom braneworld prefers a higher value of $H_0$ even without the R19 prior, thereby providing a much better fit to the local measurements. Although this braneworld model cannot fully satisfy all combinations of cosmological observables, among existing dark energy candidates the phantom brane provides one of the most compelling explanations of cosmic evolution.

astro-ph.CO↗

arXiv2021

Light scalaron as dark matter

A new cosmological scenario is proposed in which a light scalaron of $f (R)$ gravity plays the role of dark matter. In this scenario, the scalaron initially resides at the minimum of its effective potential while the electroweak symmetry is unbroken. At the beginning of the electroweak crossover, the evolving expectation value of the Higgs field triggers the evolution of the scalaron due to interaction between these fields. After the electroweak crossover, the oscillating scalaron can represent cold dark matter. Its current energy density depends on a single free parameter, the scalaron mass $m$, and the value $m \simeq 4 \times 10^{-3}\, \text{eV}$ is required to explain the observed dark-matter abundance. Larger mass values would be required in scenarios where the scalaron is excited before the electroweak crossover.

hep-ph↗

arXiv2020

Model-independent constraints in inflationary magnetogenesis

We derive a simple model-independent upper bound on the strength of magnetic fields obtained in inflationary and post-inflationary magnetogenesis taking into account the constraints imposed by the condition of weak coupling, back-reaction and Schwinger effect. This bound turns out to be rather low for cosmologically interesting spatial scales. Somewhat higher upper bound is obtained if one assumes that some unknown mechanism suppresses the Schwinger effect in the early universe. Incidentally, we correct our previous estimates for this case.

astro-ph.CO↗

arXiv2020

Chiral perturbation theory for GR

We describe a new perturbation theory for General Relativity, with the chiral first-order Einstein-Cartan action as the starting point. Our main result is a new gauge-fixing procedure that eliminates the connection-to-connection propagator. All other known first-order formalisms have this propagator non-zero, which significantly increases the combinatorial complexity of any perturbative calculation. In contrast, in the absence of the connection-to-connection propagator, our formalism leads to an effective description in which only the metric (or tetrad) propagates, there are only cubic and quartic vertices, but some vertex legs are special in that they cannot be connected by the propagator. The new formalism is the gravity analog of the well-known and powerful chiral description of Yang-Mills theory.

hep-th↗

arXiv2019

Viable inflationary magnetogenesis with helical coupling

We consider helical coupling to electromagnetism and present a simple scenario of evolution of the coupling function leading to a viable inflationary magnetogenesis without the problem of back-reaction. In this scenario, helical magnetic fields of strength of order up to $10^{- 7}\,\text{G}$, when extrapolated to the current epoch, can be generated in a narrow spectral band centered at any reasonable wavenumber by adjusting the model parameters. We discuss implications of this model for baryogenesis, which impose additional constraints on the strength and correlation length of magnetic field.

astro-ph.CO↗

arXiv2018

Versatile parametrization of the perturbation growth rate on the phantom brane

We derive an analytical expression for the growth rate of matter density perturbations on the phantom brane (which is the normal branch of the Dvali-Gabadadze-Porrati model). This model is characterized by a phantomlike effective equation of state for dark energy at the present epoch. It agrees very well with observations. We demonstrate that the traditional parametrization $f=Ω_m^γ$ with a quasiconstant growth index $γ$ is not successful in this case. Based on a power series expansion at large redshifts, we propose a different parametrization for this model: $f=Ω_m^γ\left(1+\frac{b}{\ell H}\right)^β$, where $β$ and $b$ are constants. Our numerical simulations demonstrate that this new parametrization describes the growth rate with great accuracy - the maximum error being $\leq 0.1\%$ for parameter values consistent with observations.

gr-qc↗

arXiv2018

Magnetogenesis by non-minimal coupling to gravity in the Starobinsky inflationary model

The $R^2$ term in the Starobinsky inflationary model can be regarded as a leading quantum correction to the gravitational effective action. We assume that parity-preserving and parity-violating (axial) non-minimal couplings between curvature and electromagnetic field are also present in the effective action. In the Einstein frame, they turn into non-trivial couplings of the scalaron and curvature to the electromagnetic field. We make an assessment of inflationary magnetogenesis in this model. In the case of parity-preserving couplings, amplification of magnetic field is negligibly small. In the case of axial couplings, magnetogenesis is hampered by strong back-reaction on the inflationary process, resulting in possible amplification of magnetic field at most by the factor $10^5$ relative to its vacuum fluctuations.

astro-ph.CO↗

arXiv2017

Pure-connection gravity and anisotropic singularities

In four space-time dimensions, there exists a special infinite-parameter family of chiral modified gravity theories. They are most properly described by a connection field, with space-time metric being a secondary and derived concept. All these theories have the same number of degrees of freedom as general relativity, which is the only parity-invariant member of this family. Modifications of general relativity can be arranged so as to become important in regions with large curvature. In this paper we review how a certain simple modification of this sort can resolve the Schwarzschild black-hole and Kasner anisotropic singularities of general relativity. In the corresponding solutions, the fundamental connection field is regular in space-time.

gr-qc↗

arXiv2017

Sourcing Dark Matter and Dark Energy from $α$-attractors

Recently, Kallosh and Linde have drawn attention to a new family of superconformal inflationary potentials, subsequently called $α$-attractors. The $α$-attractor family can interpolate between a large class of inflationary models. It also has an important theoretical underpinning within the framework of supergravity. We demonstrate that the $α$-attractors have an even wider appeal since they may describe dark matter and perhaps even dark energy. The dark matter associated with the $α$-attractors, which we call $α$-dark matter ($α$DM), shares many of the attractive features of fuzzy dark matter, with $V(φ) = \frac{1}{2}m^2φ^2$, while having none of its drawbacks. Like fuzzy dark matter, $α$DM can have a large Jeans length which could resolve the cusp-core and substructure problems faced by standard cold dark matter. $α$DM also has an appealing tracker property which enables it to converge to the late-time dark matter asymptote, $\langle w\rangle \simeq 0$, from a wide range of initial conditions. It thus avoids the enormous fine-tuning problems faced by the $m^2φ^2$ potential in describing dark matter.

gr-qc↗