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A. A. Starobinsky

Publications and source records attributed to A. A. Starobinsky.

At least 19 recordsLinked to original sources
arXiv2024

Hard X-ray emission from blazars associated with high-energy neutrinos

Bright blazars were found to be prominent neutrino sources, and a number of IceCube events were associated with them. Evaluating high-energy photon emission of such blazars is crucial for better understanding of the processes and regions where neutrinos are produced. Here, we focus on hard X-ray emission observed by the SRG/ART-XC telescope, by the Swift/BAT imager, and by the INTEGRAL/IBIS telescope. Their energy range ~10 keV is well-suited for probing photons that potentially participate in neutrino production by interacting with ultrarelativistic protons. We find that neutrino-associated blazars tend to demonstrate remarkably strong X-ray emission compared to other VLBI blazars in the sky. Both neutrinos and hard X-rays are found to come from blazars at cosmological distances z ~ 1, and are boosted by relativistic beaming that makes it possible to detect them on Earth. Our results suggest that neutrinos are produced within compact blazar jets, with target X-ray photons emitted from accelerated jet regions.

astro-ph.HE↗
arXiv2024

Optical Identification and Spectroscopic Redshift Measurements of 216 Galaxy Clusters from the SRG/eROSITA All-Sky Survey

We present the results of the optical identification and spectroscopic redshift measurements of 216 galaxy clusters detected in the SRG/eROSITA all-sky X-ray survey. The spectroscopic observations were performed in 2020-2023 with the 6-m BTA telescope at the Special Astrophysical Observatory of the Russian Academy of Sciences, the 2.5-m telescope at the Caucasus Mountain Observatory of the Sternberg Astronomical Institute of the Moscow State University, the 1.6-m AZT-33IK telescope at the Sayan Solar Observatory of the Institute of Solar-Terrestrial Physics of the Siberian Branch of the Russian Academy of Sciences, and the 1.5-m Russian-Turkish telescope (RTT-150) at the TÜBİTAK Observatory. For all of the galaxy clusters presented here the spectroscopic redshift measurements have been obtained for the first time. Of these, 139 galaxy clusters have been detected for the first time in the SRG/eROSITA survey and 22 galaxy clusters are at redshifts $z_{spec} \gtrsim 0.7$, including three at $z_{spec} \gtrsim 1$. Deep direct images with the rizJK filters have also been obtained for four distant galaxy clusters at $z_{spec} > 0.7$. For these observations the most massive clusters are selected. Therefore, most of the galaxy clusters presented here most likely will be included in the cosmological samples of galaxy clusters from the SRG/eROSITA survey.

astro-ph.GA↗
arXiv2024

X-Ray Variability of SDSS Quasars Based on the SRG/eROSITA All-Sky Survey

We examine the long-term (rest-frame time scales from a few months to $\sim 20$ years) X-ray variability of a sample of 2344 X-ray bright quasars from the SDSS DR14Q Catalogue, based on the data of the SRG/eROSITA All-Sky Survey complemented for $\sim 7$% of the sample by archival data from the XMM-Newton Serendipitous Source Catalogue. We characterise variability by a structure function, $SF^2(Δt)$. We confirm the previously known anti-correlation of the X-ray variability amplitude with luminosity. We also study the dependence of X-ray variability on black hole mass, $M_{\rm BH}$, and on an X-ray based proxy of the Eddington ratio, $λ_{\rm X}$. Less massive black holes prove to be more variable for given Eddington ratio and time scale. X-ray variability also grows with decreasing Eddington ratio and becomes particularly strong at $λ_{\rm X}$ of less than a few per cent. We confirm that the X-ray variability amplitude increases with increasing time scale. The $SF^2(Δt)$ dependence can be satisfactorily described by a power law, with the slope ranging from $\sim 0$ to $\sim 0.4$ for different ($M_{\rm BH}$, $λ_{\rm X}$) subsamples (except for the subsample with the lowest black hole mass and lowest Eddington ratio, where it is equal to $1.1\pm 0.4$)

astro-ph.HE↗
arXiv2023

X-ray surface brightness and gas density profiles of galaxy clusters up to 3*R500c with SRG/eROSITA

Using the data of the SRG/eROSITA all-sky survey, we stacked a sample of ~40 galaxy cluster images in the 0.3--2.3 keV band, covering the radial range up to $10\times R_{\rm 500c}$. The excess emission on top of the galactic and extragalactic X-ray backgrounds and foregrounds is detected up to $\sim 3\times R_{\rm 500c}$. At these distances, the surface brightness of the stacked image drops below $\sim 1$% of the background. The density profile reconstructed from the X-ray surface brightness profile agrees well (within $\sim30$%) with the mean gas profile found in numerical simulations, which predict the local gas overdensity of $\sim$ 20--30 at $3\times R_{\rm 500c}$ and the gas fraction close to the universal value of $\frac{Ω_b}{Ω_m}\approx 0.15$ in the standard $Λ$CDM model. Taking at face value, this agreement suggests that up to $\sim 3\times R_{\rm 500c}$ the X-ray signal is not strongly boosted by the gas clumpiness, although a scenario with a moderately inhomogeneous gas cannot be excluded. A comparison of the derived gas density profile with the electron pressure profile based on the SZ measurements suggests that by $r\sim 3\times R_{\rm 500c}$ the gas temperature drops by a factor of $\sim$ 4--5 below the characteristic temperature of a typical cluster in the sample within $R_{\rm 500c}$, while the entropy keeps growing up to this distance. Better constraints on the gas properties just beyond $3\times R_{\rm 500c}$ should be possible with a sample larger than used for this pilot study.

astro-ph.CO↗
arXiv2022

Joint reconstructions of growth and expansion histories from stage-IV surveys with minimal assumptions I: Dark Energy beyond $Λ$

Combining Supernovae, Baryon Acoustic Oscillations and Redshift-Space Distortions data from the next generation of (Stage-IV) cosmological surveys, we aim to reconstruct the expansion history up to large redshifts using forward-modeling of $f_{\mathrm DE}(z) = ρ_\mathrm{DE}(z)/ρ_\mathrm{DE,0}$ with Gaussian processes (GP). In order to reconstruct cosmological quantities at high redshifts where few or no data are available, we adopt a new approach to GP which enforces the following minimal assumptions: a) Our cosmology corresponds to a flat Friedman-Lemaître-Robertson-Walker (FLRW) universe; b) An Einstein de Sitter (EdS) universe is obtained on large redshifts. This allows us to reconstruct the perturbations growth history from the reconstructed background expansion history. Assuming various DE models, we show the ability of our reconstruction method to differentiate them from $Λ$CDM at $\gtrsim2σ$.

astro-ph.CO↗
arXiv2022

Mass estimation of the very massive galaxy cluster SRGe CL2305.2$-$2248 from strong lensing

The galaxy cluster SRGe CL2305.2$-$2248 (SPT-CL J2305$-$2248, ACT-CL J2305.1$-$2248) is one of the most massive clusters at high redshifts ($z \simeq 0.76$) and is of great interest for cosmology. For an optical identification of this cluster, deep images were obtained with the 1.5-m Russian-Turkish telescope RTT-150. Together with the open archival data of the Hubble Space Telescope, it became possible to identify candidates for gravitationally lensed images of distant blue galaxies in the form of arcs and arclets. The observed giant arc near the brightest cluster galaxies allowed us to estimate the radius of the Einstein ring, which is $ 9.8 \pm 1.3 $ arcseconds. The photometric redshift of the lensed source was obtained ($ z_s = 2.44 \pm 0.07 $). Its use in combination with the Einstein radius estimate made it possible to independently estimate the \cl2305 mass. It was done by extrapolating the strong lensing results to large radii and using the model density distribution profiles in relaxed clusters. This extrapolation leads to mass estimates $ \sim 1.5-3 $ times smaller than those obtained from X-ray and microwave observations. A probable cause for this discrepancy may be the process of cluster merging, which is also confirmed by SRGe CL2305.2-2248 morphology in the optical range.

astro-ph.CO↗
arXiv2022

Bouncing Cosmological Isotropic Solutions in Scalar-Tensor Gravity

Bouncing non-singular isotropic cosmological solutions are investigated in a simple model of scalar-tensor gravity. New families of such solutions are found and their properties are presented and analyzed using an effective potential as the main tool. Bouncing solutions are shown to exist for a Higgs-like self-interaction potential which is bounded from below, in contrast to previous solutions that appeared in the literature based on potentials which were unbounded from below. In the simplest version of a scalar field with the quartic potential and conformal coupling to gravity, bouncing spatially flat solutions either have the Hubble function diverging in the past before the bounce, but with a well-behaved future, or are globally regular but unstable with respect to anisotropic or inhomogeneous perturbations at some finite values of the scalar field and curvature. Regular solutions can only exist in the part of the parameter space where the maximum of the effective potential is larger than the first zero of the potential, and gravity becomes repulsive at the bounce.

gr-qc↗
arXiv2022

Energy-momentum tensor and helicity for gauge fields coupled to a pseudo-scalar inflaton

We study the energy-momentum tensor and helicity of gauge fields coupled through $g ϕF \tilde{F}/4$ to a pseudo-scalar field $ϕ$ driving inflation. Under the assumption of a constant time derivative of the background inflaton, we compute analitically divergent and finite terms of the energy density and helicity of gauge fields for any value of the coupling $g$. We introduce a suitable adiabatic expansion for mode functions of physical states of the gauge fields which correctly reproduces ultraviolet divergences in average quantities and identify corresponding counterterms. Our calculations shed light on the accuracy and the range of validity of approximated analytic estimates of the energy density and helicity terms previously existed in the literature in the strongly coupled regime only, i.e. for $g \dot ϕ/(2H) \gg 1$. We discuss the implications of our analytic calculations for the backreaction of quantum fluctuations onto the inflaton evolution.

gr-qc↗
arXiv2021

Observation of a very massive galaxy cluster at z=0.76 in SRG/eROSITA all-sky survey

The results of multiwavelength observations of the very massive galaxy cluster SRGe CL2305.2-2248 detected in X-rays during the first SRG/eROSITA all-sky survey are discussed. This galaxy cluster was also detected earlier in microwave band through the observations of Sunyaev-Zeldovich effect in South Pole Telescope (SPT-CL J2305-2248), and in Atacama Cosmological Telescope (ACT-CL J2305.1-2248) surveys. Spectroscopic redshift measurement, $z=0.7573$, was measured at the Russian 6-m BTA telescope of SAO RAS, in good agreement with its photometric estimates, including a very accurate one obtained using machine learning methods. In addition, deep photometric measurements were made at the Russian-Turkish 1.5-m telescope (RTT150), which allows to study cluster galaxies red sequence and projected galaxies distribution. Joint analysis of the data from X-ray and microwave observations show that this cluster can be identified as a very massive and distant one using the measurements of its X-ray flux and integral comptonization parameter only. The mass of the cluster estimated according to the eROSITA data is $M_{500}=(9.0\pm2.6)\cdot10^{14}\, M_\odot$. We show that this cluster is found among of only several dozen of the most massive clusters in the observable Universe and among of only a few the most massive clusters of galaxies at $z>0.6$.

astro-ph.CO↗
arXiv2020

Global properties of the growth index: mathematical aspects and physical relevance

We analyze the global behaviour of the growth index of cosmic inhomogeneities in an isotropic homogeneous universe filled by cold non-relativistic matter and dark energy (DE) with an arbitrary equation of state. Using a dynamical system approach, we find the critical points of the system. That unique trajectory for which the growth index $γ$ is finite from the asymptotic past to the asymptotic future is identified as the so-called heteroclinic orbit connecting the critical points $(Ω_m=0,~γ_{\infty})$ in the future and $(Ω_m=1,~γ_{-\infty})$ in the past. The first is an attractor while the second is a saddle point, confirming our earlier results. Further, in the case when a fraction of matter (or DE tracking matter) $\varepsilon Ω^{\rm tot}_m$ remains unclustered, we find that the limit of the growth index in the past $γ_{-\infty}^{\varepsilon}$ does not depend on the equation of state of DE, in sharp contrast with the case $\varepsilon=0$ (for which $γ_{-\infty}$ is obtained). We show indeed that there is a mathematical discontinuity: one cannot obtain $γ_{-\infty}$ by taking $\lim_{\varepsilon \to 0} γ^{\varepsilon}_{-\infty}$ (i.e. the limits $\varepsilon\to 0$ and $Ω^{\rm tot}_m\to 1$ do not commute). We recover in our analysis that the value $γ_{-\infty}^{\varepsilon}$ corresponds to tracking DE in the asymptotic past with constant $γ=γ_{-\infty}^{\varepsilon}$ found earlier.

astro-ph.CO↗
arXiv2020

Cosmological constraints on post-Newtonian parameters in effectively massless scalar-tensor theories of gravity

We study the cosmological constraints on the variation of the Newton's constant and on post-Newtonian parameters for simple models of scalar-tensor theory of gravity beyond the extended Jordan-Brans-Dicke theory. We restrict ourselves to an effectively massless scalar field with a potential $V \propto F^2$, where $F(σ)=N_{pl}^2+ξσ^2$ is the coupling to the Ricci scalar considered. We derive the theoretical predictions for cosmic microwave background (CMB) anisotropies and matter power spectra by requiring that the effective gravitational strength at present is compatible with the one measured in a Cavendish-like experiment and by assuming adiabatic initial condition for scalar fluctuations. When comparing these models with $Planck$ 2015 and a compilation of baryonic acoustic oscilation (BAO) data, all these models accomodate a marginalized value for $H_0$ higher than in $Λ$CDM. We find no evidence for a statistically significant deviation from Einstein's general relativity. We find $ξ< 0.064$ ($|ξ| < 0.011$) at 95 % CL for $ξ> 0$ (for $ξ< 0$, $ξ\ne -1/6$). In terms of post-Newtonian parameters, we find $0.995 < γ_{\rm PN} < 1$ and $0.99987 < β_{\rm PN} < 1$ ($0.997 < γ_{\rm PN} < 1$ and $1 < β_{\rm PN} < 1.000011$) for $ξ>0$ (for $ξ< 0$). For the particular case of the conformal coupling, i.e. $ξ=-1/6$, we find constraints on the post-Newtonian parameters of similar precision to those within the Solar System.

astro-ph.CO↗
arXiv2019

Quasi-isotropic expansion for a two-fluid cosmological model containing radiation and string gas

The quasi-isotropic expansion for a simple two-fluid cosmological model, including radiation and string gas is constructed. The first non-trivial order expressions for the metric coefficients, energy densities and velocities are explicitly written down. Their small and large time asymptotics are studied. It is found that the large time asymptotic for the anisotropic component of the metric coefficients grows faster than that of the isotropic (trace-proportional) component.

astro-ph.CO↗
arXiv2019

Global properties of the growth index of matter inhomogeneities in the universe

We perform here a global analysis of the growth index $γ$ behaviour from deep in the matter era till the far future. For a given cosmological model in GR or in modified gravity, the value of $γ(Ω_{m})$ is unique when the decaying mode of scalar perturbations is negligible. However, $γ_{\infty}$, the value of $γ$ in the asymptotic future, is unique even in the presence of a nonnegligible decaying mode today. Moreover $γ$ becomes arbitrarily large deep in the matter era. Only in the limit of a vanishing decaying mode do we get a finite $γ$, from the past to the future in this case. We find further a condition for $γ(Ω_{m})$ to be monotonically decreasing (or increasing). This condition can be violated inside general relativity (GR) for varying $w_{DE}$ though generically $γ(Ω_{m})$ will be monotonically decreasing (like $Λ$CDM), except in the far future and past. A bump or a dip in $G_{\rm eff}$ can also lead to a significant and rapid change in the slope $\frac{dγ}{dΩ_{m}}$. On a $Λ$CDM background, a $γ$ substantially lower (higher) than $0.55$ with a negative (positive) slope reflects the opposite evolution of $G_{\rm eff}$. In DGP models, $γ(Ω_{m})$ is monotonically increasing except in the far future. While DGP gravity becomes weaker than GR in the future and $w^{DGP}\to -1$, we still get $γ_{\infty}^{DGP}= γ_{\infty}^{ΛCDM}=\frac{2}{3}$. In contrast, despite $G^{DGP}_{\rm eff}\to G$ in the past, $γ$ does not tend to its value in GR because $\frac{dG^{DGP}_{\rm eff}}{dΩ_{m}}\Big|_{-\infty}\ne 0$.

astro-ph.CO↗
arXiv2019

Spectral Distortions of the CMB as a Probe of Inflation, Recombination, Structure Formation and Particle Physics

Following the pioneering observations with COBE in the early 1990s, studies of the cosmic microwave background (CMB) have focused on temperature and polarization anisotropies. CMB spectral distortions - tiny departures of the CMB energy spectrum from that of a perfect blackbody - provide a second, independent probe of fundamental physics, with a reach deep into the primordial Universe. The theoretical foundation of spectral distortions has seen major advances in recent years, which highlight the immense potential of this emerging field. Spectral distortions probe a fundamental property of the Universe - its thermal history - thereby providing additional insight into processes within the cosmological standard model (CSM) as well as new physics beyond. Spectral distortions are an important tool for understanding inflation and the nature of dark matter. They shed new light on the physics of recombination and reionization, both prominent stages in the evolution of our Universe, and furnish critical information on baryonic feedback processes, in addition to probing primordial correlation functions at scales inaccessible to other tracers. In principle the range of signals is vast: many orders of magnitude of discovery space could be explored by detailed observations of the CMB energy spectrum. Several CSM signals are predicted and provide clear experimental targets, some of which are already observable with present-day technology. Confirmation of these signals would extend the reach of the CSM by orders of magnitude in physical scale as the Universe evolves from the initial stages to its present form. The absence of these signals would pose a huge theoretical challenge, immediately pointing to new physics.

astro-ph.CO↗
arXiv2018

Pauli-Zeldovich cancellation of the vacuum energy divergences, auxiliary fields and supersymmetry

We have considered the Pauli-Zeldovich mechanism for the cancellation of the ultraviolet divergences in vacuum energy. This mechanism arises because bosons and fermions give contributions of the opposite signs. In contrast with the preceding papers devoted to this topic wherein mainly free fields were studied, here we have taken their interactions into account to the lowest order of perturbation theory. We have constructed some simple toy models having particles with spin 0 and spin 1/2, where masses of the particles are equal while the interactions can be quite non-trivial.

hep-th↗
arXiv2015

Bouncing Universes in Scalar-Tensor Gravity Models admitting Negative Potentials

We consider the possibility to produce a bouncing universe in the framework of scalar-tensor gravity models in which the scalar field potential may be negative, and even unbounded from below. We find a set of viable solutions with nonzero measure in the space of initial conditions passing a bounce, even in the presence of a radiation component, and approaching a constant gravitational coupling afterwards. Hence we have a model with a minimal modification of gravity in order to produce a bounce in the early universe with gravity tending dynamically to general relativity (GR) after the bounce.

gr-qc↗
arXiv2015

Cosmology based on $f(R)$ gravity with ${\cal O}(1)$ eV sterile neutrino

We address the cosmological role of an additional ${\cal O}(1)$ eV sterile neutrino in modified gravity models. We confront the present cosmological data with predictions of the FLRW cosmological model based on a variant of $f(R)$ modified gravity proposed by one of the authors previously. This viable cosmological model which deviation from general relativity with a cosmological constant $Λ$ decreases as $R^{-2n}$ for large, but not too large values of the Ricci scalar $R$ provides an alternative explanation of present dark energy and the accelerated expansion of the Universe. Various up-to-date cosmological data sets exploited include Planck CMB anisotropy, CMB lensing potential, BAO, cluster mass function and Hubble constant measurements. We find that the CMB+BAO constraints strongly the sum of neutrino masses from above. This excludes values $λ\sim 1$ for which distinctive cosmological features of the model are mostly pronounced as compared to the $Λ$CDM model, since then free streaming damping of perturbations due to neutrino rest masses is not sufficient to compensate their extra growth occurring in $f(R)$ gravity. Thus, we obtain $λ>8.2$ ($2σ$) with cluster systematics and $λ>9.4$ ($2σ$) without that. In the latter case we find for the sterile neutrino mass $0.47\,\,\rm{eV}$$\,<\,$$m_{ν,\,\rm{sterile}}$$\,<\,$$1\,\,\rm{eV}$ ($2σ$) assuming the active neutrinos are massless, not significantly larger than in the standard $Λ$CDM with the same data set: $0.45\,\,\rm{eV}$$\,<\,$$m_{ν,\,\rm{sterile}}$$\,<\,$$0.92\,\,\rm{eV}$ ($2σ$). However, a possible discovery of a sterile neutrino with the mass $m_{ν,\,\rm{sterile}} \approx 1.5\,$eV motivated by various anomalies in neutrino oscillation experiments would favor cosmology based on $f(R)$ gravity rather than the $Λ$CDM model.

astro-ph.CO↗