arXiv subjects

T. Kahniashvili

Publications and source records attributed to T. Kahniashvili.

7 recordsLinked to original sources

arXiv2021

The timestep constraint in solving the gravitational wave equations sourced by hydromagnetic turbulence

Hydromagnetic turbulence produced during phase transitions in the early universe can be a powerful source of stochastic gravitational waves (GWs). GWs can be modelled by the linearised spatial part of the Einstein equations sourced by the Reynolds and Maxwell stresses. We have implemented two different GW solvers into the {\sc Pencil Code} -- a code which uses a third order timestep and sixth order finite differences. Using direct numerical integration of the GW equations, we study the appearance of a numerical degradation of the GW amplitude at the highest wavenumbers, which depends on the length of the timestep -- even when the Courant--Friedrichs--Lewy condition is ten times below the stability limit. This degradation leads to a numerical error, which is found to scale with the third power of the timestep. A similar degradation is not seen in the magnetic and velocity fields. To mitigate numerical degradation effects, we alternatively use the exact solution of the GW equations under the assumption that the source is constant between subsequent timesteps. This allows us to use a much longer timestep, which cuts the computational cost by a factor of about ten.

physics.flu-dyn↗

arXiv2012

Galaxy cluster number count data constraints on cosmological parameters

[Abridged] We use data on massive galaxy clusters ($M_{\rm cluster} > 8 \times 10^{14} h^{-1} M_\odot$ within a comoving radius of $R_{\rm cluster} = 1.5 h^{-1}\Mpc$) in the redshift range $0.05 \lesssim z \lesssim 0.83$ to place constraints, simultaneously, on the nonrelativistic matter density parameter $Ω_m$, on the amplitude of mass fluctuations $σ_8$, on the index $n$ of the power-law spectrum of the density perturbations, and on the Hubble constant $H_0$, as well as on the equation-of-state parameters $(w_0,w_a)$ of a smooth dark energy component. For the first time, we properly take into account the dependence on redshift and cosmology of the quantities related to cluster physics: the critical density contrast, the growth factor, the mass conversion factor, the virial overdensity, the virial radius and, most importantly, the cluster number count derived from the observational temperature data. We show that, contrary to previous analyses, cluster data alone prefer low values of the amplitude of mass fluctuations, $σ_8 \leq 0.69 (1σC.L.)$, and large amounts of nonrelativistic matter, $Ω_m \geq 0.38 (1σC.L.)$, in slight tension with the $Λ$CDM concordance cosmological model, though the results are compatible with $Λ$CDM at $2σ$. In addition, we derive a $σ_8$ normalization relation, $σ_8 Ω_m^{1/3} = 0.49 \pm 0.06 (2σC.L.)$.

astro-ph.CO↗

arXiv2011

Mass Varying Neutrinos, Quintessence, and the Accelerating Expansion of the Universe

We analyze the Mass Varying Neutrino (MaVaN) scenario. We consider a minimal model of massless Dirac fermions coupled to a scalar field, mainly in the framework of finite temperature quantum field theory. We demonstrate that the mass equation we find has non-trivial solutions only for special classes of potentials, and only within certain temperature intervals. We give most of our results for the Ratra-Peebles Dark Energy (DE) potential. The thermal (temporal) evolution of the model is analyzed. Following the time arrow, the stable, metastable and unstable phases are predicted. The model predicts that the present Universe is below its critical temperature and accelerates. At the critical point the Universe undergoes a first-order phase transition from the (meta)stable oscillatory regime to the unstable rolling regime of the DE field. This conclusion agrees with the original idea of quintessence as a force making the Universe roll towards its true vacuum with zero Λ-term. The present MaVaN scenario is free from the coincidence problem, since both the DE density and the neutrino mass are determined by the scale M of the potential. Choosing M ~ 10^{-3} eV to match the present DE density, we can obtain the present neutrino mass in the range m ~ 10^{-2}-1 eV and consistent estimates for other parameters of the Universe.

astro-ph.CO↗

arXiv2002

Abundance and evolution of galaxy clusters in cosmological models with massive neutrino

The time evolution of the number density of galaxy clusters and their mass and temperature functions are used to constrain cosmological parameters in the spatially flat dark matter models containing a fraction of hot particles (massive neutrino) additional to cold and baryonic matter. We test the modified MDM models with cosmic gravitational waves and show that they neither pass the cluster evolution test nor reproduce the observed height of the first acoustic peak in $ΔT/T$ spectrum, and therefore should be ruled out. The models with a non-zero cosmological constant are in better agreement with observations. We estimate the free cosmological parameters in $Λ$MDM with a negligible abundance of gravitational waves, and find that within the parameter ranges $h\in (0.6, 0.7)$, $n\in (0.9, 1.1)$, (i) the value of $Ω_Λ$ is strongly affected by a small fraction of hot dark matter, $f_ν\equivΩ_ν/Ω_m\in (0, 0.2)$: $0.45 <Ω_Λ<0.7$ ($1σ$ CL), and (ii) the redshift evolution of galaxy clusters alone reveals the following explicit correlation between $Ω_Λ$ and $f_ν$: $Ω_Λ+0.5f_ν=0.65\pm 0.1$. The present accuracy of observational data allows only to bound the fraction of hot matter, $f_ν\in (0, 0.2)$ (the number of massive neutrino species remains undelimited, $N_ν=1, 2, 3$).

astro-ph↗

arXiv2001

Evolution of galaxy clusters in $Λ$MDM cosmologies

The time evolution of galaxy cluster abundance is used to constrain cosmological parameters in dark matter models containing a fraction of hot particles (massive neutrino). We test the modified MDM models with cosmic gravitational waves which are in agreement with observational data at $z=0$, and show that they do not pass the cluster evolution test and therefore should be ruled out. The models with a non-zero cosmological constant are in better agreement with the evolution test. We estimate $Ω_Λ$ and find that it is strongly affected by a small fraction of hot dark matter: $0.4 <Ω_Λ<0.8$ for $Ω_H /Ω_M <0.2$.

astro-ph↗

arXiv1999

Tensor Microwave Anisotropies from a Stochastic Magnetic Field

We derive an expression for the angular power spectrum of cosmic microwave background anisotropies due to gravity waves generated by a stochastic magnetic field and compare the result with current observations; we take into account the non-linear nature of the stress energy tensor of the magnetic field. For almost scale invariant spectra, the amplitude of the magnetic field at galactic scales is constrained to be of order 10^{-9} Gauss. If we assume that the magnetic field is damped below the Alfven damping scale, we find that its amplitude at 0.1 h^{-1}Mpc, B_λ, is constrained to be B_λ<7.9 x10^{-6} e^{3n} Gauss, for n<-3/2, and B_λ<9.5x10^{-8} e^{0.37n} Gauss, for n>-3/2, where n is the spectral index of the magnetic field and H_0=100h km s^{-1}Mpc^{-1} is the Hubble constant today.

astro-ph↗

arXiv1998

Large scale structure formation in mixed dark matter models with a cosmological constant

We study linear power spectra and formation of large scale structures in flat cosmological models with $Λ\ge 0$ and cold plus hot dark matter components (MLM). The hot component consists of massive neutrinos with cosmological density $Ω_H$. The linearized Einstein-Boltzmann equations for the evolution of the metric and density perturbations are integrated for a set of values of the cosmological parameters. For all the considered models we assume a scale-invariant primeval spectrum. The density weighted final linear power spectra are normalized to the four year COBE data and have been used to constrain the parameter space by a comparison of linear predictions with the current observational data on large scales. The consistency of MLM predictions with the observable data set is best obtained for models with one specie of massive neutrinos and $Ω_H/Ω_M \le0.2$. For this class of MLM models we obtain constraints from linear data on the present matter density. Consistency with the estimated cluster abundance can be achieved for COBE normalized MLM models with $Ω_H/Ω_M\le0.2$ and $ 0.45 \le Ω_M \le 0.75$ for $h=0.5$. If $h=0.7$ then $ 0.3 \le Ω_M \le 0.5$. These constraints are at $1σ$ level and standard MDM models are clearly ruled out. We note that the range of allowed values for $Ω_M$, that we obtain for MLM models from linear analysis, is also approximately the same range that is needed in order to consistently satisfy a variety of independent observational constraints.

astro-ph↗