Searcharxiv⌕ Search

arXiv subjects

N. A. Koshelev

Publications and source records attributed to N. A. Koshelev.

7 recordsLinked to original sources
arXiv2016

Effective dark matter fluid with higher derivative corrections

The effective field theory for hydrodynamics allows to write the action functional for fluid. In this paper, some simplest possible higher derivative terms in the fluid action and the cosmological consequences of their presence are considered. Particular attention is given to dark matter, modelled as a dust with higher derivative corrections. We study the conditions of absence of singularities in the solutions of the background and perturbed equations and investigate the evolution of perturbations in two simple models of matter dominated Universe. There is a range of parameters describing the higher derivative terms, in which the short-wavelength perturbations of dark matter are suppressed and the dark matter can be seen as fairly homogeneous on a sufficiently small scale.

gr-qc↗
arXiv2013

Spatial averaging and a non-Gaussianity

The spatial averaging used for the splitting of the local scale factor on the homogeneous background and small inhomogeneous perturbation leads to a non-local relationship between locally and globally defined comoving curvature perturbations. We study this relationship within a quasi-homogeneous, nearly spatially flat domain of the Universe. It is shown that, on scales larger than the size of the observed patch, the Fourier components of the locally defined comoving curvature perturbation are suppressed. We have also shown that the statistical properties of local and global comoving curvature perturbations are coincide on a small scale. Several examples are discussed in detail.

gr-qc↗
arXiv2013

To the study of non-Gaussianity in two-field slow-roll inflation

The general expression for the second order large scale curvature perturbation in the form of a functional over a background solution is derived. The explicit expressions was obtained for two special forms of the inflationary potential. In the considered cases, it is shown that a significant level of non-Gaussianity can be generated during the super-Hubble evolution only if nonadiabatic perturbations are non-negligible at the end of inflation.

astro-ph.CO↗
arXiv2011

Non-adiabatic perturbations in multi-component perfect fluids

The evolution of non-adiabatic perturbations in models with multiple coupled perfect fluids with non-adiabatic sound speed is considered. Instead of splitting the entropy perturbation into relative and intrinsic parts, we introduce a set of symmetric quantities, which also govern the non-adiabatic pressure perturbation in models with energy transfer. We write the gauge invariant equations for the variables that determine on a large scale the non-adiabatic pressure perturbation and the rate of changes of the comoving curvature perturbation. The analysis of evolution of the non-adiabatic pressure perturbation has been made for several particular models.

gr-qc↗
arXiv2010

On the growth of perturbations in interacting dark energy and dark matter fluids

The covariant generalizations of the background dark sector coupling suggested in G. Mangano, G. Miele and V. Pettorino, Mod. Phys. Lett. A 18, 831 (2003) are considered. The evolution of perturbations is studied with detailed attention to interaction rate that is proportional to the product of dark matter and dark energy densities. It is shown that some classes of models with coupling of this type do not suffer from early time instabilities in strong coupling regime.

gr-qc↗
arXiv2005

Adiabatic And Entropy Perturations In Inflationary Models Based On Non-linear Sigma Model

The scalar perturbations in inflationary models, based on a two-component diagonal non-linear sigma model, are considered. For inhomogeneities generated at an inflationary stage, the law of motion of the comoving curvature ${\cal R}$ is obtained (without using the slow roll approximations). A formal expressions for the power spectrum and its spectral index are obtained, which are valid and after the slow-roll stage. As an example, an inflationary model with a massive scalar field and quadratic curvature corrections is studied numerically.

astro-ph↗