Evolution Of Cosmological Density Perturbations Research Articles

Temporal evolution of cosmological density perturbations of the Bose–Einstein condensate dark matter

Dark matter is assumed to be composed of scalar boson particles that form Bose–Einstein condensate during the cosmic evolution of the Universe when the temperature of the dark matter is below the critical temperature Tcr\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$T_{cr}$$\\end{document}. At around a redshift z∼1200\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$z\\sim 1200$$\\end{document}, the normal dark matter converted to Bose–Einstein condensate dark matter through a first-order phase transition and continued to complete the condensation process for nearly 106\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$10^6$$\\end{document} years, until then, both phases coexist. In this manuscript, considering Bose–Einstein condensate dark matter as a Gross–Pitaevskii–Poisson system, we study the time evolution of density contrast of Bose–Einstein condensate dark matter using cosmological linear perturbation theory following previous works on the subject. The evolution equation contains quantum pressure due to Heisenberg’s uncertainty principle and self-interaction pressure terms. We solve the temporal density contrast equation of Bose–Einstein condensate dark matter analytically for both the Thomas-Fermi limit and the non-interacting case where the expansion rule follows similarly to that of the Einstein–de Sitter Universe. In addition, we also numerically analyze the temporal nature of the evolution of density contrast of Bose–Einstein condensate dark matter for the complete equation without applying any approximation. We find that the Bose–Einstein condensate model of dark matter could modify the temporal nature of density contrast evolution due to the presence of self-interaction and quantum pressure terms, which shows significant differences with respect to the conventional standard cold dark matter, and successfully solves the small-scale problems in the context of cosmological structure formation.

Relativistic magnetised perturbations: magnetic pressure versus magnetic tension

We study the linear evolution of magnetised cosmological perturbations in the post-recombination epoch. Using full general relativity and adopting the ideal magnetohydrodynamic approximation, we refine and extend the previous treatments. More specifically, this is the first relativistic study that accounts for the effects of the magnetic tension, in addition to those of the field’s pressure. Our solutions show that on sufficiently large scales, larger than the (purely magnetic) Jeans length, the perturbations evolve essentially unaffected by the magnetic presence. The magnetic pressure dominates on small scales, where it forces the perturbations to oscillate and decay. Close to the Jeans length, however, the field’s tension takes over and leads to a weak growth of the inhomogeneities. These solutions clearly demonstrate the opposing action of the aforementioned two magnetic agents, namely of the field’s pressure and tension, on the linear evolution of cosmological density perturbations.

Gravitational instability of the primordial plasma: Anisotropic evolution of structure seeds

We study how the presence of a background magnetic field, of intensity compatible with current observation constraints, affects the linear evolution of cosmological density perturbations at scales below the Hubble radius. The magnetic field provides an additional pressure that can prevent the growth of a given perturbation; however, the magnetic pressure is confined only to the plane orthogonal the field. As a result, the “Jeans length” of the system not only depends on the wavelength of the fluctuation but also on its direction, and the perturbative evolution is anisotropic. We derive this result analytically and back it up with direct numerical integration of the relevant ideal magnetohydrodynamics equations during the matter-dominated era. Before recombination, the kinetic pressure dominates and the perturbations evolve in the standard way, whereas after that time magnetic pressure dominates and we observe the anisotropic evolution. We quantify this effect by estimating the eccentricity ϵ of a Gaussian perturbation in the coordinate space that was spherically symmetric at recombination. For a perturbations at the sub-galactic scale, we find that ϵ=0.7 at z=10 taking the background magnetic field of order 10−9 gauss.

Phase information and the evolution of cosmological density perturbations

The Fourier transform of cosmological density perturbations can be represented in terms of amplitudes and phases for each Fourier mode. We investigate the phase evolution of these modes using a mixture of analytical and numerical techniques. Using a toy model of one-dimensional perturbations evolving under the Zel’dovich approximation as an initial motivation, we develop a statistic that quantifies the information content of the distribution of phases. Using numerical simulations beginning with more realistic Gaussian random-phase initial conditions, we show that the information content of the phases grows from zero in the initial conditions, first slowly and then rapidly when structures become non-linear. This growth of phase information can be expressed in terms of an effective entropy. Gaussian initial conditions are a maximum entropy realization of the initial power spectrum; gravitational evolution decreases the phase entropy. We show that our definition of phase entropy results in a statistic that explicitly quantifies the information stored in the phases of density perturbations (rather than their amplitudes), and that this statistic displays interesting scaling behaviour for self-similar initial conditions.

Gravitational field fluctuations in weakly clustered systems

view Abstract Citations (24) References (30) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Gravitational Field Fluctuations in Weakly Clustered Systems Antonuccio-Delogu, V. ; Atrio-Barandela, F. Abstract Stochastic fluctuations of the gravitational field in a collisionless, weakly clustered protogalactic system are induced by local fluctuations in the number density of small collapsed peaks. We calculate the probability distribution of the stochastic forces generated by these fluctuations, and we find a generalization of the Holtsmark distribution, previously studied in a stellar dynamical context by Chandrasekhar and Von Neumann and by Kandrup. We also find the probability distribution of the torques induced by these stochastic forces, introducing the analog of the Holtsmark law for the torque distribution, and we study different cases corresponding to different power laws. We apply these considerations to realistic models of protogalactic and protocluster density perturbations. The force probability distribution is remarkably influenced by the clustering of substructure: the profile is more strongly peaked, and the asymptotic decay W(F) approaches F^-5/2^ is almost suppressed, resulting in an enhanced probability near the average value. The consequences of this fact on the calculation of the dynamical evolution of cosmological density perturbations, which will be developed in a subsequent paper, are stressed. Publication: The Astrophysical Journal Pub Date: June 1992 DOI: 10.1086/171439 Bibcode: 1992ApJ...392..403A Keywords: Galactic Clusters; Galactic Evolution; Gravitational Fields; Universe; Asymptotic Methods; Chandrasekhar Equation; Dark Matter; Power Spectra; Torque; Astrophysics; COSMOLOGY: LARGE-SCALE STRUCTURE OF UNIVERSE; GALAXIES: CLUSTERING; GALAXIES: FORMATION full text sources ADS |

Evolution of Gauge-Invariant Cosmological Density Perturbations through Decoupling Era

We carefully investigate the evolution of cosmological density perturbations through decoupling era in the baryon-photon universe by a gauge-invariant method. It is found that previous results on the same problem obtained by working in the synchronous gauge are inaccurate particularly on comoving scales greater than the horizon scale at equal time. Further, the anisotropy of microwave background radiation is estimated and the result is found to be in good agreement with previous works qualitatively but in disagreement by factors of a few. Also, an attempt is made to clarify physical processes which determine the evolution of both baryon and radiation density perturbations during decoupling era and the physical interpretation is discussed.