Dark Matter Perturbations Research Articles

Parameterizing the effect of dark energy perturbations on the growth of structures

We present an analytical fit to the growth function of the dark matter perturbations when dark energy perturbations are present. The growth index γ depends upon the dark energy equation of state w, the speed of sound of the dark energy fluctuations, the dark matter abundance and the observed comoving scale. The growth index changes by O(5%) for small speed of sound and large deviations of w from −1 with respect to its value in the limit of no dark energy perturbations.

Induced Nested Galactic Bars inside Assembling Dark Matter Halos

We investigate the formation and evolution of nested bar systems in disk galaxies in a cosmological setting by following the development of an isolated dark matter (DM) and baryon density perturbation. The disks form within the assembling triaxial DM halos, and the feedback from the stellar evolution is accounted for in terms of supernovae and OB stellar winds. Focusing on a representative model, we show the formation of an oval disk and of a first generation of nested bars with characteristic subkiloparsec sizes and a few kiloparsec sizes. The system evolves through successive dynamical couplings and decouplings, forcing the gas inward, and settles in a state of resonant coupling. The inflow rate can support a broad range of activity within the central kiloparsec, from quasar to Seyfert types, supplemented by vigorous star formation as a by-product. The initial bar formation is triggered in response to the tidal torques from the triaxial DM halo, which acts as a finite perturbation. This first generation of bars does not survive for more than 4-5 Gyr; by that time, the secondary bar has totally dissolved, while the primary one has very substantially weakened, reduced to a fat oval. This evolution is largely due to chaos introduced by the interaction of the multiple nonaxisymmetric components.

Gravity of cosmological perturbations in the CMB

First, we establish which measures of large-scale perturbations are least afflicted by gauge artifacts and directly map the apparent evolution of inhomogeneities to local interactions of cosmological species. Considering nonlinear and linear perturbations of phase-space distribution, radiation intensity and arbitrary species' density, we require that: (i) the dynamics of perturbations defined by these measures is determined by observables within the local Hubble volume; (ii) the measures are practically applicable on microscopic scales and in an unperturbed geometry retain their microscopic meaning on all scales. We prove that all measures of linear overdensity that satisfy (i) and (ii) coincide in the superhorizon limit. Their dynamical equations are simpler than the traditional ones, have a nonsingular superhorizon limit and explicit Cauchy form. Then we show that, contrary to the popular view, the perturbations of the cosmic microwave background (CMB) in the radiation era are not resonantly boosted self-gravitationally during horizon entry. (Consequently, the CMB signatures of uncoupled species which may be abundant in the radiation era, e.g. neutrinos or early quintessence, are mild; albeit non-degenerate and robust to cosmic variance.) On the other hand, dark matter perturbations in the matter era gravitationally suppress large-angle CMB anisotropy by an order of magnitude stronger than presently believed. If cold dark matter were the only dominant component then, for adiabatic perturbations, the CMB temperature power spectrum C_l would be suppressed 25-fold.

Production and evolution of perturbations of sterile neutrino dark matter

Sterile neutrinos, fermions with no standard model couplings [SU(2) singlets], are predicted by most extensions of the standard model, and may be the dark matter. I describe the nonthermal production and linear perturbation evolution in the early universe of this dark matter candidate. I calculate production of sterile neutrino dark matter including effects of Friedmann dynamics dictated by the quark-hadron transition and particle population, the alteration of finite temperature effective mass of active neutrinos due to the presence of thermal leptons, and heating of the coupled species due to the disappearance of degrees of freedom in the plasma. These effects leave the sterile neutrinos with a nontrivial momentum distribution. I also calculate the evolution of sterile neutrino density perturbations in the early universe through the linear regime and provide a fitting function form for the transfer function describing the suppression of small-scale fluctuations for this warm dark matter candidate. The results presented here differ quantitatively from previous work due to the inclusion of the relevant physical effects during the production epoch.

Growth of perturbations in dark matter coupled with quintessence

We consider the evolution of linear perturbations in models with a nonminimal coupling between dark matter and scalar field dark energy. Growth of matter inhomogeneities in two examples of such models proposed in the literature are investigated in detail. Both of these models are based on a low-energy limit of effective string theory action, and have been previously shown to naturally lead to late acceleration of the universe. However, we find that these models can be ruled out by taking properly into account the impact of the scalar field coupling on the formation of structure in the dark matter density. In particular, when the transition to acceleration in these models begins, the interaction with dark energy enchances the small scale clustering in dark matter much too strongly. We discuss the the role of an effective small scale sound speed in such models with a coupled dark sector.

Analytical perfect fluid perturbation for the matter dominated epoch

We present the analytical solution to the linear evolution equation of a one component Friedmann perturbation using an equation of state of the form p = (1/3)μσ2(t), where μ is the mass density and σ(t) is the root mean square (rms) velocity in the matter dominated epoch. It is assumed that this rms velocity depends only on the time coordinate and decreases as 1/a, a being the expansion factor of the Friedmann background. The evolution equations are written for scales below the horizon using the longitudinal gauge. The general solution, in the coordinate space, of the evolution equation for the scalar mode is obtained. In the case of spherical symmetry, this solution is expressed in terms of unidimensional integrals of the initial conditions: the initial values of the Newtonian potential and its first time derivative. This perfect fluid solution is a good approximation to the evolution of warm dark matter perturbations obtained by solving the Vlasov’s equation for collisionless particles.

Leptogenesis as the source of gravitino dark matter and density perturbations

We investigate the possibility that the entropy producing decay of a right-handed sneutrino condensate can simultaneously be the source of the baryon asymmetry, of gravitino dark matter, and of cosmological density perturbations. For generic values of soft supersymmetry breaking terms in the visible sector of 1--10 TeV, condensate decay can yield the dark matter abundance for gravitinos in the mass range 1 MeV to 1 TeV, provided that the resulting reheat temperature is below ${10}^{6}\text{ }\text{ }\mathrm{G}\mathrm{e}\mathrm{V}$. The abundance of thermally produced gravitinos before and after sneutrino decay is then negligible. We consider different leptogenesis mechanisms to generate a sufficient asymmetry, and find that low-scale soft leptogenesis works most naturally at such temperatures. The condensate can easily generate sufficient density perturbations if its initial amplitude is $\ensuremath{\sim}\mathcal{O}({M}_{\mathrm{G}\mathrm{U}\mathrm{T}})$, for a Hubble expansion rate during inflation $>{10}^{9}\text{ }\text{ }\mathrm{G}\mathrm{e}\mathrm{V}$. Right-handed sneutrinos may therefore at the same time provide a source for baryogenesis, dark matter, and the seed of structure formation.

Bounds on cold dark matter and neutrino isocurvature perturbations from CMB and LSS data

Early universe models for the origin of structure typically produce a spectrum of initial fluctuations with a mixture of adiabatic and isocurvature perturbations. Using the observed anisotropies of the cosmic microwave backgound, the matter power spectra from large scale structure surveys and the luminosity distance vs. redshift relation from supernovae of type Ia, we obtain strong bounds on the possible cold dark matter/baryon as well as neutrino isocurvature contributions to the primordial fluctations in the Universe. Neglecting the possible effects of spatial curvature and tensor perturbations, we perform a Bayesian likelihood analysis with 13 free parameters, including independent spectral indexes for each of the modes and for their cross-correlation angle. We find that around a pivot wave number of $k=0.05h\text{ }\text{ }{\mathrm{M}\mathrm{p}\mathrm{c}}^{\ensuremath{-}1}$ the amplitude of the correlated isocurvature component cannot be larger than about 60% for the cold dark matter mode, 40% for the neutrino density mode, and 30% for the neutrino velocity mode, at two sigma. In the first case, our bound is larger than the WMAP first-year result, presumably because we prefer not to include any data from Lyman-$\ensuremath{\alpha}$ forests, but then obtain large blue spectral indexes for the nonadiabatic contributions. We also translate our bounds in terms of constraints on double inflation models with two uncoupled massive fields.

Probing dark energy perturbations: The dark energy equation of state and speed of sound as measured by WMAP

We review the implications of having a nontrivial matter component in the Universe and the potential for detecting such a component through the matter power spectrum and integrated Sachs-Wolfe effect. We adopt a phenomenological approach and consider the mysterious dark energy to be a cosmic fluid. It is thus fully characterized, up to linear order, by its equation of state and its speed of sound. Whereas the equation of state has been widely studied in the literature, less interest has been devoted to the speed of sound. Its observational consequences come predominantly from very large scale modes of dark matter perturbations $(kl0.01h{\mathrm{Mpc}}^{\ensuremath{-}1}).$ Since these modes have hardly been probed so far by large scale galaxy surveys, we investigate whether joint constraints can be placed on those two quantities using the recent cosmic microwave background (CMB) fluctuations measurements by the Wilkinson Microwave Anisotropy Probe as well as the recently measured CMB large scale structure cross correlation. We find only a tentative 1 sigma detection of the speed of sound, from CMB alone, ${c}_{s}^{2}l0.04$ at this low significance level. Furthermore, the current uncertainties in bias in the matter power spectrum preclude any constraints being placed using the cross correlation of CMB with the NRAO VLA Sky Survey radio survey.

Non-linear gravitational clustering in scalar field cosmologies

Non-linear gravitational clustering in a universe dominated by darkenergy, modelled by a `quintessence' scalar field, and cold dark matterwith spacetime varying mass is studied. Models of this type, wherethe variable mass is induced by dependence on the scalarfield, as suggested by string theory or extra dimensions,have been proposed as a viable solution of the coincidence problem.A general framework for the study of the non-linear phases ofstructure formation in scalar field cosmologies is provided, starting from a general relativistic treatment of the combined dark matter–dark energy system. As a first application, the mildly non-linear evolution of dark matterperturbations is obtained by a straightforward extension of the Zel'dovich approximation. We argue that the dark energy fluctuations may play an active role in cosmological structure formation if the scalar field effective potential develops a temporary spinodal instability during theevolution.

Interacting dark matter disguised as warm dark matter

We explore some of the consequences of dark-matter--photon interactions on structure formation, focusing on the evolution of cosmological perturbations and performing both an analytical and a numerical study. We compute the cosmic microwave background anisotropies and matter power spectrum in this class of models. We find, as the main result, that when dark matter and photons are coupled, dark matter perturbations can experience a new damping regime in addition to the usual collisional Silk damping effect. Such dark matter particles (having quite large photon interactions) behave like cold dark matter or warm dark matter as far as the cosmic microwave background anisotropies or matter power spectrum are concerned, respectively. These dark-matter--photon interactions leave specific imprints at sufficiently small scales on both of these two spectra, which may allow us to put new constraints on the acceptable photon--dark-matter interactions. Under the conservative assumption that the abundance of ${10}^{12}{M}_{\ensuremath{\bigodot}}$ galaxies is correctly given by the cold dark matter, and without any knowledge of the abundance of smaller objects, we obtain the limit on the ratio of the dark-matter--photon cross section to the dark matter mass ${\ensuremath{\sigma}}_{\ensuremath{\gamma}\ensuremath{-}\mathrm{DM}}{/m}_{\mathrm{DM}}\ensuremath{\lesssim}{10}^{\ensuremath{-}6}{\ensuremath{\sigma}}_{\mathrm{Th}}/(100 \mathrm{GeV})\ensuremath{\simeq}6\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}33} {\mathrm{cm}}^{2} {\mathrm{GeV}}^{\ensuremath{-}1}.$

The Search for Intergalactic Hydrogen Clouds in Voids

I present the results of a search for intergalactic hydrogen clouds in voids. Clouds are detected by their HI LyA absorption lines in the HST spectra of low-redshift AGN. The parameter with which the environments of clouds are characterized is the tidal field, which places a lower limit on the cloud mass-density which is dynamically stable against disruption. Galaxy redshift catalogs are used to sum the tidal fields along the lines of sight, sorting clouds according to tidal field upper, or lower limits. The analytical methodology employed is designed to detect gas clouds whose expansion following reionization is restrained by dark matter perturbations. End-products are the cloud equivalent width distribution functions (EWDF) of catalogs formed by sorting clouds according to various tidal field upper, or lower limits. Cumulative EWDFs are steep in voids (S ~ -1.5 \pm 0.2), but flatter in high tidal field zones (S ~ -0.5 \pm 0.1). Most probable cloud Doppler parameters are ~30 km/s in voids and ~60 km/s in proximity to galaxies. In voids, the cumulative line density at low EW (~ 15 mA) is ~ 500 per unit redshift. The void filling factor is found to be 0.87 <= f_v <= 0.94. The void EWDF is remarkably uniform over this volume, with a possible tendency for more massive clouds to be in void centers. The size and nature of the void cloud population suggested by this study is completely unanticipated by the results of published 3-D simulations, which predict that most clouds are in filamentary structures around galaxy concentrations, and that very few observable absorbers would lie in voids. Strategies for modeling this population are briefly discussed.

Evolution of cosmological dark matter perturbations

We discuss the propagation of dark matter perturbations with nonzero velocity dispersion in cosmological models. In particular a nonzero massive neutrino component may well have a significant effect on the matter power spectrum and cosmic microwave background anisotropy. We present a covariant analysis of the evolution of a dark matter distribution via a two-dimensional momentum-integrated hierarchy of multipole equations. This can be expanded in the velocity weight to provide accurate approximate equations if the matter is nonrelativistic, and we also perform an expansion in the mass to study the propagation of relativistic matter perturbations. We suggest an approximation to the exact hierarchy that can be used to calculate efficiently the effect of the massive neutrinos on the CMB power spectra. We implement the corresponding scalar mode equations numerically achieving a considerable reduction in computation time compared with previous approaches.

Linear and Second‐Order Evolution of Cosmic Baryon Perturbations below 106Solar Masses

Studies of the growth of cosmic perturbations are typically focused on galactic scales and above. In this paper we investigate the evolution of perturbations in baryons, photons, and dark matter for masses below 106 M☉ (or wavenumbers above 100 Mpc-1). Fluctuations on these scales are of interest and importance because they grow to become the earliest collapsed objects and provide the first light sources in the so-called dark ages. We investigate both the linear evolution and the second-order nonlinear effects arising from the coupling of large-scale velocity fields to small-scale perturbations in the baryon density and the electron ionization fraction. We find that this second-order nonlinear coupling dominates the growth of perturbations with masses 103 M☉ immediately after recombination, enhancing the baryon fluctuation amplitudes by a factor of ~5, but the nonlinear effect does not persist at late times.

Power-spectrum normalization from the local abundance of rich clusters of galaxies

The number density of rich galaxy clusters still provides the most robust way of normalizing the power spectrum of dark matter perturbations on scales relevant to large-scale structure. We revisit this constraint in light of several recent developments: (1) the availability of well-defined samples of local clusters with relatively accurate X-ray temperatures; (2) new theoretical mass functions for dark matter haloes which provide a good fit to large numerical simulations; (3) more accurate mass-temperature relations from larger catalogs of hydrodynamical simulations; (4) the requirement to consider closed as well as open and flat cosmologies to obtain full multi-parameter likelihood constraints for CMB and SNe studies. We present a new sample of clusters drawn from the literature and use this sample to obtain improved results on sigma_8, the normalization of the matter power spectrum on scales of 8 h^{-1} Mpc, as a function of the matter density and cosmological constant in a Universe with general curvature. We discuss our differences with previous work, and the remaining major sources of uncertainty. Final results on the 68 per cent confidence region, approximately independent of power spectrum shape, can be expressed as constraints on sigma at an appropriate cluster normalization scale R_Cl. We provide fitting formulas for R_Cl and sigma(R_Cl) for general cosmologies, as well as for sigma_8 as a function of cosmology and shape parameter Gamma. For flat models we find approximately sigma_8 \simeq 0.495^{+0.034}_{-0.037}) Omega_M^{-0.60} for Gamma=0.23, where the error bar is dominated by uncertainty in the mass-temperature relation.

Nonlinear Evolution of Very Small Scale Cosmological Baryon Perturbations at Recombination

The evolution of baryon density perturbations on very small scales is investigated. In particular, the nonlinear growth induced by the radiation drag force from the shear velocity field on larger scales during the recombination epoch, which is originally proposed by Shaviv in 1998, is studied in detail. It is found that inclusion of the diffusion term which Shaviv neglected in his analysis results in rather mild growth whose growth rate is ≪ 100 instead of enormous amplification ∼ 10 4 of Shaviv’s original claim since the diffusion suppresses the growth. The growth factor strongly depends on the amplitude of the large scale velocity field. The nonlinear growth mechanism is applied to density perturbations of general adiabatic cold dark matter (CDM) models. In these models, it has been found in the previous works that the baryon density perturbations are not completely erased by diffusion damping if there exists gravitational potential of CDM. With employing the perturbed rate equation which is derived in this paper, the nonlinear evolution of baryon density perturbations is investigated. It is found that: (1) The nonlinear growth is larger for smaller scales. This mechanism only affects the perturbations whose scales are smaller than ∼ 10 2 M⊙, which are coincident with the stellar scales. (2) The maximum growth factors of baryon density fluctuations for various COBE normalized CDM models are typically less than factor 10 for 3 − � large scale velocity peaks. (3) The growth factor depends on b.

Cosmological consequences of string-forming open inflation models

We present a study of open inflation cosmological scenarios in which cosmic strings form between the two inflationary epochs. It is shown that in these models, strings are stretched outside the horizon due to the inflationary expansion, but must necessarily re-enter the horizon before the epoch of equal matter and radiation densities. We determine the linear power spectrum of cold dark matter perturbations in these hybrid models, finding good agreement with observations for values of {Gamma}={Omega}{sub 0}h{approximately}0.3 and comparable contributions from the active and passive sources to the cosmic microwave background. Finally, we briefly discuss other cosmological consequences of these models. {copyright} {ital 1999} {ital The American Physical Society}

First Structure Formation. I. Primordial Star‐forming Regions in Hierarchical Models

We investigate the possibility of very early formation of primordial star clusters from high-\sigma perturbations in cold dark matter dominated structure formation scenarios. For this we have developed a powerful 2-level hierarchical cosmological code with a realistic and robust treatment of multi-species primordial gas chemistry, paying special attention to the formation and destruction of hydrogen molecules, non-equilibrium ionization, and cooling processes. We performed 3-D simulations at small scales and at high redshifts and find that, analogous to simulations of large scale structure, a complex system of filaments, sheets, and spherical knots at the intersections of filaments form. On the mass scales covered by our simulations (5x10^5 - 1x10^9\Ms) that collapse at redshifts z>25, we find that only at the spherical knots can enough H2 be formed (n_{H_2}/n_H > 5x10^-4) to cool the gas appreciably. Quantities such as the time dependence of the formation of H2 molecules, the final H2 fraction, and central densities from the simulations are compared to the theoretical predictions of Abel (1995) and Tegmark et al. (1997) and found to agree remarkably well. Comparing the 3-D results to an isobaric collapse model we further discuss the possible implications of the extensive merging of small structure that is inherent in hierarchical models. Typically only 5-8% percent of the total baryonic mass in the collapsing structures is found to cool significanlty. Assuming the Padoan (1995) model for star formation our results would predict the first stellar systems to be as small as ~30\Ms. Some implications for primordial globular cluster formation scenarios are also discussed.

Structure Formation with a Self-Tuning Scalar Field

A scalar field with an exponential potential has the particular property that it is attracted into a solution in which its energy scales as the dominant component (radiation or matter) of the Universe, contributing a fixed fraction of the total energy density. We study the growth of perturbations in a cold dark matter dominated $\ensuremath{\Omega}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}1$ universe with this extra field, with an initial flat spectrum of adiabatic fluctuations. The observational constraints from structure formation are satisfied as well, or better, than in other models, with a contribution to the energy density from the scalar field ${\ensuremath{\Omega}}_{\ensuremath{\varphi}}\ensuremath{\sim}0.1$ which is small enough to be consistent with entry into the attractor prior to nucleosynthesis.

The Delayed Formation of Dwarf Galaxies

One of the largest uncertainties in understanding the effect of a background UV field on galaxy formation is the intensity and evolution of the radiation field with redshift. This work attempts to shed light on this issue by computing the quasi-hydrostatic equilibrium states of gas in spherically symmetric dark matter halos (roughly corresponding to dwarf galaxies) as a function of the amplitude of the background UV field. We integrate the full equations of radiative transfer, heating, cooling, and nonequilibrium chemistry for nine species: H, H+, H-, H2, H+2, He, He+, He++, and e-. As the amplitude of the UV background is decreased, the gas in the core of the dwarf goes through three stages characterized by the predominance of ionized (H+), neutral (H), and molecular (H2) hydrogen. Characterizing the gas state of a dwarf galaxy with the radiation field allows us to estimate its behavior for a variety of models of the background UV flux. Our results indicate that a typical radiation field can easily delay the collapse of gas in halos corresponding to 1 σ cold dark matter perturbations with circular velocities of less than 30 km s-1.