Growth Of Density Perturbations Research Articles

The Time Evolution of Mutual Information between Disjoint Regions in the Universe.

We study the time evolution of mutual information between mass distributions in spatially separated but casually connected regions in an expanding universe. The evolution of mutual information is primarily determined by the configuration entropy rate, which depends on the dynamics of the expansion and growth of density perturbations. The joint entropy between distributions from the two regions plays a negligible role in such evolution. Mutual information decreases with time in a matter-dominated universe, whereas it stays constant in a Λ-dominated universe. The ΛCDM model and some other models of dark energy predict a minimum in mutual information beyond which dark energy dominates the dynamics of the universe. Mutual information may have deeper connections to the dark energy and accelerated expansion of the universe.

The growth of density perturbations in the last ∼10 billion years from tomographic large-scale structure data

In order to investigate the origin of the ongoing tension between the amplitude of matter fluctuations measured by weak lensing experiments at low redshifts and the value inferred from the cosmic microwave background anisotropies, we reconstruct the evolution of this amplitude from z ∼ 2 using existing large-scale structure data. To do so, we decouple the linear growth of density inhomogeneities from the background expansion, and constrain its redshift dependence making use of a combination of 6 different data sets, including cosmic shear, galaxy clustering and CMB lensing. We analyze these data under a consistent harmonic-space angular power spectrum-based pipeline. We show that current data constrain the amplitude of fluctuations mostly in the range 0.2 < z < 0.7, where it is lower than predicted by . This difference is mostly driven by current cosmic shear data, although the growth histories reconstructed from different data combinations are consistent with each other, and we find no evidence of systematic deviations in any particular experiment. In spite of the tension with , the data are well-described by the model, albeit with a lower value of S 8 ≡ σ8(Ωm/0.3)0.5. As part of our analysis, we find constraints on this parameter of S 8 = 0.7781 ± 0.0094 (68% confidence level), reaching almost percent-level errors comparable with CMB measurements, and 3.4σ away from the value found by .

Observing Cosmological Processes in Real Time with Repeating Fast Radio Bursts

Abstract It is noted that the duration of a fast radio burst (FRB), about 10−3 s, is a smaller fraction of the time delay between multiple images of a source gravitationally lensed by a galaxy or galaxy cluster than the human lifetime is to the age of the universe. Thus repeating, strongly lensed FRBs may offer an unprecedented opportunity for observing cosmological evolution in “real time.” The possibility is discussed of observing cosmic expansion, transverse proper motion, mass accretion, and perhaps growth of density perturbations, as a function of redshift.

Partially acoustic dark matter, interacting dark radiation, and large scale structure

The standard paradigm of collisionless cold dark matter is in tension with measurements on large scales. In particular, the best fit values of the Hubble rate $H_0$ and the matter density perturbation $\sigma_8$ inferred from the cosmic microwave background seem inconsistent with the results from direct measurements. We show that both problems can be solved in a framework in which dark matter consists of two distinct components, a dominant component and a subdominant component. The primary component is cold and collisionless. The secondary component is also cold, but interacts strongly with dark radiation, which itself forms a tightly coupled fluid. The growth of density perturbations in the subdominant component is inhibited by dark acoustic oscillations due to its coupling to the dark radiation, solving the $\sigma_8$ problem, while the presence of tightly coupled dark radiation ameliorates the $H_0$ problem. The subdominant component of dark matter and dark radiation continue to remain in thermal equilibrium until late times, inhibiting the formation of a dark disk. We present an example of a simple model that naturally realizes this scenario in which both constituents of dark matter are thermal WIMPs. Our scenario can be tested by future stage-IV experiments designed to probe the CMB and large scale structure.

Improvements on a Unified Dark Matter Model

AbstractWe study, by means of a spherical collapse model, the effect of shear, rotation, and baryons on a generalized Chaplygin gas (gCg) dominated universe. We show that shear, rotation, and the presence of baryons slow down the collapse compared to the simple spherical collapse model. The slowing down in the growth of density perturbation is able to solve the instability of the unified dark matter (UDM) models described in previous papers (e.g.

Scalar perturbations in a Friedmann-like metric with non-null Weyl tensor

In a previous work the authors have solved the Einstein equations of General Relativity for a class of metrics with constant spatial curvature, where it was found a non vanishing Weyl tensor in the presence of a primordial magnetic field with an anisotropic pressure component. Here, we perform the perturbative analysis of this model in order to study the gravitational stability under linear scalar perturbations. For this purpose, we take the Quasi-Maxwellian formalism of General Relativity as our framework, which offers a naturally covariant and gauge-invariant approach to deal with perturbations that are directly linked to observational quantities. We then compare this scenario with the perturbed dust-dominated Friedmann model emphasizing how the growth of density perturbations are enhanced in our case.

The accretion history of dark matter haloes – I. The physical origin of the universal function

Understanding the universal accretion history of dark matter halos is the first step towards determining the origin of their structure. We use the extended Press-Schechter formalism to derive the halo mass accretion history from the growth rate of initial density perturbations. We show that the halo mass history is well described by an exponential function of redshift in the high-redshift regime. However, in the low-redshift regime the mass history follows a power law because the growth of density perturbations is halted in the dark energy dominated era due to the accelerated expansion of the Universe. We provide an analytic model that follows the expression $M(z) = M_{0}(1+z)^{af(M_{0})}e^{-f(M_{0})z}$, where $M_{0} = M(z=0)$, $a$ depends on cosmology and $f(M_{0})$ depends only on the linear matter power spectrum. The analytic model does not rely on calibration against numerical simulations and is suitable for any cosmology. We compare our model with the latest empirical models for the mass accretion history in the literature and find very good agreement. We provide numerical routines for the model online.

Shear and rotation in Chaplygin cosmology

We study the effect of shear and rotation on results previously obtained dealing with the application of the spherical collapse model (SCM) to generalized Chaplygin gas (gCg) dominated universes. The system is composed of baryons and gCg and the collapse is studied for different values of the parameter $\alpha$ of the gCg. We show that the joint effect of shear and rotation is that of slowing down the collapse with respect to the simple SCM. This result is of utmost importance for the so-called unified dark matter models, since the described slow down in the growth of density perturbation can solve one of the main problems of the quoted models, namely the instability described in previous papers [e.g., H. B. Sandvik {\it et al.}, Phys. Rev. D {\bf 69}, 123524 (2004)] at the linear perturbation level.

Density growth in Kantowski–Sachs cosmologies with a cosmological constant

In this work, the growth of density perturbations in Kantowski–Sachs cosmologies with a positive cosmological constant is studied, using the 1+3 and 1+1+2 covariant formalisms. For each wave number, we obtain a closed system for scalars formed from quantities that are zero on the background and hence are gauge-invariant. The solutions to this system are then analysed both analytically and numerically. In particular, the effects of anisotropy and the behaviour close to a bounce in the cosmic scale factor are considered. We find that typically the density gradient in the bouncing direction experiences a local maximum at or slightly after the bounce.

Are dSph galaxies Galactic building blocks?

Dwarf spheroidal galaxies (dSph) are frequently assumed to represent surviving examples of a vast now destroyed population of small systems in which many of the stars now forming the Milky Way were formed. Ongoing accretion and considerable sub-structure in the outer Galactic halo is direct evidence that there is some role for stars formed in small galaxies in populating the (outer) galaxy. The evidence from stellar populations is however contradictory to this. dSph stellar populations are unlike any stars found in significant numbers in the Milky Way. The dSph are indeed small galaxies, formed over long times with low rates of star formation. Most of the stars in the Milky Way halo however seem to have formed quickly, at higher star formation rate, in gas mixed efficiently on kpc scales. The overwhelming majority of Milky Way stars, those in the Galactic thick disk and thin disk, seem to have nothing at all to do with dwarf galaxy origins. There is overwhelming and irrefutable evidence that large scale structure in the Universe is well- described by the CDM model. In this a (primordial) spectrum of perturbations with equal power on all scales generates density fluctuations which grow under self-gravity, with the dominant matter being both non-relativistic at early times, and weakly or un-coupled to the radiation which dominates the Universe at those early times. Thus the dominant matter cannot be baryonic, which couples efficiently to radiation through Thompson scattering. It is non-relativistic, does not couple to, or emit, radiation, and dominates the local gravitational field. Hence Cold Dark Matter, CDM. This model describes the Universe well on large scales, in fact on all scales large enough that the growth of density perturbations remains linear, so that local dark matter particle physics and baryonic physical processes remain unimportant. The baryonic physics of gas cooling becomes very important at scales of order 10 6 M� . There must also be some general small-scale limit, when the physical properties of whatever types of particles make up the CDM become important, and dominate the scale-free initial perturbations. Finding that scale will provide our first information on the types of matter which make up CDM, and so is of very considerable interest. A standard approach to discovering that scale is to examine when an assumed absence of scale violates observation. That is, one extrapolates the model to smaller and smaller scales until observations indicate discordance. This is the particular interest of dwarf galaxies, which form the smallest systems in which dark matter has been detected. Are they consistent with extrapolated gravitational CDM and baryons, with no evidence of additional physics? The simplest, and most extreme, CDM extrapolation posits structure in gravitationally bound CDM potential wells on all scales, with the number of such systems increasing as mass −2 . This is divergent, so must cut off somewhere. But at what scale does

Density perturbations in Kantowski-Sachs models with a cosmological constant

The growth of density perturbations in Kantowski-Sachs cosmologies with a positive cosmological constant is studied, using the 1+3 and 1+1+2 covariant formalisms. For each wave number a closed system for eight scalars is obtained. These are formed from quantities that are zero on the background and hence are gauge invariant. As an example a numerical solution describing the evolution of density perturbations on a background that experiences a bounce is presented. Typically the density gradient in the bouncing directions experiences a local maxium at or slightly after the bounce.

Clarifying the effects of interacting dark energy on linear and non-linear structure formation processes

We present a detailed numerical study of the impact that cosmological models featuring a direct interaction between the Dark Energy component that drives the accelerated expansion of the Universe and Cold Dark Matter can have on the linear and nonlinear stages of structure formation. By means of a series of collisionless N-body simulations we study the influence that each of the different effects characterizing these cosmological models - which include among others a fifth force, a time variation of particle masses, and a velocity-dependent acceleration - separately have on the growth of density perturbations and on a series of observable quantities related to linear and nonlinear cosmic structures, as the matter power spectrum, the gravitational bias between baryons and Cold Dark Matter, the halo mass function and the halo density profiles. We perform our analysis applying and comparing different numerical approaches previously adopted in the literature, and we address the partial discrepancies recently claimed in a similar study by Li & Barrow (2010b) with respect to the first outcomes of Baldi et al. (2010), which are found to be related to the specific numerical approach adopted in the former work. Our results fully confirm the conclusions of Baldi et al. (2010) and show that when linear and nonlinear effects of the interaction between Dark Energy and Cold Dark Matter are properly disentangled, the velocity-dependent acceleration is the leading effect acting at nonlinear scales, and in particular is the most important mechanism in lowering the concentration of Cold Dark Matter halos.

Decaying dark matter mimicking time-varying dark energy

A $\ensuremath{\Lambda}\mathrm{CDM}$ model with dark matter that decays into inert relativistic energy on a time scale longer than the Hubble time will produce an expansion history that can be misinterpreted as stable dark matter with time-varying dark energy. We calculate the corresponding spurious equation of state parameter, ${\stackrel{\texttildelow{}}{w}}_{\ensuremath{\phi}}$, as a function of redshift, and show that the evolution of ${\stackrel{\texttildelow{}}{w}}_{\ensuremath{\phi}}$ depends strongly on the assumed value of the dark-matter density, erroneously taken to scale as ${a}^{\ensuremath{-}3}$. Depending on the latter, one can obtain models that mimic quintessence (${\stackrel{\texttildelow{}}{w}}_{\ensuremath{\phi}}&gt;\ensuremath{-}1$), phantom models (${\stackrel{\texttildelow{}}{w}}_{\ensuremath{\phi}}&lt;\ensuremath{-}1$), or models in which the equation of state parameter crosses the phantom divide, evolving from ${\stackrel{\texttildelow{}}{w}}_{\ensuremath{\phi}}&gt;\ensuremath{-}1$ at high redshift to ${\stackrel{\texttildelow{}}{w}}_{\ensuremath{\phi}}&lt;\ensuremath{-}1$ at low redshift. All of these models generically converge toward ${\stackrel{\texttildelow{}}{w}}_{\ensuremath{\phi}}\ensuremath{\approx}\ensuremath{-}1$ at the present. The degeneracy between the $\ensuremath{\Lambda}\mathrm{CDM}$ model with decaying dark matter and the corresponding spurious quintessence model is broken by the growth of density perturbations.

Small scale structure formation in chameleon cosmology

Chameleon fields are scalar fields whose mass depends on the ambient matter density. We investigate the effects of these fields on the growth of density perturbations on sub-galactic scales and the formation of the first dark matter halos. Density perturbations on comoving scales R<1 pc go non-linear and collapse to form structure much earlier than in standard ΛCDM cosmology. The resulting mini-halos are hence more dense and resilient to disruption. We therefore expect (provided that the density perturbations on these scales have not been erased by damping processes) that the dark matter distribution on small scales would be more clumpy in chameleon cosmology than in the ΛCDM model.

Differentiating between modified gravity and dark energy

The nature of the fuel that drives today's cosmic acceleration is an open and tantalizing mystery. We entertain the suggestion that the acceleration is not the manifestation of yet another new ingredient in the cosmic gas tank, but rather a signal of our first real lack of understanding of gravitational physics. By requiring that the underlying gravity theory respect Birkhoff's law, we derive the modified gravitational force law necessary to generate any given cosmology, without reference to the fundamental theory, revealing modifications of gravity at scales typically much smaller than today's horizon. We discuss how, through these modifications, the growth of density perturbations, the late-time integrated Sachs-Wolfe effect, and even solar-system measurements may be sensitive to whether today's cosmic acceleration is generated by dark energy or modified gravitational dynamics, and are subject to imminent observational discrimination. We argue how these conclusions can be more generic, and probably not dependent on the validity of Birkhoff's law.

DAMPING TIME AND STABILITY OF DENSITY FERMION PERTURBATIONS IN THE EXPANDING UNIVERSE

The classic problem of the growth of density perturbations in an expanding Newtonian universe is revisited following the work of Bisnovatyi-Kogan and Zel'dovich. We propose a more general analytical approach: a system of free particles satisfying semidegenerate Fermi–Dirac statistics on the background of an exact expanding solution is examined in the linear approximation. This differs from the corresponding work of Bisnovatyi-Kogan and Zel'dovich where classical particles fulfilling Maxwell–Boltzmann statistics were considered. The solutions of the Boltzmann equation are obtained by the method of characteristics. An expression for the damping time of a decaying solution is discussed and a zone in which free streaming is hampered is found, corresponding to wavelengths less than the Jeans one. In the evolution of the system, due to the decrease of the Jeans length, those perturbations may lead to gravitational collapse. At variance with current opinions, we deduce that perturbations with λ≥λ J Max /1.48 are able to generate structures and the lower limit for substructures mass is M=M J max /(1.48)3≈M J max /3, where M J max is the maximum value of the Jeans mass.

Magnetized cosmological perturbations

A large-scale cosmic magnetic field affects not only the growth of density perturbations, but also rotational instabilities and anisotropic deformation in the density distribution. We give a fully relativistic treatment of all these effects, incorporating the magneto-curvature coupling that arises in a relativistic approach. We show that this coupling produces a small enhancement of the growing mode on superhorizon scales. The magnetic field generates new nonadiabatic constant and decaying modes, as well as nonadiabatic corrections to the standard growing and decaying modes. Magnetized isocurvature perturbations are purely decaying on superhorizon scales. On subhorizon scales before recombination, magnetized density perturbations propagate as magnetosonic waves, leading to a small decrease in the spacing of acoustic peaks. Fluctuations in the field direction induce scale-dependent vorticity, and generate precession in the rotational vector. On small scales, magnetized density vortices propagate as Alfve´n waves during the radiation era. After recombination, they decay slower than nonmagnetized vortices. Magnetic fluctuations are also an active source of anisotropic distortion in the density distribution. We derive the evolution equations for this distortion, and find a particular growing solution.

Growth of density perturbations with a cosmological constant

We investigate the influence both of the cosmological constant and of the physical processes that take place during the evolution of the Universe on the primordial cloud collapse. We study the evolution of a cloud with a density perturbation, b, from the recombination era to the present. As an example, we study an initial perturbation bj = 10-6 for a cloud of mass 10 105 and 106 M0 • In particular, we study the influence of the physical processes on the evolution of the peculiar velocity factor, f=d In bid Ina, where 'a' is the scalefactor ofthe Universe. We compare our results with the analytic expression for f obtained by Lahav et al. Our results show that the physical mechanisms (photon drag, photon cooling, recombination, etc.) amplify perturbations, and the cosmological constant is particularly important in a Universe dominated by it with a small matter density.

Constraints on Self-interacting Dark Matter

We consider the growth of density perturbations in the presence of self--interacting dark matter, SIDM, proposed by Carlson, Machacek and Hall (1992). We determine the range of values for the coupling constant $\lambda$ and the particle mass $m^\prime$, for which the power spectrum lies in the ``allowed" range based on constraints from the IRAS galaxy survey and damped Lyman--$\alpha $ systems. Our results show that no combination of parameters can meet both limits. We consider constraints on the $\2-2$ scatterings which keep the SIDM particles in pressure equilibrium, and we show that if such interactions maintain pressure equilibrium down to the present, they will be strong enough to disrupt galaxy mergers and may lead to stripping of galaxy halos as galaxies move through the dark matter background of these particles. Hence, we also investigate the evolution of large-scale structure in the SIDM model when the particles drop out of pressure equilibrium at some higher redshift. The resulting free-streaming leads to an additional suppression of small-scale perturbations, but it does not significantly affect our results.

Scaling properties of non-linear gravitational clustering

Hamilton et al. recently proposed the idea that the growth of density perturbations in an expanding universe is governed by a general scaling law, and showed agreement with existing numerical simulations. We examine the possible origin of this scaling behaviour in more detail. The underlying equations of motion are cast in a suggestive form, and motivate a conjecture that the scaled pair velocity, h(x, a) ≡ – [v/(ȧx)], depends on the expansion factor a and comoving coordinate x only through the density contrast |$\bar\xi(x,a)$| (the two-point correlation averaged over a sphere of radius x). This leads naturally to the proposed scaling law – the true non-linear density contrast is a universal function of the density contrast |$\bar\xi_L(l,a)$|⁠, computed in the linear theory and evaluated at a scale l which is derived to be |$l=x(1+\bar\xi)^{1/3}$|⁠. Apart from basing the proposed scaling form on an explicit dynamical hypothesis, this gives a convenient solution for the scaling function in terms of the input pair velocity. Possibilities for further elaboration of this approach in interpreting simulations of nonlinear gravitational clustering are briefly discussed.