Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userMatter Perturbations in Scaling Cosmology
86
0
0.0
(
0
)
Added by
W. S. Hipolito-Ricaldi
Publication date
2014
fields
Physics
and research's language is
English
Ask ChatGPT about the research
No Arabic abstract
A suitable nonlinear interaction between dark matter with an energy density $rho_{M}$ and dark energy with an energy density $rho_{X}$ is known to give rise to a non-canonical scaling $rho_{M} propto rho_{X}a^{-xi}$ where $xi$ is a parameter which generally deviates from $xi =3$. Here we present a covariant generalization of this class of models and investigate the corresponding perturbation dynamics. The resulting matter power spectrum for the special case of a time-varying Lambda model is compared with data from the SDSS DR9 catalogue. We find a best-fit value of $xi = 3.25$ which corresponds to a decay of dark matter into the cosmological term. Our results are compatible with the $Lambda$CDM model at the 2$sigma$ confidence level.
rate research
Read More
We show that the f(T) gravitational paradigm, in which gravity is described by an arbitrary function of the torsion scalar, can provide a mechanism for realizing bouncing cosmologies, thereby avoiding the Big Bang singularity. After constructing the simplest version of an f(T) matter bounce, we investigate the scalar and tensor modes of cosmological perturbations. Our results show that metric perturbations in the scalar sector lead to a background-dependent sound speed, which is a distinguishable feature from Einstein gravity. Additionally, we obtain a scale-invariant primordial power spectrum, which is consistent with cosmological observations, but suffers from the problem of a large tensor-to-scalar ratio. However, this can be avoided by introducing extra fields, such as a matter bounce curvaton.
We derive for the first time the growth index of matter perturbations of the FLRW flat cosmological models in which the vacuum energy depends on redshift. A particularly well motivated model of this type is the so-called quantum field vacuum, in which apart from a leading constant term $Lambda_0$ there is also a $H^{2}$-dependence in the functional form of vacuum, namely $Lambda(H)=Lambda_{0}+3 u (H^{2}-H^{2}_{0})$. Since $| u|ll1$ this form endows the vacuum energy of a mild dynamics which affects the evolution of the main cosmological observables at the background and perturbation levels. Specifically, at the perturbation level we find that the growth index of the running vacuum cosmological model is $gamma_{Lambda_{H}} approx frac{6+3 u}{11-12 u}$ and thus it nicely extends analytically the result of the $Lambda$CDM model, $gamma_{Lambda}approx 6/11$.
We study the dynamics of cosmological perturbations in models of dark matter based on ultralight coherent vector fields. Very much as for scalar field dark matter, we find two different regimes in the evolution: for modes with $k^2ll {cal H}ma$, we have a particle-like behaviour indistinguishable from cold dark matter, whereas for modes with $k^2gg {cal H}ma$, we get a wave-like behaviour in which the sound speed is non-vanishing and of order $c_s^2simeq k^2/m^2a^2$. This implies that, also in these models, structure formation could be suppressed on small scales. However, unlike the scalar case, the fact that the background evolution contains a non-vanishing homogeneous vector field implies that, in general, the evolution of the three kinds of perturbations (scalar, vector and tensor) can no longer be decoupled at the linear level. More specifically, in the particle regime, the three types of perturbations are actually decoupled, whereas in the wave regime, the three vector field perturbations generate one scalar-tensor and two vector-tensor perturbations in the metric. Also in the wave regime, we find that a non-vanishing anisotropic stress is present in the perturbed energy-momentum tensor giving rise to a gravitational slip of order $(Phi-Psi)/Phisim c_s^2$. Moreover in this regime the amplitude of the tensor to scalar ratio of the scalar-tensor modes is also $h/Phisim c_s^2$. This implies that small-scale density perturbations are necessarily associated to the presence of gravity waves in this model. We compare their spectrum with the sensitivity of present and future gravity waves detectors.
In this paper we present four simple expressions for the relativistic first and second order fractional density perturbations for $Lambda$CDM cosmologies in different gauges: the Poisson, uniform curvature, total matter and synchronous gauges. A distinctive feature of our approach is the use of a canonical set of quadratic differential expressions involving an arbitrary spatial function, the so-called comoving curvature perturbation, to describe the spatial dependence, which enables us to unify, simplify and extend previous seemingly disparate results. The simple structure of the expressions makes the evolution of the density perturbations completely transparent and clearly displays the effect of the cosmological constant on the dynamics, namely that it stabilizes the perturbations. We expect that the results will be useful in applications, for example, studying the effects of primordial non-Gaussianity on the large scale structure of the universe.
We investigate the cosmological perturbations in f(T) gravity. Examining the pure gravitational perturbations in the scalar sector using a diagonal vierbien, we extract the corresponding dispersion relation, which provides a constraint on the f(T) ansatzes that lead to a theory free of instabilities. Additionally, upon inclusion of the matter perturbations, we derive the fully perturbed equations of motion, and we study the growth of matter overdensities. We show that f(T) gravity with f(T) constant coincides with General Relativity, both at the background as well as at the first-order perturbation level. Applying our formalism to the power-law model we find that on large subhorizon scales (O(100 Mpc) or larger), the evolution of matter overdensity will differ from LCDM cosmology. Finally, examining the linear perturbations of the vector and tensor sectors, we find that (for the standard choice of vierbein) f(T) gravity is free of massive gravitons.
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
