اشترك بالحزمة الذهبية واحصل على وصول غير محدود شمرا أكاديميا
تسجيل مستخدم جديدGauge-invariant analysis of perturbations in Chaplygin gas unified models of dark matter and dark energy
213
0
0.0
(
0
)
اسأل ChatGPT حول البحث
ﻻ يوجد ملخص باللغة العربية
We exploit the gauge-invariant formalism to analyse the perturbative behaviour of two cosmological models based on the generalized Chaplygin gas describing both dark matter and dark energy in the present Universe. In the first model we consider the generalized Chaplygin gas alone, while in the second one we add a baryon component to it. We extend our analysis also into the parameter range $alpha > 1$, where the generalized Chaplygin gas sound velocity can be larger than that of light. In the first model we find that the matter power spectrum is compatible with the observed one only for $alpha < 10^{-5}$, which makes the generalized Chaplygin gas practically indistinguishable from $Lambda$CDM. In the second model we study the evolution of inhomogeneities of the baryon component. The theoretical power spectrum is in good agreement with the observed one for almost all values of $alpha$. However, the growth of inhomogeneities seems to be particularly favoured either for sufficiently small values of $alpha$ or for $alpha gtrsim 3$. Thus, it appears that the viability of the generalized Chaplygin gas as a cosmological model is stronger when its sound velocity is superluminal. We show that in this case the generalized Chaplygin gas equation of state can be changed in an unobservable region in such a way that its equivalent $k$-essence microscopical model has no problems with causality.
قيم البحث
اقرأ أيضاً
The differential age data of astrophysical objects that have evolved passivelly during the history of the universe (e.g. red galaxies) allows to test theoretical cosmological models through the predicted Hubble function expressed in terms of the reds
hift $z$, $H(z)$. We use the observational data for $H(z)$ to test unified scenarios for dark matter and dark energy. Specifically, we focus our analysis on the Generalized Chaplygin Gas (GCG) and the viscous fluid (VF) models. For the GCG model, it is shown that the unified scenario for dark energy and dark matter requires some priors. For the VF model we obtain estimations for the free parameters that may be compared with further analysis mainly at perturbative level.
Although various cosmological observations congruously suggest that our universe is dominated by two dark components, the cold dark matter without pressure and the dark energy with negative pressure, the nature and origin of these components is yet u
nknow. The generalized Chaplygin gas (gCg), parametrized by an equation of state, $p = -A/rho_{rm gCg}^{alpha}$, was recently proposed to be a candidate of the unified dark matter/energy (UDME) scenarios. In this work, we investigate some observational constraints on it. We mainly focus our attention on the constraints from recent measurements of the X-ray gas mass fractions in clusters of galaxies published by Allen et al. (2002,2003) and the dimensionless coordinate distances to type Ia supernovae and Fanaroff-Riley type IIb radio galaxies compiled by Daly and Djorgovski (2003). We obtain the confidence region on the two parameters fully characterizing gCg, $A_s equiv A/rho_{{rm gCg}0}^{(1+alpha)}$ and $alpha$, from a combined analysis of these databases, where $rho_{{rm gCg}0}$ is the energy density of gCg at present. It is found that $A_s=0.70^{+0.16}_{-0.17}$ and $alpha=-0.09^{+0.54}_{-0.33}$, at a 95% confidence level, which is consistent within the errors with the standard dark matter + dark energy model, i.e., the case of $alpha = 0$. Particularly, the standard Chaplygin gas ($alpha=1$) is ruled out as a feasible UDME by the data at a 99% confidence level.
In this paper we study the evolution of cosmological perturbations in the presence of dynamical dark energy, and revisit the issue of dark energy perturbations. For a generally parameterized equation of state (EoS) such as w_D(z) = w_0+w_1frac{z}{1+z
}, (for a single fluid or a single scalar field ) the dark energy perturbation diverges when its EoS crosses the cosmological constant boundary w_D=-1. In this paper we present a method of treating the dark energy perturbations during the crossing of the $w_D=-1$ surface by imposing matching conditions which require the induced 3-metric on the hypersurface of w_D=-1 and its extrinsic curvature to be continuous. These matching conditions have been used widely in the literature to study perturbations in various models of early universe physics, such as Inflation, the Pre-Big-Bang and Ekpyrotic scenarios, and bouncing cosmologies. In all of these cases the EoS undergoes a sudden change. Through a detailed analysis of the matching conditions, we show that delta_D and theta_D are continuous on the matching hypersurface. This justifies the method used[1-4] in the numerical calculation and data fitting for the determination of cosmological parameters. We discuss the conditions under which our analysis is applicable.
We propose a unified single-field description of the galactic Dark Matter and various uniform scalar fields for the inflation and cosmological constant. The two types of effects could originate from a fluid of both spatially and temporally varying Va
cuum Energy if the vacuum has an uneven pressure caused by a photon-like vector field (of perhaps an unstable massive boson). We propose a most general Lagrangian with a {bf N}on-{bf u}niform Cosmological Constant for this vacuum fluid (dubbed as a Nu-Lambda fluid), working within the framework of Einsteinian gravity. This theory includes a continuous spectrum of plausible dark energy theories and gravity theories, e.g., inflation, quintessence, k-essence, f(R), Generalized Einstein-Aether f(K), MOND, TeVeS, BSTV etc. theories. It also suggests new models such as a certain f(K+R) model, which suggests intriguing corrections to MOND depending of redshift and density. Some specific constructions of the Nu-Lambda fluid (e.g., Zhaos V-$Lambda$ model) closely resemble the $Lambda$CDM cosmology on large scale, but fit galaxy rotation curves as good as MOND. Perturbed Einstein Equations in a simple $f(K_4)$ model are solvable and show effects of a DM coupled to DE. Incorporating the perturbation equations here into standard simulations for cosmological structure growth offers a chance to falsify examples of the Nu-Lambda theories.
We study a class of early dark energy models which has substantial amount of dark energy in the early epoch of the universe. We examine the impact of the early dark energy fluctuations on the growth of structure and the CMB power spectrum in the line
ar approximation. Furthermore we investigate the influence of the interaction between the early dark energy and the dark matter and its effect on the structure growth and CMB. We finally constrain the early dark energy model parameters and the coupling between dark sectors by confronting to different observations.
سجل دخول لتتمكن من نشر تعليقات
التعليقات
جاري جلب التعليقات
سجل دخول لتتمكن من متابعة معايير البحث التي قمت باختيارها
