ترغب بنشر مسار تعليمي؟ اضغط هنا

Global properties of the growth index: mathematical aspects and physical relevance

101   0   0.0 ( 0 )
 نشر من قبل David Polarski
 تاريخ النشر 2019
  مجال البحث فيزياء
والبحث باللغة English




اسأل ChatGPT حول البحث

We analyze the global behaviour of the growth index of cosmic inhomogeneities in an isotropic homogeneous universe filled by cold non-relativistic matter and dark energy (DE) with an arbitrary equation of state. Using a dynamical system approach, we find the critical points of the system. That unique trajectory for which the growth index $gamma$ is finite from the asymptotic past to the asymptotic future is identified as the so-called heteroclinic orbit connecting the critical points $(Omega_m=0,~gamma_{infty})$ in the future and $(Omega_m=1,~gamma_{-infty})$ in the past. The first is an attractor while the second is a saddle point, confirming our earlier results. Further, in the case when a fraction of matter (or DE tracking matter) $varepsilon Omega^{rm tot}_m$ remains unclustered, we find that the limit of the growth index in the past $gamma_{-infty}^{varepsilon}$ does not depend on the equation of state of DE, in sharp contrast with the case $varepsilon=0$ (for which $gamma_{-infty}$ is obtained). We show indeed that there is a mathematical discontinuity: one cannot obtain $gamma_{-infty}$ by taking $lim_{varepsilon to 0} gamma^{varepsilon}_{-infty}$ (i.e. the limits $varepsilonto 0$ and $Omega^{rm tot}_mto 1$ do not commute). We recover in our analysis that the value $gamma_{-infty}^{varepsilon}$ corresponds to tracking DE in the asymptotic past with constant $gamma=gamma_{-infty}^{varepsilon}$ found earlier.



قيم البحث

اقرأ أيضاً

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 whic h 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 how the cosmological constraints from growth data are improved by including the measurements of bias from Dark Energy Survey (DES). In particular, we utilize the biasing properties of the DES Luminous Red Galaxies (LRGs) and the growth data provided by the various galaxy surveys in order to constrain the growth index ($gamma$) of the linear matter perturbations. Considering a constant growth index we can put tight constraints, up to $sim 10%$ accuracy, on $gamma$. Specifically, using the priors of the Dark Energy Survey and implementing a joint likelihood procedure between theoretical expectations and data we find that the best fit value is in between $gamma=0.64pm 0.075$ and $0.65pm 0.063$. On the other hand utilizing the Planck priors we obtain $gamma=0.680pm 0.089$ and $0.690pm 0.071$. This shows a small but non-zero deviation from General Relativity ($gamma_{rm GR}approx 6/11$), nevertheless the confidence level is in the range $sim 1.3-2sigma$. Moreover, we find that the estimated mass of the dark-matter halo in which LRGs survive lies in the interval $sim 6.2 times 10^{12} h^{-1} M_{odot}$ and $1.2 times 10^{13} h^{-1} M_{odot}$, for the different bias models. Finally, allowing $gamma$ to evolve with redshift [Taylor expansion: $gamma(z)=gamma_{0}+gamma_{1}z/(1+z)$] we find that the $(gamma_{0},gamma_{1})$ parameter solution space accommodates the GR prediction at $sim 1.7-2.9sigma$ levels.
The observational fact that the present values of the densities of dark energy and dark matter are of the same order of magnitude, $rho_{de0}/rho_{dm0} sim mathcal{O}(1)$, seems to indicate that we are currently living in a very special period of the cosmic history. Within the standard model, a density ratio of the order of one just at the present epoch can be seen as coincidental since it requires very special initial conditions in the early Universe. The corresponding why now question constitutes the cosmological coincidence problem. According to the standard model the equality $rho_{de} = rho_{dm}$ took place recently at a redshift $z approx 0.55$. The meaning of recently is, however, parameter dependent. In terms of the cosmic time the situation looks different. We discuss several aspects of the coincidence problem, also in its relation to the cosmological constant problem, to issues of structure formation and to cosmic age considerations.
We investigate the cosmology of massive spinor electrodynamics when torsion is non-vanishing. A non-minimal interaction is introduced between the torsion and the vector field and the coupling constant between them plays an important role in subsequen tial cosmology. It is shown that the mass of the vector field and torsion conspire to generate dark energy and pressureless dark matter, and for generic values of the coupling constant, the theory effectively provides an interacting model between them with an additional energy density of the form $sim 1/a^6$. The evolution equations mimic $Lambda$CDM behavior up to $1/a^3$ term and the additional term represents a deviation from $Lambda$CDM. We show that the deviation is compatible with the observational data, if it is very small. We find that the non-minimal interaction is responsible for generating an effective cosmological constant which is directly proportional to the mass squared of the vector field and the mass of the photon within its current observational limit could be the source of the dark energy.
The field equations in FRW background for the so called C-theories are presented and investigated. In these theories the usual Ricci scalar is substituted with $f(mathcal{R})$ where $mathcal{R}$ is a Ricci scalar related to a conformally scaled metri c $hat{g}_{mu u} = mathcal{C}(mathcal{R})g_{mu u}$, where the conformal factor itself depends on $mathcal{R}$. It is shown that homogeneous perturbations of this Ricci scalar around general relativity FRW background of a large class of these theories are either inconsistent or unstable.
التعليقات
جاري جلب التعليقات جاري جلب التعليقات
سجل دخول لتتمكن من متابعة معايير البحث التي قمت باختيارها
mircosoft-partner

هل ترغب بارسال اشعارات عن اخر التحديثات في شمرا-اكاديميا