Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userIs Supernovae Data in Favour of Isotropic Cosmologies?
38
0
0.0
(
0
)
Authors
Mariusz P. Dabrowski
Ask ChatGPT about the research
No Arabic abstract
Most of the observational claims in cosmology are based on the assumption that the universe is isotropic and homogeneous so they essentially test different types of Friedmann models. This also refers to recent observations of supernovae Ia, which, within the framework of Friedmann cosmologies give strong support to negative pressure matter and also weaken the age conflict. In this essay we drop the assumption of homogeneity, though temporarily leaving the assumption of isotropy with respect to one point, and show that supernovae data can be consistent with a model of the universe with inhomogeneous pressure known as the Stephani model. Being consistent with supernovae data we are able to get the age of the universe in this model to be about 3.8 Gyr more than in its Friedmann counterpart.
rate research
Read More
We present a reconstruction algorithm for cosmological models based on $f(mathcal{Q})$ gravity. We specifically focus on obtaining exact Bianchi Type-I and Friedmann-Lema^{i}tre-Robertson-Walker solutions, finding solutions that might have application in a variety of scenarios such as spontaneous isotropization of Bianchi Type-I models, dark energy, inflation as well as pre-Big Bang cosmologies.
A powerful result in theoretical cosmology states that a subset of anisotropic Bianchi models can be seen as the homogeneous limit of (standard) linear cosmological perturbations. Such models are precisely those leading to Friedmann spacetimes in the limit of zero anisotropy. Building on previous works, we give a comprehensive exposition of this result, and perform the detailed identification between anisotropic degrees of freedom and their corresponding scalar, vector, and tensor perturbations of standard perturbation theory. In particular, we find that anisotropic models very close to open (i.e., negatively curved) Friedmann spaces correspond to some type of super-curvature perturbations. As a consequence, provided anisotropy is mild, its effects on all types of cosmological observables can always be computed as simple extensions of the standard techniques used in relativistic perturbation theory around Friedmann models. This fact opens the possibility to consistently constrain, for all cosmological observables, the presence of large scale anisotropies on the top of the stochastic fluctuations.
We study general dynamical equations describing homogeneous isotropic cosmologies coupled to a scalaron $psi$. For flat cosmologies ($k=0$), we analyze in detail the gauge-independent equation describing the differential, $chi(alpha)equivpsi^prime(alpha)$, of the map of the metric $alpha$ to the scalaron field $psi$, which is the main mathematical characteristic locally defining a `portrait of a cosmology in `$alpha$-version. In the `$psi$-version, a similar equation for the differential of the inverse map, $bar{chi}(psi)equiv chi^{-1}(alpha)$, can be solved asymptotically or for some `integrable scalaron potentials $v(psi)$. In the flat case, $bar{chi}(psi)$ and $chi(alpha)$ satisfy the first-order differential equations depending only on the logarithmic derivative of the potential. Once we know a general analytic solution for one of these $chi$-functions, we can explicitly derive all characteristics of the cosmological model. In the $alpha$-version, the whole dynamical system is integrable for $k eq 0$ and with any `$alpha$-potential, $bar{v}(alpha)equiv v[psi(alpha)]$, replacing $v(psi)$. There is no a priori relation between the two potentials before deriving $chi$ or $bar{chi}$, which implicitly depend on the potential itself, but relations between the two pictures can be found by asymptotic expansions or by inflationary perturbation theory. Explicit applications of the results to a more rigorous treatment of the chaotic inflation models and to their comparison with the ekpyrotic-bouncing ones are outlined in the frame of our `$alpha$-formulation of isotropic scalaron cosmologies. In particular, we establish an inflationary perturbation expansion for $chi$. When all the conditions for inflation are satisfied and $chi$ obeys a certain boundary (initial) condition, we get the standard inflationary parameters, with higher-order corrections.
We discuss the most general field equations for cosmological spacetimes for theories of gravity based on non-linear extensions of the non-metricity scalar and the torsion scalar. Our approach is based on a systematic symmetry-reduction of the metric-affine geometry which underlies these theories. While for the simplest conceivable case the connection disappears from the field equations and one obtains the Friedmann equations of General Relativity, we show that in $f(mathbb{Q})$ cosmology the connection generically modifies the metric field equations and that some of the connection components become dynamical. We show that $f(mathbb{Q})$ cosmology contains the exact General Relativity solutions and also exact solutions which go beyond. In $f(mathbb{T})$~cosmology, however, the connection is completely fixed and not dynamical.
We investigate the qualitative evolution of (D+1)-dimensional cosmological models in f(R) gravity for the general case of the function f(R). The analysis is specified for various examples, including the (D+1)-dimensional generalization of the Starobinsky model, models with polynomial and exponential functions. The cosmological dynamics are compared in the Einstein and Jordan representations of the corresponding scalar-tensor theory. The features of the cosmological evolution are discussed for Einstein frame potentials taking negative values in certain regions of the field space.
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
