The Reduced Relativistic Gas (RRG) is a simplified version of the ideal relativistic gas, which assumes that all particles have the same momentum magnitude. Although this is a very idealized situation, the resulting model preserves the phenomenology of Maxwell-Boltzmann distribution and, in some situations, can be described as a perfect fluid, without introducing large errors in both cosmological background and first-order perturbations. The perfect fluid description of RRG model was already used to study the warmness of dark matter, massive neutrinos and interaction of baryons and photons before recombination, showing very good agreement with previous works based on the full Einstein-Boltzmann system of equations. In order to understand these results and construct a more general and formal framework for RRG, we develop a theoretical description of first-order cosmological perturbations of RRG, based on a distribution function which encodes the simplifying assumption that all particles have the same momentum magnitude. The full set of Einstein-Boltzmann equations for RRG distribution are derived and quantities beyond the perfect fluid approximation are studied. Using RRG to describe warm dark matter, we show that, for particles with m ∼ keV, the perfect fluid approximation is valid on scales k < 10 h/Mpc, for most of the universe evolution. We also determine initial conditions for RRG in the early universe and study the evolution of potential in a toy model of universe composed only by RRG.

An accurate determination of the Hubble constant remains a puzzle in observational cosmology. The possibility of a new physics has emerged with a significant tension between the current expansion rate of our Universe measured from the cosmic microwave background by the Planck satellite and from local methods. In this paper, new tight estimates on this parameter are obtained by considering two data sets from galaxy distribution observations: galaxy cluster gas mass fractions and baryon acoustic oscillation measurements. Priors from the Big Bang nucleosynthesis (BBN) were also considered. By considering the flat ΛCDM and XCDM models, and the non-flat ΛCDM model, our main results are: H0=65.9+1.5−1.5 km s−1 Mpc−1, H0=65.9+4.4−4.0 km s−1 Mpc−1 and H0=64.3+ 4.5− 4.4 km s−1 Mpc−1 in 2σ c.l., respectively. These estimates are in full agreement with the Planck satellite results. Our analyses in these cosmological scenarios also support a negative value for the deceleration parameter at least in 3σ c.l. .

Using the Reduced Relativistic Gas (RRG) model, we analytically determine the matter power spectrum for Warm Dark Matter (WDM) on small scales, k > 1 h/Mpc. The RRG is a simplified model for the ideal relativistic gas, but very accurate in the cosmological context. In another work, we have shown that, for typical allowed masses for dark matter particles, m>5 keV, the higher order multipoles, ℓ ≥ 2, in the Einstein-Boltzmann system of equations are negligible on scales k < 10 h/Mpc. Hence, we can follow the perturbations of WDM using the ideal fluid framework, with equation of state and sound speed of perturbations given by the RRG model. We derive a Mészáros-like equation for WDM and solve it analytically in radiation, matter and dark energy dominated eras. Joining these solutions, we get an expression that determines the value of WDM perturbations as a function of redshift and wavenumber. Then we construct the matter power spectrum and transfer function of WDM on small scales and compare it to some results coming from Lyman-α forest observations. Besides being a clear and pedagogical analytical development to understand the evolution of WDM perturbations, our power spectrum results are consistent with the observations considered and the other determinations of the degree of warmness of dark matter particles.