arXiv subjects

E. L. D. Perico

Publications and source records attributed to E. L. D. Perico.

6 recordsLinked to original sources

arXiv2016

Running Vacuum Cosmological Models: Linear Scalar Perturbations

In cosmology, phenomenologically motivated expressions for running vacuum are commonly parametrized as linear functions $Λ(H^2)$ or $Λ(R)$. Such kind of models assume an equation of state for vacuum given by $\,\overline P_Λ=-\,\overlineρ_Λ$, relating their background pressure $\,\overline P_Λ$ and mean energy density $\,\overlineρ_Λ\equivΛ/8πG$. This equation of state requires that the dynamic for vacuum is due to the energy exchange with the material species. Most of the approaches to background level consider only the energy exchange between vacuum and the transient dominant material component of the universe. We extend such models assuming the running vacuum as the sum of independent contributions $\,\overlineρ_Λ =\sum_i\,\overlineρ_{Λi}$, associated with (and interacting with) each of the $i$ material species. We derive the linear scalar perturbations for two running scenarios, modeling its cosmic evolution and identifying their different imprints on the cosmic microwave background anisotropies and the matter power spectrum. In the $Λ(H^2)$ scenario the running vacuum are coupled with all the material species in the universe, whereas the $Λ(R)$ description only leads to coupling between vacuum and the non-relativistic matter components; which produces different imprints of the two models on the matter power spectrum. A comparison with the Planck 2015 data was made in order to constrain the free parameters of the models. In the case of the $Λ(H^2)$ model, it was found that $Ω_Λ=0.705\pm0.027$ and $H_0=69.6\pm2.9\, km\, Mpc^{-1}\, s^{-1}$, which diminish the tension with the low redshift expectations.

astro-ph.CO↗

arXiv2015

Decaying Vacuum Inflationary Cosmologies: A Complete Scenario Including Curvature Effects

We propose a large class of nonsingular cosmologies of arbitrary spatial curvature whose cosmic history is determined by a primeval dynamical $Λ(t)$-term. For all values of the curvature, the models evolve between two extreme de Sitter phases driven by the relic time-varying vacuum energy density. The transition from inflation to the radiation phase is universal and points to a natural solution of the graceful exit problem regardless of the values of the curvature parameter. The flat case recovers the scenario recently discussed in the literature (Perico et al., Phys. Rev. D88, 063531, 2013). The early de Sitter phase is characterized by an arbitrary energy scale $H_I$ associated to the primeval vacuum energy density. If $H_I$ is fixed to be nearly the Planck scale, the ratio between the relic and the present observed vacuum energy density is $ρ_{vI}/ρ_{v0} \simeq 10^{123}$.

gr-qc↗

arXiv2013

Complete Cosmic History with a dynamical Lambda(H) term

In the present mainstream cosmology, matter and spacetime emerged from a singularity and evolved through four distinct periods: early inflation, radiation, dark matter and late-time inflation (driven by dark energy). During the radiation and dark matter dominated stages, the universe is decelerating while the early and late-time inflations are accelerating stages. A possible connection between the accelerating periods remains unknown, and, even more intriguing, the best dark energy candidate powering the present accelerating stage (Lambda-vacuum) is plagued with the cosmological constant and coincidence puzzles. Here we propose an alternative solution for such problems based on a large class of time-dependent vacuum energy density models in the form of power series of the Hubble rate, Lambda=Lambda(H). The proposed class of Lambda(H)-decaying vacuum model provides: i) a new mechanism for inflation (different from the usual inflaton models), (ii) a natural mechanism for a graceful exit, which is universal for the whole class of models; iii) the currently accelerated expansion of the universe, iv) a mild dynamical dark energy at present; and v) a final de Sitter stage. Remarkably, the late-time cosmic expansion history of our class of models is very close to the concordance LambdaCDM model, but above all it furnishes the necessary smooth link between the initial and final de Sitter stages through the radiation- and matter-dominated epochs.

astro-ph.CO↗

arXiv2013

Black Hole Formation with an Interacting Vacuum Energy Density: Curvature Effects

The gravitational collapse of a spherically symmetric massive core of a star in which the fluid component is interacting with a growing vacuum energy density filling a FLRW type geometry with an arbitrary curvature parameter is investigated. The complete set of exact solutions for all values of the free parameters are obtained and the influence of the curvature term on the collapsing time, black hole mass and other physical quantities are also discussed in detail. We show that for the same initial conditions the total black hole mass depends only on the effective matter density parameter (including the vacuum component). It is also shown that the analytical condition to form a black hole i.e. the apparent horizon is not altered by the contribution of the curvature terms, however, the remaining physical quantities are quantitatively modified.

gr-qc↗

arXiv2011

Reproducing neutrino effects on the matter power spectrum through a degenerate Fermi gas approach

Modifications on the predictions about the matter power spectrum based on the hypothesis of a tiny contribution from a degenerate Fermi gas (DFG) test-fluid to some dominant cosmological scenario are investigated. Reporting about the systematic way of accounting for all the cosmological perturbations, through the Boltzmann equation we obtain the analytical results for density fluctuation, $δ$, and fluid velocity divergence, $θ$, of the DFG. Small contributions to the matter power spectrum are analytically obtained for the radiation-dominated background, through an ultra-relativistic approximation, and for the matter-dominated and $Λ$-dominated eras, through a non-relativistic approximation. The results can be numerically reproduced and compared with those of considering non-relativistic and ultra-relativistic neutrinos into the computation of the matter power spectrum. Lessons concerning the formation of large scale structures of a DFG are depicted, and consequent deviations from standard $Λ$CDM predictions for the matter power spectrum (with and without neutrinos) are quantified.

astro-ph.CO↗

arXiv2011

Degenerate Fermi gas perturbations at standard background cosmology

The hypothesis of a tiny fraction of the cosmic inventory evolving cosmologically as a degenerate Fermi gas test fluid at some dominant cosmological background is investigated. Our analytical results allow for performing preliminary computations to the evolution of perturbations for relativistic and non-relativistic test fluids. The density fluctuation, $δ$, the fluid velocity divergence, $θ$, and an explicit expression for the dynamics of the shear stress, $σ$, are obtained for a degenerate Fermi gas in the background regime of radiation. Extensions to the dominance of matter and to the $Λ$CDM cosmological background are also investigated and lessons concerning the formation of large structures of degenerate Fermi gas are depicted.

astro-ph.CO↗