arXiv subjects

D. A. Tamayo

Publications and source records attributed to D. A. Tamayo.

4 recordsLinked to original sources

arXiv2025

Note on bulk viscosity as an alternative to dark energy

Bulk viscosity, which characterizes the irreversible dissipative resistance of a fluid to volume changes, has been proposed as a potential mechanism for explaining both early- and late-time accelerated expansion of the Universe. In this work, we investigate two distinct physical scenarios for the origin of bulk viscosity: (1) nonminimal interactions between two fluids, and (2) elastic collisions in an ideal gas. In both cases, we demonstrate that while the associated energy-momentum exchange can significantly influence fluid dynamics, overall energy-momentum conservation precludes such exchange from having any direct gravitational effect in the context of General Relativity. In case (1), we show that the standard bulk viscous energy-momentum tensor can be obtained for the two-fluid system only at the cost of the violation of all classical energy conditions: null, weak, dominant, and strong. In case (2), we consider a single fluid composed of point particles undergoing instantaneous, energy- and momentum-conserving collisions, and find that the proper pressure remains strictly non-negative, with the equation-of-state parameter confined to the interval $[0,1/3]$. In both scenarios, achieving a sufficiently negative effective pressure to drive cosmic acceleration requires assumptions that compromise the physical viability of the model. Our results highlight some of the key physical challenges involved in modeling dark energy through bulk viscous effects.

astro-ph.CO↗

arXiv2017

Production of primordial gravitational waves in a simple class of running vacuum cosmologies

The problem of cosmological production of gravitational waves is discussed in the framework of an expanding, spatially homogeneous and isotropic FRW type Universe with time-evolving vacuum energy density. The gravitational wave equation is established and its modified time-dependent part is analytically resolved for different epochs in the case of a flat geometry. Unlike the standard $Λ$CDM cosmology (no interacting vacuum), we show that gravitational waves are produced in the radiation era even in the context of general relativity. We also show that for all values of the free parameter, the high frequency modes are damped out even faster than in the standard cosmology both in the radiation and matter-vacuum dominated epoch. The formation of the stochastic background of gravitons and the remnant power spectrum generated at different cosmological eras are also explicitly evaluated. It is argued that measurements of the CMB polarization (B-modes) and its comparison with the rigid $Λ$CDM model plus the inflationary paradigm may become a crucial test for dynamical dark energy models in the near future.

astro-ph.CO↗

arXiv2016

Primordial Gravitational Waves in Running Vacuum Cosmologies

We investigate the cosmological production of gravitational waves in a nonsingular flat cosmology powered by a "running vacuum" energy density described by $ρ_Λ\equivρ_Λ(H)$, a phenomenological expression potentially linked with the renormalization group approach in quantum field theory in curved spacetimes. The model can be interpreted as a particular case of the class recently discussed by Perico et al. (Phys. Rev. D {\bf 88}, 063531, 2013) which is termed complete in the sense that the cosmic evolution occurs between two extreme de Sitter stages (early and late time de Sitter phases). {The gravitational wave equation is derived and its time-dependent part numerically integrated since the primordial de Sitter stage. The generated spectrum of gravitons is also compared with the standard calculations where an abrupt transition, from the early de Sitter to the radiation phase, is usually assumed.} It is found that the stochastic background of gravitons is very similar to the one predicted by the cosmic concordance model plus inflation except at higher frequencies ($ν\gtrsim 100$ kHz). This remarkable signature of a "running vacuum" cosmology combined with the proposed high frequency gravitational wave detectors and measurements of the CMB polarization (B-modes) may provide a new window to confront more conventional models of inflation.

astro-ph.CO↗

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.

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