Author:
Aristizabal Sierra D.,Gariazzo S.,Villanueva A.
Abstract
Abstract
Cosmological scenarios with a non-standard equation of state can
involve ultrastiff fluids, understood as primordial fluids for which p/ρ > 1. Their energy densities can dominate the Universe
energy budget at early times, in the otherwise radiation dominated
epoch. During that period the Universe undergoes a faster expansion,
that has implications for any decoupling process that takes place in
that era. Quintessence models or Ekpyrotic cosmologies are good
examples of such scenarios. Assuming the ultrastiff state to be
thermally decoupled at very early times, if ever coupled, its
observational imprints are left solely in the Universe expansion
rate and in the radiation energy density. We consider a complete set
of ultrastiff fluids and study their signatures in the neutrino
decoupling and BBN eras. Measurements of N
eff alone place
mild constraints on these scenarios, with forthcoming measurements
from the Simons Observatory in the Chilean Atacama desert being able
to test regions where still sizable effects are observable. However,
when BBN data is taken into account, those regions are proven to be
barely reconcilable with primordial helium-4 and deuterium
abundances measurements. Our findings show that measurements of the
primordial helium-4 abundance imply the tightest constraints, with
measurements of primordial deuterium being — to a certain
extent — competitive as well. We point out that a ∼ 60%
improvement on the statistical uncertainty of the primordial
helium-4 abundance measurement, will test these scenarios in the
region where they can produce sizable effects. Beyond that precision
the regions that are accessible degenerate with standard
expectations. In that case, although potentially present, neither
neutrino decoupling nor BBN observables will be sensitive probes.
Subject
Astronomy and Astrophysics