The Universe acceleration from the Unimodular gravity view point:
  Background and linear perturbations
  With the goal of studying the cosmological constant (CC) problem, we present
an exhaustive analysis of unimodular gravity as a possible candidate to resolve
the CC origin and with this, the current Universe acceleration. In this theory,
a correction constant (CC-like) in the field equations sources the late cosmic
acceleration. This constant is related to a new parameter, $z_{ini}$, which is
interpreted as the redshift of CC-like emergence. By comparing with the CC
value obtained from Planck and Supernovaes measurements, it is possible to
estimate $z_{ini}=11.15^{+0.01}_{-0.02}$ and $z_{ini}=11.43^{+0.03}_{-0.06}$
respectively, which is close to the reionization epoch. Moreover, we use the
observational Hubble data (OHD), Type Ia Supernovae (SnIa), Baryon Acoustic
Oscillations (BAO) and the Cosmic Microwave Background Radiation (CMB) distance
data to constrain the UG cosmological parameters. A Joint analysis
(OHD+SnIa+BAO+CMB), results in $z_{ini}=11.47^{+0.074}_{-0.073}$ within
$1\sigma$ confidence level consistent with our estimation from Planck and
Supernovae measurements. We also include linear perturbations, starting with
scalar and tensor perturbations and complementing with the perturbed Boltzmann
equation for photons. We show that the $00$ term in the UG field equations and
the Boltzmann equation for photons contains corrections, meanwhile the other
equations are similar as those obtained in standard cosmology.
