Cosmology in a certain vector-tensor theory of gravitation
  We study relevant cosmological topics in the framework of a certain
vector-tensor theory of gravitation (hereafter VT). This theory is first
compared with the so-called extended electromagnetism (EE). These theories have
a notable resemblance and both explain the existence of a cosmological
constant. It is shown that, in EE, a positive dark energy density requires a
Lagrangian leading to quantum ghosts, whereas VT is free from these ghosts. On
account of this fact, the remainder of the paper is devoted to study cosmology
in the framework of VT. Initial conditions, at high redshift, are used to solve
the evolution equations of all the VT scalar modes. In particular, a certain
scalar mode characteristic of VT -which does not appear in general relativity
(GR)- is chosen in such a way that it evolves separately. In other words, the
scalar modes of the standard model based on GR do not affect the evolution of
the VT characteristic mode; however, this scalar mode influences the evolution
of the standard GR ones. Some well known suitable codes (CMBFAST and COSMOMC)
have been modified to include our VT initial conditions and evolution
equations, which are fully general. One of the resulting codes -based on
standard statistical methods- has been used to fit VT predictions and
observational evidences about both Ia supernovae and cosmic microwave
background anisotropy. Seven free parameters are used in this fit. Six of them
are often used in GR cosmology and the seventh one is characteristic of VT.
From the statistical analysis it follows that VT seems to be advantageous
against GR in order to explain cosmological observational evidences.
