Constraining alternatives to a cosmological constant: generalized
  couplings and scale invariance
  We present a comparative analysis of observational low-redshift background
constraints on three candidate models for explaining the low-redshift
acceleration of the universe. The generalized coupling model by Feng and
Carloni and the scale invariant model by Maeder (both of which can be
interpreted as bimetric theories) are compared to the traditional
parametrization of Chevallier, Polarski and Linder. In principle the
generalized coupling model, which in vacuum is equivalent to General
Relativity, contains two types of vacuum energy: the usual cosmological
constant plus a second contribution due to the matter fields. We show that the
former is necessary for the model to agree with low-redshift observations,
while there is no statistically significant evidence for the presence of the
second. On the other hand the scale invariant model effectively has a
time-dependent cosmological constant. In this case we show that a matter
density $\Omega_m\sim0.3$ is a relatively poor fit to the data, and the
best-fit model would require a fluid with a much smaller density and a
significantly positive equation of state parameter.
