The variation of the fine-structure constant from disformal couplings
  We study a theory in which the electromagnetic field is disformally coupled
to a scalar field, in addition to a usual non-minimal electromagnetic coupling.
We show that disformal couplings modify the expression for the fine-structure
constant, alpha. As a result, the theory we consider can explain the non-zero
reported variation in the evolution of alpha by purely considering disformal
couplings. We also find that if matter and photons are coupled in the same way
to the scalar field, disformal couplings itself do not lead to a variation of
the fine-structure constant. A number of scenarios are discussed consistent
with the current astrophysical, geochemical, laboratory and the cosmic
microwave background radiation constraints on the cosmological evolution of
alpha. The models presented are also consistent with the current type Ia
supernovae constraints on the effective dark energy equation of state. We find
that the Oklo bound in particular puts strong constraints on the model
parameters. From our numerical results, we find that the introduction of a
non-minimal electromagnetic coupling enhances the cosmological variation in
alpha. Better constrained data is expected to be reported by ALMA and with the
forthcoming generation of high-resolution ultra-stable spectrographs such as
PEPSI, ESPRESSO, and ELT-HIRES. Furthermore, an expected increase in the
sensitivity of molecular and nuclear clocks will put a more stringent
constraint on current laboratory measurements.
