On the accelerated expansion of the cosmos
  We present a short (and necessarily incomplete) review of the evidence for
the accelerated expansion of the Universe. The most direct probe of
acceleration relies on the detailed study of supernovae (SN) of type Ia.
Assuming that these are standardizable candles and that they fairly sample a
homogeneous and isotropic Universe, the evidence for acceleration can be tested
in a model- and calibration-independent way. Various light-curve fitting
procedures have been proposed and tested. While several fitters give consistent
results for the so-called Constitution set, they lead to inconsistent results
for the recently released SDSS SN. Adopting the SALT fitter and relying on the
Union set, cosmic acceleration is detected by a purely kinematic test at 7
sigma when spatial flatness is assumed and at 4 sigma without assumption on the
spatial geometry. A weak point of the described method is the local set of SN
(at z < 0.2), as these SN are essential to anchor the Hubble diagram. These SN
are drawn from a volume much smaller than the Hubble volume and could be
affected by local structure. Without the assumption of homogeneity, there is no
evidence for acceleration, as the effects of acceleration are degenerate with
the effects of inhomogeneities. Unless we sit in the centre of the Universe,
such inhomogeneities can be constrained by SN observations by means of tests of
the isotropy of the Hubble flow.
