Is cosmic acceleration slowing down?
  We investigate the course of cosmic expansion in its `recent past' using the
Constitution SN Ia sample (which includes CfA data at low redshifts), jointly
with signatures of baryon acoustic oscillations (BAO) in the galaxy
distribution and fluctuations in the cosmic microwave background (CMB). Earlier
SN Ia data sets could not address this issue because of a paucity of data at
low redshifts. Allowing the equation of state of dark energy (DE) to vary, we
find that a coasting model of the universe (q_0=0) fits the data about as well
as LCDM. This effect, which is most clearly seen using the recently introduced
`Om' diagnostic, corresponds to an increase of Om(z) and q(z) at redshifts z
\lleq 0.3. In geometrical terms, this suggests that cosmic acceleration may
have already peaked and that we are currently witnessing its slowing down. The
case for evolving DE strengthens if a subsample of the Constitution set
consisting of SNLS+ESSENCE+CfA SN Ia data is analysed in combination with
BAO+CMB using the same statistical methods. The effect we observe could
correspond to DE decaying into dark matter (or something else). A toy model
which mimics this process agrees well with the combined SN Ia+BAO+CMB data.
