Tentative evidence for slowing down of cosmic acceleration from recent small redshift supernovae and BAO data

Abstract

An investigation of dark energy (DE) using the Constitution SnIa sample (which includes recent CfA data at low redshifts) reveals a slight inconsistency (more than 1σ) with the standard spatially flat ΛCDM model, if the assumption of a constant equation of state (w) for DE is dropped. This effect, which is most clearly seen using the recently introduced Om diagnostic, corresponds to an increase of Om(z) and w(z) at redshifts z<0.3. In geometrical terms, this suggests that cosmic acceleration may have already peaked and that we are currently witnessing its slowing down. Interestingly, such DE behaviour also provides a better fit to baryon acoustic oscillation (BAO) data. Including the cosmic microwave background (CMB) ‘shift parameter’ as another independent observable, we show that the well known CPL parametrization strains to fit the data simultaneously at low and high redshifts. This could either be because of the presence of systematics, or because the CPL ansatz is not versatile enough to catch the rapid variation in DE suggested by the data. 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 SnIa+BAO+CMB data.