Is the expansion of the universe accelerating? All signs point to yes
The accelerating expansion of the universe is one of the most profound discoveries in modern cosmology, pointing to a universe in which 70% of the mass energy density has an unknown form spread uniformly across the universe. This result has been well established using a combination of cosmological probes (e.g. Planck Collaboration et al. 2016) resulting in a "standard model" of modern cosmology that is a combination of a cosmological constant with cold dark matter and baryons. The first compelling evidence for the acceleration came in the late 1990's, when two independent teams studying type Ia supernovae discovered that distant SNe Ia were dimmer than expected. The combined analysis of modern cosmology experiments, including SNe Ia, the Hubble constant, baryon acoustic oscillations, and the cosmic microwave background has now measured the contributions of matter and the cosmological constant to the energy density of the universe to better than 0.01, providing a secure measurement of acceleration. A recent study (Tr{\o}st Nielsen et al. 2015) has claimed that the evidence for acceleration from SNe Ia is "marginal." Here we demonstrate errors in that analysis which reduce the acceleration significance from SNe Ia, and further demonstrate that conservative constraints on the curvature or matter density of the universe increase the significance even more. Analyzing the Joint Light curve Analysis supernova sample, we find 4.2{\sigma} evidence for acceleration with SNe Ia alone, and 11.2{\sigma} in a flat universe. With our improved supernova analysis and by not rejecting all other cosmological constraints, we find that acceleration is quite secure.