In the early 1900s, Edwin Hubble observed that distant galaxies are moving away from us. This surprising observation led Hubble to hypothesize that the universe is expanding; more recently, scientists have observed that not only is the universe expanding, but that the expansion is accelerating! The evidence for acceleration emerged from observations of a particular kind of supernovae (Type Ia supernovae) that have a uniform expected brightness. This uniform brightness allowed scientists to determine how far away each supernova was and to discover that distant supernovae were moving slower than expected, and conclude that the universe hasn’t always been expanding at the same rate.

Flash forward to this year – astronomers studying the same Type Ia supernovae made the contrary claim that a more rigorous analysis of the data rules out a universe in which the expansion is accelerating. A number of media outlets have sensationalized this finding, however, the new analysis only reduces the likelihood of an accelerating expansion by a mere 0.3%. That means that we can still say with 99.7% certainty that the universe is indeed accelerating. Adding to the overblown nature of these analyses, scientists have criticized the study for its failure to accurately account for a number of small, but significant differences (like intervening dust) which should be accounted for in analyzing the data.

Even if the researchers’ new analyses turn out to be wrongly criticized, Harvard Astronomy graduate student Jieun Choi remarks, “There are independent results that have now demonstrated time and time again that the universe is accelerating in its expansion. So even if we took Type Ia supernovae out of the picture, we still have sufficient evidence,” including fluctuations in the cosmic microwave background (radiation from the early universe), among others. Despite what the headlines would have you believe, it appears that all signs still point to an accelerating universe.

Acknowledgements: Many thanks to Jieun Choi, a graduate student in the Harvard Astronomy department, for her extremely helpful insight and expertise on the subject.

Managing Correspondent: Tarraneh Eftekhari

Scientific Paper: Marginal evidence for cosmic acceleration from Type Ia supernovae
Response Paper: Is the expansion of the universe accelerating? All signs point to yes

Media Coverage: No, the Universe is not expanding at an accelerated rate, say physicists – Science Alert