There have been a number of rumours of late that the evidence for dark energy is suspect, and that maybe, after all, it doesn’t exist. The stories are due to a paper which was recently published in Nature Scientific Reports, for a link to the original paper follow this link. Unlike most papers published in Nature, which are behind a paywall, this paper is available in its entirety for free. The authors argue that a larger data set of Type Ia supernovae, which were used in the 1990s as evidence for an accelerating Universe, now calls into question that whole interpretation of the data.
The 2011 Nobel prize for physics was awarded to Saul Perlmutter, Brian Schmidt and Adam Riess for their original “discovery” of cosmic acceleration, so if new data now call into question that whole idea it is, clearly, big news.
However, Adam Riess has co-authored an interesting guest blogpost in Scientific American, entitled “No, Astronomers Haven’t Decided Dark Energy Is Nonexistent”, here is a link to that article. Riess and his co-author Dan Scolnic (a cosmologist based at the University of Chicago’s Kavli Institute for Cosmological Physics) point out in this blogpost that the re-analysis by Nielsen etal. reduces the confidence that the Universe is accelerating to a 3-sigma result, which is still at a confidence level of 99.7%! So, it hardly does away with the need for acceleration, at a confidence of 99.7% it is still pretty likely. True, it now falls short of the usual 5-sigma result that scientists usually require for a “definite result”; but they also take issue with the way that Nielsen etal. have analysed their data.
Also, as Scolnic and Riess point out, evidence for cosmic acceleration is not just based on the results from surveys of Type Ia supernovae. Studies of the details of the anisotropies in the cosmic microwave background also require dark energy (thought to be responsible for cosmic acceleration), and so do surveys of the large scale structure of the Universe done by surveys such as the Sloan Digital Sky Survey and the 2 Degree Field Galaxy Redshift Survey.
This model, often called the concordance model, as it is supported by these 3 separate lines of evidence, is summarised in this figure. In this diagram, “BAO” are the results from the large scale structure surveys (the acronym stands for Baryonic Acoustic Oscillations). As the figure shows, the percentage of dark energy required to explain the results of SN, CMB and BAO is about 70% (0.7 on the y-axis).
You can read more about the three separate lines of evidence for dark matter in my book The Cosmic Microwave Background, How It Changed Our Understanding Of The Universe (follow this link to find out more about the book, including reviews).
It seems to me that this is a lot of fuss about nothing, and that the case for cosmic acceleration is as strong as ever. What do you think?