Five years ago, the Nobel Prize for physics was awarded for the discovery that the universe is not just expanding, but expanding at a faster and faster rate. Cosmology has been turned upside down to explain how this could occur, leading researchers to explore how “dark energy” could be driving this acceleration. Yet controversial new research questions whether it is happening at all.
The evidence for the accelerating rate of the universe's expansion relies on measurements of Type Ia supernovae. These enormous explosions have the useful feature of being fairly consistent in how much energy they release. By studying how bright they look, we can work out their distance. In combination with measurements of the rate at which the supernovae are moving away from us, this allows astronomers a measure of the rate of expansion in the distant universe, which can be compared with that closer to home. The difference indicates the changes over time.
Several rounds of research concluded that the differences in the speeds with which Type Ia supernovae were moving away from us could only be explained if the universe once expanded more slowly than it does today.
Professor Subir Sarkar of Oxford University examined a database of 740 Type Ia supernovae, 10 times more than previous studies relied upon. While his work does not rule out the possibility of an accelerating rate of expansion, he says his findings are also consistent with a constant rate of expansion.
Sarkar and his co-authors have not simply rerun the same analysis using more explosions. Estimates of supernova distance rely on accounting for intervening dust and other factors that could influence the brightness, and Sarkar's methods are somewhat different from those used in the past.
“Naturally, a lot of work will be necessary to convince the physics community," Sarkar said in a statement. “It is quite possible that we are being misled and that the apparent manifestation of dark energy is a consequence of analyzing the data in an oversimplified theoretical model – one that was in fact constructed in the 1930s, long before there was any real data.”
The announcement of the universe's acceleration, and the theory of dark energy to explain it, has made many people unhappy. Physics departments worldwide have been bombarded with messages, some of which have even been published, claiming they've got it all wrong. Sarkar's work, however, is an entirely different matter, with the authors hailing from some of the world's leading centers for physics research and with their study published in Scientific Reports. That doesn't make Sarkar's claims right. Other astronomers are already finding plenty to criticize in the paper, but at least it will require detailed responses.
The European Extremely Large Telescope is being built with equipment specifically designed to measure the universe's rate of expansion, but is not set to even begin operating until 2024. In the meantime, astronomers will have to debate more limited data.