Dark energy survey result dims hope for cosmological constant

A dark energy survey 10 years in the making has made a surprise finding, suggesting that the theory of the expansion of the universe might not be correct.

The final Dark Energy Survey (DES) measurement was released at the 243rd American Astronomical Society meeting in New Orleans and published on pre-print server arXiv.

Dark energy is dark matter’s even more elusive cousin. Studies have suggested that dark energy is estimated to make up almost 70% of the observable Universe and is immensely important for measurements of the acceleration of the Universe’s expansion. But, scientists also have no idea what it is.

One theory is that this dark energy should fit nicely into something called the ‘cosmological constant’ which Einstein added, and then removed, from his calculations into general relativity.

Although Einstein tossed it, it was later revived when it was realised that the universe was expanding at an accelerating rate.

“Einstein’s concept of the cosmological constant could actually explain dark energy if it had a positive value (allowing it to conform to the accelerating expansion of the cosmos),” said Professor Robert Nichol, a member of the DES collaboration in a piece for The Conversation.  Nichol is also a Pro Vice Chancellor at the University of Surrey.

If dark energy is the cosmological constant, the “equation of state” of dark energy would equal -1. The DES used the best space object we have to try and measure this equation of state – Type Ia supernovae or ‘standard candles’. These are stellar explosions which release consistently bright flashes, allowing researchers to measure how far away they are in the Universe.

“It is very exciting times to see this innovative technology to harness the power of large astronomical surveys”, says one of the researchers, Dr Anais Möller from Swinburne University of Technology.

“Not only we are able to obtain more type Ia supernovae than before, but we tested these methods thoroughly as we want to do more precision measurements on the fundamental physics of our universe.”

An international team of more than 100 scientists mapped an area almost one-eighth of the sky using the 570-megapixel Dark Energy Camera. They took data for 758 nights across six years.

But the results, weren’t what they were expecting. Instead of an equation of state of -1, the end result from the DES was -0.8.

That’s a pretty large difference in astronomy terms, and would suggest that the cosmology constant is wrong. But scientists aren’t so sure yet.

First there’s the error bars – about plus or minus 0.18. Combined with data from the ESA’s Plank telescope the uncertainty is large enough that there’s still a 5% chance of -1.

That’s only a 1 in 20 odds, not ideal, but the results aren’t sure enough for researchers to confirm either way.

“As usual, scientists want more data,” said Nichol.

“The DES results suggest that our new techniques will work for future supernova experiments with ESA’s Euclid mission (launched July 2023) and the new Vera Rubin Observatory in Chile.”