One of the most baffling results in modern physics was the discovery that the universe is tearing itself apart. In the late 1990s, astronomers realised the universe was expanding at an ever accelerating rate. This led to the idea that the universe is dominated by mysterious “dark energy”, making up 68% of the universe.
Now, new research says that this idea, which has become a pillar of modern physics, may be built on shaky foundations. An analysis of 740 exploding stars published in the journal Scientific Reports last week concluded the expansion of the universe may be constant after all.
“The evidence for accelerated expansion is marginal,” says Subir Sarkar at the University of Oxford in the UK, who led the study. If that’s the case, dark energy may not exist. So should the winners of the 2011 physics Nobel – Adam Riess, Brian Schmidt and Saul Perlmutter – hand it back?
For most of the 20th century, we’ve thought the universe was expanding at a constant rate. Then in 1998, two independent teams found apparently conclusive evidence that the universe was expanding at an increasing rate. The bottom line: something must be pushing galaxies apart from one another, though we have no idea know what this something is. As a placeholder, physicists call it “dark energy”.
The discovery of dark energy relied on a particular kind of exploding star – Type 1a supernovae – acting as a "standard candle". Astronomers assume each supernova emits the same amount of light and so can be used as distance markers – a bit like gauging the distance to a town from the brightness of its streetlights.
Type 1a supernovae arise in twin star systems, when a white dwarf star feeds off its sibling until it reaches a critical mass and explodes. Because the tipping point is the same mass each time, the explosions are identical or so goes the theory. In reality, important differences between these explosions can muddy the waters if not properly accounted for.
Over the past decade, astronomers amassed a much larger database of supernovae than was available back in 1998. And on closer inspection, the evidence for an expanding universe (and hence dark energy) appears less solid. While the work behind the 2011 Nobel prize in physics included data from just 50 such supernovae, astronomers in England, Italy and Denmark have now examined data from 740 such supernovae.
The new research still sees evidence of accelerated expansion, but at a much lower statistical significance than previously claimed. Indeed, their findings would be consistent with no acceleration at all – and hence no dark energy. Although other evidence for dark energy exists, such as in the cosmic microwave background, Sarkar says these tests are indirect. Other solutions may account for all of them without requiring dark energy.
But Brad Tucker, an astronomer at the Australian National University in Canberra, disagrees. He argues that dark energy is part of a large self-consistent picture, assembled from multiple lines of evidence.
“We know that different cosmological probes tell us different things, and it is when they work together we understand what is going on – or try to, throwing the baby out with the bath water is not the way to go.” he says.
In contrast to Sarkar’s work, Tucker was part of a team that recently measured the acceleration of the universe to greater accuracy than ever before, and found it even higher than expected. Meanwhile dark energy is not the only member of the dark side of the universe recently under question.
Last week, we reported on research led by Case Western Reserve University’s Stacy McGaugh showing that galaxy spin might be explained without invoking dark matter. But already, a pair of astrophysicists at McMaster University in Canada performed a new study (uploaded to arXiv but not yet peer-reviewed) dismissing that interpretation. They found McGaugh’s findings could be explained by a simulation incorporating the standard dark matter picture.
Challenging the prevailing theory is a normal part of the scientific process and the standard model of cosmology seems to emerge from these melees unscathed. Our universe is still dominated by the dark side.