DARK energy may not exist, according to an international team of astronomers who have used an X-ray satellite to count galaxy clusters in the early universe. If they are right, the expansion of the universe is not speeding up and the scientific community has been taken in by a huge cosmic mirage.
The idea of dark energy was first suggested in 1998 when a team led by Saul Perlmutter at the University of California at Berkeley reported that distant supernovae appeared fainter than they should, given their distance from the Earth. This could be explained by the expansion of the universe speeding up in the time the supernovae’s light had taken to reach us, driving them farther away than expected and making them appear fainter. Empty space appeared to be filled with dark energy, a weird anti-gravity stuff that was remorselessly driving the galaxies apart.
More recent observations of the dim “afterglow” of the big bang fireball, made with NASA’s Wilkinson Microwave Anisotropy Probe (WMAP) and announced earlier this year, appeared to confirm the existence of dark energy and so the astronomical community settled on a universe that was 30 per cent matter and 70 per cent dark energy. It is this conclusion that is now under attack.
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A team led by Alain Blanchard at the Astrophysics Laboratory of the Midi-Pyrénées Observatory in Toulouse used the European Space Agency’s XMM satellite to measure the X-ray spectra of galaxy clusters at very large distances and therefore far back in time.
The team found that the number of clusters in a given volume of space with a given luminosity has increased as the universe has grown older. Crucially, in a universe filled with dark energy, this number should have levelled off by now. “When dark energy gains control of the universe, its anti-gravity effect stops gravity pulling together the matter to make new clusters,” says Blanchard’s collaborator Subir Sarkar at the University of Oxford. The XMM results indicate that we live in a high-density universe, in which the total matter – visible matter plus invisible, or dark matter – nearly adds up to the critical density. “There is no need for dark energy,” says Sarkar.
The obvious question arises: how did astronomers miss all that extra matter in their telescopic surveys of the universe? The basic problem, says Sarkar, is that they cannot see dark matter directly but must infer its existence from the motion of visible material such as stars and galaxies. They could have got the ratio between dark matter and ordinary matter wrong, he says, causing them to underestimate the amount of dark matter.
Sarkar says a universe with no dark energy fits the WMAP data if we’ve got several common cosmological assumptions slightly wrong. One is that the primordial matter irregularities that led to the temperature variations seen by WMAP were not the same on all length scales. It is common practice to assume this but Sarkar points out that the widely believed theory of inflation, which describes the universe’s first split-second of existence, hints at something slightly different.
Astronomers may also have slightly over-estimated the Hubble constant – a measure of how fast space is expanding – say Sarkar and his colleagues, who include Michael Rowan-Robinson of London’s Imperial College and Marian Douspis of the University of Oxford. Rowan-Robinson has pointed out several possible sources of error in the value determined using the Hubble Space Telescope.
For the matter-driven picture to work, 12 per cent of the universe has to be in the form of neutrinos with a mass of 0.8 electronvolts. Blanchard and his colleagues spell out their alternative to the standard dark-energy driven universe in Astronomy & Astrophysics (vol 412, p 35).
The biggest pay-off from losing dark energy is that physicists would no longer have to put up with an idea that does not fit with their theories. For example, the Standard Model predicts an energy density for dark energy which is at odds with observations by a factor of 10123 – a disparity which has been described by Nobel laureate Steven Weinberg as “the worst failure of an order-of-magnitude estimate in the history of science”.
The team will face a fight to convince dark energy experts to give up on the idea, however. “In order to fit the WMAP data to a matter-dominated model, they require several new parameters, including a light neutrino,” says Princeton’s David Spergel, who has worked on the WMAP data.
If the WMAP data turns out not to support dark energy, as Blanchard’s team claims, then the sole evidence for dark energy will be the supernova observations. The tacit assumption here is that the supernovae in the distant past were the same as those today, with a standard intrinsic luminosity, but that assumption could be wrong. However, Alexei Filippenko, leader of one of the two teams at the University of California at Berkeley, whose supernova observations first suggested the existence of dark energy, stands by his original results. “The supernova observations are really pretty convincing now,” he says. “In fact, we will soon submit another paper, this time showing quite good evidence for an epoch of early cosmic deceleration followed by acceleration.”
Meanwhile England’s Astronomer Royal, Martin Rees, points out that the results could just mean that the behaviour of galaxy clusters is more complex than people imagined. They “shouldn’t be allowed to dislodge the interlocked set of arguments that imply a universe with 5 per cent ordinary matter, 25 per cent dark matter and 70 per cent dark energy”, he says.