Could accelerating Universe be a mirage?

BY KULVINDER SINGH CHADHA

ASTRONOMY NOW

Posted: 20 April, 2009

Two astronomers from Durham University have claimed doubt about the idea of an accelerating Universe in a talk at the 2009 European Week of Astronomy and Space Science at the University of Hertfordshire, at least when it comes to one line of evidence. The doubts have been raised by Doctor Utane Sawangwit and Professor Tom Shanks of Durham University, whose argument rests on a lack of something called the Integrated Sachs-Wolfe Effect (ISW) in a distribution of 800,000 galaxies.

A trio of galaxies. Image: NASA, ESA, M. Livio and the Hubble Heritage Team (STScI/AURA).

The ISW Effect is a phenomenon in which photons of the Cosmic Microwave Background radiation (CMB) are redshifted by gravitational potential wells in space. These gravity wells are created by matter (both normal and dark) and photons gain energy when they 'fall' into them, then lose energy when they come out. In a Universe dominated by matter, the overall energy change of the photons should remain the same, but when the space-stretching antics of dark energy come into play, this all changes.

As dark energy (which is thought by most cosmologists to consist of 73 percent of the energy density of the Universe, with dark matter and normal matter consisting of the rest) has the opposite effect of gravity, it stretches space, accelerating its expansion. If this was the case in our Universe then the gravitational wells would be stretched out and made shallower, argue Shanks and Sawangwit, meaning that the CMB photons would gain more energy than they lose by the time they 'climb out'. This would make for a lopsided CMB spectrum when looking at projections greater than ten degrees on the sky. A lot of data from galaxy distributions (which contain a lot of mass for making gravity wells) appears to show this, and therefore that the Universe is indeed accelerating. But could it be a mirage?

In his talk, Professor Shanks pointed out fundamental problems with the standard cosmological model, which combines the cosmological constant with cold dark matter. Namely, he says that the fact that dark matter and dark energy are required leads to new physics. Despite intensive experimentation, to date no one has directly detected dark matter particles (although there is very strong evidence for dark matter's existence from its gravitational influence in galaxy cluster mergers) and dark energy is even more mysterious. The talk described two schools of thought on the problem: the optimists who liken the current search to the search for neutrinos (which were once thought to be theoretical but are now known to be real), and the pessimists who liken it to the search for the ether (which was discredited).

Cross correlating CMB data from WMAP (Wilkinson Microwave Anisotropy Probe) with that of the Sloan Digital Sky Survey's Luminous Red Galaxies (SDSS LRG), Sawangwit and Shanks found a mixture of both positive and negative data points. In other words, the results were inconclusive and no ISW Effect was seen. As Sawangwit told Astronomy Now, "The [currently accepted] signals are small and could be a systematic effect, so they may not be as secure as they're thought."

Professor Andy Taylor of Edinburgh University, who was present at the same presentation says, “Tom’s quite right to say that evidence for the ISW is not as strong as people think it is. The effect should appear on large scales, but we’re still building surveys. Even then, the evidence is quite tentative.” However, he also says that there are caveats. “We know that there are odd effects in the CMB and because you have to remove things such as the Galactic foreground you’re playing around with that data.”

Adding to what Sawangwit says, Shanks says, "This throws doubt on previous evidence. It would also mean that Modified Newtonian Dynamics [an alternative to dark matter that consists of tweaking gravity] would also be excluded." If the pair's results hold water then the only model left according to Shanks that could explain the Universe would be the Einstein-DeSitter model (a matter-dominated, flat Universe with just enough energy to carry on expanding by itself). Taylor though says, “Other ‘pillars’ of evidence show that the Universe is accelerating and you would have to go to extremes to show otherwise, and I think Tom and Utane would agree.”

Currently Sawangwit and Shanks only have data from the Northern Hemisphere sky. Shanks is the principal investigator of the VST ATLAS survey, which will complement the SDSS results by looking at 4,500 square-degrees of the Southern Hemisphere sky. It remains to be seen whether this will confirm or deny their findings and whether current cosmological thinking will need to be modified.