Dark energy is not an illusion after all
New measurements of exploding stars are challenging an upstart theory that dark energy is just an illusion caused by our location within a giant void.
In 1998, astronomers reported that the universe's expansion seems to be faster now than it was in the past, based on measurements of supernova explosions in both nearby and distant galaxies. The latter provide a record of the past because of the time it takes their light to reach us.
That the universe's expansion could be accelerating was a surprise, since gravity should act as a brake on the expansion, slowing it with time. The most popular explanation is that energy of unknown origin – called dark energy – permeates space and acts as a repulsive force to speed up the expansion.
But some researchers have proposed an alternative: that the acceleration is an illusion that results from an uneven distribution of matter in the universe.
They accept that the expansion rate in the local universe is higher than in more distant regions. But instead of assuming the expansion rate has increased with time, they suggest our patch of the universe happens to contain less matter than average. Within this "void", the expansion rate is higher than outside because there is less gravity to slow it down.
But new, more precise measurements of supernovae, taken by the Hubble Space Telescope, clash with the simplest version of the void model. That model could be made to fit previous supernova measurements and other cosmological data, but only if the local expansion rate is about 60 kilometres per second per megaparsec or less. (One megaparsec is 3.26 million light years.)
That was within the possible error of previous measurements, but the new, more precise measurements give an expansion rate of 74 kilometres per second per megaparsec, plus or minus 2.4.
"It looks more like it's dark energy that's pressing the gas pedal," says Adam Riess of Johns Hopkins University in Baltimore, Maryland, who led the observations. The results appear in The Astrophysical Journal.
Void within a void?
But Subir Sarkar of the University of Oxford, a proponent of the void theory who was not involved in Riess's study, says the results are not a fatal blow. "The observers have done a good job, but it should be kept in mind that there is some flexibility in the alternative models, which can in fact accommodate higher values" for the local expansion, he says.
He points to a study by Tirthabir Biswas of Saint Cloud State University in Minnesota and colleagues, published in November in the Journal of Cosmology and Astroparticle Physics, which tested a variety of void models against astronomical data. Some of them allow local expansion rates as high as the new Hubble value by positing a "void within a void", where the density of matter is not constant within the void itself, but drops off steeply towards its centre.
Although such a model might seem contrived, the alternative is to invoke dark energy, whose origin is very hard to explain, says Sarkar. "I would rather believe that the universe is a little more complicated than the standard cosmological model assumes it to be," he says.