The mysterious dark matter that fills the universe could be made of the same particles that put the “big” in the big bang – explaining both inflation and dark matter in a single stroke. Cosmologists believe that the early universe went through a period of expansion, known as inflation, soon after the big bang – although they do not know exactly what caused it. Now cosmologist Andrew Liddle at the University of Sussex, UK, and his colleagues say one particle may be responsible for both inflation and the dark matter that has been perplexing astronomers. “We have these two mysteries that we don’t really understand, so we thought, why not put them together to halve the problem?” Liddle told New Scientist. “It’s so simple that at first we worried it might be ludicrous.” Liddle’s theory hinges on the notion suggested by other cosmologists that inflation is caused by a hypothetical particle, aptly named the inflaton. “The inflaton has weird pressure properties that we don’t see in everyday particles,” explains Liddle. For instance, as they are created in the early universe, they push space apart, forcing it to expand. The standard thinking goes that inflatons would have decayed into normal particles soon after inflation finished in the first fraction of a second after the big bang, with none left hanging around today. However, Liddle and his colleagues realised that if some inflaton particles survived after the intense burst of expansion during inflation, their mass would become important in attracting surrounding matter.
Collide and annihilate
“By the late stages of inflation and forever afterwards, the expansion rate has slowed down so that the particle mass becomes important, and the particles then behave as massive dark matter particles exhibiting gravitational attraction,” he explains. The problem was getting some inflatons to stick around. “It’s easy to explain why they all disappear, but why would most disappear, and only a few stay behind?” says Liddle. If inflatons can interact with normal particles, even very weakly, as normally assumed, they will easily decay whenever they collide with any particle. To get around this, Liddle suggests that inflatons only decay when they collide with other inflatons – with both members of the pair annihilating. His calculations show that while most inflatons would indeed be annihilated after inflation, a precious few would escape, as required. Finding evidence for the theory will be tough, however. Since Liddle’s inflatons don’t interact with normal matter, he says that they cannot show up in direct dark matter searches, such as the Cryogenic Dark Matter Search in Minnesota, US. While most dark matter hunters are disappointed that these searches haven’t turned up the elusive particle yet, Liddle jokingly says that he gives a “little cheer” at their failure. “Of course, if they ever do find anything, it will immediately disprove my idea,” he says.
Simple but important
Since the inflatons are now widely dispersed, there is little chance that they will come into contact with each other and annihilate today, or in the future. But if they do, they could create a detectable burst of high-energy radiation, adds Liddle, who presented the work at the COSMO 07 conference at the University of Sussex in August. Cosmologist David Lyth at Lancaster University, UK, likes the proposal. “It’s a very neat idea,” he says. “Liddle has a way of putting his finger on very simple ideas that could turn out to be important.” Lev Kofman at the Canadian Institute of Theoretical Astrophysics in Toronto also likes the theory. “It would certainly be nice if the same guy was responsible for both dark matter and inflation,” he says. Kofman worked on a similar idea in the past, but found it difficult to explain how the correct number of inflatons survived inflation. He is impressed that Liddle’s team have attempted to address this, but adds that more work needs to be done before cosmologists accept the idea. “It’s very appealing, but let’s say, I won’t bet my house on it just yet,” says Kofman.