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Magazine section: This Week 

Go-faster Universe may just be a trick of the light

New Scientist vol 172 issue 2321 - 15 December 2001, page 15


SIGNS that the expansion of the Universe is accelerating may be an illusion. Scientists think particles called axions might create the mirage by hijacking light as it travels the cosmos.

Three years ago, cosmologists found that very distant supernova explosions are fainter than they should be. This suggested that space had stretched more than expected as the light sped towards Earth, so it had to travel farther. The Universe appeared to be expanding faster and faster (New Scientist, 11 April 1998, p 26).

With scientists struggling to explain what force could drive this acceleration, a team in the US has now come up with an alternative explanation. They say the light from distant supernovae is dim because some photons turn into axions on their way to Earth.

Axions are hypothetical particles predicted by several leading theories of particle physics. Even if they exist they'd be very difficult to detect because they rarely interact with other matter. But Csaba Csáki of Los Alamos National Laboratory in New Mexico says that if axions had a tiny mass—10-21 times that of the electron—they would interact with light in the tangled magnetic fields of intergalactic space. As photons travelled to Earth, this interaction would persuade a fraction of them to flip into axions themselves, making them undetectable.

With his Los Alamos colleague John Terning and Nemanja Kaloper of Stanford University in California, Csáki has calculated that about a third of the visible photons from the most distant supernovas would effectively vanish.

"We shouldn't just accept on face value that the Universe is accelerating," Csáki concludes. "There are alternative and viable explanations to the supernova observations based on particle physics."

Hazel Muir

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