THE mysterious dark energy that is driving the expansion of the universe could be linked to neutrinos. This could account for some puzzling results from various neutrino detectors around the world and, for the first time, allow physicists to work out the source of dark energy.
In 1998, astronomers discovered that the expansion of the universe is accelerating and attributed the phenomenon to some kind of 鈥渄ark energy鈥� pushing space apart. There are several theories about the nature of this energy, but none of them can be realistically tested at the moment.
That might be about to change. Last year, Rob Fardon, Ann Nelson and Neal Weiner of the University of Washington in Seattle suggested a possible connection between dark energy and neutrinos, the lightest matter particles. Neutrinos were churned out in vast quantities in the fireball of the big bang, and are produced by stars and in nuclear reactors. They zip around the universe at enormous speeds, easily flying straight through planets and stars.
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Estimates of the energy density of neutrinos in the universe are sketchy, but they are roughly similar to estimates of the density of dark energy. The Washington team suggests this is not just a coincidence. They think dark energy is the result of neutrinos interacting with as-yet-undiscovered particles they call 鈥渁ccelerons鈥�. As the universe expands, the concentration of neutrinos falls. The new theory says the more the neutrinos spread, the greater is the energy of the acceleron field, making the neutrinos heavier. This, in turn, increases the energy density of the neutrinos, causing a rise in dark energy, which then fuels the expansion of the universe.
And Nelson, Weiner and their colleague David Kaplan also say this interaction could explain some curious results in neutrino experiments. As they travel, neutrinos tend to oscillate from one of their three types to another, with each type having a different mass. Researchers have tried to measure these differences in mass, but results from the Liquid Scintillator Neutrino Detector at Los Alamos National Laboratory in New Mexico imply that the mass differences between neutrinos are far higher than other experiments suggest.
The Washington team says these mass differences could depend upon whether neutrinos are travelling through air or rock, as they believe the acceleron field will be different in materials of different density. (). If they could confirm that the material neutrinos pass through affects their mass, it could link neutrinos to dark energy. 鈥淲e are very excited about the possible connection,鈥� says Nelson. 鈥淭his is the only theory of dark energy that relates it to particles we already know about and can do experiments with.鈥� The team鈥檚 work will appear in a future issue of the journal Physical Review Letters.
The theory has other implications. It suggests that the cosmic expansion rate will eventually slow down. That would happen when neutrinos get too heavy and too far apart to contribute to an increase in dark energy. 鈥淭he universe could continue to expand, but at an ever-decreasing rate,鈥� says Nelson.
Mark Trodden, a physicist who studies dark energy at Syracuse University in New York, welcomes the work and calls it an important step forward. 鈥淚t makes enough predictions that can be tested,鈥� he says. 鈥淚f we can probe the nature of dark energy through neutrino experiments, this would open up an important new window to the dark universe.鈥�