WHERE does the energy that makes the Universe expand come from? If a cosmologist in the US is right, it comes from nowhere at all and physicists may have to rethink the principle of energy conservation. This principle, which is one of the cornerstones of modern physics, says that energy cannot simply appear or disappear; instead, it can only be converted into another form.
Edward Harrison of the University of Massachusetts at Amherst started thinking about the origin of the energy that drives the Universe鈥檚 expansion after receiving a letter from a chemist who asked: 鈥淲hat happens in an expanding Universe if you join two bodies with a string?鈥�
Harrison devised a 鈥渢hought experiment鈥� to answer this question. He imagined a situation in which one mass is tethered to a much larger mass by a light, inelastic string. As the expansion of the Universe moves the two masses apart, a winding mechanism on the large mass plays out the string. If the string is made taut, however, this tension will slow the speed at which the smaller mass is moving relative to the larger one. It will also have a second effect: the small mass will begin to move relative to the expansion of the Universe as a whole. Eventually, the tension in the string will halt the motion of the two masses relative to each other.
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The problem for the law of conservation of energy is what happens if the small mass is then released. The equations of cosmology predict that it will once again start moving along with the expansion of the Universe, just as a branch set adrift in a river has no choice but to go at the speed of the current. This will drag the string over the winding mechanism, imparting energy to the mechanism鈥檚 cogs and wheels as it goes. 鈥淎s far as I can make out, the energy extracted does not exist previously in any identifiable form,鈥� says Harrison.鈥�
鈥淭he fact that energy can, even in principle, be harnessed from the expansion of the Universe throws into focus a problem that physicists have conveniently ignored for many years 鈥� where does the energy that drives the Universe鈥檚 expansion come from?
Cosmologists believe that energy is one of a very select band of 鈥渃onserved鈥� physical quantities which can neither be created nor destroyed. In every instance where the energy of a body appeared to change, a close examination always revealed that energy had merely been transformed from one form into another.
But Harrison argues that there are other situations which physicists have trouble squaring with the law of conservation of energy. Light waves travelling between galaxies, for instance, get stretched out by the expansion of the Universe, which increases their wavelength 鈥� a phenomenon called red shift. Light with a longer wavelength carries less energy, but it is not clear what happens to the energy that is 鈥渓ost鈥� by red shift. Harrison believes that the conservation of energy applies only locally. 鈥淓nergy is conserved in systems with boundaries but not in an unbounded system like the Universe,鈥� he says.
What does all this mean for physics? Harrison admits that he is not sure what to make of it. He will publish the details of his thought experiment in the 11 June issue of The Astrophysical Journal.