Dark energy is a mysterious force making the universe fly apart faster and faster.
Its discovery came from one of the most unexpected results in the history of astronomy: in the late 1990s, two groups studying far-off supernovae discovered that these explosions of dying, overmassive stars were consistently fainter than expected. They concluded that the intervening space through which the supernovae’s light had travelled to get to us had stretched more than expected, making them further away than supposed – and so less bright.
Dark energy is the name for whatever is causing this accelerating expansion. It dominates the cosmos, making up by the latest reckoning some 68 per cent of everything there is. But what is it? There, physicists are stumped, even more so than with dark matter, that other mysterious cosmic apparition. At least we know dark matter gravitates. The effect of dark energy seems to be to oppose gravity, whose pull would otherwise tend to make the universe contract – so it doesn’t seem to correspond to any physical phenomenon we’ve yet encountered.
One possibility is that dark energy is a vacuum energy of the sort that quantum particles might create by popping in and out of free space. This would be a resurrection of the cosmological constant that Einstein originally introduced into the equations of general relativity to make the universe neither expand nor contract, but stay static, as astronomers of the time assumed it did.
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Einstein later called this cosmological constant his “greatest blunder” when it became clear in the 1920s that the universe was actually expanding. Quantum mechanics supplies a possible identity for it, as it suggests the vacuum of space should itself have an energy. The only problem is that the theory predicts there must be 10120 times more of this energy than the amount of dark energy needed to set the universe speeding on its way – perhaps the most glaring numerical mismatch in all of physics.
Another possibility is that dark energy is perhaps a “quintessence”, an as yet undiscovered fifth force of nature. Both identities have their problems, and there could be another way out. A universe with a variable density of matter would expand at different rates in different places, possibly producing an illusion of accelerated expansion. So if we drop the cosmological principle we might possibly get rid of dark energy, too.
Astronomers hope soon to know more. The Dark Energy Survey is one project located at the Cerro Tololo Inter-American observatory in the Chilean Andes that hopes to get to grips with this cosmic mystery by mapping out distant cosmic galaxies and the history of expansion of the universe over the almost 14 billion years since the big bang. Richard Webb
