THIS year's Nobel prize for physics was awarded for what was, in a sense literally, the biggest discovery ever made in physics—that the universe is not only expanding (which had been known since the 1920s), but that the rate of expansion is increasing. Something, in other words, is actively pushing it apart.
This was worked out by two groups who, in the 1990s, were studying exploding stars called supernovae. One was the Supernova Cosmology Project, at the University of California, Berkeley, led by Saul Perlmutter. The other was the High-z Supernova Search Team, an international project led by Brian Schmidt and involving Adam Riess, both of Harvard University. It is these three gentlemen who have shared the prize.
Supernovae come in various types. One particular sort, though, known as type Ia supernovae, always explode with about the same energy and are therefore equally bright. That means they can be used to estimate quite precisely how far away they (and thus the galaxy they inhabit) are. In addition, the speed at which an object such as a star or galaxy is moving away from Earth, because of the expansion of the universe, can be worked out from its red-shift. This is a fall in the frequency of its light towards the red end of the spectrum. It is caused by the Doppler effect (something similar happens when a police car or fire engine with its siren blaring drives past you, and the pitch of the sound suddenly drops).
What both groups found was that the light from distant supernovae was fainter than predicted. In other words, the supernovae were further away than their red-shifts indicated they should be, based on the existing model of the universe. Something, then, was pushing space itself apart.
What that something is, remains conjecture. It has been labelled "dark energy", but that is really physicists' short-hand for "we haven't got a clue". It may, though, relate to a mathematical term called the cosmological constant that appears in Einstein's general theory of relativity, and which Einstein thought, before the discovery of the expansion of the universe, was necessary to stop the universe collapsing.