Mazal tov to Saul Perlmutter (Lawrence Berkeley National Laboratory), Brian Schmidt (Australian National University), and Adam Riess (Johns Hopkins University; Space Telescope Science Institute) for winning the 2011 Nobel Prize in Physics for their discovery that the universe is not only expanding, but accelerating!
Apart from the Woody Allen clip, why is this story on Jewschool? Because Perlmutter and Riess are both Jewish! They are the first Jews to be awarded a Nobel Prize since… yesterday, when Bruce Beutler and Ralph Steinman z”l won the Nobel Prize in Medicine (along with Jules Hoffmann) for their work in immunology. They are also the first Jewish physics laureates since Roy Glauber in 2005.

It has been known since the 1920s that the universe is expanding. We know this by observing distant galaxies and seeing that they are redshifted, i.e. their light is shifted towards the red end of the spectrum, to a lower frequency, indicating that they are moving away from us (in an example of the Doppler effect). By measuring the magnitude of the redshift, we can determine how fast the galaxies are moving away.
Everything has been moving away from everything else since the Big Bang. But since gravity attracts everything to everything else, one might reasonably expect that this expansion would eventually slow down and perhaps even reverse. And so our laureates, leading two different groups, set out to measure this slowing down. But that wasn’t what they found! Instead, in 1998, the two groups found that the expansion is getting faster!
How did they measure this? They were using Type Ia supernovae. When a white dwarf is part of a binary star system, it can slowly accrete mass from its companion, until it reaches a maximum mass, at which point it explodes in a supernova. Because this always happens at exactly the same mass, the resulting supernovae always have the same absolute brightness. Of course, the apparent brightness when we observe these supernovae on Earth still depends on how far away they are. (The sun appears much brighter than other stars, because it is much closer!) So we can observe the apparent brightness of Type Ia supernovae in distant galaxies, and compare this to the absolute brightness that we know all Type Ia supernovae have to be, and thereby determine how far away the galaxy is. Also, from the redshift we can determine how fast the galaxy is moving away. Putting these measurements together, the laureates found that the universe is accelerating.
So what does this all mean? No one completely knows yet, but the energy responsible for this acceleration has been termed dark energy, and models based on observations have calculated that dark energy accounts for over 70% of the universe. This has led to a revival of the cosmological constant, a fudge factor that Einstein threw into general relativity and then later abandoned. The deeper implications are still being worked out; if you can solve it, then perhaps you, too, will be featured here on Jewschool!