Congratulations to my colleagues Saul Perlmutter, Adam Riess and Brian Schmidt! They are sharing the rather lucrative Shaw Prize for their leadership in the late 1990s discovery that the Universe seems to be accelerating in its expansion. In particular, through painstaking observational campaigns over many years, they observed that distant supernovae — exploding stars whose intrinsic brightness are all roughly the same — seem to be dimmer than would be expected in the simplest universe. Dimmer, hence further away than expected; further away, then, due to an accelerated expansion of the Universe as a whole.
The most obvious mechanism for this expansion is Einstein’s Cosmological Constant. The problem is that, although we don’t have any precise way to calculate its value, the best guess is either strictly zero (so no acceleration at all) or that it’s something like 122 orders of magnitude (that’s 10122) times larger than it’s observed to be (the Planck density, for aficianados). Another possibility is that the acceleration is due to something not quite as fundamental as a cosmological constant, some sort of field pervading the Universe, often called Dark Energy or Quintessence. Unfortunately, there are no particularly compelling ways to calculate its value in that case — these are more like paradigms in search of a detailed theory. A final possibility is the so-called anthropic argument: the cosmological constant has the value that it does because, if it didn’t, we wouldn’t be able to be here to observe it. Unfortunately, any version of that argument that isn’t a tautology is unpredictive, useless, or silly, at least for now.
So let’s hope a future version of the Shaw prize goes to someone clever enough to explain the acceleration.
Thanks to Sean for pointing this out, and for noting that the ideas behind the discovery were already in the air (especially since that allows me a little self-aggrandizement). Also, in the interests of equal-time, I should point out that there are reputable astrophysicists out there who aren’t quite convinced by the Supernovae data.