Update — Mar. 17, 12:00 p.m. ET: The BICEP2 collaboration have detected gravitational waves from the early universe, providing strong evidence for cosmic inflation. The results have been released here along with an FAQ.
Today cosmologists are expected to announce the discovery of gravitational waves.
The Harvard-Smithsonian Centre for Astrophysics announced last week that they would be reporting a "major discovery" today at 12pm EDT (4pm GMT). That's pretty much all they said, but since then the cosmology rumour mill has been in overdrive about what it could be. Now it seems likely to be the discovery of primordial gravitational waves.