The on-again off-again discovery of the "smoking gun of inflation" is now firmly set to "off". The latest development is an analysis combining data from the Planck spacecraft, the BICEP2 telescope at the South Pole (and the source of the original excitement) and the Keck Array, a second South Pole experiment. The announcement was supposed to come next week, but the news leaked yesterday when the Planck mission's French website put up the press release and background information early. The page is now locked, but rather pointlessly, given how fast the news spread. And while English is the (ahem) linga franca of science, monolingual Anglophones could get the gist from Google Translate.
Back in March, the BICEP2 team claimed to have found a characteristic twist in the polarisation pattern of the microwave background -- fossil light from the big bang itself -- suggesting the universe was awash with gravitational waves. The most likely origin for these gravitational waves was inflation; a phase of accelerated expansion immediately after the Big Bang. The idea of inflaton has been around for 35 years, and the gravitational wave signal claimed by BICEP2 would convince most cosmologists inflation had really happened in our universe. The news turned the cosmology community on its head but slowly unravelled over the next few months.
Photons in the microwave background hail from the depths of intergalactic space, as they have been in flight since around 380,000 years after the Big Bang when the universe first becomes transparent. However, to get to us they pass through our Milky Way galaxy which contains a good deal of dust and gas. Some of the dust is electrically charged and interacts with the magnetic field of the galaxy, producing a similar pattern to the primordial gravitational wave signal. At the time the size of the dust signal was unknown, and the BICEP2 team accounted for it using the best estimates for its size. But those estimates were too conservative, and today's news is that the vast majority of their "signal" is from the dust.
The new information comes from combining observations at several different microwave frequencies. The map of the microwave sky made by BICEP2 is exquisitely clear, but uses a single frequency --150 gigahertz, about 1500 times higher than an FM radio signal -- so it is effectively black and white. The galactic dust has a different "colour" from the microwave background. The new analysis compares the BICEP2 signal to a map of sky made by the Planck satellite at 353 GHz and at this frequency dust is much brighter than the microwave background. If the BICEP2 looked different from the 353GHz Planck map,we would know that BICEP2 was not seeing a lot of dust - but the correlation between the maps is high, telling us that the BICEP2 signal is mainly dust and the champagne should have stayed in the fridge.
The new analysis does not rule out inflationary gravitational waves. The signal claimed by the BICEP2 team was always surprisingly large. Cosmologists use the variable "r" for the strength of a gravitational wave background. Before BICEP2 it seemed likely that r was smaller than 0.1, but the "headline" number from BICEP2 was r = 0.2, which dropped to r=0.16 when they subtracted their best-guess for the dust distribution. This table comes via Google Translate and the now hidden press release, and the upshot is that we are effectively back to where we were a year ago:
That said, BICEP2 still represents a major milestone in our ability to probe the early universe; the technology it uses gives an exquisitely clear measurements, and we can expect huge progress on the observational side in the coming years.
When the news started to unravel, it struck me that the cosmology community was in the same position as someone waking up in an unfamiliar Las Vegas hotel room with a throbbing headache, hazy memories of the night before, and a fresh tattoo reading "r=0.2". So even if r is not exactly zero, it looks like we will need a visit to this guy: