The smoking gun of cosmic inflation was a smudge of galactic dust

On 17 March 2014 the BICEP2 collaboration — a telescope at the South Pole led by John Kovac of the Harvard-Smithsonian Center for Astrophysics — announced at a Harvard press conference that it had found B-mode polarisation in the cosmic microwave background: the twist imprinted on the oldest light by gravitational waves from the first fraction of a second of the universe. The signal was 'quite a bit stronger than many scientists had dared hope', seven sigma. Inflation's smoking gun, found at last.

The reaction was a coronation. Inflation pioneer Alan Guth said he was 'completely astounded'; a researcher on the rival Planck satellite said 'if this is confirmed, it's huge'; the BBC's report noted that 'assuming the results are confirmed, a Nobel Prize seems assured'. The team said it had spent three years checking for contamination — including the one troublemaker every cosmologist knew about: dust in our own galaxy.

The doubts arrived within months. BICEP2's telescope saw the sky at one frequency, and dust shines at frequencies it could not see. In June the team lowered its confidence and its peer-reviewed paper carried an appendix on dust; in September Planck's full-sky maps showed dust alone produced a signal as strong as BICEP2's. On 30 January 2015 a joint BICEP2/Keck/Planck paper conceded the point: the signal could not be distinguished from dust, and the best estimate of primordial gravitational waves was consistent with zero. Nature's headline: 'Gravitational waves discovery now officially dead'.

The collaboration paid a smaller bill than it might have: it co-authored the refutation with its rival and kept taking data — every limit since (r < 0.12 in 2015, r < 0.036 in 2021) has found nothing, and the signal remains undetected. What died was the announcement itself: a press conference, Nobel talk and ten months of headlines built on a foreground. Brian Keating, a member of the collaboration, wrote the whole affair up in a book titled Losing the Nobel Prize.