A scientific controversy has been brewing concerning the results of BICEP2, the experiment that measured polarized microwaves coming from a patch of the sky, and whose measurement has been widely interpreted as a discovery of gravitational waves, probably from cosmic inflation. (Here’s my post about the discovery, here’s some background so you can understand it more easily. Here are some of my articles about the early universe.) On the day of the announcement, some elements of the media hailed it as a great discovery without reminding readers of something very important: it’s provisional!
From the very beginning of the BICEP2 story, I’ve been reminding you (here and here) that it is very common for claims of great scientific discoveries to disappear after further scrutiny, and that a declaration of victory by the scientific community comes much more slowly and deliberately than it often does in the press. Every scientist knows that while science, as a collective process viewed over time, very rarely makes mistakes, individual experiments and experimenters are often wrong. (To its credit, the New York Times article contained some cautionary statements in its prose, and also quoted scientists making cautionary statements. Other media outlets forgot.)
Doing forefront science is extremely difficult, because it requires near-perfection. A single unfortunate mistake in a very complex experiment can create an effect that appears similar to what the experimenters were looking for, but is a fake. Scientists are all well-aware of this; we’ve all seen examples, some of which took years to diagnose. And so, as with any claim of a big discovery, you should view the BICEP2 result as provisional, until checked thoroughly by outside experts, and until confirmed by other experiments.
What could go wrong with BICEP2? On purely logical grounds, the BICEP2 result, interpreted as evidence for cosmic inflation, could be problematic if any one of the following four things is true:
1) The experiment itself has a technical problem, and the polarized microwaves they observe actually don’t exist.
2) The polarized microwaves are real, but they aren’t coming from ancient gravitational waves; they are instead coming from dust (very small grains of material) that is distributed around the galaxy between the stars, and that can radiate polarized microwaves.
3) The polarization really is coming from the cosmic microwave background (the leftover glow from the Big Bang), but it is not coming from gravitational waves; instead it comes from some other unknown source.
4) The polarization is really coming from gravitational waves, but these waves are not due to cosmic inflation but to some other source in the early universe.
The current controversy concerns point 2.
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