It was not so very long ago (2014) that the world was shocked and amazed by the announcement that primordial gravitational waves had been found. This would have been the first observation of gravitational waves, and the data seemed to confirm a long-held theory called inflation that explained the behavior of the early Universe.

Then, disaster. The data analysis had not adequately accounted for dust in the Milky Way. Not only were no gravitational waves detected, but inflation was still unconfirmed. Fast forward four years: gravitational waves have been detected using other methods that left inflation hanging in the wind. But BICEP2 (Background Imaging of Cosmic Extragalactic Polarization) and the Keck array are back with more data and better analysis. Unfortunately, still no gravitational waves or inflation.

Inflate the Universe

The Universe presents a problem. It is, without a doubt, pretty uniform. Sure, there are stars and galaxies and even clusters of galaxies around the place. But, overall, it's pretty uniform. This is also seen in the Cosmic Microwave Background radiation (CMB). The CMB is light that has been traveling to us since the moment that the Universe became cool enough for the first atoms to form.

It is our clearest picture of the early Universe, and it is dead boring, although in an interesting way. It is nearly the same no matter where you look. But it shouldn't be the same. Imagine the Universe as a gas of particles with a temperature and pressure. For the temperature to be the same everywhere, there needs to be a way for energy to be exchanged throughout the entire Universe. This is done with photons, which travel at the speed of light.

All things being equal, the different parts of the Universe were far enough apart that light could not keep the temperatures the same. There simply wasn’t enough time for light to move everywhere. As a result, the CMB should be different everywhere we look. It is not.

To explain this, theorists have proposed a theory called inflation. The idea is that the early Universe expanded at a rate much faster than it is expanding now. The Universe started as a sphere small enough to exchange energy and reach a uniform temperature. Then inflation ensured that expansion was so rapid that there was no time to accumulate temperature differences. Hence, the CMB still reflects the uniformity of that tiny ball.

Unfortunately, inflation is not a single theory. It is more like a conglomerate of related theories that are all consistent with the data we have now. There is no easy way to choose which version of inflation might be correct.

Inflation is not silent

Fortunately, inflation had other effects on the Universe. That rapid expansion should have caused the Universe to ring like a bell, generating large gravitational waves. Like the CMB, these primordial gravitational waves should also be stretched out across the Universe and far too weak to be directly observed now.

Instead, the BICEP and Keck teams are searching for their signature in the polarization of the CMB. In particular, the CMB has two polarizations, one of which—the B-mode—is only generated by gravitational waves and gravitational lensing.

Except, of course, it's not that simple. Photons with B-mode polarization are also generated by scattering off of dust in the Milky Way. That was the problem that blew a gigantic hole in the 2014 announcement.

View is not so dusty

In the latest paper, the researchers go through several different analysis techniques and tests to ensure that the effect of dust has been accounted for. Not only that, but instead of detecting just a single microwave frequency, the data analysis used four different microwave frequencies as well as data from Planck and WMAP to ensure that the results were consistent.

In the end, the researchers concluded that they had not seen gravitational waves and could not confirm inflation. As a result, the simplest model of inflation is looking tenuous now. The latest data is strong: there is only a very narrow range for which the simplest model might still fit the data.

More importantly: now the data analysis chain seems like it works pretty well. BICEP3 is on the way, and the rate at which data is accumulating is growing fast. The researchers expect that, within five years, primordial gravitational waves will be detected. Then we might be able to pick a good model for inflation.

Physical Review Letters, 2018, DOI: 10.1103/PhysRevLett.121.221301 (About DOIs)