It was not so long ago (2014) that the world was shocked and stunned by the announcement of the discovery of the original gravitational waves. This would have been the first observation of gravitational waves, and the data seemed to confirm a long-cherished theory called inflation, which explains the behavior of the early universe.
Then catastrophe. The data analysis had not sufficiently considered dust in the Milky Way. Not only were no gravitational waves discovered, but inflation was not yet confirmed. Fast forward of four years: gravitational waves were discovered 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 is problematic. It is undoubtedly quite consistent. Certainly there are stars and galaxies and even galaxy clusters in the area. But overall, it's pretty consistent. This is also evident in cosmic microwave background radiation (CMB). The CMB is light that has traveled to us since the moment when the universe was cool enough to form the first atoms.
It's our clearest picture of the early universe, and it's totally boring, albeit in an interesting way. It is almost the same, no matter where you look. But it should not be the same. Imagine the universe as a particle gas with a temperature and a pressure. For the temperature to be the same everywhere, energy needs to be exchanged throughout the universe. This happens with photons that propagate at the speed of light.
Under the same conditions, the different parts of the universe were so far apart that the light could not keep up with the temperatures. There simply was not enough time to move light everywhere. Consequently, the CMB should be different wherever we look. This is not the case.
To explain this, theorists have proposed a theory called inflation. The idea is that the early universe expanded much faster than it does now. The universe began as a sphere small enough to exchange energy and reach a uniform temperature. Then inflation made the expansion so fast that there was no time for temperature differences. Therefore, the CMB still reflects the uniformity of this little ball.
Unfortunately, inflation is not a single theory. It's more like a conglomerate of related theories, all of which are consistent with the data we have now. It is not easy to decide which version of inflation might be right.
Inflation is not quiet
Fortunately, inflation had a different impact on the universe. This rapid expansion would have made the universe ring like a bell and produce large gravitational waves. Like the CMB, these original gravitational waves should be stretched across the universe and too weak to be directly observed.
Instead, the teams of BICEP and Keck are looking for their signature in the polarization of the CMB. In particular, the CMB has two polarizations, one of which ̵
Except, of course, it's not that easy. Photons with B-mode polarization are also produced by dust scattering in the Milky Way galaxy. That was the problem that blew a giant hole in the 2014 announcement.
The visibility is not so dusty
In the latest work, researchers go through various analytical procedures and tests to make sure that the effects of dust have been taken into account. In addition, data analysis did not use a single microwave frequency but 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 came to the conclusion that had seen no gravitational waves and could not confirm the inflation. Therefore, the simplest inflation model now seems weak. The latest data is strong: there is only a very narrow range for which the simplest model could still fit into the data.
More importantly, the data analysis chain now seems to work pretty well. BICEP3 is on the way, and the rate at which data accumulates is growing rapidly. The researchers expect to see ancient gravitational waves within five years. Then we might be able to choose a good model for inflation.
Physical Review Letters 2018, DOI: 10.1103 / PhysRevLett.121.221301 (About DOIs)