It was not so 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-term theory called inflation that explained the behavior of the primordial universe.
So, disaster. The analysis of the data had not adequately taken into account the dust in the Milky Way. Not only were no gravitational waves detected, but inflation was not yet confirmed. Fast forward four years: gravitational waves were 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 wave or gravitational inflation.
Inflate the Universe
The Universe presents a problem. It is, without a doubt, fairly uniform. Of course, there are stars and galaxies and even groups of galaxies around the place. But, on the whole, it is quite uniform. This is also seen in the cosmic microwave background radiation (CMB). The CMB is the light that traveled to us from the moment the Universe cooled enough to allow the formation of the first atoms.
It is our clearest image of the primordial universe, and it is boringly boring, though in an interesting way. It is almost the same regardless of where you look. But it should not be the same. Imagine the Universe as a particle gas with a temperature and pressure. Because the temperature is the same everywhere, there must be a way to exchange energy throughout the 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 away that the light could not keep the same temperatures. There simply was not enough time to move the light anywhere. As a result, the CMB should be different everywhere we look. It is not.
To explain this, the theorists proposed a theory called inflation. The idea is that the primitive universe was expanding at a much faster rate than it is now expanding. The Universe began as a sphere small enough to exchange energy and reach a uniform temperature. So inflation assured that the expansion was so rapid that there was no time to accumulate temperature differences. Thus, the CMB still reflects the uniformity of that ball.
Unfortunately, inflation is not a single theory. It's 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 could be correct.
Inflation does not keep quiet
Fortunately, inflation has had other effects on the Universe. This rapid expansion should have made the universe sound like a bell, generating large gravitational waves. Like CMB, these primordial gravitational waves should also be spread out over the Universe and too weak to be directly observed now.
Instead, the BICEP and Keck teams 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 simple. Photons with B-mode polarization are also generated by the dispersion of dust in the Milky Way. This was the problem that shocked a gigantic hole in the announcement of 2014.
The view is not so dusty
In the last article, researchers tackle different techniques of analysis and testing to ensure that the The effect of dust has been taken into account for. Not only that, but instead of detecting only a single microwave frequency, the data analysis used four different microwave frequencies and data from Planck and WMAP to ensure that the results were consistent.
In the end, the researchers concluded that he had not seen gravitational waves and could not confirm inflation. As a result, the simplest inflation model now seems tenuous. The latest data are strong: there is only a very narrow range for which the simplest model could still adapt to the data.
Even more important: now the data analysis chain seems to work quite well. BICEP3 is coming and the speed with which data is accumulating is growing rapidly. Researchers expect that primitive gravitational waves will be detected within five years. So we may be able to choose a good model for inflation.
Physical Review Letters 2018, DOI: 10.1103 / PhysRevLett.121.221301 (Information on DOI)