by GENE HOWINGTON
Critics have pointed to another possible explanation of the gravitational waves detected by BICEP2: phase changes after inflation as the early universe started to cool. As reported by the Physics arXiv Blog:
But today, James Dent at the University of Louisiana at Lafayette and a couple of pals say the BICEP2 team has overlooked something. They say one possibility is that the polarisation of the cosmic microwave background was indeed caused by gravitational waves just as the BICEP2 team claim, but that these waves formed after inflation, not before it.
Here’s how. For the last few years, cosmologists have discussed what happened in the moments after inflation, as the universe began to cool. It was during this time that the universe we know ‘condensed’ out of the high energy maelstrom generated in the Big Bang.
In particular, as the universe cooled, the fundamental forces we see now, such as the weak and strong nuclear forces and the electromagnetic force, formed in processes called phase changes, just as ice forms as water cools below a critical temperature or as a magnetic field within a material aligns as it cools below a critical temperature.
What’s interesting about phase changes is that they don’t form across the entire material at the same instant. In a cooling magnet, for example, the magnetic field forms in different regions which then spontaneously align when the temperature cools below the critical point. Only then does a uniform field fill the entire material.
Many cosmologists think that the same kind of phases changes occurred in the universe after inflation. Each phase change began in different regions at slightly different times.
But as the entire universe cooled, the fields in these regions would have spontaneously aligned, filling the universe with the same properties at that instant.
This self-ordering process would have been hugely violent, generating its own gravitational waves that rippled through spacetime, albeit after inflation. Could this process be responsible for the polarisation that the BICEP2 team has measured?
According to Dent and co, it could. “Unfortunately, the [BICEP2 measurement] falls just short of ruling out this other source as the dominant contribution of the observed effect,” they say.
So the big announcement last week was premature. And before it can be confirmed, the BICEP2 team has some work ahead of it to rule out the possibility that self ordering in the early universe could be responsible.”
While this is an alternative explanation for the observation, it is couched as a possibility. Unless evidence of a phase change inducing the wave is found, inflation is still the most likely explanation. Time will tell. This does not change the importance of the discovery or diminish the excitement about it but instead offers a chance to ask more questions and refine our knowledge further. In itself, that is exciting. Questions are doorways.
Science is a journey down the path of knowledge, not a destination, but that is all part of the attraction.