First detection of primordial gravitational waves?

There was a big press conference last week to announce the discovery of evidence of primordial gravitational waves from the formation of the Universe in the Big Bang. It was carried by much of the UK media [TV and print] so I thought a few words appropriate.

The currently accepted theory of the Big Bang requires a process called inflation to explain what we observe today. Inflation is the faster than light expansion of space that smoothed out space, amongst other things, ultimately leaving its fingerprint on the remnant radiation from the later part of the Big Bang presently observable.

This remnant of the Big Bang is  the 3 Kelvin temperature cosmic microwave background (CMB) radiation first observed by Penzias and Wilson in the mid 1960’s, for which they were awarded the Nobel Prize for Physics in 1978. More recently the CMB has been mapped in great detail, first by the COBE satellite and more recently the WMAP satellite. The all-sky map of temperature is remarkably even at approximately 2.7 degrees above absolute zero. The important result though is the fluctuations about this temperature are remarkably small, a few hundred millionth’s of a degree – a picture [below] we are probably quite familiar with now.

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This small variations can be readily explained by the Big Bang model with inflation. It is believed that these fluctuations indicate tiny temperature and density variations that ultimately led to the structures we see in the Universe today – stars, galaxies and clusters of galaxies.

However, the inflation model also suggests the production of gravitational waves and these gravitational waves will leave a particular signature on the CMB – it will polarise the radiation. The BICEP2 experiment at the South Pole has looked at a small region of sky. The results announced at the press conference seem to show polarisation that can only be explained by gravitational waves. The picture below shows the familiar temperature pattern with the added extra of the lines showing the polarisation._73634672_73634671

The twisted form of the polarisation is the expected signature of the primordial gravitational waves. If so, this is really an important observation suggesting that the inflation model is correct. Some are saying a Nobel Prize level discovery. The BICEP2 team say that the only other possible polarisation source is dust in our galaxy and synchrotron radiation but that they can eliminate these ( at some level) using earlier observations.

Other experiments are trying to find the same effect but have yet to announce any results. The Planck satellite is said to have lots of data awaiting analysis which hopefully will show the effect. Quite a few very knowledgeable people are slightly sceptical as these results need extensive confirmation.  One prominent UK scientist said “If…and it’s a big if…this is true, it would be spectacular evidence for what happened at the Big Bang.” There are discrepancies between this new data and the older WMAP and Planck observations which need investigation and explanation. Time will tell if this first observation of the effect of gravitational waves is correct.

One odd thing is that the results were announced at a press conference and not published in a peer reviewed journal.  Peer review and confirmation by another experiment is the real test of any scientific discovery. The cynical will wonder why a press conference: is it just because it is so important; is another group about to announce a similar discovery; or is their funding up for renewal? Many physicists of my age remember the last great ‘discovery’ announced by a press conference – cold fusion, the fusion that never was… I saw a draft of the paper on that work and if it had been a year 1 undergraduate student effort, it would have got 3/10 and the words ‘You need to be more precise and more scientific in your writing’! Hopefully, this discovery will turn out to be the real deal.

1 thought on “First detection of primordial gravitational waves?

  1. Sadly, my scepticism seems to have been justified. For several weeks now, there has been quite a bit of informed talk about there being no detection of gravitational waves…. a large loop of foreground dust had been detected by other experiments which the BICEP2 team were unaware of. Although the BICEP2 team tried using earlier data to allow for dust, they didn’t allow for such a large dust loop right through their observation area. The concensus seems to be that BICEP2 hasn’t seen primordial gravitational waves. Oh well, the perils of premature publication…

    Ian

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