In search of the Cosmic Plasterer

Why does the Universe look the way it does? Associate Professor of Theoretical Physics Diederik Roest would very much like to know the answer to that question. He just got funding for a project to look at inflation, ‘the Cosmic Plasterer’.

It happened just after the Big Bang. In no time at all (by normal standards), the Universe got a whole lot bigger. Some 1,000,000,000,000,000,000,000 times bigger, give or take. ‘The result of this process, which we call ‘inflation’, was that everything in the Universe was smoothed out’, Roest explains. ‘The expansion was extreme and acted like a sort of “Cosmic Plasterer”: afterwards, the entire Universe looked the same everywhere, with everything evenly spread out .’ That is, except for quantum fluctuations.

These quantum fluctuations produce pairs of ‘virtual particles’ out of nothing. ‘Pairs of particles and anti-particles come into existence all the time, but they almost immediately recombine and annihilate’, says Roest. They are gone before you can notice them. But when the Universe expanded during inflation, these quantum pairs were torn apart, so they couldn’t find each other again to recombine. As a result, they were not annihilated.

This meant that after inflation, the Universe was totally smoothed out – except for myriads of lost subatomic particles. ‘What happened next was that gravity took over. Even though these particles are extremely small, they exert some gravitational pull.’ This meant they started clumping together and forming larger and larger structures. They were the seeds out of which galaxies and all other structures in our Universe would eventually form.

How do we know all this? Well, the early fluctuations caused by those lost particles left an imprint on the Cosmic Microwave Background (CMB), the first light that escaped when the Universe stopped being opaque, some 300,000 years after the Big Bang. ‘There are two types of fluctuation in the CMB’, Roest explains. ‘There are density fluctuations, which we can measure as temperature differences, but there should also be gravitational waves that affect the polarity of CMB light.’ The polarity is the orientation in which the light waves oscillate.

Scientist have studied the temperature differences for decades. The latest and most accurate measurements are from the Planck satellite. ‘Based on the measurements of the temperature differences, which are really tiny, physicists have created theoretical models for inflation.’ There are multiple models which each make predictions on how inflation could have shaped the Universe.

These models also predict the effect of the gravity waves – which have never yet been observed. And this is the source of some recent excitement. American scientists operating the BICEP2 experiment in Antarctica reported just over a year ago they had seen polarization in the CMB which they linked to the gravity waves caused by inflation. However, it recently turned out their conclusions were incorrect.

‘Now, this isn’t the end for inflation’, Roest cautions. ‘There have been multiple confirmations of inflation, and the fact that in the end BICEP2 didn’t find the traces of those gravity waves is actually giving us important information: we know now that those gravity waves were too weak for this experiment to detect, so we have an upper limit for the power of these waves.’ In fact, Roest is in a sense quite glad the BICEP2 results were wrong. ‘Our preferred models for inflation didn’t allow for the strength of gravity waves that the BICEP2 team claimed to have found!’

Roest has recently received a EUR 225,000 grant from the FOM Foundation for Fundamental Research on Matter to strengthen the theoretical basis of models for inflation. The project is entitled A scale model for the early Universe . ‘The Planck data on temperature difference in the CMB and the BICEP2 results provide constraints for theoretical models. We now have a class of different models describing inflation that fall within these constraints. But why do different theories produce similar results?’ This might point to a deeper, underlying theoretical framework.

On top of that, the gravity waves resulting from inflation are a quantum phenomenon. ‘Which means we need a theory for quantum gravity to explain them.’ And quantum gravity has proven to be extremely elusive; theoretical physicists have been working on this for decades. ‘It may be that string theory can provide a solution.’

For the FOM project, one PhD student will work with Roest to tackle these very big questions. ‘In the end, we want to understand why the Universe is what it is!’ The handiwork of the Cosmic Plasterer – temperature differences and polarization in the CMB – should contain the answers.

Diederik Roest works at the Van Swinderen Institute for Particle Physics and Gravity

See also this related article: The Universe, life and everything Interview with University of Groningen honorary doctor Renata Kallosh and husband Andrei Linde.

Read more about the University of Groningen international physics bachelor or master.