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The Universe as a test bed

27 November 2013
Diederik Roest
Diederik Roest

What do you do when even the largest particle accelerator is too small for your experiments? Simple: you turn to the Big Bang, the biggest explosion in history. That’s what Diederik Roest, a theoretical physicist at the University of Groningen, has done in his quest to find out how it all works. He also likes to visit schools to explain what he does. And at the age of 35, he has just been elected to The Young Academy, an elite group of 50 young scientists in the Netherlands.

It is quite an honour to be selected for the Young Academy. ‘First, you have to be nominated, then you have to write a motivational letter and to round it off there’s an interview’, says Roest. He will be inaugurated in March for a five-year term. ‘I’ve already participated in an outreach programme called “The Young Academy on Wheels”, where we visited schools. That’s something I already do a lot, and it was brilliant.’

Science communication will be one of Roest’s focal points at the Young Academy. ‘The other one will be science policy. Science has taken a bit of a beating in recent years, both in terms of funding cuts and in terms of its standing. Too often, it is seen as just another opinion.’ Promoting and defending science is therefore going to be his mission. ‘But in all honesty, I haven’t really worked out a proper plan yet.’

Diederik Roest
Diederik Roest

His work for the Young Academy should take about half a day and a couple of evenings a week. This will mean less time for science. ‘Yes it might boil down to one less paper a year, but being part of this community of young scientists, all at roughly the same stage in their careers, will be a fantastic experience’, says Roest. ‘I expect to learn a great deal about managing your own group and things like that.’

Roest established his own group in String Cosmology after landing a VIDI grant in 2008. ‘That’s almost finished now, but things are going well.’ His line of research is theoretical high energy physics. ‘We’re studying String Theory, the theory which describes fundamental particles as infinitesimal small strings. The properties of such a string depend on the frequency at which it vibrates.’

String Theory, which has been around for some 40 years, describes matter on an ultra-small scale. ‘This means that to study it you need extreme energies, which are impossible to generate on Earth.’ That’s why String Theory is sometimes seen as untestable. ‘But that’s not the case.’ Out there in the Universe, very high energy events do happen, and the mother of all high energy events is the Big Bang, of course.

‘There are two important points in time when we can take measurements’, Roest explains. ‘The first one is the present. This has already taught us a lot about how the Universe works. We can explain how “ordinary matter” works, but that only takes up four percent of the energy content of the Universe.’ The second point in time for measurements is 300,000 years after the Big Bang, the moment when the first light emerged from the boiling plasma.

‘This is what we call the Cosmic Microwave Background (CMB), the uniform radiation that would have filled the entire universe.’ This radiation has a temperature of just 2.7 Kelvin (roughly minus 271 degrees Celsius). ‘And what we can see is that it is almost the same however you look at it.’ This was an enigma: how could the Universe be so uniform? The concept of inflation provided the solution.

‘We now believe that in the first second after the Big Bang, the entire Universe expanded extremely rapidly, which would explain the smoothness of the CMB.’ But although the background radiation was extremely smooth, it wasn’t completely uniform. Measurements with different satellites have confirmed that there are small variations in temperature in the order of tens of a millionth of a degree. ‘So the fifth number behind the decimal point may vary.’

Planck measurements of the CMB.
Planck measurements of the CMB.

These small variations were caused by quantum fluctuations, events that happened during inflation. ‘Without these fluctuations, no stars would have formed because gravity would have been the same everywhere, so particles wouldn’t have clumped together. The fluctuations were the primordial seeds of the structures we now observe in the Universe.’

As the quantum fluctuations are governed by the laws of physics, they form a test bed for String Theory. ‘There are different versions of String Theory, and they predict different quantum fluctuations.’ These different versions can therefore be tested against the real data on the CMB.

Roest and his colleagues are therefore eagerly awaiting new results from the Planck space telescope, designed to measure the CMB with unprecedented precision. ‘The temperature variations in the CMB as measured by Planck have already been published, but the polarization results are due next year.’ Photons are polarized, and polarization is a measure of the orientation of the vibration of light waves compared with their direction of travel.

‘If the polarization is just a random scatter, it will be disappointing, but if the light polarization from the CMB shows a pattern, this will suggest that it is influenced by quantum gravity.’ And this will mean that gravity can be described as a quantum phenomenon, something that has eluded physicists so far. ‘We know that three of the four fundamental forces are quantized, but gravity has eluded us so far.’

The Planck telescope
The Planck telescope

Such a result would pave the way for a unified Theory of Everything, in which one quantum theory describes all four fundamental forces. This is the Holy Grail of theoretical physics, but why is it so significant?

Roest smiles. ‘There is no practical benefit in knowing this, as far as we can tell now. For me, the benefit is the beauty of being able to comprehend and verify how things work. It brings us closer to answering the fundamental question people have been asking for centuries: how does everything work?’ Another thing that appeals to Roest, who studied Physics at the University of Groningen, is how his work combines Mathematics, Physics and Astronomy.

How does he explain such esoteric concepts to schoolchildren? ‘I don’t give a big lecture, but start out by asking them questions, making them think for themselves. How big is the Universe? And what does that mean? Can there be an edge to the Universe? Considering such questions really piques their interest in what I have to say.’

For Roest, taking people on a trip to the ragged edges at the limits of what we know is part of his vocation as a scientist. The next five years will give him plenty of opportunity to do just that.

Last modified:01 July 2021 2.50 p.m.
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