Focus on research: theoretical physicist Jan Pieter van der Schaar
Van der Schaar became interested in astronomy when he was only thirteen - night after night he would stare at the sky from his telescope in the garden. Eventually he decided to study theoretical physics. ‘An apple falling from a tree and the earth revolving around the sun are explained by the same theory', says Van der Schaar. ‘That notion, that there are universal laws that you can apply to the universe as a whole but also around your own home, the ast scope/range of those theories, was very appealing to me.'
This theoretical scope/range continues to arouse Van der Schaar's curiosity, and logically led him to the field of string theory research. String theory represents an attempt to unify all the fundamental forces of nature. There are various theories in physics that can each be applied individually to different situations on their own scale. For example, we use the theory of gravity in daily life, and quantum mechanics to describe physics on a minute level. String theory attempts to combine these theories into a single framework that also describes gravity on a very small scale: quantum gravity.
String theory regards the particles that make up everything around us not as small points, but as vibrating strings. Particles with greater mass are described by strings that vibrate more vigorously. Although it may sound simple, there is one problem: the strings are so incredibly small that we would not be able to see them even with the world's most powerful microscope. String theory cannot be tested.
After completing his PhD in Groningen, Van der Schaar went on to postdoctoral research in America and Switzerland. ‘I defined my own research programme', says Van der Schaar. ‘I wanted to think about what string theory has to say about cosmology.' Cosmology describes the evolution of the universe. Astronomical observations show that the universe is expanding - we can see galaxies from a great distance moving away from us. Reasoning backwards, this leads to the current accepted theory that the universe used to be much smaller, and that it was created around 13.7 billion years ago in a large explosion, or the big bang.
Van der Schaar's main interest lies in the initial moments following the Big Bang. ‘The nice thing about cosmology is that you can regard it as the best microscope we have. With a particle accelerator you can make particles collide at high speeds, and in so doing, learn about their substructure. The particle accelerator works like a microscope, it shows us what the particles are made of.' The greater the energy used to fire the particles into each other in the accelerator, the smaller the structures that become visible, or the stronger the microscope.
‘The accelerator that is about to start work in Switzerland achieves enormous energy levels. But that's actually nothing compared to the nascent universe. As you get closer to the beginning of the universe, around 13.7 billion years ago, distances become smaller and smaller, and energy levels higher and higher. For energies that are comparable to the Planck scale, gravity becomes just as important as the other fundamental forces and it is believed that the effects of quantum gravity start to play an important part at these levels. And the problem with string theory is that it is precisely on these fantastically small scales that it shows its true colours. For years, this has been one of the most significant points of criticism: string theory is all very well and good, but how are we ever going to test it?'
Answering this question is one of Van der Schaar's major challenges. ‘Shortly after the big bang, the universe was amazingly small and so incredibly dense that both gravity and quantum mechanics played an important part. String theory is a proposal for a unification of these theories, and at the same time should describe all other forces and particles.' If string theory was indeed significant in those early beginnings, it may have had an effect on everything that happened thereafter.
Van der Schaar is searching for this possible effect in cosmic background radiation, or the ‘afterglow' of the big bang. ‘Immediately following the big bang came a very hot period during which there was a type of plasma made up of charged particles. Inside the plasma, light particles (photons) bounced back and forth as if in a pinball machine. For as soon as a photon meets a charged particle, it changes direction.' Given that everything in the plasma was charged, the photons could not travel far. But the universe continued to expand, allowing it to cool down. Electrons became bound to protons, creating neutral atoms. From that point on the light (i.e. photons) was no longer dispersed. The universe became transparent, and the photons could move around uninhibited. ‘So there was a very hot period, and at a certain point the light was switched on. And that moment is still visible to us today - as cosmic background radiation.'
At the point in time when the light was released the universe was exceptionally hot, and the photons possessed very high energy. But the expansion of the universe has meant that wavelengths have become elongated, reducing the energy of the photons. The light that still reaches us from this phase in the universe's evolution has cooled down immensely, from 3000 to 2.7 degrees. And while starlight comes from a certain direction, we receive cosmic background radiation from all directions in the sky. This background radiation is even visible on a normal television - one percent of television noise originates from the nascent universe. ‘This is consistent with the calculations made by early theoreticians, in which the ancient universe was much hotter, smaller and denser. In short, this background radiation is the most convincing evidence for the big-bang theory.'
‘If you look very closely, there is an amazing wealth of information hidden in cosmic background radiation', says Van der Schaar. The hot plasma contained subtle differences in temperature due to small variations in density, which are still observable in background radiation. Over billions of years, these tiny differences in the original distribution of matter grew into entire galaxies. ‘And I'm really talking about tiny differences - ripples the size of one-hundredth of a millimetre inside a swimming pool a few metres deep. But they can be measured, and they provide information about the density distribution of the plasma, three hundred thousand years after the big bang.'
A model exists that can explain the small differences in plasma density: cosmic inflation. Cosmic inflation was a period immediately following the big bang, in which the universe expanded at an exponential rate. ‘In a period of roughly 10-30 seconds (a ridiculously short amount of time), it increased in size by a factor of around 1030. That's inconceivable!' says a very animated Van der Schaar. ‘Quantum mechanics states that particles are being created constantly. Normally they disappear again straight away - matter and antimatter particles meet, and annihilate or destroy each other. But during the period of cosmic inflation, the particles were ripped apart so quickly that they never saw each other again. This meant that the distribution of particles was no longer purely homogenous.'
Small differences in density emerged in the plasma, which were able to grow, slowly, due to gravity. An important part was also played by radiation pressure - the photons trapped in the clusters exerted pressure in the opposite direction. ‘Matter will shrink until radiation pressure becomes too great, then it expands and then shrinks again, creating "acoustic" oscillations in the plasma.' Van der Schaar demonstrates the plasma's oscillating motion with his hands. ‘The original fluctuations from the period of cosmic inflation were further enhanced by the acoustic oscillations in the plasma, which are now observable in cosmic background radiation. In order to see the original fluctuations, you first need to separate the background radiation from the acoustic oscillations. And those fluctuations provide us with a lot of information.'
Although background radiation gives Van der Schaar an understanding of the geometry of the universe, more importantly it contains information on string theory. Graphing the deviation of all parts of the sky from the average background radiation will produce a symmetrical bell-shaped curve, also called a Gaussian distribution. ‘Some string-theory models state that this graph should not be completely Gaussian. For example, there ought to be slightly more colder areas than warmer ones. The effect is quite minimal, but it is possible that it will be measured in the future.'
Next year will see the launch of the Planck satellite, which will measure background radiation much more precisely. ‘We are currently preparing to receive the data. It would be revolutionary if we were to observe non-Gaussian effects.' It is not the case that the data will be able to negate or prove the whole of string theory. However, there is an entire class of models that do not predict any non-Gaussian effects, which could be refuted by the data straight away. ‘The more precisely we can measure background radiation, the more selective we can be in identifying a particular cosmic inflation model, which could give an indication of whether string theory is involved or not. This is one of the important things I am now working on.'
In addition to explaining the universe using string theory in order to prove directly that it is indeed the one theory that describes all natural phenomena, Van der Schaar also works on matters that he describes as real, ‘hard-core' string theory. ‘Subjects that relate to the structure of the theory itself.' Reluctantly, he admits: ‘But that's the point when it really gets tough to explain to the general public.'