Drillingsraum: Besides dust, gas, stars, black holes and dark matter there is another thing needed to simulate the evolution of the universe, something that is thought to be its today's major component: How do you handle the dark energy in the simulations?

Prof. Dr. Simon White: The dark energy in the simulations is treated just as a cosmological constant. It is something which stays uniform, it doesn't participate directly in the growth of structure. Its only effect is to change the overall expansion rate of the universe, so at recent times it causes the expansion of the universe to accelerate.

Drillingsraum: There are different attempts to describe the nature of dark energy. Besides the model of a Cosmological Constant, dynamical scalar fields are also being discussed, which could have the effect of an accelerated expansion. Both cases have their problems. Let's have a look to these dynamical fields: There we find some disagreements between predictions coming from renormalisation and some properties of the scalar fields. What exactly is renormalization and why does it lead to a problem here?

Prof. Dr. Simon White: That I think is beyond my competence (laughs). You ask the wrong person for this particular high energy physics issue. What appears to be well established is that the current expansion of the universe is accelerated. There is something which pushes rather than pulls, so the pressure effects exceed the gravitational attraction effects and so the expansion of the universe is accelerated. From fluctuations in the microwave background we know, that the geometry of the universe is euclidian, or very close to euclidian. It has a

completely independent routes which lead us to infer that there is something like a cosmological constant. The arguments against the Cosmological Constant are essentially philosophical. It is possible to put it in the theory, it is consistent. But it is unclear how to link its current size with anything else in physics. The reason people don't like the cosmological constant is because it is a number which has no natural explanation and no reference to anything else in physics. So, they have naturally looked for things which can act today like a cosmological constant, but which is in some kind linked to other physics. Dynamical scalar fields are an example of this. To say they link to other physics is still something in exaggeration, because currently there are no such scalar fields known, so this is still an extension of physics. But it is at least something where you can write down interactions and evolution equations, you can treat it in the language of high energy physics theory. There are still issues about why the size of the cosmological constant should be what it is today. People have various ideas how it could be linked to other aspects of physics: A manner of the cosmological constant might be related somehow to the balance between matter and radiation. Matter and radiation is similar to within a factor of about 10000, they are within a few orders of magnitude to each other. Maybe the dark energy becomes somehow related to that. Maybe it is related in somehow to neutrinos: For some reasons the total mass of neutrinos in the todays universe is within a couple of powers of ten of the mass of ordinary matter. This is completely not understood. Some people think the properties of the dark energy are related to this. Other people think that it is entirely disconnected of scalar fields. They think they can get around some of the coincidences which seem so surprising, particularly the size of the cosmological constant. You can imagine it is a new dynamical field like the scalar fields we just were talking about. You could imagine that it perhaps is Einstein's theory of gravity itself which needs modifications, so people are investigating possible extensions to Einstein's theory to see if they could include something which would behave like the dark energy. And there is a small set of people who believe

large scale structure of space time, and so cause the acceleration. I think the great majority of the community thinks this effect is either absent or far too small to cause the apparent acceleration, but there is no complete consensus yet. Some people who are conform to relativity theory still think there are open questions to be answered, so we don't understand which of these route is possible. Another route might be that the observations are mistaken. But now they've got the noble price, I guess we don't have to think about that anymore. (laughs)

Drillingsraum: Hopefully.

Prof. Dr. Simon White: In principle it is a difficult observation. It is a very good thing to have two independent routes that lead to the same conclusions. All we know is, that supernovae at different distances from us - which corresponds to different times in the universe's history - have an apparently different maximum brightness. This could either be due to the geometry of space time and the acceleration, or it could just be that the properties of supernovae at earlier times are different from supernovae nearby. And since they are very complicated objects, we can't be sure. In principle it is impossible to separate these two effects, so we need other ways than just the supernovae to measure things to be sure that the cosmic acceleration is real. The microwave background argument on the geometry of the universe in comparison with it is an independent argument and should lead to the same conclusions.

Drillingsraum: Dark Energy will play a major role in the future evolution of the universe. But to predict this future, we first need to understand the past. In 2009 the Planck-Space-Observatory began to map the cosmic microwave background with a higher resolution compared to COBE and WMAP, besides that Planck is less sensitive to disturbing signals.

Prof. Dr. Simon White: It also has a much wider frequency range, which is important.

Drillingsraum: Ok. So, what information can we expect from Planck, that COBE and WMAP couldn't give us?

It will measure significantly finer scales. The signal-to-noise ratio Planck achieves on each element of the sky is higher, so the temperature mapping of the microwave background is essentially perfect. We do not expect structures on very small scales in the primordial structure anyway, because they are smoothed out by the finite depth of the surface we are looking at. For the fluctuations that come from the initial conditions we do not

you can see shadows of galaxy clusters against the microwave background through a compton scattering effect which is known as the Sunyaev-Zel'dovich-Effect. Ground based experiments are already able to see distant clusters better than Planck, but they cannot cover such a high region of the sky. Planck is the first satellite to measure the polarisation of the microwave background, but it is unclear whether it will be sensitive enough to mine all the information which is in the polarisation signal. Here future missions will improve upon Planck. Physicists will not report Plancks cosmological results until the beginning of 2013 - and even then it is unclear whether they will be ready to publish the polarisation results, because the polarisation effect is even smaller than the temperature fluctuations. There also are different problems with foreground effects, for example from polarized emissions in our galaxy. Therefore it will take a considerable time to track down all the systematic issues and the foreground issues to be sure of the result. We hope, this will be possible in 2013, but we will have to wait and see.

Drillingsraum: When we plot the intensity of the microwave background radiation as a function of the wavenumber, we can see that its spectrum is comparable to that of a blackbody radiation. Why is this fact so important in relation to the exploration of the early universe?

Prof. Dr. Simon White: It is if you plot the intensity as a function of a frequency, than it looks like a black body. If you plot it as a function of spatial scale, then you basically see the modulation of the amplitude of the primordial soundwaves as a function of their wavelengths. What we see are soundwaves propagating through the universe at 400.000 years after the big bang, and these are influenced by three factors: One is the geometry of the universe, because that translates angles on the sky into centimeters on the surface we are looking at. The result there is that the universe appears to be flat. The second factor that the fluctuations are sensitive to is the medium in which the soundwaves are propagating, so they are sensitive to the content of the universe. From that we learn, that the ordinary matter is a small fraction of the total gravitating matter, even when the universe was only 400.000 years old. This tells us, that the dark matter problem was already there before there were any structures like