What is the visible universe

"Possible deviations from the standard model"

In the young cosmos - a few hundred thousand years after the Big Bang - matter was still very evenly distributed in the universe. Over time, however, they clumped together and stars, galaxies and eventually even extensive galaxy clusters formed. In an interview with Welt der Physik, the cosmologist reports how Hendrik Hildebrandt from the University of Bochum and his colleagues research the structure and density of matter in the universe and what amazing results they came across.

World of physics: You deal with matter in the universe. What are you interested in exactly?

Hendrik Hildebrandt

Hendrik Hildebrandt: Today's universe is filled with matter, from planets and stars to galaxies and entire galaxy clusters. This visible matter makes up about twenty percent of the matter content of the universe. The rest of the matter - the so-called dark matter - is invisible and is only noticeable through gravity. In my research, I am interested in how matter - both the visible and the invisible - is distributed in our universe. We want to find out, for example, whether matter is relatively uniformly distributed in space or whether it clumps together in many places - such as in galaxy clusters.

How do such lumps of matter arise?

According to our current state of knowledge, matter was still very regularly distributed in the universe shortly after the Big Bang. This is shown by measurements of the so-called cosmic background radiation, which was released around 380,000 years after the Big Bang and which still fills the universe today. However, at that time there were already tiny fluctuations in the density of matter. In the places where the matter was a bit denser, even more matter was attracted from the environment due to the force of gravity. Tiny condensations at the beginning of the universe have developed into larger lumps of matter over time.

How can this be investigated?

To describe the history and development of our universe, physicists use the so-called standard model of cosmology. With this model we can calculate the distribution of matter at different points in time in the universe - based on the distribution of matter in the young cosmos. The aim of our research group is to check these predictions. We do this by measuring the distribution of matter in the universe several billion years after the Big Bang. In this way we can find out whether our models actually describe the structure of the universe correctly.

How did you go about it?

Since most of the matter in our universe consists of dark matter, the lumps of matter cannot be detected directly with telescopes. Nevertheless, we can observe them indirectly - through the effect of their gravitational field. Because according to the general theory of relativity, light is deflected by the gravitational field of a large mass. This so-called gravitational lensing effect means that the light of a galaxy - if there is a massive object between the galaxy and us - does not hit our earth directly. The light from the galaxy is deflected and distorted so that our telescopes take a distorted image of the galaxy.

And through the distorted image, can you identify the lumps of matter?

Gravitational lensing

Exactly, but the effect is usually not visible to the naked eye. We have to analyze the shape of a great many galaxies that are in the same area of ​​the sky. If the galaxies are on average distorted in a certain direction, we can conclude that there must be a greater mass in front of the galaxies. In order to determine how large this mass is, however, we still need to know how far away the light sources are from us. Just like with an optical lens, the image through the gravitational lens also depends on the position of the light source.

How do you determine this distance?

We use another physical phenomenon to determine the distance between galaxies: the further away a galaxy is from us, the faster it moves away from us due to the expansion of the universe. Because of this movement, the light from the galaxies reaches the earth at a lower frequency and is shifted into the red color range. This effect is similar to the Doppler effect that occurs with sound waves when, for example, a fire engine with a siren drives past us and the frequency of the sound changes. With the help of telescopes, we can make the redshift visible by viewing the galaxies with different color filters and examining their relative brightness. The further away the galaxies are from us, the redder they appear to us. From the combination of the distorted images of the galaxies and their redshift, we can then investigate how much matter is in the observed area.

What did you find out?

We observed several million galaxies and, based on this data, calculated a value that takes into account the density of matter in our universe and its tendency to form clumps of matter. The amazing thing about the result is that our value deviates significantly from what we had expected from the measurements of the cosmic background radiation - extrapolated with the standard model of cosmology. We have had indications of this deviation for a number of years, which we have now been able to confirm again with our latest measurements.

What does this deviation mean?

Statistically, the probability that the two values ​​can still be reconciled is around one percent. In the future we want to evaluate data from other galaxies in order to increase our informative value. At the same time, we want to exclude possible systematic errors when determining the two values. If the discrepancy between our measurements and the theoretical calculations persists, this could mean that the universe has developed differently than we have previously assumed based on the measurements of the background radiation and the standard model of cosmology.

So it could be that the standard cosmological model is being discarded?

That is an option. Scientists are already working on numerous alternatives - from expanding the standard model to a completely new theory. For example, the so-called dark energy - which ensures the accelerated expansion of the universe - could change over time. This could possibly explain the discrepancy between our results of the matter distribution and the projections that result from the background radiation and the standard model of cosmology. But the cosmological standard model has proven itself time and again in the past and can explain most of our observations. In order for another model to prevail, we therefore need more powerful information. Hopefully, the evaluation of further data will show whether we actually need a new model to describe our cosmos and, if so, what this model should look like.