Studying the Cosmic Bubble Bath

The visible matter in the universe is arranged in a pattern: galaxies and gas clouds form a kind of foam in which filaments of matter surround voids. But alongside this visible matter are vast quantities of ‘dark matter’ and ‘dark energy’, both of which are invisible. University of Groningen PhD student Patrick Bos has demonstrated that it is possible to study the structure and nature of these invisible building blocks.

How do you study voids in something that you cannot see? This is the basic problem that Patrick Bos set out to solve, his supervisor and Professor of Astronomy Rien van de Weijgaert explains. ‘We can see that the visible matter forms a kind of cosmic web: high-density clusters that are linked by threads, and between them a void. Patrick studied how we can use this visible web to find out more about the universe as a whole.’

Only about four percent of the entire universe is made up of visible matter. Cosmologists believe that the rest can be divided into dark matter (a quarter) and dark energy (the rest). And we know very little about these, which is rather dissatisfying.

Voids

In his study, Bos focused mainly on the voids. Van de Weijgaert: ‘There is great interest in these cosmic voids because small deviations in gravity, for instance, can have a huge effect on them.’ Dark energy, which is considered to be responsible for the accelerating expansion of the universe, also has a greater effect there. ‘Dark energy is an extremely weak force, so the less normal matter there is, the greater the effect of this force.’ After all, a gentle breeze has a greater effect on a feather than on a brick.

The voids took shape as the universe evolved from the formation of the first stars down to the present day. Bos’s first step was to study these voids in detail. ‘How do you define the boundaries of a void? And just as in foam, larger voids contain smaller ones.’ Van de Weijgaert’s group developed a technique that can clearly, and above all objectively, identify the boundaries of the voids, the Watershed Void Finder Method, which is based on an image processing technology.

Initial conditions

Bos’s second step was to use information about the spatial distribution of galaxies and clusters of galaxies to determine the exact structure of the cosmic web (or bubble bath). This enabled him to reach a conclusion about the initial conditions of those parts of the universe that he studied. This may sound easy, but it was a very complex task. ‘He developed an ingenious statistical formula that, on the basis of a limited number of observed positions of galaxies or clusters, can determine what the initial conditions must have been in the area where these objects were situated.’ Van de Weijgaert is keen to emphasize how the research is thus a prime example of the research theme ‘Data Science and Systems Complexity’ of the Faculty of Mathematics and Natural Sciences of the University of Groningen.

Bos then carried out simulations that involved feeding his initial conditions into the models and calculating how these would have influenced the development of the universe. The results were a prediction of the distribution of visible and invisible matter in today’s universe. Bos could test the accuracy of his models by comparing them with observations.

Verlinde

Bos’s simulations proved that his method worked. ‘What is more, he is the first person to calculate how different proposed forms of dark energy would affect the development of voids in the cosmic web.’ Bos therefore used various models of dark energy to show how it influences the formation of voids. This is the first step to a better understanding of dark energy, despite University of Amsterdam physicist Erik Verlinde’s claim that this is unnecessary. Does Bos’s work have anything to say about Verlinde’s theory?

Van de Weijgaert thinks for a moment. ‘His theory does not contain any dark energy or dark matter, which is elegant, because despite decades of research we still have no idea what they are made up of.’ However, Verlinde is not the first to come up with an alternative theory of gravity. ‘These alternatives have not fared any better than Einstein’s thus far. The best model in all comparisons is generally his theory of relativity, with the cosmic constant that determines the expansion.’ Although it will not be easy to test Verlinde’s ideas in Bos’s system, it would be possible.

Data

Bos’s work is important primarily because of the flood of data that will come from new telescopes. ‘We have now determined the distribution of matter in a small part of the universe. In the decades to come we will gain much more information about the present universe and the early one.’ The further you look into the universe, the further back in time you see, because the light from distant galaxies takes billions of years to reach us. All this information can be used for a detailed study of dark matter and energy with the method that Bos describes in his thesis.

Patrick Bos was awarded his PhD on 5 December for his thesis entitled Clusters, voids and reconstructions of the cosmic web.