Simulating the Cosmic Web

If you could zoom out on the universe - so far that entire galaxies containing hundreds of billions of stars are just tiny points of light - you would find that the universe is ordered in complex and interconnected structures. If you could then turn back the clock on the universe - so far that the Big Bang was just tens of thousands of years previous, instead of billions - you would instead find a universe that looks almost exactly the same everywhere, with hardly any structures. The way in which the smooth and homogeneous early universe evolved, through gravity pulling mass together while the fabric of space itself expands, into the "cosmic web" of filaments, clusters, and voids we see today is the subject of my research at the University of Portsmouth Institute of Cosmology and Gravitation.

Even in a good budget situation, it is not entirely feasible to test models of structure formation by creating a universe and measuring what happens as it evolves. Instead, cosmologists in my field (also known as "large-scale structure") rely on computer simulations that solve the complex nonlinear equations of gravity for different cosmological models. We then can compare the results of these simulations to the distribution of galaxies observed with our telescopes and determine which model produces the best match to the observations.

A slice through the Millennium XXL simulation showing the cosmic web of structures. The bright yellow regions are the high density clusters that form at the intersection of filaments. For more, check out the interactive Millennium XXL browser!

But there is a problem. It turns out that most of the "stuff" in the universe can't be seen by telescopes at all, but only inferred through the effect it has on the movements of stars and gas via gravity. Since this stuff, called "dark matter", makes up most of the mass in the universe, we can understand structure formation on large scales by simulating the gravitational evolution of dark matter and ignoring the formation of stars and galaxies. Then, comparing these simulations to observations in order to test cosmological models first involves identifying the structures in the simulation that have fully collapsed - called dark matter halos - that are the sites of galaxy formation.

The method that my colleagues and I have developed to identify these structures - the halos, filaments, and voids of the cosmic web - is called ORIGAMI. As the initially smooth distribution of matter in the universe collapses under gravity to form structures, the dark matter particles cross paths, forming folds in a phase-space diagram of velocity vs. position. The figure below shows the final configuration of an initially flat sheet of dark matter particles, with lines connecting particles having very similar initial y-positions. In the figure, the large velocity spikes on the top correspond to the gravitational collapse on the bottom.

Phase-space (top) and real-space (bottom) final conditions of initial dark matter sheet.

In our three-dimensional universe, some structures have collapsed along only one dimension (walls), some along two (filaments), and full three-dimensional collapse forms halos. Areas that are not dense enough for gravitational collapse instead expand to form voids. The figure below shows the ORIGAMI-identified void particles in a simulation slice, with each void given a different random color, plotted over (small and black) particles in halos, filaments, and walls. Most of the volume of the universe is void!

ORIGAMI voids from a dark matter simulation.

At the ICG, I'm using these methods of detecting structures in simulations to look for observable signatures of different cosmological models. In particular, I am looking at models that modify general relativity on large scales as a way of explaining the accelerated expansion of the universe. The discovery of this acceleration has fundamentally changed our picture of cosmology and was acknowledged with a recent Nobel Prize in Physics. Simulations and observations of the cosmic web of large-scale structure will hopefully help us determine whether the acceleration is caused by "dark energy", is the signal of a problem with general relativity, or something else entirely. One alternative - and I'm being perfectly serious - is that the gravitational force weakens as it leaks out along a 4th spatial dimension! It is an exciting time to be in the field of cosmology.