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Large-scale structure and Cosmology


The physical description of the cosmos, based on physics and science has evolved into an important discipline of astronomy. Astrophysical cosmology overlaps with theoretical physics in studying the properties of space and time, their intimate connection with gravitation, and the formation of structure in the universe. On the other hand, observational cosmology relies on making direct observations of astronomical targets such as galaxies and analysing the dependence of their properties with distance and epoch to constrain the history of the universe. To reflect this, Leiden Observatory offers an MSc program in cosmology together with the Institute Lorentz.

Over the past decade or so, cosmology has been revolutionized thanks to ever better observations, such as those of the cosmic microwave background, the relic radiation of the hot big bang which occurred 13.7 billion years ago, when the entire universe was compressed into an unimaginable tiny volume. The universe has, however, proven to be a rather bizarre place. For instance, observations of the motion of gas in galaxies and clusters of galaxies have provided strong evidence for the existence of dark matter. In addition, the discovery in 1998 that the expansion of the universe is accelerating suggests the existence of "dark energy". These findings provide some of the strongest evidence for the existence of new physics beyond the standard model of particle physics. By improving observational constraints on the constituents of the universe, cosmologists help constrain new physical theories. Understanding how the "simple" early universe evolved into the complex web of galaxies we see today is another main aspect of modern cosmology and research at Leiden Observatory. Several research projects at Leiden are directed towards studying the early universe. We here briefly mention a few examples:

  • Approximately 300,000 years after the Big Bang the universe was cool enough for neutral hydrogen to form and for photons to travel essentially unobstructed. The cosmic microwave background correspond to the last stages of this process. However, the current universe contains very little neutral hydrogen, which means something must have re-ionized the universe. When and how this happened is an important area of research. Leiden staff are involved both in theoretical studies of this "epoch of reionization" and in the development of the detection experiment with the Low Frequency Array (LOFAR). An important goal of LOFAR is to search for the radio signal from the first observable hydrogen in the Universe, during the period when the universe was emerging from the so-called "dark ages" and the first galaxies were beginning to emerge.
  • The study of the distribution of dark matter in the universe can tell us much about the properties of the dark energy, which drives the accelerated expansion of the universe. Furthermore, by comparing the masses of galaxies and clusters of galaxies to their visible properties, we can better understand their formation. One of the most promising ways forward uses the fact that such massive structures perturb the paths of photons emitted by distant galaxies: it is as if we are viewing these galaxies through a piece of glass with a spatially varying index of refraction. As a result the images of the galaxies appear distorted, which can be measured. This phenomenon, called gravitational lensing, allows us to make pictures of the dark matter distribution directly. Leiden staff play leading roles in this field of research and are involved in a number of large international projects. The latest of these is the KiloDegree Survey (KiDS).
  • Under the influence of gravity, primordial irregularities seeded by quantum fluctuations grew into the structure that we see around us today. As soon as these fluctuations become non-linear, their evolution becomes difficult to track analytically. Leiden staff carry out large computer simulations of the formation of cosmic structures, with an emphasis on the effects of baryonic physics. The models are used both to improve our understanding of the underlying physics and to help interpret observational data.
  • Galaxies with active/ explosive nuclei can easily be observed out to large distances. They are powerful probes of the early Universe and signposts of emerging galaxy clusters. There is a large group at Leiden devoted to studies of these early clusters and the properties of their constituent galaxies. They use a wide variety of space and ground-based facilities.

Faculty active in this area

Brinchmann, Franx, Hoekstra, Icke, Kuijken, Miley, Röttgering, Schaye

Projects and Collaborations