Department of Physics and Astronomy

Cosmology

Dark matter

About 85% of the matter in the Universe seems to be in some form that neither emits nor absorbs light, and shows no resemblance to the matter that we encounter in everyday life. This dark matter remains unidentified, but plays a key role in our current understanding of cosmology and the emergence of large-scale structure in the Universe. Every large galaxy, including our own home galaxy – the Milky Way – is believed to be surrounded by a huge dark matter halo – an order of magnitude more massive and extended than the galaxy itself.

One of the unsolved problems of modern cosmology concerns the distribution of dark matter on scales smaller than individual galaxies. The currently favoured dark matter paradigm predicts that galaxies like the Milky Way should be surrounded by large numbers of dark matter clumps (so-called subhalos) in the dwarf-galaxy mass range, yet only a few tentative detections of such dark subhalos have so far been made. Our team is using observations to study the properties of these subhalos, through the gravitational lensing distortions they produce in the images of distant galaxies and quasars.

The luminous parts of the galaxy 4144, a disk galaxy fairly similar to our own home galaxy, the Milky Way. Like all large galaxies, NGC 4144, is believed to be sitting inside a halo of dark matter, with large numbers of dark matter clumps (subhalos) in the dwarf-galaxy mass range. Image: NASA

Gravitational lensing

Galaxies and galaxy clusters can curve the path of light from background objects, thereby making them appear distorted and magnified. This gravitational lensing effect allows astronomers to study faint, distant light sources that would otherwise be beyond the reach of our telescopes, and to study the distribution of matter within the lensing objects themselves. Using a combination of observations and simulations, members of our group are exploiting gravitational lensing effects to hunt down galaxies in the early Universe, and to study dark matter on subgalactic scales.

By curving spacetime, the yellow galaxy in the centre curves distorts the appearance of a more distant, blue galaxy and makes it appear as a large, horseshoe-shaped structure. Gravitational lensing effects of this type can be used to study distant astronomical objects at large magnification, and to study the distribution of matter within the lens galaxy itself. Image: ESA/Hubble & NASA

Contact: Erik Zackrisson