We cannot see one of the universe’s primary constituents: dark matter. The reason is simple: it's dark. However, we can infer where it is located from observations of distant galaxies because of a key property of light, namely that it does not always travel in straight lines. For example, regions of the Universe with higher densities of dark matter, which host galaxies or clusters of galaxies, deflect, or “bend,” light rays from these far-off objects as the light passes close by, focusing the rays somewhat like a lens does. In some cases the deflection is large enough that it is possible for multiple images of the same background source to appear. This effect is known as “gravitational lensing” and is used by researchers at KIPAC to map out the distribution of mass – both dark and luminous – around clusters and galaxies.
By measuring the time delays between the multiple images of a flickering quasar, we can even measure the distance to the lens and hence the overall scale of the universe, otherwise known as Hubble's constant. KIPAC scientists are currently measuring the distances to several of these rare objects in collaboration with the COSMOGRAIL team, using data from the Hubble Space Telescope (among others). We are also preparing to extend this investigation to much larger samples, hundreds or even thousands of lenses, using the Dark Energy Survey and, later, the Large Synoptic Survey Telescope. With enough data points, researchers should be able to measure the effects of the dark energy.
Multiple image systems - strong lenses - are rare events. The more lenses we have, the more science we can do, and so KIPAC researchers are engaged in various searches for more gravitational lenses. You can help too, at the citizen science project Space Warps. LSST will discover many thousands of new lenses, presenting a significant data mining challenge.
Gravitational lensing effects are detectable in all distant galaxies: their images are all slightly distorted as they are lensed by the massive structures in the Universe lying between us and them. This weak lensing signal will be precisely measured using DES and LSST, allowing us to learn about the growth of structures in the universe, and the dark energy driving the expansion of the space between them. KIPAC scientists are engaged in trying to make these measurements as accurately as possible, understanding the telescope and camera hardware, and writing analysis software to extract the maximal amount of information from the data.