We cannot see one of the Universe’s primary constituents: dark matter. The reason is simple: it doesn’t emit or absorb light in any wavelength. However, dark matter deflects, or “bends” light passing it and we can measure the distortions imprinted in the distant galaxies whose light is passing through dense regions of dark matter, a phenomenon predicted by General Relativity and known as gravitational lensing. 

Gravitational lensing effects are imprinted 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, allowing us to learn about the growth of structures in the Universe, and the dark energy driving the expansion of the space between them. This weak lensing signal is currently being analyzed using the Dark Energy Survey, and upcoming observations from the Vera Rubin Observatory will allow us to make these measurements much more precisely to learn more about the nature of dark matter and dark energy and their impact on the evolution of cosmic structure. KIPAC scientists are playing a leading role in making these measurements as accurately as possible, understanding the telescope and camera hardware, simulating what we expect to see from telescopes, and writing analysis software to extract the maximal amount of information from the data. 

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 “strong 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 or an exploding star, we can even measure the distance to the lensing objects and hence the overall scale of the Universe, and determine the current expansion rate of the Universe, otherwise known as Hubble's constant. KIPAC scientists are currently measuring the distances to several of these rare objects and play a leading role in the TDCOSMO collaboration, using data from the Hubble Space Telescope and the largest optical telescopes on Earth. We are currently investigating several newly discovered lenses from the Dark Energy Survey and we are preparing to extend this investigation to much larger samples, hundreds or even thousands of lenses, using the Vera Rubin Observatory. You can help us find new lenses too -- citizen science projects like Space Warps are playing a key role in this search. With enough data points, researchers should be able to measure the effects of the dark energy over time and decisively determine whether or not predictions from the early Universe are consistent with measurements in the late Universe, contributing resolution of the so-called  “Hubble tension.”