April 10 – Public Monthly Meeting – How to Measure Velocities of Distant Galaxy Clusters, and Why

This NASA/ESA Hubble Space Telescope image features the galaxy LRG-3-817, also known as SDSS J090122.37+181432.3. The galaxy, its image distorted by the effects of gravitational lensing, appears as a long arc to the left of the central galaxy cluster. Gravitational lensing occurs when a large distribution of matter, such as a galaxy cluster, sits between Earth and a distant light source. As space is warped by massive objects, the light from the distant object bends as it travels to us and we see a distorted image of it. This effect was first predicted by Einstein’s general theory of relativity. Strong gravitational lenses provide an opportunity for studying properties of distant galaxies, since Hubble can resolve details within the multiple arcs that are one of the main results of gravitational lensing. An important consequence of lensing distortion is magnification, allowing us to observe objects that would otherwise be too far away and too faint to be seen. Hubble makes use of this magnification effect to study objects beyond the sensitivity of its 2.4-metre-diameter primary mirror, showing us the most distant galaxies humanity has ever encountered. This lensed galaxy was found as part of the Sloan Bright Arcs Survey, which discovered some of the brightest gravitationally lensed high-redshift galaxies in the night sky. 

How to Measure Velocities of Distant Galaxy Clusters, and Why


Arthur Kosowsky, Ph.D.

Sunday, April 12, 2022

7:30 PM to 9:00 PM EST

Online event


Monthly Meeting – Public Invited



Galaxy clusters are the most massive gravitationally bound objects in the universe. Their hot ionized gas scatters photons of the cosmic microwave background, creating a microwave spectrum distortion (called the Thermal Sunyaev-Zeldovich Effect) which recent experiments have used to identify thousands of clusters. Velocities of these clusters with respect to the cosmic rest frame can be probed using a related hot-gas scattering effect (the Kinematic Sunyaev-Zeldovich Effect) or a subtle gravitational lensing effect (the transverse lensing effect). I will describe these effects and their detection prospects in upcoming cosmic microwave background experiments. Velocities of galaxy clusters reflect the growth of structure in the universe, and so provide one route to probing whether the accelerating expansion of the universe is due to some mysterious dark energy or to a modification of general relativity on cosmological scales.


Arthur Kosowsky is Professor and Chair of Physics and Astronomy at the University of Pittsburgh. He received his Ph.D. from the University of Chicago in 1994 and spent three years as a Junior Fellow at Harvard University before taking a faculty job at Rutgers University. He moved to Pittsburgh in 2005. Major research interests have included the cosmic microwave background radiation, properties of the universe, and gravitational waves. Kosowsky was a founding member of the Atacama Cosmology Telescope collaboration and the Simons Observatory collaboration.

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