Cosmology from Sunyaev-Zeldovich selected Galaxy Clusters

hdf Hubble Deep Field (Image credit NASA).

Current work in cosmology focuses on understanding the universe through the lens of the current standard model of cosmology, as well as extensions to this model.

Today’s standard model of cosmology includes a description of both dark matter and dark energy, and is capable of predicting observations of the universe with tremendous accuracy. However, the nature of dark matter and dark energy is completely unaddressed by this model; dark energy could be an actual energy density which permeates the universe, or, it could be the result of our theory of gravity being incomplete. Testing whether one of these explanations is correct is at the forefront of modern cosmology.

The standard model of cosmology can also be extended in other ways; the properties of neutrinos, a type of elementary particle, affect the evolution of the universe. Originally these particles were thought to be massless, but now cosmological models include nonzero neutrino masses. However, neutrinos are some of the lightest particles that are not massless known to us, and their masses are not known particularly well. Because cosmological evolution is sensitive to neutrino masses, we can use experiments in cosmology to probe these masses and set bounds on their sum.

Cosmology from Galaxy Clusters

galaxyclusters Galaxy cluster MACS J0454.1-0300 (Image credit ESA/Hubble).

These and other extensions can be probed with galaxy clusters, huge conglomerations of many galaxies which become bound together by their gravity. These clusters formed late in the universe, and their formation is influenced by the properties of gravity at large scales, as well as by dark energy. This makes measuring their clustering properties, as well as their abundances throughout the history of the universe, a good way to test our models of gravity and dark energy, as well as constrain neutrino masses. By looking at galaxies further and further away, we peek into the past due to light’s finite speed, allowing us to measure the properties of clusters at various times in the history of the universe.

A major difficulty with using galaxy clusters comes in actually picking them out of data. Optical surveys performed with visible light have the disadvantage that they are subject to statistical biases which we still struggle to understand. Fortunately, there is another way to select clusters which is not as statistically problematic, which utilizes the effect clusters have on the Cosmic Microwave Background, or CMB.

structuregrowth Growth of structure simulations showing how groups of galaxies can form from uniform distributions of matter. Simulations performed at the National Center for Supercomputer Applications (credit Andrey Kravtsov and Anatoly Klypin).

Selecting Clusters using the Sunyaev-Zeldovich Effect

The CMB was the first light which propagated in the universe, once it cooled enough that light could avoid scattering off of the hot, dense gas which made up our universe in its youth. For the most part, this light was able to propagate freely through the universe, only stopping when it hits something like a planet (for example, Earth). However, galaxy clusters contain large clouds of electron gas, which imprint a slight hotspot into CMB light before it reaches us, in a process called the thermal Sunyaev-Zeldovich (or tSZ) effect. By looking for hotspots like this, we identity locations of clusters on the sky, and can then use optical telescopes to learn more about the properties of these clusters.

By utilizing the tSZ effect to find clusters, we can use the cluster catalogs to improve our knowledge about the standard model of cosmology, as well as constrain the properties of dark energy, gravity, and massive neutrinos. I hope to use data from the currently active Atacama Cosmology Telescope (ACT), as well the Simons Observatory in the near future (it is due for first-light in 2021), which will both observe the CMB and find thousands of galaxy clusters, to utilize the tSZ effect for cosmology.

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