Haiman Group

Black hole astrophysics and cosmology

Over the past decades, it has become clear that most galaxies have a massive black hole in their nucleus, with masses ranging from a million to a billion solar masses. The energetic radiation and material jets emanating from these black holes play a significant role in sculpting galaxies over cosmic time, but their origin remains unknown. Observations have shown that these massive black holes already existed in early times, when the universe was less than a billion years old, i.e. they possibly pre-date galaxies.
We also know that galaxies are built up over cosmic time by a series of mergers, from small early protogalaxies with perhaps a million solar masses to present-day galaxies as large as tens of trillions of solar masses. Massive black holes in galactic nuclei are naturally expected to merge and to produce copious gravitational waves in the process. These early massive black hole mergers are the prime targets for forthcoming space-based low-frequency gravitational wave detectors, such as the European Space Agency’s Laser Interferometer Space Antenna (LISA), planned for launch in the 2030s.
Galaxies and their black holes form and evolve in an expanding background universe, whose expansion has recently speeded up. This overall cosmic acceleration can be attributed to a hitherto unidentified component of our universe (dubbed “dark energy”), to a finite energy density of the quantum vacuum, or else to our lack of understanding of gravitation deviating from Einstein’s theory on cosmic scales. These different explanations cause subtle differences in how the average universe has evolved over time for the past several billion years, as well as the types of structures that formed and the speed at which these structures grew nonlinear. These differences can be revealed statistically through gravitational lensing, an effect of general relativity, which distorts the apparent shapes of galaxies—so-called weak gravitational lensing.
Zoltan Haiman’s group investigates questions related to the above phenomena through semi-analytic approaches, numerical simulations, and interpreting observations. How and when did the first objects (including stars and black holes) form in the universe, and what impact did they have on the rest of the universe? Are mergers a major contributor to the growth of black holes, and how do such mergers unfold in galactic nuclei in the presence of stars and gas? How can we use observations of both gravitational waves and electromagnetic radiation in tandem to answer these questions? Finally, while weak lensing is a promising cosmological tool, its full potential is yet to be realized. Haiman’s group has been developing new statistical approaches, including through machine learning, to utilize lensing data from large forthcoming surveys that will include measurements of the shapes of up to billions of galaxies.