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James Bullock - UC Irvine. Irvine, CA, US

James Bullock

Dean, School of Physical Sciences, Professor Physical Sciences, Physics & Astronomy | UC Irvine


James Bullock studies how galaxies and their constituent dark matter halos have formed and evolved over billions of years of cosmic time.





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Putting Together the Pieces of the Universe


UCI Podcast: School of Physical Science Dean James Bullock on Solutions That Scale



Professor Bullock received a B.S. in both Physics and Math from The Ohio State University in 1994 and a Ph.D. in Physics from the University of California, Santa Cruz in 1999. After postdoctoral positions at The Ohio State University and Harvard University, he came to UC Irvine as an Assistant Professor in 2004. He was elected Fellow of the American Association for the Advancement of Science in 2008. Professor Bullock served as the 17th Chair of the UCI Physics and Astronomy Department from 2017-2019 before becoming the 9th Dean of the UCI School of Physical Sciences in 2019.

Aided by super-computer simulations and analytic models, Professor Bullock studies how galaxies and their constituent dark matter halos have formed and evolved over billions of years of cosmic time. By analyzing data that astronomers have collected using the Hubble Space Telescope, the Keck Observatory, and other ground and space telescopes, he works to understand how galaxies, including the Milky Way and its Local Group of galaxies, emerged from the primordial universe. One of his long-standing interests has been the use of astrophysical observations to constrain the microphysical nature of dark matter.

Professor Bullock currently serves as Chair of the James Webb Space Telescope User’s Committee. Previously he was Chair of the working group that recommended the Hubble Frontier Fields Program, which is responsible for galaxy cluster image on the top of this page. He is passionate about science outreach and appears regularly on the Science Channel’s How the Universe Works.

Areas of Expertise (5)

Dark Matter

Star Formation


Galaxy Dynamics


Education (2)

University of California, Santa Cruz: PhD, Physics 1999

The Ohio State University: BS, Physics and Math 1994

Media Appearances (8)

Dark matter does exist, simulations indicate

Futurity  online


“Observed galaxies seem to obey a tight relationship between the matter we see and the inferred dark matter we detect, so much so that some have suggested that what we call dark matter is really evidence that our theory of gravity is wrong,” says coauthor James Bullock, professor of physics at UC Irvine and dean of the UCI School of Physical Sciences. “What we showed is that not only does dark matter predict the relationship, but for many galaxies it can explain what we see more naturally than modified gravity. I come away even more convinced that dark matter is the right model.”

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Explore space science at the Discovery Cube

The Orange County Register  online


Guests may venture out even further in the Solar System Encounter, a popular returning exhibit that features a colorful, large-scale model of the sun, planets and other objects at its center. Here, visitors may participate in space-themed experiments, find out what they’d weigh on other planets, and – new this year – view images from NASA’s James Webb Space Telescope provided through a collaboration with James Bullock, an astrophysicist and dean of UC Irvine’s School of Physical Sciences.

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Galaxies without dark matter might be possible after all

Futurity  online


James Bullock, an astrophysicist at the University of California, Irvine, described how he and the team didn’t build their model just so they could create galaxies without dark matter—something he says makes the model stronger, because it wasn’t designed in any way to create the collisions that they eventually found. “We don’t presuppose the interactions,” says Bullock.

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Galaxies Exist Without Dark Matter, Research Says

Azo Quantum  online


James Bullock, from UCI, an astrophysicist who is a world-renowned expert on low-mass galaxies, explained how he and the team did not construct their model just so they can make galaxies without dark matter — something he said makes the model powerful since it was not developed in any way to establish the collisions that they eventually found. “We don’t presuppose the interactions,” stated Bullock.

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Scientists discover how galaxies can exist without dark matter

Phys.org  online


UCI's James Bullock, an astrophysicist who's a world-renowned expert on low-mass galaxies, described how he and the team didn't build their model just so they could create galaxies without dark matter—something he said makes the model stronger, because it wasn't designed in any way to create the collisions that they eventually found. "We don't presuppose the interactions," said Bullock.

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James Bullock to become new dean of UCI School of Physical Sciences

UCI News  online


James Bullock, professor and chair of physics & astronomy at the University of California, Irvine, has been appointed dean of the School of Physical Sciences. He will assume the new role on July 1, upon the retirement of Dean Kenneth Janda.

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UCI celestial census indicates that black holes pervade the universe

UCI News  online


“We think we’ve shown that there are as many as 100 million black holes in our galaxy,” said UCI chair and professor of physics & astronomy James Bullock, co-author of a research paper on the subject in the current issue of Monthly Notices of the Royal Astronomical Society.

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Dark Matter May Be More Complex Than Physicists Thought

Wired  online


About four years ago, James Bullock, a professor of physics and astronomy at the University of California, Irvine, began to wonder whether the standard view of dark matter was failing important empirical tests. “This was the point where I really started thinking hard about alternatives,” he said.

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Articles (5)

Accurate mass estimates from the proper motions of dispersion-supported galaxies

Monthly Notices of the Royal Astronomical Society

Alexandres Lazar, James S Bullock

2020 We derive a new mass estimator that relies on internal proper motion measurements of dispersion-supported stellar systems, one that is distinct and complementary to existing estimators for line-of-sight velocities. Starting with the spherical Jeans equation, we show that there exists a radius where the mass enclosed depends only on the projected tangential velocity dispersion, assuming that the anisotropy profile slowly varies.

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A profile in FIRE: resolving the radial distributions of satellite galaxies in the Local Group with simulations

Monthly Notices of the Royal Astronomical Society

Jenna Samuel, Andrew Wetzel, Erik Tollerud, Shea Garrison-Kimmel, Sarah Loebman, Kareem El-Badry, Philip F Hopkins, Michael Boylan-Kolchin, Claude-André Faucher-Giguère, James S Bullock, Samantha Benincasa, Jeremy Bailin

2019 While many tensions between Local Group (LG) satellite galaxies and Λ cold dark matter cosmology have been alleviated through recent cosmological simulations, the spatial distribution of satellites remains an important test of physical models and physical versus numerical disruption in simulations.

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Astro2020 APC White Paper: Theoretical Astrophysics 2020-2030

arXiv preprint arXiv:1912.09992

Juna A Kollmeier, Lauren Anderson, Andrew Benson, Tamara Bogdanovic, Michael Boylan-Kolchin, James S Bullock, Romeel Dave, Federico Fraschetti, Jim Fuller, Philip F Hopkins, Manoj Kaplinghat, Kaitlin Kratter, Astrid Lamberts, M Coleman Miller, James E Owen, E Sterl Phinney, Anthony L Piro, Hans-Walter Rix, Brant Robertson, Andrew Wetzel, Coral Wheeler, Andrew N Youdin, Matias Zaldarriaga

2019 The past two decades have seen a tremendous investment in observational facilities that promise to reveal new and unprecedented discoveries about the universe. In comparison, the investment in theoretical work is completely dwarfed, even though theory plays a crucial role in the interpretation of these observations, predicting new types of phenomena, and informing observing strategies.

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Stars made in outflows may populate the stellar halo of the Milky Way

arXiv preprint arXiv:1912.03316

Sijie Yu, James S Bullock, Andrew Wetzel, Robyn E Sanderson, Andrew S Graus, Michael Boylan-Kolchin, Anna M Nierenberg, Michael Y Gurdić, Philip F Hopkins, Dušan Kereš, Claude-André Faucher-Giguère

2019 We study stellar-halo formation using six Milky Way-mass galaxies in FIRE-2 cosmological zoom simulations. We find that 5−40% of the outer (50−300 kpc) stellar halo in each system consists of in-situ stars that were born in outflows from the main galaxy. Outflow stars originate from gas accelerated by super-bubble winds, which can be compressed, cool, and form co-moving stars.

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Be it therefore resolved: cosmological simulations of dwarf galaxies with 30 solar mass resolution

Monthly Notices of the Royal Astronomical Society

Coral Wheeler, Philip F Hopkins, Andrew B Pace, Shea Garrison-Kimmel, Michael Boylan-Kolchin, Andrew Wetzel, James S Bullock, Dušan Kereš, Claude-André Faucher-Giguère, Eliot Quataert

2019 We study a suite of extremely high-resolution cosmological Feedback in Realistic Environments simulations of dwarf galaxies (⁠Mhalo≲1010M⊙⁠), run to z = 0 with 30M⊙ resolution, sufficient (for the first time) to resolve the internal structure of individual supernovae remnants within the cooling radius. Every halo with Mhalo≳108.6M⊙ is populated by a resolved stellar galaxy, suggesting very low-mass dwarfs may be ubiquitous in the field.

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