Colloquium: ΛCDM: Large-Scale Triumphs and Small-Scale Challenges (Joel Primack, UC-Santa Cruz, Sep. 6, 2012)

报告题目Title: ΛCDM: Large-Scale Triumphs and Small-Scale Challenges

报告人Speaker: Joel Primack, Physics Department, University of California, Santa Cruz.

地点(Location)Room 111, Physics Building

时间Time Sep. 6, 2012 (Thursday) 15:00 (3pm)


ΛCDM has become the standard cosmological model because its predictions agree so well with observations of the cosmic microwave background and with the large-scale structure of the universe as shown by comparison of the latest cosmological simulations with observations. However ΛCDM has faced challenges on smaller scales.

Some of these challenges, including the "angular momentum catastrophe" and the absence of density cusps in the centers of small galaxies, may be overcome with improvements in simulation resolution and feedback. Recent simulations appear to form realistic galaxies in agreement with observed scaling relations. Although dark matter halos start small and grow by accretion, the existence of a star-forming band of halo masses naturally explains why the most massive galaxies have the oldest stars, a phenomenon known as known as galactic "downsizing." The discovery of many faint galaxies in the Local Group is consistent with ΛCDM predictions, as is the increasing evidence for substructure in galaxy dark matter halos from gravitational lensing flux anomalies and gaps in cold stellar streams. The "too big to fail" (TBTF) problem, which challenges ΛCDM, arose from analyses of the Aquarius and Via Lactea very high resolution ΛCDM simulations of dark matter halos like that of the Milky Way. Each simulated halo had ~10 sub-halos that were so massive and dense that they would appear to be too big to fail to form lots of stars. The TBTF problem is that none of the observed satellites of the Milky Way or Andromeda have stars moving as fast as would be expected in these densest sub-halos. This may indicate the need for a more complex theory of dark matter -- but the latest simulations suggest that a better understanding of baryonic physics may resolve this problem.



Distinguished Professor of Physics

Director, University of California systemwide High-Performance Astro-Computing Center, 2010-

Princeton University A.B. 1966 Physics, (Summa cum laude, valedictorian)

Ph.D. Stanford University, 1970 Physics

Junior Fellow of the Society of Fellows, Harvard University, 1970-73

A.P. Sloan Foundation Research Fellowship, 1974

Fellow of the American Physical Society and of the American Association for the Advancement of Science 

Alexander von Humboldt Foundation Senior Award, 1999


         Dr. Joel R. Primack specializes in the formation and evolution of galaxies and the nature of the dark matter that makes up most of the matter in the universe. After helping to create what is now called the "Standard Model" of particle physics, Primack began working in cosmology in the late 1970s, and he became a leader in the new field of particle astrophysics.  His 1982 paper with Heinz Pagels was the first to propose that a natural candidate for the dark matter is the lightest supersymmetric particle.  He is one of the principal originators and developers of the theory of Cold Dark Matter, which has become the basis for the standard modern picture of structure formation in the universe.  With support from the National Science Foundation, NASA, and the Department of Energy, he is currently using supercomputers to simulate and visualize the evolution of the universe and the formation of galaxies under various assumptions, and comparing the predictions of these theories to the latest observational data.