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)

摘要（Abstract）:

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.

个人简介（Biography）:

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.