Colloquium
Colloquium

ΛCDM Cosmology: Successes, Challenges, and Opportunities for Progress 

Joel Primack

University of California, Santa Cruz

 

The modern standard cosmological theory ΛCDM is remarkably successful in predicting the cosmic microwave background and large-scale structure, and ΛCDM parameters have been determined with only mild tensions between different types of observations.  Hydrodynamical simulations starting from cosmological initial conditions are increasingly able to capture the complex interactions between dark matter and baryonic matter in galaxy formation.  Simulations with relatively low resolution (EAGLE, Illustris) now succeed in describing the overall galaxy population.  It once seemed that galaxies are pretty smooth, that they generally grow in size as they evolve, and that they are a combination of disks and spheroids. But recent HST observations combined with high-resolution hydrodynamic simulations are showing that most star-forming galaxies are very clumpy; that galaxies often undergo compaction, which reduces their radius and increases their central density; and that most lower-mass star-forming galaxies are not spheroids or disks but are instead elongated when their centers are dominated by dark matter.  I will also review ΛCDM challenges on smaller scales: cusp-core, “too big to fail,” and substructure issues.  Although starbursts can rapidly drive gas out of galaxy centers and thereby reduce the dark matter density, it remains to be seen whether this or other baryonic physics can explain the observed rotation curves of the entire population of dwarf and low surface brightness galaxies.  If not, perhaps more complicated physics such as self-interacting dark matter may be needed.  But standard ΛCDM appears to be successful in predicting the dark matter halo substructure that is now observed via gravitational lensing and breaks in cold stellar streams, and any alternative theory must do at least as well.