PIRSA:13090066 Scientific Series Cosmology

DOI10.48660/13090066

Series: Cosmology and Gravitation

Primack, J. (2013). ΛCDM Large Scale Triumphs and Small Scale Challenges. Perimeter Institute. https://pirsa.org/13090066

Primack, Joel. ΛCDM Large Scale Triumphs and Small Scale Challenges. Perimeter Institute, Sep. 17, 2013, https://pirsa.org/13090066 

@misc{ pirsa_PIRSA:13090066,
  doi = {10.48660/13090066},
  url = {https://pirsa.org/13090066},
  author = {Primack, Joel},
  keywords = {Cosmology},
  language = {en},
  title = {ΛCDM Large Scale Triumphs and Small Scale Challenges},
  publisher = {Perimeter Institute},
  year = {2013},
  month = {sep},
  note = {PIRSA:13090066 see, \url{https://pirsa.org}}
}

Abstract

ΛCDM has become the standard cosmological model because its predictions agree so well with observations of the cosmic microwave background and the large-scale structure of the universe. 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 galactic “downsizing." The discovery of many faint galaxies in the Local Group is consistent with the large number of subhalos in ΛCDM simulations.  There is increasing evidence for such substructure in galaxy dark matter halos from gaps in cold stellar streams in the Milky Way and Andromeda and from gravitational lensing flux anomalies, with the prospect of rapidly increasing data on that from ALMA. The “too big to fail" (TBTF) problem is the latest apparent challenge to ΛCDM. It arose from analysis of the Aquarius and Via Lactea very-high-resolution ΛCDM simulations of Milky-Way-mass dark matter halos. Each simulated halo has ∼10 subhalos 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 dwarf satellite galaxies of the Milky Way or Andromeda have stars moving as fast as would be expected in these densest subhalos. This may indicate the need for a more complex theory of dark matter – but several recent papers have shown that subhalos in pure dark matter simulations like Aquarius or Via Lactea are significantly modified when baryonic effects are included, so as to solve the TBTF problem.  Higher resolution simulations are needed to verify this.

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