Triumphs and tribulations of ΛCDM, the double dark theory

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 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. However, the "too big to fail" (TBTF) problem challenges ΛCDM. It arose from analysis of the Acquarius and Via Lactea very high-resolution ΛCDM simulations of dark matter halos like that of the Milky Way. Each simulated halo has ∼ 10 subhalos 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 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 -or perhaps just better understanding of dark matter simulations and/or baryonic physics.

Parkes Pulsar Timing Array constraints on ultralight scalar-field dark matter

“…The noisy features in the (purple) solid curve are due to the amplitude modulation of pulsar terms; see Eq. (9).…”

“…The signal parameters in question are: the amplitude of dark matter induced gravitationalpotential oscillations Ψ c , oscillation frequency f , phase parameters α e and θ p ; see Eq. (9). It turns out that the statistic can be analytically maximized over Ψ c and thus the parameter space that needs to be numerically searched over is N p +2 dimensional.…”

The Phenomenology of Right Handed Neutrinos

“…On one hand, only little is known about how much structure actually exists at small (sub-galactic) scales. While lensing flux and stellar streams suggest the existence of subhalos [492], there is no direct observation of such structures and their existence is disfavoured by some stability considerations [570]. On the other hand there are no systematic studies of structure formation in the nonlinear regime that include the nonthermal component.…”

Terrestrial and martian heat flow limits on dark matter

“…Strongly-interacting DM can appreciably raise the temperature of the Earth and Mars through capture and subsequent annihilation. Simulations indicate that galaxies like the Milky Way exist within a virialized and spherical DM halo [19,20]. From this it follows that so long as DM is a light particle (in this case lighter than a small asteroid), there will be a constant flux of DM incident upon the Earth as it orbits the Galaxy.…”