Cold dark matter: Controversies on small scales

Abstract

The cold dark matter (CDM) cosmological model has been remarkably successful in explaining cosmic structure over an enormous span of redshift, but it has faced persistent challenges from observations that probe the innermost regions of dark matter halos and the properties of the Milky Way's dwarf galaxy satellites. We review the current observational and theoretical status of these "small-scale controversies." Cosmological simulations that incorporate only gravity and collisionless CDM predict halos with abundant substructure and central densities that are too high to match constraints from galaxy dynamics. The solution could lie in baryonic physics: Recent numerical simulations and analytical models suggest that gravitational potential fluctuations tied to efficient supernova feedback can flatten the central cusps of halos in massive galaxies, and a combination of feedback and low star formation efficiency could explain why most of the dark matter subhalos orbiting the Milky Way do not host visible galaxies. However, it is not clear that this solution can work in the lowest mass galaxies, where discrepancies are observed. Alternatively, the small-scale conflicts could be evidence of more complex physics in the dark sector itself. For example, elastic scattering from strong dark matter self-interactions can alter predicted halo mass profiles, leading to good agreement with observations across a wide range of galaxy mass. Gravitational lensing and dynamical perturbations of tidal streams in the stellar halo provide evidence for an abundant population of low-mass subhalos in accord with CDM predictions. These observational approaches will get more powerful over the next few years.dark matter | cosmology | galaxy formation T he cold dark matter (CDM) hypothesis-that dark matter consists of a weakly interacting particle whose velocity dispersion in the early universe was too small to erase structure on a galactic or subgalactic scale-emerged in the early 1980s and quickly became a central element of the theory of cosmic structure formation. Influential early papers include Peebles' calculation of cosmic microwave background (CMB) anisotropies and the matter power spectrum (1), discussions of galaxy formation with particle dark matter by Bond et al. (2) and Blumenthal et al. (3,4), and Davis et al.'s (5) numerical simulations of galaxy clustering. By the mid-1990s, the simplest CDM model with scale-invariant primordial fluctuations and a critical matter density of ðΩ m = 1Þ had run afoul of multiple lines of observational evidence, including the shape of the galaxy power spectrum, estimates of the mean matter density from galaxy clusters and galaxy motions, the age of the universe inferred from estimates of the Hubble constant, and the amplitude of matter clustering extrapolated forward from the fluctuations measured in the CMB. Many variants on "canonical" CDM were proposed to address these challenges, and by the turn of the century, the combination of supernova evidence for cosmic acceleration and CMB evidence for ...

Structure formation with suppressed small-scale perturbations

“…Implicitly, the WDM regime is defined to lie in the rather narrow band where the suppression of perturbation has a significant effect on dwarf galaxy formation but is still in agreement with observations. Furthermore, WDM is generally assumed to reduce potential small-scale inconsistencies, such as the missing satellite or the too big to fail problem, occurring in a ΛCDM universe (see Weinberg et al 2013, for a review on these topics). Recent studies point out that both requirements -passing observational constraints and alleviating small scale inconsistencies -seem impossible to combine because constraints from Lyman-α are prohibitively strong in the case of WDM (Viel et al 2013;Schneider et al 2014).…”

Reionization and Galaxy Formation in Warm Dark Matter Cosmologies

“…With its property of clustering on all scales, CDM has been remarkably successful in predicting the large-scale structure of the universe, the temperature anisotropies measured by the cosmic microwave background (CMB) and Lyα forest statistics (e.g., Peebles 1971;Bond & Szalay 1983;Blumenthal et al 1984;Lange et al 2001;Cole et al 2005;Hinshaw et al 2013;Slosar et al 2013;Planck Collaboration et al 2014a). However, as recently reviewed by Weinberg et al (2015), CDM exhibits a number of small-scaleproblems, including(1) producing halo profiles that are cuspy as opposed to the observationally preferred constant density cores (Navarro et al 1997;Subramanian et al 2000), (2) over-predicting the number of satellite and field galaxies as compared to observations-the "missing satellite problem" (Klypin et al 1999;Moore et al 1999), (3) predicting massive (Large Magellanic Cloud mass) concentrated Galactic sub-halos inconsistent with observations (e.g., Boylan-Kolchin et al 2012), and (4) facing difficulties in producing typical disks due to ongoing mergers down to z;1 (Wyse 2001). The limited success of baryonic feedback in solving these smallscale problems (e.g., Boylan-Kolchin et al 2012;Teyssier et al 2013) has prompted questions regarding the nature of dark matter itself.…”

Constraints on anharmonic corrections of fuzzy dark matter

“…Even though most DM models mimic CDM on cosmological scales, their predictions usually differ on smaller scales [5] so they could be discriminated based only on their gravitational effects. In fact, there exist three long-standing debates, questioning the agreement between observations and the CDM theoretical predictions [6,7], the so-called 'too big to fail ' [8], 'missing satellites' [9] and especially the 'core-cusp' problem [10]. The 'core-cusp' problem refers to the discrepancy between the density profiles of CDM halos obtained in N -body simulations, that tend to be cuspy in the center, and the ones inferred from observations, that point to the existence of a central…”