The core-cusp problem revisited: ULDM vs. CDM

Abstract

The core-cusp problem is often cited as a motivation for the exploration of dark matter models beyond standard CDM [cold dark matter]. One such alternative is ULDM [ultra-light dark matter]; particles exhibiting wavelike properties on kiloparsec scales. ULDM dynamics are governed by the Schrödinger-Poisson equations, which have solitonic ground state solutions consisting of gravitationally-bound condensates with no internal kinetic energy. Astrophysically realistic ULDM halos would consist of larger NFW-like configurations with a possible solitonic core. We describe a parameterisation for the radial density profiles of ULDM halos that accounts for the environmental variability of the core-halo mass relation. We then compare the semi-analytic profiles of ULDM and CDM with astrophysical data, and find that a ULDM particle mass of 10 −23 eV can yield a reasonably good fit to observed rotation data, particularly at small radii. This ULDM mass is in tension with other constraints but we note that this analysis ignores any contribution from baryonic feedback.It is widely agreed that non-baryonic dark matter constitutes the majority of the mass of the observable universe, but its precise nature remains an open question. Many dark matter models have been proposed, with particle CDM [Cold Dark Matter] being the most widely studied. This scenario successfully accounts for the large scale structure of the universe [1] and the spectrum of anisotropies in the microwave background [2-8], but the so-called "smallscale crisis" remains a challenge [9]. A key issue is the tension between the central density profiles of dark matter halos in simulations containing only gravitationally interacting CDM, and those inferred from observational data. Simulations tend to produce 'cuspy' central density profiles [10], which grow as 1/r at small radii, but observational data appears to favour flattened central cores [11]. This so-called core-cusp problem has been the focus of much recent attention [12][13][14].The seriousness of the core-cusp problem is the subject of ongoing debate, and may be ameliorated by adding baryonic matter to CDM simulations [15]. Nevertheless, the wider category of "small-scale" problems in standard CDM along with tighter constraints from direct-detection experiments [16] motivates the study of alternative dark matter models. One scenario which has gained substantial traction is ultra-light dark matter [ULDM], also known as scalar-field dark matter, Ψ dark matter, axion dark matter, BEC dark matter and fuzzy dark matter. As reviewed by Hui et al. [17], ULDM consists of an axion-like particle whose very small mass (O(∼ 10 −22 eV )) corresponds to a kiloparsec-scale de Broglie wavelength. ULDM thus exhibits novel wave-like behaviour on astrophysically interesting scales and can form soliton-like gravitationally confined Bose-Einstein condensates. ULDM simulations suggest that realistic astrophysical halos have an inner core consisting of a kiloparsec scale Bose-Einstein condensate or soliton, while the outer halo is ...

Schrödinger-Poisson Solitons: Perturbation Theory

“…Structure formation with ULDM reproduces the successes of ΛCDM on large scales while producing cored halos and substructure that are potentially more consistent with observations on small scales [8,11,12]. In addition to dark matter, the Schrödinger-Poisson system of equations governing ULDM dynamics emerges in other systems of interest, including boson stars [13][14][15][16] and the very early universe [17][18][19].…”

“…In this final section, we investigate the eigenstate decomposition and evolution of a ULDM halo. A ULDM halo profile can be parametrized into a solitonic core and NFW skirt [11]…”

High-precision search for dark photon dark matter with the Parkes Pulsar Timing Array

“…Currently there is no consensus on the constraints on the mass of the fuzzy dark matter. The stellar kinematics of dwarf spheroidal galaxies tend to favor a lower mass [56,57] (a few times of 10 −23 eV), while the halo mass function derived from the Lyman-α forest suggests a higher lower limit [16,27] ( 10 21 eV). Most of these constraints rely on as- Frequency (Hz) 10 9 10 8 10 7…”

Ultra-light dark matter

“…Safarzadeh and Spergel (2019), using the half-mass radius and the slope the mass profile of Fornax and Sculptor dwarf spheroidal galaxies, obtained a different bound for the FDM mass m 10 −21 eV, to have the expected density profile and halo mass for those dSphs from observations like dynamical friction. Kendall and Easther (2020) pointed out yet another possible limitation of the above analysis. They show that the presence of cores with flat density profile in the center of the NFW-halos can actually make the density of large ULDM halos larger than the CDM ones, making the cusp-core problem worse.…”