Massive prompt cusps: A new signature of warm dark matter

Every dark matter halo and subhalo is expected to have a prompt $\rho\propto r^{-1.5}$ central density cusp, which is a relic of its condensation out of the smooth mass distribution of the early universe. The sizes of these prompt cusps are linked to the scales of the peaks in the initial density field from which they formed. In warm dark matter (WDM) models, the smoothing scale set by free streaming of the dark matter can result in prompt cusps with masses of order $10^7$ M$_\odot$. We show that WDM models with particle masses ranging from 2 to 6 keV predict prompt cusps that could detectably alter the observed kinematics of Local Group dwarf galaxies. Thus, prompt cusps present a viable new probe of WDM. A prompt cusp's properties are highly sensitive to when it formed, so prospects can be improved with a better understanding of when the haloes of the Local Group dwarfs originally formed. Tidal stripping can also affect prompt cusps, so constraints on satellite galaxy orbits can further tighten WDM inferences.Comment: 5 pages, 6 figures; accepted by MNRAS Letters. Includes more detail on the sampling of prompt cusp

Simulations of Gravitational Heating Due to Early Matter Domination

In cosmologies with an early matter-dominated era (EMDE) prior to Big Bang nucleosynthesis, the boosted growth of small-scale matter perturbations during the EMDE leads to microhalo formation long before halos would otherwise begin to form. For a range of models, halos can even form during the EMDE itself. These halos would dissipate at the end of the EMDE, releasing their gravitationally heated dark matter and thereby imprinting a free-streaming cut-off on the matter power spectrum. We conduct the first cosmological $N$-body simulations of the formation and evaporation of halos during and after an EMDE. We show that in these scenarios, the free-streaming cut-off after the EMDE can be predicted accurately from the linear matter power spectrum. Although the free streaming can erase much of the EMDE-driven boost to density perturbations, we use our findings to show that the (re-)formation of halos after the EMDE nevertheless proceeds before redshift $\sim 1000$. Early-forming microhalos are a key observational signature of an EMDE, and our prescription for the impact of gravitational heating will allow studies of the observational status and prospects of EMDE scenarios to cover a much wider range of parameters.Comment: 33 pages, 16 figures. Comments welcom

Inner cusps of the first dark matter haloes: Formation and survival in a cosmological context

We use very high resolution cosmological zoom simulations to follow the early evolution of twelve first-generation haloes formed from gaussian initial conditions with scale-free power spectra truncated on small scales by a gaussian. Initial collapse occurs with a diverse range of sheet- or filament-like caustic morphologies, but in almost all cases it gives rise to a numerically converged density cusp with $\rho = Ar^{-3/2}$ and total mass comparable to that of the corresponding peak in the initial linear density field. The constant $A$ can be estimated to within about 10 per cent from the properties of this peak. This outcome agrees with earlier work on the first haloes in cold and warm dark matter universes. Within central cusps, the velocity dispersion is close to isotropic, and equidensity surfaces tend to align with those of the main body of the halo at larger radii. As haloes grow, their cusps are often (but not always) overlaid with additional material at intermediate radii to produce profiles more similar to the Einasto or NFW forms typical of more massive haloes. Nevertheless, to the extent that we can resolve them, cusps survive at the smallest radii. Major mergers can disturb them, but the effect is quite weak in the cases that we study. The cusps extend down to the resolution limits of our simulations, which are typically a factor of several larger than the cores that would be produced by phase-space conservation if the initial power spectrum cutoff arises from free streaming.Comment: 23 pages, 28 figures; to be submitted to MNRA

Prompt cusps and the dark matter annihilation signal

Dark matter is the dominant form of matter in today's universe. Its gravitational effects drive the formation of galaxies and all larger structure, yet its nature is unknown. As gravitational collapse creates the first cosmic objects, a dark matter cusp forms immediately at every initial density maximum. Such prompt cusps have a density profile $\rho\propto r^{-1.5}$ extending up to a limiting density dependent on the nature of the dark matter. Numerical simulations and theoretical arguments suggest that the bulk of these cusps survive until the present day. Here we show that if dark matter is a thermally produced weakly interacting massive particle, many thousands of prompt cusps with individual masses similar to that of the earth should be present in every solar mass of dark matter. This radically alters predictions for the amount and spatial distribution of dark matter annihilation radiation, substantially tightening observational constraints on the relevant cross sections. In particular, the cross section required to explain the observed $\gamma$-ray excess near the Galactic Centre predicts prompt cusp emission from the Milky Way's outer halo and from extragalactic dark matter at levels in tension with the observed diffuse $\gamma$-ray background.Comment: 19 pages, 10 figures; submitte

Probing the Early Universe Using Dark Matter Minihalos

Through their observable properties, the first and smallest dark matter halos represent a rare probe of subkiloparsec-scale variations in the density of the early Universe. These density variations could hold clues to the nature of inflation, the postinflationary cosmic history, and the identity of dark matter. However, the dynamical complexity of these minihalos hinders their usage as cosmological probes. A theoretical understanding of the minihalo-cosmology connection demands numerical simulation, but minihalos are too small and dense to simulate up to the present day in full cosmological context. This dissertation meets this challenge by using controlled numerical simulations to develop (semi)analytic models of dark matter structure. These models describe the formation of the first halos and their subsequent evolution as they accrete onto larger systems, both through tidal forces and encounters with other objects. I also explore two observational applications of the minihalo-cosmology connection: breaking a degeneracy between the properties of thermal-relic dark matter and the postinflationary history, and probing inflation's late stages via the small-scale primordial power spectrum.Doctor of Philosoph

How an era of kination impacts substructure and the dark matter annihilation rate

An era of kination occurs when the Universe's energy density is dominated by a fast-rolling scalar field. Dark matter that is thermally produced during an era of kination requires larger-than-canonical annihilation cross sections to generate the observed dark matter relic abundance. Furthermore, dark matter density perturbations that enter the horizon during an era of kination grow linearly with the scale factor prior to radiation domination. We show how the resulting enhancement to the small-scale matter power spectrum increases the microhalo abundance and boosts the dark matter annihilation rate. We then use gamma-ray observations to constrain thermal dark matter production during kination. The annihilation boost factor depends on the minimum halo mass, which is determined by the small-scale cutoff in the matter power spectrum. Therefore, observational limits on the dark matter annihilation rate imply a minimum cutoff scale for a given dark matter particle mass and kination scenario. For dark matter that was once in thermal equilibrium with the Standard Model, this constraint establishes a maximum allowed kinetic decoupling temperature for the dark matter. This bound on the decoupling temperature implies that the growth of perturbations during kination cannot appreciably boost the dark matter annihilation rate if dark matter was once in thermal equilibrium with the Standard Model.Comment: 23 pages, 18 figures. References added; matches accepted versio