[Paper Review] Prompt cusps and the dark matter annihilation signal

M. Sten Delos, Simon D. M. White|arXiv (Cornell University)|Sep 22, 2022

Dark Matter and Cosmic Phenomena4 citations

TL;DR

This paper proposes that thermally produced WIMPs form dense, primordial 'prompt cusps' at the centers of early dark matter peaks, which survive to the present day. These cusps boost dark matter annihilation signals by at least an order of magnitude—especially outside galactic centers—making the isotropic gamma-ray background and large-scale structure prime targets for indirect detection, challenging prior assumptions focused on galactic centers.

ABSTRACT

As the first dark matter objects gravitationally condense, a density cusp forms immediately at every initial density maximum. Numerical simulations and theoretical arguments suggest that these prompt cusps can survive until the present day. 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 may be present in every solar mass of dark matter. This radically alters predictions for the amount and spatial distribution of dark matter annihilation radiation. The annihilation rate is boosted by at least an order of magnitude compared to previous predictions, both in the cosmological average and within galaxy-scale halos. Moreover, the signal is predominantly boosted outside of the centers of galactic halos, so alternative targets become significantly more attractive for indirect-detection searches. For example, prompt cusps present new opportunities to test the dark matter interpretation of the Galactic Center gamma-ray excess by searching for similar spectral signatures in the isotropic gamma-ray background and large-scale cosmic structure.

Motivation & Objective

• To investigate the survival and properties of primordial density cusps formed during the collapse of the first dark matter halos.
• To assess how these prompt cusps affect the predicted dark matter annihilation signal, especially in comparison to standard halo substructure models.
• To re-evaluate indirect detection strategies by quantifying the spatial distribution and boost factor of annihilation radiation from prompt cusps.
• To test the dark matter interpretation of the Galactic Center gamma-ray excess by predicting its signature in the isotropic gamma-ray background.

Proposed method

• Modeling the formation of prompt cusps via numerical simulations and theoretical arguments, showing they form with ρ ∝ r⁻¹.⁵ at collapse and survive tidal stripping.
• Using cosmological initial conditions to derive the mass and size of each prompt cusp from the linear density field of early peaks.
• Applying tidal stripping models to estimate the survival fraction of prompt cusps in larger halos, assuming pericenter radii ~ r/2.
• Calculating the annihilation boost factor by integrating over the spatial distribution of cusps and comparing to smooth halo contributions.
• Deriving the isotropic gamma-ray background flux using redshifted energy spectra and cosmological integration, accounting for cusp density and survival fraction.
• Comparing predicted fluxes from galactic halos and extragalactic structures to Fermi-LAT measurements of the isotropic background.

Experimental results

Research questions

• RQ1 What is the spatial distribution and survival rate of prompt cusps formed during the collapse of the first dark matter halos?
• RQ2 How do prompt cusps modify the predicted dark matter annihilation signal compared to standard substructure models?
• RQ3 What is the expected boost factor for annihilation radiation from prompt cusps within galactic halos and in the extragalactic background?
• RQ4 Can the Galactic Center gamma-ray excess be explained by prompt cusp annihilation, and what would that imply for the isotropic gamma-ray background?
• RQ5 Are large-scale cosmic structures and the isotropic gamma-ray background more promising targets for indirect detection than galactic centers?

Key findings

• Prompt cusps formed in early WIMP halos survive tidal stripping and can persist with intact inner regions, contributing significantly to the present-day dark matter distribution.
• The annihilation rate in galactic halos is boosted by at least a factor of 10 due to prompt cusps, with the boost being strongest outside galactic centers.
• The spatial distribution of annihilation radiation is more extended than previously assumed, as prompt cusps dominate the signal in low-density regions.
• If the Galactic Center gamma-ray excess is due to dark matter annihilation, a similar spectral signature should be detectable in the isotropic gamma-ray background.
• Extragalactic structures and the large-scale cosmic web become significantly more attractive targets for indirect detection, given the enhanced signal from unresolved prompt cusps.