[Paper Review] Cosmological simulations with rare and frequent dark matter self-interactions

Moritz S. Fischer, M. Brüggen+5 more|arXiv (Cornell University)|May 4, 2022

Dark Matter and Cosmic PhenomenaPhysics and Astronomy127 references25 citations

TL;DR

This study presents the first cosmological N-body simulations of frequent dark matter self-interactions (fSIDM), comparing them to standard rare self-interaction models (rSIDM). Despite similar large-scale behavior, fSIDM shows a significantly suppressed satellite abundance—offering a key observational discriminator when combined with halo density or shape profiles—while also providing the first upper limits on cross-sections for frequent scattering. - meta_description: First cosmological simulations of frequent dark matter self-interactions reveal suppressed satellite counts and new cross-section limits, distinguishing fSIDM from rSIDM via combined observables. - objective: - Investigate the phenomenological differences between frequent (small-angle) and rare (large-angle) dark matter self-interactions in a cosmological context. - Address the lack of constraints on frequent self-interactions, which are theoretically motivated but previously untested in full cosmological simulations. - Identify observable signatures that can discriminate between fSIDM and rSIDM models. - Provide the first upper limits on momentum-transfer cross-sections for frequent self-interactions. - method: - Conducted DM-only cosmological N-body simulations using a full-box and zoom-in techniques to model fSIDM and rSIDM. - Implemented a novel numerical scheme that treats frequent small-angle scatterings as continuous momentum diffusion, enabling simulation of high-frequency, low-angle scattering events. - Compared fSIDM and rSIDM with standard CDM across multiple diagnostics: matter power spectrum, two-point correlation function, halo and subhalo mass functions, and density/shape profiles. - Used the momentum-transfer cross-section as a consistent comparison metric, ensuring equal energy and momentum transfer per scattering event. - Analyzed halo shapes via principal axis analysis and density profiles via spherical averaging. - Derived upper limits on the cross-section for frequent self-interactions from the absence of observable deviations in the simulations. - research_questions: - How do frequent self-interactions (fSIDM) affect the matter power spectrum and large-scale structure compared to rSIDM and CDM? - What are the differences in halo and subhalo mass functions between fSIDM and rSIDM with the same momentum-transfer cross-section? - How do the density and shape profiles of dark matter haloes differ between fSIDM and rSIDM? - Can the abundance of satellites be used to distinguish fSIDM from rSIDM, especially when combined with other observables? - What are the first upper limits on the cross-section for frequent self-interactions in a cosmological setting? - key_findings: - On large scales, fSIDM and rSIDM are indistinguishable from CDM, but both deviate significantly on small scales due to self-interaction effects. - The suppression of small-scale structures—evident in the matter power spectrum, density PDF, and two-point correlation function—is very similar between fSIDM and rSIDM for the same momentum-transfer cross-section. - The abundance of subhaloes (satellites) is strongly suppressed in fSIDM compared to rSIDM, even with identical cross-sections, indicating a key observational discriminant. - Halo density and shape profiles are more sensitive to self-interaction physics at large radii, where baryonic effects are minimal, making them promising for constraining DM models. - The combination of satellite counts with halo shape or density profile measurements can break degeneracy between fSIDM and rSIDM, offering a multi-observable strategy for model discrimination. - This work provides the first upper limits on the momentum-transfer cross-section for frequent self-interactions, setting a benchmark for future observational constraints.

ABSTRACT

Dark matter (DM) with self-interactions is a promising solution for the small-scale problems of the standard cosmological model. Here we perform the first cosmological simulation of frequent DM self-interactions, corresponding to small-angle DM scatterings. The focus of our analysis lies in finding and understanding differences to the traditionally assumed rare DM (large-angle) self scatterings. For this purpose, we compute the distribution of DM densities, the matter power spectrum, the two-point correlation function and the halo and subhalo mass functions. Furthermore, we investigate the density profiles of the DM haloes and their shapes. We find that overall large-angle and small-angle scatterings behave fairly similarly with a few exceptions. In particular, the number of satellites is considerably suppressed for frequent compared to rare self-interactions with the same cross-section. Overall we observe that while differences between the two cases may be difficult to establish using a single measure, the degeneracy may be broken through a combination of multiple ones. For instance, the combination of satellite counts with halo density or shape profiles could allow discriminating between rare and frequent self-interactions. As a by-product of our analysis, we provide - for the first time - upper limits on the cross-section for frequent self-interactions.

Motivation & Objective

• Investigate the phenomenological differences between frequent (small-angle) and rare (large-angle) dark matter self-interactions in a cosmological context.
• Address the lack of constraints on frequent self-interactions, which are theoretically motivated but previously untested in full cosmological simulations.
• Identify observable signatures that can discriminate between fSIDM and rSIDM models.
• Provide the first upper limits on momentum-transfer cross-sections for frequent self-interactions.

Proposed method

• Conducted DM-only cosmological N-body simulations using a full-box and zoom-in techniques to model fSIDM and rSIDM.
• Implemented a novel numerical scheme that treats frequent small-angle scatterings as continuous momentum diffusion, enabling simulation of high-frequency, low-angle scattering events.
• Compared fSIDM and rSIDM with standard CDM across multiple diagnostics: matter power spectrum, two-point correlation function, halo and subhalo mass functions, and density/shape profiles.
• Used the momentum-transfer cross-section as a consistent comparison metric, ensuring equal energy and momentum transfer per scattering event.
• Analyzed halo shapes via principal axis analysis and density profiles via spherical averaging.
• Derived upper limits on the cross-section for frequent self-interactions from the absence of observable deviations in the simulations.

Experimental results

Research questions

• RQ1 How do frequent self-interactions (fSIDM) affect the matter power spectrum and large-scale structure compared to rSIDM and CDM?
• RQ2 What are the differences in halo and subhalo mass functions between fSIDM and rSIDM with the same momentum-transfer cross-section?
• RQ3 How do the density and shape profiles of dark matter haloes differ between fSIDM and rSIDM?
• RQ4 Can the abundance of satellites be used to distinguish fSIDM from rSIDM, especially when combined with other observables?
• RQ5 What are the first upper limits on the cross-section for frequent self-interactions in a cosmological setting?

Key findings

• On large scales, fSIDM and rSIDM are indistinguishable from CDM, but both deviate significantly on small scales due to self-interaction effects.
• The suppression of small-scale structures—evident in the matter power spectrum, density PDF, and two-point correlation function—is very similar between fSIDM and rSIDM for the same momentum-transfer cross-section.
• The abundance of subhaloes (satellites) is strongly suppressed in fSIDM compared to rSIDM, even with identical cross-sections, indicating a key observational discriminant.
• Halo density and shape profiles are more sensitive to self-interaction physics at large radii, where baryonic effects are minimal, making them promising for constraining DM models.
• The combination of satellite counts with halo shape or density profile measurements can break degeneracy between fSIDM and rSIDM, offering a multi-observable strategy for model discrimination.
• This work provides the first upper limits on the momentum-transfer cross-section for frequent self-interactions, setting a benchmark for future observational constraints.