Dark Matter Self-interactions and Small Scale Structure (1705.02358v2)

Abstract: We review theories of dark matter (DM) beyond the collisionless paradigm, known as self-interacting dark matter (SIDM), and their observable implications for astrophysical structure in the Universe. Self-interactions are motivated, in part, due to the potential to explain long-standing (and more recent) small scale structure observations that are in tension with collisionless cold DM (CDM) predictions. Simple particle physics models for SIDM can provide a universal explanation for these observations across a wide range of mass scales spanning dwarf galaxies, low and high surface brightness spiral galaxies, and clusters of galaxies. At the same time, SIDM leaves intact the success of $\Lambda$CDM cosmology on large scales. This report covers the following topics: (1) small scale structure issues, including the core-cusp problem, the diversity problem for rotation curves, the missing satellites problem, and the too-big-to-fail problem, as well as recent progress in hydrodynamical simulations of galaxy formation; (2) N-body simulations for SIDM, including implications for density profiles, halo shapes, substructure, and the interplay between baryons and self-interactions; (3) semi-analytic Jeans-based methods that provide a complementary approach for connecting particle models with observations; (4) merging systems, such as cluster mergers (e.g., the Bullet Cluster) and minor infalls, along with recent simulation results for mergers; (5) particle physics models, including light mediator models and composite DM models; and (6) complementary probes for SIDM, including indirect and direct detection experiments, particle collider searches, and cosmological observations. We provide a summary and critical look for all current constraints on DM self-interactions and an outline for future directions.

• The paper identifies dark matter self-interactions as a promising solution to small-scale anomalies such as the core-cusp, missing satellites, and too-big-to-fail problems.
• It applies hydrodynamical simulations, semi-analytical methods, and the Jeans equation to connect SIDM particle physics with observed galaxy density profiles.
• The study suggests a self-interaction cross-section of 0.1–1 cm²/g and explores light mediator models, linking astrophysical observations to particle physics experiments.

Dark Matter Self-interactions and Small Scale Structure

The paper by Sean Tulin and Hai-Bo Yu provides an extensive review of the current understanding and theoretical framework related to dark matter (DM) self-interactions, particularly focusing on the small scale structure of the universe. This work examines the impacts of Self-interacting Dark Matter (SIDM) on astrophysical observations and juxtaposes it with the traditional collisionless Cold Dark Matter (CDM) paradigm. Due to certain discrepancies observed in small scale structures, such as the core-cusp problem and the missing satellites issue, SIDM emerges as a compelling alternative to CDM.

SIDM posits that dark matter particles interact with each other through elastic scatterings, distinguished by a cross-section parameter per unit mass ($\sigma/m$). The paper elucidates how these interactions can potentially explain anomalous observations, such as the flat density profiles in dwarf galaxies, high surface brightness spiral galaxies, and galaxy clusters, which are not consistent with the predictions of collisionless CDM. Additionally, the presence of SIDM aligns with the successes of the $\Lambda$CDM model on larger cosmological scales.

The paper systematically reviews the small scale structure issues encountered with CDM:

• Core-cusp problem: CDM simulations predict a steep slope in the inner density profiles of halos, contrary to observations indicating cored profiles in certain galaxies.
• Missing satellites problem: The number of observed dwarf satellites in the Milky Way is fewer than predicted by CDM simulations.
• Too-big-to-fail problem: The most luminous satellites should reside in massive subhalos according to CDM, yet these subhalos do not align with current observations.
• Diversity problem: Variations in rotation curves across galaxies of similar mass are more pronounced than CDM predictions.

The authors detail how hydrodynamical simulations, semi-analytical approaches, and the Jeans equation are used to better understand the connection between particle physics models of SIDM and astrophysical observations. A key finding is that a self-interaction cross section in the range of $0.1 - 1 \; \text{cm}^2/\text{g}$ could successfully explain the observed discrepancies on dwarf scales, while being consistent with cluster observations.

Furthermore, the paper explores potential particle physics models that could underpin dark matter self-interactions. These models typically introduce a light mediator between dark matter particles that results in a velocity-dependent interaction cross-section. This nuanced interaction accounts for the varied observations at different astrophysical scales and addresses the requirement of consistent results from dwarf galaxies to clusters.

Implications and Future Directions:

The suggested parameters for SIDM provide a promising avenue for resolving numerous small scale issues while retaining the larger scale predictions of CDM models. Research into dark matter self-interactions has broad implications:

• Astrophysical Impacts: A deeper understanding of SIDM could refine models of galaxy formation and evolution, providing more accurate predictions of structures observed in the universe.
• Particle Physics Prospects: Exploring models of DM self-interactions, especially those involving light mediators, could potentially connect astrophysical observations with direct detection experiments and accelerator tests.
• Cosmological Considerations: SIDM requires revisiting the thermal history of the universe, given that dark matter self-interactions would affect the early universe dynamics differently from CDM.

The paper ultimately positions SIDM as a complementary theory to the conventional CDM paradigm, capable of providing insights into the longstanding discrepancies between theoretical predictions and observations in the structure of the universe. While SIDM models are not yet conclusive, they invite robust experimental and observational efforts to further explore the nature of dark matter interaction. Future developments will likely focus on refining experimental techniques to probe these interactions more precisely and extending simulations to encompass baryonic physics alongside self-interacting dark matter.