Double-Disk Dark Matter

Abstract: Based on observational constraints on large scale structure and halo structure, dark matter is generally taken to be cold and essentially collisionless. On the other hand, given the large number of particles and forces in the visible world, a more complex dark sector could be a reasonable or even likely possibility. This hypothesis leads to testable consequences, perhaps portending the discovery of a rich hidden world neighboring our own. We consider a scenario that readily satisfies current bounds that we call Partially Interacting Dark Matter (PIDM). This scenario contains self-interacting dark matter, but it is not the dominant component. Even if PIDM contains only a fraction of the net dark matter density, comparable to the baryonic fraction, the subdominant component's interactions can lead to interesting and potentially observable consequences. Our primary focus will be the special case of Double-Disk Dark Matter (DDDM), in which self-interactions allow the dark matter to lose enough energy to lead to dynamics similar to those in the baryonic sector. We explore a simple model in which DDDM can cool efficiently and form a disk within galaxies, and we evaluate some of the possible observational signatures. The most prominent signal of such a scenario could be an enhanced indirect detection signature with a distinctive spatial distribution. Even though subdominant, the enhanced density at the center of the galaxy and possibly throughout the plane of the galaxy can lead to large boost factors, and could even explain a signature as large as the 130 GeV Fermi line. Such scenarios also predict additional dark radiation degrees of freedom that could soon be detectable and would influence the interpretation of future data, such as that from Planck and from the Gaia satellite. We consider this to be the first step toward exploring a rich array of new possibilities for dark matter dynamics.

• The paper presents the PIDM framework where a fraction of dark matter exhibits self-interactions, altering galactic dynamics.
• It demonstrates that cooling processes can lead to the formation of a thin dark disk, producing density spikes that enhance indirect detection signals.
• The study outlines observational signatures and gravitational effects that offer new avenues to constrain and explore dark matter’s complex nature.

The Double-Disk Dark Matter Model: Dynamics and Observational Signatures

The paper "Double-Disk Dark Matter" by JiJi Fan, Andrey Katz, Lisa Randall, and Matthew Reece proposes an alternative model of dark matter, introducing the concept of Partially Interacting Dark Matter (PIDM), a scenario that accommodates a more complex dark sector than the standard cold, collisionless dark matter model. Within this framework, the authors present the concept of Double-Disk Dark Matter (DDDM), where a subdominant self-interacting component of dark matter forms a disk analogous to the baryonic disk within galaxies.

Key Elements and Results

1. Partially Interacting Dark Matter (PIDM): This novel scenario proposes that a fraction of dark matter has significant self-interactions, deviating from the cold dark matter paradigm that treats dark matter as collisionless. This component, while not dominant, can influence galactic dynamics and structure.
2. Double-Disk Dark Matter (DDDM): The DDDM scenario allows for energy dissipation, resulting in the formation of a thin disk-like structure within galaxies. The model predicts that even though the DDDM constitutes only a small fraction of total dark matter, its interactions create potentially observable effects, notably the density enhancement in the galactic center, which could lead to increased indirect detection signals.
3. Cooling Dynamics: The formation of a dark disk is contingent upon effective cooling processes that mirror molecular cooling in baryons. The paper demonstrates that under certain conditions, the subdominant dark matter component can cool sufficiently to form a disk within the galaxy.
4. Observational Signatures and Implications:
   • Indirect Detection Enhancements: The enhanced density of DDDM could lead to significant boost factors in indirect detection experiments. A notable example is the Fermi gamma-ray line at 130 GeV, where the observed signal might be explained by DDDM configurations resulting in density spikes near the galactic center.
   • Gravitational Effects on Galactic Structures: The existence of a dark disk can impact rotational curves and gravitational lensing, thus providing potential observational avenues to detect or constrain the model.
5. Constraints and Future Prospects:
   • The authors discuss existing constraints on the abundance of interacting dark matter from cluster interactions and halo shapes, emphasizing that subdominant components are subject to weaker constraints.
   • Future observations, particularly those probing deviations in the cosmic microwave background or large-scale structure, could further constrain or support the presence of a rich dark matter sector.

Theoretical and Practical Implications

The DDDM model not only invigorates the discussion around the complexity of dark matter but also suggests new paradigms for interpreting galactic dynamics and cosmic structure formation. By acknowledging a scenario where dark matter exhibits complex interactions, akin to visible matter, the model opens new channels for research into the nature of dark matter, potentially providing explanations for observed galactic phenomena inconsistent with cold dark matter predictions.

Moving forward, the primary avenues for exploration include refining theoretical models to better predict cooling rates and disk dynamics, coupled with high-resolution simulations that can accurately capture the multi-scale interactions between baryons and complex dark matter sectors. Additionally, more sophisticated analyses of current and future astrophysical data may either lend support to the DDDM model or help further delineate the boundaries of viable dark matter theories.

Overall, the introduction of DDDM represents a step towards a more nuanced understanding of the dark sector, challenging researchers to extend beyond traditional models and consider the profound implications of dark matter's potential complexities. The paper sets a foundation for future work that could lead to significant developments in both theoretical particle physics and cosmology, driving forward the quest to unlock the secrets of dark matter.