Wave Dark Matter (2101.11735v1)

Abstract: We review the physics and phenomenology of wave dark matter: a bosonic dark matter candidate lighter than about 30 eV. Such particles have a de Broglie wavelength exceeding the average inter-particle separation in a galaxy like the Milky Way, and are well described as classical waves. We outline the particle physics motivations for them, including the QCD axion and ultra-light axion-like-particles such as fuzzy dark matter. The wave nature of the dark matter implies a rich phenomenology: (1) Wave interference leads to order unity density fluctuations on de Broglie scale. A manifestation is vortices where the density vanishes and around which the velocity circulates. There is one vortex ring per de Broglie volume on average. (2) For sufficiently low masses, soliton condensation occurs at centers of halos. The soliton oscillates and random walks, another manifestation of wave interference. The halo/subhalo abundance is suppressed at small masses, but the precise prediction from numerical wave simulations remains to be determined. (3) For ultra-light ~$10^{-22}$ eV dark matter, the wave interference substructures can be probed by tidal streams/gravitational lensing. The signal can be distinguished from that due to subhalos by the dependence on stream orbital radius/image separation. (4) Axion detection experiments are sensitive to interference substructures for moderately light masses. The stochastic nature of the waves affects the interpretation of experiments and motivates the measurement of correlation functions. Current constraints and open questions, covering detection experiments and cosmological/galactic/black-hole observations, are discussed.

• The paper reveals that the wave nature of light bosonic dark matter produces distinct interference patterns and soliton cores in galactic halos.
• It employs theoretical analysis, numerical simulations, and astrophysical observations to link particle physics motivations with cosmological phenomena.
• The study outlines experimental pathways, including gravitational lensing and axion detection, as critical tools to constrain these dark matter candidates.

Wave Dark Matter: An Overview

The paper presented by Lam Hui on "Wave Dark Matter" explores the intriguing concept of wave-like behaviors in light bosonic dark matter candidates, particularly focusing on those with particle masses less than approximately $30$ eV. This category includes compelling cases such as the QCD axion and ultra-light axion-like particles, often termed "fuzzy dark matter." The research discusses the particle physics motivations, phenomenological implications, and various observational constraints that explore the potential role of these candidates as components of dark matter in the universe.

Conceptual Framework

Wave dark matter distinguishes itself by the large de Broglie wavelengths of these particles, which surpass the average inter-particle separation in galaxies like the Milky Way, thus manifesting as classical waves rather than discrete particles. Several motivations are outlined, including the well-established QCD axion, which arises from theoretical attempts to address the strong CP problem. The potential existence of ultra-light axions or axion-like-particles is further supported by predictions from string theory frameworks, which often suggest a multitude of such particles with varying characteristics.

Phenomenological Aspects

The paper details several key phenomenological features implicated by the wave nature of these particles:

1. Wave Interference: Interference patterns created by these wave-like particles lead to density fluctuations of order unity on the scale of their de Broglie wavelengths. Vortices, where the density vanishes, appear as a characteristic feature with circulating velocities surrounding them.
2. Soliton Cores: In halos, these wave dynamics lead to the formation of soliton cores at the centers. These solitons present as coherent structures where quantum pressure counteracts gravitational forces. For ultra-light particles, these structures are theoretically and numerically observed to oscillate and evolve dynamically.
3. Substructure Probing: The interference of substructures along with solitons provides a test bed for future exploration through gravitational lensing and the dynamics of tidal streams, distinguishing these features from other types of subhalos.

Theoretical and Practical Implications

The exploration into wave dark matter not only broadens the theoretical landscape of dark matter models but also poses significant implications for astrophysical structures and cosmological phenomena. It raises questions about the role of such particles in resolving potential discrepancies at small scales observed in conventional CDM models, including galaxy formation cues. The wave nature offers unique interference effects that necessitate reconsideration of typical density distributions and dynamical friction phenomena in galaxies.

Experimental Probes and Constraints

Currently, the constraints on wave dark matter candidates are multifaceted. Astrophysical observations, such as those from the Lyman-alpha forest, galaxy halo observations, and gravitational lensing efforts, have placed lower bounds on the mass of these particles. Direct detection experiments, notably axion detection setups, also provide a pathway to constrain the existence of these particles within specific mass ranges. The stochastic nature of wave dark matter implies interesting experimental consequences, including the measurement of correlation functions that reveal the oscillatory, wave-like behaviors over time and space.

Conclusion

Hui’s paper offers a comprehensive review of the state of wave dark matter research, highlighting both the promising aspects and the challenges of elucidating the identity of dark matter. By detailing the particle physics basis, numerical simulations, and observational constraints, it delineates a structured roadmap for future research endeavors and experimental probes in this domain, thereby enriching the broader understanding of dark matter and its diverse manifestations in the cosmos. This paper, therefore, invites continued exploration, especially in improving simulation techniques and developing sophisticated experimental methodologies to further probe the interaction dynamics and physical properties of these elusive, wave-like dark matter candidates.