Ultralight scalars as cosmological dark matter (1610.08297v2)

Abstract: An intriguing alternative to cold dark matter (CDM) is that the dark matter is a light ( $m \sim 10^{-22}$ eV) boson having a de Broglie wavelength $\lambda \sim 1$ kpc, often called fuzzy dark matter (FDM). We describe the arguments from particle physics that motivate FDM, review previous work on its astrophysical signatures, and analyze several unexplored aspects of its behavior. In particular, (i) FDM halos smaller than about $10^7 (m/10^{-22} {\rm eV})^{-3/2} M_\odot$ do not form. (ii) FDM halos are comprised of a core that is a stationary, minimum-energy configuration called a "soliton", surrounded by an envelope that resembles a CDM halo. (iii) The transition between soliton and envelope is determined by a relaxation process analogous to two-body relaxation in gravitating systems, which proceeds as if the halo were composed of particles with mass $\sim \rho\lambda^3$ where $\rho$ is the halo density. (iv) Relaxation may have substantial effects on the stellar disk and bulge in the inner parts of disk galaxies. (v) Relaxation can produce FDM disks but an FDM disk in the solar neighborhood must have a half-thickness of at least $300 (m/10^{-22} {\rm eV})^{-2/3}$ pc. (vi) Solitonic FDM sub-halos evaporate by tunneling through the tidal radius and this limits the minimum sub-halo mass inside 30 kpc of the Milky Way to roughly $10^8 (m/10^{-22} {\rm eV})^{-3/2} M_\odot$. (vii) If the dark matter in the Fornax dwarf galaxy is composed of CDM, most of the globular clusters observed in that galaxy should have long ago spiraled to its center, and this problem is resolved if the dark matter is FDM.

• The paper demonstrates that ultralight scalar fuzzy dark matter (~10^-22 eV) can produce solitonic core structures in galactic halos as an alternative to conventional CDM models.
• It reveals how the quantum pressure from long de Broglie wavelengths suppresses small-scale structure formation, addressing inconsistencies in traditional dark matter scenarios.
• It outlines how integrating theoretical particle physics with astrophysical observations refines our understanding of cosmic structure formation and galactic dynamics.

Ultralight Scalars as Cosmological Dark Matter

The paper "Ultralight Scalars as Cosmological Dark Matter" provides a comprehensive analysis of fuzzy dark matter (FDM) as an alternative to the conventional cold dark matter (CDM) models in cosmology, focusing on the potential role of ultralight scalars. This concept arises from the limitations of CDM models, particularly on small scales, and suggests that dark matter could comprise ultralight bosons with masses around $10^{-22}$ eV. The discussion encompasses various theoretical motivations, astrophysical implications, and observational consequences of such a dark matter model.

Theoretical Motivation and Particle Physics

The proposed FDM model is rooted in theoretical particle physics where ultralight scalar fields, akin to axions, arise naturally due to an approximate shift symmetry. Such fields are compelling candidates for dark matter as they can have macroscopic de Broglie wavelengths—on the order of a kiloparsec—which influence structure formation at small scales. The paper details mechanisms through which such light masses might arise, focusing on periodic scalar fields exhibiting a shift symmetry that breaks to generate a non-zero, but small, mass. This paradigm allows for a cosmic abundance close to observed dark matter density with parameters that are realistic within some grand unified theories.

Cosmological and Astrophysical Implications

Astrophysical phenomena are profoundly impacted by the characteristics of FDM. The long de Broglie wavelength associated with such light particles effectively introduces a quantum pressure which suppresses small-scale halo formation, addressing several issues faced by CDM. Key findings include:

• Structure of FDM Halos: Unlike CDM halos with central cusps, FDM halos showcase a solitonic core—a stable, ground-state solution to the Schrödinger–Poisson equation—surrounded by an NFW-like envelope. This results in core-dominated structures, providing a natural explanation for the absence of density cusps observed in dwarf galaxies.
• Solitonic Feature and Relaxation: The solitonic core can dynamically relax into its ground state through mechanisms resembling gravitational cooling, influenced significantly by wave interference patterns. The process competes with CDM-like halo formation, resulting in mixed structures where the soliton dominates central dynamics.
• Galaxy Formation and Evolution: While FDM suppresses the formation of the smallest structures, its large-scale predictions remain consistent with observed high-redshift galaxies' properties. The suppressed small-scale power ameliorates the excess satellite galaxy problem in CDM and aligns the cosmic reionization epoch with observational constraints.
• Dynamical Friction and Galactic Cores: FDM weakens the dynamical friction experienced by objects like globular clusters and supermassive black holes, due to the ultraviolet behavior of its wave function, potentially alleviating the "too big to fail" problem.

Observational Consequences and Tensions

Current observational evidence, particularly from the Lyman-alpha forest, constrains the mass of FDM particles. While constraints suggest particle masses $m \gtrsim 10^{-22}$ eV, tension exists with lower mass scenarios due to suppressed small-scale power, necessitating sophisticated theoretical treatments of reionization and non-linear structure growth to reconcile these observations.

Future Directions and Theoretical Investigation

The domain of ultralight scalars as dark matter posits numerous theoretical and observational avenues. Detailed numerical simulations remain critical to refining predictions of solitonic structures and their influence on galaxy dynamics and morphology. Additionally, further probing of the interplay between FDM-induced dynamics and galactic feedback mechanisms promises to illuminate a crucial aspect of cosmic structure formation.

In summary, the paper offers a robust picture of FDM as an intriguing collation of particle physics and astrophysical evidence, positing that the inclusion of ultralight particles could significantly refine our understanding of cosmic structure and dark matter’s role within it. The interplay between theoretical modeling and future observational data will be vital in ascertaining FDM's viability as the prevailing cosmological dark matter candidate.