Ultralight scalars as cosmological dark matter

Journal Article · Tue Feb 28 00:00:00 EST 2017 · Physical Review. D.

DOI:https://doi.org/10.1103/physrevd.95.043541· OSTI ID:1536231

Hui, Lam ^[1]; Ostriker, Jeremiah P. ^[2]; Tremaine, Scott ^[3]; Witten, Edward ^[3]

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1. Columbia University, New York, NY (United States)
2. Columbia University, New York, NY (United States); Princeton University, NJ (United States)
3. Princeton University, NJ (United States)

Many aspects of the large-scale structure of the Universe can be described successfully using cosmological models in which 27 ± 1% of the critical mass-energy density consists of cold dark matter (CDM). However, few—if any—of the predictions of CDM models have been successful on scales of ~10 kpc or less. This lack of success is usually explained by the difficulty of modeling baryonic physics (star formation, supernova and black-hole feedback, etc.). An intriguing alternative to CDM is that the dark matter is an extremely light (m ~10^-22 eV) boson having a de Broglie wavelength λ ~1 kpc, often called fuzzy dark matter (FDM). Here 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 or subhalos smaller than about 10⁷(m/10^-22 eV)^-3/2 M_⊙ do not form, and the abundance of halos smaller than a few times 10¹⁰(m/10^-22 eV)^-4/3 M_⊙ is substantially smaller in FDM than in CDM. (ii) FDM halos are comprised of a central core that is a stationary, minimum-energy solution of the Schrödinger-Poisson equation, sometimes called a “soliton,” surrounded by an envelope that resembles a CDM halo. The soliton can produce a distinct signature in the rotation curves of FDM-dominated systems. (iii) The transition between soliton and envelope is determined by a relaxation process analogous to two-body relaxation in gravitating N-body systems, which proceeds as if the halo were composed of particles with mass ~ ρλ³ where ρ is the halo density. (iv) Relaxation may have substantial effects on the stellar disk and bulge in the inner parts of disk galaxies, but has negligible effect on disk thickening or globular cluster disruption near the solar radius. (v) Relaxation can produce FDM disks but a FDM disk in the solar neighborhood must have a half-thickness of at least ~300 (m/10^-22 eV)^-2/3 pc and a midplane density less than 0.2(m/10^-22 eV)^2/3 times the baryonic disk density. (vi) Solitonic FDM subhalos evaporate by tunneling through the tidal radius and this limits the minimum subhalo mass inside ~30 kpc of the Milky Way to a few times 10⁸(m/10^{- 22} eV)^-3/2 M_⊙. (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. (viii) FDM delays galaxy formation relative to CDM but its galaxy-formation history is consistent with current observations of high-redshift galaxies and the late reionization observed by Planck. If the dark matter is composed of FDM, most observations favor a particle mass ≳ 10^-22 eV and the most significant observational consequences occur if the mass is in the range 1 -10 × 10 ^{- 22} eV. There is tension with observations of the Lyman-α forest, which favor m ≳10 - 20 × 10^-22 eV and we discuss whether more sophisticated models of reionization may resolve this tension.

• Research Organization: Columbia Univ., New York, NY (United States)
• Sponsoring Organization: USDOE Office of Science (SC); National Aeronautics and Space Administration (NASA); National Science Foundation (NSF)
• Grant/Contract Number: SC0011941; NNX14AM24G; NXX16AB27G; AST-1406166; PHY-1606531
• OSTI ID: 1536231
• Alternate ID(s): OSTI ID: 1345282
• Journal Information: Physical Review. D., Vol. 95, Issue 4; ISSN 2470-0010
• Publisher: American Physical Society (APS)
• Country of Publication: United States
• Language: English

Related Subjects

79 ASTRONOMY AND ASTROPHYSICS
cosmology
dark matter
large scale structure of the universe