fuzzy cold dark matter
{{short description|Hypothetical form of cold dark matter proposed to solve the cuspy halo problem}}
Fuzzy cold dark matter is a hypothetical form of cold dark matter proposed to solve the cuspy halo problem. It would consist of extremely light scalar particles with masses on the order of eV; so a Compton wavelength on the order of 1 light year. Fuzzy cold dark matter halos in dwarf galaxies would manifest wave behavior on astrophysical scales, and the cusps would be avoided through the Heisenberg uncertainty principle.{{cite journal|title=Cold and Fuzzy Dark Matter|doi=10.1103/PhysRevLett.85.1158|year=2000|journal=Physical Review Letters|volume=85|issue=6|pages=1158–61|pmid=10991501|arxiv=astro-ph/0003365|last1=Hu|first1=Wayne|last2=Barkana|first2=Rennan|last3=Gruzinov|first3=Andrei|bibcode = 2000PhRvL..85.1158H |s2cid=118938504 }}
The wave behavior leads to interference patterns, spherical soliton cores in dark matter halo centers,{{cite journal|last1=Schive|first1=Hsi-Yu|last2=Chiueh|first2=Tzihong|last3=Broadhurst|first3=Tom|title=Cosmic structure as the quantum interference of a coherent dark wave
|journal=Nature Physics|volume=10|pages=496–499|year=2014|issue=7|doi=10.1038/nphys2996|arxiv=1406.6586|bibcode=2014NatPh..10..496S|s2cid=118725080}} and cylindrical soliton-like cores in dark matter cosmic web filaments.{{cite journal|last1=Mocz|first1=Philip|last2=Fialkov|first2=Anastasia|last3=Vogelsberger|first3=Mark|last4=Becerra|first4=Fernando|last5=Amin|first5=Mustafa A.|last6=Bose|first6=Sownak|last7=Boylan-Kolchin|first7=Michael|last8=Chavanis|first8=Pierre-Henri|last9=Hernquist|first9=Lars|last10=Lancaster|first10=Lachlan|last11=Marinacci|first11=Federico|last12=Robles|first12=Victor H.|last13=Zavala|first13=Jesús|title=First Star-Forming Structures in Fuzzy Cosmic Filaments|journal=Physical Review Letters|volume=123|issue=14|pages=141301|year=2019|issn=0031-9007|doi=10.1103/PhysRevLett.123.141301|pmid=31702225|arxiv=1910.01653|bibcode=2019PhRvL.123n1301M|s2cid=203734641}}
Fuzzy cold dark matter is a limit of scalar field dark matter without self-interaction.{{Cite journal|author1=Bohua Li |author2=Tanja Rindler-Daller |author3=Paul R. Shapiro |title=Cosmological Constraints on Bose-Einstein-Condensed Scalar Field Dark Matter|journal=Phys. Rev. D|volume=89 |issue=8 |pages=083536 |doi=10.1103/PhysRevD.89.083536|arxiv = 1310.6061 |bibcode = 2014PhRvD..89h3536L |year=2014 |s2cid=118654592 }}{{Cite journal|last=Lee|first=Jae-Weon|date=2018|title=Brief History of Ultra-light Scalar Dark Matter Models|journal=EPJ Web of Conferences|volume=168|pages=06005|bibcode=2018EPJWC.16806005L|arxiv=1704.05057|doi=10.1051/epjconf/201816806005|s2cid=54649264}} Fuzzy cold dark matter is sometimes called wave DM, or simply fuzzy dark matter (FDM).{{cite journal |last1=Ferreira |first1=Elisa G. M. |title=Ultra-light dark matter |journal=The Astronomy and Astrophysics Review |date=December 2021 |volume=29 |issue=1 |doi=10.1007/s00159-021-00135-6|arxiv=2005.03254 }} It is governed by the Schrödinger–Poisson equation.
Fuzzy dark matter models are the simplest class of the ultralight dark matter models; the only free parameter is the particle mass. (In "ultralight dark matter models", the dark matter of a galaxy condenses into a superfluid. This requirement greatly constrains the particle mass; for example, the QCD (Peccei–Quinn) axion is considered too heavy to condense.) A second approach, where FDM is modified to have simple self-interaction, has been suggested with theories such as self-interacting fuzzy dark matter (SIFDM), repulsive DM, scalar field DM, and fluid dark matter. A third approach, called the "DM superfluid model", focuses on the empirical data for a large-scale MOND relation, and then works backwards to determine what types of complicated self-interactions would best produce such a distribution.
New research (2023) has uncovered evidence that fuzzy dark matter, specifically ultralight axions, may better fit gravitational lens data than WIMP dark matter.{{cite web |last=Timmer |first=John |title=No WIMPS! Heavy particles don't explain gravitational lensing oddities |website=Ars Technica |date=2023-04-21 |url=https://arstechnica.com/science/2023/04/gravitational-lensing-may-point-to-lighter-dark-matter-candidate/ |access-date=2023-06-08}}