Cosmological constraints on Bose-Einstein-condensed scalar field dark matter

Abstract

(Abridged) Despite the success of Cold Dark Matter (CDM) in explaining a wide range of observations, the microscopic nature of dark matter is still unknown. Attempts to detect WIMPs, the most commonly studied form, have not yet succeeded. Some predictions are even in apparent conflict with observations (e.g. cuspy-cored halos and `missing satellite problem'). Therefore, it is important to consider different forms of CDM. We focus on dark matter comprised of ultralight bosons that form a Bose-Einstein Condensate (BEC), described by a complex scalar field, and solve the Klein-Gordon and Einstein field equations to evolve the Friedmann-Robertson-Walker universe. We find, in addition to phases of radiation-, matter- and \Lambda-domination, an earlier phase of scalar-field-domination. Moreover, while WIMP CDM is non-relativistic at all times after it decouples, BEC scalar field dark matter (SFDM) is relativistic at early times, evolving from stiff to radiation-like, before it becomes non-relativistic (CDM-like) at late times. The timing of transitions between these phases yields fundamental constraints on SFDM model parameters, particle mass m and self-interaction coupling strength \lambda. We derive the range of particle parameters required to match observations of the evolving background universe, including the CMB and abundances of light elements produced by BBN, characterized by N_eff, the effective number of neutrino species, and the epoch of matter-radiation equality z_eq. This yields m >= 2.4*10^{-21} eV/c^2 and 9.5*10^{-19} eV^{-1}cm^3 <= \lambda/(mc^2)^2 <= 4*10^{-17} eV^{-1}cm^3. Our model accommodates current observations in which N_eff at BBN is higher than at z_eq, as probed by the CMB, otherwise unexplained by WIMP CDM. SFDM without self-interaction (`Fuzzy Dark Matter') cannot satisfy current BBN constraints within 68% confidence and is therefore disfavored.