Ultra-light dark matter

Astron Astrophys Rev (2021) 29:7 https://doi.org/10.1007/s00159-021-00135-6 REVIEW ARTICLE Ultra-light dark matter Elisa G. M. Ferreira1,2 Received: 17 May 2020 / Accepted: 4 June 2021 © The Author(s) 2021 Abstract Ultra-light dark matter is a class of dark matter models (DM), where DM is composed by bosons with masses ranging from 10−24 eV < m < eV. These models have been receiving a lot of attention in the past few years given their interesting property of forming a Bose–Einstein condensate (BEC) or a superfluid on galactic scales. BEC and superfluidity are some of the most striking quantum mechanical phenomena that manifest on macroscopic scales, and upon condensation, the particles behave as a single coherent state, described by the wavefunction of the condensate. The idea is that condensation takes place inside galaxies while outside, on large scales, it recovers the successes of ΛCDM. This wave nature of DM on galactic scales that arise upon condensation can address some of the curiosities of the behaviour of DM on small-scales. There are many models in the literature that describe a DM component that condenses in galaxies. In this review, we are going to describe those models, and classify them into three classes, according to the different non-linear evolution and structures they form in galaxies: the fuzzy dark matter (FDM), the self-interacting fuzzy dark matter (SIFDM), and the DM superfluid. Each of these classes comprises many models, each presenting a similar phenomenology in galaxies. They also include some microscopic models like the axions and axion-like particles. To understand and describe this phenomenology in galaxies, we are going to review the phenomena of BEC and superfluidity that arise in condensed matter physics, and apply this knowledge to DM. We describe how ULDM can potentially reconcile the cold DM picture with the small-scale behaviour. These models present a rich phenomenology that is manifest in different astrophysical consequences. We review here the astrophysical and cosmological tests used to constrain those models, together with new and future observations that promise to test these models in different regimes. For the case of the FDM class, the mass where this model has an interesting phenomenology on smallscales ∼ 10−22 eV, is strongly challenged by current observations. The parameter space for the other two classes remains weakly constrained. We finalize by showing B Elisa G. M. Ferreira 1 Max-Planck-Institut für Astrophysik, Karl-Schwarzschild-Str. 1, 85748 Garching, Germany 2 Instituto de Física, Universidade de São Paulo, C.P. 66318, São Paulo, SP 05315-970, Brazil 0123456789().: V,-vol 123 7 Page 2 of 186 E. G. M. Ferreira et al. some predictions that are a consequence of the wave nature of this component, like the creation of vortices and interference patterns, that could represent a smoking gun in the search of these rich and interesting alternative class of DM models. Keywords Ultra-light dark matter · Fuzzy dark matter · Superfluid dark matter · Bose–Einstein condensate · Superfluid Contents 1 Introduction and motivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 Small-scale challenges of cold dark matter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1 Dark matter halos and substructures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2 Discrepancies in comparison with observations . . . . . . . . . . . . . . . . . . . . . . . . 2.2.1 Cusp–core . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.2 Missing satellites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.3 Diversity vs. regularity: scaling relations . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.4 What the small-scales tell us . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 Bose–Einstein Condensation and Superfluidity . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1 Non-interacting ideal gas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2 Landau’s superfluid model and criteria for superfluidity . . . . . . . . . . . . . . . . . . . . 3.3 Weakly interacting Bose gas superfluid . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.1 Field theory description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4 Effective field theory of a superfluid . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.5 Rotating superfluid—quantum vortices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.6 BEC in wave turbulence—kinetic theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.7 Summary and discussion: what is a condensate? . . . . . . . . . . . . . . . . . . . . . . . . 4 Ultra-light dark matter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1 FDM and SIFDM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1.1 Formation: ALPs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1.2 Cosmological evolution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1.3 Evolution on small-scales . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1.4 Description of the condensate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1.5 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1.6 Cosmological and astrophysical consequences of the FDM . . . . . . . . . . . . . . . 4.1.7 Addressing the small-scale challenges . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2 DM superfluid . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2.1 Conditions for DM condensation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2.2 Superfluid dynamics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2.3 Halo profile . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2.4 Observational consequences . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2.5 Validity of the EFT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2.6 Relativistic completion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2.7 Cosmology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3 Simulating ULDM models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4 ULDM as dark energy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4.1 Fuzzy DM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4.2 Superfluid DM—unified superfluid dark sector . . . . . . . . . . . . . . . . . . . . . 5 Cosmological and astrophysical constraints, and new windows of observation . . . . . . . . . . . 5.1 Cosmological constraints: CMB and LSS . . . . . (...truncated)