Strong Constraints on Fuzzy Dark Matter from Ultrafaint Dwarf Galaxy Eridanus II.

The fuzzy dark matter (FDM) model treats DM as a bosonic field with an astrophysically large de Broglie wavelength. A striking feature of this model is O(1) fluctuations in the dark matter density on time scales which are shorter than the gravitational timescale. Including, for the first time, the effect of core oscillations, we demonstrate how such fluctuations lead to heating of star clusters and, thus, an increase in their size over time. From the survival of the old star cluster in Eridanus II, we infer m_{a}≳0.6→1×10^{-19} eV within modeling uncertainty if FDM is to compose all of the DM and derive constraints on the FDM fraction at lower masses. The subhalo mass function in the Milky Way implies m_{a}≳0.8×10^{-21} eV to successfully form Eridanus II. The region between 10^{-21} and 10^{-20} eV is affected by narrow band resonances. However, the limited applicability of the diffusion approximation means that some of this region may still be consistent with observations of Eridanus II.

Proposal to Detect Dark Matter using Axionic Topological Antiferromagnets.

Antiferromagnetically doped topological insulators (ATI) are among the candidates to host dynamical axion fields and axion polaritons, weakly interacting quasiparticles that are analogous to the dark axion, a long sought after candidate dark matter particle. Here we demonstrate that using the axion quasiparticle antiferromagnetic resonance in ATIs in conjunction with low-noise methods of detecting THz photons presents a viable route to detect axion dark matter with a mass of 0.7 to 3.5 meV, a range currently inaccessible to other dark matter detection experiments and proposals. The benefits of this method at high frequency are the tunability of the resonance with applied magnetic field, and the use of ATI samples with volumes much larger than 1 mm^{3}.

Searching for the QCD Axion with Gravitational Microlensing.

The phase transition responsible for axion dark matter (DM) production can create large amplitude isocurvature perturbations, which collapse into dense objects known as axion miniclusters. We use microlensing data from the EROS survey and from recent observations with the Subaru Hyper Suprime Cam to place constraints on the minicluster scenario. We compute the microlensing event rate for miniclusters, treating them as spatially extended objects. Using the published bounds on the number of microlensing events, we bound the fraction of DM collapsed into miniclusters f_{MC}. For an axion with temperature-dependent mass consistent with the QCD axion, we find f_{MC}<0.083(m_{a}/100 µeV)^{0.12}, which represents the first observational constraint on the minicluster fraction. We forecast that a high-efficiency observation of around ten nights with Subaru would be sufficient to constrain f_{MC}≲0.004 over the entire QCD axion mass range. We make various approximations to derive these constraints, and dedicated analyses by the observing teams of EROS and Subaru are necessary to confirm our results. If accurate theoretical predictions for f_{MC} can be made in the future, then microlensing can be used to exclude or discover the QCD axion. Further details of our computations are presented in a companion paper [M. Fairbairn, D. J. E. Marsh, J. Quevillon, and S. Rozier (to be published)].

Tensor interpretation of BICEP2 results severely constrains axion dark matter.

The recent detection of B modes by the BICEP2 experiment has nontrivial implications for axion dark matter implied by combining the tensor interpretation with isocurvature constraints from Planck observations. In this Letter the measurement is taken as fact, and its implications considered, though further experimental verification is required. In the simplest inflation models, r=0.2 implies HI=1.1×10(14) GeV. If the axion decay constant fa1 accounts for theoretical uncertainty). If fa>HI/2π then vacuum fluctuations of the axion field place conflicting demands on axion DM: isocurvature constraints require a DM abundance which is too small to be reached when the backreaction of fluctuations is included. High-fa QCD axions are thus ruled out. Constraints on axionlike particles, as a function of their mass and DM fraction, are also considered. For heavy axions with ma≳10(-22) eV we find Ωa/Ωd≲10(-3), with stronger constraints on heavier axions. Lighter axions, however, are allowed and (inflationary) model-independent constraints from the CMB temperature power spectrum and large scale structure are stronger than those implied by tensor modes.

Axion dark matter: What is it and why now?

The axion has emerged in recent years as a leading particle candidate to provide the mysterious dark matter in the cosmos, as we review here for a general scientific audience. We describe first the historical roots of the axion in the Standard Model of particle physics and the problem of charge-parity invariance of the strong nuclear force. We then discuss how the axion emerges as a dark matter candidate and how it is produced in the early universe. The symmetry properties of the axion dictate the form of its interactions with ordinary matter. Astrophysical considerations restrict the particle mass and interaction strengths to a limited range, which facilitates the planning of experiments to detect the axion. A companion review discusses the exciting prospect that the axion could be detected in the near term in the laboratory.