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Axion dark matter thermalizes by gravitational self-interactions and forms a Bose-Einstein condensate. We show that the rethermalization of the axion fluid during the initial collapse of large scale overdensities near cosmic dawn transports angular momentum outward sufficientlly fast that black holes form with masses ranging from approximately $10^5$ to a few times $10^{10}~M_\odot$., Comment: 16 pages, 3 figures

We investigate the effect of a dark matter caustic passing through the Solar System. We find, confirming a previous result, that the Sun tracks the caustic surface for some time. We integrate numerically the equations of motion of the Sun and a comet for a large number of initial conditions and of caustic passage properties. We calculate the probability for the comet to escape the Solar System and the probability for it to fall within 50 A.U. of the Sun, given the initial semimajor axis and eccentricity of its orbit. We find that the average probability for a comet to fall within 50 A.U. of the Sun is of order $3 \times 10^{-4}$ and that comets which are initially at a distance larger than about $10^5$ A.U. have a probability of order one to be ejected from the Solar System.

A contribution to Frank Wilczek's 70th birthday's festschrift, this brief note considers how much power can be extracted from dark matter., Comment: 5 pages, no figures

The Gaia map of the Milky Way reveals a pair of triangular features at nearly symmetric locations on opposite sides of the Galactic Center. In this paper we explore the implications of these features assuming they are manifestations of a caustic ring in the dark matter distribution of the Milky Way halo. The existence of a series of such rings is predicted by the Caustic Ring Model. The model's phase-space distribution is that acquired by a rethermalizing Bose-Einstein condensate of axions or axion-like particles. We show that dust is gravitationally entrained by cold axion flows and propose this as an explanation for the sharpness of the triangular features. The locations of the features imply that we on Earth are much closer to the fifth caustic ring than thought on the basis of pre-Gaia observations. Most likely we are inside its tricusp cross-section. In that case the dark matter density on Earth is dominated by four cold flows, termed Big, Little, Up and Down. If we are outside the tricusp cross-section the dark matter density on Earth is dominated by two cold flows, Big and Little. We use the triangular features in the Gaia map, and a matching feature in the IRAS map, to estimate the velocity vectors and densities of the four locally dominant flows., Comment: 40 pages, 14 figures. An analysis of the statistical significance of the Gaia features has been added, and other smaller changes made. Version published in Physics of the Dark Universe

Electromagnetic radiation with angular frequency equal to half the axion mass stimulates the decay of cold dark matter axions and produces an echo, i.e. faint electromagnetic radiation traveling in the opposite direction. We propose to search for axion dark matter by sending out to space a powerful beam of microwave radiation and listening for its echo. We estimate the sensitivity of this technique in the isothermal and caustic ring models of the Milky Way halo, and find it to be a promising approach to axion, or axion-like, dark matter detection., Comment: 5 pages, 2 figures, includes changes suggested by PRL referees

Caustic rings of dark matter with $\textit{tricusp}$ cross-section were predicted to lie in the galactic disk. Their radii increase on cosmological time scales at a rate of order $1$ kpc/Gyr. When a caustic ring passes through the orbit of a star, the orbit is strongly perturbed. We find that a star moving in a nearly circular orbit is first attracted towards the caustic ring, then moves with and oscillates about the caustic for approximately $1$ Gyr before returning to its original orbit. As a result, a stellar overdensity forms around the caustic ring. We predict such overdensities to be of order $120\%$ near the 2nd caustic ring where the Monoceros Ring is observed and of order $45$, $30$ and $15\%$ near the 3rd, 4th and 5th caustic rings, respectively. We show that the associated bulk velocities of the stars near the caustic rings are less than a few km/s. We also determine the density profile of interstellar gas near the 5th caustic ring assuming it is in thermal equilibrium in the gravitational potential of the caustic and of the gas itself., Comment: 19 pages, 9 figures

The infall of cold dark matter onto a galaxy produces cold collisionless flows and caustics in its halo. If a signal is found in the cavity detector of dark matter axions, the flows will be readily apparent as peaks in the energy spectrum of photons from axion conversion, allowing the densities, velocity vectors and velocity dispersions of the flows to be determined. We discuss the evolution of velocity dispersion along cold collisionless flows in one and two dimensions. A technique is presented for obtaining the leading behaviour of the velocity dispersion near caustics. The results are used to derive an upper limit on the energy dispersion of the Big Flow from the sharpness of its nearby caustic, and a prediction for the dispersions in its velocity components., Comment: 19 pages, 2 figures

Dark matter axions form a rethermalizing Bose-Einstein condensate. This provides an opportunity to distinguish axions from other forms of dark matter on observational grounds. I show that if the dark matter is axions, tidal torque theory predicts a specific structure for the phase space distribution of the halos of isolated disk galaxies, such as the Milky Way. This phase space structure is precisely that of the caustic ring model, for which observational support had been found earlier. The other dark matter candidates predict a different phase space structure for galactic halos., Comment: 4 pages, no figures