http://arxiv.org/abs/1808.01879
If the symmetry breaking leading to the origin of the axion dark matter field occurs after the end of inflation and never restores, then overdensities in the axion field collapse to form dense objects known in the literature as axion miniclusters. The mass of the minicluster is set by the energy density of the axion field inside the horizon at the onset of coherent oscillations, and the radius is fixed by the details of the collapse at matter-radiation equality. In the standard cosmological scenario, axion miniclusters are expected to be as massive as the heaviest asteroids in the Solar System and with a size comparable to an astronomical unit; yet, their density would be millions of times larger than the local dark matter density. These estimates strongly depend on the details of the cosmology at which the onset of axion oscillations begin. A modification to these initial conditions would alter the expected mass and radius of an axion minicluster by various orders of magnitudes. Assessing the properties of miniclusters is crucial for experimental interest, since the direct detection of axions at current and future facilities would be sensibly altered if the axion field clumps into substructures instead of existing as a smooth component. In turn, this could potentially lead to an alternative method for exploring the content of the Universe before BBN occurred, the earliest time from which we have direct data. We have considered some modified exotic cosmologies during which the Universe could have been dominated by some form of energy other than radiation, down to a reheating temperature which is bounded by BBN considerations. If the axion field starts to evolve during this exotic cosmological period then various properties would be altered compared to the standard expectations, including the number of miniclusters formed, their size and mass, and the chance of an encounter with the Earth
L. Visinelli
Tue, 7 Aug 18
32/68
Comments: 16 pages, 4 figures, 1 table
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