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순수물리이론 연구단
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Axion as a fuzzy dark matter candidate: Proofs in different gauges

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Title
Axion as a fuzzy dark matter candidate: Proofs in different gauges
Author(s)
Jai-chan Hwang; Hyerim Noh
Publication Date
2022-03
Journal
Journal of Cosmology and Astroparticle Physics, v.2022, no.3
Publisher
IOP Publishing Ltd
Abstract
© 2022 IOP Publishing Ltd and Sissa Medialab.Axion as a coherently oscillating massive scalar field is known to behave as a zero-pressure irrotational fluid with characteristic quantum stress on a small scale. In relativistic perturbation theory, the case was proved in the axion-comoving gauge up to fully nonlinear and exact order. Our basic assumption is that the field is oscillating with Compton frequency and the Compton wavelength is smaller than the horizon scale. Here, we revisit the relativistic proof to the linear order in the other gauge conditions. We show that the same equation for density perturbation known in the non-relativistic treatment can be derived in two additional gauge conditions: the zero-shear gauge and the uniform-curvature gauge. The uniform-expansion gauge fails to get the aimed equation, and the quantum stress term is missing in the synchronous gauge. For comparison, we present the relativistic density perturbation equations in the zero-pressure fluid in these gauge conditions. Except for the comoving and the synchronous gauge, the equations strikingly differ from the axion case. We clarify that the relativistic analysis based on time averaging is valid for scales larger than the Compton wavelength. Below the Compton wavelength, the field is not oscillating, and our oscillatory ansatz does not apply. We suggest an equation valid in all scales in the comoving gauge. For comparison, we review the non-relativistic quantum hydrodynamics and present the Schrödinger equation to first-order post-Newtonian expansion in the cosmological context.
URI
https://pr.ibs.re.kr/handle/8788114/11358
DOI
10.1088/1475-7516/2022/03/001
ISSN
1475-7516
Appears in Collections:
Center for Fundamental Theory(순수물리이론 연구단) > 1. Journal Papers (저널논문)
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