Oscillating scalar field as dark matter through the cosmological epochs

Abstract

An oscillating scalar field can behave like cold dark matter in the expanding Universe. In relativistic cosmology, this perspective is primarily reasoned by an evolution of a background scalar field and its subhorizon perturbations in a matter-dominated era. A corresponding relativistic description is usually based on the ansatz that the perturbations oscillate in the same way as the background field and on time averaging to find the oscillation amplitudes. In contrast, the presented approach makes no initial assumptions about the perturbation behavior and does not use time averages. It is shown that although the previous treatments are valid only well inside the Hubble scale, the scalar field reproduces the CDM scenarios on the superhorizon scale and also throughout the evolution of the Universe from the beginning of the oscillations. The misunderstanding about the Jeans length scale in the matter-dominated epoch is also clarified. An analog of the Jeans effect is found in a radiation-dominated era. It can prevent an overproduction of small halos for the scalars with $m\sim 10^{-13}$~eV.

11 pages, 1 figure

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