Cosmological implications of ultralight axionlike fields
Abstract
Cosmological observations are used to test for imprints of an ultralight axionlike field (ULA), with a range of potentials V (ϕ )∝[1 cos (ϕ /f )]^{n} set by the axionfield value ϕ and decay constant f . Scalar field dynamics dictate that the field is initially frozen and then begins to oscillate around its minimum when the Hubble parameter drops below some critical value. For n =1 , once dynamical, the axion energy density dilutes as matter; for n =2 it dilutes as radiation and for n =3 it dilutes faster than radiation. Both the homogeneous evolution of the ULA and the dynamics of its linear perturbations are included, using an effective fluid approximation generalized from the usual n =1 case. ULA models are parametrized by the redshift z_{c} when the field becomes dynamical, the fractional energy density f_{zc}≡Ω_{a}(z_{c})/Ω_{tot}(z_{c}) in the axion field at z_{c}, and the effective sound speed c_{s}^{2}. Using Planck, BAO and JLA data, constraints on f_{zc}are obtained. ULAs are degenerate with dark energy for all three potentials if 1 +z_{c}≲10 . When 3 ×10^{4}≳1 +z_{c}≳10 , f_{z c} is constrained to be ≲0.004 for n =1 and f_{zc}≲0.02 for the other two potentials. The constraints then relax with increasing z_{c}. These results have implications for ULAs as a resolution to cosmological tensions, such as discrepant measurements of the Hubble constant, or the EDGES measurement of the global 21 cm signal.
 Publication:

Physical Review D
 Pub Date:
 October 2018
 DOI:
 10.1103/PhysRevD.98.083525
 arXiv:
 arXiv:1806.10608
 Bibcode:
 2018PhRvD..98h3525P
 Keywords:

 Astrophysics  Cosmology and Nongalactic Astrophysics;
 High Energy Physics  Phenomenology
 EPrint:
 22 pages, 12 figures, comments welcome