Papers › Cosmological implications of ultra-light axion-like fields
Cosmological implications of ultra-light axion-like fields
Vivian Poulin, Tristan L. Smith, Daniel Grin, Tanvi Karwal, Marc Kamionkowski
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Cosmological observations are used to test for imprints of an ultra-light axion-like field (ULA), with a range of potentials V(ϕ)∝[1-cos(ϕ/f)]ⁿ set by the axion-field 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 parameterized by the redshift z_c when the field becomes dynamical, the fractional energy density f_(z_c) ≡Ωₐ(z_c)/Ωₜₒₜ(z_c) in the axion field at z_c, and the effective sound speed cₛ². Using Planck, BAO and JLA data, constraints on f_(z_c) are obtained. ULAs are degenerate with dark energy for all three potentials if 1+z_c ≲10. When 3×10⁴ ≳1+z_c ≳10, f_(z_c) is constrained to be ≲0.004 for n=1 and f_(z_c) ≲0.02 for the other two potentials. The constraints then relax with increasing z_c. These results strongly constrain 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.
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