Tensor Detection Severely Constrains Axion Dark Matter [CL]


The recent detection of tensor modes by BICEP2 has non-trivial implications for axion dark matter implied by combining the tensor detection with isocurvature constraints from Planck. In this paper the measurement is taken as fact, and its implications considered, though further experimental verification is required. In the simplest inflation models $r=0.2$ implies $H_I=1.1\times 10^{14}\text{ GeV}$. If the axion decay constant $f_a>H_I$ then isocurvature constraints effectively rule out the QCD axion as dark matter for $m_a \lesssim 0.06\mu$ eV, contributing only a fraction $\Omega_a/\Omega_d\lesssim 10^{-11}(f_a/10^{16}\text{ GeV})^{5/6}$ (where $\Omega_i$ is the fraction of the critical density), with misalignment angle $\theta_i\lesssim 3.1 \times 10^{-8}(f_a/10^{16}\text{ GeV})^{-1/6}$ . Implications of this fine tuning are discussed. Constraints on axion-like particles, as a function of their mass and dark matter fraction are also considered. For heavy axions with $m_a\gtrsim 10^{-22}\text{ eV}$ we find $\Omega_a/\Omega_d\lesssim 10^{-3}$, with stronger constraints on heavier axions. For lighter axions, however, direct constraints from the CMB temperature power spectrum and large scale structure are stronger.

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D. Marsh, D. Grin, R. Hlozek, et. al.
Tue, 18 Mar 14