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Tensor Detection Severely Constrains Axion Dark Matter

David J. E. Marsh, Daniel Grin, Renee Hlozek, Pedro G. Ferreira

Abstract

The recent detection of B-modes by BICEP2 has non-trivial implications for axion dark matter implied by combining the tensor interpretation 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 $f_a<H_I/2π$ constraints on the dark matter (DM) abundance alone rule out the QCD axion as DM for $m_a \lesssim 52χ^{6/7}\,μ\text{eV}$ (where $χ>1$ accounts for theoretical uncertainty). If $f_a>H_I/2π$ then vacuum fluctuations of the axion field place conflicting demands on axion DM: isocurvature constraints require a DM abundance which is too small to be reached when the back reaction of fluctuations is included. High $f_a$ QCD axions are thus ruled out. Constraints on axion-like particles, as a function of their mass and DM fraction, are also considered. For heavy axions with $m_a\gtrsim 10^{-22}\text{ eV}$ we find $Ω_a/Ω_d\lesssim 10^{-3}$, with stronger constraints on heavier axions. Lighter axions, however, are allowed and (inflationary) model-independent constraints from the CMB temperature power spectrum and large scale structure are stronger than those implied by tensor modes.

Tensor Detection Severely Constrains Axion Dark Matter

Abstract

The recent detection of B-modes by BICEP2 has non-trivial implications for axion dark matter implied by combining the tensor interpretation 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 implies . If the axion decay constant constraints on the dark matter (DM) abundance alone rule out the QCD axion as DM for (where accounts for theoretical uncertainty). If then vacuum fluctuations of the axion field place conflicting demands on axion DM: isocurvature constraints require a DM abundance which is too small to be reached when the back reaction of fluctuations is included. High QCD axions are thus ruled out. Constraints on axion-like particles, as a function of their mass and DM fraction, are also considered. For heavy axions with we find , with stronger constraints on heavier axions. Lighter axions, however, are allowed and (inflationary) model-independent constraints from the CMB temperature power spectrum and large scale structure are stronger than those implied by tensor modes.

Paper Structure

This paper contains 18 equations, 1 figure.

Figures (1)

  • Figure 1: Constraints in axion parameter space: regions below curves are allowed. The solid red line shows the result of the present work which constrains axions using the measured value of $r=0.2$ ($^{+0.07}_{-0.05}$ shown in thin lines) and the Planck constraint on axion isocurvature, $A_I/A_s<0.04$. The dashed red line approximates the loosening of this constraint due to suppression of the axion isocuvature power when $m_a<H_{\rm eq}$. We also show the 95% exclusion contours of Ref. amendola2005 from CMB (WMAP1) and CMB+Lyman-alpha forest power spectra, which are significantly stronger than the tensor/isocurvature constraint for intermediate mass axions, and are independent of the inflationary model.