title: Tensor interpretation of BICEP2 results severely constrains axion dark matter.
abstract: The recent detection of B modes by the BICEP2 experiment has nontrivial implications for axion dark matter implied by combining the tensor interpretation with isocurvature constraints from Planck observations. In this Letter 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 HI=1.1×10(14)  GeV. If the axion decay constant fa<HI/2π, constraints on the dark matter (DM) abundance alone rule out the QCD axion as DM for ma≲52χ6/7  μeV (where χ>1 accounts for theoretical uncertainty). If fa>HI/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 backreaction of fluctuations is included. High-fa QCD axions are thus ruled out. Constraints on axionlike particles, as a function of their mass and DM fraction, are also considered. For heavy axions with ma≳10(-22)  eV we find Ωa/Ωd≲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.
authors.affiliation: Perimeter Institute, 31 Caroline Street N, Waterloo, Ontario N2 L 6B9, Canada.
authors.forename: David J E
authors.initials: DJ
authors.lastname: Marsh
authors.affiliation: Department of Astronomy and Astrophysics, University of Chicago, Chicago, Illinois 60637, USA.
authors.forename: Daniel
authors.initials: D
authors.lastname: Grin
authors.affiliation: Department of Astronomy, Princeton University, Princeton, New Jersey 08544, USA.
authors.forename: Renée
authors.initials: R
authors.lastname: Hlozek
authors.affiliation: Astrophysics, University of Oxford, DWB, Keble Road, Oxford OX1 3RH, United Kingdom.
authors.forename: Pedro G
authors.initials: PG
authors.lastname: Ferreira