Using the full power of the cosmic microwave background to probe axion dark matter

Abstract

The cosmic microwave background (CMB) places strong constraints on models of\ndark matter (DM) that deviate from standard cold DM (CDM), and on initial\nconditions beyond the scalar adiabatic mode. Here, the full \\textit{Planck}\ndata set (including temperature, $E$-mode polarisation, and lensing deflection)\nis used to test the possibility that some fraction of the DM is composed of\nultralight axions (ULAs). This represents the first use of CMB lensing to test\nthe ULA model. We find no evidence for a ULA component in the mass range\n$10^{-33}\\leq m_a\\leq 10^{-24}\\text{ eV}$. We put percent-level constraints on\nthe ULA contribution to the DM, improving by up to a factor of two compared to\nthe case with temperature anisotropies alone. Axion DM also provides a\nlow-energy window onto the high-energy physics of inflation through the\ninterplay between the vacuum misalignment production of axions and isocurvature\nperturbations. We perform the first systematic investigation into the parameter\nspace of ULA isocurvature, using an accurate isocurvature transfer function at\nall $m_{a}$ values. We precisely identify a "window of co-existence" for\n$10^{-25}\\text{ eV}\\leq m_a\\leq10^{-24}\\text{ eV}$ where the data allow,\nsimultaneously, a $\\sim10\\%$ contribution of ULAs to the DM, and $\\sim 1\\%$\ncontributions of isocurvature and tensors to the CMB power. ULAs in this window\n(and \\textit{all} lighter ULAs) are shown to be consistent with a large\ninflationary Hubble parameter, $H_I\\sim 10^{14}\\text{ GeV}$. The window of\nco-existence will be fully probed by proposed CMB-S4 observations with\nincreased accuracy in the high-$\\ell$ lensing power and low-$\\ell$ $E$ and\n$B$-mode polarisation. If ULAs in the window exist, this could allow for two\nindependent measurements of $H_I$ in the CMB using the axion DM content and\nisocurvature, and the tensor contribution to $B$-modes.\n