Axion hot dark matter bounds after Planck

TL;DR

The paper updates cosmological bounds on thermally produced axions that behave as hot dark matter by combining Planck CMB data with large-scale structure and local $H_0$ measurements. Using a $\Lambda$CDM framework extended by both axion and neutrino HDM components and marginalising over $\Sigma\,m_\nu$, the authors find $m_a<1.01$ eV from CMB alone, improving to $m_a<0.86$ eV with SDSS data and to $m_a<0.78$ eV when including the local $H_0$ measurement; the final joint dataset yields $m_a<0.67$ eV. In contrast, the neutrino mass sum is tightly constrained, reaching $\Sigma\,m_\nu<0.27$ eV with all data, and it is anti-correlated with $H_0$, whereas the axion bound is robust to the $H_0$ tension. The work highlights that cosmological HDM bounds on axions remain around the eV scale, with future surveys like Euclid potentially extending sensitivity, and underscores the complementary role of helioscope experiments in exploring the axion parameter space.

Abstract

We use cosmological observations in the post-Planck era to derive limits on thermally produced cosmological axions. In the early universe such axions contribute to the radiation density and later to the hot dark matter fraction. We find an upper limit m_a < 0.67 eV at 95% C.L. after marginalising over the unknown neutrino masses, using CMB temperature and polarisation data from Planck and WMAP respectively, the halo matter power spectrum extracted from SDSS-DR7, and the local Hubble expansion rate H_0 released by the Carnegie Hubble Program based on a recalibration of the Hubble Space Telescope Key Project sample. Leaving out the local H_0 measurement relaxes the limit somewhat to 0.86 eV, while Planck+WMAP alone constrain the axion mass to 1.01 eV, the first time an upper limit on m_a has been obtained from CMB data alone. Our axion limit is therefore not very sensitive to the tension between the Planck-inferred H_0 and the locally measured value. This is in contrast with the upper limit on the neutrino mass sum, which we find here to range from 0.27 eV at 95% C.L. combining all of the aforementioned observations, to 0.84 eV from CMB data alone.

Figures (8)

• Figure 1: Top: Axion number density $n_a$ as a function of the axion mass $m_a$. Bottom: Present-day axion energy density $\omega_a$ as a function of $m_a$.
• Figure 2: Two-dimensional marginal 68% and 95% probability density contours in the $(\Sigma\,m_\nu,m_a)$-plane derived from Planck temperature and WMAP polarisation (Planck+WP) measurements (blue shading) and in combination with the halo power spectrum (red shading).
• Figure 3: CMB temperature angular power spectra for three different cosmological models, all with the same total dark matter density $\omega_{\rm dm} = \omega_{\rm cdm} + \omega_{\rm \nu} + \omega_{\rm a} = 0.112$. The blue/dot-dash line corresponds to the reference $\Lambda$CDM model ($\omega_{\rm cdm}=0.112$), the green/dashed line an axion HDM scenario with $m_a=2.4$ eV ($\omega_{\rm cdm}=0.099$, $\omega_a=0.013$), and the red/solid line the case of three degenerate neutrinos with masses summing to $\Sigma \, m_\nu=1.2$ eV ($\omega_{\rm cdm}=0.099$, $\omega_\nu=0.013$). In the top panel we hold all other cosmological parameters fixed, while in the bottom panel we further adjust $H_0$ in order to match the peak positions---the corresponding $H_0$ values are, in units of ${\rm km} \ {\rm s}^{-1} \ {\rm Mpc}^{-1}$, 70, 72, and 64.5 for the $\Lambda$CDM, the axion and the neutrino model respectively.
• Figure 4: Same as figure \ref{['fig:mamnu']}, but in the $(\Sigma\,m_\nu,H_0)$-plane.
• Figure 5: Same as figure \ref{['fig:mamnu']}, but in the $(m_a,H_0)$-plane.