Post-inflationary axion isocurvature perturbations facing CMB and large-scale structure

Abstract

Dark matter comprised of axion-like particles (ALPs) generated by the realignment mechanism in the post-inflationary scenario leads to primordial isocurvature fluctuations. The power spectrum of these fluctuations is flat for small wave numbers, extending to scales accessible with cosmological surveys. We use the latest measurements of Cosmic Microwave Background (CMB) primary anisotropies (temperature, polarization) together with CMB lensing, Baryonic Acoustic Oscillations (BAO) and Sunyaev Zel'dovich (SZ) cluster counts to measure the amplitude and tilt of the isocurvature component. We find preference for a white-noise isocurvature component in the CMB primary anisotropies; this conclusion is, however, weakened by current large-scale structure (LSS) data. Interpreting the result as a conservative upper limit on the isocurvature component, the combined bound on the ALP mass from all probes is ma ≳ 10−19 eV, with some dependence on how ma evolves with temperature. The expected sensitivity of cosmic shear and galaxy clustering from future LSS experiments and CMB lensing suggests improved bounds of ma ≳ 10−18–10−13 eV, depending on scale cuts used to avoid non-linearities and the ALP mass-temperature dependence.

Observing Invisible Axions with Gravitational Waves

“…42 The precise numerical bound depends on the cosmological data that is combined. Interestingly the analysis in [79] shows a mild preference for a non-zero isocurvature component.…”

“…For m a 10 −28 eV, k CMB is inside the k 3 part of ∆ 2 a (k) and we can directly apply the limit f iso < 0.64 obtained in [79]. 42 In general the axions from scaling only comprise a fraction of the total dark matter, so there is a factor of Ω st a /Ω DM in their contribution to f iso .…”

“…previously been carried out for the QCD axion [77] and axion-like particles in [78,79], and we comment in Appendix F on the differences with our approach. The perturbations in the axion energy density ρ a are most easily studied using the overdensity field δ a (x) ≡ (ρ a (x) − ρ a )/ ρ a , where the brackets indicate the spatial average.…”

“…without extrapolation, unlike simulations of the physical system in (1), which must include modes with k m r ). 79 Let us now discuss what changes if H crit < H . In this case the number density at H = H crit is approximately n a = 8πξ log H crit f 2 a /x 0,a .…”

Dark Matter Physics from the CMB-S4 Experiment

“…Alternatively, if the ULA U (1) symmetry is broken after the end of inflation, a whitenoise power spectrum of isocurvature would be produced. Depending on details of ULA production (such as thermal corrections to the ULA mass), CMB-S4 sensitivity to isocurvature fluctuations (when combined with CMB lensing, and independent probes of galaxy clustering and lensing shear) could test ULA masses as high as m φ 10 −17 eV [147].…”