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Measurements of cosmic microwave background (CMB) anisotropies provide strongevidence for the existence of dark matter and dark energy. They can also testits composition, probing the energy density and particle mass of differentdark-matter and dark-energy components. CMB data have already shown thatultra-light axions (ULAs) with mass in the range $10^{-32}~{\rm eV} \to10^{-26}~{\rm eV}$ compose a fraction $< 0.01$ of the cosmological criticaldensity. Here, the sensitivity of a proposed CMB-Stage IV (CMB-S4) experiment(assuming a 1 arcmin beam and $< 1~\mu K{\rm-arcmin}$ noise levels over a skyfraction of 0.4) to the density of ULAs and other dark-sector components isassessed. CMB-S4 data should be $\sim 10$ times more sensitive to the ULAenergy-density than Planck data alone, across a wide range of ULA masses$10^{-32}< m_{a}< 10^{-23}~{\rm eV}$, and will probe axion decay constants of$f_{a}\approx 10^{16}~{\rm GeV}$, at the grand unified scale. CMB-S4 couldimprove the CMB lower bound on the ULA mass from $\sim 10^{-25}~{\rm eV}$ to$10^{-23}~{\rm eV}$, nearing the mass range probed by dwarf galaxy abundancesand dark-matter halo density profiles. These improvements will allow for amulti-$\sigma$ detection of percent-level departures from CDM over a wide rangeof masses. Much of this improvement is driven by the effects of weakgravitational lensing on the CMB, which breaks degeneracies between ULAs andneutrinos. We also find that the addition of ULA parameters does notsignificantly degrade the sensitivity of the CMB to neutrino masses. Theseresults were obtained using the axionCAMB code (a modification to the CAMBBoltzmann code), presented here for public use.

Future CMB tests of dark matter: ultra-light axions and massive neutrinos