  Cosmological structure formation simulations of ultralight axion-like dark
matter have shown that an axion star forms at the center of every dark matter
halo in the Universe. These axion stars would then form in large numbers during
the dark ages, $z \lesssim 70$. Axion stars would represent the densest axion
environments in the Universe, and as such they can trigger collective processes
that cannot otherwise occur for axions in vacuum. In particular, even though
the lifetime of individual sub-eV axions decaying into a pair of photons is
much larger than the age of the Universe, axion stars can decay into photons on
very short time scales due to parametric resonance. In this talk, based on
arXiv:2302.10206 and arXiv:2301.09769, I will discuss the cosmological
implications of such decays. We show that massive enough axion stars will decay
into a large number of radio photons which will in turn lead to heating and
ionization during the dark ages which is strongly constrained by Planck. As a
result, we find that couplings $10^{-14}\,{\rm GeV}^{-1} \lesssim
g_{a\gamma\gamma} \lesssim 10^{-10}\,{\rm GeV}^{-1}$ are excluded by Planck for
$10^{-14}\,{\rm eV}\lesssim m_a\lesssim 10^{-8}\,{\rm eV}$ within our benchmark
model of axion star abundance. We also highlight that future measurements of
the 21 cm line can have sensitivity to couplings at least one order of
magnitude smaller.
