Cosmological limits on axions and axion-like particles
  The axion is a pseudo-Nambu-Goldstone boson. It appears after the spontaneous
breaking of the Peccei-Quinn symmetry, which was proposed to solve the
strong-CP problem. Other pseudo-Nambu-Goldstone bosons, postulated in some
extensions of the standard model of particle physics, are called axion-like
particles (ALPs) if they share certain characteristics with the axion, in
particular a coupling to two photons. Thus far, axion and ALP searches have
been unsuccessful, indicating that their couplings have to be extremely weak.
However, axions and ALPs could be responsible for some observable effects in
astrophysics and cosmology, which can also be exploited to constrain the
parameter space of these particles. We focus on limits coming from cosmology,
which is an optimal field for studying axions and ALPs. In particular, we first
investigate the possibility of a primordial population of axions and ALPs
arising during the earliest epochs of the universe. The importance of this
analysis lies on the fact that axions and ALPs are ideal dark matter candidates
because of their faint interactions and their peculiar production mechanisms.
Finally, we consider the consequences of the decay of such a population on
specific cosmological observables, namely the photon spectrum of galaxies, the
cosmic microwave background, the effective number of neutrino species, and the
abundance of primordial elements. Our bounds constitute the most stringent
probes of early decays and exclude a part of the ALP parameter space that is
otherwise very difficult to test experimentally.
