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.