Axions comprise a broad class of particles that can play a major role in explaining the unknown aspects of cosmology .
They are also well-motivated within high energy physics , appearing in theories related to CP -violation in the standard model , supersymmetric theories , and theories with extra-dimensions , including string theory , and so axion cosmology offers us a unique view onto these theories .
I review the motivation and models for axions in particle physics and string theory .
I then present a comprehensive and pedagogical view on the cosmology and astrophysics of axion-like particles , starting from inflation and progressing via BBN , the CMB , reionization and structure formation , up to the present-day Universe .
Topics covered include : axion dark matter ( DM ) ; direct and indirect detection of axions , reviewing existing and future experiments ; axions as dark radiation ; axions and the cosmological constant problem ; decays of heavy axions ; axions and stellar astrophysics ; black hole superradiance ; axions and astrophysical magnetic fields ; axion inflation , and axion DM as an indirect probe of inflation .
A major focus is on the population of ultralight axions created via vacuum realignment , and its role as a DM candidate with distinctive phenomenology .
Cosmological observations place robust constraints on the axion mass and relic density in this scenario , and I review where such constraints come from .
I next cover aspects of galaxy formation with axion DM , and ways this can be used to further search for evidence of axions .
An absolute lower bound on DM particle mass is established .
It is m _ { a } > 10 ^ { -24 } \text { eV } from linear observables , extending to m _ { a } \gtrsim 10 ^ { -22 } \text { eV } from non-linear observables , and has the potential to reach m _ { a } \gtrsim 10 ^ { -18 } \text { eV } in the future .
These bounds are weaker if the axion is not all of the DM , giving rise to limits on the relic density at low mass .
This leads to the exciting possibility that the effects of axion DM on structure formation could one day be detected , and the axion mass and relic density measured from cosmological observables .