Venue: Maths 103
I will discuss axions as dark matter candidates; in particular light axions with large decay constants as motivated by string theory. If the recent BICEP measurement of B-modes in the CMB is taken to indicate tensor perturbations from inflation, the large energy scales necessary induce large axion isocurvature perturbations. In order that these perturbations are not in conflict with Planck adiabaticity constraints, generalised axions must be sub-dominant in dark matter, while the QCD axion with large decay constant is completely ruled out. I will present these constraints and demonstrate how they are complimentary over a wide range of parameter space, spanning ~15 orders of magnitude in axion mass, with other constraints, in particular those from large scale structure and the CMB temperature and lensing. In large scale structure, light axions provide an elegant solution to the "small-scale crises" of CDM. I will review this solution, and how it is distinct from, and preferable to, a WDM solution. If axions do play a role in structure formation (or indeed in laboratory processes), we have an opportunity to search for them in a manner independent from the inflationary interpretation of BICEP, and thus evidence for or against their existence can help to confirm or refute this paradigm. In this direction, I will present preliminary results on a wide range of cosmological constraints to ultra-light axions.