This week we have David (Doddy) Marsh (PI) to tell us about:
Axions are hypothetical particles that emerge in many different contexts. There may be many different types of axion spanning many orders of magnitude in mass. In cosmology and astrophysics, they are our best friends, and can play major roles in inflation, dark matter (DM), dark energy, and even in the neutrino sector. In this talk I will focus on their role as DM. The axion mass defines a characteristic length scale. For 1e-33 eV<m< 1e-20 eV this length scale is cosmological and axion dark matter can be distinguished from CDM. The magnitude of these effects is set by the axion relic density. The axion relic density and initial perturbations are established in the early universe and by physics at or beyond the GUT scale. I will present multiple probes as constraints of axions: the Planck temperature power spectrum, the WiggleZ galaxy redshift survey, Hubble ultra deep field, the epoch of reionisation as measured by cmb polarisation and the density profiles of dwarf spheroidal galaxies. Together these probe the entire 13 orders of magnitude in axion mass where axions are distinct from CDM in cosmology. The observations hint that axions in the range 1e-22 eV<m<1e-20 eV may play an interesting role in structure formation, and evidence for this could be found in the future surveys AdvACT (2015), JWST, and Euclid (>2020). Given time, I will also discuss how observing axion DM can offer a window onto theories of inflation and and the string theory landscape.
