Abstract

I will discuss two unexplored parts of isocurvature parameter space. In one, the compensated isocurvature mode (a fluctuation in the cosmic baryon fraction), the usual linear theory observables are hidden, and the model remains unconstrained using familiar analyses. Instead, the hypothesis can be tested using the CMB trispectrum (put alternatively, using off-diagonal correlations). After developing the formalism, I’ll describe a WMAP search for such an isocurvature mode. I’ll then discuss the possibility of probing ultra-light axions (ULAs) as a dark matter/dark energy candidate, using CMB anisotropy observables, the matter power spectrum, and in particular, the imprint of isocurvature in a ULA . I will also spend some time discussing the technical challenges that result when implementing this possibility in a Boltzmann code or a Markov Chain analysis.