Tristan L. Smith
The cosmic microwave background (CMB) has provided us the laboratory in which we entered the
era of 'precision cosmology'. By and large these observations have confirmed the basic, standard,
cosmological model which supposes a universe seeded with Gaussian initial fluctuations and filled
with non-relativistic matter, photons, and 3 neutrino species. As observations have become more
accurate it has become increasingly pertinent to test some of the assumptions that have gone into
building the standard cosmological model. In this talk I will present some of my recent work on testing
First I will discuss how recent measurements of the small-scale CMB have hinted at the presence of extra relativistic energy density within the universe. Any interpretation of these results will necessitate the measurement of some of the properties of any extra relativistic energy density. Using recent small-scale measurements of the CMB, along with my colleagues, I have been able to constrain the clustering properties of the extra relativistic energy density. We show that current observations disfavor the interpretation that any extra relativistic energy consists of standard neutrinos.
Second, there has been an increasing interest in testing whether the initial fluctuations follow Gaussian statistics. These tests of 'primordial non-Gaussianity' use estimators constructed from either the CMB three-point (bispectrum) or four-point (trispectrum) correlation functions. Usually an estimator constructed from data is assumed to have a Gaussian probability distribution (PDF) as a result of the central limit theorem. However, in this case the central limit theorem does not apply since the number of terms that appear in these estimators are much greater than the number of measurements. A complete knowledge of the shape of the PDF is necessary in order to properly assign confidence limits to any constraint. I will discuss how Monte Carlo simulations show that the PDF for these estimators are highly non-Gaussian themselves, necessitating more care in using these estimators, especially for future observations with the Planck satellite. I will also show how the constraining power of these estimators may be improved by using a realization-dependent normalization.