Probing large-angle correlations with the microwave background temperature and lensing cross-correlation

A lack of correlations in the microwave background temperature between sky directions separated by angles larger than 60A degrees has recently been confirmed by data from the Planck satellite. This feature arises as a random occurrence within the standard I > cold dark matter (I > CDM) cosmological model less than 0.3 per cent of the time, but so far no other compelling theory to explain this observation has been proposed. Here, we investigate the theoretical cross-correlation function between microwave background temperature and the gravitational lensing potential of the microwave background, which in contrast to the temperature correlation function depends strongly on gravitational potential fluctuations interior to our Hubble volume. For standard I > CDM cosmology, we generate random sky realizations of the microwave temperature and gravitational lensing, subject to the constraint that the temperature correlation function matches observations, and compare with random skies lacking this constraint to illustrate the value added in considering simulations that match observations (rather than standard I > CDM with cosmic variance). The distribution of large-angle temperature-lensing correlation functions in these two cases is different, and the two cases can be clearly distinguished in around 40 per cent of model realizations. We present an a priori procedure for optimizing statistics for large-angle correlations between other types of data, using unconstrained I > CDM as generic model for comparison, to determine whether the lack of large-angle correlations is a statistical fluke or points to a shortcoming of the standard cosmological model.