Tensor Minkowski Functionals: first application to the CMB

Abstract

Tensor Minkowski Functionals (TMFs) are tensorial generalizations of the usual Minkowski Functionals which are scalar quantities. We introduce them here for use in cosmological analysis, in particular to analyze CMB maps. They encapsulate information about the shapes and the orientation of structures. We focus on one of the TMFs, namely $W_2^{1,1}$, which is the generalization of the genus. The ratio of the eigenvalues of the average of $W_2^{1,1}$ over all structures, $\alpha$, encodes the net orientation; and the average of the ratios of the eigenvalues of $W_2^{1,1}$ for each structure, $\beta$, encodes the net anisotropy. We have developed a code that computes $W_2^{1,1}$, and from it $\alpha$ and $\beta$, for a set of structures on the plane. We compute $\alpha$ and $\beta$ as functions of threshold levels for simulated Gaussian and isotropic CMB fields. We obtain $\alpha$ to be one for both temperature and $E$ mode, which means that we recover the input statistical isotropy of density fluctuations in the simulations. The level of net anisotropy of hotspots and coldspots in the CMB fields is quantified by $\beta\sim 0.62$. Then we compute $\alpha$ and $\beta$ for data from PLANCK. We find that the temperature field agrees with the standard LCDM prediction of no net orientation within $3-\sigma$. However, we find that $E$ mode data shows a net orientation that deviates from the theoretical expectation at $14-\sigma$. The possible origin of this deviation may be instrumental effects or other sources and needs to be investigated further. For the net anisotropy we obtain values of $\beta$ for both temperature and $E$ mode that are consistent with the expectations from the standard LCDM simulations. Accurate measurements of $\alpha$ and $\beta$ can be used to test the standard model of cosmology and to search for deviations from it.