Résultats de recherche pour « Cosmological correlations »

ABSTRACT Cross-correlations between data sets are used in many different contexts in cosmological analyses. Recently, k-nearest neighbour cumulative distribution functions (kNN-CDF) were shown to be sensitive probes of cosmological (auto) clustering. In this paper, we extend the framework of NN measurements to describe joint distributions of, and correlations between, two data sets. We describe the measurement of joint kNN-CDFs, and show that these measurements are sensitive to all possible connected N-point functions that can be defined in terms of the two data sets. We describe how the cross-correlations can be isolated by combining measurements of the joint kNN-CDFs and those measured from individual data sets. We demonstrate the application of these measurements in the context of Gaussian density fields, as well as for fully non-linear cosmological data sets. Using a Fisher analysis, we show that measurements of the halo-matter cross-correlations, as measured through NN measurements are more sensitive to the underlying cosmological parameters, compared to traditional two-point cross-correlation measurements over the same range of scales. Finally, we demonstrate how the NN cross-correlations can robustly detect cross-correlations between sparse samples – the same regime where the two-point cross-correlation measurements are dominated by noise.

Abstract Current cosmological tensions motivate investigating extensions to the standard Λ cold dark matter (ΛCDM) model. Additional model parameters are typically varied one or two at a time, in a series of separate tests. The purpose of this paper is to highlight that information is lost by not also examining the correlations between these additional parameters, which arise when their effects on model predictions are similar even if the parameters are not varied simultaneously. We show how these correlations can be quantified with simulations and Markov Chain Monte Carlo methods. As an example, we assume that ΛCDM is the true underlying model, and calculate the correlations expected between the phenomenological lensing amplitude parameter, A L , the running of the spectral index, n run , and the primordial helium mass fraction, Y P , when these parameters are varied one at a time along with the ΛCDM parameters in fits to the Planck 2015 temperature power spectrum. These correlations are not small, ranging from 0.31 ( A L − n run ) to −0.93 ( n run – Y P ). We find that the values of these three parameters from the Planck data are consistent with ΛCDM expectations within 0.9 σ when the correlations are accounted for. This does not explain the 1.8–2.7 σ Planck preference for A L > 1, but provides an additional ΛCDM consistency test. For example, if A L > 1 was a symptom of an underlying systematic error or some real but unknown physical effect that also produced spurious correlations with n run or Y P our test might have revealed this. We recommend that future cosmological analyses examine correlations between additional model parameters in addition to investigating them separately, one a time.