Cross-correlations of the Cosmic Neutrino Background: HR-DEMNUni simulation analysis

TL;DR

The paper investigates cross-correlations between the Cosmic Neutrino Background and large-scale structure using high-resolution HR-DEMNUni simulations. It constructs full-sky maps of CDM density, neutrino density, neutrino velocity, deflection angle, and weak-lensing convergence, and computes their auto- and cross-power spectra to identify where signals are strongest. Two detection scenarios are explored: direct Earth-based cross-correlations requiring space-time delays, and co-located neutrino-induced photon emissions, with both showing nonzero large-scale signals for a neutrino mass sum of $igl\sum m_\nu\bigr = 0.16\,\mathrm{eV}$. The results highlight that cross-correlations can be as informative as auto-correlations, offering a robust path to probe cosmological neutrinos as observational capabilities improve.

Abstract

We use the high-resolution HR-DEMNUni simulations to compute cross-correlations of the Cosmic Neutrino Background quantities, like neutrino density, deflection angle, and velocity, with other quantities, like cold dark matter density and effective weak lensing convergence, by accounting for the space-time delay between signals on Earth. We provide this to theoretically illustrate how much can be learned from these cross-correlation signals, once the cosmic neutrino background is detected with instruments in multiple locations. Against a naive expectation of null cross-correlation, we show that the signal is non zero, specially at the largest scales. We also discuss the scenario of co-located cross-correlations between dark matter, weak lensing and a future neutrino-induced photon emission signal. As cross-correlations will be comparable to auto-correlations of the cosmic neutrino background itself and are less affected by cosmic variance and shotnoise, these might be the ones to be measured first. Our predictions thus provide the imprint of what cosmological massive neutrinos, with total mass $\sum{m_ν} \sim 0.1$ eV, should look like from cosmological observations.

Figures (7)

• Figure 1: Full-sky map of 250$h^{-1}\mathrm{Mpc}$ comoving radius around an observer placed at the centre of CDM surface overdensity (top left), neutrino velocity modulus (top right), neutrino surface overdensity (bottom left), and neutrino mean cosine of deflection angle (bottom right).
• Figure 2: Cross-correlations power spectra (off-diagonal panels) along with autocorrelation power spectra (diagonal panels) of CDM and neutrino maps. The CDM density contrast ($\delta_{\rm \Sigma_{\rm CDM}}$) map is constructed from a shell at a radius of $250\,h^{-1}$Mpc from an observer on the Earth, while the neutrino density contrast ($\delta_{\rm \Sigma_\nu}$), neutrino velocity ($v_{\rm\nu}/c$) and cosine of the neutrino deflection angle ($\cos\theta_{\rm\nu}$) are obtained from a shell at a radius of $30\,h^{-1}$Mpc from the same observer.
• Figure 3: Cross-correlation power spectra (off-diagonal panels) along with autocorrelation power spectra (diagonal panels) for the maps shown in \ref{['fig:maps_0']}, excluding the $\cos\theta_\nu$ map due to the loss of directionality in neutrino-induced photon emission processes. Unlike the previous case, these spectra are computed using maps constructed from the entire simulation box at $z=0$, rather than from radial shells.
• Figure 4: WL map constructed as described in the text by considering the total contribution of neutrinos and CDM, together with the corresponding total power spectrum.
• Figure 5: WL cross-correlation power spectra with CDM and neutrino density contrast. The cross-correlation spectra found in this case shows features similar to the one that cross-correlates the dark matter. This is expected, as the weak lensing convergence is basically a re-weighting of the dark matter distribution given a $n(z)$.