(Mis-)Interpreting supernovae observations in a lumpy universe

TL;DR

This paper investigates how tiny beams from supernovae interact with the lumpy universe, revealing that under-dense lines of sight significantly affect observations and modeling of cosmic distances, which is crucial for precision cosmology.

Contribution

It demonstrates the importance of accurately modeling ultra-narrow beams in cosmology, highlighting the impact of matter inhomogeneities on supernova observations and distance measurements.

Findings

Most lines of sight are significantly under-dense even over Gpc scales.

Standard models predict large variance in beam observations, complicating interpretation.

Uncertainty in expansion rate modeling affects whether under-dense regions dim or brighten sources.

Abstract

Light from `point sources' such as supernovae is observed with a beam width of order of the sources' size - typically less than 1 AU. Such a beam probes matter and curvature distributions that are very different from coarse-grained representations in N-body simulations or perturbation theory, which are smoothed on scales much larger than 1 AU. The beam typically travels through unclustered dark matter and hydrogen with a mean density much less than the cosmic mean, and through dark matter halos and hydrogen clouds. Using N-body simulations, as well as a Press-Schechter approach, we quantify the density probability distribution as a function of beam width and show that, even for Gpc-length beams of 500 kpc diameter, most lines of sight are significantly under-dense. From this we argue that modelling the probability distribution for AU-diameter beams is absolutely critical. Standard analyses predict a huge variance for such tiny beam sizes, and nonlinear corrections appear to be non-trivial. It is not even clear whether under-dense regions lead to dimming or brightening of sources, owing to the uncertainty in modelling the expansion rate which we show is the dominant contribution. By considering different reasonable approximations which yield very different cosmologies we argue that modelling ultra-narrow beams accurately remains a critical problem for precision cosmology. This could appear as a discordance between angular diameter and luminosity distances when comparing SN observations to BAO or CMB distances.