Beyond H_0 and q_0: Cosmology is no longer just two numbers

TL;DR

This paper challenges the traditional focus on H_0 and q_0 in cosmology, demonstrating that q_0 cannot be directly measured accurately or precisely with current methods, and proposing alternative approaches for better parameter estimation.

Contribution

It shows that q_0 cannot be directly measured accurately or precisely and suggests using H_0 and Omega_M as better parameters for cosmological models.

Findings

q_0 cannot be directly measured accurately or precisely with current distance indicators

H_0 can be measured accurately and precisely, but with a small bias

Using H_0 and Omega_M provides a better parameter pair for cosmology

Abstract

For decades, H_0 and q_0 were the quest of cosmology, as they promised to characterize our "world model" without reference to a specific cosmological framework. Using Monte Carlo simulations, we show that q_0 cannot be directly measured using distance indicators with both accuracy (without offset away from its true value) and precision (small error bar). While H_0 can be measured with accuracy and precision, to avoid a small bias in its direct measurement (of order 5%) we demonstrate that the pair H_0 and Omega_M (assuming flatness and w = -1) is a better choice of two parameters, even if our world model is not precisely LambdaCDM. We illustrate with analysis of the Constitution set of supernovae and indirectly infer q_0 = -0.57+/-0.04. Finally, we show that it may be possible to directly determine q_0 with both accuracy and precision using the time dependence of redshifts ("redshift drift").