The Hubble series: Convergence properties and redshift variables

TL;DR

This paper investigates the convergence properties of series expansions in cosmological redshift variables, demonstrating limitations of z-based expansions at high redshift and proposing y=z/(1+z) as a more effective parameterization.

Contribution

It provides a theoretical analysis of the convergence limits of redshift series expansions and advocates for using the y-redshift to improve cosmological data analysis.

Findings

z-series converges only for z ≤ 1, limiting high-redshift applications

y-series converges for y ≤ 1, suitable for early universe studies

y-parameterization remains valid back to the big bang

Abstract

In cosmography, cosmokinetics, and cosmology it is quite common to encounter physical quantities expanded as a Taylor series in the cosmological redshift z. Perhaps the most well-known exemplar of this phenomenon is the Hubble relation between distance and redshift. However, we now have considerable high-z data available, for instance we have supernova data at least back to redshift z=1.75. This opens up the theoretical question as to whether or not the Hubble series (or more generally any series expansion based on the z-redshift) actually converges for large redshift? Based on a combination of mathematical and physical reasoning, we argue that the radius of convergence of any series expansion in z is less than or equal to 1, and that z-based expansions must break down for z>1, corresponding to a universe less than half its current size. Furthermore, we shall argue on theoretical grounds for the utility of an improved parameterization y=z/(1+z). In terms of the y-redshift we again argue that the radius of convergence of any series expansion in y is less than or equal to 1, so that y-based expansions are likely to be good all the way back to the big bang y=1, but that y-based expansions must break down for y<-1, now corresponding to a universe more than twice its current size.