Minimal cosmography

TL;DR

This paper explores a generalized, non-metric approach to cosmography, proposing a framework based on linear connections that can modify standard cosmological observables and potentially be tested through experiments.

Contribution

It introduces a minimal, non-metric cosmography framework using linear connections compatible with homogeneity and isotropy, extending traditional metric-based cosmology.

Findings

Velocities of free-falling bodies decrease over time due to expansion

Redshift may be independent of the scale factor in this framework

Standard geometric observables are modified and testable

Abstract

The minimal requirement for cosmography - a nondynamical description of the universe - is a prescription for calculating null geodesics, and timelike geodesics as a function of their proper time. In this paper, we consider the most general linear connection compatible with homogeneity and isotropy, but not necessarily with a metric. A light-cone structure is assigned by choosing a set of geodesics representing light rays. This defines a "scale factor" and a local notion of distance, as that travelled by light in a given proper time interval. We find that the velocities and relativistic energies of free-falling bodies decrease in time as a consequence of cosmic expansion, but at a rate that can be different than that dictated by the usual metric framework. By extrapolating this behavior to photons redshift, we find that the latter is in principle independent of the "scale factor". Interestingly, redshift-distance relations and other standard geometric observables are modified in this extended framework, in a way that could be experimentally tested. An extremely tight constraint on the model, however, is represented by the blackbody-ness of the Cosmic Microwave Background. Finally, as a check, we also consider the effects of a non-metric connection in a different set-up, namely, that of a static, spherically symmetric spacetime.