Model independent constraints on the cosmological expansion rate

TL;DR

This study develops a model-independent method to analyze the universe's expansion rate using supernova and large-scale structure data, confirming late-time acceleration without assuming specific cosmological models.

Contribution

A new model-independent approach to constrain the cosmic expansion history, avoiding assumptions about dark energy or gravity theories.

Findings

Late-time acceleration is confirmed at >5 sigma from supernova data.

Deceleration at high redshifts is supported only under flat or negatively curved universe assumptions.

Expansion acceleration today is detected at >12 sigma using Taylor expansion of the scale factor.

Abstract

We investigate what current cosmological data tells us about the cosmological expansion rate in a model independent way. Specifically, we study if the expansion was decelerating at high redshifts and is accelerating now, without referring to any model for the energy content of the universe, nor to any specific theory of gravity. This differs from most studies of the expansion rate which, e.g., assumes some underlying parameterised model for the dark energy component of the universe. To accomplish this, we have devised a new method to probe the expansion rate without relying on such assumptions. Using only supernova data, we conclude that there is little doubt that the universe has been accelerating at late times. However, contrary to some previous claims, we can not determine if the universe was previously decelerating. For a variety of methods used for constraining the expansion history of the universe, acceleration is detected from supernovae alone at >5 sigma, regardless of the curvature of the universe. Specifically, using a Taylor expansion of the scale factor, acceleration today is detected at >12 sigma. If we also include the ratio of the scale of the baryon acoustic oscillations as imprinted in the cosmic microwave background and in the large scale distribution of galaxies, it is evident from the data that the expansion decelerated at high redshifts, but only with the assumption of a flat or negatively curved universe.