Suggestions of decreasing dark energy from supernova and BAO data

TL;DR

This study uses supernova and BAO data to explore how a scalar field's potential energy might decrease over time, challenging the standard cosmological constant model and suggesting dynamic dark energy evolution.

Contribution

It introduces a linear and second-order approximation to scalar field potentials and analyzes their compatibility with observational data, highlighting a preference for decreasing potential energy.

Findings

Likelihood analysis favors models with decreasing potential energy.

$ m Lambda$CDM model is disfavored in this context.

Wide range of potentials fit supernova data, including non-accelerating models.

Abstract

The potential energy from a time-dependent scalar field provides a possible explanation for the observed cosmic acceleration. In this paper, we investigate how data from supernova and bary acoustic oscillation surveys constrain the possible evolution of a single scalar field over the period of time (roughly half the age of the universe) for which these data are available. Taking a linear approximation to the scalar potential $V(\phi) = V_0 + V_1 \phi$ around the present value, a likelihood analysis appears to significantly prefer models with a decreasing potential energy at present, with approximately $99.99 \%$ of the $\exp(-\chi^2/2)$ distribution having $V_1 > 0$ in a convention where $\dot{\phi} \le 0$ at present. The models favoured by the distribution typically have an order one decrease $\langle |{\rm Range}[V(\phi(t))] / V(t_0)| \rangle \approx 0.36$ in the scalar potential energy over the time frame corresponding to $z < 2$. According to the likelihood analysis, the $\Lambda$CDM model with no variation in dark energy appears to be significantly disfavoured in the context of the linear potential model, but this should be interpreted cautiously since model selection criteria that make use of $\Delta \chi^2$ while ignoring parameter space volumes still favour $\Lambda$CDM. Working with a second order approximation to the potential, the supernova data can be fit well for a wide range of possible potentials, including models where the universe has already stopped accelerating.