The effective Lagrangian of dark energy from observations

TL;DR

This paper uses observational H(z) data to constrain the effective Lagrangian of dark energy, finding it consistent with a cosmological constant and supporting a flat potential, with no need for additional parameters beyond LCDM.

Contribution

It demonstrates that the effective Lagrangian of dark energy can be constrained in a model-independent way using observational data, supporting the cosmological constant model.

Findings

The potential of dark energy is consistent with being flat within 6%.

Current data do not require extra parameters beyond the LCDM model.

Effective theory approach is justified by observational constraints.

Abstract

Using observational data on the expansion rate of the universe (H(z)) we constrain the effective Lagrangian of the current accelerated expansion. Our results show that the effective potential is consistent with being flat i.e., a cosmological constant; it is also consistent with the field moving along an almost flat potential like a pseudo-Goldstone boson. We show that the potential of dark energy does not deviate from a constant at more than 6% over the redshift range 0 < z < 1. The data can be described by just a constant term in the Lagrangian and do not require any extra parameters; therefore there is no evidence for augmenting the number of parameters of the LCDM paradigm. We also find that the data justify the effective theory approach to describe accelerated expansion and that the allowed parameters range satisfy the expected hierarchy. Future data, both from cosmic chronometers and baryonic acoustic oscillations, that can measure H(z) at the % level, could greatly improve constraints on the flatness of the potential or shed some light on possible mechanisms driving the accelerated expansion. Besides the above result, it is shown that the effective Lagrangian of accelerated expansion can be constrained from cosmological observations in a model-independent way and that direct measurements of the expansion rate H(z) are most useful to do so.