The Dark Energy Cosmic Clock: A New Way to Parametrise the Equation of State

TL;DR

This paper introduces a novel dark energy equation of state parametrisation using the cosmic clock $\,\Omega_e$ and orthogonal polynomials, outperforming existing models and linking to quintessence theories.

Contribution

It presents a new $\,\Omega_e$-based parametrisation of dark energy, demonstrating improved performance and robustness over traditional models, with potential for high redshift data adaptation.

Findings

Outperforms CPL and GE parametrisations in data fitting.

Robust to prior choices in parameter estimation.

Links dark energy models directly to the new parametrisation.

Abstract

We propose a completely new parametrisation of the dark energy equation of state, which uses the dark energy density, $\Omega_e$ as a cosmic clock. We expand the equation of state in a series of orthogonal polynomials, with $\Omega_e$ as the expansion parameter and determine the expansion coefficients by fitting to SNIa and $H(z)$ data. Assuming that $\Omega_e$ is a monotonic function of time, we show that our parametrisation performs better than the popular Chevallier--Polarski--Linder (CPL) and Gerke and Efstathiou (GE) parametrisations, and we demonstrate that it is robust to the choice of prior. Expanding in orthogonal polynomials allows us to relate models of dark energy directly to our parametrisation, which we illustrate by placing constraints on the expansion coefficients extracted from two popular quintessence models. Finally, we comment on how this parametrisation could be modified to accommodate high redshift data, where any non--monotonicity of $\Omega_e$ would need to be accounted for.