How early is early dark energy?

TL;DR

This paper examines how constraints on early dark energy (EDE) depend on its epoch of influence, showing that bounds weaken if EDE becomes significant later, with implications for understanding dark energy's role in cosmic history.

Contribution

The study introduces a new parametrization of EDE with fewer parameters and compares different models to analyze how CMB constraints vary with the epoch of EDE influence.

Findings

EDE bounds weaken if EDE becomes significant at lower redshifts.

Tight constraints (~1-2%) are maintained if EDE affects last scattering.

CMB primarily constrains EDE presence at emission, less so on subsequent structure growth.

Abstract

We investigate constraints on early dark energy (EDE) from the Cosmic Microwave Background (CMB) anisotropy, taking into account data from WMAP9 combined with latest small scale measurements from the South Pole Telescope (SPT). For a constant EDE fraction we propose a new parametrization with one less parameter but still enough to provide similar results to the ones previously studied in literature. The main emphasis of our analysis, however, compares a new set of different EDE parametrizations that reveal how CMB constraints depend on the redshift epoch at which Dark Energy was non negligible. We find that bounds on EDE get substantially weaker if dark energy starts to be non-negligible later, with early dark energy fraction Omega_e free to go up to about 5% at 2 sigma if the onset of EDE happens at z < 100. Tight bounds around 1-2% are obtained whenever dark energy is present at last scattering, even if its effects switch off afterwards. We show that the CMB mainly constrains the presence of Dark Energy at the time of its emission, while EDE-modifications of the subsequent growth of structure are less important.