Shifting the Universe: Early Dark Energy and Standard Rulers

TL;DR

This paper investigates how early dark energy affects the cosmic sound horizon and can bias cosmological measurements if not properly accounted for, highlighting the importance of fitting for the absolute ruler scale.

Contribution

It reveals the degeneracy in the CMB spectra caused by early dark energy and proposes fitting for the absolute ruler scale to avoid bias in cosmological parameters.

Findings

Early dark energy can hide in CMB spectra due to degeneracy.

Using the sound horizon as a standard ruler can bias parameter estimates.

Fitting for the absolute ruler scale reduces bias but weakens BAO constraints.

Abstract

The presence of dark energy at high redshift influences both the cosmic sound horizon and the distance to last scattering of the cosmic microwave background. We demonstrate that through the degeneracy in their ratio, early dark energy can lie hidden in the CMB temperature and polarization spectra, leading to an unrecognized shift in the sound horizon. If the sound horizon is then used as a standard ruler, as in baryon acoustic oscillations, then the derived cosmological parameters can be nontrivially biased. Fitting for the absolute ruler scale (just as supernovae must be fit for the absolute candle magnitude) removes the bias but decreases the leverage of the BAO technique by a factor 2.