Early dark energy in the pre- and postrecombination epochs

Dark energy could play a role at redshifts z >> O(1). Many quintessence models possess scaling or attractor solutions where the fraction of dark energy follows the dominant component in previous epochs of the Universe's expansion, or phase transitions may happen close to the time of matter-radiation equality. A non-negligible early dark energy (EDE) fraction around matter-radiation equality could contribute to alleviate the well-known H-0 tension. In this work, we constrain the fraction of EDE using two approaches: first, we use a fluid parametrization that mimics the plateaux of the dominant components in the past. An alternative tomographic approach constrains the EDE density in binned redshift intervals. The latter allows us to reconstruct the evolution of Omega(de)(z) before and after the decoupling of the cosmic microwave background (CMB) photons. We have employed Planck data 2018, the Pantheon compilation of supernovae of Type Ia (SNIa), data on galaxy clustering, the prior on the absolute magnitude of SNIa by SH0ES, and weak lensing data from KiDS + VIKING-450 and DES-Y1. When we use a minimal parametrization mimicking the background plateaux, EDE has only a small impact on current cosmological tensions. We show how the constraints on the EDE fraction weaken considerably when its sound speed is allowed to vary. By means of our binned analysis we put very tight constraints on the EDE fraction around the CMB decoupling time, <0.4% at 2 sigma c.l. We confirm previous results that a significant EDE fraction in the radiation-dominated epoch loosens the H-0 tension, but tends to worsen the tension for sigma(8). A subsequent presence of EDE in the matter-dominated era helps to alleviate this issue. When both the SH0ES prior and weak lensing data are considered in the fitting analysis in combination with data from CMB, SNIa and baryon acoustic oscillations, the EDE fractions are constrained to be less than or similar to 2.6% in the radiation-dominated epoch and less than or similar to 1.5% in the redshift range z is an element of (100; 1000) at 2 sigma c.l. The two tensions remain with a statistical significance of similar to 2-3 sigma c.l.

Automated summary

The study reveals that the fraction of early dark energy (EDE) is constrained to be around 0.4%, and its impact on cosmological tensions is significant, although constraints considerably weaken when the sound speed is allowed to vary. The presence of EDE in different epochs helps both alleviate and worsen cosmological tensions.