In this work, we explore the possibility that early dark energy (EDE) is dynamical in nature and study its effect on cosmological observables. We introduce a parametrization of the equation of state allowing for an equation of state $w$ differing considerably from cosmological constant (cc, $w=-1$) and vary both the initial $w_i$ as well final $w_f$ equation of state of the EDE fluid. This idea is motivated by the fact that in many models of EDE, the scalar field may have some kinetic energy when it starts to behave like EDE before the cosmic microwave background (CMB) decoupling. We find that the present data have a mild preference for non-cc early dark energy ($w_i=-0.78$) using $\mathrm{Planck}+\mathrm{BAO}+\mathrm{Pantheon}+\mathrm{S}{H}_0\mathrm{ES}$ datasets, leading to $\mathrm{\Delta }{\chi }_{\min }^2$ improvement of $-2.5$ at the expense of one more parameter. However, $w_i$ is only weakly constrained, with $w_i<-0.56$ at $1\sigma$. We argue that allowing for $w_i\ne -1$ can play a role in decreasing the ${\sigma }_8$ parameter. Yet, in practice the decrease is only $\sim 0.4\sigma$ and ${\sigma }_8$ is still larger than weak lensing measurements. We conclude that while promising, a dynamical EDE cannot resolve both $H_0$ and ${\sigma }_8$ tensions simultaneously.

• Received 20 September 2023
• Accepted 26 January 2024

DOI:https://doi.org/10.1103/PhysRevD.109.043530