CONSTRAINING PERTURBATIVE EARLY DARK ENERGY WITH CURRENT OBSERVATIONS

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In this work, we study a class of early dark energy (EDE) models, in which, unlike in standard dark energy models, a substantial amount of dark energy exists in the matter-dominated era. We self-consistently include dark energy perturbations, and constrain these models using current observations. We consider EDE models in which the dark energy equation of state is at least wm ≳ −0.1 at early times, which could lead to an EDE density of up to . Our analysis shows that marginalizing over the non-DE parameters such as Ωm, H0, andns, current CMB observations alone can constrain the scale factor of transition from EDE to late-time dark energy to at ≳ 0.44 and width of transition to Δt ≲ 0.37. The equation of state at present is somewhat weakly constrained to w0 ≲ −0.6, if we allow H0 < 60 km s−1 Mpc−1. Taken together with other observations, such as SNe, Hubble Space Telescope, and Sloan Digital Sky Survey luminous red galaxies, w0 is constrained much more tightly to w0 ≲ −0.9, while redshift of transition and width of transition are also tightly constrained to at ≲ 0.19 andΔt ≲ 0.21. The evolution of the equation of state for EDE models is thus tightly constrained to ΛCDM-like behavior at low redshifts. Incorrectly assuming dark energy perturbations to be negligible leads to different constraints on the equation of state parameters—w0 ≲ −0.8, at ≲ 0.33, andΔt ≲ 0.31, thus highlighting the necessity of self-consistently including dark energy perturbations in the analysis. If we allow the spatial curvature to be a free parameter, then the constraints are relaxed to w0 ≲ −0.77, at ≲ 0.35, andΔt ≲ 0.35 with −0.014 < Ωκ < 0.031 for CMB + other observations. For perturbed EDE models, the 2σ lower limit on σ8 (σ8 ⩾ 0.59) is much lower than that in ΛCDM (σ8 ⩾ 0.72), thus raising the interesting possibility of discriminating EDE from ΛCDM using future observations such as halo mass functions or the Sunyaev–Zeldovich power spectrum.