The Second Peak: The Dark-Energy Density and the Cosmic Microwave Background

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Supernova evidence for a negative-pressure dark energy (e.g., cosmological constant or quintessence) that contributes a fraction ΩΛ≃0.7 of closure density has been bolstered by the discrepancy between the total density, Ω tot≃1, suggested by the location of the first peak in the cosmic microwave background (CMB) power spectrum and the nonrelativistic-matter density Ωₘ≃0.3 obtained from dynamical measurements. Here we show that the impending identification of the location of the second peak in the CMB power spectrum will provide an immediate and independent probe of the dark-energy density. As an aside, we show how the measured height of the first peak probably already points toward a low matter density and places upper limits to the reionization optical depth and gravitational-wave amplitude.