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

Supernova evidence for a negative-pressure dark energy (e.g., cosmological constant or quintessence) that contributes a fraction $\Omega_\Lambda\simeq0.7$ of closure density has been bolstered by the discrepancy between the total density, $\Omega_{\rm tot}\simeq1$, suggested by the location of the first peak in the cosmic microwave background (CMB) power spectrum and the nonrelativistic-matter density $\Omega_m\simeq0.3$ obtained from dynamical measurements. Here we show that the impending identification of the location of the {\it 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.