  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.
