Supernova evidence for a negative-pressure dark energy ( e.g. , cosmological constant or quintessence ) that contributes a fraction \Omega _ { \Lambda } \simeq 0.7 of closure density has been bolstered by the discrepancy between the total density , \Omega _ { tot } \simeq 1 , suggested by the location of the first peak in the cosmic microwave background ( CMB ) power spectrum and the nonrelativistic-matter density \Omega _ { m } \simeq 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 .