The measured distances of type Ia (hydrogen-deficient) supernovae as a function
of redshift (z) have shown that the expansion of the Universe is currently
accelerating, probably due to the presence of repulsive dark energy (X) such
as Einstein's cosmological constant (Lambda). Combining all of the data with
existing results from large-scale surveys, we find a best fit for Omega_M and
Omega_X of 0.28 and 0.72 (respectively), in excellent agreement with the values
(0.27 and 0.73) recently derived from WMAP measurements of the cosmic
microwave background radiation. A number of possible systematic effects (dust,
supernova evolution) thus far don't seem to eliminate the need for Omega_X > 0.
Most recently, analyses of supernovae at z = 1.0-1.7 reveal an early epoch of
deceleration, followed by acceleration. Several groups are now in the process
of measuring hundreds of supernovae with z = 0.2-0.8, to determine the equation of
state of the dark energy, w_X = P/(rho c^2); thus far, the best-fit value
is w_X = -1, and the data are consistent with dw/dz = 0, suggesting
that the dark energy may indeed be the cosmological constant or something
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