  We present a method for reducing cosmological data to constraints on the
amplitudes of modes of the dark energy density as a function of redshift. The
modes are chosen so that (1) one of them has constant density and (2) the
others are non-zero only if there is time-variation in the dark energy density
and (3) the amplitude errors for the time-varying modes are uncorrelated with
each other. We apply our method to various combinations of three-year WMAP
data, baryon acoustic oscillation data, the 'Gold' supernova data set, and the
Supernova Legacy Survey data set. We find no significant evidence for a
time-varying dark energy density or for non-zero mean curvature. Although by
some measure the limits on four of the time-varying mode amplitudes are quite
tight, they are consistent with the expectation that the dark energy density
does not vary on timescales shorter than a Hubble time. Since we do not expect
detectable time variation in these modes, our results should be viewed as a
systematic error test which the data have passed. We discuss a procedure to
identify modes with maximal signal-to-noise ratio.
