  Type Ia supernova data have recently become strong enough to enable, for the
first time, constraints on the time variation of the dark energy density and
its equation of state. Most analyses, however, are using simple two or
three-parameter descriptions of the dark energy evolution, since it is well
known that allowing more degrees of freedom introduces serious degeneracies.
Here we present a method to produce uncorrelated and nearly model-independent
band power estimates of the equation of state of dark energy and its density as
a function of redshift. We apply the method to recently compiled supernova
data. Our results are consistent with the cosmological constant scenario, in
agreement with other analyses that use traditional parameterizations, though we
find marginal (2-sigma) evidence for w(z) < -1 at z < 0.2. In addition to easy
interpretation, uncorrelated, localized band powers allow intuitive and
powerful testing of the constancy of either the energy density or equation of
state. While we have used relatively coarse redshift binning suitable for the
current set of about 150 supernovae, this approach should reach its full
potential in the future, when applied to thousands of supernovae found from
ground and space, combined with complementary information from other
cosmological probes.
