  We present a 2.5 sigma detection of the Integrated Sachs-Wolfe (ISW) effect
and discuss the constraints it places on cosmological parameters. We
cross-correlate microwave temperature maps from the WMAP satellite with a 4000
deg^2 luminous red galaxy (LRG) overdensity map measured by the Sloan Digital
Sky Survey. Accurate photometric redshifts allow us to perform a reliable
auto-correlation analysis of the LRGs, eliminating the uncertainty in the
galaxy bias, and combined with cross correlation signal, constrains
cosmological parameters -- in particular, the matter density. We find a 2.5
sigma signal in the Ka, Q, V, and W WMAP bands, after combining the information
from multipoles 2 <= l < 400. This is consistent with the expected amplitude of
the ISW effect, but requires a lower matter density than is usually assumed:
the amplitude, parametrized by the galaxy bias assuming \Omega_M=0.3,
\Omega_\Lambda=0.7 and \sigma_8=0.9, is b_g = 4.05 \pm 1.54 for V band, with
similar results for the other bands. This should be compared to b_g = 1.82 \pm
0.02 from the auto-correlation analysis. These data provide only a weak
confirmation (2.5 sigma) of dark energy, but provide a significant upper limit:
\Omega_\Lambda=0.80_{-0.06}^{+0.03} (1 sigma)_{-0.19}^{+0.05} (2 sigma),
assuming a cosmology with \Omega_M+\Omega_\Lambda=1, \Omega_b = 0.05, and
\sigma_8=0.9, and w=-1. The weak cross-correlation signal rules out low matter
density/high dark energy density universes and, in combination with other data,
strongly constrains models with w<-1.3. We provide a simple prescription to
incorporate these constraints into cosmological parameter estimation methods
for (\Omega_M, \sigma_8,w). (abridged)
