  We present a new compilation of Type Ia supernovae (SNe Ia), a new dataset of
low-redshift nearby-Hubble-flow SNe and new analysis procedures to work with
these heterogeneous compilations. This ``Union'' compilation of 414 SN Ia,
which reduces to 307 SNe after selection cuts, includes the recent large
samples of SNe Ia from the Supernova Legacy Survey and ESSENCE Survey, the
older datasets, as well as the recently extended dataset of distant supernovae
observed with HST. A single, consistent and blind analysis procedure is used
for all the various SN Ia subsamples, and a new procedure is implemented that
consistently weights the heterogeneous data sets and rejects outliers. We
present the latest results from this Union compilation and discuss the
cosmological constraints from this new compilation and its combination with
other cosmological measurements (CMB and BAO). The constraint we obtain from
supernovae on the dark energy density is $\Omega_\Lambda=
0.713^{+0.027}_{-0.029} (stat)}^{+0.036}_{-0.039} (sys)}$, for a flat, LCDM
Universe. Assuming a constant equation of state parameter, $w$, the combined
constraints from SNe, BAO and CMB give
$w=-0.969^{+0.059}_{-0.063}(stat)^{+0.063}_{-0.066} (sys)$. While our results
are consistent with a cosmological constant, we obtain only relatively weak
constraints on a $w$ that varies with redshift. In particular, the current SN
data do not yet significantly constrain $w$ at $z>1$. With the addition of our
new nearby Hubble-flow SNe Ia, these resulting cosmological constraints are
currently the tightest available.

Astrophysics