We review the use of Type Ia supernovae for cosmological distance determinations .
Low-redshift SNe Ia ( { { { { z \mathrel { \mathchoice { \lower 3.0 pt \vbox { \halign { \cr } $ \displaystyle \hfill < $ \cr$% \displaystyle \hfill \sim$ } } } { \lower 3.0 pt \vbox { \halign { \cr } $ \textstyle \hfill < $% \cr$ \textstyle \hfill \sim$ } } } { \lower 3.0 pt \vbox { \halign { \cr } $ \scriptstyle \hfill% < $ \cr$ \scriptstyle \hfill \sim$ } } } { \lower 3.0 pt \vbox { \halign { \cr } $% \scriptscriptstyle \hfill < $ \cr$ \scriptscriptstyle \hfill \sim$ } } } } 0.1 ) demonstrate that the Hubble expansion is linear , that H _ { 0 } = 65 \pm 2 ( statistical ) km s ^ { -1 } Mpc ^ { -1 } , and that the properties of dust in other galaxies are similar to those of dust in the Milky Way .
We find that the light curves of high-redshift ( z = 0.3 –1 ) SNe Ia are stretched in a manner consistent with the expansion of space ; similarly , their spectra exhibit slower temporal evolution ( by a factor of 1 + z ) than those of nearby SNe Ia .
The luminosity distances of our first set of 16 high-redshift SNe Ia are , on average , 10–15 % farther than expected in a low mass-density ( \Omega _ { M } = 0.2 ) universe without a cosmological constant .
Preliminary analysis of our second set of 9 SNe Ia is consistent with this .
Our work supports models with positive cosmological constant and a current acceleration of the expansion .
We address the main potential sources of systematic error ; at present , none of them appears to reconcile the data with \Omega _ { \Lambda } = 0 and q _ { 0 } \geq 0 .
The dynamical age of the Universe is estimated to be 14.2 \pm 1.7 Gyr , consistent with the ages of globular star clusters .