Exploding Stars and the Accelerating Universe

Observations of exploding stars halfway across the Universe show that the expansion of the Universe is speeding up.  We attribute this to a pervasive "dark energy" whose properties we would like to understand.  I will show the evidence from supernovae, and outline the present state of knowledge on dark energy, which is completely consistent with a modern version of the cosmological constant, but with a ridiculously low value.  Then I will discuss ways to use infrared observations to make the supernova measurements with better accuracy and higher precision.  Finally I'll discuss how the matrix of evidence from other observations, including the cosmic microwave background and the growth of structure in the universe, can help us understand whether modifications to general relativity or a time-varying component of dark energy can be ruled out.