Exploding stars halfway across the universe show that the expansion of the universe is speeding up. Astronomers detected this runaway universe with the Hubble Space Telescope (HST) in the 1990s and they are using it now to understand the dark energy that causes cosmic acceleration. Is the dark energy a modern version of Einstein's discarded cosmological constant or is it something else? We don't know, but we do know how to find out through observations with HST and its successors.

The John N. Bahcall Lecture is sponsored by the Space Telescope Science Institute and the Hubble Space Telescope Project/NASA Goddard Space Flight Center.