The Hubble Space Telescope imaged the most distant type 1a supernova (exploding stellar remnant) found to date.  As astronomers push these detections to larger distances (and therefore further back in time), the size and expansion history of the Universe gets constrained more and more.

The lower panels in the image above (zooms of the top image) show the appearance of the supernova between the time the first one was taken (left) and the second.  Here is the full press release.