 One of the most interesting uses of strong galaxy cluster gravitational lensing involves distant supernova. Multiple light paths through the cluster produce multiple arrival times here on Earth. Here are a few examples. In 2014, a team of astronomers found a supernova in this galaxy cluster over 5 billion light years away. The supernova actually happened in a galaxy 4 billion light years beyond that, making it 9 billion light years away. The huge mass of the foreground galaxy and galaxy cluster bent the light from the distant supernova, creating four separate images of the same explosion. The images are arranged around an elliptical galaxy in a formation known as an Einstein Cross. Following this discovery, astronomers modeled several possible gas and dark matter distributions in the galaxy cluster. Each model predicted that another image of this supernova will appear in the cluster, but they had different time estimates ranging from 2015 through 2025. In December 2015, it appeared for the first time in history, the time and location of a supernova was accurately predicted. We actually saw the supernova happen. Instead of detecting a flash in the sky and turning telescopes to its location, we had the telescopes already focused on the correct area and recorded the event from beginning to end. This also constitutes a measure of the speed of light without the use of mirrors. Here's another one. Previews of the same supernova appear in this 2016 image left, taken by the Hubble Space Telescope, but they're gone in the 2019 image. The distant supernova named Requim is embedded in a giant galaxy cluster four billion light years away. The cluster gravitationally lens the light from the supernova located in the galaxy far behind it ten billion light years away. It also split the supernova's light into multiple mirror images highlighted by the white circles in the 2016 image. Based on the foreground galaxy's dark matter distribution, researchers predict that a reappearance of the same supernova will happen in 2037. The predicted location of that fourth image is highlighted by the yellow circle at the top left. And like we did with Flickering quasars, time delayed supernova images can also be used to calculate the Hubble constant. In 2017, a Swedish-led team of astronomers used the Hubble Space Telescope to analyze multiple images of a gravitationally lens type 1A supernova. This had never been done before. Here we see the lens galaxy in the middle frame. It's over two billion light years away. The four images of the supernova can be seen in the most frame. It originated over four billion light years away. These four images of the exploding star and the time difference in their light profiles can be used to measure the Hubble constant in a completely different way. Since the light travel times for the various images are unequal, intrinsic variations in the source would be observed at different times in the images. The delay between images is proportional to the difference in the light path lengths through the lens and galaxy space time, which in turn is proportional to one over a Hubble constant. So by measuring red shifts and time delays, and by producing an accurate model for the lens and galaxy, the Hubble constant can be calculated.