 You may recall that we used Redshift to determine a galaxy's receding velocity along with the inverse square law for standard candles like C-feed variables and Type 1A supernova to determine a galaxy's distance. Combined, they gave us Hubble's law and 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 rightmost 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 time delay between images is proportional to the difference in the light path lengths through the lensing galaxy's 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 lensing galaxy, the Hubble constant can be calculated. But measuring the lens properties of a galaxy billions of light years away is difficult. This is where most of the ongoing research is focused.