 We covered the Hubble constant in-depth in the How Far Away Is It? video book. It was developed by Edwin Hubble in the late 1920s with his studies of nearby galaxies. He used the cosmic distance ladder, CVID variables, and type 1A supernova standard candles inside distant galaxies to determine receding velocities. His constant gives us the rate at which the universe is expanding. Today's current value, from distance ladder measurements, is 22.7 km per second. For every million light-years further away, a celestial object is. In 2016, a group of astronomers used the Hubble Space Telescope and others to study five galaxies, gravitationally lensing five quasars, in order to arrive at an independent measurement of the Hubble constant. The strong gravitational lensing creates multiple images of the background quasars. Quasars are incredibly luminous galaxy cores. It is thought that they are massive black holes, actively accreting huge amounts of matter. Some of these quasars flicker. While the relative time between two flickers is correctly represented in this animation, in reality the delays are on the range of two days to two weeks. Because the lens quasar is not perfectly aligned with the lensing galaxy, and the lensing galaxy is not perfectly spherical, the light from the different images of the background quasar follow paths which have different lengths. The delays between them depends on the lengths of the paths the light has taken. It is possible to determine the Hubble constant from the flicker delay time. Here's the lens equation we covered earlier. If we mark the length from the observer to the first flicker, and mark the length from the observer to the second flicker, we create an angle. The distance between these two points will be equal to the light travel time delay multiplied by the speed of light, adjusted for the redshift of the lensing galaxy. Analysis of the lens itself also gives us a measure of the distance based on the lens mass distribution geometry. It's based on an optics principle first discovered by the French mathematician Pierre Fermat in 1662. In fact, Fermat's principle holds in the general theory of relativity where gravitational lenses replace glass. These two distance quantities are responsible for the measured time delay, so they are equal. You can now solve for the Hubble constant. With accurate measurements of the time delays between the multiple images, as well as computer models for the lensing galaxy, the team determined the Hubble constant. It is in agreement with the Type 1A supernova method.