How Two Supernovae Helped Measure the Expansion Rate of the Universe ✛ ✦
Light from a distant supernova is like a traffic light’s signal reflected in several storefronts: copies of the explosion reach us with different delays. In space, the 'storefronts' are galaxy clusters: their gravity warps space and creates multiple images of a single explosion. This effect, gravitational lensing, was predicted by Einstein, and Zwicky realized it could be used to study distant objects. Later, Hubble discovered that the universe is expanding, and now astronomers use lenses to measure the rate of that expansion.
Recently, two supernovae that exploded 10 billion years ago were ‘reflected’ by the massive cluster MACS J0138. The Hubble and James Webb telescopes saw several flashes, separated by years. Analyzing the delay with spectroscopy and photometry, and accounting for dark matter and dark energy, scientists calculated the expansion rate of the universe. It turned out to be close to data from the reddening of ancient light, but it diverges from local measurements. This discrepancy is a mystery, hinting that we don’t yet understand everything.
Two different supernovae in one distant galaxy is an incredibly rare event. Their light traveled to us for more than 10 billion years, and the time difference between their images is a full 11 years, allowing astronomers to plan observations in advance.