USAAAO — 2026 First Round

Suppose the Honeyhive galaxy and the Gold Leaf galaxy are diametrically opposite when viewed from the Earth, and the following sequence of events happens: 1. A type Ia supernova happens in the Honeyhive galaxy. 2. Astronomers on Earth measure the redshift of (the spectral lines) in the Honeyhive supernova to be z = 0.5. At the same time, another supernova happens in the Milky Way galaxy. 3. Aliens in the Gold Leaf galaxy measure the redshifts of the supernovae in the Honeyhive and Milky Way galaxies. If the alien astronomers measure the redshift of the Milky Way supernova to be z = 0.8, what redshift would they measure for the Honeyhive supernova? You may assume that the local velocities of the galaxies are negligible, that the universe is simple, homogeneous, and isotropically expanding, and that redshift depends only on the scale factor and is the same function of distance for all observers.