Recedra

Meet Recedra, a galaxy drifting away from us, and from everyone else, too.

In the 1920s astronomers found that the light of distant galaxies is redshifted, and that the shift grows with distance. Georges Lemaître (1927) and Edwin Hubble (1929) linked the two: on large scales a galaxy's recession speed v is proportional to its distance d, with the constant H₀. The natural explanation is that space itself is expanding, carrying galaxies apart.

H₀ is usually quoted in kilometres per second per megaparsec. A value near 70 means a galaxy 100 megaparsecs away, about 326 million light-years, recedes at roughly 7,000 km/s. One over H₀ gives a rough timescale for the expansion, on the order of 14 billion years.

Every observer in a uniformly expanding universe sees the same pattern, so we are not at a centre, and neither is Recedra.

Open question: measurements disagree. The cosmic microwave background implies about 67 km/s/Mpc, while nearby distance-ladder measurements give about 73. This 'Hubble tension' is unresolved. Nearby galaxies also have their own motions, so the law holds only on large scales.