who discovered that the universe is expanding

Edwin Hubble is the most widely credited figure for establishing that the universe is expanding, but the discovery was built on earlier work by others, notably Vesto Slipher. Slipher’s measurements of the redshifts of distant galaxies in the 1910s and early 1920s demonstrated that many galaxies are receding from the Milky Way, providing essential evidence for an expanding cosmos. Hubble, using Slipher’s redshift data along with his own distance measurements to galaxies, formulated what is now known as Hubble’s law: the greater the distance to a galaxy, the faster it appears to be moving away. This empirical relationship firmly established the expansion of the universe in observational cosmology. The recognition of who “discovered” the expansion has nuances: Slipher provided the first observational hints, Lundmark presented earlier observational support in 1924, and Lemaître (a Belgian priest and physicist) connected theory to observation in 1927, with Hubble’s 1929 work giving the clearest, widely accepted demonstration and a robust calibration of the expansion rate. In short, Slipher’s redshifts started the thread; Lundmark and Lemaître contributed crucial observational/theoretical steps; and Hubble’s subsequent measurements and publication solidified the expansion paradigm.