Hubble's law and the expanding universe

Examine the light from distant galaxies and, with only rare nearby exceptions, its characteristic spectral lines are shifted toward longer (redder) wavelengths compared with the same lines measured in a laboratory on Earth — a redshift, closely related to the Doppler shift familiar from sound and from light emitted by objects receding through space. In the 1920s, Edwin Hubble measured both the redshift and the distance of a number of galaxies and found a striking relationship: redshift grows directly in proportion to distance, a relationship now called Hubble's law. A galaxy twice as far away shows roughly twice the redshift, a galaxy ten times as far away shows roughly ten times the redshift, and so on, essentially without exception for sufficiently distant galaxies.

The straightforward reading of this pattern — that every distant galaxy is physically flying away from us through static, unchanging space, faster the farther away it already is — raises an immediate puzzle: why would we happen to be sitting at the one special centre that everything is fleeing from? The resolution is that this reading is not the correct physical picture. Instead, general relativity describes the redshift-distance relationship as arising from the expansion of space itself: the physical distance between any two sufficiently separated points in the universe grows over time, not because either point is moving through space in the ordinary sense, but because the space between them is stretching. A photon of light travelling across this stretching space has its wavelength stretched right along with it, redshifted by an amount that grows with how much space it has crossed — and since more distant galaxies have more expanding space between them and us, their light is stretched, and therefore redshifted, correspondingly more, reproducing Hubble's distance-proportional relationship directly.

This "expanding space" picture resolves the apparent puzzle completely: because the expansion happens uniformly everywhere, not outward from any particular point, every observer anywhere in the universe — not just an observer on Earth — sees every sufficiently distant galaxy receding from them in exactly the same distance-proportional way, with no special centre or privileged vantage point required. Hubble's law, and the broader expansion it reveals, is the central observational pillar supporting the Big Bang model of cosmology: running the observed expansion backward in time implies the universe was once far smaller and denser than it is today, a conclusion later corroborated independently by the discovery of the cosmic microwave background radiation, the cooled relic of the early universe's own intense heat.