Hubble's law

Point a telescope at galaxy after galaxy and measure their light, and a startling pattern appears: almost every galaxy is rushing away from us, and the farther one lies, the faster it flees. Edwin Hubble found this in 1929, and it answers one of the oldest questions we can ask, is the universe static or changing? The honest reading is not that we sit at the center of a cosmic explosion; it is that space itself is stretching, carrying the galaxies apart like raisins in rising dough, so that from any galaxy the view is the same.

The law states that a galaxy's recession speed v is proportional to its distance d, written v = H_0 d, where H_0 is the Hubble constant, the present-day expansion rate, measured at roughly 70 km/s per megaparsec (a megaparsec is about 3.26 million light-years). The proportionality is the fingerprint of uniform expansion: if every distance in the universe grows by the same fractional amount each second, then more distant objects recede proportionally faster, which is exactly what v = H_0 d says. The inverse 1/H_0 sets a rough timescale for the expansion, of order the age of the universe, about 13.8 billion years.

A crucial caveat: the recession is not ordinary motion through space, so for distant galaxies v can formally exceed the speed of light without violating relativity, because it is space between us that is expanding, not the galaxies sliding through it. H_0 is a constant across space today but not across time, and it is genuinely contested: distance-ladder measurements give about 73 while the cosmic microwave background implies about 67, a discrepancy called the Hubble tension that may be pointing at new physics.