Hubble constant
/ H-naught /
If the universe is expanding, the obvious question is: how fast? The Hubble constant, written H0 and read 'H-naught', is the single number that answers it. It is the present-day rate of cosmic expansion — the proportionality factor in Hubble's law that tells you how much faster a galaxy recedes for every extra unit of distance away it sits.
Its standard unit looks odd at first: kilometres per second per megaparsec (km/s/Mpc), where a megaparsec is about 3.26 million light-years. A value of, say, 70 km/s/Mpc means that for every megaparsec of distance, a galaxy recedes about 70 km/s faster. So a galaxy at 10 Mpc recedes near 700 km/s, at 100 Mpc near 7,000 km/s. Strip away the distances and the unit reduces to one over time, which is why the Hubble constant secretly sets the age and size scale of the whole universe. Despite its name it is not truly constant over cosmic history — the expansion rate has changed as the universe aged — so H0 specifically denotes the value today, and cosmologists call the more general time-varying quantity the Hubble parameter.
Pinning down H0 has been one of the great quests of modern astronomy, because nearly every cosmic distance and the age of the universe hinge on it. For decades estimates ranged wildly from 50 to 100; today they cluster near 67 to 73 km/s/Mpc. But the best methods now disagree at a level larger than their stated errors — the famous Hubble tension — so even this bedrock number carries an honest, unresolved uncertainty.
Take H0 = 70 km/s/Mpc. A galaxy in the Virgo cluster, about 16 Mpc away, should recede at roughly 70 x 16 = 1,120 km/s from expansion alone — close to what is observed, once Virgo's own internal motions are averaged out.
H0 turns a galaxy's distance into its expansion-driven recession speed.
Despite the name, H0 is not a fixed cosmic constant; it is the expansion rate today and has changed over time. Its present value is still debated at the level of the Hubble tension.