Vesto Slipher’s observations of spiral nebulae in the early 20th century led him to conclude that most spirals, with the exception of a few (such as Andromeda), were receding from Earth. Edwin Hubble confirmed that the spiral nebulae were actually separate galaxies in 1925 when he discovered Cepheid variable stars in Andromeda. These pulsating stars obey a period-luminosity law—essentially, the longer the period of light variation, the brighter the star—which makes it possible to determine their distances. Hubble found that Andromeda was much too distant to be part of our galaxy, confirming what other astronomers previously suspected: the spiral nebulae were actually separate galaxies.
Using data from Slipher and others, Georges Lemaître tentatively determined in 1927 that the more distant the galaxy, the faster it’s speeding away from us. However, he published his paper in French, and it wasn’t widely seen. Hubble independently (and with better data) published the same relation in 1929, and he’s usually given the credit for discovering what’s now called Hubble’s Law. He expressed this law in the equation v = Hod, where v is the galaxy’s speed in kilometers per second, and d is the galaxy’s distance from Earth in megaparsecs, where 1 megaparsec is 3.25 million light-years.
Ho is a constant with the units kilometers per second per megaparsec. That mouthful means that, for every megaparsec of distance between Earth and a galaxy, that galaxy’s speed in km/s increases by a constant amount. The speed is not actually the galaxy’s speed through space, but rather a measure of the expansion of space itself.
Determining the exact value of Ho has proven difficult, largely because of uncertainties in measuring a galaxy’s distance. Hubble initially assigned a value of 500 km/s/Mpc, due to errors in his distance calculations, but astronomers soon narrowed it down to some value between 50 and 100 km/s/Mpc.
Currently astronomers have two conflicting values for Ho. An intensive project, using the Hubble Space Telescope to determine distances to far-flung galaxies by observing Cepheid variable stars, sets the value at 73.8 +/- 2.4 km/s/Mpc. But this value disagrees slightly with the value based on the Planck spacecraft’s measurements of the cosmic microwave background (CMB): 67.80 +/- 0.77. Astronomers don’t know yet why the discrepancy exists.