Spectroscopy, which can identify objects via unique combinations of so-called “spectral lines,” can also be used to measure velocity, thanks to the Doppler effect. Leavitt used the “identification” properties of spectroscopy to find her law, but to find his, Hubble needed to focus on velocity measurements as well. He made extensive use of the measurements of Vesto Slipher, who was the first astronomer to measure Doppler shifts to higher velocity (known as “redshifts”) for distance galaxies, and to realize that meant the galaxies were moving away from us.
Thus the stage was set for the revolution that Hubble’s Law would effectively begin. Astronomers were still arguing about the true size of the Universe, and most, including Einstein, wanted it to be static. But, then came Hubble’s 1929 result, showing a “linear” trend for galaxy velocity to increase with the separation of that galaxy from our own. The simplest explanation for galaxies moving faster and faster the farther apart they are is an expanding Universe--one where space and time are connected, and space itself expands as time moves forward. No single galaxy, or place, is the “center” of the expansion, as Hubble’s Law would look the same as viewed from any galaxy.
Raisin Cake Video: https://youtu.be/-xrg8Ikanf8
A traditional analogy used to describe the motions of galaxies in an expanding Universe is to a raisin in a baking cake. This is a great analogy in that each raisin moves, on average, away from all the other raisins as the cake bakes and expands. BUT, this is a terrible analogy when it comes to the two questions we are trying to address here:
If the Universe had a beginning, what was here before the big bang?
If space is expanding, what’s it expanding into? what lies beyond?
“Before” and “beyond” have meaning for a baking cake that exists in a 3-D pan in space, and that changes in time, where time as a separate dimension from space. But “before” and “beyond” do not have analogous meanings in 4D space-time that is constantly expanding in both space and time.
To really understand the Universal expansion Hubble’s 1929 Law implied that it took very open minds--ones able to think in abstract mathematical terms about 4D space time. Eventually, nearly all 20th-century astronomers came to grips with a huge, and expanding Universe.