Less well known is that Hubble might not have been the first the person to make this momentous discovery.
A Belgian priest and cosmologist named Georges Lemaitre published a paper reaching very similar conclusions two years earlier. It's a contentious issue among cosmologists, needless to say.
The problem was, Lemaitre's paper was in French, and appeared in a rather obscure journal: Annals of the Brussels Scientific Society. This limited its distribution throughout the scientific community (at least initially).
Yet even when his paper was finally translated and broadly disseminated, certain key elements went missing, sparking rumors that prominent scientists - Sir Arthur Eddington, perhaps, or even Hubble himself - had deliberately "censored" Lemaitre's paper to ensure Hubble's scientific legacy.
What happened? The answer might lie in a new article in Nature by cosmologist and author Mario Livio.
It's a long, complicated story, but here's the CliffsNotes version...
In the late 1920s, astronomer Edwin Hubble was studying distant galaxies at the Carnegie Observatories in Pasadena, home of the spanking new 100-inch Hooker telescope on Mount Wilson.
He measured the brightness of so-called Cepheid variable stars - a type of periodically pulsing star - based on the "Period-Luminosity Relation" discovered by Henrietta Swan Leavitt. Basically, if you know how long it takes for the star to go from bright to dim, this will tell you how bright it actually is. And once you know that, you have a means of measuring distance.
So Hubble was able to deduce the relative distance of the galaxies. He combined those observations with data collected in 1912 by Vesto Slipher. Slipher is usually credited with being the first to notice that the light the galaxies emitted had a pronounced "shift" toward the red end of the electromagnetic spectrum, indicating that they were moving away from earth.
Next Hubble plotted the velocity (indicated by the redshift) against relative distance, to get the graph at the top of this post. To a casual observer, it might seem like a random number of points scattered about, with some clustering hinting at a possible pattern.
But Hubble wasn't a casual observer, he was a frickin' genius. He looked at that graph and drew a straight line through all those data points. As telescope resolutions improved over the ensuing decades, Hubble's half-intuitive leap proved correct. Plot the same data today, and the points will fall neatly along the line Hubble drew.
In mathematical terms, that straight line indicates a linear function. That is, the redshift of distant galaxies increased as a linear function of their distance. Hubble reasoned (correctly) that the longer the light has been traveling, the more time there has been for space to expand, and hence the greater the red shift of the light's wavelength.
So he proposed a law: the greater the distance between any two galaxies, the greater their relative speed of separation. Based on that law, he arrived at an inescapable conclusion: the cosmos was still expanding. And that, of course, changed everything in the field of cosmology.
Now back to Lemaitre.
The academic quibbling usually hinges on whether Lemaitre fully derived Hubble's law on his own from actual observational data, or limited his analysis to theoretical predictions. Lemaitre did rely on data, it turns out - the same redshift data from Slipher's observations, combined with estimates of galaxy distances inferred from Hubble's own observations, published in 1926. And he also correctly concluded that this meant the universe was expanding, not static.
Sean Carroll wrote about this over at Cosmic Variance back in 2007:
Lemaitre didn't have very good data (and what he did was partly from Hubble, I gather). And for whatever reason, he did not plot velocity vs. distance. Instead, he seems to have taken the average velocity (which was known since the work of Vesto Slipher to be nonzero) and divided by some estimated average distance! If Hubble's Law - the linear relation between velocity and distance - is true, that will correctly get you Hubble's constant, but it's definitely not enough to establish Hubble's Law. If you have derived the law theoretically from the principles of general relativity applied to an expanding universe, and are convinced you are correct, maybe all you care about is fixing the value of the one free parameter in your model. But I think it's still correct to say that credit for Hubble's Law goes to Hubble - although it's equally correct to remind people of the crucial role that Lemaitre played in the development of modern cosmology.