We’ve previously celebrated Father Georges-Henri Lemaitre on this very blog, for taking seriously the idea of the Big Bang. His name has come up again in the post expressing thanks for Hubble’s Law — several commenters, including John Farrell, who wrote the book and should know — mentioned that it was actually Lemaitre, not Hubble, who first derived the law. That offered me a chance to haughtily dismiss these folks as being unable to distinguish between a theoretical prediction (Lemaitre was one of the first to understand the equations governing relativistic cosmology) and an observational discovery. But it turns out that Lemaitre did actually look at the data! Shows you how much you should listen to me.
I received an email from Albert Bosma that cleared up the issue a bit. Indeed, it was not just a theoretical prediction (as I had wrongly presumed) — Lemaitre definitely used data to estimate Hubble’s constant in a 1927 paper. He obtained a value of about 625 km/sec/Mpc, not too different from Hubble’s ultimate value. Of course, Lemaitre’s paper was in French, so it might as well be in Martian. Arthur Eddington translated the paper for the Monthly Notices of the Royal Astronomical Society, in 1931, but left out the (one-sentence) discussion of the data!
Here is a slide from Albert containing the original paragraphs. Click for a larger version, and you can compare the French to the English. Thanks to Albert for sending it along.
However — 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.
Further update: Albert has now sent me more snippets from Lemaitre’s original paper (one, two, three), and the English translation. (All jpg images.)
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I hope you have started a tradition with something in physics for which you are thankful, even though a year is a long wait!
Other than our apparent intent to screw up life on the planet, what a lovely neighborhood is 3 Sunnyside Place.
Belated but Happy Thanksgiving to you.
Thanks for posting this. It truly was remarkable how early on Lemaitre started visiting with anyone and everyone he thought might provide him with data pertaining to expansion. He was over in the states on several trips between 1924 and 1927 when he wrote the paper you discuss above.
Well, that does it for me: the Hubble-Lemaitre law.
“Of course, Lemaitre’s paper was in French, so it might as well be in Martian”
It’s a good thing that language wasn’t so much of a drawback for Einstein’s papers though…
I believe German was the de facto language for science at the time Einstein published most of his works.
This actually clears something up for me. As I’ve noted many times while teaching Intro-Astro, it’s straightforward to deduce that universal expansion leads to the Hubble Law, but it is much less obvious to start with the Hubble Law and realize that it implies universal expansion.
Hubble wasn’t that much of a theorist, and I never quite understood how he pulled it off. Having a prediction of the effect before observing it makes much more sense.
I’m not sure that Hubble either deduced it or heard it from somewhere else (although either is quite possible). I think that he never insisted that he had discovered “the expansion of the universe,” only that he had found the linear redshift/distance relation. He left those interpretational leaps to the theorists. (And I think it was first de Sitter who predicted the redshift in an expanding universe.)
Hello,
just a note to say there was a nice discussion of Lemaitre’s early version of Hubble’s law in the magazine Astromony Now last year. If I remember correctly, the article shows that Lemaitre did have a crude velocity vs distance graph, cobbled together from data given to him by Slipher….and it was this early graph that gave rise to his idea of an infinitely hot, dense beginning for the U……Cormac
Hello All,
Yes, Hubble was sceptical that he had discovered the
“expansion of the universe”. I recall a talk given by Grote Reber
in the late 80’s where he mentioned meeting Hubble in the early 1950’s. He said that Hubble was still sceptical that that the velocity – redshift law implied cosmic expansion.
As an aside, I recall reading that Hubble was not the first to look for a velocity – redshift relationship. Lundmark looked for the relationship around 1915 or 1916, but did not find anything because his sample included both galaxies and some of the globular clusters in our galaxy. Before Shapley came up with his model for our galaxy in 1917, some people thought that globular clusters could be extragalactic.
I know it’s nowhere near the point of this post, but all the references to velocity re-enforces what I think is the most confusing aspect of cosmology for laymen like myself. I still see articles talking about galaxies speeding away from each other, and the red shift ascribed to the Doppler effect. Until I understood that the universe is incredibly homogenous, and the big bang was not an explosion radiating out from a central point most aspects of modern cosmology didn’t make sense (e.g. CMB). I wouldn’t be surprised if this has already been dealt with in cosmic variance, but if not you would do your readers a big favor by decoding what the “effective velocity” really is.