Robert McNees
Physicist & astronomer Georges Lemaître was born #OTD in 1894.
In 1927 he proposed a model of an expanding Universe. Then, in 1931, he ran the clock backwards and hypothesized its origin as a “unique atom” or “unique quantum,” paving the way for modern ideas about the Big Bang.
Lemaître and the expanding universe were the subject of a nifty Google doodle back in 2018.
Photo: Bettmann
Gif: The Google Doodle team
Robert McNees
Lemaître became a diocesan priest after serving in WWI. Later, he studied with Eddington at Cambridge, working on cosmology and stellar astronomy, applying insights from the still-new field of relativity. His studies took him to Harvard and MIT.
In 1927 he published a paper in Annales de la Société Scientifique de Bruxelles, claiming that relativity implies an expanding Universe. His proposal assumed a once-static configuration (preferred by Einstein at the time) that suddenly begins expanding.
Robert McNees
Lemaître was not the first to propose an expanding universe. For example, Alexander Friedman’s proposal had appeared five years earlier, in his paper “On the Curvature of Space.”
inspirehep.net/literature/8573
Robert McNees
But Lemaître correctly pointed out that his model would imply an expansion velocity for galaxies — what we now refer to as Hubble's Law.
The idea that other galaxies exist outside our Milky Way had only been confirmed two years prior to this!
Einstein was skeptical of this claim. After all, he'd added a cosmological constant term to the equations of general relativity in 1917 to avoid exactly this sort of behavior.
Robert McNees
The work of Edwin Hubble, Vesto Slipher, Annie Jump Cannon, Milton Humason, and others eventually established a clear relationship between galactic distance and redshift. Between that and scientists like Eddington promoting Lemaître's idea, Einstein came around and agreed that our Universe must be dynamical and expanding.
Robert McNees
Okay, since a few people have asked: When Edwin Hubble presented evidence in 1925 that M31 was a galaxy outside the Milky Way, Betty White was about to turn three years old. The current oldest living person, a woman in France, was 18 or 19.
There are plenty of people alive today who were born into a world where we didn't yet have proof of anything beyond the Milky Way.
Here's a thread! https://mastodon.social/@mcnees/109614690601381175
Robert McNees
In 1931, Lemaître pursued a radical line of thought. He ran the clock of his expanding Universe model backwards and concluded that there must be some sort of beginning.
This would be an extremely dense and energetic phase which he described as "very different from the present order of Nature."
In a short letter published in Nature, Lemaître used the terms “unique quantum" and "unique atom” to refer to this hypothetical early state.
His letter is available here: https://www.nature.com/articles/127706b0.pdf
Robert McNees
Many years later, in 1949, Fred Hoyle would dismissively refer to models of this sort as beginning with a "Big Bang."
Hoyle was a staunch opponent of the idea that the Universe could have a beginning. He was defending his Steady State Theory on the BBC, and criticized “the hypothesis that all matter of the Universe was created in one big bang.”
Nowadays, there are two main ways that the term "Big Bang" is used. Lemaître's original idea shares features with both of them.
Robert McNees
First, the "hot Big Bang" is the idea that if we follow the evolution of the universe backwards, we necessarily arrive at a very hot, dense, and energetic early phase.
Our physical theories can be pushed to very high energies and densities, but they only allow us to extrapolate back so far.
You run the clock forward from there and find that much of what we see in the universe (cosmological expansion, CMB, primordial abundance of elements, etc) follows from that.
Robert McNees
Notice that there is no mention of a “beginning” here. The hot Big Bang appreciates that our physical theories have limits.
The other notion of "Big Bang" is that the Universe has a definite beginning, by which I mean a describable phase before the hot Big Bang.
What this early phase might be isn't clear; we all have our speculations. Most physicists agree that these questions are the province of quantum gravity.
Robert McNees
Lemaître doesn't say anything about quantum gravity. But he does insist that notions of space and time must fail to have meaning during this phase. That is a very sophisticated idea for 1931!
Robert McNees
The last paragraph of Lemaître's 1931 paper is a poetic rumination on determinism in a Universe that has a beginning, and is described by physical laws.
It sings to me.
Robert McNees
"The whole matter of the world may have been present at the beginning, but the story it has to tell may be written step by step."
— Georges Lemaître
alastair
@mcnees this is to ignore the Interpenetrative aspects of the Prime Mover being the cause of all effects as a direct antecedent
Greg Whitehead
@mcnees Veritasium's recent video on entropy had an interesting bit about running the clock back to the Big Bang https://youtu.be/DxL2HoqLbyA
Robert McNees
@Rog Not stupid at all! Ultimately physics is an experimental science, and speculative ideas need to have observable consequences so they can be tested. Physicists spend lots of time trying to work out how to do this for various proposed ideas.
One pretty well accepted idea about this earlier phase is that it involved a period of “Cosmic Inflation.” Inflation predicts statistical patterns for small variations in the cosmic microwave background which we have observed.
Rog
Stupid question maybe, but is the expectation that descriptions of what happened before will be testable?
STEMcoding Project
@mcnees At Ohio State last year we unveiled the first planetarium show on Georges Lemaitre. I totally forgot it was his birthday!
https://planetarium.osu.edu/events/georges-lemaitre-and-discovery-expanding-universe
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