History of Cosmology: 1915-1929 | Einstein, Hubble & GR

Hi, I've recently developed an interest for the history of the development of cosmology and find it very interesting. The key events I have been reading up on are:

1915 - Einstein's theory of General Relativity was published.

1923 - Hubble discovered a Cepheid variable in the Andromeda nebula and was able to conclude there are other galaxies out there.

1927 - The Fifth Solvay Conference on physics. Arguably the greatest meeting of world leading physicists ever. George Lemaitre who recently did his PhD at MIT attended to ask Einstein about work he was doing on his new theory of GR. He used Einstein's theory to show the universe must be expanding. Einstein acknowledged the maths but said something along the lines of 'The physics and conclusion however are abominable" thus rejecting his idea.

1929 - Hubble announced his finding on red shift of galaxies showing evidence for the expansion of the universe.

The confusion I had, that I was hoping someone would clarify, is what the cosmological constant Einstein inserted was actually intended for? Some web sources say he introduced it so his equations represented a universe that wasn't expanding i.e. to halt the expansion. Other sources say that because Einstein thought the universe was static (and therefore has always been static) then the gravitational attraction of matter would cause the universe to collapse in on itself. Therefore the cosmological constant served as a kind of 'anti gravity'. Both of these justifications for including the cosmological constant at the time seem the opposite; one saying its used as 'anti gravity' to stop the collapse of the universe and one to stop the expansion of the universe. They both seem like opposite effects?

Any interesting facts anyone has about the history of cosmology around this time would be greatly appreciated. Thanks.

I don't know much about the history, but I can tell you that the problem with the explanations of the cosmological constant are that it doesn't really fit neatly into a "gravity or anti-gravity" model.

Lemaitre's work showed that if you have a universe everywhere filled with uniform density matter and radiation then it either expands forever, or it expands and then collapses again. There is no way to have a "steady state" where everything remains more or less the same forever - the only "neither expanding nor contracting" solution is the instant between expanding and collapsing phases of the one that eventually collapses. I think Einstein really didn't like the "universe with a beginning" model - and to be fair, it opens complex questions around what "before time existed" can even mean. Not yet knowing the evidence for cosmological redshift, an eternal solution must have been attractive.

So, mathematically at least, you can conjure a static universe into existence, but it will immediately start collapsing. But if you add a cosmological constant into the mix then it doesn't collapse - it stays the same forever. And that "stays the same forever" works backwards as well - the cosmological constant would mean that the universe neither expanded nor contracted, and would be eternal. So it doesn't stop the universe exanding or collapsing so much as allow you to describe a universe which neither expanded nor collapsed in the first place.

The theoretical problem with this is that the cosmological constant has to be very carefully fine tuned to cause this result. Any variation at all from the exact required value gives you back modified versions of the expand-forever or expand-and-collapse models. And the universe is not exactly uniform anyway, so this kind of dancing on a razor edge feels really like replacing "there are awkward philosophical questions about the Big Bang" with "there are awkward philosophical questions about the cosmological constant". And the practical problem with it is that Hubble's work rendered it all academic anyway.

Modern usage of the cosmological constant/dark energy does not presume a static universe so does not require the fine tuning that Einstein needed.

Einstein introduced the cosmological constant term in his 1917 paper as a means to make gravitational interaction vanish at large distances. That its physical interpretation evolved into the tendency of empty space to expand was a somewhat later (but not much later) development.
See e.g. this historical review: https://arxiv.org/abs/1701.07261

As a curious fact, I note that Einstein already hints at the cosmological constant in a footnote of his 1916 paper 'The foundation of the general theory of relativity', as can be seen in the following link:
https://einsteinpapers.press.princeton.edu/vol6-trans/192

Jaime Rudas said:

As a curious fact, I note that Einstein already hints at the cosmological constant in a footnote of his 1916 paper 'The foundation of the general theory of relativity', as can be seen in the following link:
https://einsteinpapers.press.princeton.edu/vol6-trans/192

Yeah, he's being careful to define what he means by "vacuum" there, I think. Mathematically the constant is uncontroversial - it's just its physical interpretation that's interesting. If you are not claiming to be studying a vacuum you can always take the cosmological constant term across to the other side and subsume it in the stress-energy tensor as just another source of gravity (albeit a weird one) and discuss it there. But there he's talking about vacuum, so the stress-energy tensor is zero and he can't defer discussion of the constant by lumping it with the other sources. So he notes he's setting it to zero.

As an aside: history should start with the observation that the milky way was not the entire universe. Other galaxies exist.

mathman said:

As an aside: history should start with the observation that the milky way was not the entire universe. Other galaxies exist.

Agree. Immanuel Kant and Pierre Laplace among several others, contributed significantly to cosmology even as details change and theories evolve. Extracted from the introduction to an online discussion of the nebular hypothesis originally formalized in late 18th Century:

The nebular hypothesis is the most widely accepted model in the field of cosmogony to explain the formation and evolution of the Solar System (as well as other planetary systems). It suggests the Solar System is formed from gas and dust orbiting the Sun which clumped up together to form the planets. The theory was developed by Immanuel Kant and published in his Universal Natural History and Theory of the Heavens (1755) and then modified in 1796 by Pierre Laplace. Originally applied to the Solar System, the process of planetary system formation is now thought to be at work throughout the universe.

Jimmy87 said:

Hi, I've recently developed an interest for the history of the development of cosmology and find it very interesting.

Hi, when I read this I just wanted to mention an interesting person in history in case you haven't heard of him (he is somewhat lesser known, I think): Giordano Bruno.

Wikipedia said:

He is known for his cosmological theories, which conceptually extended to include the then novel Copernican model. He proposed that the stars were distant suns surrounded by their own planets, and he raised the possibility that these planets might foster life of their own, a cosmological position known as cosmic pluralism. He also insisted that the universe is infinite and could have no "center".

Einstein set the CC to a non-zero value to prevent the stars from collapsing in to one another. It was believed at the time that the Universe was in a steady state. The existence of other galaxies was not yet known.

Thanks to all for your insights, it is much appreciated! @Hornbein - yes that's how I interpreted it and that makes sense to set the CC to a non-zero value to prevent the universe collapsing for an eternal static universe. This is what almost all the sources say. Some said that he didn't like an expanding universe so used CC to create a static universe from expanding. These are two different siuations and motives so would require two different CC. That was the angle I was coming from. I think @Ibix explained it elegantly by saying these two situations are just time reverses of each other. I just wondered if Einstein used two CC values; one to stop it collpasing and one to stop it expanding (disprove Lemaitres working on GR).

These professors seem to say it was the latter and this is what got me on to starting this thread.

Jimmy87 said:

I just wondered if Einstein used two CC values; one to stop it collpasing and one to stop it expanding

There aren't two CC values. The Einstein static universe is an unstable equilbrium, like a pencil balanced on its point, between collapsing and expanding. It has exactly one CC value that keeps it at that equilibrium.

Jimmy87 said:

I just wondered if Einstein used two CC values; one to stop it collpasing and one to stop it expanding (disprove Lemaitres working on GR).

No, there's just one value. The cosmological constant is just one "dial" on your model universe that sets one number. Whatever setting you choose applies to the whole history of the model, and what it does is adjust the way the universe expands and contracts. There is one specific setting (the exact value of which depends on how much matter and radiation you plugged into the model universe) that makes it so that the model never expands or contracts, so models a universe that has always existed and always will. It doesn't stop or start expansion or collapse - it just exactly cancels out the effects that would cause expansion or collapse so neither ever happens.

Ibix said:

It doesn't stop or start expansion or collapse - it just exactly cancels out the effects that would cause expansion or collapse so neither ever happens.

This might be a bit misleading.

Strictly speaking, "expansion" or "collapse" by themselves are not what are affected by either ordinary matter or a cosmological constant. What are affected are "accelerations"--rates of change of the rate of expansion or collapse.

At a rough heuristic level, ordinary matter causes expansion to decelerate and collapse to accelerate. A cosmological constant causes expansion to accelerate and collapse to decelerate.

The Einstein static universe is a solution in which the density of ordinary matter and the cosmological constant are exactly balanced so that there is zero acceleration/deceleration. So if such a universe is static at some instant of time, it will be static for all time (we are talking here of an idealized solution where there are no small perturbations or fluctuations).

Note that it is possible for a solution to have such a "zero acceleration" balance at some instant of time and not be static. Indeed, that is what our best current model says happened in our actual universe a few billion years ago: the density of ordinary matter, which decreases as the universe expands, reached a point where, for an instant, it just balanced the cosmological constant. This marked the transition from a matter dominated, decelerating expansion to a cosmological constant (aka dark energy) dominated, accelerating expansion. But in a non-static solution, such a balance can only exist for an instant, since the density of ordinary matter changes with expansion whereas the cosmological constant remains the same.

Ibix said:

I don't know much about the history, but I can tell you that the problem with the explanations of the cosmological constant are that it doesn't really fit neatly into a "gravity or anti-gravity" model.

The cosmological constant perfectly fits into GR. If you think about GR in terms of how an action should look like to derive the field equations from the principle of least action, given that the equivalence principle can be satisfied by assuming a pseudo-Riemannian spacetime manifold, you come to the conclusion that the lowest-dimension Lagrangians in terms of the metric as the fundamental field is the Riemann scalar ##R## and a constant, and indeed ##R## gives Einstein's original GR, and the constant is just another possibility, which cannot be ruled out within this line of thought. So it's natural to include it and check the consequences.

Historically, indeed Einstein introduced the cosmological constant to be able to derive a static universe. Unfortunately (a) the corresponding solution is unstable under small perturbations and (b) contradicts the observation of Hubble-Lemaitre expansion (in terms of the redshifts of radiation from far-distant galaxies as predicted by Lemaitre and observed by Hubble). That's why Einstein famously considered the introduction of the cosmological constant as his "biggest blunder", but I don't think it was a blunder, because it's a natural parameter of GR in the sense argued about above. Since there are no plausible principles, ruling it out, it's just one more parameter to be fixed with observations, and the current status indicates that a substantial part (around 3/4) of the energy content of the universe indeed is "dark energy", the modern interpretation of the cosmological constant, being responsible for the "accelerated expansion" of the universe.

vanhees71 said:

... (b) contradicts the observation of Hubble-Lemaitre expansion (in terms of the redshifts of radiation from far-distant galaxies as predicted by Lemaitre and observed by Hubble).

In this regard, it is worth noting that the de Sitter cosmological model of 1917 predicted the redshift of distant galaxies, and this redshift was observed by Vesto Slipher in 1922.

In fact, the redshifts used by Hubble in his 1929 paper were the ones observed by Slipher. Additionally, in that paper, Hubble attributes that redshift to the de Sitter effect.

I don't think stars ever radiate coherent light. However gas clouds can. Masers occur naturally in space and have been detected. I'm sure lasers also occur though they are more difficult to see.

Jimmy87 said:

1929 - Hubble announced his finding on red shift of galaxies showing evidence for the expansion of the universe.
[...]
Any interesting facts anyone has about the history of cosmology around this time would be greatly appreciated.

Hubble didn't find the redshift of galaxies. Between 1912 and 1922, Vesto Slipher found the vast majority of redshifts that Hubble included in his 1929 paper.
On the other hand, in that article, Hubble doesn't attribute his discovery to the expansion of the universe, but to the De Sitter effect.

Yes. Do remind us of your beef with Hubble again next month.

vanhees71 said:

That's why Einstein famously considered the introduction of the cosmological constant as his "biggest blunder", but I don't think it was a blunder, because it's a natural parameter of GR in the sense argued about above.

Mathematically the CC arises naturally. It would be a blunder to exclude it. However Einstein's game was physics, not so much math. The whole focus is on describing the world as it is, usually by persuading that some math corresponds to the real world. The blunder was to not realize that a static universe was infeasible and his choice of CC was a feeble Bandaid on the problem. It should have been obvious but he had a lot on his mind and millenia of tradition are not easily cast aside, not even for a champion caster-asider such as AE.

Jimmy87 said:

Hi, I've recently developed an interest for the history of the development of cosmology and find it very interesting. The key events I have been reading up on are:

1915 - Einstein's theory of General Relativity was published.

1923 - Hubble discovered a Cepheid variable in the Andromeda nebula and was able to conclude there are other galaxies out there.

1927 - The Fifth Solvay Conference on physics. Arguably the greatest meeting of world leading physicists ever. George Lemaitre who recently did his PhD at MIT attended to ask Einstein about work he was doing on his new theory of GR. He used Einstein's theory to show the universe must be expanding. Einstein acknowledged the maths but said something along the lines of 'The physics and conclusion however are abominable" thus rejecting his idea.

1929 - Hubble announced his finding on red shift of galaxies showing evidence for the expansion of the universe.

The confusion I had, that I was hoping someone would clarify, is what the cosmological constant Einstein inserted was actually intended for? Some web sources say he introduced it so his equations represented a universe that wasn't expanding i.e. to halt the expansion. Other sources say that because Einstein thought the universe was static (and therefore has always been static) then the gravitational attraction of matter would cause the universe to collapse in on itself. Therefore the cosmological constant served as a kind of 'anti gravity'. Both of these justifications for including the cosmological constant at the time seem the opposite; one saying its used as 'anti gravity' to stop the collapse of the universe and one to stop the expansion of the universe. They both seem like opposite effects?

Any interesting facts anyone has about the history of cosmology around this time would be greatly appreciated. Thanks.

There is an interview with Georges Lemaître, explained here.
I think it is on pf somewhere, perhaps in TIL.
It is in French but there is a transcript in English mentioned in the video.
Lost till 2022
Some of the other big players along the way if you are interested in the history.
Bear in mind this is history rather than robust science, BB “explosion” “from a small point” not correct for instance.