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Was the universe uniform prior to expansion? (self.askscience)
submitted 3d ago by Das_Guet
I know I'm going to flub this question but...
If the universe's expansion points to a moment in the past where it existed as a singularity (or something close to it anyway) such a thing would, in theory, be uniform. If that is the case, how would areas of higher and lower gravity be able to form?
Edit: it's been about a day since I posted this and I want to thank everyone who commented, and I do read all of them, and helped me understand where I was either missing something or misinterpreting something about this particular question.
If the universe's expansion points to a moment in the past where it existed as a singularity (or something close to it anyway) such a thing would, in theory, be uniform. If that is the case, how would areas of higher and lower gravity be able to form?
Edit: it's been about a day since I posted this and I want to thank everyone who commented, and I do read all of them, and helped me understand where I was either missing something or misinterpreting something about this particular question.
Putinator 43 points 3d ago
No. There are *extremely small* quantum fluctuations in density/temperature, which were enhanced during/by inflation. Basically, there is very small randomness due to quantum mechanics, and the density distribution during inflation was essentially 'frozen in.'
Side note because the terminology is confusion, there are two separate similar terms: **expansion** and **inflation**. Colloquially they have similar meanings, but in the context of cosmology they are different.
* Expansion just refers to, well, expansion. We can measure it in various ways, and by fitting data to models have a pretty good idea of the expansion rate going back to just after inflation ended.
* [Inflation](https://en.wikipedia.org/wiki/Inflation_(cosmology)) is a period of extremely rapid expansion just after the Big Bang. In a fraction of a second (much shorter than any timescale we can measure) the Universe multiplied in scale many times. This was initially devised as a mechanism to resolve several issues unexplained by the standard cosmological model, including the fact that the early Universe (even after inflation) is very uniform -- this is referred to as the 'horizon problem'. There are some observables predicted that we can use to test inflationary theories, but they are tough and current results are inconclusive.
Side note because the terminology is confusion, there are two separate similar terms: **expansion** and **inflation**. Colloquially they have similar meanings, but in the context of cosmology they are different.
* Expansion just refers to, well, expansion. We can measure it in various ways, and by fitting data to models have a pretty good idea of the expansion rate going back to just after inflation ended.
* [Inflation](https://en.wikipedia.org/wiki/Inflation_(cosmology)) is a period of extremely rapid expansion just after the Big Bang. In a fraction of a second (much shorter than any timescale we can measure) the Universe multiplied in scale many times. This was initially devised as a mechanism to resolve several issues unexplained by the standard cosmological model, including the fact that the early Universe (even after inflation) is very uniform -- this is referred to as the 'horizon problem'. There are some observables predicted that we can use to test inflationary theories, but they are tough and current results are inconclusive.
nicuramar 5 points 3d ago
By the way, I don’t think we have any evidence that can put an upper limit on how long inflation lasted? (Or if it happened at all?)
elwebst 6 points 3d ago
Could expansion just be inflation that slowed down naturally, meaning the same mechanism?
Jew-fro-Jon 15 points 3d ago
The effects are dramatically different in order of magnitude. Maybe 10^100 different, give or take a few zeroes in the exponent.
Inflation happened very fast at very small scale.
Expansion happens proportionally to the distance. So small scale it is basically nothing, but at large scales it can be faster than light.
Inflation happened very fast at very small scale.
Expansion happens proportionally to the distance. So small scale it is basically nothing, but at large scales it can be faster than light.
Old-Fishing1199 3 points 3d ago
Thank you so much I never knew the difference!
xxDankerstein 9 points 3d ago
The slightest possible variation from uniform symmetricality, and I mean literally the smallest possible variance, would cause a chain reaction that would lead to massive variance. This could be as simple as the wavelength of the smallest unit of energy not matching up perfectly to 360 degrees (as in if you laid out all of the smallest packets of energy together in a 360 degree array in a circle, there could be some overlap between the "particles").
quantum_splicer 6 points 3d ago
So you have small imperfections in the early universe that when you scale them with expansion it scales up the imperfections.
Also you get areas where gravity can be stronger because there is simply more mass and more mass creates more gravity , which draws more mass.
At the beginning of the universe you had gigantic starts (10,000 times bigger than the sun in mads) that burned out quickly and created blackholes ; these blackholes would have been attracted to each other as well and they of been gravity sinks where hydrogen and interstellar dust would be attracted to these blackholes forming galaxies.
When you observe parts of our observable universe , you see that actually it's similar in every direction.
To answer your gravity question; this link may help https://www.universetoday.com/113150/what-is-the-great-attractor/
Also you get areas where gravity can be stronger because there is simply more mass and more mass creates more gravity , which draws more mass.
At the beginning of the universe you had gigantic starts (10,000 times bigger than the sun in mads) that burned out quickly and created blackholes ; these blackholes would have been attracted to each other as well and they of been gravity sinks where hydrogen and interstellar dust would be attracted to these blackholes forming galaxies.
When you observe parts of our observable universe , you see that actually it's similar in every direction.
To answer your gravity question; this link may help https://www.universetoday.com/113150/what-is-the-great-attractor/
gerryflint 3 points 3d ago
The galaxies formed where dark matter already gathered beforehand. It's not simply where more baryons were because they could not agglutinate in the early universe due to the high energy photons. Also the velocity of galaxies is too low to explain their formations on a bigger scale. Filaments and voids can only exist because the gravitational seed already was there in the early universe long before stars - in the form of dark matter.
Baron_Bosc 1 points 3d ago
Not a physicist, but I think I have something to say about this.
The Cosmic Microwave Background (CMB) shows that the universe was not entirely uniform... at least not at that point in its history. The high and low temperature variations the CMB reveals speak to the eventual evolution of galaxy clusters we see today. What caused the temperature variation is not well understood (from what I can tell), though I believe that quantum fluctuations in the very, very early universe could tell a part of the story.
How that relates to inflation (if it happened), dark matter, primordial black holes (if they exist), or any other of the myriad of mysteries surrounding the origin of the universe is still an open question.
Actual physicists, did I get that about right?
The Cosmic Microwave Background (CMB) shows that the universe was not entirely uniform... at least not at that point in its history. The high and low temperature variations the CMB reveals speak to the eventual evolution of galaxy clusters we see today. What caused the temperature variation is not well understood (from what I can tell), though I believe that quantum fluctuations in the very, very early universe could tell a part of the story.
How that relates to inflation (if it happened), dark matter, primordial black holes (if they exist), or any other of the myriad of mysteries surrounding the origin of the universe is still an open question.
Actual physicists, did I get that about right?
ObligatoryOption 1 points 3d ago
I don't see how a "uniform" universe could be said to exist at all. If there is no difference of any kind between here and there, which would imply no difference at all between then and now (since it's all uniform) then how can anything be said to exist? We know things exist because they are different: my foot is something different from the ground. If my foot and the ground were the same thing then would there be a foot at all? Would there be a ground? For something to exist, it has to be distinguishable from its surroundings, otherwise there is nothing there but the surrounding. And if the entire universe has no distinguishable part then no part of it can be said to exist. And then of course, since the universe is defined as "all that exists" but nothing exists within it then such a universe cannot exist either.
cronedog 5 points 3d ago
Why do you think existence ceases with homogeneity? If you have a box with only electrons in it, all electrons are the same....do they stop existing somehow?
ObligatoryOption 3 points 3d ago
> Why do you think existence ceases with homogeneity?
That's not quite what I said. I said a homogeneous universe cannot exist in the first place, for the reasons I provided.
> If you have a box with only electrons in it, all electrons are the same....do they stop existing somehow?
No, because the electrons are different from the non-electron part of the universe (the vacuum in between them) so you can tell they exist from that difference, as well as from the distinct positions they occupy within the box. The box also exists since it is neither electron not vacuum, so all these things exist distinctly, so this is not a homogeneous universe.
That's not quite what I said. I said a homogeneous universe cannot exist in the first place, for the reasons I provided.
> If you have a box with only electrons in it, all electrons are the same....do they stop existing somehow?
No, because the electrons are different from the non-electron part of the universe (the vacuum in between them) so you can tell they exist from that difference, as well as from the distinct positions they occupy within the box. The box also exists since it is neither electron not vacuum, so all these things exist distinctly, so this is not a homogeneous universe.
quantum_splicer 1 points 3d ago
I think semantically what your saying is logical and make senses.
On a theoretical point : if the universe is uniform In the totality (is uniform now) ; that means nowhere in the universe has any distinguishable properties. By implication there could be no question of whether the universe exists because there would no one to question it.
Also following on your point pragmatically ; blackholes , they have three properties ; mass , spin , charge. ; If two blackholes share exactly the same properties they are the same and indistinguishable .
On a theoretical point : if the universe is uniform In the totality (is uniform now) ; that means nowhere in the universe has any distinguishable properties. By implication there could be no question of whether the universe exists because there would no one to question it.
Also following on your point pragmatically ; blackholes , they have three properties ; mass , spin , charge. ; If two blackholes share exactly the same properties they are the same and indistinguishable .
ObligatoryOption 2 points 3d ago
> If two blackholes share exactly the same properties they are the same and indistinguishable.
You can distinguish them from their distinct positions, so this is not a homogeneous universe.
Edit: also of course, there is a difference between what is a black hole and the rest, so again, not homogeneous. Homogeneous means wherever you look, you cannot distinguish any difference, any feature. If you can spot anything different then it's not homogeneous.
You can distinguish them from their distinct positions, so this is not a homogeneous universe.
Edit: also of course, there is a difference between what is a black hole and the rest, so again, not homogeneous. Homogeneous means wherever you look, you cannot distinguish any difference, any feature. If you can spot anything different then it's not homogeneous.
quantum_splicer 2 points 3d ago
Agreed on that point but those aren't features inherent to the blackholes themselves ; but attributes to the specific space the blackholes occupy.
But I agree on the homogeneous point ; way we had a hypothetical universe B ; that due to slightly different physics ; early in its evolution it was dominated by blackholes that merged early on to make a blackhole the size of universe B that would be homogeneous.
But I guess there is a fundamental question; blackholes have an event horizon ; where we cannot see past. our current understanding of black holes beyond the event horizon are contradictory and inconsistent ; where physicists keep proposing numerous theories to resolve our incomplete understanding
But I agree on the homogeneous point ; way we had a hypothetical universe B ; that due to slightly different physics ; early in its evolution it was dominated by blackholes that merged early on to make a blackhole the size of universe B that would be homogeneous.
But I guess there is a fundamental question; blackholes have an event horizon ; where we cannot see past. our current understanding of black holes beyond the event horizon are contradictory and inconsistent ; where physicists keep proposing numerous theories to resolve our incomplete understanding
ObligatoryOption 2 points 3d ago
Keep in mind that the topic OP raised is about the universe before its expansion, it's not about black holes, not about any particular part of it, but about the whole thing. Could the universe have been homogeneous? This is the question I am addressing.
Had the universe been homogeneous, we could not say that it even existed, and it could not have given rise to today's non-homogeneous universe because that would have required forces to apply here but not there, which would have required a cause that applied here but not there, which would refute the premise that it was homogeneous. Spontaneous quantum fluctuations? Ok, but then the nature of quantum events cannot be homogeneous (whatever its innate nature may be), and if quantum effects are real then they are part of the universe (all that exists) which is therefore not homogeneous. I don't see any way around it.
Had the universe been homogeneous, we could not say that it even existed, and it could not have given rise to today's non-homogeneous universe because that would have required forces to apply here but not there, which would have required a cause that applied here but not there, which would refute the premise that it was homogeneous. Spontaneous quantum fluctuations? Ok, but then the nature of quantum events cannot be homogeneous (whatever its innate nature may be), and if quantum effects are real then they are part of the universe (all that exists) which is therefore not homogeneous. I don't see any way around it.
florinandrei -4 points 3d ago
"Singularity" actually means: this is where the equations crash like a video game having a bug, and our math doesn't really work there anymore, so we just call it some word to label it that way. It should not be used as a synonym for "small point". That wrong usage is due to poor explanations of science on social media.
If you want to say "small point", then say small point.
Anyway, there was no universe before the Big Bang, so the question is not meaningful. Something that doesn't exist, is it uniform? It's just a bunch of words.
If you want to say "small point", then say small point.
Anyway, there was no universe before the Big Bang, so the question is not meaningful. Something that doesn't exist, is it uniform? It's just a bunch of words.
Tryingsoveryhard 10 points 3d ago
“There was no universe before the big bang”. This is not known or implied by the standard model, and in fact by any accepted theory. There is a fair bit of evidence to the contrary however. When you scorn people for being wrong, be more careful to be right.
ElderWandOwner 2 points 3d ago
>Anyway, there was no universe before the Big Bang
I don't think this statement is accurate. There are plenty of physicists who believe there was a universe before the big bang.
I don't think this statement is accurate. There are plenty of physicists who believe there was a universe before the big bang.
TheBasementGames 3 points 3d ago
How many believe that the current matter has always existed, though? An overwhelming majority accept that the universe has existed for a finite amount of time. Before that? Nothing, or else, what?
LacedVelcro 6 points 3d ago
The correct answer is "I don't know", followed by "it's not clear what the word 'before' means in this context". Not "There definitely wasn't a universe there."
Could have been a universe-sized black hole that reached some sort of threshold that resulted in it reversing, forming a white hole that became our universe. Maybe we're still inside that universe-sized black hole.......
Could have been a universe-sized black hole that reached some sort of threshold that resulted in it reversing, forming a white hole that became our universe. Maybe we're still inside that universe-sized black hole.......
GruntLife0369 0 points 2d ago
Not to knock anyone but its comical that we think people existing in this moment actually think they know. Sure we have telescopes and rays and simulated models (filled with data that we've only had for decades), but its all theory. Noone knows, which is why its so great to hear the ideas people have.
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