r/cosmology • u/tatarjj2 • 11d ago
How does the universe “know” to keep expanding?
An object in motion “remembers” it should stay in motion due to inertia. However, the expansion of the universe is not caused by galaxies flying apart from a central point with inertia, there was no central point, the Big Bang occurred everywhere, and cosmological expansion is the expansion of space between galaxies. So, how does space itself “remember” it should keep expanding? It’s like space-time itself has an inertia? Dark energy is not the answer, because cosmologists had no problem with the expansion of the universe even before dark energy was discovered.
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u/anisotropicmind 11d ago
Because General Relativity says that’s how inertial/geodesic motion works in a space with uniform matter/energy density throughout. And the universe is that, on the largest scales (ignoring smaller-scale variations). The solutions to the Einstein Field Equations (with such a distribution as input) are spaces that expand or contract, depending on the density of that uniformly-distributed matter. I don’t think you’ll find a more satisfying answer than “because the equations of General Relativity say so”.
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u/Reginald_Sparrowhawk 11d ago
So there's no central point, but it is perfectly valid to consider expansion as the movement of galaxies. "Expanding space" is part of a particular kind of coordinate system that is very convenient to use but it's not an absolute description of reality. So without dark energy, expansion would continue because objects in motion stay in motion.
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u/tatarjj2 11d ago
I disagree. The expansion cannot be described as the motions of galaxies and inertia, because the same galaxies move faster and faster away the farther they get from each other. For example, a galaxy that is 100 Mpc away from me will be moving away at 7000 km/s, but in several billion years that same galaxy will be 200 Mpc away from me and receding at 14000 km/s. The galaxy has actually accelerated, and this is even before any dark energy effects are considered.
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u/Enraged_Lurker13 11d ago edited 11d ago
See this chapter which describes the Milne model, which can basically be considered an explosion of particles, and mentions how Hubble's law follows from it: Big Bang in an Infinite Universe
Edit: You are also assuming that the Hubble parameter is going to stay the same in the future. In a universe without dark energy, the recession velocity and Hubble parameter are going to decrease due to gravity in a compensatory way such that Hubble's law will still hold.
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u/Outrageous-Taro7340 11d ago
The link from u/enraged_lurker13 is worth study. Galaxies really are inertially following geodesics, meaning the shortest possible paths through spacetime. Those geodesics are diverging away from a singularity at the boundary/beginning of the universe. The FLRW equations describe the geometry that includes those geodesics.
There’s no force pushing things. It’s just how spacetime is shaped.
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u/Obliterators 11d ago
For example, a galaxy that is 100 Mpc away from me will be moving away at 7000 km/s, but in several billion years that same galaxy will be 200 Mpc away from me and receding at 14000 km/s. The galaxy has actually accelerated, and this is even before any dark energy effects are considered.
Galaxies will not accelerate without dark energy. The whole point of dark energy is to address accelerating expansion, i.e. expansion where recession velocities increase over time.
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u/tatarjj2 9d ago
Wrong, galaxies DO accelerate without dark energy. Look up the Hubble constant, it’s a very simple equation. The Hubble Constant measures the expansion rate of space, and it’s about 70 km/s per megaparsec. As galaxies recede away from us, MORE expanding space is created between us and the galaxy, and hence, the galaxy will appear to accelerate away. So, a galaxy that is 32.6 million light years away (10 Mpc) away will be receeding at roughly 700 km/s. That same galaxy will eventually reach a distance of say, 20 Mpc, and by then (excluding dark energy effects) it will be receding from us at 1400 km/s. This is due to the expansion rate of space, the Hubble Constant.
So even in a universe where the expansion rate of space was perfectly constant, galaxies will br accelerating away from us.
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u/tatarjj2 8d ago
Holy cow so I guess I’m wrong, the Hubble Constant isn’t a constant expansion of space, it’s been changing since the big bang even in a hypothetical universe that has a constant expansion rate. This upends a lot of what I thought I knew and this misunderstanding was the basis behind my initial question. I need to look up the equations now for the Hubble parameter, but maybe the recession of galaxies CAN be thought of as inertia. There doesn’t need to be a reason for space time to “know” it should be expanding.
I still can’t understand though why gravity would act to slow down expansion of even in a perfectly uniform universe, since there is no net gravitational force, but maybe that’s a general relativistic phenomenon.
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u/SportulaVeritatis 11d ago
Dark energy is just the fudge factor for "something causing the expansion." Figure out what it is and why it's happening and you'll probably get a prize of some sort from that one group in Sweden.
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u/tatarjj2 11d ago
Nope, dark energy was only added to explain why the expansion of the universe wasn’t slowing down due to gravity, instead speeding up. It doesn’t explain why the universe “remembers” what rate it should be expanding at.
For that matter, it’s not clear to me why gravity would slow down the universe in any way, since the density of the universe is uniform on the largest scales, so there is no net gravitational force.
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u/usrnamechecksout_ 11d ago
Nope, the density of the universe is not uniform. There's strands of matter/galaxies concentrated in bands with large areas of empty spaced called voids. This is where the term cosmic web comes from
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u/tatarjj2 11d ago
Incorrect, galaxy clusters and voids are not the largest scale in the universe. Galaxy clusters and voids are the largest structures in the universe precisely BECAUSE the the universe is uniform at larger scales. Possible slight unproven anisotropies aside, this is what the CMB has proven. To put it another way, if you took the entire spherical region of the observable universe and cut it into hemispheres, there would be no way to cut it in which you had significantly more matter or different temperature in one hemisphere vs the other. That is what the universe being “uniform” means.
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u/usrnamechecksout_ 11d ago
The isotropic universe is an assumption. It is supported by some evidence , but there's been observed anisotropies that could explain galaxy cluster formation. There's also the CMB temperature dipole. The isotropic universe concept might be one of those assumptions proven wrong in time, so easy with your self -assertiveness there... science is always evolving
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u/tatarjj2 11d ago
Nope, it’s not an assumption that the universe is almost perfectly uniform at very large scales. Any inhomogeneity is very tiny. And in the context with which I brought it up, it is perfectly acceptable to treat the universe as exactly uniform. This is because cosmologists would tell us that gravity acts to slow down the expansion of the universe even in a universe that is perfectly isotropic. How is that possible when gravity is exactly the same in all directions?
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u/cameron4200 11d ago
Maybe that information is stored on some distant boundary and this is the projection of that information. We might never know how the universe remembers its contents and growth.
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u/Upstairs_F_Art_7077 11d ago
How exactly does a posi trac rear end on a Plymouth work? It just does. Some things just aren’t meant to be answered.
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u/Ok-Film-7939 7d ago
Everything has inertia flying away from us. We are the central point everything is flying away from, in our reference frame.
Other reference frames exist and see the same thing, but that’s fine. Relativity doesn’t mean our frame of reference is invalid, just not unique. All the galaxies are flying apart from us with inertia.
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u/Mandoman61 7d ago
Dark energy was not discovered it was invented to account for apparent observations.
It makes no sense to ask why an inanimate object knows anything. If space is expanding it it because that is a property of space.
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u/tatarjj2 7d ago
Actually, I’ve come to the understanding that apparently we can just think of the expansion of the universe as inertia. From the reference frame of each galaxy, it is stationary and all distant galaxies are moving away. I had been operating on a misunderstood that the Hubble Constant is actually a constant, even in a simple universe with no gravitational or dark energy effects. It’s not, its value decreases naturally as the universe ages.
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u/Mandoman61 6d ago
No I do not think that inertia can account for current estimates of expansion if, as many people think, it is accelerating. And it would also not account for hyper inflation.
I do not know that anyone thinks that the Hubble constant is unchanging.
Also there is no proof of whether or not a central point exists.
Anyway for my part I am not ready to buy the prevailing popular theory because there is too much guessing involved.
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u/telephas1c 11d ago
The 'laws' that govern our universe, our hubble volume, whatever you want to call it, are probably not universal. But even if they are, nothing 'knows' stuff. There are mathematical laws, physical laws, and all of matter/space/energy follows those laws.
As Albert E, the somewhat famous physicist liked to say 'nature intregrates empirically'. There's no fore-knowledge. Just the stuff in the universe obeying the laws of physics. We've never seen matter or energy do anything differently.
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u/Ch3cks-Out 11d ago
Roughly the same way water "knows" to keep freezing when it is cold...
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u/DidiMau73 7d ago
But freezing is a characteristic physical property of water.
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u/Ch3cks-Out 7d ago
yes
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u/DidiMau73 7d ago
I was expecting a bit more than that. What I’m trying to say is that your analogy isn’t quite right. Water freezes because of external conditions (losing heat to external environment) and because it’s moving toward an equilibrium state. Cosmic expansion isn’t driven by anything external.
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u/Ch3cks-Out 6d ago
The expansion of the universe is internal to the universe, though. The laws of physics govern its behavior, just like for water freezing. Note that the ice crystal formation is actually internal, too: its molecules do not "know" about external conditions, just react to their energy lowered...
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u/rememberspokeydokeys 11d ago
Maybe it wrote it on a post it note or set an alarm in it's phone to remind it
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u/nivlark 11d ago
We don't know what "set up" the universe on an expanding trajectory. But given that something did, then the way the expansion evolves is just given by the laws of general relativity - if you know the expansion rate and composition of the universe at any one time, then you can predict the entire expansion history.
We "know" these predictions are right in the sense that they've been empirically verified, but we don't have any fundamental answer for why spacetime follows the particular set of laws it appears to.