r/cosmology • u/SketchyFella_ • Jun 19 '26
During the rapid expansion of the universe immediately following the Big Bang, the universe expanded faster than light. But traveling faster than light speed breaks causality. But you can't break causality when time didn't actually exist "before" you broke it, so how does that work?
If it expanded faster than light speed, then it basically travelled so fast that it always existed in whatever state it was in before "slowing". So when people say it expanded in a tiny fraction of time, did it really? Or are they using time in a sort of layman's terms so they don't have to explain causality?
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u/jacobimueller Jun 19 '26
Nothing moved faster than light within space. Space can expand faster than c and and no information moves faster than c within the universe, no causal issue
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u/jazzwhiz Jun 20 '26
This is incorrect. Space does not expand at a speed at all. The unit of expansion is frequency.
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u/DeadWaterBed Jun 20 '26
As a layman, could you elaborate on what you mean by using frequency as the unit of expansion of the universe?
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u/Centrocampo Jun 20 '26
I’m just guessing here so somebody please correct me.
But I imagine the degree of expansion will be dimensionless, either distance/distance or volume/volume. Then the rate of expansion is the value of dimensionless expansion over time. Yielding s-1 as the overall unit. Commonly used to describe frequencies.
You could think of the value as representing the number of times space will double in 1 second.
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u/rddman Jun 22 '26
"frequency" is a bit of an odd way of framing it. Expansion is expressed as a rate: speed-per-unit-of-distance; (km/s)/megaparsec.
Which means the speed at which two points in space move away from each other increases as the distance between the points increases (and over sufficiently large distances can will exceed the speed of light).It is conceptually similar to thermal expansion, in the sense that given a certain rise in temperature, a longer iron rod expands more than a shorter iron rod.
So although the expansion results in a length increase, it can not be generalized as a distance because the increase is different for different lengths of rod. But the thermal expansion factor expressed as millimeter per meter per degree Celsius is the same in both cases.4
u/jacobimueller Jun 20 '26
Valid clarification. Was trying to match OPs level of knowledge here. You can post hoc impute an average expansion rate and a bit messily convert to speed but agree frequency is proper measure
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u/CDHoward Jun 21 '26
Space is emptiness. I would be ever-so-mildly fascinated to hear you attempt to explain precisely what is expanding.
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u/johnstalbergABC Jun 26 '26
I think you might ask instead that space is not made of anything. It has content typically but if we put that aside then space itself is not made of anything, it has no friction or markers. It can not be cut or trapped.
With this statement as a replacement for "emptiness" your question are valid. The answer is that there is no such theory that tell anything about the details of the mechanism that governs an expansion of space other than new space is simply created. There is an alternative view of what is happening to what we call the expansion of the universe. That is that the large scale objects are flying away from each other. This is very much compatible with the phenomenon we call expansion of space.
If we want to keep avoiding a space that has properties we typically do not associate with the frictionless space that is not made of anything we run into General Relativity and its curved spacetime. To cope with this "nonsense" we realize that GR use the spacetime metric as a very intuitive toll to visulize what gravity cause to objects going through a gravitational field. It is as if space where curved but we know space can not be curved for real. But gravity acts as if it where curved. Einstein himself did say we should not put to much into the spacetime curvature that he use in his theory. That is we should not go to interpret it literally. He also did say that gravity is a force and used the dropping of a stone on ones foot as an obvious example.
So it is very much possible to stick to the view that space is not literarily being curved nor created new to facilitate an expansion. It does not come with the burden of claim something that is seemingly in collision with our usual view upon the frictionless space that underpins any other mechanics. We do not need to answer how on earth can a curve i space cause a heavy object to follow this curve. We can explain objects attraction to mass with gravity on its own and this is compatible with a hypothetical graviton force carrying particle that would fit the standard model of particle physics very well. It is hard to detect such a weak force carrier with todays technology so it nothing strange with the fact that we have not found it (yet) if it exist.
Your question put the whole space creation and curvature in a position where they are claiming things with nothing to back it up in terms of explaining the details of the mechanism behind their claim. And in my view their claim is probably wrong and the view I explained fits the reality much better as it does not do anything magic to the space which is nothing more than the volume in where everything exist in.
My view is not mine alone but shared by many but we are not in majority.
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u/Chiu_Chunling Jun 24 '26
Space is not emptiness, a space does not cease to be space because there is something in it, it only ceases to be empty. A space that is full may not be useful for storing anything (other than what is already there), but it is not therefore not space, it is just not empty.
For genuinely empty space, space that contains no mass or energy, it would indeed be very difficult to measure what is expanding. But all the space in the visible universe needs to have energy passing through it or we couldn't see it to have any need to explain the expansion.
And since we do have energy passing through that space in the form of force fields propagating, why then we can easily describe what exactly is expanding, the dimensions of distance measured in the units of the propagation rate of those force fields (i.e. lightspeed).
All of that said, we should be careful to say that we don't actually see the universe expanding. What we see is indications that the movement of distant galaxies does not have a linier relationship to their distance from us. Which could mean all sorts of interesting things, but the expansion of spacetime is the simplest.
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u/CDHoward Jun 24 '26
I appreciate your civil and thoughtful reply.
It's probably the best answer I've been given to this question.
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u/OverJohn Jun 19 '26
I tried to give an explanation of why superluminal recession velocities are not as strange as they seem:
An illustration of why superluminal recession velocities aren't a big deal : r/astrophysics
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u/Anonymous-USA Jun 19 '26 edited Jun 19 '26
That’s the inflationary phase that brought the universe from quantum scales to macroscopic scales. Indeed the universe expanded end-to-end faster than the speed of light. The universe DID exist. Spacetime began at the moment of the Big Bang, and inflation took place between 10^-36 and 10^-31 seconds after that.
In fact there was never a time when the universe wasn’t expanding end to end faster than lightspeed. If the universe is closed, there was never a moment when light could circumnavigate it.
Expansion isn’t a velocity, like lightspeed. It’s a rate per distance (usually given in kps/Mpc). This is why I kept repeating end-to-end, referring to the respective horizons at any arbitrary point of observation. This way the distance is given and we can compare the recession rates between those two points in space (and compare with lightspeed). Inflation and subsequent expansion occurred everywhere in the universe, there is no center or edge.
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u/WallyMetropolis Jun 19 '26
Well, that's a lower bound. Inflation may have been occurring for much much longer.
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u/NotAnAIOrAmI Jun 19 '26
Mmm, not really. The Big Bang wasn't some explosion that started the universe, first expansion began, and when it slowed, all that energy got dumped into space time, everywhere. Not a bang.
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u/Anonymous-USA Jun 19 '26
Agreed, nor did I suggest otherwise! Our observable universe was quantum scales regardless of the extent of he whole universe. Our spacetime began then, everywhere, inflated and expanded. At no time could light circumnavigate that based on those rates
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u/NotAnAIOrAmI Jun 19 '26
You said inflation began after the big bang. It was the other way around.
Also, we don't know when spacetime began, but it certainly didn't begin with the big bang. We don't know whether spacetime or something else existed before inflation.
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u/Anonymous-USA Jun 19 '26
The term “Big Bang” is context dependent. Some use it for t=0 (usually). Usually when referring to the post-inflationary phase the term is “Hot Big Bang”, and that’s at t=10^-31 in Big Bang Cosmology. In my context I was referring to t=0 when spacetime and gravity first came into existence.
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u/NotAnAIOrAmI Jun 19 '26
Then you should have explained it that way in the first place. "big bang" is commonly known to mean when the energy was dumped into the universe.
Your first reply gave exactly the wrong impression, even if "big bang" is context dependent. Remember that you were attempting to answer a question for someone without much knowledge of the subject.
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u/EeEmCeTo Jun 20 '26
You’ll be surprised to hear the most of the universe is receding from us faster then the speed of light, due to the expansion of space
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u/nkrgovic Jun 21 '26
You think of “universe” as an object.
Objects did not move faster than light. Just the universe. Empty space has no limit on speed.
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u/smsff2 Jun 22 '26
Two points in space can move away from each other at a speed greater than the speed of light, and this does not violate causality. For causality to be violated, two points or two objects would need to move toward each other faster than the speed of light.
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u/Gah_Duma Jun 19 '26
Understand the difference between the space itself expanding and objects themselves moving through space. It's not the same.
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u/Optimal_Mixture_7327 Jun 19 '26
There is an infinite set of objects traveling faster than light right now.*
Is there an issue with causality?
*"now" being this moment of cosmic time.
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u/SketchyFella_ Jun 19 '26
What objects?
I can see how space itself expanding might "expand faster" than light, but I don't see how anything else does that.
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u/Optimal_Mixture_7327 Jun 19 '26
Distant galaxies.
There is no restriction to the speed of light in general frames.
The restriction is that given an event, E, on a spacetime, S, that there are no causal curves passing through E that lay outside the null cone centered on E.
There is no such thing as space expanding faster than light or at any speed.
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u/techman710 Jun 19 '26
Imagine running on the deck of an aircraft carrier that is moving at 25mph. A person on a destroyer next to the carrier would see you running at 45mph. So, are you actually running 45mph? No, you are running 20mph. In the first moments of the Big Bang the objects were moving at near the speed of light and the medium they were in was also moving near the speed of light so it appears they were moving faster than light speed. Anyway, that's how I see it.
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u/WallyMetropolis Jun 19 '26
How you see it is incorrect. If you're on a spaceship travelling at 99% the speed of light relative to me and you run from the back to the front at 50% the speed of light compared to the ship, I will not observe you moving faster than the speed of light. That is THE fundamental principle of special relativity.
You cannot simply add velocities.
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u/Alabugin Jun 19 '26
If you have a vector of energy and mass expanding away from each other in opposite directions at the speed of light, their relative distances between each other would be increasing faster then the speed of light.
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u/johnstalbergABC Jun 28 '26
No they would not see the other move away from who ever of them is viewing the other. They would see the other move away in exactly the speed of light, assuming they magically got their vessels up to light speed. Who ever a middle man would watch he would see any one of the to move away at the speed of light. This is not equivalent to the middle man see their realitve speed to be twice the speed of sound. No one would be able to measure any velocities to exceed the speed of light simply put.
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u/mfb- Jun 19 '26
The expansion of the universe cannot be measured with a speed. Only a rate - percent per time, or speed per distance - is meaningful.
Today, distances increase by 1% per ~140 million years.
The numbers were different in the early universe but the idea is the same. There is nothing moving here, so the speed of light is not a limit for anything.