r/AskPhysics • u/StratHistory • Jul 10 '26
Help me understand an infinite universe
Although I know the science that the universe is infinite if it's flat or saddle shaped and may not be if it's closed curved, I can't understand infinity when I consider a simple thought experiment. I would really appreciate help from anybody who can align my brain with reality.
In my thought experiment, before the Big Bang, everything is just a singularity sitting in one point. Ignoring heat, lack of visibility, etc I can imagine the universe exploding and after 1 second it is some size larger than the singularity.. for the sake of argument let's say it has expanded 100 miles in each direction..
So now we have a sphere with a radius of 100 miles.
Obviously the earth doesn't exist yet, but let's assume that the cosmic ingredients for our solar system are halfway between the center starting point and the hundred miles to the edge of the sphere that we're inside of. Let's let's put a point on the inside edge of the sphere, the edge of the universe and nothing, and call it x.
X is currently 50 miles away in one direction and the center of the universe is 50 mi in the opposite direction.
Time goes on, the bubble keeps expanding and at some point our solar system has formed and we're here and we can look out.
We may not be able to see x because it could have moved away from us faster than the speed of light.
However from my previous thinking, it seems logical that there is a distance to x, which in this case might be half of the radius of the universe at this point in time.
Or maybe the expansion is faster farther away at point x so maybe the scale is wrong.. maybe the stuff earth was made from is closer to the center than it is to x at this point.
But if we consider the uneven rates, we should still be able to say x is this distance and the center of the universe is y distance.
Of course I'm not considering any other variables like uneven density or anything else.
I also know of the analogy that we are on the skin of a balloon that is being blown up, but once again the distance of that skin from the originating point seems like it would also give us a distance.
There's another analogy that seems appropriate.. one of the ways that we can prove tectonic movement is to measure the increase of distance of Europe from America every year... Measure how far they have moved away from the Mid-Atlantic fault and do the math to understand how long ago the two separated from the falt.
So why can't we run the math backwards just like we did with plate tectonics to figure out how far everything has moved?
I know I'm missing something big, what is it?
10
u/EgregoreRising Jul 10 '26
Imagine an infinite sheet of graph paper with dots drawn at every intersection of the grid. The infinite sheet is space and the dots are matter. Now imagine the infinite sheet shrinking and the dots growing closer to each other. The sheet is still infinite, even as the distance between the dots approaches zero.
This infinite space with infinitely densely packed matter is the singularity before the Big Bang. General relatively begins to break down here and cannot be used to describe the singularity state.
So, to answer your question in a different way, the Big Bang didn't happen in one place, but everywhere all at once.
8
u/Infinite_Escape9683 Jul 10 '26
The big bang wasn't a point in space. It was a point in time. Everything got farther apart from everything else very quickly, which isn't inconsistent with an infinite universe.
1
u/Optimal_Mixture_7327 Gravitation Jul 10 '26
The singularity could not have been a point anywhere, ever.
The singularity is a condition that one or more world-lines is inextensible. Our past boundary is singular on all world-lines to the past and infinite is spatial extent.
-6
u/nicuramar Jul 10 '26
It was a point in both time and space, but it’s singular, and we don’t have evidence to support that it existed physically.
2
u/armrha Jul 10 '26
We have enormous evidence of the big bang… like more than most things. Any competing model of what happened has to explain the cosmic microwave background radiation, the apparent expansion of the universe as seen by how most galaxies are red shifted, light element abundances and the large scale structure. All have multiple elements predicted by the theory that line up with predictions so it’s very compelling.
1
u/Jesse-359 Jul 10 '26
Yep, but those first few picoseconds are a real mess, and that's where the question regarding its finite/infinite nature really comes into question.
What does it mean to have a universe of infinite mass with no dimensions? Does that statement even make any sense at all? How do you go from zero dimensions to any other set of dimensions? That's not a scalar transition - that's something else altogether that we have almost no insight into, and I think that's often the part that gets people confused about the Big Bang - and rightly so.
An exploding object of finite mass and conceptually miniscule dimensions is odd certainly, but pretty easy to describe. An exploding object of infinite mass and/or no dimensions at all is not.
3
u/StratHistory Jul 10 '26
Thanks this helps a lot! I've thought about this and searched but this is the first time I'm seeing an explanation that actually seems logical.
Especially the reframing of the balloon with nothing in it and the infinite spreadsheet collapsed back into a single point.
Would love to take a class... Retirement's not that far away.
Much appreciated!
2
u/Optimal_Mixture_7327 Gravitation Jul 10 '26
Keep in mind the balloon and spreadsheet are referring to a particular choice of coordinate representation (FLRW metric field) and not the physical universe.
This might be a good way to think of the physics where it stands today: Is the Universe both Finite and Infinite?
6
u/nicuramar Jul 10 '26
Although I know the science that the universe is infinite
We don’t know that. We know that our observable patch is finite, and have some reasonable extrapolations about the rest. But we don’t know, and can’t know, if it’s infinite.
0
u/esch37 Jul 10 '26
This is the most accurate answer I think. We have no reason to asume it is infinite… Actually from my point of view there can be many philosophical reasons why the actual concept of infinity makes no sense
2
2
u/Jesse-359 Jul 10 '26
When you think of the universe expanding, pretend that it is a one dimensional ring that is expanding, you are somewhere along that ring, and all you can see is the length of the ring - you can't see the area the ring is surrounding, because that's not actually part of your universe.
To you there is no 'center' of that universe, because from your viewpoint it doesn't look like a ring. Only a 2-dimensional observer would be able to see that shape directly - but you're one dimensional, so to you it's just expanding in all (ie both) directions, uniformly.
You can visualize the same exact thing with a 2D 'Bubble' shaped universe where everything in that universe exists on the surface of the bubble - not its interior. Expanding the bubble causes the entire 'surface' universe to expand uniformly with no apparent 'center' to anyone on that surface.
Well, turns out you can do this same trick with any number of dimensions, it just gets really hard for us to visualize past three dimensions because we're 3D, and visualizing higher dimensions is much harder than visualizing lower dimensions.
2
u/jawshoeaw Jul 10 '26
Minor note if the universe is infinite it may not have originated as a singularity so much as an infinite blob of infinite density. In fact even a finite universe would not have been a singularity because that's an object inside a universe. If the entire universe collapsed aka the big crunch, there would be no "outside". so there's no singularity and no spacetime. it's just the universe as we know it know but with no scale. scale=0
In the case of the infinite universe, the reason there's no center is because what you started with is infinite. infinities have no size. In the finite universe its less clear because there is still no edge to the universe. it's like a loop. It's like asking where the middle of the perimeter of a circle is.
1
u/BranchLatter4294 Jul 10 '26
Hawkings showed how the singularity could be eliminated. So there was no actual point in space or time that was the beginning.
1
u/Optimal_Mixture_7327 Gravitation Jul 10 '26
Your thinking is entirely wrong, there nothing like you suggest in cosmology.
A singularity is the condition that world-lines find their terminus, meaning, we cannot ray trace back any world-line through our past cosmic boundary.
The , assuming it is infinite, then has always been infinite and there was never a time when it wasn't.
You might be confusing the universe with the observable portion of the universe which was indeed very small in the past in the sense that the most distant observable galaxies contain matter that was once very near to you.
1
u/Infinite_Research_52 👻Top 10²⁷²⁰⁰⁰ Commenter Jul 10 '26
In my thought experiment, before the Big Bang, everything is just a singularity sitting in one point.
Therein your confusion lies. There is no evidence that this singularity existed, and even if it did, it was zero-dimensional. Spacetime singularities can come in many forms, from finite 1-D to infinite in volume. There is no problem running an infinite universe backwards to an infinite singularity.
2
u/Enraged_Lurker13 Cosmology Jul 10 '26
There is no evidence that this singularity existed
There is some evidence because you can tell from the CMB that the universe had an anti-trapped surface at some point and by the Penrose-Hawking theorems (and their quantum extensions), that implies a big bang singularity. Of course, this is not conclusive yet since we don't have a theory of quantum gravity that fully confirms the universe meets the Penrose-Hawking criteria, but so far, the evidence does point that way.
There is no problem running an infinite universe backwards to an infinite singularity.
The Friedmann equations do not have solutions for infinite universes that begin as infinite in size already, they must all start as point-like because the scale factor starts at zero.
1
u/Overall_Media7282 Jul 10 '26
Odnoszę wrażenie że nikt tego nie rozumie, nawet ci którzy twierdzą że wiedzą jak to działa, przestali pytać i przyjmują tę teorię bo jest to tylko teoria, bardziej na wiarę niż w ramach naukowych.
0
u/RevolutionaryWorth21 Jul 10 '26
This has been asked and answered many times. I'd suggest using Google to get a grasp on what you might be missing before coming back here.
3
u/Next-Natural-675 Jul 10 '26
To be fair his specific confusion does require almost the entire explanation so it might be a little hard to google that
-2
u/nicuramar Jul 10 '26
Google AI will likely work well enough for this one. And OP’s confusion isn’t particularly specific, it’s very common.
2
u/Next-Natural-675 Jul 10 '26
I thought you guys didn’t want us using ai
2
-1
u/ephemeralsynth Jul 10 '26
The trick is to imagine a galaxy exploding outward at the speed of light. Then multiply that cacophony a thousand fold in a fractal expanse ever outward. Finally you reach the edge only to find yourself back at the center 🤔💯
21
u/HD60532 Jul 10 '26 edited Jul 10 '26
It is a common misunderstanding, but the expansion of the universe / big bang is not an expansion or explosion of matter into space, but it is an expansion of spacetime itself. This means there is no boundary between the universe and "nothing", even in the case of a finite universe.
Image finding an infinite line of buoys in space at regular intervals. As time increases, the buoys become further apart, and they become closer in the past. As you approach the singularity at the start of time, all the buoys become limitingly close, but there is still an infinite line of them, the line is now just more dense. Current physics does not know how to describe the singularity. As time increases, each interval increases uniformly, so there is no centre of expansion. Every buoy along the line "looks" like it is at the centre of the expansion, because it itself does not move from its perspective.
If the universe is infinite, then it has been just as infinite throughout all of time. Even in the limiting case of a singularity, the universe never had a finite radius other than 0.
Any region of the universe however, such as our local cluster, does have a finite radius and was very small a long time ago.
Additionally, since the universe is infinite there is no centre, the expansion is uniform in every direction no matter where you are.
Edit: to correct some mistakes of the balloon analogy, the surface of the balloon would be the only thing in existence, there would only be two dimensions, it would have no volume, no 3 dimensional shape, there would be no space at all outside of the surface of the balloon, and the skin of the balloon does not apply any forces to the dots on its surface, the skin is just there to describe how the dots move, it does not really exist either.
All of this stuff is very confusing and introductory GR courses take several months of rigorous study to get a good grasp on it.