No, it didn’t expand from a single point at all. The Big Bang happened everywhere in space. If you’re not getting that, you’re misunderstanding things.
No, that’s not true. It’s not just that we don’t know where it is. The notion of a “center” simply does not apply to any geometry of the universe in GR. You’re implicitly assuming that a finite universe must have an “edge”, from which we could (in principle) measure to the center. But that’s not the way things are. No edge, no center. The problem is that we don’t have good intuition for visualizing this, hence the various analogies with their inherent limitations.
This doesn’t sound right to me. AIUI, no matter where you are in the universe, there is the same amount of space from you in every direction. Therefore, no matter where you are, you are in the “center”.
The observable universe is defined with the observer (usually us) at the center, and has an “edge” that’s simply the limit of information for that particular observer. There’s no implication that that’s a “real” physical edge to the universe or that space “ends” there.
I think that that’s what I was trying to say. Even if you were at the edge of the observable universe (from our perspective), your perspective would show you to be in the center since your observable universe would be different than ours.
Aside from the fact that it’s not known whether space is finite or not, this simply isn’t true. There are lots of finite geometries for space that don’t have anything that can reasonably be called a center. While it’s possible to have a geometry with a defined ‘center’, I’m not aware that any such geometries fit what we’ve observed of the universe so far, so they’re likely not accurate.
Yet, to be fair to those of us not sold on this “no center” concept: Astronomers will claim every now and then they have found objects on the edge of the universe, peering back in time 3 minutes after the Big Bang, or such. To me, that implies there is a point of origin they are looking back to. Clearly, they are not tracing objects backward only to converge on earth as said above the balloon model suggests.
Also, I’m not convinced the dots on the balloon would all appear to be receding from each other. While I know this is the accepted dogma, wouldn’t the dots all appear to be radiating outward from the mouthpiece of the balloon?
Again, this is the widespread confusion between the entire universe and the observable universe. The astronomers are talking about the edge of our observable universe. They are saying that they are seeing things that are close to the most distant point that from which we could in principle acquire information. That boundary also corresponds to the beginning of time, the Big Bang, since looking a long way also means seeing things further in the past. It does not correspond to a physical “edge” in the sense that space “ends” there. And astronomers are certainly not looking back at anything that corresponds to a point in the usual sense. We can, in principle, see the Big Bang in every direction that we look. In practice, we can only see times somewhat after the Big Bang - but we see that in every direction we look, as would any other observer.
No, you are just stretching the balloon analogy to a point where it fails. Forget the balloon analogy, it’s misleading. Either go 1-d as I described above, or if you want a 2-D analogy use the stretchy flat sheet as in the Krauss video linked above.
There’s some ambiguity in what one refers to as The Universe. Most of the time, it refers to the observable universe, the universe we can currently see. It is a finite amount, because it has been around for a finite amount of time, and light travels at a finite speed. By definition, we are at the center of it.
One may also refer to the Universe as everything, including stuff we can’t see and will never be able to see. Unfortunately, because the universe is expanding, we will never be able to see the entirety of everything. We don’t know how much further the universe goes beyond the borders of our visible universe, but it seems likely that it goes on forever.
Yeah, the balloon analogy isn’t great. As a classroom demo, I use a slinky, with paperclips hanging from it. Stretch out the slinky, and the paperclips move further apart, with the ones further from each other moving even further apart than ones close together. You can also hold one paperclip stationary as you stretch it, and you’ll see the same thing no matter which one you choose, so each paperclip can claim they are the center. But the paperclips never actually move along the slinky, they are only moving away from each other because the slinky is stretching.
As a bonus, since the slinky looks like a wave, it also demonstrates how the wavelengths of light get stretched out.
There is no perfect analogy but some are better than others. The problem with the rubber sheet analogy is that there is obviously a perimeter, observable even by a 2D creature living on its 2D surface, and an empirically definable real center. Same with a rubber band, in 1D. The balloon analogy, if one thinks of it from the standpoint of a 2D creature living on its 2D surface, much more accurately conveys the idea of the universe as “finite but unbounded”, intuitively demonstrating the no-boundary and no-center concepts common to most of the models.
Sure they would, by virtue of the simple fact that rubber of the balloon is stretching everywhere at the same rate. The “mouthpiece” of the balloon is just a discontinuity in a real-life balloon that can be eliminated either by imagining the inflation point to be infinitesimally small or by imagining a perfect inflated rubber sphere that is spontaneously expanding as the air pressure in the room is reduced.
I made the suggestion in a recent thread that it may be better to try a different attack on visualising things.
Rather than the universe expanding, imagine that you are shrinking, and everything else is shrinking as well. And you keep on shrinking, and shrinking, and shrinking.
Even things close to you at the start will appear to be further away from you (relative to your size) and and time goes on, and you get smaller, the time it would take you to travel to another object increases, and thus the speed you would need to move at relative to your size, gets greater. Eventually there will be things that were once a few steps away, that you could never reach again because your constantly diminishing size means you can’t run fast enough to make progress towards it.
You could consider that the fastest you can run is analogous to the speed of light. Eventually there will be objects that you cannot ever reach, and someone at that object could never reach you.
You can imagine this on a plane, or in 3D space.
Where it breaks down is that there is clearly a defined centre that is immobile. So it doesn’t help you there. You could imagine standing on a Möbius strip or a Klein bottle, but that probably stretches the value of the idea too much.
The way to get rid of the “center of the universe” idea is to imagine the universe was infinite in size BEFORE the big bang. Which is a perfectly valid assumption given the facts. The universe was infinitely dense and then became much less dense, as space opened up between all the things that were once adjacent. In that scenario, there’s no need for a center. Things were close and now they aren’t. And that doesn’t require a center point.
Er, yeah. True. What I was trying to say was that the initial state of the system has a centre, and that centre remains where it is - so it is immobile - as well as everything else. The immobility is rather redundant (to say the least )
In short, don’t imagine stuff expanding into space. Imagine space itself expanding, and taking stuff along for the ride: Stuff at point A was once right up next to stuff at point B; stuff at point A is still at point A, and stuff at point B is still at point B, but now, there’s more distance between points A and B.
Look at it from the perspective of the stuff at point A: The stuff at point B is moving away, possibly quite quickly. In fact, every other piece of stuff more than some minimum distance away is receding, at a speed proportional to how far away the stuff is. If you shift from point A to point B, the stuff at point B sees the exact same thing. Physics is not changed by moving around in space. This a very deep result, which leads to the amazingly beautiful results of Noether’s Theorem if you use it and a bit of basic calculus. What’s more, it works equally well at the quantum scale as at the scale of the whole Universe. Reality is elegant; it does a lot with a little.
Project this backwards, and you get a time when there was no distance at all between points A and B, a time when every point was the same point, because there was no distance at all in the Universe. That’s the singularity prior to the Big Bang. Project this forwards, and you get a time when there’s some arbitrarily gigantic distance between A and B, but no substantial amount of stuff (or energy) came along to fill it. It’s just cold, dark, empty space. That’s the Heat Death scenario, the currently accepted model for how the Universe will end.
I think the choice of the term “big bang” was perhaps an unfortunate one in that it is suggestive of explosions, which have a central point and expand outwards in a different way than the inflation of the early universe. To imagine space itself compressed into a single point with one dimension is very difficult when you’re unused to thinking in this way.
If we carry the balloon analogy a bit further, it is amusing to me to think of a singularity as the result of someone popping that balloon, and letting all the space out.
“The Big Bang” was originally coined by a detractor of the theory, who felt it was unrealistic and wanted a somewhat insulting name for it. It caught on in spite of not being really accurate.
It is ~mostly true. Fred Hoyle was an die-hard opponent to the theory. He did coin/popularize the term. His claims that it wasn’t intended to be derogatory are taken with a grain of salt.
I like this explanation, though I have no idea if it describes the actual situation of the universe. As many others have mentioned, the balloon analogy fails because as 3D creatures we can see that there is more to the balloon than just it’s 2D surface, that it has an interior. If we move up from the 2D balloon expanding into a 3rd dimension to a 3D space expanding into a 4th dimension, the question arises as to what is the nature of this dimension. In the 2D to 3D conversation we all know that we go from length and width to length, width, and depth. Trying to picture a 4th spatial dimension is impossible for me to picture, but if we say that time is this 4th dimension and that the center of the universe is not a place 13.82 billion light years away from in distance, but that the Big Bang itself, a time 13.82 billion years in the past, is the center, that seems a lot more intellectually satisfying. Of course that means we have no hope of reaching this center any more than a 2D creature on a balloon could reach the spatial center of the balloon.
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