From inflation theory and other theories explaining the birth of the universe, the universe is expanding and recently it's been discovered that the expansion has been accelerating. My question is simple, what is happening on the boundary of our inflating universe and what is that 'space' that is experiencing that boundary?
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$\begingroup$ Related: astronomy.stackexchange.com/questions/5960/…, astronomy.stackexchange.com/questions/2137/…, physics.stackexchange.com/questions/128271/… and links therein. $\endgroup$– HDE 226868 ♦Commented Feb 19, 2015 at 2:14
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$\begingroup$ also see giyf.com/what+is+our+universe+expanding+into $\endgroup$– YukterezCommented Feb 19, 2015 at 2:27
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5 Answers
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It's a rather difficult concept to grab but, in short, the image of expansion you have in your mind is wrong. It's not your fault, it's everywhere out there, the inflatable balloon etc. And it's terrible :)
Another way to look at it is that the universe properties are changing. Whether it is infinite or finite, its size (as measured within itself) changes. Things are pulled appart. You could imagine that free vacuum keeps being generated between all objects but that would be a misrepresentation of the theory.
The most simple way I see it is actually what the equations say. It's lame but sometimes the math are the simplest way to understand a phenomenon. The equation say the metric is expanding. It means two point in the fabric of the universe are measured at increasing distances over time. As a result, unless objects are pulled to each other with sufficient force, they tend to drift appart. It's a property of the universe that has not been explained yet (at least not completely), and we still are on the fence as to what influences the expansion rate.
Ultimately, this concept is very easy to accept when you cease to represent the universe from outside of itself. The universe is everything that is so if any image in your mind represents the universe from outside, it won't give you a good picture. Balloons are especially bad :). If you need an image, draw a grid or take a notebook, draw some points here and there on it and imagine the grid keeps getting bigger and bigger. But don't look at the whole page, forget the page limits, focus on the grid. And keep in mind that the points do not expand, only the grid does...
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$\begingroup$ +10. I had this exact question come to mind today, and this is an excellent answer. As a statistician, I appreciate the mathematical explanation too! $\endgroup$ Commented Mar 11, 2019 at 16:59
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There is no boundary there is no outside to be expanding into. Only slightly misleading is the usual analogy of the expansion of the universe is that of the surface of a balloon as it is blown up, which just gets larger but has no boundary.
One problem with the balloon analogy is that with a balloon we can see that it is embedded in a 3-space, but there is no need for such a 4-space embedding for the Universe. Another problem is that the surface of the balloon is very definitely finite, we don't really know if that is the case or not for the Universe.
answered Feb 19, 2015 at 11:39
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$\begingroup$ What is the state of that expansion? If it's infinite expansion then it beats the logic of a beginning. $\endgroup$– SycheCommented Feb 19, 2015 at 16:22
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$\begingroup$ @Ken I don't know what you mean by "infinite expansion", please be more precise. Are you asking is the universe finite or infinite? Will the expansion go on forever (whatever that might mean)? Or what? $\endgroup$ Commented Feb 19, 2015 at 16:49
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$\begingroup$ #Ken, probably but forever is a very long time and I would not guarantee what will happen in the very far future. $\endgroup$ Commented Feb 19, 2015 at 17:00
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The universe isn’t expanding into anything. It is just the distance between the galaxies is increasing continuously, but the galaxies are relatively still but space between them is moving. However the universe is infinitely big enough to begin with and remain infinite, it’s size doesn’t change. For e.g. consider 1,2,3... upto infinity. If you multiply it with 2, then 2,4,6.... upto infinity. Here we can say that distance between adjacent has increased, but total extent of numbers remain the same.
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Most of the Astronomers and the community of Astrophysics, will agree that the fate of planet earth is "The Big rip, most comparable to The Big Bang;but polar opposites.Where the universe is expanding to it's breaking point, total expansion of the know universe.
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3$\begingroup$ The Big Rip is entirely hypothetical and inconsistent with the standard ΛCDM cosmological model. It's theoretically possible, but I severely doubt that "most" of the astrophysics community will agree that it's remotely likely. $\endgroup$ Commented Feb 19, 2015 at 7:59
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2$\begingroup$ You are making up your own question, this does not address the question being asked here $\endgroup$ Commented Feb 19, 2015 at 11:37
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$\begingroup$ Tbe (hypothetical) Big Crunch is generally considered the be the opposite of the Big Bang, not the Big Rip. $\endgroup$– HDE 226868 ♦Commented Feb 19, 2015 at 16:18
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$\begingroup$ Thank you HDE226868 for your correctional suggestions it kept me awake last night, wondering if I clarified both the rip and the bang!! :) $\endgroup$ Commented Feb 19, 2015 at 21:19
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Look, you can get the answer just from thinking about what we already know. You can infer the model without even creating any new concepts.
Everybody agrees the universe started off at a single point in space and time.
Then it expanded outward. If everything expanded from the center at the same rate, it necessarily forms a sphere, a 3 dimensional one called a hypersphere. With the big bang in the center.
As time progressed, it got bigger and bigger.
That's what everybody already believes, and it's all you have to know for this to be a picture of the expanding universe:
It's all you have to believe to know that the universe is expanding into future time. It creates the future just by continuing to exist.
The source of the expansion is the passage of time. The universe expands to make room for the past.
You can validate this by noticing that the surface of the hypersphere expands at 2 Pi light-seconds per second for the age of the universe.
A single division yields the megaparsec's share of that new space, and the predicted expansion rate is exactly what we observe it to be, within 0.25%.
The simple, intuitive model would appear to be validated.
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4$\begingroup$ "Everybody agrees the universe started off at a single point in space and time." No they don't! See physics.stackexchange.com/q/136860/123208 $\endgroup$– PM 2RingCommented Aug 13 at 0:14
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1$\begingroup$ "If the global curvature of the universe isn't positive, then the size of the universe is infinite, (and it's always been infinite since the dawn of time, at the Big Bang), assuming the topology of the universe is trivial". astronomy.stackexchange.com/a/31795/16685 $\endgroup$– PM 2RingCommented Aug 13 at 0:33
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$\begingroup$ @PM2Ring That objection assumes that the expansion occurred in space, And that some parts of the universe would be closer to the center than others. But it expands into future time. And the entire universe at a given time is the same distance from the center: the age of the universe. $\endgroup$ Commented Aug 13 at 0:33
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$\begingroup$ But in this model, the global curvature is positive and the circumference of the universe is its age times 2 pi or 87 billion light years. That's approximately the same as another frequently cited estimate of about 90 billion my years. $\endgroup$ Commented Aug 13 at 0:35
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$\begingroup$ Sure, if the universe has positive curvature (and simple topology), then it's a 4D cone, with time as the axis of the cone. That picture doesn't quite work with non-positive curvature, but you can still treat time as an axis. $\endgroup$– PM 2RingCommented Aug 13 at 0:38