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The simple answer is time. As we look out into the universe we see distant galaxies all moving away from us like we are the center of the universe. We are not unique because every place in the universe looks like it is the center. If we could go a billion light years from here in any direction it would also look like the center. The universe has no single center or any edge away from that center. There is no outside for the universe to expand into. In a way, every point in the universe today is the place where the universe began, the center. Also in a way, every point in the universe is the edge where the universe is expanding. The edge is not somewhere out there. The edge, like the center, is everywhere. The universe is larger today than it was in the past and it will be larger in the future. The edge that is expanding is every point in the universe and the dimension that it is expanding into is the future. So the simple answer to, what is the universe expanding into, is that the universe is expanding into time.
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Unfortunately things aren't that simple.
The universe appears to be a 4-manifold, where time is just one of the vairables of the manifold. It is true that the 4-volume of the universe appears to increase (expand) with time. However, it is not technically correct to say the universe is expanding into anything - the universe is not a gas expanding into a balloon. For something to expand into another thing there must be a boundary between the two and there must be a way to measure the difference - where's the boundary and how do we distinguish between "universe" and "not universe."
It is natural to want the universe to expand into something else - that is what fits our everyday experiences and expectations. Instead of trying to make the universe fit your expectations, we need to make our expectations fit the universe as it is - something that most people find very hard. -
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Hi Troy, I know that there isn't anything for the universe to expand into and there is no boundary between universe and not universe. I just liked the idea that the expansion was into the future. To me it is a simple way to not think about a boundary out there. In truth it is a nonsense question, like what is north of the north pole.
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<...It is natural to want the universe to expand into something else - that is what fits our everyday experiences and expectations. Instead of trying to make the universe fit your expectations, we need to make our expectations fit the universe as it is - something that most people find very hard.>
Thanks, Troy.
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Response
It is a “visualization” to say that the universe is expanding into time. It is only saying that the universe will be larger in the future. It requires no center or boundary in space. It does not even require the future to be anywhere for the universe to expand into it.
I realize that I am taking some liberty with my use of the word “into”. Random House, into, 5, used to indicate a continuing extent in time … , lasted into… Five is kind of low on the list.
Dots on an expanding balloon is also a visualization used to represent the expanding universe. It has some value and some problems.
What I have said in my presentation is as follows:
That every place in the universe looks like the center.
That there is no single center or edge out there.
That the expansion takes place over time and not into any existing space.
I think that all of these presentations are in agreement with our best understanding of the universe.
I Still feel that it is a good visualization. Apply some imagination. -
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Unsu...
That's a good question. It is "common knowledge" that in General Relativity, time only has a definition within the known universe. You always see these "how the universe evolved" images/posters, and it makes you wonder how physicists were able to show the visualisation. Are these visualisations just a feeble attempt ? -
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You have the good question. "...just a feeble attempt ?" I think all our visualizations are just an attempt to understand..
Some may be better than others. -
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All theories start with some axioms that eventually collapse to some level of observation and there is more than one axiomatic path leading to those observations for the given accuracy level. If you can slap a meta-spatial dimension at the cosmological level and make it work...more power to you. You know, like the flatlander living on a sphere. It's not that much different than adding 3 or 4 spatial dimensions for string theory and extrapolation upward in scale.
Is it mathematically possible to have a closed, say 3 spatial dimensions, space that exists wholly in 3D space? Do we need a minimum of that 4th dimension to wrap it onto it self?
Me thinks things will shake out much differently in the future, these theories will fall to dust like the baroque beasts that they are.
-dean -
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Unsu...
Baroque beasts? Which theories are you talking about? QM & GR? What are closed 3d spatial dimensions?
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"Is it mathematically possible to have a closed, say 3 spatial dimensions, space that exists wholly in 3D space? Do we need a minimum of that 4th dimension to wrap it onto it self? "
In the real world we could not have our universe without time. We need the time dimension for our universe to exist. There has to be change. Without change there could be no photons, no heat, etc. etc. etc. Yes we need the time dimension for our universe to exist. -
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"Is it mathematically possible to have a closed, say 3 spatial dimensions, space that exists wholly in 3D space? Do we need a minimum of that 4th dimension to wrap it onto it self? "
Strictly mathematics here:
First there is a need to rewrite the question a bit so as to clarify it.
Can a closed (loosely defined as having a finite surface area) n-surface (IE an n-manifold, where n is the degrees of freedom) be embedded in an n-euclidean space?
No, at the absolute minimum a closed n-surface can be parametrically embedded in an (n+1)-euclidean space (think about a sphere in 3-euclidean space).
Ok, what about an open n-surface (IE an n-manifold) be embedded in an n-euclidean space?
Yes, but only if it spans the n-euclidean space (this really is just a change of coordinates).
In general and depending if certian conditions are met, an n-surface requires a maximum of a (2n)-euclidean space to be embedded into (sorry but I've forgotten the name of the theorem at the moment). I believe I've seen reference to a less restrictive theorem that states that an n-surface can always be embedded in a (3n+2)-euclidean space - saw it once but haven't been able to find it again.
One of the advantages of these ideas is that they allow people to work out physical theorems in high-dimensional spaces, where the math is easier, to parametrically describe our 4-spacetime. Lost you? String theorem with its ten or eleven dimensions could really just be a the parametric description of our 4-spacetime where each of the dimensions in string theory is really just a function of 4 variables - unfortuantely no one else seems to be thinking along those lines.
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