Does the universe have an edge?
I've heard the universe is just an empty cavity. Does the universe have any edge or stop or is it just an infinite empty cavity?
As far as theories I've heard of, we can't see the edge of the universe (if truly exists) because the farther you look out into space through a telescope, the farther back in time you are actually looking. For example, if you are looking at a star 20 light-years away... you're actually currently observing what was happening on that star 20 years ago... the theory is that there is only so far you can look out into the universe because there is only so much history post Big Bang.
Image above: What is the furthest we can see? In 2003, NASA's WMAP satellite took images of the most distant part of the universe observable from Earth. The image shows the furthest we can see using any form of light. The patterns show clumps of matter that eventually formed into galaxies of stars. Credit: NASA/WMAP Science Team
Don't understand why the down vote. [link]http://www.nasa.gov/audience/foreducators/5-8/features/F_How_Big_is_Our_Universe.html
Galaxies extend as far as we can detect... with no sign of diminishing.There is no evidence that the universe has an edge. The part of the universe we can observe from Earth is filled more or less uniformly with galaxies extending in every direction as far as we can see - more than 10 billion light-years, or about 6 billion trillion miles. We know that the galaxies must extend much further than we can see, but we do not know whether the universe is infinite or not. When astronomers sometimes refer (carelessly!) to galaxies "near the edge of the universe," they are referring only to the edge of the OBSERVABLE universe - i.e., the part we can see
If we know the universe is expanding, we are saying the universal boundary is expanding. Before the big bang(s), there was "nothing". Which seems an impossibility, because something happened. So we have the big bang(s) and the boundary begins to move outward, at least at the speed of light (knowing there is debate about variable speeds of light, I am keeping this open). That means that according to our understanding thus far, there is an edge to the universe.
I try to keep in mind that as a creature with everyday experience in three dimensions, it is next to impossible to really understand all the multidimensional possibilities which may make up "the edge". Just like I cannot imagine a time from my viewpoint where there was a "nothing". I believe that the answer lies in a better understanding of multi-dimensions than we have now.
We are so very, very early in our use of a scientific method of exploration, and it seems that every year we are bowled over by the implications of a new discovery - especially in quantum physics.
I believe the question is about a possible edge of the "whole" Universe, not the _observable_ Universe which is what you're referring to (and which increases its distance to us not only at $c$, but at $4.3c$).
Great comment. I guess I'm extrapolating that as we see the observable universe expand, the entire universe is also expanding. Presumptuous perhaps, but isn't anything we can measure "observable" (directly or indirectly)? In either case, a bang from "nothing" (which I have a hard time with) would seem to create a boundary between that nothing and something...observable or not. And if we are questioning the speed of light, and if it may be variable, how can we convincingly state that the universe expands at 4.3c?
I don't think assuming that the rest of the Universe is expanding is presumptuous. Anyway, a bang from nothing doesn't necessarily create a boundary. The Universe is either finite of infinite, but certainly neither implies that is has an edge. If it's finite, then it "curves back on itself", like the surface of a balloon. In "mainstream physics" we don't question the speed of light, and by Hubble's law galaxies the edge of the observable Universe (which is not a physical edge) recede at $v = H_0 d \simeq 3.3c$.
Because light from increasingly far away will reach us, the distance to the edge increases by another factor of $c$, so the obs. Uni. increases its radius at $v\simeq 4.3c$.
But, if you're floating in the inside of a balloon, and travel to the balloon itself, that is an edge to whatever is beyond it. I totally get the "edge" idea. But I don't think we can answer it any more than we can answer "what was there before the big bang?", and if the speed of light is indeed variable (or slowing down), that has major implications in our understanding of all kinds of things...age of universe, how relativity would work, where the edge of the universe is, and how long that light would indeed take to reach us.
The balloon analogy is a 2D analogy of a 3D space. You can't be "inside" the ballon, since then you leave the 2D space. But just as you can travel the surface of the balloon and never reach an edge, you can travel 3D space and never reach an edge. If space is homogeneous and isotropic (the "mainstream" assumption) we _can_ answer the question: Space is either _infinite_, in which case there's no edge and you can travel forever and meet new galaxies, or it is _finite_, in which case there is _also_ no edge, but you can (in principle) travel and get back to the beginning (like on the balloon).
As for the variable speed of light hypothesis, that's not mainstream physics and, to my knowledge, has no experimental verification whatsoever. As for "how long that light would take to reach us", that's quite easily calculated from the Friedman equation. I will leave it as an exercise :)
Thanks Pela, very informative! I caught on that OP was referring to the observable universe, to which there obviously is an "edge" of sorts. Wasn't getting that the balloon was a 2D analogy, sorry. Makes much more sense now. Maybe the question is better thought of as a membrane on the universe. What could lie beyond it. I wish I could come back in 1,000 years to see what we've discovered, if we haven't completely annihilated the earth.
Oh, and the question of how long light would take to reach us wasn't something I mentioned. Obviously we can calculate that if we accept the older assumption of a constant speed of light. I personally have major doubt about that, and like Magueijo and Afshordi's theory because the uniformity of temperature and matter in the universe are puzzling under our current assumptions. Hawking's expansionist theory is hard to accept without knowing the mechanism which triggered that expansion. It really feels like the theory was massaged to fit the observation of uniformity. Digressed, sorry.
Dark energy is driving the accelerated expansion of our universe.
Is it accelerating into itself?
Or is it accelerating into nothing, An outer void?
If we could see the furthest away accelerating object that would be the edge of the Universe. Thus there is an edge to the Universe.
Remember, just because we can't look and see today doesn't mean it doesn't exist.