# Light Waves at Edge of Universe Question

by justwondering
Tags: edge, light, universe, waves
 P: 46 Since the outer boundry(s) of our expanding universe are moving at a less than the speed of light rate there must be light waves generated in our universe that travel to the limits of our expanding universe and reach the "edge" of it. What takes place when they reach this edge? Can they "bust through" to the unknown outside of our universe or are they stopped dead or reflected back into our universe, or whatever? Something must happen to them and I don't think that they can go beyond the edge of our universe. If this question is not clear enough I am glad to try to explain it further.
 Mentor P: 21,669 There is no outer boundary. There is no edge. Much like if you were to fly around the surface of the earth, if you flew far enough and fast enough in one direction in space, eventually you'd end up back where you started.
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 Quote by russ_watters Much like if you were to fly around the surface of the earth, if you flew far enough and fast enough in one direction in space, eventually you'd end up back where you started.
Sorry Russ but I think that comparison is not quite corect.

Even if one assumes a (semi-) closed universe it is not true that you can end back where you started.

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## Light Waves at Edge of Universe Question

 Quote by russ_watters There is no outer boundary. There is no edge. Much like if you were to fly around the surface of the earth, if you flew far enough and fast enough in one direction in space, eventually you'd end up back where you started.
IMO that is a good comparison because the surface of a sphere is a 2D space with no boundary.

we don't need to push the analogy to a travel scenario, it is just a good illustration of a space without boundary

but suppose just for fun we WANT to push the comparison

we can make our home base be a galaxy that is not moving with respect to the CMB, that is the one we want to leave from and come back to.

and we can suppose that it is a modest-size universe like ours may have been long ago, but let's have it not expand as fast as ours, so we have some chance of going full circle without having to fight huge rates of expansion

well it is sort of imaginable maybe with a very fast ship one could make it around---if, merely for purposes of illustration, it was a small, slowly expanding, S3 universe
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 Quote by justwondering ... What takes place when they reach this edge? Can they "bust through" to the unknown outside of our universe or are they stopped dead or reflected back into our universe, or whatever? ...
as far as anyone knows there is no boundary to space, no edge. Like there is no edge of the world to fall off of.

but sometimes people speak of the "observable universe" which is the part that we are already receiving light from, so the things in it are observable by us

that's a very different business from the whole universe! and the observable universe DOES have a boundary
it is a bit like your horizon when you are outdoors on flat plains, not exactly, but has the idea of "as far as the eye can see"

that observable part of the universe is always expanding as light reaches us from more and more galaxies reaches us.

CAUTION: please don't confuse that expansion of our observable part (as we hear from more and more outlying precincts, so to speak) with the "expansion of space" you hear about connected with standard bang consmology.

In any case, no need to worry about what happens when a bit of light reaches that kind of horizon-like boundary. It is just ordinary space there. The light just keeps right on going. A horizon is not like a real material wall it is just a feature of where we are and what we can see from here.

Keep asking questions, Justwondering. we havent even got to the bottom of all you raised in this one post! but maybe it's good to stop and see if you want to pursue it or have any other questions
 P: 46 After doing a Web search I found one site that stated "Various observations imply that the expansion of the Universe is accelerating. If so, this strongly suggests that the Universe is geometrically "flat". " Perhaps the model of the "expanding balloon surface" Universe analogy is not our reality.
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 Quote by justwondering After doing a Web search I found one site that stated "Various observations imply that the expansion of the Universe is accelerating. If so, this strongly suggests that the Universe is geometrically "flat". "
I don't think so. Not to me anyway. You often find misleading stuff on the web---it would help if you would give me the URL and I can see who said it. And when. The way cosmologists are talking has changed somewhat since March 2006, when the WMAP three-year data came out.

Accelerating expansion is a feature of the current consensus mainstream model called LambdaCDM and the current data is compatible both with a perfectly flat (infinite) version and a large nearly flat ("balloon-like" finite) version.

My sources on this are cosmologists at the top of their profession. I'm not expert myself, I just read the professional-journal papers that they write and watch the field.

If somebody in their league said recently, not in a popular account but in a peer-review scietific paper, that positive lambda "strongly suggests [perfectly] flat" then I would be surprised and delighted. It is always interesting to get a difference of opinion.

 Perhaps the model of the "expanding balloon surface" Universe analogy is not our reality.
The expanding balloon surface analog, but in 3D, is EXPLICITLY being included by pros as a serious possibility.

Perhaps it is perhaps it isn't, our reality. Keep posted
It is a cliffhanger. The thirdyear WMAP satellite data was highly relevant.
Both finite and infinite are in the running!
 P: 46 The Web site says "recent observations (such as BOOMERANG and MAXIMA cosmic microwave background radiation results and various supernova observations)" imply this. The "recent" term may not be very recent.
 PF Patron Sci Advisor P: 22,351 Since i asked you for a URL it's only fair if I offer one. Here's a favorite paper, very recent (Jan 2007) http://arxiv.org/abs/astro-ph/0701584
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 Quote by russ_watters There is no outer boundary. There is no edge. Much like if you were to fly around the surface of the earth, if you flew far enough and fast enough in one direction in space, eventually you'd end up back where you started.
The universe is so large, that light itself would not be able to go around it in the lifetime of the universe if it was closed.
 P: 1 It is my view (considering E=MC2)...if there is zero matter at edge of universe then there is no energy (vis: E = 0 x C2...therefore E=0). But just before you reach "the edge"..you would tranisition thru Bose-Einstien condensate (BEC) layer...very little matter...very little energy...very very cold..near 00Kelvin.
 P: 3 I think, from what I have read, that space-time warps around mass, so that the universe is, in effect, in every direction no matter where the light is. A light-wave would theoretically only get so far from known mass before a straight line in any direction would direct no further away. Light doesn't curve, space-time warps - the light is still effective following a straight line. If I am wrong here, please be specific as to why.
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 Quote by russ_watters There is no outer boundary. There is no edge...
You heard the man
Russ is a PF mentor who has been around here since 2003. If you don't want to listen to him then listen to this: I've been following cosmology for several decades and I've never come across a professional cosmic model with an edge. There are some models with uneven density, but they are not typical. Working cosmologists dont use them. And even those models don't have any edges.

Typically, however much space volume you have, it is all uniformly filled with matter, roughly same average density everywhere. If the total volume of all space is finite then the amount of matter is finite. If one is infinite the other is.

 Quote by Toxicuser It is my view...if there is zero matter at edge of...
There is no edge, Toxic. Back to the drawing board. Try to invent a world model with no edge.

 Quote by mathius A light-wave would theoretically only get so far from known mass before a straight line in any direction would direct no further away... If I am wrong here, please be specific as to why.
If a light wave got out of the KNOWN region then it would necessarily have to start looping back. It could continue on in a part of space which we haven't mapped but which we assume has the same average density of matter----the same number of galaxies per volume. Your picture seems to have the light tethered to the region of the universe which we have seen and mapped out. No reason it should be anchored here.

But you've got a grain of an idea there. Space may be finite volume-----analogous to the 2D surface of a balloon (but nothing inside or outside, only the surface). The term for the 3D analog is a 3-sphere. Space might be a 3-sphere.

In that case IF it were not expanding, light could after a very very long time make it around full circle. So that is a little like being tethered.

But it is not like being anchored to one known region, because there is always as much matter ahead as there is behind and the light just keeps going on and on, more or less straight ahead. till it circumnavigates the 3-sphere.

Recent WMAP data report gave the minimum circumference as 6.28 x 108 billion lightyear.
That is about 680 billion lightyear. The farthest stuff we have gotten light from, and observed, is 46 billion lightyear away at the present time. so you can see that a round trip would take you MUCH farther than what is KNOWN. And that is the minimum estimate.

And in practice you couldnt do a roundtrip like that anyway because the thing would be expanding as you were trying to circumnavigate it====defeating your efforts.

Magellan's ship would never have made it around the earth if the earth had been expanding faster than the ship itself could sail. But still there is no edge
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 Quote by marcus But still there is no edge
And on that note, it's time to put this revived thread to bed.

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