What happens to light when it reaches the edge of the universe?

In summary: The universe is curved, so if you keep walking in one direction on Earth, eventually you will reach your starting point. Similarly, light traveling in one direction through the universe will eventually reach its starting point (if it travels long enough and outruns any expansion of the universe). However, if the universe is expanding (as is currently believed), then light could never reach the edge.In summary, the "edge" of the universe is the distance to which this light has gotten, and if the universe is the same everywhere - as stephen hawking says it must be to make calculations relevant -then light can't reach an "edge" because an edge would be different from the rest of the universe.
  • #1
fbsthreads
36
1
What happens to light when it reaches the edge of the universe?

it couldn't keep on traveling into nothingingness could it? becuase that nothingness would be space.

it couldn't reflect off something unless there was matter at the edge to reflect off.

the light must do something, it can't disappear can it? that would contradict the 'can't create or destory energy' law.
 
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  • #2
It would keep going. "The edge of the universe" is the distance to which this light has gotten, (at least).
 
  • #3
If the universe is the same everywhere - as stephen hawking says it must be to make calculations relevant -then light can't reach an "edge" because an edge would be different from the rest of the universe! On the event horizon of a black hole the
kinetic energy of a mass = potential energy and this is the same for the furthest distance light is from the Earth - using the Newtonian idea that the universe is a sphere.The universe isn't a black hole, this just shows that the universe is like a black hole in that it won't allow a mass to escape from it.
 
  • #4
maybe space is so curved that light just gets bent when it reaches the edge and flows along the edge of the universe.
 
  • #5
The universe (in all credibel cosmological models) doesn't have an edge, it's as simple as that.
 
  • #6
Curvature of the universe

fbsthreads said:
What happens to light when it reaches the edge of the universe?
The universe's three dimensional space is curved back on itself in the same way that the 2-dimentional surface of the Earth is curved back on itself. If you keep walking in one direction on Earth, eventually you will reach your starting point. Similarly, light traveling in one direction through the universe will eventually reach its starting point (if it travels long enough and outruns any expansion of the universe).

Because the universe is curved, if you could see far enough, and if there was a clear view in front of and behind you, you would see the back of your head. If you could see far enough and looked up, you would see the other side of the earth.

The implication here, though, is that whatever direction you look in you will see your starting place. Since this Earth-in-the-sky would logically have to be continuous, wherever you happen to be located at any given moment will be continuously projected in reverse against the "sky" of the universe.
 
  • #7
hitssquad's description applies if the universe is closed; if it's open you wouldn't be able to see yourself (and flat is in between). There's the small matter of the speed of expansion, which is not a question of the geometry of the universe.

The WMAP site has a nice alien illustration of closed, flat and open.
 
  • #8
Correct me if I am wrong, but there is no requirement for the universe itself to be moving(expanding) at or less than the speed of light, and with the recent discoveries concerning dark energy, it appears that its expansion is accelerating. Hence, the "edge" of the universe could be moving faster than the speed of light, and a "beam" of light couldn't reach it!
 
  • #9
geometer said:
Hence, the "edge" of the universe could be moving faster than the speed of light, and a "beam" of light couldn't reach it!

but if it is accelerating all the time then there must have been a time when light could easily reach the edge.

also, if big bang is to be believed then any light from the big bang 'explosion' would have been at the edge of the small universe in no time.

if light could never reach the edge, you could still see the big bang if you were at the edge.
 
  • #10
The initial expansion of the universe was much faster than the speed of light (the inflationary period), and so the "edge" has probably always been receeding at faster than the speed of light.
 
  • #11
geometer said:
The initial expansion of the universe was much faster than the speed of light (the inflationary period), and so the "edge" has probably always been receeding at faster than the speed of light.

No:

It is possible for objects in the visible universe (with respect to some obsever) to have recession velocities faster than the speed of light, but, you have to relaize that the recession velocity of an object is depenednet on the distance of the observer from that object. A beam of light emitted by the observer would never recah your hypothetical edge, but it would be possible for a beam of light within the observable universe that is considerably closer to the edge to reach and go past that edge.

As I said before, whatever the geometry of the universe is (whether it is postive, negative, or flat curavture or whether it is infinite or finite), it doesn't have an edge, it really is as simple as that.
 
  • #12
Basically expansion is global, not local.
 
  • #13
When the vu the BB-scientists calculate the universe to expand in equals the speed of light, c, the ether will fall back to it's previous energylevel, and the gates god built will not stay in the condition it had earlier.
 
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  • #14
Sariah, though I siad exapnsion, is global not local, the measure rate of expansion is local not global, so the expansion rate is not a velocty, but a function of distance and velocity (it will give you the recession velocity v of two objects in a co-moving sphere of radius r) and therefore can't equal c as it has different units.
 
  • #15
time went and radius grew. r/t is not c yet.
 
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  • #16
though the universe is not really expanding, i still think this is what happens when dr/dt = c
 
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  • #17
The rate of expansion of the universe is measure by Hubble's constant, which is equal to vr/d, so you should be able to see that even if Hubbles constant is very small, as long as the distance between the two objects (d)is large enough the recession velocity (vr) will exceed c. In an infinite expanding universe there must be a distance where vr exceeds c.
 
  • #18
hitssquad said:
The universe's three dimensional space is curved back on itself in the same way that the 2-dimentional surface of the Earth is curved back on itself. If you keep walking in one direction on Earth, eventually you will reach your starting point. Similarly, light traveling in one direction through the universe will eventually reach its starting point (if it travels long enough and outruns any expansion of the universe).

if this was the case, would the darkness of space even exist? wouldn't there be everlasting light in every single direction at every single point of time?
 
  • #19
fbsthreads said:
What happens to light when it reaches the edge of the universe?

it couldn't keep on traveling into nothingingness could it? becuase that nothingness would be space.

it couldn't reflect off something unless there was matter at the edge to reflect off.

the light must do something, it can't disappear can it? that would contradict the 'can't create or destory energy' law.

What happens to the value of a fraction as its denominator approaches infinity? This is the same thing that happens to light - but at a rate inverse to dist^2.

BTW...what makes you think the Universe has an edge? And what is on the other side of that edge?
 
  • #20
I ment dr/dt ofcourse, pardon.
 
  • #21
I thought that with new experiments revealing the density of the universe it was exactly enough for a flat universe, not a curved one.
 
  • #22
cangus said:
if this was the case, would the darkness of space even exist? wouldn't there be everlasting light in every single direction at every single point of time?
In a closed universe, only if the expansion were slower than c.
 
  • #23
Matrixman13 said:
I thought that with new experiments revealing the density of the universe it was exactly enough for a flat universe, not a curved one.
The question as to whether the universe is open or flat is still ... open :tongue: It shouldn't be surprising if this is still under some debate 50 or 100 years from now ...
 
  • #24
cangus said:
if this was the case, would the darkness of space even exist? wouldn't there be everlasting light in every single direction at every single point of time?

Not if the expanding universe was finite in age (light from distant objects may not have reached you yet).
 
  • #25
fbsthreads said:
but if it is accelerating all the time then there must have been a time when light could easily reach the edge.

also, if big bang is to be believed then any light from the big bang 'explosion' would have been at the edge of the small universe in no time.

In a closed universe, you may get a situation like hitssquad mentioned (space being curved such that the light beam reaches its starting point and goes around again).

In an open or flat universe, space is infinite (or at least boundless), so there is no edge. Offhand, I'm not quite sure how that works at the instant following the Big Bang...infinite space from the very beginning I think (or a temporary closed topology?)

if light could never reach the edge, you could still see the big bang if you were at the edge.

In any of the models (closed, open, flat) there is no edge. The Big Bang was not an explosion of matter/light/energy into empty space. It was the rapid expansion of all of existence.

The Big Bang happened about 13.7 billion years ago & due to the finite speed of light, we could see the Big Bang if we could see 13.7 billion light years away (we can get close with current technology, but not that close). But if we were to magically transport ourselves 13.7 billion light years away, we would not be in the middle of the Big Bang. Things would look there as they do here because time has passed there too.
 
  • #26
An infinite unievres must of always been infinite, at the big bang in an infite unievres, you basically have an inifte, but infinitely dense space, which expands (in other words becomes less dense).

The universe is flat within experimental error, no-one is ever going to be able to say it's completely flat as that means measuring it's curvature precisely. Many of the interesting new cosmologies that make the mainstream media, (like for example the 'soccer ball' universe) have a curvature that is within the experimental error of the measured value, but are not flat. Flat unieverses needn't be infinite in space, howver I've been told that topologies which give flat finite universe are extremely non-trivial so the bog-standard infinite topology must be preferred.

The problem with phrases like an 'open universe' is technically it refers to a Friedmann-Lemaitre universe whioch is infinite in space and with either negative or Euclidian (i.e. no) curvature, that will contiune to expand forever. An open universe is an example of an 'open space' which is a infite space with no boundaries. It's gets even more confusing as people sometimes use the word 'open', rightly or wrongly to refer to other kinds of cosmologies that shares certain features with a Friedmann-Lemaitre open universe.
 
  • #27
It is possible for objects in the visible universe (with respect to some obsever) to have recession velocities faster than the speed of light,...

This is a little confusing to me - I thought it was not possible for any object with a real rest mass to have a velocity equal to the speed of light, let alone exceed it.
 
  • #28
Take a look in the General Astronomy & Cosmology sticky (A&C reference library); there are several papers which cover this topic, in considerable detail. Look for "Lineweaver".
 
  • #29
fbsthreads said:
but if it is accelerating all the time then there must have been a time when light could easily reach the edge.

also, if big bang is to be believed then any light from the big bang 'explosion' would have been at the edge of the small universe in no time.

if light could never reach the edge, you could still see the big bang if you were at the edge.
What do you not understand? The universe has no edge, regardless if it's closed or not.
 
  • #30
geometer said:
It is possible for objects in the visible universe (with respect to some obsever) to have recession velocities faster than the speed of light,...

This is a little confusing to me - I thought it was not possible for any object with a real rest mass to have a velocity equal to the speed of light, let alone exceed it.

There's a difference between velocity and recession velocity, it's only in special relativity that the observed velocity will be the same as the relative veloctiy with respect to c. In general relativity your dealing with spaces that aren't Euclidian and that have a dynamic geometry and special relativity becomes the local case only. Recession velocity isn't due to any actual movement of the objects themselves, but the expansion of spacetime, so there is no barrier to it being greater than c.
 
  • #31
Space curvature is nice, but to really understand how things work you need to know what's going on at the quantum level, which is anyones guess.

meemoe_uk's law is a simple idea which deals with the edges of space with QM.
- every traveling quantum must have a start and end point
so no light can be sent off the edge on an infinate journey.

Quasars are the most distant objects we can detect, so they are our current best bet for 'objects on the edge'. By meemoe_uk's law, all their radiation must be transmitted back into the universe. This of course makes them seem to radiate much more intensely then we'd expect if we thought their radiation was free to fly off in any direction. Also this may explain why the universe appears to expand. Galaxys and quasars effectively have radiation thrusters which accelarate them into the least dense space, in the case of quasars this is off the edge of the universe! lol
 
  • #32
Flatland said:
What do you not understand? The universe has no edge, regardless if it's closed or not.
Some people tend to overlook the fact that the very premise of their argument is flawed. Be patient.
 
  • #33
there is no 'edge' of the universe. only spatially bound objects have edges and the universe in not an 'object'.. it is all of spacetime. to think of it as an object requires the existence of non-object regions [i.e. spacetime continuums that originated independent of our universe]. it also requires them to interact [have observable effects] with this universe to be of any theoretical consequence. we have not yet observed any phenomenon in this universe that require the existence of other universes to be explained.

there is however a theoretical limit to the observable universe. hubbles constant predicts objects at a distance around 15 billion light years would be receding at the speed of light. we could therefore say the observable universe is a sphere 30 billion light years in diameter. of course, you would arrive at this same conclusion no matter where you were in the universe. this imaginary edge only exists in your local reference frame. under general general relativity, no matter where you are or what speed you travel, the 'edge' of the universe will always appear to be 15 billion light years [according to whatever clock you happen to have along] away from your current position. the long and the short of it is you can't get there from here, or from anywhere else for that matter.
 
  • #34
Chronos said:
...a theoretical limit to the observable universe. hubbles constant predicts objects at a distance around 15 billion light years would be receding at the speed of light. we could therefore say the observable universe is a sphere 30 billion light years in diameter...

Chronos you might enjoy reading "Expanding Confusion" by Tamara Davis. I will put a link, in case you want to.
http://arxiv.org./abs/astro-ph/0310808
It addresses some misconceptions about the expansion of the universe and the dimensions of what is observable.

You are right that there is a bound to the observable universe. But I don't think it is 15 billion LY.
Hubble's parameter has not been constant over time.
One cannot go by the present value of it.

Indeed, it turns out that
the light reaching us from many of the galaxies we see now was emitted at a time when that galaxy was receding from us at faster than the speed of light---and yet the light managed to reach us, curiously enough.
The Davis and Lineweaver article explains how this can be.

For example, galaxies are routinely observed at redshifts greater than 3.

In fact, one was recently detected to have z = 10 (by Roser Pello's group).

A galaxy observed at z = 3 must have been receding from us, at the time it emitted the light we are now receiving from it, at a speed greater than light.

using the standard "Sky and Telescope" calculator at S. Morgan's website
http://www.earth.uni.edu/~morgan/ajjar/Cosmology/cosmos.html [Broken]

you can see that a galaxy observed with z = 3 would have emitted the light
when it was receding at 1.6 times c
and it would now be at a distance of 21 billion LY
and currently receding at a speed of 1.5 times c.

If you want to use the online calculator, put in 0.73 for dark energy (lambda)
and 0.27 for matter (omega), and z = 3 or whatever you want the redshift to be.

BTW it looks like you, flatland and Thor are all saying no edge to the U
and I can only say amen to that! I'm not certain myself there is even a back fence in the time direction :smile:, but for sure nobody I know thinks there's a spatial boundary
 
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  • #35
i've always wondered about "expanding" space
are the objects (protons, photons, whatever) embedded? do they expand along with space? or do little distance markers move away from each other? Does matter expand along with space? isn't it part of spacetime or is it totally separate? Does space "expanding" simply mean that MORE space got put in between two objects? If 2 objects are attracted to each other gravitationally (sitting in little spacetime wells) then wouldn't the "expanding" space between them affect their gravitational pull on one another?
 
<h2>What happens to light when it reaches the edge of the universe?</h2><p>1. Is the edge of the universe the same as the observable universe?</p><p>No, the observable universe is the portion of the universe that we can see and measure, while the edge of the universe refers to the theoretical boundary beyond which we cannot observe or measure.</p><h2>What happens to light when it reaches the edge of the universe?</h2><p>2. Does light travel forever in the universe?</p><p>Yes, light travels at a constant speed and does not dissipate or disappear. However, it may be affected by the expansion of the universe and the presence of matter and energy.</p><h2>What happens to light when it reaches the edge of the universe?</h2><p>3. Can light escape the edge of the universe?</p><p>There is no definitive answer to this question as we do not know the exact nature of the edge of the universe. Some theories suggest that the edge of the universe is a boundary beyond which light cannot escape, while others propose that light can continue to travel beyond the edge.</p><h2>What happens to light when it reaches the edge of the universe?</h2><p>4. Will light eventually reach the edge of the universe?</p><p>Again, this depends on the nature of the edge of the universe. If it is a boundary that light cannot escape, then light will not reach it. However, if the universe continues to expand, the edge may also expand, allowing light to continue its journey.</p><h2>What happens to light when it reaches the edge of the universe?</h2><p>5. How does the edge of the universe affect the speed of light?</p><p>The speed of light is a constant in the universe, so it is not affected by the edge of the universe. However, the expansion of the universe and the presence of matter and energy can affect the path that light takes, potentially altering its perceived speed.</p>

What happens to light when it reaches the edge of the universe?

1. Is the edge of the universe the same as the observable universe?

No, the observable universe is the portion of the universe that we can see and measure, while the edge of the universe refers to the theoretical boundary beyond which we cannot observe or measure.

What happens to light when it reaches the edge of the universe?

2. Does light travel forever in the universe?

Yes, light travels at a constant speed and does not dissipate or disappear. However, it may be affected by the expansion of the universe and the presence of matter and energy.

What happens to light when it reaches the edge of the universe?

3. Can light escape the edge of the universe?

There is no definitive answer to this question as we do not know the exact nature of the edge of the universe. Some theories suggest that the edge of the universe is a boundary beyond which light cannot escape, while others propose that light can continue to travel beyond the edge.

What happens to light when it reaches the edge of the universe?

4. Will light eventually reach the edge of the universe?

Again, this depends on the nature of the edge of the universe. If it is a boundary that light cannot escape, then light will not reach it. However, if the universe continues to expand, the edge may also expand, allowing light to continue its journey.

What happens to light when it reaches the edge of the universe?

5. How does the edge of the universe affect the speed of light?

The speed of light is a constant in the universe, so it is not affected by the edge of the universe. However, the expansion of the universe and the presence of matter and energy can affect the path that light takes, potentially altering its perceived speed.

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