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

[fbsthreads]

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

it couldn't keep on traveling into nothingingness could it? because 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.

 

[LURCH]

It would keep going. "The edge of the universe" is the distance to which this light has gotten, (at least).

 

[kurious]

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.

 

[fbsthreads]

maybe space is so curved that light just gets bent when it reaches the edge and flows along the edge of the universe.

 

[jcsd]

The universe (in all credibel cosmological models) doesn't have an edge, it's as simple as that.

 

[hitssquad]

Curvature of the universe

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-dimensional 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.

 

[Nereid]

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.

 

[geometer]

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!

 

[fbsthreads]

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.

 

[geometer]

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.

 

[jcsd]

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 positive, negative, or flat curavture or whether it is infinite or finite), it doesn't have an edge, it really is as simple as that.

 

[jcsd]

Basically expansion is global, not local.

 

[Sariaht]

When the v_u 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.

 

[jcsd]

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.

 

[Sariaht]

time went and radius grew. r/t is not c yet.

 

[Sariaht]

though the universe is not really expanding, i still think this is what happens when dr/dt = c

 

[jcsd]

The rate of expansion of the universe is measure by Hubble's constant, which is equal to v_r/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 (v_r) will exceed c. In an infinite expanding universe there must be a distance where v_r exceeds c.

 

[cangus]

The universe's three dimensional space is curved back on itself in the same way that the 2-dimensional 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?

 

[Thor]

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

it couldn't keep on traveling into nothingingness could it? because 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?

 

[Sariaht]

I ment dr/dt ofcourse, pardon.

 

[Matrixman13]

I thought that with new experiments revealing the density of the universe it was exactly enough for a flat universe, not a curved one.

 

[Nereid]

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.

 

[Nereid]

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 It shouldn't be surprising if this is still under some debate 50 or 100 years from now ...

 

[Phobos]

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).

 

[Phobos]

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.

 

[jcsd]

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.

 

[geometer]

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.

 

[Nereid]

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".

 

[Flatland]

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.

 

[jcsd]

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.