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? 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.

 

[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-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.

 

[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 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.

 

[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-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?

 

[Thor]

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?

 

[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.

 

[meemoe_uk]

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

 

[Thor]

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.

 

[Chronos]

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.

 

[marcus]

...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

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 , but for sure nobody I know thinks there's a spatial boundary

 

[shrumeo]

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?

 

[Chronos]

marcus: 15 billion light years is the average of what is currently measured as the distance the hubble's constant approaches the speed of light [between 10 and 20 billion light years]. this approximation is supported by stellar evolution models that predict it would take no less than 10 billion years [by local inertial reference frame clocks] and no more than 20 billion years to result in globular clusters composed mainly of white dwarf stars. the fact these clusters are only observed at the extreme fringes of galaxies strongly suggests they are the most ancient gravitationally condensed collections of matter in any galaxy.

regarding recessional velocities. you are right. hubble's constant is not constant over time. the fact the red shift increase with distance, and therefore over time, insists the universe expanded more rapidly in the past than it does now. that is no surprise. the early universe had to expand with more force than the attractive force of gravitation. it would otherwise have collapsed upon itself before we had the opportunity to observe and ask the question 'what happened?'. we know gravity is attractive, hence, the existence of a repulsive [anti-gravity] force is virtually assured. the main question is which force will prevail? i would guess neither. the universe, as we perceive, will eventually reach a state of equilibrium. when the matter density exactly balances the energy density, the universe will acquire a state of perfect equilibrium. this implies other consequences, but, we don't have to deal with that for at least another 6 billion years.

 

[Chronos]

shrumeo: i think the real problem here [and with quantum gravity] is that gravity ceases to be a 'player' at the subatomic level. number one, the other 3 forces [strong, weak, and electromagnetic] are vastly more powerful than gravity at short distances. i think gravity is nonexistent at such short distances. quantum field theory requires forces to act at integer distances. from what i have seen, the wave length of gravity must be a recipocal of c^2. this is not consistent with the theoretical distance between an electron and proton in atomic hydrogen.

 

[Hydr0matic]

Doesn't a flat and infinite universe imply there's a center ?

Assume the universe is flat and infinite.
Is there an infinite amount of matter covering this infinite universe ? Not according to big bang, right ?
So is this limited amount of matter spread across the entire universe ? No, cause then there'd be infinite distance between these "pieces" of matter, right ?

The only alternative is that there is a limited amount of matter localised somewhere in the universe. So I guess it's a matter of definition, right ? Do we define "the center" as the center of all space (i.e. there is none), or do we define it as the gravity center of all matter/energy ? Or perhaps the point where the sum of all coordinates of matter equals zero ?
.. or what ?

If there is to be no center (of any kind) to the universe, isn't a closed borderless one the only option ?

 

[jcsd]

Doesn't a flat and infinite universe imply there's a center ?

Assume the universe is flat and infinite.
Is there an infinite amount of matter covering this infinite universe ? Not according to big bang, right ?
So is this limited amount of matter spread across the entire universe ? No, cause then there'd be infinite distance between these "pieces" of matter, right ?

The only alternative is that there is a limited amount of matter localised somewhere in the universe. So I guess it's a matter of definition, right ? Do we define "the center" as the center of all space (i.e. there is none), or do we define it as the gravity center of all matter/energy ? Or perhaps the point where the sum of all coordinates of matter equals zero ?
.. or what ?

If there is to be no center (of any kind) to the universe, isn't a closed borderless one the only option ?

No a falt nfinite unievrse doe not imply a centre, the big bang means that if the unievrse is infinite in space it is infinite in energy.

 

[Hydr0matic]

hmm.. ok ?
So how did this energy get from being localised in a single point to being distributed in an infinite space with finite density ? Did this happen in an inifinitesimal timeframe ? Was there ever a moment where the energy was somewhere in between the singularity and the infinite distribution ?

Feels like I've got some reading to do ...

 

[marcus]

hmm.. ok ?
So how did this energy get from being localised in a single point to being distributed in an infinite space ...

Hydr0 there is actually a linguistic (not physics) problem here
that confuses people again and again

In ordinary non-technical English, "singularity" means peculiarity, or oddness, or abnormality

(it has no connotation of happening at a single point!)

a theoretical model can experience a singularity at an infinite set of points

it simply means that there is a boundary or limit to its applicability

it means you cannot push the model past a certain limit, because (say) it blows up and fails to compute, or it computes infinities or meaningless numbers----then there is a singularity

this limit could be pictured as a 3D hypersurface bounding a 4D region where the model works well----a little bit like the 2D crust on a 3D loaf of bread is a boundary of the bread (no, I cannot think of a good image, all the images seem to make it more confusing)

But because "singularity" sounds like the word "single"
it suggests to many people that there is a single point
where the singularity happens!
so they imagine a single isolated point
this is a wrong image and leads to much confusion

the BB singularity may have been confined in a small point-like region, but this is not the prevailing view. It could also have occurred at every point of an infinite 3D hypersurface---this is nowadays a very common view

 

[Hydr0matic]

Ok, I see. So the difference between now and the beginning is simply a matter of energy density, where "the critical density" marks the point to which the theory is limited.

right ?

 

[marcus]

not right yet

the "critical density" is a very nice density of about 0.83 joules per cubic kilometer

which is either exactly (or else very near) the actual density
of the universe right at this moment!


at the bigbang time the energy density was probably "off the chart"
infinities are usually a sign that the model is breaking down
and in the usual model
things like density and curvature go off to infinity as one approaches
the instant that expansion began
and so one really must admit that if one wants to approach
that instant and continue computing them one needs a new model!


perhaps you are wondering what is the "critical density"
it is the density the U would need to have (in the simplified Friedmann picture) in order to be exactly spatially flat, at this moment

too much mass/energy and she will be positively curved
too little and she will be negatively curved
and critical is just right
and since the U is observed to be indistinguishable from flat, spatially, it is usually assumed that the actual density, smoothed out to a uniform average level, would be equal to or very near the critical

 

[Hydr0matic]

Ok .. "critical" sounded more dramatic so I assumed it was the point when the model brakes down.

thnx for clearing things up

 

[shrumeo]

shrumeo: i think the real problem here [and with quantum gravity] is that gravity ceases to be a 'player' at the subatomic level. number one, the other 3 forces [strong, weak, and electromagnetic] are vastly more powerful than gravity at short distances. i think gravity is nonexistent at such short distances. quantum field theory requires forces to act at integer distances. from what i have seen, the wave length of gravity must be a recipocal of c^2. this is not consistent with the theoretical distance between an electron and proton in atomic hydrogen.

hmm, maybe I'm missing something...(total non-physicist here trying to understand these things in plain english)

i didn't really mean to imply anything about gravity among subatomic particles. it was just a string of nonsequential questions. But still, let's say we are at the subatomic level and we are "looking" (forgive me Heisenberg) at an electron surrounding a proton. Now, space is expanding even at this scale, right? Now, are the electron and proton something that is not part of the fabric of spacetime? In other words is there more "vacuum" being added to the existing "vacuum" that spearates them? is the electromagnetic force constantly overcoming the expansion OR are they wrinkles in spacetime that also expand along with the vacuum?

On the other scale, say a planet and a moon. Is the space between them expanding to where gravity is constantly overcoming all the new space between them?

Also, there is amount of space between the atoms and molecules that make up the planet and moon (forgot tht scale). Is the electromagnetic force constantly overcoming the expansion of space between them?

If objects and matter remain intact (which they appear to do), do any equations that, say, deal with their, say, group velocity, for example, automatically have built in them, from experiment or derivation, a term that overcomes the expansion of space?

 

[mouseonmoon]

http://www.space.com/scienceastronomy/mystery_monday_040524.html

Universe Measured: We're 156 Billion Light-years Wide!
By Robert Roy Britt
Senior Science Writer

...

Stretching reality

The universe is about 13.7 billion years old. Light reaching us from the earliest known galaxies has been travelling, therefore, for more than 13 billion years. So one might assume that the radius of the universe is 13.7 billion light-years and that the whole shebang is double that, or 27.4 billion light-years wide.

But the universe has been expanding ever since the beginning of time, when theorists believe it all sprang forth from an infinitely dense point in a Big Bang.

"All the distance covered by the light in the early universe gets increased by the expansion of the universe,"

The scientists studied the cosmic microwave background (CMB), radiation unleashed about 380,000 years after the Big Bang, when the universe had first expanded enough to cool and allow atoms to form...

...findings have shown "no sign that the universe is finite, but that doesn't prove that it is infinite."

The results do render impossible a "soccer ball" shape for the universe...



"If the universe was finite, and had a size of about 4 billion to 5 billion light-years, then light would be able to wrap around the universe, and with a big enough telescope we could view the Earth just after it solidified and when the first life formed," Cornish said. "Unfortunately, our results rule out this tantalizing possibility."


"The problem is that funny things happen in general relativity which appear to violate special relativity (nothing traveling faster than the speed of light and all that).

=====
might help-might not

 

[Chronos]

how did that guy earn a doctorate? i agree GR has potential flaws, but, not on the basis of such patently flawed arguments. you cannot look into a telescope and see the back of your head. that violates causality. i can play along with any kind of universe anyone predicts, but, i absolutely reject any model that violates causality. even GR forbids that. god may play dice, but a universe without causality is impossible to observe.

 

[shrumeo]

how did that guy earn a doctorate? i agree GR has potential flaws, but, not on the basis of such patently flawed arguments. you cannot look into a telescope and see the back of your head. that violates causality. i can play along with any kind of universe anyone predicts, but, i absolutely reject any model that violates causality. even GR forbids that. god may play dice, but a universe without causality is impossible to observe.

i think you are agreeing with the article(?)

 

[GodAdoresU]

According to the most accurate measurements of the position of "planet earth" in relation to the rest of the universe, we are in the Dead center. And so one could assume 2 things about the universe: 1) we are extremely lucky. or 2) the universe is closed. The reason being, if we do indeed live in a closed universe, then it would appear that we we're in the center. For example, Imagine yourself standing on planet Earth (pretty hard huh? XD) But without all of the mountains, valleys, trees, grass etc. Just you and a nice two dimensional surface. If you assumed that the Earth was flat (and if it was), then you would observe that you were in the very center (just as we observe). However, it is very unlikely that we are at the very center of the universe. Therefore, there is only one other possibility:We live in a closed universe. However, this is only hypothetical. And I don't think that we will know for sure until we are able to venture out into the universe and make more observations. So the answer to your question in my opinion is that Light propagating from stars and galaxies eventually comes back around to the point of origin.

 

[EMFsmith]

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.

I though the speed of light was the speed at which "Stuff" (Pardon my ignorance here) of 0 mass can travel, ie photons. So how can there be ANYTHING that travels faster than this?