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Is the observable universe limited by an expansion event horizon or an opaque primordial plasma?

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- Thread starter mrspeedybob
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Is the observable universe limited by an expansion event horizon or an opaque primordial plasma?

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Simon Bridge

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It can't. Well...I have heard it said that galaxies exist which are beyond the observable universe because the expansion causes them to be receding at super-luminal velocity. How can this be?

Neither.We can see all the way back to the surface of last scattering, when the universe was just dense plasma. The limit to the observable universe cannot be both.

Is the observable universe limited by an expansion event horizon or an opaque primordial plasma?

The observable universe is the bit you can see. There exist galaxies et al beyond this because light from those galaxies has not reached us yet. It follows that someone in Andromeda can see bits of the Universe we cannot see and will not for two and a half million years. (The light has reached them, but they are 2.5mil ly away.)

Each year we can see 1ly further in space, but no further back in time because the bit of the Universe right here is a year older.

Back to that "Well..."

http://arxiv.org/abs/1107.5168

... some people argue the Hubble expansion can result in superluminal relative speeds because it is a geometric expansion of space rather than the usual idea of speed involving travel in space. Considering that the limit of observable universe is retreating from us at the speed of light, then this suggests that some galaxies will never be observed (they are moving faster than that wrt us even though their speed wrt to objects closer to them is not faster than light.)

This paper argues that the Hubble expansion is only good for nearby objects.

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- #3

Chronos

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The surface of last scattering [CMB] IS the limit of the observable universe. Your basic premise is correct - it is obviously illogical to suggest we can ever observe entities older than the universe itself.

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http://arxiv.org/PS_cache/arxiv/pdf/1107/1107.5168v1.pdf

"As expected, Hubble’s receding velocity can be

thought to give the relative velocity between two

close comoving objects. It is also possible to

reinterpret (13) by reversing the logic. Since

Hubble’s receding velocity is supposed to give

the relative velocity, it is natural to define vrel

by parallel-transportation along the straight line

joining two comoving objects at a fixed time, at

least for nearby objects. For larger distances,

one can imagine infinitesimally separated comoving

observers placed in between A and B. Nearby

observers can measure their relative velocities at

a fixed time. From that information, relative velocity

between distant objects A and B can be

determined by integration, which indeed corresponds

to parallel-transportation along the finite

line-segment and thus we obtain (11). Consequently,

vrel can be seen to generalize the usual

concept of relative velocity in a cosmological context.

In summary, we believe that the best way to

resolve concerns about superluminal expansion

speeds is to emphasize that Hubble’s law does

not make sense for large distances. We showed

that if the time derivative of the distance between

two objects is naively identified as the relative

velocity, then faster than light speeds can

also be found in special relativity. Therefore,

we need to be careful in determining the correct

physical meaning of a mathematical quantity in

a relativistic theory, which is also the main issue

with Hubble’s law. These examples can be used

to convince students that there is nothing wrong

with a naive superluminal expansion speed since

it has nothing to do with relative velocity or as

a matter of fact it has no direct physical significance.

Moreover, we pinned down the correct

differential geometrical meaning of the Hubble’s

receding velocity as the rapidity of a local

Lorentz transformation. With the derivation of

this last result, there must not arise any further

issue with faster than light expansion speeds."

Just as soon as I had began to accept the unlikely possibility of superluminal velocities then some one suggests it is a fallacy.

Do we all agree that the matter which emitted the CMB was moving away from us at almost at the speed of light? Objects further away from must surely have to be travelling away from us faster than this? Also I believe that the dark energy expansion makes the relative velocities higher. Perhaps a deeper understanding of 4D spacetime and relativity can solve this?

"As expected, Hubble’s receding velocity can be

thought to give the relative velocity between two

close comoving objects. It is also possible to

reinterpret (13) by reversing the logic. Since

Hubble’s receding velocity is supposed to give

the relative velocity, it is natural to define vrel

by parallel-transportation along the straight line

joining two comoving objects at a fixed time, at

least for nearby objects. For larger distances,

one can imagine infinitesimally separated comoving

observers placed in between A and B. Nearby

observers can measure their relative velocities at

a fixed time. From that information, relative velocity

between distant objects A and B can be

determined by integration, which indeed corresponds

to parallel-transportation along the finite

line-segment and thus we obtain (11). Consequently,

vrel can be seen to generalize the usual

concept of relative velocity in a cosmological context.

In summary, we believe that the best way to

resolve concerns about superluminal expansion

speeds is to emphasize that Hubble’s law does

not make sense for large distances. We showed

that if the time derivative of the distance between

two objects is naively identified as the relative

velocity, then faster than light speeds can

also be found in special relativity. Therefore,

we need to be careful in determining the correct

physical meaning of a mathematical quantity in

a relativistic theory, which is also the main issue

with Hubble’s law. These examples can be used

to convince students that there is nothing wrong

with a naive superluminal expansion speed since

it has nothing to do with relative velocity or as

a matter of fact it has no direct physical significance.

Moreover, we pinned down the correct

differential geometrical meaning of the Hubble’s

receding velocity as the rapidity of a local

Lorentz transformation. With the derivation of

this last result, there must not arise any further

issue with faster than light expansion speeds."

Just as soon as I had began to accept the unlikely possibility of superluminal velocities then some one suggests it is a fallacy.

Do we all agree that the matter which emitted the CMB was moving away from us at almost at the speed of light? Objects further away from must surely have to be travelling away from us faster than this? Also I believe that the dark energy expansion makes the relative velocities higher. Perhaps a deeper understanding of 4D spacetime and relativity can solve this?

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- #5

Simon Bridge

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But is that what "observable universe" usually means in this context?Chronos said:The surface of last scattering [CMB] IS the limit of the observable universe. Your basic premise is correct - it is obviously illogical to suggest we can ever observe entities older than the universe itself.

http://en.wikipedia.org/wiki/Observable_universe

- #6

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http://en.wikipedia.org/wiki/Observable_universe

A 2% delta is a reasonable approximation?

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I think that's the layman doesn't really explain anything explanation :P

- #8

DrChinese

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I think that's the layman doesn't really explain anything explanation :P

Welcome to PhysicsForums, Stimpon!

Here is a good article that covers this subject:

http://space.mit.edu/~kcooksey/teaching/AY5/MisconceptionsabouttheBigBang_ScientificAmerican.pdf [Broken]

Keep in mind that new evidence is coming in all the time, and a lot of new science in this area has appeared in the past 20 years.

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- #9

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https://www.physicsforums.com/forumdisplay.php?f=69

Therefore presumably all we can say is that our measurement of the distance between two very distant galaxies is increasing. And for galaxies beyond our observable universe the distance is increasing so fast that light from the distant galaxy will no longer be able to reach us. Why this happens is unknown, but the effect has been given the name dark energy.

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