# The limit of the observable universe

## Main Question or Discussion Point

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? 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?

Simon Bridge
Homework Helper
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?
It can't. Well...
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?
Neither.

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.

Last edited:
Chronos
Gold Member
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.

Last edited:
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
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?

Last edited:
Simon Bridge
Homework Helper
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.
But is that what "observable universe" usually means in this context?

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

The current comoving distance to the particles which emitted the CMBR, representing the radius of the visible universe, is calculated to be about 14.0 billion parsecs (about 45.7 billion light years), while the current comoving distance to the edge of the observable universe is calculated to be 14.3 billion parsecs (about 46.6 billion light years),[1] about 2% larger.

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

A 2% delta is a reasonable approximation?

Essentially it's not so much that the galaxies themselves are moving at superluminal speed but rather that spacetime is itself expanding everywhere which causes things far away from us to recede at superluminal speed.

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

DrChinese
Gold Member
Essentially it's not so much that the galaxies themselves are moving at superluminal speed but rather that spacetime is itself expanding everywhere which causes things far away from us to recede at superluminal speed.

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:

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.

Last edited by a moderator:
In this thread it was said that space has no objective existance itself, and that space is not created the same as ponderable matter.

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.