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davebahr
2008-Oct-29, 03:55 PM
I have often wondered... Is there a location somewhere in the universe that one could be placed such that all known matter would be in front of you and there would be nothing behind you?

Argos
2008-Oct-29, 04:00 PM
The universe is homogeneous and isotropic (http://en.wiktionary.org/wiki/isotropic) on large scales. So, no.

John Mendenhall
2008-Oct-29, 04:41 PM
The universe is homogeneous and isotropic (http://en.wiktionary.org/wiki/isotropic) on large scales. So, no.

Agreed. No sides, no top, no bottom, no front, no back, and . . . no center.

cosmocrazy
2008-Oct-29, 05:46 PM
As far as we can tell based on current observation and evidence. ;)

mugaliens
2008-Oct-29, 06:17 PM
The universe is homogeneous and isotropic (http://en.wiktionary.org/wiki/isotropic) on large scales. So, no.

I second cosmocrazy. The observable universe holds to this. But that boundary lies where the velocity of spacetime expansion at the distant edge renders more distant stuff beyond... well, beyond our ability to know it's there. We only surmise it is there, as we appear to be at the exact center of the universe, and given the statistical unlikelihood of this happening, the logical conclusion is that we're simply at the center of the observable universe only, which does stand to reason.

So, we don't know what's beyond the observable universe. But that's not to say your comment is wayward, Argos, as it's also reasonable to assume that the isotropism continues from the visible to beyond.

astromark
2008-Oct-29, 06:26 PM
This is not a subject we can answer without starting a descusion...
As the previous answers have implied the universe does not have an edge. A place where all mater is in front of you. We are expected to understand that the universe most probably had a beginning. That that great expansion began from a singularity. It is continuing to accelerate its expansion rate making reaching any perceived edge impossible. If there is one.

thorkil2
2008-Oct-30, 09:10 AM
At great risk of offending the ATM rule keepers, I have suggested in the past that in a 4-dimensional Universe, the most logical place for an edge is now. But I dare not pursue that further here. :)

Delvo
2008-Oct-30, 12:30 PM
In order for the answer to be "No", one of two conditions would need to apply:

1. The universe is infinite.

or

2. The universe is finite but has some weird way of wrapping back around onto itself like the paths at the top, bottom, and sides of a Pac-Man maze.

Neither of these conditions is actually known to be the case in reality.

John Mendenhall
2008-Oct-30, 02:21 PM
At great risk of offending the ATM rule keepers, I have suggested in the past that in a 4-dimensional Universe, the most logical place for an edge is now. But I dare not pursue that further here. :)

4 spatial and 1 time? Not likely, no observations to support it. 3 spatial, 1 time, matches observations, we see no evidence of any edge.

alainprice
2008-Oct-30, 02:53 PM
In order for the answer to be "No", one of two conditions would need to apply:

1. The universe is infinite.

or

2. The universe is finite but has some weird way of wrapping back around onto itself like the paths at the top, bottom, and sides of a Pac-Man maze.

Neither of these conditions is actually known to be the case in reality.

Nor can those options be discounted.

aurora
2008-Oct-30, 02:54 PM
I did see an article recently about a discovery that the galactic clusters were tending to move in the same direction, perhaps influenced by something that is outside our visible universe. It was either in Science News or Scientific American, I was trying to get caught up with my reading last night.

davebahr
2008-Oct-30, 03:17 PM
Is the universe defined by the "stuff" that is in it? (atoms, photons, etc.) Is there any reason to believe that it can't extend beyond the "stuff"?

Argos
2008-Oct-30, 04:09 PM
The visible universe is defined by the surface of last scattering [sometimes called the 'comoving distance'] which is roughly 46 billion LY away, and is comprised by the photons emitted at the time of the recombination, some 400,000 years after the big-bang. So, the points comprising that surface are in the past.

If we were to use the famous baloon analogy of the universal expansion, then the present time [we, right now] is at the edge of the universe. Nonetheless it is still isotropic.

Anything beyond the horizon has no causal relationship with us, and cannot be fathomed by physics, being irrelevant.

IŽm aware of the article mentioned by Aurora. But all I can say is it is highly speculative.

Jeff Root
2008-Oct-30, 04:19 PM
At great risk of offending the ATM rule keepers, I have suggested in
the past that in a 4-dimensional Universe, the most logical place for
an edge is now. But I dare not pursue that further here. :)
3 spatial, 1 time, matches observations, we see no evidence of any edge.
I think he does mean 3 dimensions of space, one of time, but the
edge he suggests is in time rather than space. Nobody can see
anything after "now". There is a definite cutoff at "now".

-- Jeff, in Minneapolis

John Mendenhall
2008-Oct-30, 04:45 PM
Nobody can see
anything after "now". There is a definite cutoff at "now".

-- Jeff, in Minneapolis

Argos said this also, a good point, and someone made the point in another thread that time has a starting point (BB). Looking at it this way, time has a start and a cutoff.

Of course, the cutoff is as perceived by each of us individually . . . ouch, that makes my head hurt to think of it.

Jeff Root
2008-Oct-30, 04:46 PM
Argos,

The part of the Universe with which ours is causally connected is larger
than the part we can see, which is limited by the surface of last scattering
of the cosmic background radiation. Light before the time of last scattering
was absorbed again, but at least some of it had time to reach us by now
if it hadn't been absorbed. Neutrinos from that early time could be passing
through us and around us. So could gravity waves, and/or gravitons, if they
exist. Dark matter from that time, beyond the surface of last scattering,
would also be causally connected with us.

The "flatness" of the Universe's overall spacetime geometry suggests that
it is considerably larger than the portion we can see. If the Universe wraps
around on itself (which I think is extremely unlikely), it must do so on a
scale vastly larger than what we can see. If the Universe really is isotropic
and homogenous on the largest scales, and does not wrap around on itself,
then it must be infinite in extent. An infinite universe which has finite age
could not be all causally connected. Parts of a universe which are not
causally connected should have nothing in common. An overall expansion
is occuring in the part of the Universe we can see. That expansion would
not be occuring in any part of the Universe which is not causally connected
with our part. Carbon atoms (for example) exist in the part of the Universe
we can see. There would not be any such thing as carbon atoms in a part
of the Universe which is not causally connected with our part.

-- Jeff, in Minneapolis

Argos
2008-Oct-30, 04:59 PM
Argos,

The part of the Universe with which ours is causally connected is larger
than the part we can see, which is limited by the surface of last scattering
of the cosmic background radiation. Light before the time of last scattering
was absorbed again, but at least some of it had time to reach us by now
if it hadn't been absorbed. Neutrinos from that early time could be passing
through us and around us. So could gravity waves, and/or gravitons, if they
exist. Dark matter from that time, beyond the surface of last scattering,
would also be causally connected with us.

Yeah, no doubt. I oversimplified my point. I stand corrected. :)

thorkil2
2008-Oct-30, 06:33 PM
4 spatial and 1 time? Not likely, no observations to support it. 3 spatial, 1 time, matches observations, we see no evidence of any edge.

I meant 3 spatial dimensions and 1 time dimension. The edge would not be a where, but a when, which is "now".