APOD: The Antennae (2010 May 07)

[neufer]

The Big Bang describes an "explosion" in four dimensions, not three. So there is no sphere of expansion, and never was. The expansion we see is taking place on the three-dimensional surface of a four-dimensional hypersphere. The center is at t=0 - a direction we can't see.

I thought that the center at t=0 is in the ONLY direction that we can see.

[Chris Peterson]

I thought that the center at t=0 is in the ONLY direction that we can see.

Not at all. As we look at distant objects, we see their three-dimensional state at some time t<now, but that isn't the same as seeing along the time axis of spacetime.

[neufer]

I thought that the center at t=0 is in the ONLY direction that we can see.

Not at all. As we look at distant objects, we see their three-dimensional state at some time t<now, but that isn't the same as seeing along the time axis of spacetime.

But that is not what you said!

Everywhere one looks is in the direction of the big bang center at t=0; isn't it?

[Chris Peterson]

But that is not what you said!

Everywhere one looks is in the direction of the big bang center at t=0; isn't it?

If you like. I don't really see any problem with what I said originally, though. We can't see the center because it is on the time axis, and we can't see along that axis.

[neufer]

Everywhere one looks is in the direction of the big bang center at t=0; isn't it?

If you like. I don't really see any problem with what I said originally, though.
We can't see the center because it is on the time axis, and we can't see along that axis.

We can't see along the time axis or along any space axis...but that is not really the issue.

The reason we can't see the center because the CMBR plasma gets in the way.

[Chris Peterson]

We can't see along the time axis or along any space axis...but that is not really the issue.

We can certainly see along the spatial axes. We can see any point in the observable universe, as some (x,y,z) at t=now (in the most common sense, that we always see now; you can adjust for distance if you like, it doesn't change the argument). The point is, any given point (x,y,z) we can only see at one time, whatever you want to call that time. We can't explore that point in the t dimension. We can explore the entire Universe in the x, y, and z directions.

The reason we can't see the center because the CMBR plasma gets in the way.

We still wouldn't see a center in the sense the OP was asking about. Because there is no center that can be defined in three dimensions for our universe.

[neufer]

We can't see along the time axis or along any space axis...but that is not really the issue.

We can certainly see along the spatial axes. We can see any point in the observable universe, as some (x,y,z) at t=now (in the most common sense, that we always see now; you can adjust for distance if you like, it doesn't change the argument). The point is, any given point (x,y,z) we can only see at one time, whatever you want to call that time.

I am quite happy to call the past light cone as "now" and claim that the true "visible" universe is 13.7 billion light years in radius.

However, the general terminology states that our telescopes are virtual time machines that allow us to see into the past.
Hence, our telescopes do NOT allow us to see the much larger "visible?" universe that is "now".

The reason we can't see the center because the CMBR plasma gets in the way.

We still wouldn't see a center in the sense the OP was asking about. Because there is no center that can be defined in three dimensions for our universe.

On that we agree.

[Chris Peterson]

However, the general terminology states that our telescopes are virtual time machines that allow us to see into the past.
Hence, our telescopes do NOT allow us to see the much larger "visible?" universe that is "now".

That's fine. The point remains, we can only see each three-dimensional point at one time. That is very different than the way we see space, where every (x,y,z) is accessible to us.

[Rabelad]

Please explain how galactic collisions could occur when the Big Bang theory says that all matter in the universe was ejected from a giant explosion. If that is the case then all matter would be flying outward from the central point of the primordial explosion and each would be ejected at it's own unique angle. If that be the case then and none should interfere with another, as the Big Bang would have directed each at an angle that could not cross the path of any other. Ok, so gravitational forces somehow get to affect the galaxies, forces that are perhaps much stronger than the momentum of the ejected matter from the Big Bang. So if that be the case then such collisions should have happened much earlier in the evolution of the universe because they were all so much closer to each other, say, a couple of billion years back and not in these later times.

[Chris Peterson]

Please explain how galactic collisions could occur when the Big Bang theory says that all matter in the universe was ejected from a giant explosion. If that is the case then all matter would be flying outward from the central point of the primordial explosion and each would be ejected at it's own unique angle. If that be the case then and none should interfere with another, as the Big Bang would have directed each at an angle that could not cross the path of any other. Ok, so gravitational forces somehow get to affect the galaxies, forces that are perhaps much stronger than the momentum of the ejected matter from the Big Bang. So if that be the case then such collisions should have happened much earlier in the evolution of the universe because they were all so much closer to each other, say, a couple of billion years back and not in these later times.

Most galaxies are part of clusters- groupings that are gravitationally bound. That means that the cluster components orbit one another in a complex dance (and are not getting farther apart as the Universe expands). This can go on for billions of years with no collision taking place. Therefore, we can see recent collisions, and there are collisions that will occur in the future (such as between the Milky Way and the Andromeda galaxy). There is also solid evidence in looking at the structure of galaxies that many of them have been involved in collisions in the distant past.

[Osage]

Thanks, I have a habit of thinking in linear three dimensional terms. That explains it nicely.