Shape of the universe

[JMFNYC]

And every star in the Milky Way other than the sun may have exploded last week yet we have that nice map from National Geographic. My original question factored in a suddenly static universe, just as the Nat. Geo. map is a snapshot. Assume that for the next 13 billion years each galaxy stays exactly where it is...RIGHT NOW. ok so what will this map look like then?

 

[SpeedFreek]

And every star in the Milky Way other than the sun may have exploded last week yet we have that nice map from National Geographic. My original question factored in a suddenly static universe, just as the Nat. Geo. map is a snapshot. Assume that for the next 13 billion years each galaxy stays exactly where it is...RIGHT NOW. ok so what will this map look like then?

A map of the galaxy is relatively easy, as it is only 100,000 light years across, so what we see of the Milky Way in the sky will only be up to 100,000 years out of date. A galaxy is a relatively stable and predictable structure over that sort of timescale, so that picture of the Milky Way is probably pretty accurate. You say the Nat. Geo. map is a "snapshot". Is that a snapshot of what we see (which gets progressively out of date with distance, leading to a "bent" model), or an idealised view of what we think the galaxy would look like from a distance?

But unlike a stable galaxy which we only have look across 100,000 years for, the universe has been dynamic over its 13,700,000,000 year history. We think it is 46 billion light-years in radius, based on the Cosmic Microwave Background Radiation. We think that the region of space that the CMBR we currently detect was emitted from was originally around 42 million light years away but is now 46 billion light years away.

We think that, just as galaxies have formed in this part of the universe, galaxies will have formed throughout, so that there will be galaxies that are around 46 billion light years away right now at the edge of our observable universe, but all we see from those regions of space is the background radiation that was emitted perhaps half a billion years before any galaxies even formed!

The most distant (in time) galaxy we have detected was around 3.5 billion light years away when it emitted the light we see. It emitted that light nearly 13 billion years ago. Just like stars come and go, so do galaxies (through mergers etc). We might extrapolate that galaxies position so it is where we think it might be today, which is something like 28 billion light years away, but we have no way of knowing if it even exists.

The most distant (in terms of how far away they were when they emitted the light we see) galaxies we have detected were over 5.7 billion light years away, but they emitted that light only 9 billion years ago and they are now thought to be something a little under 14 billion light years away today, if they are still where they were relative to the galaxies around them.

But the evolution and mergers of galaxies over the history of the universe would be such a dynamic process that the possible margin for error would be so large at the scales we are talking about that they would be essentially meaningless, except as a model example of what a generic universe like ours might look like, which would be as accurate as that picture from the atlas of the universe I linked earlier in this thread.

Across the history of the universe, galaxies are swirling around in clusters, sometimes hitting each other and spinning off in wild directions towards other galaxies, all of which are in turn swirling around in larger clusters, which are in turn becoming more distant from each other, with the gravity of those clusters tugging all the galaxies at the edges around and stretching connecting "filaments" of galaxies between them.

And when I say the universe is dynamic at the large scales, that is an understatement. Cosmologists use idealised generic models when talking in terms of "now" across the universe, as the concept of "now" only ever really applies at the very local scale. At the larger universal scales, concepts of time and space are so intertwined that the idea of "now" really does have no meaning at all.

For instance, did you know that, from your point of view, when you pace back and forth in your living room and consider a distant galaxy that you happen to be walking towards, any notion of "now" you might have for that galaxy actually leaps backwards and forwards by hundreds of years as you change direction! This sounds ludicrous, I know. Of course, as you pace back and forth time isn't actually leaping backwards and forwards in that distant galaxy (except from your point of view), but the notion of "now" across space and time (space-time), the simultaneity between events at large distances, is an entirely dynamic relationship.

I have heard of a few 3D model simulations of an expanding universe that would be as good a "map" as any, the Millenium Simulation being one of the best. It uses billions of "test particles" to represent galaxies.

Probably the best proper map we have is from the Sloan Digital Sky Survey. The map on the front page covers a radius of only 2 billion light-years.

 

[JMFNYC]

Speed if the universe is 13.7 billion years old, how can ANYTHING be 46 billion light years away from us? Isn't the diameter of the universe thus 27.4 billion light years? And therefore the maximum distance anything could be from us 27.4 billion light years?

 

[Saiph]

the key to your last question is that expansion isn't limited by the speed of light. Space can expand or contract, or twist or whatever, at any speed it wants. The speed of light limit is ONLY for objects moving THROUGH space.

 

[JMFNYC]

Ok. So right now are the furthest STARS from earth, at the other side of the universe

A. 27.4 billion light years away
B. More than 27.4 Billion light years away but we cannot yet calculate exactly how far.
or C. it is impossible to calculate.

 

[SpeedFreek]

Ok. So right now are the furthest STARS from earth, at the other side of the universe

A. 27.4 billion light years away
B. More than 27.4 Billion light years away but we cannot yet calculate exactly how far.
or C. it is impossible to calculate.

B. Something around 46 billion light years away, at the edge of our observable universe, which is something over 90 billion light-years across.

We can see the CMBR radation emitted from points on opposite sides of the sky, radiation emitted from regions of space we now think will be around 46 billion light-years away in each direction. We are halfway between those regions, we are at the edge of the observable universe of any galaxies currently in those regions.

Have a read of this PDF article - it will explain some of what we are saying here:

Misconceptions about the Big Bang.

 

[JMFNYC]

Speed thank you for that link - the article is of great interest and help. I am having trouble with the concept of "expansion" I think, because I have always had a false picture of the big bang as a bomb or explosion going off. Question: Is it impossible for the universe to slow down and cease expansion altogether at some point in the future?

 

[SpeedFreek]

Question: Is it impossible for the universe to slow down and cease expansion altogether at some point in the future?

I wouldn't say it was impossible, but it seems highly unlikely in light of recent observations that the expansion is accelerating. It all depends on what is causing that apparent acceleration as to whether the acceleration will continue or not. If it continues, then the expansion will never cease.

 

[Saiph]

it is possible, indeed it was considered the likely state of affairs only a few years ago. However recent evidence shows an accelerating expansion, and no indication of a change. That doesn't mean there won't be a slowing/contraction, only that as it stands, we wouldn't understand how or why it would happen (heck, we don't understand the accelerated expansion either!!)