JMFNYC":1js1bp40 said:
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as it is only 100,000 light years across, so what we see of the Milky Way in the sky will only be up to 100,000 years out of date.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 Is that a snapshot of what we see (which gets progressively out of date with distance, leading to a "bent" model), or an idealised view of what we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.
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