Red Shift and Expanding Universe
Date: Fall 2011
It is about red shift and the outer galaxies. I do have a grasp of the concept of expansion. What I do not understand is and it bothers me: It is said that the outer galaxies of our observable universe are expanding between them faster than closer ones. First I am correct in saying that? Second, if I interpreted what I read correctly, would an observer at the edge of our observable universe, looking towards our galaxy see the opposing situation: an observable universe that is reducing in expansion, with that other observer's outer galaxies progressively expanding less (as this would be the galaxies closer to us)?
Hi Ivan -- If you were in that far-away galaxy, you would make the
same observation about galaxies near the milky way: that the
farther-away galaxies are moving away from you faster than galaxies
closer to you.
I think the best analogy to understand this is to blow up a balloon. Take a large balloon, and blow it up a little. Then, draw some dots on it (representing galaxies). Now blow up the balloon more -- notice how the dots move apart as you blow up the balloon? This is like how galaxies move apart in the expanding universe. Suppose you made a darker dot representing "here" -- and then compared the distance of farther dots as you blew up the balloon. You would notice that a dot that started farther from your 'here' dot ends of proportionally farther from the far-away do than it does from a closer dot. This is because there's more rubber (balloon) to expand between it.
Space can be thought of similarly -- the more 'space' there is between two galaxies, the more there is expanding, and therefore the faster the galaxies appear to be moving apart.
Hope this helps,
I am not up on all the latest, but do not forget that
looking far out in space includes looking far back in time.
If there has been any gravitational deceleration of an original big-bang,
and one maps all the velocities by observing red-shifts,
there should be some appearance of farther galaxies separating from each other at higher speeds.
The observer at the other end would be looking at our distant past,
not at us per se,
so he too would probably see faster expansion at great distance.
A symmetry of appearance like this also comes up in special-relativity.
If two parties pass each other at speeds approaching light,
each appears to the other to be more massive, foreshortened, and evolving slowly.
Modulations on the questions of
how fast gravity is decelerating expansion
and how far is the farthest extent of space in any observed direction
(i.e., is the edge of what we see the end of time and/or space,
and if not, how far past the presently observable horizon
farther does/did the universe go on...)
are what is being debated and explored lately.
Personally I am not too sure how much of what astrophysicists think they know at this point
will really be the surviving view in the future.
We might end up with big revelations or discouraging, bland limitations.
I would not yet try to interpret it all too tightly.
But faster expansion in the past just makes sense.
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Update: June 2012