The following is an answer to a question about the size and expansion of the Universe. As time permits, I will alter its structure, so that it becomes a stand-alone discussion, but for now, it is presented as-is. The reason for this post, aside from its general interest, is that some of the topics covered here are not always correctly presented, even by astronomers (that was, in fact, the point of an interesting Scientific American article of a few years' back).
Question: What is the size of the Universe (in light years)? Is the size of the
Universe expanding, and if so, at what rate? Will it continue to do so, or will it stop at some point, then begin to contract?
There are some complications involved in answering your question, so a
simple answer isn't necessarily correct, and a correct answer may be more
complicated than you want. So I'll start with simple statements, and expand on them.
(1) The size of the Universe is not known, but is thought to be hundreds or
thousands of times larger than the size of the "observable" Universe, which is the
portion small enough that light has had time to get from "there" to here. The radius
of the observable Universe in light years is therefore the same as its age in years,
or about 14 billion.
(2) Both the overall and observable Universes are expanding. The observable Universe
is expanding at one light year per year, because we can see that much further each
year. The overall Universe is expanding much faster than that, depending upon how
much bigger it is.
(3) The local rate of expansion of the Universe is now roughly constant, at about
one light year of expansion for each light year of distance per 10 to 15 billion
years, and should remain near that value for the rest of eternity. So the Universe
will expand forever.
(4) Although the rate of expansion of the observable Universe is constant at one
light year per year, and the local rate of expansion is more or less constant at one
light year expansion per light year of distance per 10 to 15 billion years, the rate
at which individual objects move away from us is accelerating, because they are
getting further away from us. An object now 200 million light years away is moving
away from us at about 200 million light years per 10 to 15 billion years. But in 10
to 15 billion years, it will be 400 million light years away, and since it will be
twice as far away, it will be moving away from us twice as fast. In other words, the
local rate of expansion is constant, but distant objects move away from us faster
and faster, over (very) long periods of time.
(5) The discussion in (4) only applies to things that are far away. Things less than
a few tens of millions of light years distance (such as the local supercluster) are
gravitationally bound, and do not move away from us. They will always be about the
same distance from us that they now are. But in the very distant future, all things
beyond that "nearby" region will be unimaginably far away, moving away from us at
faster-than-light speeds, and the observable Universe, though much larger than now,
will be completely empty, save for the gravitationally bound region close to us.
(6) When we say that the observable Universe has a radius in light years equal to
its age in years, that is based on the idea that the light must have been traveling
that long to get here. But since the Universe was expanding while the light
traveled, the distance it has had to travel is larger than the distance between us
and its source at the time the light was emitted. So the most distant observable
regions were only about half the 14 billion year distance quoted in (1), at the time
the light we now see left them.
(7) As a corollary to (6), the objects which we now observe at the "edge" of the
observable Universe are now well beyond the 14 billion year distance, moving away
from us at more than the speed of light, and we will never see the light they are
now emitting, because the space between us and them is expanding faster than light
can travel through it. This is why, in (5), the contents of the observable Universe
seem to shrink over time, even though its size increases.
(8) Note that in any discussion of faster-than-light expansion, the speed-of-light
speed limit is not violated. That only applies to the motion of light or physical
objects through "local" space. There is no limit to how fast the empty space between
distant objects and us can expand; it just depends upon how far away they are, and
the local rate of expansion. The latter is constant, but as noted in (4), the former
is increasing at an accelerating rate.
(9) Finally, for now, all the above has to be modified for regions moving away from
us so fast that they have an expansion speed close to the speed of light, and
slightly modified for regions closer than that. Such distant regions are
"foreshortened" by their rapid motion away from us, so that the actual distance
between one part of the very distant observable Universe and a still more distant
part is larger than their separation as measured by us.