Thanks Davala-ga.
This was not only a challenging question, but a fascinating one as
well. It was great fun to research.
If anything I explain (or should I say...attempt to explain) below
isn't fully clear, just post a Request for Clarification and I'm happy
to provide additional detail.
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One of the sites I came across in my research was the Schlumberger
Science Center at:
http://www.slb.com/seed/en/notes/cos_gui.htm
which begins with the following understatement: "Cosmology is hard to
understand."
The site is aimed at science teachers. It goes on to discuss (a bit)
about the expanding universe, and encourages teachers to use that
classic educational tool -- an inflated balloon -- to illustrate how,
as the balloon expands, all points will grow apart from all others.
Two "galaxies" on the surface of the balloon grow further and further
apart the more the balloon is inflated.
But wait a minute. What happens to the galaxies themselves? And the
individual stars and planets? And the people living on those planets?
This is the gist of your question: Are we expanding too? And if we
are, how can we possibly know it, since our measuring tools -- all our
yardsticks -- are expanding too?
There are three ways to envision the expanding balloon universe that I
find helpful in answering your question:
1. Draw galaxies on the balloon's surface with a marker pen. As the
balloon inflates, the galaxies increase in size as well.
2. Put little galaxies (say, made of clingy plastic) on the surface
of the balloon. As the balloon inflates, the galaxies grow farther
apart, *but* each galaxy stays its original size.
3. Repeat #2, but this time, attach some of the galaxies together
with a string. Now, as the balloon inflates, not only do the galaxies
stay the same size, but the remain the same distance apart since the
string holding them together doesn't stretch, even as the balloon's
surface stretches apart underneath it.
So...which "balloon" do we live in? On a large scale, we live on
balloon #2. On a human scale, balloon #3. But we don't -- under any
circumstances -- occupy balloon #1. That is, space expands...we
don't!
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Let me digress just a bit here. You may be familiar with the Coriolis
Effect (sometimes called the Coriolis Force, although, strictly
speaking, it is not a force at all). Coriolis gives moving objects in
the Northern Hemisphere a counterclockwise twist, and is a very
important determinant for the behavior of winds and ocean currents.
But wait a minute. If Coriolis is everywhere, how come when I'm
walking down the street, or driving my car, I'm not being pulled to
the right, like the winds and the waters are?
(Coriolis doesn't even effect water in the toilets -- for a discussion
of this, you can check out the discussion at The Straight Dope):
http://www.straightdope.com/classics/a1_161.html
Turns out, Coriolis only operates at a macroscale, but doesn't effect
everyday human existence. For different reasons, the expansion of the
universe operates in much the same way -- on a broad, cosmic scale,
everything's being tugged apart by expanding space. But on a more
human scale, we stay the same size.
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Back to the balloons.
On balloon #2, as the balloon stretches, our little plastic galaxies
stay the same size. Why? The molecular forces in the plastic are so
strong as to completely overwhelm any "attachment" the galaxy has to
the balloons surface, so it does not get stretched along with the
balloon. (In contrast, the galaxies made of ink used on balloon #1
are very tightly bound, and do, in fact, stretch as the balloon
stretches).
Similarly, on balloon #3, the galaxies not only do not expand, but the
stay at a fixed distance. The molecular forces in the string are not
influenced by the strewtching of the balloon.
Now, the expansion of the universe isn't a physical "force" in the
conventional sense. Not is Coriolis. But both can appear to act like
forces, when there aren't any competing forces to overwhelm their
effect. For galaxies that are thousands of light years apart from one
another, there is no appreciable gravitational attraction between
them. The only "force" at play is the expansion of the universe. So
-- at these macroscales -- expansion is "felt' and the galaxies drift
apart.
At smaller scales, however, gravity becomes increasingly important,
and at still smaller scales, electromagnetic and atomic forces come
into play as well. Simply put, they hold things together. They are
the equivalent of the string in balloon #3, or the strength of the
plastic galaxy in balloon #2.
So....bottom line is, you and I and earth and the solar system aren't
expanding, even as the universe as a whole is expanding. And that, I
believe, answers the first part of your question.
But as you noted, the second question (how fast are we expanding) is
contingent on the answer to the first. Since we're not expanding, the
question becomes moot -- the only answer that makes sense would be to
say we're expanding at a rate of zero (choose your own units).
I've included some links below -- along with some material excerpted
from the sites -- that discuss your question in a bit more detail.
Since this is bound to raise some controversy, as well, I also want to
invite my fellow researchers -- as well as those in the peanut gallery
-- to post additional information and opinions as comments.
And again, let me repeat that if anything here is not clear (like they
said...cosmology ain't easy), just post a Request for Clarification to
let me know.
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University of Virginia, Department of Astronomy
http://astsun.astro.virginia.edu/~jh8h/Foundations/quest10.html
Question:
In the expansion of the universe, what is actually expanding? Is it
dark space where no matter exists? If so, how can "nothing" expand. If
"everything" expands, then how would we know that it is really
expanding? Wouldn't everything seem relative?
Answer:
In general relativity space isn't exactly "nothing" - it is curved by
mass and it tells mass how to move. It is physical and can have
properties, e.g., intrinsic geometric curvature. This can be defined
in a real, measurable and reproducable way. Expansion can be defined
phenomenologically: the worldlines of freefalling observers in the
cosmos will diverge with time. But this is significant only over large
distances and after large times. Further, all particles in the cosmos
are not freefalling with respect to the cosmos as a whole. For example
if I was floating in space between galaxies the atoms of my body might
like to participate in the Hubble flow, but the intermolecular forces
would hold me together - I don't expand any more than I am stretched
out by the tidal force difference between my head to my feet on the
Earth. Galaxies, clusters, solar systems are held together by their
own gravity.
Question:
I don't understand why, if all of space is expanding, galaxies and
clusters of galaxies don't expand along with it.
Answer:
Think of the tendency to expand as an expansion force. Free particles
will respond to this expansion force. However, the expansion force is
very weak - it only becomes significant over megaparsec scales.
Galaxies, stars, etc. can easily hold themselves together against this
force through their own self-gravity.
The scale at which gravitationally bound structures end and expansion
takes over is an interesting question. For example, as clusters of
galaxies become bigger, they are more loosely held together by
self-gravity, so it may be that at some stage, these huge apparent
clusters are in fact not bound together and will spread out over time.
Question:
If the universe is expanding, won't the solar system eventually expand
apart?
Answer:
No. The solar system's self-gravity is far stronger than the expansion
effect. The Milky Way stopped expanding when its mass was locally
great enough to pull itself together despite expansion.
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Another University of Virgina site: [you may want to visit here to
look at the illustrations]
http://astsun.astro.virginia.edu/~jh8h/Foundations/sphere.html
Expansion: A 2D Spherical Example
Consider a two-dimensional spherical surface upon which we have a
collection of 2D galaxies. Notice the lines of latitude: these are our
comoving coordinates. Now let's have this sphere expand a bit:
Notice that the galaxies stay the same size and in the same position
relative to the lines of latitude and longitude (comoving coordinates
stay constant). But because the sphere has expanded, there is now a
greater distance between each galaxy. The further away two galaxies
were at the start, the greater their separation now. This is the
Hubble Law.
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North Carolina State University
http://www.ncsu.edu/felder-public/kenny/papers/cosmo.html
A problem arises when we consider an expanding universe. Suppose
everything in the universe were to double in size. The distances
between galaxies would double, the size of the Earth would double, the
size of all our meter sticks would double, and so on. It would seem to
an observer (who will also have doubled in size) as if nothing had
happened at all. So what do we mean by saying the universe expands?
In fact, not everything grows as the universe expands. In the example
of the rubber sheet, the distance between thumbtacks keeps increasing
but the thumbtacks themselves remain the same size. Similarly, while
distant galaxies are pulled away from each oth er by the expansion,
smaller objects like meter sticks, people, and the galaxies themselves
are held together by forces that prevent them from expanding. So we
expect that billions of years from now galaxies will still be roughly
the same size they are to day, but the distances between them will on
average be much larger.
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search strategy: "expanding universe"
I also based my answer on personal knowledge |
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