Re: Our Expanding Universe

From: Alf P. Steinbach (alfps_at_start.no)
Date: 12/02/04 

Date: Thu, 02 Dec 2004 00:03:14 GMT

* BluMax:
> I have always wanted an answer that I can understand to the following question.
>
> Simply asked, "What is our Universe expannning into"?
>
> Please explain it assuming I am an *ordinary* 13 years old.

Oh, well.

 
> I finally found this question and its answer, in a FAQ called
> "Frequently Asked Questions in Cosmology":
>
> http://www.astro.ucla.edu/~wright/cosmology_faq.html

Presumably you mean

  <url: http://www.astro.ucla.edu/~wright/cosmology_faq.html#XIN>

You might also want to visit

  <url:
http://math.ucr.edu/home/baez/physics/Relativity/GR/expanding_universe.html>

 
> It is all "%^$@^$%@^" to me. :-(
>
> Thanks in advance to "anyone/everyone" who can explain it to
> me so that I understand.

To be honest there are issues regarding that expansion that I don't
understand -- I mention some of them in the discussion below.

First off, let's dispense with the incorrect idea that the _Universe_,
also known as "everything", is expanding. If everything were expanding,
then there would be nothing to measure the expansion against, and it
would look like a non-expanding Universe to us. So what's not expanding
is ordinary matter, like atoms, planets, solar system, galaxies and so
on up to some limit (and don't ask, because the nature of that
non-expansion and the limit to it is one of those issues that neither I
nor anyone I've ever asked understand; generally it's sweeped under the
carpet by some handwaiving and a selection of exorcist phrases like
"bound system"). And what's expanding is space, the distances between
particles, above the expansion limit. So summing up this paragraph: the
apparent expansion is not an expansion of the Universe, because that
would be impossible to measure, but a continuing
internal-to-the-Universe change of proportion between matter and space.

If you already have a nice, Newtonian "through space" view of the
expansion you might find it hard to believe that's correct, but read on.

Now consider a finite Universe. OK, I don't think it's on as physical
reality, but consider it. The question here is (regarding your "into"):
can a finite Universe exist without having a boundary, an outside?

The answer to that question is yes. If the Universe were
two-dimensional it could then be like the surface of a ball. If some
Flatlanders on that surface started expanding a small circle it would
get larger and larger until it circumscribed the ball (equator), and
then when they expanded it even more it would get _smaller_ and smaller
until it shrunk to just a point at the opposite side of the ball.

With three dimensions the analogous situation is a kind of wrapping of
space where, if you cut the finite Universe in two, the cut is the
surface of a ball, and each half is a ball. Instead of a circle you can
here expand a ball. When you have expanded it to half the size of the
Universe it's of the greatest size and corresponds to a half-and-half
cut, when you expand it even more it shrinks (like the circle)...

Possibly you fell off the wagon here, but such a warped space _is_
possible to visualize also for three dimensions, just very difficult.

For an infinite Universe it's not necessary to wrap/warp space to avoid
having a boundary, and that's partly why an infinite Universe is so much
nicer to think about (for those who are not shy about infinities).

Even those who are not thirteen might find it amusing at this point to
consider the idea of a fixed-size space where all matter is _shrinking_.
Instead of a Big Bang blowout the Universe (or any particular region of
it) stays the same size all the time. Only the matter is shrinking,
yielding apparently ever greater distances as measured by us... Instead
of the Big Bang, the Big Implosion (viewpoint). Or BIMP, as I call it.

Now to the question you could have been asking. If the Universe is
expanding at a fixed rate (Hubble's law), uniform constant expansion,
how come we say it's 13.6 billion years old (or whatever)? Wouldn't it
_always_ have been expanding, then?

Well, the "local expansion" idea is probably the most common
misconception about the universal expansion, and the question above the
most common never-asked natural question about that misconception.
Here's the misconception: that for any given time inverval, distances
between things are multiplied by some factor proportional to the time
interval. That _not_ how uniform constant expansion works. This is how
uniform constant expansion works: above the expansion limit things (such
as superclusters of galaxies) are speeding away from each other at, for
each pair, constant relative velocity, and the Big Bang is the time
where, if you run the film backwards, they would have collided at a
single point which then would have been the whole Universe (or in the
BIMP view: the point in time when space was so small compared to matter
that every little particle filled out the whole infinite Universe).

As you can see, constant expansion is not really constant like every
local little bit of space expanding on its own, being that many percent
larger each second: the universal rate of expansion of space is varying
_just so_ that it maintains constant relative speed between things that
are far away from each other (and yes: I don't understand that). That
speed increases with distance because it was always greater (at least,
that's the theory) between things that are now at greater distance. If
the relative speed between thing X and thing Y is now, say, V m/sec,
then it was V m/sec also at the Big Bang -- or thereabouts.

There is now firm evidence, e.g. from Hubble's ultra deep view, that the
expansion is not really really constant but a bit varying, and currently
accelerating. That's difficult to swallow because it dispenses with any
classical notion of how the Universe works, and makes it even more
difficult to visualize. But for practical purposes it can still be
regarded as constant, which means just Newton's law of inertia: when two
things have some relative speed, and nothing acts on them, they will
continue to have that relative speed (a comfortable "through space"
picture that is not and can not be true, but is, well, comfortable).

Here's how accelerated expansion could be a fundamental problem: mostly
everything is space, including your body, the inside of the atoms in
your body, and the inside of the particles in each atom. With
accelerated expansion that space is expanding, right now. Which means
that at every level -- fundamental particle, atom, molecule, human
body, planet Earth, solar system, our local galaxy, and so on up to the
expansion limit mentioned earlier -- there is an exact compensation.
Various systems are bound by different forces (depending on the system
size) that counteract the expansion. Just like you're now in the grip
of a balance of forces that keeps you on the surface of the planet
instead of sinking into it or sailing freely out into space. But there
could be measurable effects, especially over, say, 5 billion years. 

-- 
A: Because it messes up the order in which people normally read text.
Q: Why is it such a bad thing?
A: Top-posting.
Q: What is the most annoying thing on usenet and in e-mail?

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