Darren New <dne### [at] sanrrcom> wrote:
> I've never understood whether the expansion of space also implies the 
> expansion of the particles in that space.

  The current theory of the expansion of the universe is not a simple one.

  This is how I have understood it after struggling with it for quite some
time (errors are possible):

  Galaxies do not expand because their gravity keeps them in shape. In
other words, gravity inside (and near) galaxies is strong enough to
"resist" the expansion. Consequently nothing inside galaxies expands.
Dark matter halos surrounding galaxies might play a role in this too,
although this is only my own speculation.

  In intergalactic space, far away from galaxies, the gravity caused by
galaxies is too weak to "resist" expansion and thus space expands there.
This is the reason why galaxies recede from us, but stars in our galaxy
don't.

  The expansion of the universe doesn't imply movement of the galaxies.
The space itself expands, and distances get larger. (No, I don't know
for sure what is it exactly that causes this. I'm told that according
to GR this would happen even without dark energy.)

  This is also the reason why galaxies can recede from us faster than c.
They are not actually moving, it's just that "new space" is forming between
us and those galaxies, making the distance between us and them larger.
There's nothing stopping this from making the distance grow faster than c,
not even in GR.

  This causes, among other things, for it to be possible for there to be
a so-called cosmological horizon: From certain distance forward from us
there's universe which we can never see, no matter how long we wait.
Nothing from there ever arrives here, and we can never go there. This is
because those parts are receding from us faster than c. The actual size
of the universe is pure speculation (it's impossible for us to know it).
Conjectures range from the universe actually being much smaller than what
is observable to it being staggeringly larger.

  Another slightly difficult thing to grasp is that a constant expansion
of the universe actually means that galaxies recede from us at an exponential
rate. At each certain unit of time space distances double, which means that
the distance between two galaxies grows exponentially with time. (If galaxies
receded at a constant velocity that would actually imply a logarithmic
expansion of the universe.)

  Now, what baffled scientists was that the expansion is not constant nor
slowing down, but actually accelerating (which I suppose means that galaxies
are actually receding faster than exponentially from us).

  Dark energy has been postulated as an explanation for this. It somehow
causes an inflationary effect on the universe, making it to expand in an
accelerated way.

  I have understood this as being a bit like this: While galaxies "pull"
the geometry of space, contracting it, and keeping it static, dark energy
has the opposite effect and "pushes" the geometry of space, inflating it,
a bit like a gas. Close to galaxies this "pushing force" is not strong
enough to overcome the "pulling" effect of gravity, but far away from the
galaxies it overcomes it and causes intergalactic space to expand.

  (All this is not based on professional literature, so don't take it
for granted. It's just how I have understood it with me extremely limited
understanding.)

-- 
                                                          - Warp

Post a reply to this message