"Sylvia Else" wrote in message
...
Greg Neill wrote:
Sylvia Else wrote:
"Greg Neill" wrote in message
. ..
Ken S. Tucker wrote:

Are you guys painting yourselves into a corner? I think so.
Sam, when you say, "cannot see", you're presuming no EMR can
be received from Galaxy 1 to 2, yet Hubbles constant only red shifts.
We shouldn't find them moving at relative speeds greater than "c",
otherwise toss out SR and the Conservation of Mass-Energy Law,
as has been already done.
The Hubble constant tells us how fast space at a
given distance is expanding away from space at our
location. The matter in space moves along with this
so-called "Hubble Flow". This is why we say that
space is expanding.

Relativity does not place constraints upon how fast
regions of space may be moving with respect to
each other, only on how fast anything may move *in*
space.
It places constraints on how fast things can be moving relative to us.
In particular, that they cannot be moving at more than c relative to us,
though two objects in our frame of reference may be separating at mroe
than c (thought not more than 2c).

The only constraint is on the motion of things in space.
Things beyond the cosmic horizon can certainly be moving
away at greater than c, but of course we will never be
able to observe them since the space they're in is moving
away at greater than c and nothing can get 'here' from
'there'.

Space beyond about 13.7 billion light years in any
direction is moving away from us at greater than c,
so light from anything past that distance will never
get here. That is our 'cosmic horizon'.
They may be outside our light cone as a result of inflation in the early
universe, but that doesn't mean they're receding at more than c - just
that the light from them hasn't had time to reach us since the big bang.

No, light from there can *never* get here no matter how long
you wait. The space its in is being carried away faster
than the light can move towards us.


Doesn't sound right. It would imply that there are closer places that it
can reach, but that having reached them, it can't reach us. For such a
model to work, the universe would have to contain discrete subspaces.

Sylvia.


No, what he says is scientific orthodoxy.

The subspaces do not have to be discreet. Imagine you have a car that can go
100 kph, and its on a giant racetrack (say 10,000 kms around). Now here is
the even more unlikely scenario - the racetrack is expanding by getting
1,000 kms longer every hour. Wherever you start on the racetrack, there are
some places you can potentially drive to, and other which you can't. You
couldn't possibly ever reach the opposite point on a circular track, because
it is receding at 500 kph and you can only drive at 100 kph. You could
certainly however drive to a point only a few metres away. Everybody has
their own private universe of points they can effectively drive to.

(Breath)

This begs quite a few questions, and on first viewing has some butt ugly
aspects.

The first is that if the Universe is expanding, how can we talk about some
point in space at time t_1 being the same point in space at time t_2 ? Well,
in some sense we can't, but that doesn't really matter to the maths.

The second is the concept that objects are moving relative to us at faster
than the speed of light, which they are. Indeed, this is another one of
those delightful examples of "things" that appear to move faster than light,
but do not involve information (causality) moving faster than light, and so
in a sense validate Special Relativity (other examples are in quantum
physics).

The final one for me is the philosophical problem that there are places we
can't in principle see or visit. That's not a short term problem. And that
doesn't mean we can know nothing about it; when the Universe was 10^(-20)
seconds old it was tiny, and there was information transfer. That is still
detectable; indeed its why we think it exists at all, as the gravity models
suggest the Universe is much larger than we can see.

So, yes, unfortunately it is highly likely that most of the Universe is
invisible to a telescope of any size, and we have no way even in principle
of seeing them.