Modern cosmology has introduced the concept of
'expanding space'. Cf. the inflation theory of
creation, and recently the dark energy 'negative
gravity' hypothesis. In these scenarios, it is
posited that the expansion can actually be faster than light.

This bugs me, because from an instrumental viewpoint,
things move apart, right? All motion is relative.
Then, what is the difference between 'A and B are
separating by moving through space', and 'A and B
are separating as space expands'?

(Some will object to the concept of moving through
space, but I think it's clear in context)

Let's put it another way. Suppose we observe
Andromeda flying away at .8 c. Is it possible to
say, "Andromeda is really moving at only .6 c;
the extra .2 c comes from space expanding".
Is that meaningful in any way? I don't get it.

In particular, if space expands at greater than
c, wouldn't this represent a catastrophe for relativity?


--
Rich

RichD wrote:
Modern cosmology has introduced the concept of
'expanding space'. Cf. the inflation theory of
creation, and recently the dark energy 'negative
gravity' hypothesis. In these scenarios, it is
posited that the expansion can actually be faster than light.

This bugs me, because from an instrumental viewpoint,
things move apart, right? All motion is relative.

Let me stop you right there Rich.
If a ship takes off from mars moving - through space -
toward the earth you can't say that the earth is moving
through space toward the ship. Can you?
One motion is absolute the other is only relative.
Unfortunately Einstein didn't see the difference.
But it is there.

Then, what is the difference between 'A and B are
separating by moving through space', and 'A and B
are separating as space expands'?

(Some will object to the concept of moving through
space, but I think it's clear in context)

Let's put it another way. Suppose we observe
Andromeda flying away at .8 c. Is it possible to
say, "Andromeda is really moving at only .6 c;
the extra .2 c comes from space expanding".
Is that meaningful in any way? I don't get it.

In particular, if space expands at greater than
c, wouldn't this represent a catastrophe for relativity?


--
Rich

You are right to bring it up. We are not finished with
it. We have just begun.
Mitch Raemsch -- Light Falls --

"RichD" wrote in message
ups.com...
Modern cosmology has introduced the concept of
'expanding space'. Cf. the inflation theory of
creation,

Careful, it's not a 'Creation' theory.

and recently the dark energy 'negative
gravity' hypothesis. In these scenarios, it is
posited that the expansion can actually be faster than light.

This bugs me,

What bugs you? Inflation, or the fact that expansion can
occur at speeds greater than light speed?

because from an instrumental viewpoint,
things move apart, right? All motion is relative.
Then, what is the difference between 'A and B are
separating by moving through space', and 'A and B
are separating as space expands'?

On difference is that bodies in the universe can be not
only separating, but accelerating with repect to one another,
yet they will not experience any net force!


(Some will object to the concept of moving through
space, but I think it's clear in context)

Let's put it another way. Suppose we observe
Andromeda flying away at .8 c. Is it possible to
say, "Andromeda is really moving at only .6 c;
the extra .2 c comes from space expanding".
Is that meaningful in any way? I don't get it.

Andromeda is next door. It's gravitationally bound in our
local group, so it won't participate in universal
expansion (relative to us).


In particular, if space expands at greater than
c, wouldn't this represent a catastrophe for relativity?

Not at all. General Relativity does not place a speed limit
on how fast space can expand.

RichD wrote:

In particular, if space expands at greater than
c, wouldn't this represent a catastrophe for relativity?

Good question, I see Greg Neill has offered some help.. others probably will as
well, just a helpful hint, don't respond or listen to anything people from the
Operation Troll Shoulder list throw at you.. they'll only confuse you much much
more.

Uncle Al, Sam Wormley, Bjoern Fuerbacher (spelling?), and Franz Heymann, are 4
very good sources to listen to.

Seeya

Rich, M'n'M is the likes of Modern Day Crusades.
Not everybody on his list is what he says they are.
He is the self apointed authority of authority on this board.
But you can see that for yourself.
Just watch what he does.
He doesn't have the guts to post any physics.
He just tags along hoping to effect people.
Mitch Raemsch -- Light Falls --

RichD wrote:
Modern cosmology has introduced the concept of
'expanding space'. Cf. the inflation theory of
creation, and recently the dark energy 'negative
gravity' hypothesis. In these scenarios, it is
posited that the expansion can actually be faster than light.

Yes.


This bugs me, because from an instrumental viewpoint,
things move apart, right? All motion is relative.
Then, what is the difference between 'A and B are
separating by moving through space', and 'A and B
are separating as space expands'?

There is no difference -- "moving through space" cannot be defined in a
measurable way (only "moving relative to these coordinates" can be).

In these cosmological models, spacetime is modeled as one of the FRW
manifolds, in which the entire manifold is filled with noninteracting
"dust" particles, which are identified with the galaxies (or galactic
clusters). The cosmological expansion is measurable because as
cosmological time increases, the isochronous distance between any pair
of dust particles is increasing.

Note some such models have a period of expansion, followed by
an equally-long period of contraction, down to a big crunch.
But today is in the expansion period.


(Some will object to the concept of moving through
space, but I think it's clear in context)

Unfortunately, "moving through space" cannot be defined well enough to
be useful (despite the many attempts around here to discuss it). But see
above -- moving relative to coordinates defined by the other dust
particles can be well defined.


Let's put it another way. Suppose we observe
Andromeda flying away at .8 c.

Actually the Andromeda galaxy is approaching the Milky Way. It is very
close to us (on a cosmological scale), and its local motion exceeds the
general cosmological expansion.


Is it possible to
say, "Andromeda is really moving at only .6 c;
the extra .2 c comes from space expanding".

No.


In particular, if space expands at greater than
c, wouldn't this represent a catastrophe for relativity?

No.

In GR the speed limit of c applies to the velocity of timelike objects
relative to locally-inertial coordinates. There is no constraint on this
cosmological expansion. In the standard model of cosmology, there is a
period of inflation just after the big bang that exceeds c by several
orders of magnitude.


See the FAQ for sci.astro for more details....


Tom Roberts

"RichD" wrote in message
ups.com...
Modern cosmology has introduced the concept of
'expanding space'. Cf. the inflation theory of
creation, and recently the dark energy 'negative
gravity' hypothesis. In these scenarios, it is
posited that the expansion can actually be faster than light.

The assertion of expansion of empty space (inflation) is designed to avoid
the implication that objects can move faster than the speed of light. It is
designed to save the SR postulate that no object can move faster than the
speed of light.

This bugs me, because from an instrumental viewpoint,
things move apart, right? All motion is relative.
Then, what is the difference between 'A and B are
separating by moving through space', and 'A and B
are separating as space expands'?

The difference is that the observed horizon of our universe appears to be
larger than the age of our universe. For this to happen, the objects must
move apart faster than the speed of light to acheive such a large horizon.
That's why the astronomers came up with the assertion that it is not the
objects in the universe are moving apart but rather it is space that is
expanding between the objects. I have a new model of the universe that will
avoid the horizon problem and at the same time explains the accelerated
expansion of the universe.
A paper entitled "Unification of Physics" is available at the following
link. It includes a new theory of gravity and it unites gravity with the
electromagnetic and nuclear forces naturally. Also, it includes a new
proposed experiment to detect physical space.
http://www.journaloftheoretics.com/L...apers/Seto.pdf

Ken Seto

RichD wrote:
Modern cosmology has introduced the concept of
'expanding space'.


I would not say it was modern cosmology that introduced it. Some
believe in it and some don't.


Cf. the inflation theory of
creation,


No, it is not a theory of creation, it is an addition to the BB Theory
invented so as to plug some of the holes left there by the BBT.


and recently the dark energy 'negative
gravity' hypothesis. In these scenarios, it is
posited that the expansion can actually be faster than light.


No, they have nothing to do with the Inflationary Period theory. The
IPT was posited long ago as an event which occurred way faster than c
and deflated just as fast, apparently. As you learn more about
Theoretical Physics, you will see that impossible obstacles like c mean
nothing when creating unfalsifiable concepts.

One of those is the expansion of space theory, which is tied in with
the concept that space came out of the BB along with all the other
stuff. The BB exploded into a so-called "Great Void" which was and
must still be a place which has nothing in it - not even space!?!


This bugs me, because from an instrumental viewpoint,
things move apart, right? All motion is relative.
Then, what is the difference between 'A and B are
separating by moving through space', and 'A and B
are separating as space expands'?

(Some will object to the concept of moving through
space, but I think it's clear in context)

Let's put it another way. Suppose we observe
Andromeda flying away at .8 c. Is it possible to
say, "Andromeda is really moving at only .6 c;
the extra .2 c comes from space expanding".
Is that meaningful in any way? I don't get it.

In particular, if space expands at greater than
c, wouldn't this represent a catastrophe for relativity?


Yes, indeedy, it would and it does.
TomGee