T Wake wrote:
"Nick" wrote in message
oups.com...
what is the geometry of stretching space?


What do you mean by "stretching space?"

IIRC the expansion of space was not a stretch, more an increase in the
distance between large scale structures.

Depending on who you talk to. If you talk to me, that's what I would
tell you. I would also tell you that "space stretching" is a misnomer,
but a lot of people don't know that. They use dots on a polka-dot
balloon to show that the center of an expanding two dimensional surface
is not necessarily on the surface, then go on to say that it is the
same with our universe.

The analogy is very vague, and it must break down at some point. Does
the universe wrap around on itself like the surface of the balloon?
Does the center of the universe lie on a vector perpendicular to all
spatial directions? Do the dots on the surface of the balloon expand,
or do they shrink relative to the surface of the balloon, being held
together by gravity and molecular forces? If the dots on the surface
expand, then of course, it would not seem like the universe were
expanding at all, since our meter sticks would expand at the same rate.

Finally there is no cause and effect related to this balloon model. We
don't see some evidence pointing to the balloon and only the balloon.
All we see is that objects in the universe are moving away from us. We
also see that there is a dim all-pervasive black-body radiation called
the CMB. The CMB has been carefully mapped, and resembles an acoustic
signature of a roughly spherical object (for instance, a balloon)... As
long as you don't look to close. An acoustic signature would, of
course, involve molecules bouncing off each other and exchanging energy
across the surface of the sphere, whereas the surface of this sphere
has a radius of at least 13 billion light years.

However, to a number-cruncher, an acoustic signature is an acoustic
signature, and currently the money (whatever tiny amount there is), and
the interest (also pretty small, I think), is in looking at the
anomolies in the acoustic signature, and trying to find local causes
for the lowest frequency elements of the acoustical signature to be
missing.

The issue of these missing frequencies, according to the August issue
of Scientific American "could send us back to the drawing board about
the early universe." I've not been able to understand what is on the
current drawing board, but I do know one thing that is missing: A
proper appreciation for the relativistic effects due to the momentum of
the receding objects.

Why are people ignoring these effects? I gather that people believe
"The galaxies aren't really receding. The space is expanding between
the galaxies making them appear to recede." Which returns us to Nick's
question. What is the geometry of stretching space?

The clearest answer is what T Wake has given. Space is not stretching.
The objects within it are moving apart. However, this is in direct
conflict with the belief that the galaxies are not really receding.

Finally, another issue obfuscates this further. There is more than one
way to define simultaneity in cosmology.

The first way is what I would think is most obvious, to simply take a
photograph, and anything on a surface equidistant from the lens that
shows up in the photo should be considered simultaneous, making some
minor corrections for differences in gravitational potential of the
source image. *totally obvious, right? actually it sounds a lot more
complicated than I thought.*

The second way to define simultaneity is by measurement of any
particular object's proper age, that is the time it would measure
itself aging since, for instance the big bang. It is in this model,
where if you plot positions versus time, you actually get a universe
which is infinite in extent, completely homogeneous, and expanding over
time. This seems to me to be an odd way of defining simultaneity, but
it does make some of the other often repeated statements about
cosmology make more sense.

So there are a lot of arguments caused because there is more than one
context in which to describe the phenomena, analogies are useful in one
context and not in another, and the mathematics are sometimes suggested
without reference to specific problems. The answers are vague, and the
questions are even more vague, and the more people know about the
topic, the less they want to say, because they are more aware of what
they don't know.

This is just the way it seems to me this morning.