Re: Expanding Space

Dear Jack:

On Jan 24, 3:30 pm, "Jack" <cawo...@xxxxxxxxx> wrote:
Can anyone explain to me what "expanding space"
(a la Big Bang theory) actually means?

It means the volume of the Universe increases with time. Not that the
Universe is expanding into something, but that the relationship between
any bound collection of mass and any other is constantly changing.

If we notice 2 objects moving away from each other we
don't say space is expanding so why is this any different
when the objects are 2 galaxies or all galaxies?

If we look in opposite directions, at equal distances, we see objects
moving away from us at equal "speeds". What is more, the recession
"speed" increases with distance. This requires that each of those
galaxies see the Universe exactly the same way. This doesn't mean that
galaxies don't have non-expansion relative velocity... they do. It
just means that the distance vs. recession velocity is not due to some
"explosion", or some "kinetic flow".

Surely it's more appropriate to say that the physical
universe (matter, energy and what not) is expanding -
into space. Why do we say that space itself expands?

So you say, "surely it's more appropriate to say that the physical
universe ... is expanding - into space". We look in every direction,
and in every direction, galaxies are receding from us. How can we be
expanding unto some void, when there is stuff moving away from us in
every direction, no void detectable / observable? There is no
substance "space". What spacetime represents is the relationship
between each mass / energy center and *every other* such center. I
know that doesn't help much.

Pre-Einstein space seemed fairly intuitive but now it
seems to have been materialized/physicalized.

Only in popular press.

If space itself were indeed expanding would that not
mean that the length "1 meter" is ambiguous unless
a time is specified?

Actually no. Any standard length is based on physical law. The
physcial laws aren't changing, or are changing less than the current
Hubble parameter, so you obtain a meter, a second, and a kilogram just
as you always did.

Would we even be able to detect space expanding if
all out instruments are expanding with it?

Sure. Just look where we observe everything moving (more or less)
uniformly away from us. Jim Black recommended Ned Wright's site. An
excellent recommendation.

David A. Smith

.

Relevant Pages

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