[[Mod. note -- It seems the news server to/from which I post moderated
articles no insists on having a valid domain name for the From: line,
so I had to edit the author's spam-proofing. My apologies for any spam
which gets through now which might have been blocked by the original!
-- jt]]
In article <mt2.0-9640-1050410…@sshserv.aei.mpg.de>, Kent Betts
<kent_be…@hotmail.com> writes:
> This article about very old supernovae states that in the early few
> billions of years that the expansion of the Universe was slowing down.
> http://www.spaceflightnow.com/news/n0304/10supernova/
> It has now been observed that the expansion of the Universe is accelerating.
> I was wondering whether any sort of masking effect on gravity has been
> proposed. Has it been suggested that perhaps the mutual attraction might
> eventually diminish over great distances?
People have suggested all kinds of things as alternatives to "standard"
cosmology. Almost always, such suggestions create more problems than
they solve.
Cosmology has been transformed, in just the last few years, from an
armchair pursuit to a data-driven science. Classical cosmology can be
defined as working out the dependence of some observational quantity,
such as the brightness of a standard candle, on redshift for a given set
of cosmological parameters. By comparing theory to observation, one
fits for the cosmological parameters. At the moment, many different
types of cosmological tests result in the same values for the
cosmological parameters. This lends a lot of credibility to the basic
picture. Any alternative suggestion, in order to be taken seriously at
all, would have to make equally quantitative predictions AND have those
agree with observations AND show that the predictions could be obtained
without knowing the observations in advance.
> If gravitation were somehow masked, or less efficient, over great distances,
> wouldn’t the observed effect be an increase in the rate of expansion? I ask
> this because it is usually ascribed to mysterious repulsive forces.
It seems to me that masking gravitation would be much more mysterious
than repulsive forces. I’m sure some GR experts will point out why it
is very improbably that gravity can be masked. (H.G. Wells, IIRC, wrote
a story in which a material called Cavorite was used to mask gravity.
But that is (old) science fiction.)
At the moment, all the observations are consistent with the repulsive
force being a "traditional" cosmological constant ($\omega = -1$ for the
experts). In some sense, we don’t know "what that is", but in more or
less the same sense we don’t know WHY gravity exists.
> Thx for info. (I did not find a cosmology newsgroup on my server, BTW.)
Sci.astro.research and sci.physics.research are probably the most
appropriate.
——————————————————————————-
CALIFORNIA magazine, in an article on "The Man Who Invented Time Travel", even
ran a photograph of me doing physics in the nude on Palomar Mountain. I was
mortified—not by the photo, but by the totally outrageous claims that I had
invented time machines and time travel.
—Kip Thorne
"Kent Betts" <kent_be…@hotmail.com> wrote in message
news:mt2.0-9640-1050410670@sshserv.aei.mpg.de…
> This article about very old supernovae states that in the early
few billions of
> years that the expansion of the Universe was slowing down.
> http://www.spaceflightnow.com/news/n0304/10supernova/
> It has now been observed that the expansion of the Universe is
accelerating.
Would it be meaningful to say, rather than everything is
accelerating away from everything else, that space is created
between masses and the rate of creation of space is increasing? In
other words, space or the fundamental structure of the universe
itself is expanding like a balloon blowing up. This process is a
small enough factor that it is grossly noticeable only at
interstellar and intergalactic distances. Of course if it is true,
it should be measureable even within solar system distances with
careful observation.
I think of the speed of light as the rate of information
propagation across this structure.
I realize the Michaelson Morley experiment was supposed to
disprove the idea of an ether, but of course what it proved was
that the fundamental structure did not interact with the
propagation of light.
Brad Jensen
[[Mod. note -- Is there any operational way to tell the difference
between the two statements
(a) "the distance between a and b is such-and-such a function of time"
(b) "a and b are at rest, but space is being created between them"
I can't offhand think of any experiment or observation to distinguish
these statements...
-- jt]]
In article <mt2.0-1319-1053107…@sshserv.aei.mpg.de>, b…@elstore.com
writes:
> [[Mod. note -- Is there any operational way to tell the difference
> between the two statements
> (a) "the distance between a and b is such-and-such a function of time"
> (b) "a and b are at rest, but space is being created between them"
> I can't offhand think of any experiment or observation to distinguish
> these statements...
> -- jt]]
Imagine this R(t) diagram.
^—
/
/
/
____/
X Y
Light is emitted at t=X and received at t=Y. In case a), there will be
no redshift measured. In case b), there will be.
I recommend the "Redshifts" chapter in Edward Harrison’s COSMOLOGY, THE
SCIENCE OF THE UNIVERSE. (Indeed, I recommend the whole book.)
In article <mt2.0-4555-1053128…@sshserv.aei.mpg.de>,
Phillip Helbig—remove CLOTHES to reply <helbig-CLOT…@astro.multivax.de> wrote:
- Hide quoted text — Show quoted text -
>In article <mt2.0-1319-1053107…@sshserv.aei.mpg.de>, b…@elstore.com
>writes:
>> [[Mod. note -- Is there any operational way to tell the difference
>> between the two statements
>> (a) "the distance between a and b is such-and-such a function of time"
>> (b) "a and b are at rest, but space is being created between them"
>> I can't offhand think of any experiment or observation to distinguish
>> these statements...
>> -- jt]]
>Imagine this R(t) diagram.
> ^—
> /
> /
> /
>____/
>X Y
>Light is emitted at t=X and received at t=Y. In case a), there will be
>no redshift measured. In case b), there will be.
I’m sure that I don’t disagree with you about the physics, but I think
I do disagree with this verbal description of it. To me, both (a) and
(b) are reasonable (although incomplete) ways of describing the same
physics, namely the physics of the Robertson-Walker metric (according
to which, of course, there is a redshift in this situation).
You seem to be assuming that (a) is describing a situation
in which things are moving around in ordinary Minkowski space
or some such thing. But to me, (a) certainly doesn’t imply that.
In particular, (a) is clearly a literally true statement in
the standard general-relativistic Robertson-Walker model.
>I recommend the "Redshifts" chapter in Edward Harrison’s COSMOLOGY, THE
>SCIENCE OF THE UNIVERSE. (Indeed, I recommend the whole book.)
It’s been a while since I looked at this book; I’ll have to go back
and read it again to see exactly what Harrison says.
-Ted
–
[E-mail me at n...@domain.edu, as opposed to n...@machine.domain.edu.]
In article <mt2.0-3175-1053244…@star.bris.ac.uk>,
eb…@lfa221051.richmond.edu writes:
> >> [[Mod. note — Is there any operational way to tell the difference
> >> between the two statements
> >> (a) "the distance between a and b is such-and-such a function of time"
> >> (b) "a and b are at rest, but space is being created between them"
> >> I can’t offhand think of any experiment or observation to distinguish
> >> these statements…
> I’m sure that I don’t disagree with you about the physics, but I think
> I do disagree with this verbal description of it. To me, both (a) and
> (b) are reasonable (although incomplete) ways of describing the same
> physics, namely the physics of the Robertson-Walker metric (according
> to which, of course, there is a redshift in this situation).
> You seem to be assuming that (a) is describing a situation
> in which things are moving around in ordinary Minkowski space
Right, I was assuming that the moderator’s note used "distance between a
and b is…" as short-hand for "moving around in ordinary Minkowski
space".
Of course, if one uses the term "distance" in cosmology, one must first
define it. It is possible that one type of distance increases with time
while another type decreases.
Where things get confusing (and a lot of people get confused) is when
one has "ordinary" motion through space coupled with the expansion of
the universe. See astro-ph/0104349 for some interesting thoughts on
this.
> > [[Mod. note — Is there any operational way to tell the difference
> > between the two statements
> > (a) "the distance between a and b is such-and-such a function of time"
> > (b) "a and b are at rest, but space is being created between them"
> > I can’t offhand think of any experiment or observation to distinguish
> > these statements…
Of course, as Ted points out, in an expanding Robertson-Walker
cosmological model, both a) and b) can be valid (if incomplete)
descriptions of the same phenomenon.
More interesting is perhaps the question how one can distinguish between
the two cases a) where the distance (to be specific, the proper
distance) between two objects is increasing due to the expansion of the
universe (i.e. "space being created between them", "at rest in space")
and b) where it is increasing since they are moving through space apart
from each other. Of course, in general, since b) takes a finite time,
a) will be present as well.