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Warp <war### [at] tag povrayorg> wrote:
> > I think it was more popular before people found dark energy and thought we
> > were heading for a "big crunch" or something?
>
> I think the word "found" is wrong here. It hasn't been measured; it's a
> hypothesis. (It could perfectly well be the *correct* hypothesis, but until
> it's measured it's just that.)
Yeah; AFAIU it, 'dark matter' and 'dark energy' are somewhat ad-hoc
postulates--there was a 'hole' in the current cosmological model for awhile,
where certain observations about the universe just didn't match the model; hence
*something* must be missing from it (if it's to better match the observations.)
DM and DA would fill that hole; but whether they truly exist or not is another
matter. The observed effects could be something else entirely, something even
stranger (IMO).
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On 11/18/2010 12:13 PM, Darren New wrote:
> (OK, I see that gravity fell out very early on...)
Gravity, having fallen out in early childhood from its siblings Strong,
Weak, and the twins Electro and Magneto, remains estranged to this day.
Regards,
John
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Warp wrote:
> I have hard time believing that.
It's just what I've heard. Maybe you wouldn't notice the tides if you didn't
look out the window, is what they meant. (I think I heard it from Sagan or
Asimov or something like that.)
Your argument is good, tho. Thinking on it, I can't imagine how you wouldn't
notice. :-)
> However, this horizon is caused by the expansion of the Universe, not
> by gravity, and it's always relative to where the observer is to begin
> with.
I was talking about overall. (You may be taking me too seriously. :-)
>> I think it was more popular before people found dark energy and thought we
>> were heading for a "big crunch" or something?
>
> I think the word "found" is wrong here. It hasn't been measured; it's a
> hypothesis. (It could perfectly well be the *correct* hypothesis, but until
> it's measured it's just that.)
For "found" read "found the affects attributed to..."
--
Darren New, San Diego CA, USA (PST)
Serving Suggestion:
"Don't serve this any more. It's awful."
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Kenneth wrote:
> DM and DA would fill that hole; but whether they truly exist or not is another
> matter. The observed effects could be something else entirely, something even
> stranger (IMO).
Well, there's really convincing evidence for DM, at least. They found two
galaxies that had crashed together, and the stars were all jumbled up in the
middle, but the Dark Matter kept going (as indicated by gravitational
lensing from the DM).
Oh, there it is. http://www.abc.net.au/science/news/stories/s1720848.htm
--
Darren New, San Diego CA, USA (PST)
Serving Suggestion:
"Don't serve this any more. It's awful."
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Warp wrote:
> IIRC the Universe only expands (ie. new space is formed) at intergalactic
> space, not inside galaxies. The gravity of galaxies is high enough to stop
> (or at least greatly diminish) expansion from happening inside them.
FWIW, that's not what I've heard. However, I'm not sure anyone actually
knows the answer there. :-)
--
Darren New, San Diego CA, USA (PST)
Serving Suggestion:
"Don't serve this any more. It's awful."
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Darren New <dne### [at] sanrrcom> wrote:
> Warp wrote:
> > IIRC the Universe only expands (ie. new space is formed) at intergalactic
> > space, not inside galaxies. The gravity of galaxies is high enough to stop
> > (or at least greatly diminish) expansion from happening inside them.
> FWIW, that's not what I've heard. However, I'm not sure anyone actually
> knows the answer there. :-)
Thinking about it, I can think of one argument of why that would be so:
If space was expanding evenly everywhere, it would mean that there should
be an extra "force" pulling planets away from the Sun, and an even "stronger
force" pulling stars away from thir galaxy's center, and this extra "pulling"
factor would have to be taken into account in orbit equations. But is it?
(OTOH maybe the expansion inside our solar system is so minuscule that
it basically unnoticeable and doesn't affect orbits in practice...)
Wikipedia (the always trustworthy source) seems to confirm this idea.
For example at http://en.wikipedia.org/wiki/Metric_expansion_of_space it
says:
"Metric expansion is a key feature of Big Bang cosmology and is
modeled mathematically with the FLRW metric. This model is valid in
the present era only at relatively large scales (roughly the scale of
galactic superclusters and above). At smaller scales matter has
clumped together under the influence of gravitational attraction and
these clumps do not individually expand, though they continue to
recede from one another."
"In addition to slowing the overall expansion, gravity causes local
clumping of matter into stars and galaxies. These stars and galaxies
do not subsequently expand, there being no force compelling them to do
so."
"However the only locally visible effect of the accelerating expansion
is the disappearance (by runaway redshift) of distant galaxies;
gravitationally bound objects like the Milky Way do not expand."
The page http://en.wikipedia.org/wiki/Hubble%27s_law could perhaps have
more on this, but I can't easily find anything to corroborate either claim,
except perhaps for:
"In systems that are gravitationally bound, such as galaxies or our
planetary system, the expansion of space is a much weaker effect than
the attractive force of gravity."
--
- Warp
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nemesis <nam### [at] gmailcom> wrote:
> Try sci.astro and have old wolves devour your idea.
I'm reading some of the latest posts there. Over half of the posts are
spam (*sigh*, I still remember the time in the mid-90's when there was
virtually *no* spam in usenet, as incredible as that might sound today)
and most of the rest seems to consist of pseudoscientists writing miles-long
essays on their newest fads and denigrating established theories and known
scientists.
I'm guessing that if I posted there, nobody would actually answer my
question and instead I would get a few totally useless "the Big Bang
theory is BS" replies.
--
- Warp
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Warp <war### [at] tagpovrayorg> wrote:
> nemesis <nam### [at] gmailcom> wrote:
> > Try sci.astro and have old wolves devour your idea.
>
> I'm reading some of the latest posts there. Over half of the posts are
> spam (*sigh*, I still remember the time in the mid-90's when there was
> virtually *no* spam in usenet, as incredible as that might sound today)
> and most of the rest seems to consist of pseudoscientists writing miles-long
> essays on their newest fads and denigrating established theories and known
> scientists.
>
> I'm guessing that if I posted there, nobody would actually answer my
> question and instead I would get a few totally useless "the Big Bang
> theory is BS" replies.
yes, lots of loonies (and spam) there. Still, those are rapidly torn apart by
actual astrophysicists or wannabes with quite some knowledge. Some interesting
discussions follow from time to time...
sci.astro is much better than alt.astronomy BTW. ;)
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Warp wrote:
> "In systems that are gravitationally bound, such as galaxies or our
> planetary system, the expansion of space is a much weaker effect than
> the attractive force of gravity."
Yeah, I was interpreting that kind of statement as "gravity holds things
together in spite of the expansion of space trying to pull them apart." On
the other hand, it could be that the "dark energy" (i.e. vacuum energy) or
expansion of space or whatever is actually suppressed (in some sense) by the
presence of normal energy or the presence of curved space. I don't know at
this point any more.
--
Darren New, San Diego CA, USA (PST)
Serving Suggestion:
"Don't serve this any more. It's awful."
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Darren New <dne### [at] sanrrcom> wrote:
> Warp wrote:
> > "In systems that are gravitationally bound, such as galaxies or our
> > planetary system, the expansion of space is a much weaker effect than
> > the attractive force of gravity."
> Yeah, I was interpreting that kind of statement as "gravity holds things
> together in spite of the expansion of space trying to pull them apart." On
> the other hand, it could be that the "dark energy" (i.e. vacuum energy) or
> expansion of space or whatever is actually suppressed (in some sense) by the
> presence of normal energy or the presence of curved space. I don't know at
> this point any more.
Has it even been hypothesized *why* the expansion of the universe
happens? Or is it just another one of those "it just happens, we don't
really know why" things?
--
- Warp
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