two Expanding Universe Questions

How would you tell the difference between an expanding universe and
light slowing down over time? I would think since light is your
yardstick, they would look the same.

Second question, is there any evidence on a local scale of an
expanding universe, or does it all come from the Doppler shift of
distant galaxies? Is there any way to notice the meter bar in Paris
has grown a tad, or anything comparable?
--
Roedy Green Canadian Mind Products
http://mindprod.com

"The industrial civilisation is based on the consumption of energy resources that are inherently limited in quantity, and that are about to become scarce. When they do, competition for what remains will trigger dramatic economic and geopolitical events; in the end, it may be impossible for even a single nation to sustain industrialism as we have know it in the twentieth century."
~ Richard Heinberg, The Party’s Over: Oil, War, and the Fate of Industrial Societies

"Roedy Green" wrote in message
...
How would you tell the difference between an expanding universe and
light slowing down over time? I would think since light is your
yardstick, they would look the same.

It's not quite that simple, but you are on the right track.
http://www.androcles01.pwp.blueyonder.co.uk/Doolin'sStar.GIF
As each star in a galaxy moves toward us, the light it emits travels
faster than when it was moving away. Faster light passes the slower
light emitted earlier if the galaxy is far enough away. However, for
MOST of the period we only see the slow light, as you can see by
the spacing along the top of the graph.
The light isn't slowing as it travels, it was already slow when it left,
so all the lines are straight. But they are not parallel as they would
be if all the light had the same speed.



Second question, is there any evidence on a local scale of an
expanding universe, or does it all come from the Doppler shift of
distant galaxies?

It only comes from a mistaken belief that the speed of light is
tied to the ectoplasm or aether or whatever it is called instead
of the emitting body.

Match the caption with the gif:

--
A) http://tinyurl.com/lv2fl7
B) http://tinyurl.com/njgouh
C) http://tinyurl.com/klkfc9
D) http://tinyurl.com/l6lt4g
1) applies to light (in vacuum) and sound (in air)
2) applies to light but not sound
3) applies to sound but not light
4) applies to neither light nor sound

(If you think A) and 1) match, write down "A1".)


Is there any way to notice the meter bar in Paris
has grown a tad, or anything comparable?

Not a snowball's chance in hell.

On Mon, 20 Jul 2009 15:15:58 +0100, "Androcles"
wrote, quoted or indirectly quoted
someone who said :

Is there any way to notice the meter bar in Paris
has grown a tad, or anything comparable?

Not a snowball's chance in hell.


I want to get some idea how fast the universe is expanding. What
units would you measure it in? What does the function for the size of
the universe given time T look like? Is there a sort of compound
interest going on, or is the growth approximately linear?
--
Roedy Green Canadian Mind Products
http://mindprod.com

"The industrial civilisation is based on the consumption of energy resources that are inherently limited in quantity, and that are about to become scarce. When they do, competition for what remains will trigger dramatic economic and geopolitical events; in the end, it may be impossible for even a single nation to sustain industrialism as we have know it in the twentieth century."
~ Richard Heinberg, The Party’s Over: Oil, War, and the Fate of Industrial Societies

"Roedy Green" wrote in message
...
On Mon, 20 Jul 2009 15:15:58 +0100, "Androcles"
wrote, quoted or indirectly quoted
someone who said :

Is there any way to notice the meter bar in Paris
has grown a tad, or anything comparable?

Not a snowball's chance in hell.


I want to get some idea how fast the universe is expanding.


It isn't, but being a ****wit you'd make a stupid statement like that
and snip the reply I gave you earlier because you didn't read it.
**** off.
*plonk*

Do not reply to this generic message, it was automatically generated;
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subjects to a sci. newsgroup, attempting cheapskate free advertising
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Boringly stupid is the most common cause of kill-filing, but because
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There is no appeal, I have despotic power over whom I will electronically
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This should not trouble you, many of those plonked find it a blessing
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I hope you find this explanation is satisfactory but even if you don't,
damnly my frank, I don't give a dear. Have a nice day.

Dear Roedy Green:

On Jul 20, 9:08*am, Roedy Green
wrote:
....
I want to get some idea how fast the universe
is expanding. *What units would you measure
it in?

The usual way is "km/sec / kparsec", also known a "Hubble's constant".

*What does the function for the size of the
universe given time T look like?

Fairly complex, with wide errorbars. For the first few hundred
thousand years, it was *very* high, then drops to a very small value,
then within the last few billion years, increases to its current value
("acceleration of expnasion").

*Is there a sort of compound interest going on,
or is the growth approximately linear?

Approximately linear over short time periods. I'd suggest you review
the links I provided for what current observation supports on this
topic.

David A. Smith

On Mon, 20 Jul 2009 12:25:44 -0700 (PDT), dlzc wrote,
quoted or indirectly quoted someone who said :

Fairly complex, with wide errorbars. For the first few hundred
thousand years, it was *very* high, then drops to a very small value,
then within the last few billion years, increases to its current value
("acceleration of expnasion").

That is quite a surprise. I would have expected something much
simpler. Thank you.
--
Roedy Green Canadian Mind Products
http://mindprod.com

"The industrial civilisation is based on the consumption of energy resources that are inherently limited in quantity, and that are about to become scarce. When they do, competition for what remains will trigger dramatic economic and geopolitical events; in the end, it may be impossible for even a single nation to sustain industrialism as we have know it in the twentieth century."
~ Richard Heinberg, The Party’s Over: Oil, War, and the Fate of Industrial Societies

On Mon, 20 Jul 2009 12:25:44 -0700 (PDT), dlzc wrote,
quoted or indirectly quoted someone who said :
Fairly complex, with wide errorbars. For the first few hundred
thousand years, it was *very* high, then drops to a very small value,
then within the last few billion years, increases to its current value
("acceleration of expnasion").

In article ,
Roedy Green writes:
That is quite a surprise.

Yes, it is a surprise because it's completely wrong. See
http://www.astro.ucla.edu/~wright/CosmoCalc.html
for a calculator that will let you compute the expansion rate, though
it wasn't designed for that. (You will have to see how comoving
radial distance changes with age for your favorite cosmological
model, assuming it's a relatively conventional one.) Be sure to look
at the explanations and formulas so you know what definitions are
used and what you are calculating.

--
Help keep our newsgroup healthy; please don't feed the trolls.
Steve Willner Phone 617-495-7123
Cambridge, MA 02138 USA

On Mon, 20 Jul 2009 12:25:44 -0700 (PDT), dlzc wrote,
quoted or indirectly quoted someone who said :

The usual way is "km/sec / kparsec", also known a "Hubble's constant".

The dimension is then distance/time /distance right?

So Hubble's constant is a frequency?

or are parsecs also a unit of time? making Hubble an acceleration?

--
Roedy Green Canadian Mind Products
http://mindprod.com

"The industrial civilisation is based on the consumption of energy resources that are inherently limited in quantity, and that are about to become scarce. When they do, competition for what remains will trigger dramatic economic and geopolitical events; in the end, it may be impossible for even a single nation to sustain industrialism as we have know it in the twentieth century."
~ Richard Heinberg, The Party’s Over: Oil, War, and the Fate of Industrial Societies

"Roedy Green" wrote in message
...
On Mon, 20 Jul 2009 12:25:44 -0700 (PDT), dlzc wrote,
quoted or indirectly quoted someone who said :

The usual way is "km/sec / kparsec", also known a "Hubble's constant".

The dimension is then distance/time /distance right?

So Hubble's constant is a frequency?

or are parsecs also a unit of time? making Hubble an acceleration?


He means km/sec/megaparsec.

The dimensionality of H is 1/time, so if you convert to SI units, 1/H is of
order the age of universe. (There's some factor [2/3?] in there depending on
the cosmological model used).

--
Mike Dworetsky

(Remove pants sp*mbl*ck to reply)

Dear Roedy Green:

On Jul 23, 3:04*am, Roedy Green
wrote:
On Mon, 20 Jul 2009 12:25:44 -0700 wrote,
quoted or indirectly quoted someone who said :

The usual way is "km/sec / [megaparsec]", also known
a "Hubble's constant".

Correction noted by Mike Dworetsky.

The dimension is then distance/time /distance
*right?

Actually velocity / distance, but same net-net.

So Hubble's constant is a frequency?

No, Hubble's constant expresses an apparent recessional velocity,
based on the source's distance. That you can cook the meaning out of
it by looking at only units, does not mean you won't wander off into
the woods and get lost.

Hubble in other units is roughly a change of 1 part in 10^11 per year,
or 1 part in 10^18 per second. About half the lunar recession rate
(and no, lunar recession is entirely described by momentum transfer
from the Earth, and Universal expansion doesn't work in this "system",
if GR describes expansion correctly).

or are parsecs also a unit of time? making
Hubble an acceleration?

A parsec is the distance light travels in a period of time (3.26163
years) in empty space.

David A. Smith