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Speaking of Statistical Significance!



 
 
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  #31  
Old June 28th 15, 04:39 AM posted to sci.astro.research
Phillip Helbig (undress to reply)[_2_]
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Posts: 273
Default Speaking of Statistical Significance!

In article , Steve Willner
writes:

Consider the standard version of the standard model, or any other
universe with non-zero matter content...


Thus proving how hard it is to discuss this stuff in words. In the
earlier discussion, I thought it was clear from the context that we
were discussing _only_ the effect of the cosmological constant, not
anything about matter.


OK.

Now considering an empty universe with the dark energy being a
cosmological constant and nothing else:
The limiting case, when matter is too thin to matter (pun, as
always, intended), is the de Sitter universe, which has exponential
expansion.


Exponential in what I've called "comoving coordinates." Divide by
the scale factor to get what I called "metric coordinates," and the
result is...?


OK, I see the reason for the confusion. The usual usage is that
"comoving" is the "physical" ("metric"?) distance divided by the scale
factor, i.e. the other way around. Thus a distance which changes with
time due only to the change in the scale factor will be a constant
comoving distance.
  #32  
Old June 30th 15, 06:47 PM posted to sci.astro.research
Steve Willner
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Posts: 1,172
Default Speaking of Statistical Significance!

In article ,
"Phillip Helbig (undress to reply)" writes:
The usual usage is that "comoving" is the "physical" ("metric"?)
distance divided by the scale factor,


You mean multiplied, right? "Physical" is probably a better word
than "metric," but I'd expect either to be understood.

Equations are easier than words. :-)

Thus a distance which changes with time due only to the change in
the scale factor will be a constant comoving distance.


Yes.

--
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Steve Willner Phone 617-495-7123
Cambridge, MA 02138 USA
  #33  
Old June 30th 15, 08:54 PM posted to sci.astro.research
Phillip Helbig (undress to reply)[_2_]
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Posts: 273
Default Speaking of Statistical Significance!

In article , Steve Willner
writes:

The usual usage is that "comoving" is the "physical" ("metric"?)
distance divided by the scale factor,


You mean multiplied, right? "Physical" is probably a better word
than "metric," but I'd expect either to be understood.


Suppose I have a physical distance, now, in meters. After the universe
expands, it will be larger, in meters. Thus, I need to divide each
quantity in meters by the corresponding scale factor, in meters, to get
a quantity which doesn't change with the expansion of the universe, i.e.
a comoving quantity.

Equations are easier than words. :-)


Indeed.

Thus a distance which changes with time due only to the change in
the scale factor will be a constant comoving distance.


Yes.


We of course agree here.
  #34  
Old July 2nd 15, 09:17 PM posted to sci.astro.research
Jos Bergervoet
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Posts: 126
Default Speaking of Statistical Significance!

On 6/30/2015 9:54 PM, Phillip Helbig (undress to reply) wrote:
In article , Steve Willner
writes:

The usual usage is that "comoving" is the "physical" ("metric"?)
distance divided by the scale factor,


You mean multiplied, right? "Physical" is probably a better word
than "metric," but I'd expect either to be understood.


Suppose I have a physical distance, now, in meters. After the universe
expands, it will be larger, in meters. Thus, I need to divide each
quantity in meters by the corresponding scale factor, in meters, to get
a quantity which doesn't change with the expansion of the universe, i.e.
a comoving quantity.


But it would be unwise to call it a comoving distance,
since you then use the same word for two different
things. It was already used for the distance that was
increasing.. (Perhaps better would be using something
like "comoving seperation, or something similar).

Equations are easier than words. :-)


Indeed.


If you use the same symbol for different things the same
confusion will arise! (Like the Omega and Lambda game.)

The situation is comparable to the use of "mass" in both
rest mass and kinetic mass, but of course there wisdom
has prevailed (after some time, I guess) and we now use
mass only as a synonym for rest mass, while the velocity-
dependent mass isn't called mass but energy.

--
Jos
 




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