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It is good practice to calculate the dark energy density. I remember doing it a few years back and later posting the result here at PF.
As I recall it was about 0.6 joule per cubic kilometer----so if you were being not overly finicky and giving a rough approx, it was about HALF a joule per cubic km.
Who here has done this calculation on their own? It is simple and the reason it is good practice is that it uses the grav. const. G and also the Hubble parameter H, and the speed of light and that is about it.
Would anybody who HASN'T done the calculation themselves like to go thru it with me?
If you do then here is what you need
https://www.physicsforums.com/showthread.php?p=136418#post136418
this is post #5 of the "A and C Reference Thread". as you can see it came rather early in that thread sometime in January 2004.
What we assume as GIVEN is that H is 71 km/sec per Megaparsec
and that the dark energy density is 73 PERCENT OF TOTAL energy density.
(A) that 73 percent is a commonplace we are all familiar with these days----we are constantly being told that dark energy is "about 70 percent" or "about 3/4" of the total energy density----the current best estimate is around 0.73 and people round that off various ways so as not to sound finicky.
(B) and we are constantly reminded that the universe is approx spatially FLAT which means that the TOTAL energy density is about equal to CRITICAL.
so for a pretty good estimate of DARK DENSITY WE MERELY CALCULATE CRITICAL (from the Hubble parameter H) AND THEN TAKE 73 PERCENT OF IT.
THAT LINK TO A&C REFERENCE THREAD WILL GIVE A FORMULA FOR CRITICAL DENSITY. And if I remember it will come out to about 0.85 joules per cubic kilometer. Then you take 0.73 of that and you get around 0.6 joules per cubic kilometer for dark density.
that's why I say it roughly half a joule----not to put too fine a point on it.
=====================
someone recently asked me about this, so I thought maybe some other people might want to work the exercise. does anyone have any questions?
Did anyone find a different answer? If you get something different please
let me know and i will check my work.
As I recall it was about 0.6 joule per cubic kilometer----so if you were being not overly finicky and giving a rough approx, it was about HALF a joule per cubic km.
Who here has done this calculation on their own? It is simple and the reason it is good practice is that it uses the grav. const. G and also the Hubble parameter H, and the speed of light and that is about it.
Would anybody who HASN'T done the calculation themselves like to go thru it with me?
If you do then here is what you need
https://www.physicsforums.com/showthread.php?p=136418#post136418
this is post #5 of the "A and C Reference Thread". as you can see it came rather early in that thread sometime in January 2004.
What we assume as GIVEN is that H is 71 km/sec per Megaparsec
and that the dark energy density is 73 PERCENT OF TOTAL energy density.
(A) that 73 percent is a commonplace we are all familiar with these days----we are constantly being told that dark energy is "about 70 percent" or "about 3/4" of the total energy density----the current best estimate is around 0.73 and people round that off various ways so as not to sound finicky.
(B) and we are constantly reminded that the universe is approx spatially FLAT which means that the TOTAL energy density is about equal to CRITICAL.
so for a pretty good estimate of DARK DENSITY WE MERELY CALCULATE CRITICAL (from the Hubble parameter H) AND THEN TAKE 73 PERCENT OF IT.
THAT LINK TO A&C REFERENCE THREAD WILL GIVE A FORMULA FOR CRITICAL DENSITY. And if I remember it will come out to about 0.85 joules per cubic kilometer. Then you take 0.73 of that and you get around 0.6 joules per cubic kilometer for dark density.
that's why I say it roughly half a joule----not to put too fine a point on it.
=====================
someone recently asked me about this, so I thought maybe some other people might want to work the exercise. does anyone have any questions?
Did anyone find a different answer? If you get something different please
let me know and i will check my work.
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