Who here has calculated the dark energy density?

[marcus]

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.
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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.

 

[AndyW]

I would like to address this forum post and clarify a few points. While it is good practice to calculate the dark energy density, it is important to note that this calculation is based on several assumptions and approximations. Therefore, the result should not be taken as a precise value, but rather as an estimate.

Firstly, the value of 0.6 joules per cubic kilometer mentioned in the post is not a universal or exact value. The actual value of dark energy density is still a topic of ongoing research and debate in the scientific community. Different studies and observations have yielded slightly different values, ranging from 0.3 to 0.9 joules per cubic kilometer.

Secondly, the calculation of dark energy density is based on the assumption that the universe is flat and that dark energy makes up 73% of the total energy density. While this is a commonly accepted assumption, it is important to note that it is still an assumption and not a proven fact.

Furthermore, the calculation also relies on the Hubble parameter, which is a measure of the expansion rate of the universe. The value of the Hubble parameter can vary depending on the method used to measure it, leading to some uncertainty in the final result.

In conclusion, while it is a good exercise to calculate the dark energy density using the formula provided in the post, it is important to keep in mind that the result is an estimate and subject to some uncertainties and assumptions. As scientists, we should always be open to new data and continue to refine our understanding of the universe.