# Dark energy and cosmological constant

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1. Aug 11, 2015

### yazanhomsi

Hello everyone.
I am having some problem with dark energy and the cosmological constant. The cosmological constant is a proposed form of dark energy. Alright, now the cosmological constant is is estimated by cosmologists to on the order of 10^-26 kilograms per cubic meters. But the density of dark energy calculated from our so far observable universe is 6.91 x 10^-29 kilograms per cubic meters. If cosmological constant is a proposed form of dark energy, shouldn't the density of dark energy be the same as the cosmological constant? Please I need further explanation on this. Thank you!

2. Aug 11, 2015

### Orodruin

Staff Emeritus
Please provide references to where you have obtained those numbers.

3. Aug 11, 2015

### Chalnoth

There is no theoretical prediction of the value of the cosmological constant. At least, none that makes sense. The only measurement we have on dark energy (or the cosmological constant) currently is the change over time of the rate of expansion. In the models where the cosmological constant is the explanation, it is dark energy, and they take the same value.

4. Aug 11, 2015

### yazanhomsi

, at 4:30 the guy states the cosmological constant.
http://hyperphysics.phy-astr.gsu.edu/hbase/astro/dareng.html around the last paragraph the guy states that the dark energy density is 73 percent that of the critical density. Which means the 73 percent of 9.47 x 10^-27 kg/m3 equals 6.9131x10^-27 kg/m3

5. Aug 11, 2015

### yazanhomsi

But if they take the same value, which means the densities of the cosmological constant and dark energy are the same. How come they have come up with different densities for both, like even if these calculations aren't correct but it should show that there is something missing?

6. Aug 11, 2015

### Orodruin

Staff Emeritus
I have not looked at it because my current connection does not allow it, but you need to put far less faith in what you see on YouTube.

7. Aug 11, 2015

### yazanhomsi

Have you checked the website? It has a similar explanation somehow..

8. Aug 11, 2015

### George Jones

Staff Emeritus
When you refer to "the guy" from the video, you mean Ed Copeland, a well-respected physicist at the University of Nottingham. Copeland, at about 4:35 in the video say "is about $10^-29$ grams per centimetre cubed". The "about" means that Copeland is giving an order of magnitude estimate, and

$$10^{-29}\frac{g}{cm^3} = 10^{-29}\frac{10^{-3}kg}{\left(10^{-2}m\right)^3} = 10^{-26}\frac{kg}{m^3,}$$

which (to an order of magnitude), agrees with the website.

9. Aug 11, 2015

### yazanhomsi

Wait but the density of dark energy is 6.91 x 10 ^ -27. It is still isnt an order of magnitude of 10^-26.

10. Aug 11, 2015

### George Jones

Staff Emeritus
The video says " about 10^-26". The website says "6.91x10^-27". And you think think that these two references contradict each other???

To an order of magittude 6.91 is the same as 10, and 10x10^-27 = 10^-26.

Another way to look at this is to solved for $x$ in

$$6.91 \times 10^{-27} = 10^x$$

11. Aug 11, 2015

### yazanhomsi

Alright I got it. Thanks a lot you were the only one able to understand my question. Sorry for showing any form of naiveness.

12. Aug 11, 2015

### Orodruin

Staff Emeritus
(My emphasis)
I think the most important lesson of this thread is this one. In physics it is very important to get the units right. This is an insight that will serve you well in all areas of physics.