# Why "Dark Energy density" instead of CC?

1. Mar 3, 2015

### wabbit

Not worrying about the term DE exactly - my question really is, why is the CC described as an energy density, instead of just a nonlocal quantity ? Why bother with a constant scalar field when a constant would do ? After all we do not hear talk of a local gravitational strength density which by the way happens to be constant - so what is the fundamental difference between the two cases? Or is is just convention / usage, with no deep meaning ?

Then, if this is indeed necessary, how should we interpret this in a relational view of spacetime ?

Thanks

Edit : I realize this allows an identification with QFT vacuum energy, but with 120 orders of magnitude to cross, this isn't really compelling - or is it ?

Last edited: Mar 3, 2015
2. Mar 3, 2015

### Staff: Mentor

It is a mathematical equivalence, there is no physical difference between the two concepts in current theories.
If we find some sort of interaction with such a field, the difference might become relevant.

3. Mar 3, 2015

### wabbit

Thanks - conceptually /intuitively it does change, as the density would be a local property of space (which happens not to vary, one more thing to explain), while a constant is well, just a constant.
It's reassuring that this is just a choice of term so I can continue thinking only of CC : ).

4. Mar 3, 2015

### wabbit

Follow-up related question : in the statement "the universe is made of 70 % DE, 25 % DM and 5 % Matter" There is no particular meaning to the 100 %? It would be just as meaningful to leave the CC aside and say "80 % DM and 20 % matter", correct ? Or is there some kind of balancing betwwen CC and the rest that makes the first statement more meaningful ?

5. Mar 3, 2015

### phyzguy

I think the reason that people started talking of "dark energy" was because we don't really know that it is described by a cosmological constant, i.e. a value that is constant in space and time. It's true that today all of our measurements are consistent with a cosmological constant, but we should at least consider the possibility that this value varies in space and time. So we refer to it as "dark energy", and we are in the process of measuring just how constant it really is. If, after all of the data is in, we decide that it is truly a constant, then probably the term dark energy was a mistake. But if we always refer to it as the "cosmological constant", then we tend not to even entertain the possibility that it might vary.

6. Mar 3, 2015

### Staff: Mentor

Well, we don't know, and we can search for deviations.

The relative influence on the expansion of the universe is measurable. "More DE than other contributions" => the expansion is accelerating.

7. Mar 3, 2015

### wabbit

Presumably this would be more natural than a non-constant gravitational constant i.e, a less drastic change in the EFE, wouldn't break things as much ?

Ah yes, fair enough. OTOH the M/DM split is constant in time so.it's a more structural aspect of our universe...but yes I see your point.

8. Mar 3, 2015

### wabbit

To add, about possible variation of CC this can either fall in the category "possible variation of fundamental constants" or in the category "measuring a field at different places to see if it is constant" - not necessarily a highly operational difference, but still at least intuitively, a different kind of question

9. Mar 3, 2015

### phyzguy

There is indeed a meaning to the 100%. They are typically quoted as fractions of the critical density, which is the density required to make the universe flat. Since our measurements today tell us that the universe is quite close to being flat, the densities add up to 100%. But this would not need to be the case.

10. Mar 3, 2015

### ChrisVer

CC?

11. Mar 3, 2015

### wabbit

Cosmological Constant, sorry.

12. Mar 3, 2015

### wabbit

Thanks for the reminder, I thought it was expressed as % of total energy-matter content.

13. Mar 3, 2015

### ChrisVer

I think saying 80% DM and 20% matter will only destroy the universe we are observing....
You can for example try setting the CC contribution to CMB background to 0, and the densities you proposed to check whether your result fits the actual data.
Of course it will not:

Your generated data ($\Omega_{cdm}=0.8$, $\Omega_b =0.2$ and $\Omega_{\Lambda}=0$):

A close to measured data CMB angle spectrum ($\Omega_{cdm}=0.224$, $\Omega_b =0.046$ and $\Omega_{\Lambda}=0.7$):

The age of the Universe can also be affected by making that assumption (drops from 13.72Gyr to 9.311Gyr).

If on the other hand you want to put that much DM to the hot component, then you would destroy the structure formation.

Figures generated by:
http://lambda.gsfc.nasa.gov/toolbox/tb_camb_form.cfm

14. Mar 3, 2015

### wabbit

No no I said 80/20 leaving aside DE, by which I meant the same universe but refering to a different total, 80=25/30 etc.. I am not trying to set CC=0 sorry for the confusion:)
But thanks, nice pics

15. Mar 3, 2015

### ChrisVer

The 100% corresponds to the cosmic sum rule.
The Friedmann Eqs are:
$H^2 + \frac{k}{a^2} - \frac{\Lambda}{3} = \frac{8 \pi G}{3} ( \rho_m + \rho_r )$
At today $t=t_0$ this equation is:
$H^2_0 + \frac{k}{a^2_0} - \frac{\Lambda}{3} = \frac{8 \pi G}{3} ( \rho_{m0} + \rho_{r0} )$
And the critical density $\rho_c= \frac{3H^2}{8 \pi G}$ is given at time $t_0$ as: $\rho_{c0} = \frac{3 H_0^2}{8 \pi G}$ . Dividing with it you get:

$\Omega_{m} + \Omega_{r} + \Omega_\Lambda + \Omega_k = 1$
With $\Omega_{m,r} = \frac{\rho_{m,r 0}}{\rho_{c0}}$
$\Omega_k = -\frac{3k}{8 \pi G a_0^2}= -\frac{k}{H_0^2 a_0^2}$
$\Omega_\Lambda = \frac{\Lambda}{8 \pi G \rho_{c0}}= \frac{\Lambda}{3H_0^2}$
the 100% corresponds to the 1 on the RHS.
If you say that DM+matter=1 then $\Omega_\Lambda=0$ (since the universe is spatially flat and $\Omega_k \approx 0$)

16. Mar 3, 2015

### wabbit

Thanks - that's not what I meant, hope it's clarified above. I am not touching the LCDM parameters, just refering to different ones, bad idea sorry
To clarify again all my questions in this thread are about interpretation and context, not about the results of standard cosmology.

17. Mar 3, 2015

### ChrisVer

OK sorry...

18. Mar 3, 2015

### wabbit

Not at all it was my fault for beeing unclear, I appreciate your responding and trying to help.

19. Mar 3, 2015

### wabbit

Thanks everyone for the responses this was very helpful - or How I Learned to Stop Worrying and Love the CC :)

20. Mar 3, 2015

### Chalnoth

The cosmological constant is a specific idea that can be considered a parameter of the gravitational field equations, or a constant energy density.

Dark energy is a concept that encompasses the cosmological constant as well as a variety of other models with similar properties.

Saying "dark energy" instead of "cosmological constant" is the equivalent of saying, "Well, we don't really know what this is....maybe it's a cosmological constant, maybe it's something else."

21. Mar 3, 2015

### slatts

Sorry, I shouldn't have said "dead stars"--more like "little neutrinos on a totally different scale, with vastly less mass than the known ones".

22. Mar 3, 2015

### wabbit

Agree completely about this, so I must admit a bias for the standard cosmological model : ) but you are right : in a context where the model is unknown DE is the better or only term. What I find surprising is that within standard cosmology where the model is a special case of GR, DE/density wording is very common. Your comment leads me to think this may be a matter of perspective as working cosmologist are not necessarily committed to that standard model, they just use it because that's what works now, so may prefer a formulation that is more model independent. My perspective is more narrow minded in the sense that I would see an interest in a variable CC only after a demonstrable need for it arises - I tend to view varying CC in the same light as varying the fine-structure constant : an interesting possibility but not one you mention in every equation. But that is just that, a matter of perspective, not of substance.

Edit: the story behind my bias is in part that it took me a long time before I realized that this "mysterious form of dark energy permeating the universe" I was hearing about was in fact, as far as accepted models where concerned, fully accounted for by just one of the two standard scalar parameters of the EFE.

Last edited: Mar 3, 2015
23. Mar 3, 2015

### Chalnoth

Well, saying that it's a standard parameter of the EFE is no less mysterious, because its value in natural units is around $10^{-122}$, which begs for some sort of explanation.

24. Mar 3, 2015

### wabbit

I suppose it does, but we don't have an explanation for why gravity (specifically the other scalar in EFE) is so weak either - admittedly not so dramatically. Still a different kind of issue, saying "observations now support a very tiny positive CC" vs "what is this mysterious substance that revolutionizes our basic understanding of the universe".
Anyway, kind of a pet peeve, that's all. I still had some lingering doubts as to whether there might be some substantial reason to using DE beyond the possibility that another model might be needed at a later stage, which is why I asked, and the responses in this thread have now laid that to rest, so I stopped worrying:)

Last edited: Mar 3, 2015
25. Mar 3, 2015

### Chalnoth

A fairly common perspective is that such an absurdly small number is unphysical, and therefore it's more likely to be a field of some sort or other that is dynamically set to the small value, and the cosmological constant itself is set to zero by some symmetry or other. The most common models for this fall under the label "Quintessence", which is a dynamical field which "freezes out" in such a way that it seems to solve the coincidence problem.