# Is this the end of dark energy

1. Jan 16, 2015

### wolram

This link should take you to the paper

Xiv:1501.0349Xiv:1501.03491

In this paper we investigate the limits imposed by thermodynamics to a dark energy fluid. We obtain the heat capacities and the compressibilities for a dark energy fluid. These thermodynamical variables are easily accessible experimentally for any terrestrial fluid. The thermal and mechanical stabilities require these quantities to be positive. We show that such requirements forbid the existence of a cosmic fluid with negative constant EoS parameter which excludes vacuum energy as a candidate to explain the cosmic acceleration. We also show that the current observational data from SN Ia, BAO and H(z) are in conflict with the physical constraints that a general dark energy fluid with a time-dependent EoS parameter must obey which can be interpreted as an evidence against the dark energy hypothesis. Although our result excludes the vacuum energy, a geometrical cosmological term as originally introduced by Einstein in the field equations remains untouched.

This paper seems to good to be true.

2. Jan 16, 2015

### Chalnoth

It does.

The first thing I'd wonder is whether or not they managed to correctly carry through all of the implications of a negative equation of state parameter. It's conceivable, for instance, that they assumed a stability condition that is required for terrestrial fluids, but that this stability condition actually needs to be modified in the presence of a negative equation of state.

I'm not sure I'd be able to detect any shortcomings in the paper, but I'm sure a number of theorists will respond to the paper soon. A paper like this is not likely to go without response.

3. Jan 16, 2015

### Garth

Of course the paper is talking about dark energy, not the cosmological constant.
What is the difference between $\Lambda$, the cosmological constant, and dark energy (DE)?

A lot.

If cosmic 'acceleration' is due to DE then there is something there - a remarkable "hypothetical fluid with (enormous) negative pressure, dubbed dark energy" with $p = -\rho c^{2}$.

If it is $\Lambda$, the cosmological constant, it is nothing. It is an 'integration constant' thrown into the gravitational field equation. It simply implies that at short range (say <100 Megaparsecs) gravity attracts in a Newtonian kind of way whereas at cosmological ranges it repels.

Garth

4. Jan 16, 2015

### Chronos

I also found this paper interesting, but, I couldn't shake the uneasy feeling that no other researchers have apparently taken notice of the inconsistencies uncovered by the authors. Some possibly relevant observations: the paper has apparently not been submitted for publication, and the authors have a weak publication history. This leads to suspicion over the thoroughness of their analyses. I certainly feel there is room to question the rather provocative argument that GR may not apply at sufficiently large scales. It will be interesting to see if it receives any attention from the mainstream community.

5. Jan 16, 2015

### OCR

Why do these people use the term...?
There's dark energy, dark flow, and dark fluid. I see no... dark energy fluid.

They don't seem too awfully proud of the paper, either... https://arxiv.org/auth/show-endorsers/1501.03491

Or, is that the way it usually works at arXiv ?

Any way, and by no means am I knowledgeable about the subject, well, not much...I'm skeptical...

http://en.wikipedia.org/wiki/Dark_energy

http://hubblesite.org/hubble_discoveries/dark_energy/de-what_is_dark_energy.php

6. Jan 16, 2015

### marcus

The geometrical cosmological term, denoted Λ, is a small intrinsic curvature---one over length squared, a reciprocal area in other words. I agree with quite a lot of what Garth said. And it squares with a paper that came out in 2010. (google "why prejudice against constant?")

It's normal procedure in integral calculus to include the constant of integration (you get marked off if you don't ) and normal in physics to include constants which are allowed by the symmetries of the theory. Then it's up to experimentalists to measure them and find out if they are zero or not. Λ definitely belongs in the GR equation, as the other constant allowed by general covariance, the symmetry of the theory. When it was finally measured it was found to not be zero. It is a curvature constant. A reciprocal area.

Algebraically one can convert an inverse area term to a pressure (or an energy density) simply by multiplying by a force. But in the context of the classical theory there is no need to do that. Λ is just a small curvature (reciprocal area) constant which belongs in the equation. No mystery about it.
Ascribing it to a mysterious "energy" field is an artificial, unnecessary obfuscation---at least this is what I take to be the message of Bianchi Rovelli's paper:
http://arxiv.org/abs/1002.3966
Why all these prejudices against a constant?
Eugenio Bianchi, Carlo Rovelli
(Submitted on 21 Feb 2010)
The expansion of the observed universe appears to be accelerating. A simple explanation of this phenomenon is provided by the non-vanishing of the cosmological constant in the Einstein equations. Arguments are commonly presented to the effect that this simple explanation is not viable or not sufficient, and therefore we are facing the "great mystery" of the "nature of a dark energy". We argue that these arguments are unconvincing, or ill-founded.

Last edited: Jan 16, 2015
7. Jan 16, 2015

### strangerep

Just because Barboza has not yet authored enough published papers to qualify him an endorser, why would you think he's not proud of this paper??

Last edited: Jan 16, 2015
8. Jan 16, 2015

### strangerep

Yes -- I was about to start a thread and mention that, but wolram and you got in first.

The (origin of) the sign of Λ still seems a bit of a mystery. (Theoretically, that is. -- Experimentally the sign is clear).

9. Jan 17, 2015

### cristo

Staff Emeritus
This looks like an interesting paper. I haven't had time to read it yet, but will add it to the pile.

Well, in cosmology we describe energy content as fluids; we can write an equation of state such that $P=w\rho$, Then each fluid has a different equation of state parameter, w. For matter and dark matter, w=0, for radiation, w=1/3. The dark energy equation of state is unknown, but w must be negative, and is probably close to -1.

This just means that the haven't formally claimed authorship through the arxiv website. I don't think too many people do this..

10. Jan 17, 2015

### marcus

I've wondered about the sign too. Could it be like this? Λ is the intrinsic vacuum curvature so it is the part that doesn't have to be attributed to matter (the RHS).

so the LHS is giving us a DIFFERENCE, which is the actual curvature MINUS the vacuum curvature that you get for free. And only that difference (what there is over and above the intrinsic vacuum curvature) is what goes over to the RHS and gets converted to energy and momentum etc. does this make sense?
So on the LHS you have a minus sign in front of the Lambda term.

Strangerep, I think when you said "got in first" you were quoting where I mentioned googling "why prejudices against constant?" in order to get the 2010 Bianchi Rovelli paper.
So you know that one, and were planning to post the link :) I'm glad someone else is familiar with http://arxiv.org/abs/1002.3966
I think it is an excellent paper. And since 2010 my hunch is we've seen a drift in the cosmology literature towards just talking about the cosmological constant and not referring so much to "dark energy".

11. Jan 17, 2015

### cristo

Staff Emeritus
I don't think this is true. The next generation of galaxy surveys have a goal of determining the dark energy equation of state parameter, w. In the theory community there has been a huge amount of literature in the last few years on modified gravity theories, theories with screening mechanisms, bimetric theories, or even the "most general" allowed theories producing second order field equations (so there are no ghost degrees of freedom), dubbed Horndeski theories. Theorists are trying to find a way to parametrize these theories such that they can be tested with the upcoming surveys.

So, to summarize, the community is far from happy with just considering a cosmological constant!

12. Jan 17, 2015

### ShayanJ

I just found this paper. It seemed interesting to me so I mention it here for others.

13. Jan 18, 2015

### Haelfix

Which is certainly true. Although In some sense there isn't much of a difference anyway for most of the cosmic history, as in order to match experiment quintessence (or any exotic matter obeying the equations of state) and the cosmological constant are for all operational purposes basically the same thing. It's like the difference between a line on a paper, and a line with a completely unobservably tiny gradient. It only makes a moral difference b/c one can imagine that the quintessence field can do something else at some point deep in the past of our cosmological history, and yes we can imagine certain tiny subspaces of the parameter space (where we parametrize the eos equations in some way) to have certain configurations that might have some observational consequences that distinguishes them.

Either way, insisting that one has a pure cosmological constant instead of dark energy makes absolutely no difference to resolving the apparent paradox of the smallness of the experimentally determined quantity. That is still very much a big mystery, that no one (including all of papers linked in this thread) have even really addressed.

Last edited: Jan 18, 2015
14. Jan 18, 2015

### strangerep

Why is it any different from asking "why is the speed of light so fast?" (which is a question I almost never hear any more).

Last edited: Jan 18, 2015
15. Jan 18, 2015

### strangerep

You seem to be talking about 3 ingredients: "geometric curvature", "vacuum curvature", and the stress-energy.

I doubt that we should separate ordinary geometric curvature from "vacuum curvature". The former is based on the local invariance group of an inertial observer being Poincare, but the elementary relativity principle admits a deSitter-like group (or possibly a variant thereof). If the latter is nature's choice, we should be constructing quantum fields as deSitter reps rather than Poincare reps -- and then the distinction between "ordinary" and "vacuum" curvature disappears.

16. Jan 19, 2015

### marcus

As I recall "vacuum curvature" was a term Bianchi&Rovelli used in the "Why all these prejudices against a constant?" paper. I think you probably know the paper. the intuitive idea is that this is a kind of baseline intrinsic curvature that is there in the absence of matter---and the matter stress-energy tensor (on RHS) is only responsible for the ADDITIONAL CURVATURE over and above the intrinsic vacuum curvature.

By 'geometric' curvature do you mean the total? that would be what Bianchi Rovelli called vacuum (i.e. Lambda) plus the contribution from stress-energy tensor, or matter.

We were wondering about the minus sign. that is one way of explaining it. But I'm not insisting on any particular interpretation. Tell me yours, I might like it better. "Vacuum curvature" is not an important term, just something Bianchi Rovelli used at one point as an intuitive name for Lambda the cosmological constant. If you find it unhelpful we can discard it and just say Lambda.

As they use it "vacuum curvature" has no relation to field theory "vacuum energy". AFAIK. It is just an intrinsic curvature constant throughout all geometry that other stuff is added on top of. A "zero point curvature".

17. Jan 19, 2015

### Haelfix

So for a statement of the problem, the canonical reference (which takes a bit of googling to find a .pdf) is
(The cosmological constant problem, Steven Weinberg
Rev. Mod. Phys. 61, 1 – Published 1 January 1989)

and a more modern one that I happen to find very succinct (much recommended):
http://arxiv.org/abs/hep-th/0603249

The difference between the cc problem, and "why the speed of light is what it is", is that we actually make a prediction for what the cosmological constant ought to be, whereas the speed of light is basically an experimental input. The fundamental problem is that our best theoretical answer is so incredibly wrong, that its hard to see how we could modify the theory in a way that could possibly make the answer right.

Silly papers like the one Rovelli wrote, don't really address the problem. You see, its perfectly fine to just treat the cc to be of the simplest type, and to make it an integration constant that can be whatever you what. Everyone has known that forever. The problem is that what he doesn't say is that he must tacitly set the quantum vacuum contribution to zero in order for it to be a solution.

But that's just restating the problem! Ideas like this are very old and often go by the name 'degravitating the quantum vacuum'. Great, fine, but you haven't done anything until you actually *show* this explicitly, and (in passing) you have to explain why this doesn't violate the equivalence principle, which is the foundation of the very thing that we are assuming.

18. Jan 19, 2015

### strangerep

Oh, that.

Your use of the word "paradox" threw me. I would have thought it was just incredibly good experimental proof that the "quantum vacuum" explanation is total rubbish.