# 'Smoking Gun' for Dark Energy?

## What evidence do you think is needed to confirm the existence of Dark Energy?

4 vote(s)
14.3%

6 vote(s)
21.4%

14 vote(s)
50.0%
4. ### Don't know

4 vote(s)
14.3%
1. Apr 29, 2007

### Wallace

This has come up in a few threads as a side question, but I'd like to have a central discussion on this issue.

The question I have is what evidence do you think will be needed to elevate the existence of Dark Energy to the same kind of level as the existence of things in the standard model, so say electrons for instance.

The cosmological evidence makes a very strong case for the existence of Dark Energy, except of course that this conclusion is reliant on the theoretical framework of General Relativity. However, all observations in science rely on a theoretical framework in order to give them meaning.

I've often heard it said that DE is merely in the theory to 'save the appearances' or some such phrase, since it hasn't been 'directly' detected in a lab. What I would like to know then is do you think that any amount of observational evidence not involving lab experiments will ever be able to settle the issue?

I certainly don't think current observations are sufficient to make DE a robust theory, but I'm trying to decide whether hypothetical future observations could be sufficient either. If DE was something that literally had zero coupling to baryons, how could we ever detect it in a lab anyway? In this case the Universe is our lab, since the only way to 'see' DE is by large scale gravitational effects.

Someone once told me that in many ways we are as far from the wavefunction of an electron on the lab as we are from a Quasar in space, and for the most part I think this is true. Lab experiments after all still rely on theoretical framework and are nothing more than observations of the action of physical laws. Why then are lab experiments held to be superior to the same kind of observations of the workings of physical laws we get from looking at the Cosmos?

2. Apr 29, 2007

### Kea

Dark Energy does not exist.

3. Apr 29, 2007

### George Jones

Staff Emeritus
I believe (like marcus?) that a quantum theory of gravity is necessary (but maybe not sufficient) for a convincing intrepretation of the data.

Last edited: Apr 30, 2007
4. Apr 29, 2007

### turbo

I concur. Dark Energy is a parameter invented to fix a disconnect between concordance cosmology and observations.

5. Apr 29, 2007

### Wallace

This may well be true. Am I correct in assuming then that you think DE is not a thing so much as a parametrization of our ignorance of the true theory of gravity? In other words that there really is only matter in the Universe, but that the way gravity works gives us the appearance of DE if we interpret the results in terms of GR?

If so then this is a good point but not quite what I'm driving at. We would need more data than we currently do in order to come up with a quantum theory of gravity. In the end the question remains, do you think that cosmological observations can ever be enough by themselves to be able to construct such a theory, or would we absolutely need lab results as well?

But this is science is it not? Theory does not match observations so the theory is updated to fit the observation. It puzzles me that when particle physics makes up particles and forces in order to save the appearances (which is how the standard model has been built up) people don't question it, but when cosmology does the same it is merely an 'invention' that cannot exist in reality

Last edited: Apr 29, 2007
6. Apr 30, 2007

### Garth

There are many hypothetical particles in particle physics. Many of them have been subsequently discovered 'in the laboratory'. When they are so discovered then their status changes.

DE may be 'discovered' locally, for example as the Casmir force once the ~10120 'Lambda' problem has been resolved, and at that time its status will change.

Until that happens an open mind is essential and alternative interpretations sought. What 'is science' is not just the ability to accept a standard theory but more importantly the willingness, if necessary, to reject it.

Garth

Last edited: Apr 30, 2007
7. Apr 30, 2007

### Wallace

I'm not referring to hypothetical particles like axions and so on, I'm referring to the experiments in which the standard model particles are discovered and studied. What is done in particle physics experiments is events are observed and by applying the laws of physics to interpret those events the existence of particles and forces are inferred. What I want to know is why are the observations made in a detector on a particle accelerator any more valid than observations of the motions of objects in the Universe? The motions of distant SN combined with the observations of CMB anisotropies and galaxy clustering, when interpreted in the framework of GR tell us that there should be this stuff we call DE having certain properties.

How is this fundamentally different to analyzing the energy and paths of particles in accelerator experiments to determine, via the framework of physical laws to infer the existence and properties of particles? As far as I can see there is no fundamental difference. Lab experiments are no more 'direct' than cosmological observations, that only difference is that you can run them multiple times and change the initial conditions. In principle though any one experiment in which a particle is produced can give you the answers you need.

I couldn't agree more, though the above comment seems to be painting me a light that is not supported by my statements. I am not advocating that we accept any theory at present. What I am asking is what are the ground rules for validating theories by using cosmological observations.

One of the reasons I am interested in this questions is precisely that I am concerned that if are insist on a theory only being valid if we can test is 'in the lab' then are we being closed minded to the possibility of extending the reach of what we will ever be able to know if there really is some DE type stuff in the Universe that cannot be played around with in a lab. If we insist on laboratory confirmation without thinking through what that means we are not in any way being open minded.

Anyway it's my pet hate when people try and win an argument by appealing to moral high-ground of open-mindedness, it's a funny concept that can be twisted in any number of ways to support a position. So I'll stop doing that myself.

The key question here that I would really like opinions on is why are observations made in a lab given so much more weight than observations of the Universe when they are in essence the same process?

Last edited: Apr 30, 2007
8. Apr 30, 2007

### Garth

In particle physics there is always the possibility that discoveries in a particle accelerator have been mis-identified as they too are 'theory dependent'. When considering the DM particle, rather than DE, one could accept its existence to a high level of confidence if it is discovered both in the laboratory and by cosmological observations with the necessary properties. With DE you are correct, it may only be possible to detect it cosmologically, if it is just the cosmological constant for example.

I am not accusing you, Wallace in person, rather the general unwillingness in the cosmological community to seriously question the model and work on alternatives.

The appeal for 'an open mind' wasn't a claim for the high moral ground just an acknowledgement that 96% of the content of the universe is at present unknown.

Garth

9. Apr 30, 2007

### Wallace

The 96% isn't unknown though, we know quite a lot about the properties that it must have, we just don't have the full picture. You could argue that electrons are unknown since the standard model of particle physics is incomplete. We do however know a lot about their properties.

I guess it depends on which cosmologists we have each met and interacted with but in my experience professional cosmologists are all about questioning the current model. You could argue that there is a lot of attention on DE and the alternatives are being neglected, however DE is the most promising theory and the harder we push it the more likely it is to break if there are fundamental problems. Particularly in terms of observations campaigns, any study focused on DE is almost as applicable for testing the predictions of alternative theories so even though the next gen probes (SNAP,PLANCK etc) are being promoted as DE instruments, once the data is out all the alternative models can be checked against it just as well as the DE models.

A quick glance of astro-ph on any given day usually reveals as may 'alternative' theory papers in cosmology than it does 'standard' papers. I think the field has a good level of balance in terms of focus on the most promising theory to test the details as well as plenty of activity on the fringes providing alternatives that may prove to be better theories in the future.

I have yet to meet a cosmologist that would be unwilling to question the LCDM model, but if there are any that you have met Garth then I agree wholeheartedly with your criticism of this position. I do however strongly disagree with the statement that the community generally holds this view. I just don't see that in the papers written or in discussions with different people.

10. Apr 30, 2007

### Garth

I think we know a little more about the electron than we do about DE or the DM particle.

Garth

Last edited: Apr 30, 2007
11. Apr 30, 2007

### Garth

Certainly there are large numbers of alternative theories published on the physics ArXiv, though only a relative few make it to publication in a refereed journal. I have collated a few that make predictions of the Gravity Probe B experiment.

The question is: "How seriously are these alternatives taken?"

I agree that there are several endorsed papers on the ArXiv about alternative theories and some can be dismissed fairly easily, however once an alternative has been published in a refereed journal then it ought to be either seriously refuted or endorsed as a viable challenge to the standard model.

I remember the Steady State/Big Bang controversy of the '60's and it seemed that then, although the different camps were somewhat entrenched, because there were alternative interpretations there was a healthier atmosphere of scepticism about the conclusions.

Perhaps I am just smarting that my recent journal published SSC papers have been given the 'silent treatment' instead of being seriously criticised. But now of course that theory in its present form has been falsified by GP-B.

However another, related, example from the past would be Kolb's 1989 paper A Coasting Cosmology in which he introduced "K-matter" with an equation of state $p = -\frac{1}{3}\rho$ to deliver a linear expanding universe. Such a universe would not have suffered from the horizon, density and smoothness problems that otherwise require Inflation.

Although the hypothesis of K-matter might seem 'ad hoc' it might be seen to be no more so than the introduction of DE.

When Permutter's et al.’s paper Measurements of Omega and Lambda from 42 High-Redshift Supernovae was published nine years later DE was accepted as being the cause of cosmic acceleration.

However at that time it should have also been recognised that Kolb's k = -1 model also fitted the data as well and K-matter seen to be just as likely an explanation as DE. The concordance of the freely coasting model was pointed out in Permutter's paper (figure 2), but that point was never taken up.

Of course later data from higher red-shift SNe Ia's, if taken as standard candles in the early universe, did not support the Kolb model, and we have discussed this as well elsewhere, but at the time it should have been flagged up.

I don't think the linearly expanding universe is finished yet, the Coincidence of Universe age in LambdaCDM and Milne cosmologies, which we have also discussed elsewhere, is another indicator IMHO.

Note: If the age A of the universe is expressed in units of the present Hubble time then, with a flat universe and DE, A could lie anywhere 0.667 < A < $\infty$.

In fact, with the best accepted values where the age of the universe is 13.81 Gyears and Hubble Time is 13.89 Gyears, A = 0.994 very close to 1. Coincidence? Or is the universe 'trying to tell us something'?

I offer this just as an example of where the community may have been reluctant to seriously consider other models and, as K-matter may offer an alternative explanation, I vote that some 'laboratory' confirmation is necessary as a "smoking gun" to confirm the existence of DE.

Garth

Last edited: Apr 30, 2007
12. Apr 30, 2007

### Nereid

Staff Emeritus
Mine is the (so far) only 'current evidence is sufficient' vote .

However, my reasons for choosing this may be somewhat different from what Wallace had intended; let me explain.

To me, 'Dark Energy' is merely a label, a placeholder, that succinctly summarises a pretty clear signal in the relevant astronomical data.

Like just about all signals in just about all astronomical data*, the 'DE signal' is not theory-free. Near the top of the tree, there are theories about Type 1a SNe, about the CMB, and about galaxies (how well can their observed distribution be used to detect signals re large-scale structure? for example).

However, front and centre is GR; in one sense, Dark Energy owes its entire existence to GR.

But if you choose to use, as the core theory in your cosmology, a theory of gravity other than GR, does DE still exist?

Here's why I voted 'yes': the 'DE signal' is still in the astronomical data, no matter which theory of gravity you base your cosmology on.

So, what might lead to the death of DE, to its being relegated to the same dustbin of science history wherein lie phlogiston, N-rays, and so on? For starters, a far better understanding of the nature of Type 1a SNe; for seconds, discovery of a far richer source of signals in the CMB. While I think we will, over the next two decades or so, certainly come to understand Type 1a SNe much better, and will probably discover that the CMB is more complex than in our present interpretation of the data, I also think that the 'DE signal' will not disappear.

That's why I voted for 'current evidence is sufficient'.

*Other than, perhaps, what you can see with your unaided eyes.

13. Apr 30, 2007

### Wallace

Good point Nereid, I should have stated clearly that I was referring to specific theories in which the DE is a real material, as opposed to a different explanation for the effects seen. I agree with the general approach you outlined of treating DE as a way of "summaris(ing) a pretty clear signal in the relevant astronomical data".

14. Apr 30, 2007

### Wallace

I don't get why this gets any attention. It is a simple co-incidence. The age of the Universe is a derived result based on the experimentally determined cosmological parameters. It is the result of an integral over a(t).

If two curves have very different trajectories, and those trajectories can be tracked (as we can do with a(t)) then noting that the final integrated answer is this same has no relevance . The Milne model is a truly awful fit to all data, not just supernovae.

I work in structure formation and it is very easy to see that a Milne universe has much less structure formation than a LCDM one, we simply cannot get the kinds of structures we see in the Universe if we take a Milne model. You can pick up a piece of the Universe and have a look at the date written on the back of it, the age is merely a derived quantity based on the set of observable quantities. If a model has the observables completely whacked but gets the same integrated answer do you really expect the community to take this seriously?

Why is it that you think the community is focused of DE instead of alternatives Garth? What in your opinion was the reason that the Perlmutter approach was more accepted than the Kolb one? I'm sure you are implying something but I'm not sure what it is?

15. Apr 30, 2007

### marcus

In that case we need to observe it in the lab---or the some outdoor equivalent like cosmic ray research.

George rightly guessed my hunch too. Sorry for responding to your poll so belatedly

If it is somehow observed in lab, fine! If instead it turns out to be the effect of a successful tested quantum gravity, excellent!
If neither happens, it will be a troubling quandary. I feel one or the other must happen (nature subtle but not malicious).

Last edited: Apr 30, 2007
16. Apr 30, 2007

### turbo

I believe that there is a disconnect at GR. Einstein believed that the structure of space (he called it the ether) varies in its properties due to the matter embedded in it. He went to great pains in his 1920 book on relativity to explain that the refraction due to "gravitational" lensing cannot occur unless the speed of light through a vacuum varies from location to location. Since then people have blithely ignored him and have assumed that the assumption that he made for the sake of the special theory of relativity ("c" is fixed) is universally applicable. He explained in his book in 1920, and in his Leyden speech of 1920, and in his 1924 essay "On the Ether" why the speed of light in a vacuum cannot be constant, yet this concept is routinely buried, to the detriment of us all. We cannot posit the existence of dark matter or dark energy until we have re-examined GR in light of another ~100 years of observations. Who here would dare to claim that if Einstein was aware of the flat rotation curves of spiral galaxies, the excess lensing of clusters, and the excess binding energy of clusters, he would have blithely proclaimed that Newton's approximation to gravity was correct and exotic forms of matter and energy were responsible for the the deficits? I don't believe that for an instant. He believed that space could be conditioned in its properties by the matter embedded in it, and he would probably have been far more amenable to accepting variable "g", variable inertial effects, and variable refractive effects, based on the amount and distribution of matter in the space under study.

17. Apr 30, 2007

### Nereid

Staff Emeritus
Thanks.

So did you intend to differentiate among quintessence, (a particular value of) Λ, or any (other) 'DE as (a) real material'?

18. Apr 30, 2007

### Wallace

Any of those (except perhaps for interpreting $$\Lambda$$ as a geometric term) to me are a real material, and genuine 'energy'. So the question becomes can you, by cosmological observations alone, determine the nature of this material?

19. May 1, 2007

### Garth

Pelmutter simply reported on the relative faintness of distant SNe Ia. It was his paper that said, (Figure 2), comparing the empty (linearly expanding) universe with the $\Lambda$CDM universe, "Note that this plot is practically identical to the magnitude residual plot for the best-fit unconstrained cosmology of Fit C,"

It is important to remember the path along which we have travelled.

At the time, at the end of the '90's', Inflation (supported by COBE) seemed to demand that $\Omega = 1$, therefore the community jumped on the $\Lambda$CDM model, which required DE, and not the Kolb model, which required K-matter. (Note: 1. the Kolb model could be closed/flat/open with $\Omega$>=< 1 depending on the amount of K-matter, 2. there are other ways of producing scale-invariant anisotropies, 3. 'Kolb' has 5 x amount of baryonic matter than normal and this could produce large scale structure.)

What I am implying is the momentum of the Inflation hypothesis (which the 'Kolb' model doesn't need) determined the path the community took.

The problem is that the $\Lambda$CDM model not only required undiscovered DE and DM but also an undiscovered Higgs/Inflaton particle. (Here I am restricting the word 'discovery' to mean 'discovery in the 'laboratory'')

The entities were building up and the requirement for one created the momentum that demanded the other.

Now, all these entities ('Higgs'/exotic DM particle/DE) may well exist and they may well be discovered 'tomorrow' with the required properties, but on the other hand they may not be. Therefore until they are discovered it is best to keep an open mind - IMHO.

It is because these particles are yet to be discovered, and their cosmological observations are theory dependent, that I would argue the confirmation of the existence and nature of DE requires detection in the 'laboratory'.

Garth

Last edited: May 1, 2007
20. May 2, 2007

### Garth

If DE is simply the cosmological constant, not false vacuum energy, nor 'quintessence', nor anything else, then it may be impossible to confirm its existence in the laboratory.

In this case, even though its existence is confirmed by several different cosmological observations, as each of those observations are theory dependent they could be subject to revision if the underlying theory were significantly modified.

In that situation , and it could well be the case that DE is the CC, then all we can do is test that theory (GR) in as many ways as possible and over as many orders of magnitude as possible.

Even so there would always the possibility that 'tomorrow' it will be found wanting and replaced by an alternative, say a quantum gravity theory, with possible consequential changes in the interpretation of those DE observations.

Therefore perhaps we will never be able to confirm the existence of DE.

Garth

Last edited: May 2, 2007
21. May 2, 2007

### Chronos

Words get in the way sometimes. DE is pretty much the non-photonic counterpart to non-baryonic matter. Theories evolve and that is a very natural process. I would argue that nearly all theories begin as ad-hoc additions to 'accepted' theories. No news there. I've not yet seen a theory that claims to be 'complete'. Exceptions are the rule and can always be accomodated in the mathematical structure of a 'true' theory. SN1e is the 'smoking gun' [IMO] in the case for DE. Adding DE seems a lot more practical and sensible, at least for now, than rewriting GR. Einstein realized from the beginning that GR not only permitted, but nearly insisted on the addition of something that looked very much like DE.

22. May 5, 2007

### Chronos

The Perlmutter study is the gold standard for both DE and expansion. Debunk that and the Nobel is yours for the taking.

23. May 5, 2007

### Garth

That study and other more recent ones rely on the assumption that SNe Ia are standard candles over cosmological time scales.

The observation that the distant SNe Ia are fainter than expected led to the conclusion that the flat $\Omega =1$ universe has accelerated in its expansion and this cosmic acceleration, together wiith 73% of cosmological density, are caused by negative pressure DE.

But how can this 'standard candle' assumption be verified?

The light curve of SNe Ia peaks with the radioactive decay of Ni56.

Nickel itself is produced in the SN explosion by nucleo-synthesis from iron.

If there is an iron and consequently Ni56 deficiency in early stars, compared with nearby contemporary ones, then the distant and early SNe Ia may indeed be intrinsically less luminous than nearby ones.

This may be the case because as iron is the last element to be produced by fusion processes there could be a tendency for an iron deficiency in young systems.

Now where do I claim my Nobel prize?

Garth

Last edited: May 5, 2007
24. May 5, 2007

### Nereid

Staff Emeritus
This is putting it a little too strongly, I feel; the importance of SNe 1a as standard candles means that their 'standardness' is not simply assumed, but also tested.

You can certainly examine the work - past and on-going, observational and theoretical - done to test those 'assumptions'; there's a considerable literature on it.
There are quite a few possible systematic effects that could lead to the observed SNe 1a signal; for example:

* selection effects, e.g. the more distant the SNe, the smaller the sample of the distribution of SNe (by max magnitude) we are selecting (or perhaps it's by colour?); there are quite a few of these kinds of possible selection effects

* variation within 'SNe 1a' itself varies with distance/time - this is a generalisation of your question

* contamination with rare SNe that 'look like' 1a; these 'rogue' SNe may be quite rare locally (so our local calibration is unaffected) but common in the past

* Average SNe 1a environments that have changed over time - how much does the average path of the SNe 1a photons through the local galaxy/cluster vary? how much do the different paths affect the observed/derived SNe 1a parameters? This could be 'grey' absorption, or reddening, or ... Of course, this can also lead to a selection effect.

I'm sure others PFers can add more ...

25. May 5, 2007

### Garth

Unfortunately we can only test their 'standardness' with nearby, low z, SN. What is difficult to test for is a systematic effect over cosmological time scales back to z ~1 and further.
Indeed, one such other selection effect might be a bias in detection. The SN are detected after the first sudden increase in brightness, but how soon after?

They are more difficult to detect if they are further away and fainter. Therefore, the more distant they are the greater the delay might be after maximum brightness and that maximum therefore thought to be fainter than standard. Now this bias is well known and allowance made, but has the allowance been made correctly?

Garth

Last edited: May 5, 2007