Dark Energy Discoevery [NASA Teleconference 16 Nov]

[neutrino]

Dark Energy Discovery [NASA Teleconference 16 Nov]

The latest fad among cosmologists...

via Asymptotia

NASA will host a media teleconference with Hubble Space Telescope astronomers at 1 p.m. EST Thursday, Nov. 16, to announce the discovery that dark energy has been an ever-present constituent of space for most of the universe's history.

Looks like cosmology is accelarating.

 

[George Jones]

I don't know what's going to be announced, but it seems that one possible take on it is that there is no dark energy.

Of course, this is a very non-standard interpretation.

 

[Garth]

How would such DE be compatible with the WMAP results and BBN?

Better wait until tomorrow and see what the announcement actually says...

Garth

 

[Kea]

Hmmm...given the participants, it would seem they are going to prematurely claim a victory for a Cosmological Constant, without acknowledging the correct answer, which is that DE does not exist.

 

[Chronos]

A bold assertion, Kea. I admire your strength of conviction, but, am skeptical. I think the case for dark energy is strong, if not compelling. If DE is tossed out of the 'equation of state' [so to speak], cosmology has a ponderable mass of a mountain to climb. We live in interesting times.

 

[CarlB]

The web of physics is woven with very long threads so it's hard to reject one part of it without setting yourself up for a multi-year crusade. Eventually you'll end up tilting at wind generators.

 

[EL]

Hmmm...given the participants, it would seem they are going to prematurely claim a victory for a Cosmological Constant, without acknowledging the correct answer, which is that DE does not exist.

How can you conclude DE does not exist?

You want to make a bet again?
(You'll get much better odds this time...)

 

[neutrino]

You may get a better report here after the conference...
http://cosmicvariance.com/2006/11/16/dark-energy-has-long-been-dark-energy-like/

 

[Garth]

While we wait it might be pertinent to recall that there may be problems modelling SN Ia as standard candles.

Howell et al's paper Journal-ref: Nature 443 (2006) 308: The type Ia supernova SNLS-03D3bb from a super-Chandrasekhar-mass white dwarf star

Here we show that the high redshift supernova SNLS-03D3bb has an exceptionally high luminosity and low kinetic energy that both imply a super-Chandrasekhar mass progenitor. Super-Chandrasekhar mass SNe Ia should preferentially occur in a young stellar population, so this may provide an explanation for the observed trend that overluminous SNe Ia only occur in young environments. Since this supernova does not obey the relations that allow them to be calibrated as standard candles, and since no counterparts have been found at low redshift, future cosmology studies will have to consider contamination from such events.

(Emphasis mine)
(Supplementary information available at www.nature.com/nature)

Garth

 

[neutrino]

While we wait it might be pertinent to recall that there may be problems modelling SN Ia as standard candles.

http://www.nasa.gov/mission_pages/hubble/news/dark_energy.html

Looks like they are using SNs.

I like their balloon+anvil analogy, though.

 

[neutrino]

http://hubblesite.org/newscenter/archive/releases/2006/52/

Hubble news release.

 

[Kea]

It was just as I thought. No discussion of QG possibilities.

 

[Kea]

Varying-speed-of-light cosmologies predicted the apparent acceleration before it was ever observed. It also predicted precisely the result just seen. I suggest people start considering all the alternatives before they decide to pass judgement. Come on, does anybody seriously believe that a fudge factor in GR has any explanatory power? It's unrelativistic. Einstein must be turning in his grave.

 

[SpaceTiger]

I think a lot of folks are missing the point of the article. For the purposes of the study they're describing, it doesn't much matter what the reason for the acceleration is, they're just parameterizing the behavior of "dark energy". If dark energy is really just a manifestation of modified gravity, then the result is still relevant. They're likely working with this parameterization:

$$P=w(z)\rho$$

In other words, above is the equation of state (or effective equation of state) of the "fluid" plugged into Einstein's equation. To first order, it can be expanded as

$$w(z)=w(z=0)+\frac{dw}{dz} z$$

For a cosmological constant, w=-1 and dw/dz=0. It appears that their measurements are consistent with a cosmological constant, though at least three things should be kept in mind:

1) The result doesn't rule out any modified gravity theory that mimics a cosmological constant.
2) The error bars on dw/dz are probably quite large.
3) Any supernova measurement is potentially subject to a large number of systematic errors.

Nonetheless, it is an interesting result. Whatever the dark energy turns out to be, it seems to behaving much like a cosmological constant. A great deal of money is currently being spent to check this statement to ever higher precision.

 

[Wallace]

SpaceTiger is on the right track. 'Dark Energy' is actually a catch-all phrase that describes a host of models including modified gravity. There are plenty of 'dark energy' models that actually do not contain any new exotic energy. It's just a somewhat misleading collective name.

For more detailed info on how the equation of state parametrization includes the generic set of models see http://arxiv.org/abs/astro-ph/0305286" .

For instance you could phrase a varying c theory in terms of the Dark Energy equation of state w(z). It seems odd but all it says is that such a theory is degenerate with the existence of dark energy with that equation of state model.

This degeneracy can be broken by non-SN methods such as growth measures, so the theories can be still be distinguished at some level. The fact that growth measure are consistent with Cosmological constant (or dark energy with similar properties) and cold dark matter universe but not so consistent with the varying c theories is the reason the physical dark energy picture is preferred (though it's far from a closed case).

 

[Kea]

This degeneracy can be broken by non-SN methods such as growth measures, so the theories can be still be distinguished at some level.

Hi Wallace

Did these varying $c$ analyses you refer to properly take into account adjusted luminosities, or do they assume the same standard candles? Which theories have been so tested?

 

[Chronos]

Kea, your insights are sharp, as usual. My main objection to variable c is it fails to explain cosmological redshift without wrecking almost every other independent distance indicator model.

The basic premise is, if I understand correctly, that c increases over time - thus implying the permittivity and permissivity of empty space has 'relaxed' since the big bang. Would not 'alpha' also vary? If all the fundamental constants of nature conspire to create such an illusion, this conjecture cannot be falsified. That does not mean it is wrong, merely irrelevant.

 

[Wallace]

Hi Wallace

Did these varying $c$ analyses you refer to properly take into account adjusted luminosities, or do they assume the same standard candles? Which theories have been so tested?

I'm not quite sure I understand what you mean. If SN1A are not standard candles then we'll have to simply throw them out as being useful cosmological probes entirely.

I'm not sure what you mean by 'adjusted luminosities'? The apparent luminosity you expected an (assumed standard) SN1A to have at a given redshift depends on the cosmological model, with dark energy parametrized by the equation of state w. As I said, a varying c theory will be degenerate with a given dark energy model (i.e. with the equation of state matched to achieve this degeneracy), in terms of the expected luminosity-redshift relation from SN1A observations.

To the question of which theories have been tested then, ALL varying c theories are constrained through the current constraints of the dark energy equation of state, since they can be mapped to degenerate dark energy models. I don't know the specifics of which c models exist, what there equivalent equation of state is and therefore which are ruled out and which are allowed.

Remember the the parameter(s) w(z) are a mathematical convenience. If dark energy is a real exotic energy then it refers to the physical equation of state that it has. In other 'dark energy' theories, which by that I mean 'theories that explain the SN1A results' then the parameter(s) w(z) don't have the same physical meaning, but the effect of the theory, be in modified gravity, varying speed of light etc, can be represented with these parameters.

I strongly encourage you to read the Linder & Jenkins paper I linked in my previous post. Not all of it, just the first 2 sections (less than 2 pages). That should explain in detail what I mean.

That all being said, varying speed of light theories fail to explain the observed cosmic structure, so there is a good reason they are disfavored. Plus the limits of variations to the fine structure constant pretty tightly constrain any variation in the speed of light.

 

[Kea]

To the question of which theories have been tested then, ALL varying c theories are constrained through the current constraints of the dark energy equation of state, since they can be mapped to degenerate dark energy models.

The theory I have in mind has an effective Barrett-Crane spin foam description with the correct equation of state, which follows from the Kashaev volume conjecture. But in the more mathematical version of this M-theory (via twistor Strings) the Riofrio cosmology should work, because it is based on the correct Machian principles. Of course there are still standard candles...that is the point, after all...but the real luminosity depends on the cosmology. I somehow doubt the Riofrio cosmology has been properly checked by professionals. It should be, because she gets a fit to the WMAP power spectrum, whereas the Standard scenario doesn't do too well on this count.

Oh, and by the way, c is slowing down, not speeding up.

 

[cesiumfrog]

Regards the standard candles, note that SN1a aren't all just assumed to be the same - the original luminosity is more accurately estimated from a model which depends on the colour of the supernovae.

This announcement is interesting because, by looking to deeper redshifts, it should be able to eliminate most of the likely systematic errors (eg. possible grey dust just making distant candles look dull) from the SN1a data that initially produced the acceleration discovery.

A readable (and authorative) explanation, by Brian Schmidt, is "Measuring global curvature and cosmic acceleration with supernovae", Class. Quantum Grav., 19, 3487 (2002).

 

[Wallace]

I somehow doubt the Riofrio cosmology has been properly checked by professionals. It should be, because she gets a fit to the WMAP power spectrum, whereas the Standard scenario doesn't do too well on this count.

Why do you doubt this? Just curious. Can you point me a technical reference for this particular varying c theory you advocate? I'd like to have a look at some of the details.

The standard scenario, by definition, fits the WMAP power spectrum. If it didn't it wouldn't be the standard scenario since the WMAP results are our best current cosmological probe and these results have been used to decice the current best guess cosmology

The are still some interesting details (see Garth's recent post on the Axis of Evil for instance) but these are at the few percent level. The basics of the cosmologt are pretty well defined by the trio of CMB, SN1A and Galaxy Clustering measurements. There are theories that, by introducing extra parameters, fit one of these three better than the standard, but none that fits all three nearly so well. If there were, they would by now be the standard theory

That's not to say we know all, there is still some room within the current constraints for a variety of different models (for instance, the exact nature of dark energy) and in time perhaps a radically new idea could fundamentally change the prevaling view, but the Cold Dark Matter + dark energy that's quite similar to a cosmological constant fits all the dats sets remarkably well.

 

[George Jones]

I don't know what's going to be announced, but it seems that one possible take on it is that there is no dark energy.

Of course, this is a very non-standard interpretation.

What I meant here is that I thought w = -1 would be consistent with the announced results, which it is. However, as Space Tiger points out the error bars are huge.

Even if w = -1, then one possibility is that it is purely due a non-zero cosmological constant. In this interpretation, gravity is a theory that depends on three fundamental constants - G, c, and Lambda. When Lambda is positive, gravity becomes repulsive on cosmological scales, and no mysterious antigravity or dark energy (as a contributer to T) is needed - it's just geometry. We didn't notice it before, because we didn't look on large enough scales. In this intrepretation, an empty universe has spacetime curvature.

The standard interpretation of w = -1 is vacuum energy due to quantum field theory/quantum gravity, and it would be quite exciting ito see a first-principles calculation that agrees with observations.

 

[Mike2]

http://hubblesite.org/newscenter/archive/releases/2006/52/

Hubble news release.

If the energy desity that produced the negative pressure on the universe so it expands is constant, then constant density means that this energy is not produced by particles whose density decreases as the universe expands. The only alternative is that this energy density is a property of spacetime itself - a cosmological constant. So is this now confirmed constant energy density proof of the existence of a Zero Point Energy?

 

[SpaceTiger]

So is this now confirmed constant energy density proof of the existence of a Zero Point Energy?

No, as was already discussed in posts https://www.physicsforums.com/showpost.php?p=1162906&postcount=14".

 

[Mike2]

It appears that their measurements are consistent with a cosmological constant, though at least three things should be kept in mind:

1) The result doesn't rule out any modified gravity theory that mimics a cosmological constant.
2) The error bars on dw/dz are probably quite large.
3) Any supernova measurement is potentially subject to a large number of systematic errors.

Nonetheless, it is an interesting result. Whatever the dark energy turns out to be, it seems to behaving much like a cosmological constant. A great deal of money is currently being spent to check this statement to ever higher precision.

As for 1), this sounds like a rose by a different name. But wouldn't you have to agree that if it is a constant density as the universe expands, then it must be a property of space itself. And wouldn't any constant energy density be equivalent to a type of zero point energy?

And as far as 2) and 3), of course if the data is skewed, no conclusions ca be made. My question is IF the data DOES indicate a constant energy density, then isn't this proof of the controversial Zero Point Energy?

 

[Kea]

And wouldn't any constant energy density be equivalent to a type of zero point energy?

How many times do we have to say NOOOO!

 

[hellfire]

I do not understand why this should rule out a zero-point energy as source for the accelerated expansion. Sorry but I have seen no mention of this in posts #14 and #15. A zero-point energy would have a constant energy density (at least according to current QFT) and would behave as a cosmological constant, as George points out in post #22. It is however possible to interpret the cosmological constant as a pure geometrical phenomenon.

How many times do we have to say NOOOO!

So, at least once more for me, please. I did not get it.