Is Dark Energy Just a Myth in Modern Science?

[Chalnoth]

A couple of quick technical points that might clear up a little confusion. The term WIMP, as I understand it, was originally meant to imply "interacting under the weak force, but no other standard model forces." I presume this is not the sense of "weakly interacting" that Chalnoth presumes to be using. In this original sense, it is quite possible that dark matter is not weakly interacting - it's interactions with standard model particles could be through some other, as yet unknown, force. (In the particle physics literature, you can find quite a few such models, the most conventional of which have dark matter interacting either through scalar fields or through a new heavy neutral gauge boson, generically referred to as a Z'.) Such a new force is, still, expected to be weak compared with EM or the strong force.

A second possible point of contention regarding the particle physics is the identification of dark matter as a "particle." There's been quite a bit of discussion in the particle physics literature over the past few years about the idea of what are called "unparticle" models, which are a sort of non-local quantum field theories. The hallmark of such fields are particle-like excitations with continuous mass spectra.

These are both small technical quibbles; but, they demonstrate that the degree to which we can say that it's essentially settled that dark matter is WIMPs depends on how strictly we're interpreting what it means to have a weakly interacting particle.

Right, and let me be perfectly clear that these sorts of exotic possibilities, such as "unparticle" models would fit very much within the most general sort of definition I laid out. Yes, particle theorists are the people to ask for what sorts of specific models are out there. But those models must all be in line with the cosmological evidence that states two things:

1. It has mass.
2. It has no electromagnetic or strong force interactions.

When you look into the details of the various experiments to detect dark matter, you get further restrictions, but they're not worth going into here.

 

[diggy]

I think this is a pretty fair question. What is more believable, that our universe model in GR is wrong (or at least not the full story) or that we don't know what *actually* makes up 95% of our universe... or both.

Its been a while, but the last talk I saw on dark energy was pretty much a laundry list of "this didn't even come close to working" ideas. Dark matter is on "firmer" grounds, but no one can show you a piece of dark matter.

 

[IcedEcliptic]

Um, no. Dark energy might be a bit that way, but dark matter is much more concrete, in the sense of having a range of observations and data giving useful constraints on real properties. Hence there is a strong trend in the literature to say that dark matter is most like in the form of WIMPS rather than MACHOS or other possibilities. It's not all settled of course, but it is one heck of a lot more than pure conjecture. Scientists are not just conjecturing, but testing the alternative ideas and reaching conclusions on the basis of available empirical observations.

Cheers -- sylas

It is not concrete, but I am not saying that dark matter does not exist. I believe that GR explains enough that such a gap would be unlikely, but that does not make Dark Matter anything but a title for a something that we are yet to detect directly, or even through interaction or decay. Sterile Neutrinos, The Tau, WIMPs, and more... what is a WIMP other than a title for "we don't know, but something is there that has mass and does not radiate." It is a guess, a conjecture to explain gaps in our observations, but everyone always has the explanation just about to be revealed, and it never works.

 

[Chalnoth]

I think this is a pretty fair question. What is more believable, that our universe model in GR is wrong (or at least not the full story) or that we don't know what *actually* makes up 95% of our universe... or both.

This is an extremely misleading measure, though, because it's a measure that changes with time. We know the primary makeup of our universe a few seconds after reheating, for example: in the very early universe, nearly all of the energy density was made up of radiation. Not long after that, and until a few billion years ago, nearly all of the energy density was normal matter and dark matter. A few billion years from now, nearly all of it will be dark energy.

Its been a while, but the last talk I saw on dark energy was pretty much a laundry list of "this didn't even come close to working" ideas. Dark matter is on "firmer" grounds, but no one can show you a piece of dark matter.

It should be noted that the first idea on "dark energy" is still the most in accord with experiment: that it's a cosmological constant. And as for dark matter, just because it's hard to detect directly doesn't mean we can't be extremely confident it exists.

 

[IcedEcliptic]

This is an extremely misleading measure, though, because it's a measure that changes with time. We know the primary makeup of our universe a few seconds after reheating, for example: in the very early universe, nearly all of the energy density was made up of radiation. Not long after that, and until a few billion years ago, nearly all of the energy density was normal matter and dark matter. A few billion years from now, nearly all of it will be dark energy.


It should be noted that the first idea on "dark energy" is still the most in accord with experiment: that it's a cosmological constant. And as for dark matter, just because it's hard to detect directly doesn't mean we can't be extremely confident it exists.

I may be miscommunicating in this case; I personally believe that dark matter is composed of WIMPs, or Sterile Neutrinos, or both. I do not believe GR has such holes in it, that we're just "off the mark." Dark energy, I do not know. It seems there is a positive cosmological constant, and vacuum expectation energy would explain both, but it is far from being shown to be. I am disputing your certainty, not your conclusions.

 

[Chalnoth]

I may be miscommunicating in this case; I personally believe that dark matter is composed of WIMPs, or Sterile Neutrinos, or both. I do not believe GR has such holes in it, that we're just "off the mark." Dark energy, I do not know. It seems there is a positive cosmological constant, and vacuum expectation energy would explain both, but it is far from being shown to be. I am disputing your certainty, not your conclusions.

Slight caveat: I think we can be quite certain that GR is wrong on some level, but we can also be extremely confident that the way in which it's wrong cannot explain dark matter. Our confidence that the way in which it's wrong doesn't explain dark energy is a little less certain, but it still seems likely that the answer is not a modification of gravity.

 

[diggy]

This is an extremely misleading measure, though, because it's a measure that changes with time. We know the primary makeup of our universe a few seconds after reheating, for example: in the very early universe, nearly all of the energy density was made up of radiation. Not long after that, and until a few billion years ago, nearly all of the energy density was normal matter and dark matter. A few billion years from now, nearly all of it will be dark energy.

Then we will know what makes up approximately 0% of the universe -- much better :-)

I think the point is that if you model y=a*x, and measure x, you can only infer y through the model. That is all the OP is saying, and he is correct, imo. You however are taking the model as gospel, which is dangerous even if you (not necessarily you specifically) are right.

 

[Chalnoth]

You however are taking the model as gospel, which is dangerous even if you (not necessarily you specifically) are right.

Hardly. I'm just saying we have cause to be a lot more confident about specific features of the models than many people in this thread seem to think. The evidence for dark matter, in particular, spans a wide variety of mutually-corroborating data sets. From the data we have available to us today, there just isn't much of any wiggle room left.

Dark energy is much more up in the air. It could still be a modification of gravity, though that is seeming increasingly unlikely. The possibility of it just being a misunderstanding of how to deal with inhomogeneities properly is exceedingly far-fetched now. That leaves a cosmological constant or some time-varying field with the right properties (specifically having a pressure/energy density ratio close to -1 for late times, and not having too much energy density at early times).

 

[diggy]

Hardly. I'm just saying we have cause to be a lot more confident about specific features of the models than many people in this thread seem to think. The evidence for dark matter, in particular, spans a wide variety of mutually-corroborating data sets. From the data we have available to us today, there just isn't much of any wiggle room left.

Dark energy is much more up in the air. It could still be a modification of gravity, though that is seeming increasingly unlikely. The possibility of it just being a misunderstanding of how to deal with inhomogeneities properly is exceedingly far-fetched now. That leaves a cosmological constant or some time-varying field with the right properties (specifically having a pressure/energy density ratio close to -1 for late times, and not having too much energy density at early times).

Fair enough.

New question, if somewhere/somehow we were able to prove that either dark matter or dark energy was non-existent, i.e. the our universe model was no longer consistent with GR and evidence, would you still be an advocate of the other missing quantity?

 

[Chalnoth]

Fair enough.

New question, if somewhere/somehow we were able to prove that either dark matter or dark energy was non-existent, i.e. the our universe model was no longer consistent with GR and evidence, would you still be an advocate of the other missing quantity?

Yes. Why wouldn't I?

 

[twofish-quant]

New question, if somewhere/somehow we were able to prove that either dark matter or dark energy was non-existent, i.e. the our universe model was no longer consistent with GR and evidence, would you still be an advocate of the other missing quantity?

If you show that the existence of dark energy or dark matter was inconsistent with observations and general relativity, it could be that GR is wrong, or that we fundamentally misunderstand our observations.

Also I think you are setting the bar way too high. Dark energy is a crazy, stupid, silly explanation for how the universe works, and dark matter only slightly less so. All you have to do in order to convince me that dark energy and dark matter is not the best explanation is to come up with something slightly less crazy, stupid, and silly.

It's harder than it sounds...

The other thing is that whether dark energy is a good explanation or not is not some sort of religious or political issue. Give me one paper to read, and I might think it's a nutty idea, and then two weeks later, I might change my mind, and then two weeks later, I might change my mind again.

 

[twofish-quant]

I base my disbelief solely on philosophical intuition

That's a bad, bad way of doing physics. People have come up with alternative explanations for the observations, and right now the alternative explanations are worse than "dark matter."

One thing that is very educational is to go through the original paper that presented evidence that the universe is accelerating. They went through every possibility that people could think of for attributing this to some sort of observational weirdness and came up with reasons why that didn't work.

It troubles me deeply that we talk about dark energy as if it is a concretely proven phenomena

Except that we don't. A lot of what you read in theoretical papers are of the form "if dark energy exists then..." The point of talking about something that you don't know exists as if it did was so that you can then figure out what the consequences of it existing are.

yet we have no real basis for even believing it exists, other than if we don't know the answer, it's easier to just invent an explanation. To me that isn't science, that's myth making.

That's science. Invent an explanation. See if it works. If it doesn't, invent another explanation. Repeat.

To this effect, I'm also doubtful about dark matter. Not that there is unseen matter out there which has a gravitational effect, but the idea that dark matter is some unknown type of matter we've never seen before.

The problem is that if you assume that dark matter is "normal matter" you run into a lot of problems. Galaxies clump in the wrong way, and all the elemental abundances are wrong. People have tried really, really hard to come up with explanations of this, but the explanations are even weirder than dark matter.

 

[twofish-quant]

You know Maxwell discovered Electromagnetism based solely on his philosophical intuition that the universe should be symmetrical.

Intuition is wonderful for creative inspiration, but science involves matching intuition with cold hard facts.

You claim I should base my opinions on evidence, yet you seem (judging by your rude comment) to be a believer in dark energy.

For things at the frontiers of science, I hate the term "believe" or "disbelieve". Right now, it seems to me that dark energy is the best explanation anyone has come up with for what we are seeing. It's possible that this evening I'll read some paper on the Los Alamos Preprint Server and change my mind about this.

What evidence do you have for this beliefs?

There are review papers that people can point you to. Personally I'd start with wikipedia, since the articles there are often surprisingly good,

You can claim that you observe (or at least accept that others do) that the universe is expanding at an accelerating rate, but I can just as easily claim that such an observation is far too difficult to be certain about, and that your methods and instruments are flawed.

You can claim that. You can't *easily* claim that.

http://adsabs.harvard.edu/abs/1998AJ...116.1009R

Go through Section Five, where they go through all of the obvious explanations for the results and knock them down. If you can think of something they didn't include, feel free to mention it.

The fact is, no one has any explanation of what or how dark energy may work, and in the end they accept it not on scientific grounds, but on philosophical ones.

[/QUOTE]

You come up with an explanation, figure out the consequences, fit to observations. Right now dark energy is the "least bad" of the explanations.

 

[twofish-quant]

I strongly suspect that the Pioneer anomaly has more to do with the spacecraft itself or with how we are measuring its position than with fundamental physics.

Same here. If the Pioneer anomaly were due to fundamental physics, then we should be able to see weird things happening with other things.