Can Extended Theories of Gravity Explain Dark Matter and Dark Energy?

[CWatters]

If WIMPs are the explanation for dark matter should it clump together to form dark matter "planets"?

 

[Vanadium 50]

No. What would bind the WIMPS? They have no chemistry.

 

[CWatters]

Forgive my ignorance but wouldn't gravity do it?

 

[Nabeshin]

Forgive my ignorance but wouldn't gravity do it?

In principle you could have a gravitationally bound object composed only of WIMPs. However, to see why in practice this is very unlikely to happen one should first look at how normal bound objects form. In general, the process goes like this: A bunch of matter slowly starts falling in on itself under mutual gravitational attraction. As the matter collapses it becomes more dense and heats up, losing energy to thermal radiation. This loss of energy slows down the collapse until, eventually, there exists a small bound object.

For WIMPs, the entire part about heating up and losing energy is absent. So the cloud would just collapse, but pass right through itself (classically oscillating like that forever).

 

[Vanadium 50]

Furthermore, normal bound objects stick together, and these start to accrete even more matter. WIMPs aren't sticky, so this doesn't happen.

 

[Bobbywhy]

This Physics Forums thread from the “Cosmology” section in October, 2011 seems to answer the OP’s question:
https://www.physicsforums.com/showthread.php?t=540971

Interesting to note that in the above thread, in post number five, the following paper is referenced:
http://www.mpa-garching.mpg.de/galfo...um/0504097.pdf

Near the end of this paper it is claimed that during galaxy formation the dark matter halo is “virialized” in an isothermal sphere. This is not really “clumped” as the OP asked about, but it is a kind of an over-density structure held together cohesively by self-gravity.

For a recent review paper see: “The missing matter problem: from the dark matter search to alternative hypotheses”
http://arxiv.org/pdf/1110.5026

 

[CWatters]

Thanks for all the replies. Interesting.

 

[Light Bearer]

What I've gathered from this thread is that WIMPS only interact gravitationally with themselves, matter and antimatter?

 

[Passionflower]

What I've gathered from this thread is that WIMPS only interact gravitationally with themselves, matter and antimatter?

Well that is very convenient as this theory cannot be falsified.

If our gravitation laws do not match with experiment and we simply introduce 'invisible pink WIMPS' that have no other observables than adjusting for the experimental discrepancy with theory I'd say we do not have a valid theory.

 

[Nabeshin]

Well that is very convenient as this theory cannot be falsified.

If our gravitation laws do not match with experiment and we simply introduce 'invisible pink WIMPS' that have no other observables than adjusting for the experimental discrepancy with theory I'd say we do not have a valid theory.

Ah, we have a MOND believer among us!

 

[Passionflower]

Ah, we have a MOND believer among us!

Not really.

 

[Chronos]

If you find dark matter to be objectionable, your options are limited - like MACHOS, black holes, or invoke new physics such as MOND, mirror matter, or exotic matter. The only problem is they replace the perceived pink fairies of DM with pink elephants.

 

[Passionflower]

If you find dark matter to be objectionable, your options are limited

Not really, we can always state that we do not currently have a scientific explanation for it. There is no shame in that.

 

[Drakkith]

Well that is very convenient as this theory cannot be falsified.

If our gravitation laws do not match with experiment and we simply introduce 'invisible pink WIMPS' that have no other observables than adjusting for the experimental discrepancy with theory I'd say we do not have a valid theory.

What constitutes a "valid" theory? Was Newtons law of universal gravitation valid? Obviously we probably have a long ways to go in understand what dark matter is, but is our current theory valid? Since it matches observations to some degree I'd argue that it is.

And I'm not sure why you're saying that the theory cannot be falsified. We may not have the capability of detecting WIMPS at the moment, but the theory can be falsified in principle.

Not really, we can always state that we do not currently have a scientific explanation for it. There is no shame in that.

That's not how science works. We can't fully explain anything, as that would take us into a possible infinite series of 'why' questions. What we can do is form theories in an attempt to understand how our universe works, and using these theories come up with models and methods to check them. Every theory goes through a point in time where you just don't have the data to say yay or nay on it.

By your train of thought we should just say we have no idea how anything works because we don't fully understand it.

 

[Passionflower]

By your train of thought we should just say we have no idea how anything works because we don't fully understand it.

Not at all, we can present theories that in principle can be falsified and have predictive values.

However making numbers fit by introducing entities whose only observables are used to make the numbers fit is no theory at all in my opinion, we simply could not falsify it because there are no other observables than making the numbers fit.

I think we just have to agree to disagree, personally I find the postulation of further undetectable matter only on the ground that it will match the numbers a low in the history of science.

 

[Chronos]

For starters, WMAP and Planck strongly constrain the amount of baryonic matter in the universe to only a fraction of the total matter content of the universe. The bullet cluster appears to be a strong candidate for observational evidence of dark matter. The DAMA/Libra experiment has already claimed direct detection of DM. The Alpha Magnetic Spectrometer [AMS-2] is regarded as likely to confirm this result - and number of other direct detection experiments are also under way. It currently appears much more a question of when, not, if DM will be confirmed. Suggesting that DM is little more than numerolgy is, at best, uninformed.

 

[ImaLooser]

What I've gathered from this thread is that WIMPS only interact gravitationally with themselves, matter and antimatter?

Sort of. Matter interacts gravitationally with space itself, then WIMPS matter and antimatter go along for the ride. It least that is the way that I see it.

 

[ImaLooser]

Not at all, we can present theories that in principle can be falsified and have predictive values.

However making numbers fit by introducing entities whose only observables are used to make the numbers fit is no theory at all in my opinion, we simply could not falsify it because there are no other observables than making the numbers fit.

I think we just have to agree to disagree, personally I find the postulation of further undetectable matter only on the ground that it will match the numbers a low in the history of science.

Clumps of dark matter can be observed via bending of light. So things have progressed beyond the purely numerical stage.

 

[Bobbywhy]

Dark Matter was invoked to explain a large number of observations. But as yet, no dark matter particle signatures (evidence) has been detected or observed. Using the simplest or most economic argument without the introduction of exotic ingredients, Einstein’s General Relativity at large scales may need to be revised and extended to account for all the observed dynamics. The very existence of Dark Matter remains an open question until empirical evidence from observations becomes available.

Bobbywhy

 

[mrspeedybob]

Not really, we can always state that we do not currently have a scientific explanation for it. There is no shame in that.

That's no different then the "dark matter" that you are objecting to.

Observations were made that seemed to indicate the presence of something that we couldn't see. No body knows what it is, but it's awkward to call it "an unexplained source of apparent space-time curvature" so we call it dark matter instead. We may discover at some point that it's not matter at all, but some other effect. Until we know what it is, we need something to call it. "Dark matter" is as good a name as any for an unexplained phenomena.

 

[Passionflower]

That's no different then the "dark matter" that you are objecting to.

Observations were made that seemed to indicate the presence of something that we couldn't see. No body knows what it is, but it's awkward to call it "an unexplained source of apparent space-time curvature" so we call it dark matter instead. We may discover at some point that it's not matter at all, but some other effect. Until we know what it is, we need something to call it. "Dark matter" is as good a name as any for an unexplained phenomena.

I disagree, I am not saying that it is the case but it may be also true that the laws of gravitation work slightly different at low strengths. Or is that perhaps a sacrilegious idea?

From a phenomenological point of view we are confronted with a situation that does not rhyme with theory. That could mean that the theory needs to be adjusted or that could mean that we missed a factor. To assert by default that it is the latter is in my opinion a bias that unscientific. Not only that, many people seem to have no problem to completely ignore the standard model and just pose some new kind of matter, as if it is some new flavor of chips. As I wrote before I am of the opinion that an extraordinary claim requires extraordinary proof.

But, apparently it is not done on this forum to simply question dark matter, if one does people resort to grave insults, so we should probably end the discussion about this as there is no point.

 

[Drakkith]

I disagree, I am not saying that it is the case but it may be also true that the laws of gravitation work slightly different at low strengths. Or is that perhaps a sacrilegious idea?

Not at all, there have been theories involving modified gravity before. They simply don't fit the available evidence as well as dark matter does. This may change in the future.

From a phenomenological point of view we are confronted with a situation that does not rhyme with theory. That could mean that the theory needs to be adjusted or that could mean that we missed a factor.

Of course. No one's arguing against this.

To assert by default that it is the latter is in my opinion a bias that unscientific.

This is what you aren't getting. We haven't 'asserted by default' that dark matter is more likely. What we have done is build up numerous theories and models involving multiple types of dark matter and modified gravity and compared them against the available evidence. Current theories on dark matter simply fit better than modified gravity.

Not only that, many people seem to have no problem to completely ignore the standard model and just pose some new kind of matter, as if it is some new flavor of chips. As I wrote before I am of the opinion that an extraordinary claim requires extraordinary proof.

And modified gravity requires less proof than a new type of matter? The fact is that we have observed something that can't be fully explained. We have multiple theories and models all competing against each other to explain it. None of them are intrinsically better than any of the others. It all comes down to matching observations with predictions and seeing which ones hold out the longest. Modified gravity simply isn't the one.

But, apparently it is not done on this forum to simply question dark matter, if one does people resort to grave insults, so we should probably end the discussion about this as there is no point.

I don't know why you believe this. Multiple posters have said that we don't know what dark matter is and that within current science the possible explanations are limited at this time. The only thing you've said so far is that you don't think we should call it dark matter, yet you cannot give a good reason that argues from real science and reason and not personal opinion.

In short, you've come to this thread, made claims from personal opinion based on ignorance of the real science of dark matter, and then tried to make us feel bad by claiming we've insulted you because we have corrected you. No one's insulted you and I don't feel bad. If you think otherwise, you may need to grow thicker skin and not try to read into the text so much. It doesn't carry any of the verbal/nonverbal communication people are used to, which makes it very difficult to tell what the underlying feelings of someone are.

 

[ImaLooser]

Dark Matter was invoked to explain a large number of observations. But as yet, no dark matter particle signatures (evidence) has been detected or observed. Using the simplest or most economic argument without the introduction of exotic ingredients, Einstein’s General Relativity at large scales may need to be revised and extended to account for all the observed dynamics. The very existence of Dark Matter remains an open question until empirical evidence from observations becomes available.

Bobbywhy

Well, General Relativity is already the simplest and most economic argument. There is not much latitude to adjust it.

Besides, some galaxies have dark matter and some don't. I don't see how fiddling with GR could help you with that.

 

[Passionflower]

Well, General Relativity is already the simplest and most economic argument. There is not much latitude to adjust it.

People seem to forget (or sometimes not even know) that there is a proportionality constant in Einstein's field equations. This constant is not derived from GR but plugged in by assuming Newton's law is always true for the weak field and low speed.

Is that true or not?

 

[Bobbywhy]

Well, General Relativity is already the simplest and most economic argument. There is not much latitude to adjust it.

Besides, some galaxies have dark matter and some don't. I don't see how fiddling with GR could help you with that.

Here is the first part of the Abstract from the below paper:

“Extended Theories of Gravity”

“Extended Theories of Gravity can be considered a new paradigm to cure shortcomings of General Relativity at infrared and ultraviolet scales. They are an approach that, by preserving the undoubtedly positive results of Einstein’s Theory, is aimed to address conceptual and experimental problems recently emerged in Astrophysics, Cosmology and High Energy Physics. In particular, the goal is to encompass, in a self-consistent scheme, problems like Inflation, Dark Energy, Dark Matter, Large Scale Structure and, first of all, to give at least an effective description of Quantum Gravity. We review the basic principles that any gravitational theory has to follow. The geometrical interpretation is discussed in a broad perspective in order to highlight the basic assumptions of General Relativity and its possible extensions in the general framework of gauge theories.”

http://arxiv.org/abs/1108.6266v2