If WIMPs are the explanation for dark matter should it clump together to form dark matter "planets"?
No. What would bind the WIMPS? They have no chemistry.
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).
Furthermore, normal bound objects stick together, and these start to accrete even more matter. WIMPs aren't sticky, so this doesn't happen.
This Physics Forums thread from the “Cosmology” section in October, 2011 seems to answer the OP’s question:
Interesting to note that in the above thread, in post number five, the following paper is referenced:
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”
Thanks for all the replies. Interesting.
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.
Ah, we have a MOND believer among us!
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.
Not really, we can always state that we do not currently have a scientific explanation for it. There is no shame in that.
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.
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.
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.
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.
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.
Clumps of dark matter can be observed via bending of light. So things have progressed beyond the purely numerical stage.
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.
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.
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