What if all Dark Matter searches like CDMS II remain negative?

ensabah6

Is there any reason why DM has to be a particle rather than, say, a "fluid" like helium-3 superfluid, out of neutrinos or something else, or a field, perhaps baryonic matter has a previously unknown degree of freedom acting over long ranges, or a property of "dark energy"?

phyzguy

Now we're getting somewhere. It sounds like you're beginning to see the reasons why physicists believe dark matter must exist, and you have passed onto the "What is it?" question, which is where the physics community is working on today. There are lots of possibilities. Some of the candidates are:
(1) MACHOs - Massive Compact Halo Objects - i.e. substellar bodies such as Jupiter sized planets which are not large enough to emit light. Searches for these have all been negative, and the BBN (Big Bang Nucleosynthesis) data says conclusively that there are not enough baryons in the universe (by a factor of ~10) to account for the dark matter, so it appears that dark matter cannot be composed of ordinary matter, no matter how it is packaged.
(2) WIMPs - Weakly Interactive Massive Particles - This looks to be the best possibility today - i.e. some undiscovered elementary particle that interacts only weakly with ordinary matter. Thus all of the searches that you have referenced. However, until we actually find these particles in some other way, this hypothesis remains unproven.
(3) Neutrinos - there are a lot of neutrinos out there, but given what we know about their masses, there are not even close to being enough to explain dark matter. Also, the way the dark matter "clumps" in forming galaxies indicates that it is cold (hence CDM - cold dark matter). Neutrinos, which are very light and travel at near the speed of light, would not clump in the center of galaxies the way we see.

Personally, I think the WIMP hypothesis is the best one, but as you point out, until we find them it is an unproven hypothesis. That's what makes the search exciting! When you say a fluid or a field, what exactly do you mean? All fluids and fields that we know of today are ultimately composed of elementary particles.

ensabah6

Hey thanks,
is dark energy composed of elementary particles?

phyzguy

Dark energy is much less well understood than dark matter. It appears to be a uniform "tesnion" everywhere in space. Today's data is completely explained by what is called the cosmological constant, which is constant throughout space and time. However, there are many experiments underway to determine if it is really constant or has some structure. In my judgement, no, it is not composed of elementary particles, but is more a property of space-time.

nismaratwork

I'm sure that this has been said, but to the OP: we look harder, with finer instruments unless a better and more predictive theory emerges. Dark matter seems like a terribly safe bet, but dark energy could be what phyzguy said, or vacuum expectation energy, or something else entirely. I don't think DE will be discovered, or the effect explained in our lifetimes, but I suspect we'll find DM soon (soon being 5 to 10 years).

ensabah6

that's what I'm talking about, that what we call "dark matter" could be a property of space-time in the presence of baryonic matter. Perhaps baryonic matter interacts with dark energy to produce stronger than expected gravitational effects.

ensabah6

well superCDMS, the successor of CDMSII will be online, along with several other research groups, and have the sensitivity to detect nearly the entire parameter space of SUSY-DM like neurtralinos. Then there's the LHC. CDM might not be neutralinos but if it has the mass and cross sectional flux of neutralinos, it should be detected.

nismaratwork

I certainly hope so, and frankly I wouldn't be surprised if the LHC was the key to unlocking this mystery. Observations are going to need to be screened for noise, and what better tool than the LHC? I think between SCDMS, the LHC, LIGO, and LISA are going to make this a very exciting decade to be alive.

nismaratwork

That doesn't match observations, and would require radical rethinking of existing theories. Do you have a source for this notion?

ensabah6

http://arxiv.org/abs/0804.1588

Dark Fluid: Towards a unification of empirical theories of galaxy rotation, Inflation and Dark Energy
Authors: HongSheng Zhao (SUPA, St Andrews) Baojiu Li (DAMTP, Cambridge)
(Submitted on 10 Apr 2008)

Abstract: Empirical theories of Dark Matter like MOND gravity and of Dark Energy like f(R) gravity were motivated by astronomical data. But could these theories be branches rooted from a more general hence natural framework? Here we propose the natural Lagrangian of such a framework based on simple dimensional analysis and co-variant symmetry requirements, and explore various outcomes in a top-down fashion. Our framework preserves the co-variant formulation of GR, but allows the expanding physical metric be bent by a single new species of Dark Fluid flowing in space-time. Its non-uniform stress tensor and current vector are simply functions of a vector field of variable norm, resembling the 4-vector electromagnetic potential description for the photon fluid, but is dark (e.g., by very early decoupling from the baryon-radiation fluid). The Dark Fluid framework naturally branches into a continuous spectrum of theories with Dark Energy and Dark Matter effects, including the $f(R)$ gravity, TeVeS-like theories, Einstein-Aether and $\nu\Lambda$ theories as limiting cases. When the vector field degenerates into a pure Higgs-like scalar field, we obtain the physics for inflaton and quintessence. In this broad setting we emphasize the non-constant dynamical field behind the cosmological constant effect, and highlight plausible corrections beyond the classical MOND predictions. Choices of parameters can be made to pass BBN, PPN, and causality constraints. The Dark Fluid is inspired to unify/simplify the astronomically successful ingredients of previous constructions: the desired effects of inflaton plus quintessence plus Cold DM particle fields or MOND-like scalar field(s) are shown largely achievable by one vector field only.

nismaratwork

Thanks very much, I'll give the whole shebang a read.

TrickyDicky

Flatness of earth and ptolemaic epicycles were quite succesfull too, if not so empty.

"If I could have my pick, I would like to learn that Newton's laws must be modified in order to correctly describe gravitational interactions at large distances.That's more appealing than a universe filled with a new kind of sub-nuclear particle."

This statement is by Vera Rubin. But what does she know about this anyway.


It might even not be necesary to look for any modification.

"General Relativity Resolves Galactic Rotation Without Exotic Dark Matter" http://arxiv.org/PS_cache/astro-ph/p.../0507619v1.pdf

Regards

Ich

Thanks, I referred to that paper in a different thread but couldn't find it anymore. It's errorneous.

Yeah, If...
You don't understand the difference between one's preferences and cruel reality? The question is not what people like to have as the explanation, the question is which explanation is vialble. Vera Rubin definitely knows that.

TrickyDicky

You're welcome. What exactly is wrong in the paper? That kind of assertion is not very scientific.

Ha Ha, Vera Rubin definitely knows it, the problem is you don't. Try and aply it to your blind beliefs about physics. But I can see reality must indeed be cruel to you so I understand your resistance.

Regards

TrickyDicky

I just saw the references in the other (rather interesting) thread. I'll take a look at them.

Regards