Dark Matter

Scientists have proposed Dark Matter as a solution for why there seems to be more gravity than visible matter holding together galaxies.

From what I understand of vaccuum fluxuations/virtual particle pairs, a particle and antiparticle will spontaneously spring into existence…then immediately annihilate each other, all before the rest of the universe notices.

Also from what I understand, antimatter has mass like matter, therefore generates a gravitational field like matter.

Has particle/antiparticle pairs been researched as an explanation for dark matter?

Before an actual astrophysicist or cosmologist weighs in, I believe dark matter possibilities have been deduced down to mostly non-baryonic matter candidates.

This isn’t to say they could’t have their own anti-particle pairs, but it’s thought dark matter wouldn’t interact with ordinary matter electromagnetically; only gravitationally or through the weak nuclear force.

All that to say, dark matter, according to observances and hypothesis, should be an altogether different form of matter than all the ordinary matter we can and have currently interacted with.

Nope, vacuum energy functions like a cosmological constant (i.e. dark energy), with the slight problem in that it naively 10[sup]120[/sup] larger than the observed dark energy content.

Or to expand slightly cold dark matter is a pressureless fluid, but vacuum energy would be a fluid with negative pressure.

If it’s only 10[sup]120[/sup] larger, why even worry about it [grin].

Not that there’s any pressure, anyhow.

Waiting for Chronos to chime in.

Well, I’m not Chronos, but I’ll reply anyway.

Some points in no particular order…

  • The dark matter that is needed to explain all observations has to be something that has mass and sticks around long enough to have dynamics. It’s not a static gravitational source that happens to be centered on each galaxy. Rather, it’s some stuff that moves around under the influence of gravity. In the early universe, for instance, dark matter clumped up along with regular matter, influencing large-scale structure and patterns in the cosmic microwave background fluctuations. A virtual pair of particles just goes ‘pop’ and doesn’t have time to do anything dynamic.

  • If you want to look only at static galaxies and not the broader picture: The dark matter halo of a galaxy has a radial profile. Vacuum fluctuations happen everywhere uniformly and have no reason to have a dependence on distance from a galaxy’s core.

  • Gravity results not only from mass. The full stress-energy tensor determines what happens to spacetime. In the case of a rock floating still in space, the only piece of the tensor that matters is the energy density, which is directly related to the mass of the rock. For vacuum fluctuations, the stress-energy tensor is more complicated and does not produce the sort of gravitational effects required of dark matter.

  • Vacuum fluctuations could in principle be an excellent explanation for dark energy, so named because it is consistent with a constant energy density that permeates the universe to cause an accelerating expansion of spacetime. If you take our current understanding of particles and their interactions, and you calculate whether vacuum fluctuations could make up part of this observed apparent energy density, you do not get a sensible answer. In fact, the prediction comes out 10[sup]107[/sup] times larger than observation. This is what Asymptotically fat was referring to.

The key point to realize is this: the energy density of the vacuum must be Lorentz invariant, otherwise there would be a preferred reference frame for the vacuum which is not allowed. This immediately requires that vacuum energy has a negative pressure which is equal to the energy density (multiplied by a constant).

This just doesn’t produce the effects of dark matter and in fact observation indicates that dark matter has zero pressure or possibly a positive pressure that is much, much smaller than the energy density (multiplied by the same constant as above).

Now as we’re talking about vacuum fluctuations rather than vacuum energy itself you could argue that I haven’t been complete as I’ve ignored the quantum aspect of vacuum energy. However this doesn’t really make any difference, the expectation value for the energy of the vacuum must still be Lorentz invariant and all the other arguments therefore apply.

Asympotically Fat has it right, and there’s not much I can add to it: Vacuum fluctuations produce something qualitatively like dark energy, but quantitatively absolutely insanely ludicrously absurdly I’m-out-of-adverbs-ily wrong.

All I can really add is that the numbers that are so wrong aren’t exactly a calculation, but more of a back-of-the-envelope estimate, which explains the significant variation in numbers you might see quoted… Still, it’s very hard to imagine even a back-of-the-envelope estimate being that far wrong.

Bringing the discussion back toward dark matter, how much will really nailing down the physics of the Higgs field based on the energies we find these particles at aid in pinning down DM?

Have putative modified-gravity theories such as MOND, scalar-tensor-vector gravity et al been discarded in favor of dark matter as the only explanation for observed cosmological effects?

How can we speculate on DM theories when the quantum nature of gravity is yet to be correctly modeled and understood?

Though on the other hand it isn’t implausible that there are missing terms that can cancel the calculated vacuum energy density. The real puzzle is why the cancellation should be so exact or even worse almost but not quite exact. Still the fact that we don’t observe such vacuum energy density in the form of a much, much larger cosmological constant tells us that there is something going on that we’re not fully aware of.

MOND simply doesn’t fit the evidence as well as general relativity + dark matter and quantum gravity must recover general relativity within the correct limits, which dark matter exists well inside of.

Right; if it turned out that everything canceled out exactly, that’d be worth a “huh, that’s interesting”, but it wouldn’t really be particularly surprising. Cancelling out to over 100 decimal places but not quite exactly, though, is crazy.

Personally, I suspect that the quantum vacuum energy really does exactly cancel out to exactly zero, and that there’s some other completely unrelated phenomenon (possibly time-varying) that accounts for the cosmological effects we see. But that’s just my guess.

Very little, in all probability. The Higgs boson itself is not a viable candidate for dark matter; it decays far, far too quickly. I suppose it’s just barely conceivable that we could discover that the Higgs boson can decay into dark matter particles somehow, but this seems exceedingly unlikely to me.

The Large Hadron Collider is more likely to help with the study of dark matter by creating dark matter particles directly in one of its collisions. But that doesn’t relate to the Higgs boson except inasmuch as they would have been discovered by the same experiment.

Here’s a related post from last year. Non-baryonic non-relativistic dark matter works for all the bits of evidence so far. Things like MOND only address some.

Indeed, and unfortunately no one is going to take the other side of that potential wager. I’d give excellent odds to anyone that wanted to bet on fine-tuning (…neverminding that we’d likely not live to see its resolution).

Dark matter and dark energy constitute the majority of stuff in the universe and we don’t understand it/them. Does anyone talk about needing another Einstein and a new physics? BTW, I can find my ass in the dark with both hands but that’s all the math I know…

Not quite in those terms, but certainly some people believe that a resolution to the “dark energy” problem will require some fundamentally new physics. Here’s a nature article from last year about some of the ideas being bandied about these days concerning the structure of space and time.