Is dark matter real?

Chipper Q
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Hmm. Dark matter, apparently

Hmm. Dark matter, apparently in some initial noncollapsible maximum density, first attracts normal matter gravitationally to form the galaxies, and is subsequently displaced from the galactic interior, into a halo around the galaxy?

Thanks for the help with the terms, Solomon. My understanding of the weak force is that it's sort of a residual effect of the strong force (due to greater distance), and that it's analogous to the van der Waals force (the EM force at a greater distance). What's the reason that dark matter should interact with the weak force?

Tullio mentioned axions. I ran across this SciAm article about them. In the effort to detect them coming from the sun with null results, wouldn't axions be formed along the polar axis only, and hence be detectable only above the N and S magnetic poles, and not in the equatorial plane?

With regard to regions in the universe at a temperature below the CMB blackbody radiation, I thought that the cooling process might work through evaporation, taking place on the interior side of the halo, which cools the halo sufficiently well below the lambda point of the helium. (I got the idea from reading an article about the surface temperature of Pluto being colder than what was expected, and colder than that of Charon, which has no halo, -er atmosphere :) )

Solomon
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There's no particular reason

There's no particular reason that dark matter should interact through the weak force; but, my understanding is that observations have not yet ruled out such interactions either, whereas they have ruled out electromagnetic and strong interactions.

As for galaxy formation; it's not so much that dark matter distributions can't collapse. It's that, not interacting electromagnetically, the time scales for such collapse are enormously larger than for regular matter. Hence, regular matter starts clumping where the dark matter is more dense and then collapses down to a much greater density much faster than the dark matter can, leaving the dark matter in a halo.

Chipper Q
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So if the main constituent of

So if the main constituent of dark matter is superfluid helium, what would we see? Possibly a faint glow in the microwave band, as the surface electrons drop back down to their ground state? (Ref. figure 1 of this pdf document, 'Microwave saturation of the Rydberg states of electrons on helium')

This might also explain why ~75% of the nearby galaxies (that should cast a shadow) apparently aren't casting a shadow in the CMB (see this E@H post by HomeGnome).

Another constituent of the dark matter could also be frozen bit-sized chunks (or chunk-sized bits, up to the limit Ben pointed out for MACHOs) of solid hydrogen, no?

Solomon
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Once again, given that the

Once again, given that the CBR is at 2.7 K, how do you propose that this Helium dark matter would remain cold enough to be superfluid, given that the superfluid phase does not exist at above 2.2 K? And, for that matter, is it even possible to have a coherent BEC state spanning such large distance scales? It seems to me that that would cause causality issues.

Mike Hewson
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RE: So if the main

Message 38356 in response to message 38354

Quote:
So if the main constituent of dark matter is superfluid helium...


Chipper, m'lud .... superfluid Helium is not a state that occurs simply because the temperature becomes low enough. The whole kit'n'kaboodle must remain very undisturbed as well. The Helium atoms behave as bosons. Although the protons ( 2 ), neutrons ( 2 ) and electrons ( 2 ) in Helium are individually fermions, when they are together ( 'pairing', roughly ) allows them to behave conjointly as a single rather heavy boson. Now bosons, when they only have little/no options as to their quantum state ( the 'wave function' if you like ) will tend to congregate in that state. In contrast fermions ( eg. electrons ) are quite snobby and prefer not to. So in the highly artificial circumstances of near-vacuum chambers, with shielding from all sorts of rigamarole, Helium II ( the phase ) will exhibit some tremendous properties because there are few quantum state options available - but the situation degrades very readily if disturbed. Varying the energy ( or temperature ) is only one way to alter the possible states, and thus lose the coherence of their behaviour. They have to hang around within a few tens of atomic radii of each other too. Impurities, anything at all actually, will expand quantum options also. I expect deep space is full of events ready to spill the teacup! Quite apart from any other influence the cosmic microwave background is, and has always thus far been, hotter than the maximum temperature allowable for HeII.
Cheers, Mike.

( edit ) Chuckle .. I cracked a funny! Read 'maximum temperature allowable for He II' instead of 'maximum temperature allowable for HeII.' It is not, in fact, hotter than hell! :-)

I have made this letter longer than usual because I lack the time to make it shorter. Blaise Pascal

Chipper Q
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RE: Once again, given that

Quote:
Once again, given that the CBR is at 2.7 K, how do you propose that this Helium dark matter would remain cold enough to be superfluid, given that the superfluid phase does not exist at above 2.2 K?


Evaporation, similar to the way sweat cools the body. The evaporation takes place on the interior of the halo, and cools the bulk. If this is the case, then we might expect to find that helium in the local interstellar medium is ionized to a greater degree than the hydrogen, even though it has a higher ionization potential than the hydrogen. The following quote is a bit dated (9/2000), and may or may not be critical to supporting the idea of evaporation as a cooling mechanism, but IMHO it illustrates how much specialized hardware we need, just to see the stuff that's nearby. The quote is from 'JOP129: The Gravitational Focusing Cone of Interstellar Helium':

Quote:
Of the components of the local interstellar medium (LISM) helium is the species for which the neutral parameters can be determined without alteration from inside the heliosphere. On the other hand He also seems to be very intriguing because of recent results that it appears to be more highly ionized than H, in spite of its higher ionization potential. Currently, this has not been reconciled yet with the local radiation environment in the LISM.

The difference in surface temperatures between Pluto and Charon is a good example of the evaporative cooling mechanism. Charon, with no atmosphere, has a surface temperature of what you would expect when you consider the material composition, surface area, and incident radiation. But when you apply the same reasoning to Pluto, you get a temperature that is greater than the actual one; the surface of Pluto is colder, because it has an atmosphere where evaporation is occurring, and cooling the bulk.

In the case with Pluto, the working material is nitrogen, and the cooling process was described as an 'anti-greenhouse effect'. The nitrogen evaporates off the icy surface, carrying away heat. It radiates the heat in the thin nitrogen atmosphere, condenses, and falls back to the surface. In the case with SFHe dark matter, the helium will be in the same constant flux (at the "surface" of the interior side of the region) as the nitrogen is on Pluto, but when the helium condenses, I don't know whether it's proper to say that it "rises" from the galaxy back into the dark matter, or that it simply "falls" back into the dark matter; surely some gravitationally meteorological combination of both.

Is there any reason to expect that the temperature of the dark matter would be hotter than 2.7K? If not, then it's not too far from there to the limit of how cold anything could ever get. Helium's the only thing that will remain as a liquid down to that limit.

Quote:
And, for that matter, is it even possible to have a coherent BEC state spanning such large distance scales? It seems to me that that would cause causality issues.

Is there some rule prohibiting a macroscopic quantum mechanical system? As Mike pointed out, there are surely plenty of things to 'spill the teacup'. The question is what does the spilled tea look like, and how rapidly is it drawn back into the cup (by the temperature and pressure of the surroundings)? The pressure is near vacuum, and temperature is a combination of gentle gravitational-inertial confinement of material that's continually cooled by evaporation, and that very same cooling process is also the primary 'shield' (for the bulk of the dark matter) from most of the galactic 'rigamarole'. Is that possible?

The question Mike raised about impurities brings up an interesting point. There's no reason I can think of that helium would be the sole constituent of dark matter. There would have to be, probably in primordial proportions, also hydrogen, carbon, oxygen, and nitrogen, etc. But the helium wouldn't clump, and just about everything else (being diatomic) would clump, into frozen-solid chunks, suspended in the superfluid liquid. Consequently, not being a gas, these solid clumps fall into the MACHO class of objects: if the clump isn't bigger than something about Jupiter-sized, then we (currently) aren't able to see it. Recalling the article about SFHe mimicking vacuum for a molecule suspended in it, if there's a sufficient amount of helium to completely surround the impurity, coherence is maintained and the impurity acts as it does in vacuum except that it has a greater moment of inertia.

Nereid
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If there were sufficient

If there were sufficient superfluid He, in the form of 'droplets', to comprise a signicant proportion (mass-wise) of the DM, wouldn't it be opaque to light (in the optical, UV, and NIR wavebands)?

And wouldn't there be some interesting regions of gaseous He, and He plasma, around the hot stars and galaxies deeply embedded in regions where we know DM hangs out?

And wouldn't we see some interesting He signatures in planetary nebulae that are way out in the DM halos of galaxies?

Once evaporated, how would the He recombine to form a superfluid again?

Mike Hewson
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RE: Evaporation, similar to

Message 38359 in response to message 38357

Quote:
Evaporation, similar to the way sweat cools the body ...... coherence is maintained and the impurity acts as it does in vacuum except that it has a greater moment of inertia.


Interesting stuff Chipper. One of the issues with cosmology, so 'evident' that it is often forgotten, is that our best and most reliable data is only really for nearby behaviour in space and time. All else is ( quite sensibly ) an extrapolation from that.
So if we recieve, say, light from the Andromeda galaxy then we analyse it based upon our modelling of what light has done locally. It's not a bad assumption to consider that light propagates from Andromeda all the way to here in the same fashion as it does at Earth. But it is an assumption none the less - meaning no one has been able to map the path light took, by actually going to check that our laws are valid over there too ( and return to report ). This is not to say that we can't hypothecate some variance from local laws and then extract an observable signature of that to check against, but it remains a unalterable weakness. All our measurements of distant stuff ( say beyond the solar system ) are really a proxy. I don't mean to get too 'existential' here.
Something certainly unusual is going on, otherwise we would not be discussing a concept like 'dark matter', and a priori ideas are falsifiable by new data.
Cheers, Mike.

I have made this letter longer than usual because I lack the time to make it shorter. Blaise Pascal

Ben Owen
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Chipper Q, Dark matter is

Chipper Q,

Dark matter is not superfluid helium, for all the reasons people have mentioned in this thread plus many more they have not.

I think you got confused by the statement in that article you originally linked to that the "superfluid helium acts like vacuum..." All they meant was that it has no viscosity, that is it doesn't resist when something's trying to move through it. In pretty much every other respect, superfluid helium is not like a vacuum at all.

By the way, we know that dark matter can't be just ordinary matter that happens to be hidden somehow because of something called the "nucleosynthesis bounds." All the very hot ordinary matter in the early universe should have started as hydrogen. You can work out, as a function of how much ordinary matter there is in the universe, how far nuclear reactions would have proceeded in the early hot days and therefore the relative abundances of various light elements in the modern universe. (In other words, how much of the initial hydrogen got "burned" into other elements.) The observed abundances today imply that we've seen just about all the ordinary matter. Given the error bars you could say we've missed a little bit, but not that we've missed it by a factor of 5-6 which is what you need if you want to claim the dark matter is just ordinary matter in the form of lots of Jupiters or something else that wouldn't shine.

And yes, it is possible to have macroscopic quantum phenomena. That's exactly what superfluidity is, and superconductivity. Come to think of it, the neutron stars Einstein@Home is looking for (~10km radius) are the most macroscopic phenomena in the modern universe. They're (mostly) superconducting and superfluid, and they're held up by degeneracy pressure which is yet another macroscopic quantum effect.

Hope this helps,

Ben

P.S. Oops, just realized white dwarfs are bigger and they're held up by electron degeneracy pressure. But you could still argue for neutron stars, because the neutrons and protons in WDs (99.9% of the mass) are not degenerate; nor are they sueprconducting or superfluid.

Ben Owen
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P.P.S.: Regarding the

P.P.S.: Regarding the business about "new space appearing between the Earth and the Sun." First, the cosmological expansion is better described by space stretching rather than new space appearing. And the distance between the Earth and Sun is just about unaffected by cosmological expansion.

The standard cosmological expansion comes from solving the Einstein equations assuming the matter in the universe is evenly distributed. On very large length scales, averaging over many clusters of galaxies, voids, and filaments, that's true. On the scale of the solar system, it's clearly not true. When you're close to lumps of stuff much denser than the average density of the universe, as we are, the spacetime dynamics are completely dominated by the lumps of dense stuff and do not experience the cosmological expansion to an appreciable extent.

Hope this helps,

Ben

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