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Physicists Resurrect an Old, Strange Dark Matter Theory
New submitter rossgneumann writes: Dark matter might not be nearly as exotic as most theories suggest. Instead, it could be macroscopic clumps of material formed from common particles already found within the Standard Model of particle physics. This argument comes courtesy of physicists at Case Western University (PDF). Dark matter is usually thought of in terms of exotic, so-far undiscovered particles. The leading candidates are known as weakly interacting massive particles, or WIMPs. But the Case Western theory suggests that there are no dark matter particles, at least none that exist outside of current knowledge. Instead, there are baseball-sized clumps of "regular" matter formed from unexpected combinations of Standard Model particles.
So it's strange that this matter may not be exotic?
"For every expert, there is an equal and opposite expert"
Excuse the oversimplification here but....
What I'm getting is, if they take a bunch of particles together in the right combination, then they no longer emit or react to photons? A) huh? B) so invisibility cloak anyone?
While I agree that something is odd with gravity, the certainty that many scientists seem to have that it must be an exotic particle or form we have not discovered seems misguided. It could be something exotic and new that doesn't fit with any previously discovered science... or not. Dark matter just fails Occam's Razor in my opinion.
I'm not saying it doesn't exist either... just that I think we need to be more open to alternative theories like this. I'd love to see this particular question answered in my lifetime.
"I will trust Google to 'do no evil' until the founders no longer run it." Hello Alphabet.
we're talking about clumps of matter with a density of a hundred billion tons per cc that would collide (likely passing straight through with catastrophe on both sides) with the earth at least once a year....that would be VERY noticeable. Even moreso noticeable if the velocity was insufficient to leave the other side, we'd have a growing degenerate matter "star" in the center of our planet, which could only end badly.
Wikipedia has the answer! http://en.wikipedia.org/wiki/Cthulhu
Nothing like an invisibility cloak. If I understand the paper correctly they're just heavy enough that there are few enough that we haven't seen them.
Glad they didn't say "football-sized" or we'd have to go down that whole units thread yet again.
Have gnu, will travel.
Does this imply that there might be an Oort-style cloud, or bubble-like sphere, on the galactic edges made of small clumped matter, the total mass of which is many times that of the observable galaxy?
That's not what dark matter is.
If dark matter existed in normal form, the gravity associated with it would be vastly distorting the universe as we know it.
Pretty much, we know that dark matter can't be normal - as you would consider it - matter. All other theories as to what it might be are just as unproven and have just as many holes.
There was always an assumption that rogue or orphaned planets could contribute. Cross referencing against observations and known theories yielded a contribution that exists, but is far too small. These are what they are refering to as MACHOs.
There needs to be *a lot* of whatever is causing the discrepancies, and that much "normal" matter would probably be easily measured as the effect that the mysterious matter has is significant enough to require an equivalent mass to something like 20-30% of the mass of the universe. It would be difficult to believe we could be off on our observations by that much.
Cold Dark Matter: http://en.wikipedia.org/wiki/C...
MACHOs or Massive Compact Halo Objects are large, condensed objects such as black holes, neutron stars, white dwarfs, very faint stars, or non-luminous objects like planets. The search for these consists of using gravitational lensing to see the effect of these objects on background galaxies. Most experts believe that the constraints from those searches rule out MACHOs as a viable dark matter candidate.[5][6][7][8][9][10]
Also:
http://en.wikipedia.org/wiki/D...
Studies of big bang nucleosynthesis and gravitational lensing have convinced most scientists[10][80][81][82][83][84] that MACHOs of any type cannot be more than a small fraction of the total dark matter.[8][80] Black holes of nearly any mass are ruled out as a primary dark matter constituent by a variety of searches and constraints.[80][82] According to A. Peter: "...the only really plausible dark-matter candidates are new particles."[81]
The various limits on dark matter actually limit the ratio of the scattering cross section and the mass of whatever is making up the dark matter (this obviously does not apply to MOND type theories, which are different).
So, there are two ways to have a more-or-less non-interacting dark matter - have a small mass, and a very, very small cross section (as in WIMPs), or have a large mass, and a high density (as in quark matter DM theories). The large mass means that the scattering cross section can be more or less anything, and, specifically, can be what you would expect for regular matter.
Before the CMB was emitted, the entire universe was an extremely smoothly-distributed ionized plasma. There were no galaxies or stars or planets: just a smooth plasma whose temperature varied from place to place by about one part in 100,000. We can see an image of the universe when this plasma cooled to the point it became a gas. This image shows a very clear signature of dark matter (in fact, it's the most sensitive detection of dark matter density that exists).
This proposal has the same sort of problem: how would you produce such extremely dense objects when the matter was distributed so evenly?
We would have seen them via gravitational microlensing. This provides a graph of the limits on dark matter, and the planetary mass range is excluded.
Also, planets (unlike condensed dark matter) would not evade the Big Bang Nucleosynthesis limits on baryonic matter, which rule this out for any mass range.
To follow up, I'd like to point out plot 2 in the article under discussion (go on, have a look. Opening a PDF isn't that painful). It is a plot of part of the parameter space for dark matter particle candiates, with weakly interacting, relatively light particles in the lower left corner and strongly interacting very heavy particles in the top right corner. MACHOs live to the right in this plot, and WIMPs near and below the bottom. The interesting thing about the plot is that it shows all the regions that have been excluded, color coded by how they were excluded. MACHO territory is basically completely excluded by microlensing. That doesn't mean that MACHOs don't exist - they definitely do (the earth basically qualifies, since it's compact and doesn't shine), but there can't be anywhere enough of them for their gravity to be important.
If you make the MACHOs smaller so that they aren't as good at lensing, you have to compensate by having more of them to get enough gravity, so microlensing can exclude a pretty wide parameter range. But if things get too light the lensing effect gets too small for us to detect, ending the microlensing exlusion range at a particle mass of about 10^24 g, about 1/10000 of the Earth's mass. But if they get a bit smaller, then can then be detected using lensing interferometry (=nanolensing), and for even lighter objects, by their imprints on crystals found in deep mines that act as natural particle detectors.
Anyway, I encourage everybody to read the paper: It details all the different techniques used to exclude models. The paper is really quite the opposite of what the [rant]typical Slashdotter anti-science prejudice[/rant] is. It's not somebody pulling some hypothesis out of thin air and then not bothering to test it. As the plot shows, this is really a case of eliminating slice after slice of the model space, with 75% of the area in the figure already being excluded.
I was very suspicious when I saw the vixra.org link, but you've actually found a non-crackpot vixra article (if a very short one)! I guess it goes to show that one shouldn't be too quick to judge something by its company.
(Some context for other readers. arxiv.org is where all scientific papers in the fields of astronomy, particle physics and related fields are posted and read by working scientists. In these fields it has in practice supplanted traditional journals - on still submits articles to them, but nobody actually reads them, since articles appear on arxiv much earlier, and arxiv is free to everybody and much more convenient than dozens of scattered journals. But not everybody can post on arxiv. One must either be part of an academic institution or be endorsed by somebody who is. vixra was formed as a completely open alternative where anybody could post. But it quickly drowned in a deluge of crackpots. I've sampled it at several points (mostly the astronomy section), and did not succeed in finding a single remotely worthwhile paper in several pages of listing in any of the attempts. Hence my surprise this time.)
How likely do you think it is that scientists haven't thought of clumping of dark matter or gravitational time dilation in galaxies? It sounds like you really believe that all dem stoopid scientists and their entire field of research have missed your "novel" and "revolutionary" points. I think usually when it seems that way, the natural thing to do is to assume that you've misunderstood something, at least until you've properly researched the issue.
And it clumps together forming the scaffolding for the galaxies, but it also somehow separates out to only show up as a halo around the outer edge of the galaxies.
Have you thought about why our galaxy is the size it is? Gravity is definitely pulling inwards, so why doesn't it just collapse? The answer is velocity and angular momentum. The stars are all in free fall, but they keep missing the center of the galaxy because of their tangential velocity. Even if you start with very low angular velocity, as something collapses its angular velocity grows (just like figure skaters rotate faster when pulling in their arms). And to form galaxy-size objects you already have to get rid of a lot of angular momentum, and the way you do that is through pressure and friction. They baryonic gas that makes up the raw materials for the galaxy has some of this, and is therefore better at collapsing to small objects than dark matter is. This isn't just a handwavy argument - when you put dark matter and baryons into detailed physical simulations and let them run from a start state corresponding to our pictures of the primordial universe, you actually end up with galaxies embedded in dark matter halos.
Plus, nowhere do they ever say they account for time dilation in the galactic rotation speed. If gravity is more intense in the center of the galaxies, then time there will be slower. Which will appear as the outer edge of the galaxy rotating faster than it should. Time is moving faster for the matter there, so it moves further from our viewpoint. Our most accurate clock shows a difference from moving the clock from the floor to putting it up on the wall, I think there would be somewhat more of a difference when you move towards the center of a galaxy.
The Schwartzchild radius of our galaxy (counting the visible matter) is 0.0003 light years. 1 light year away, the gravitational time dilation would be a tiny 0.015% compared to the outside of the galaxy. At 1% of the radius of the galaxy it would be 3e-7. At our position it would be 5e-9. So if you ignored it, you would still be 99.9999% correct about velocities in almost all the galaxy, and only do slightly worse at the core. So why can we measure this effect on earth? Because we have ridiculously accurate clocks.
She also says how it passes through when two galaxies collide without interacting, but in some cases it collides and stays in the middle while the galaxies pass through and are on opposite sides.
No it doesn't. You seem to be talking about the explanation of the Bullet Cluster here. The bullet cluster (like any galaxy cluster) is believed to consist of three components: Stars (which we can observe directly), diffuse gas (which we can observe directly) and dark matter (which does not emit light). Stars are compact and practically never hit each other. When two galaxy clusters collide, none of the stars hit each other due to the enormous distances between each individual star, so the stellar part of all the galaxies pass straight through each other as if nothing happened. But the stars aren't the main component of the galaxies. There is much more diffuse gas (the same kind of stuff stars are made of, but not yet collapsed to form stars). This gas has a pressure, and fee