The Bullet Cluster's Dark Matter

What Can The (Dark) Matter Be?

1 Mar , 2010 by

What better place to look for dark matter than down a mine shaft? A research team from the University of Florida have spent nine years monitoring for any signs of the elusive stuff using germanium and silicon detectors cooled down to a fraction of a degree above absolute zero. And the result? A couple of maybes and a gritty determination to keep looking. 

The case for dark matter can be appreciated by considering the solar system where, to stay in orbit around the Sun, Mercury has to move at 48 kilometers a second, while distant Neptune can move at a leisurely 5 kilometers a second. Surprisingly, this principle doesn’t apply in the Milky Way or in other galaxies we have observed.  Broadly speaking, you can find stuff in the outer parts of a spiral galaxy that is moving just as fast as stuff that is close in to the galactic centre. This is puzzling, particularly since there doesn’t seem to be enough gravity in the system to hold onto the rapidly orbiting stuff in the outer parts – which should just fly off into space. 

So, we need more gravity to explain how galaxies rotate and stay together – which means we need more mass than we can observe – and this is why we invoke dark matter. Invoking dark matter also helps to explain why galaxy clusters stay together and explains large scale gravitational lensing effects, such as can be seen in the Bullet Cluster (pictured above). 

Computer modeling suggests that galaxies may have dark matter halos, but they also have dark matter distributed throughout their structure – and taken together, all this dark matter represents up to 90% of a galaxy’s total mass. 

An artist's impression of dark matter, showing the proportional distribution of baryonic and non-baryonic forms (this joke never gets old).

Current thinking is that a small component of dark matter is baryonic, meaning stuff that is composed of protons and neutrons – in the form of cold gas as well as dense, non-radiant objects such black holes, neutron stars, brown dwarfs and orphaned planets (traditionally known as Massive Astrophysical Compact Halo Objects – or MACHOs). 

But it doesn’t seem that there is nearly enough dark baryonic matter to account for the circumstantial effects of dark matter. Hence the conclusion that most dark matter must be non-baryonic, in the form of Weakly Interacting Massive Particles (or WIMPs). 

By inference, WIMPS are transparent and non-reflective at all wavelengths and probably don’t carry a charge. Neutrinos, which are produced in abundance from the fusion reactions of stars, would fit the bill nicely except they don’t have enough mass. The currently most favored WIMP candidate is a neutralino, a hypothetical particle predicted by supersymmetry theory. 

The second Cryogenic Dark Matter Search Experiment (or CDMS II) runs deep underground in the Soudan iron mine in Minnesota, situated there so it should only intercept particles that can penetrate that deeply underground. The CDMS II solid crystal detectors seek ionization and phonon events which can be used to distinguish between electron interactions – and nuclear interactions. It is assumed that a dark matter WIMP particle will ignore electrons, but potentially interact with (i.e. bounce off) a nucleus. 

Two possible events have been reported  by the University of Florida team, who acknowledge their findings cannot be considered statistically significant, but may at least give some scope and direction to further research.

By indicating just how difficult to directly detect (i.e. just how ‘dark’) WIMPs really are – the CDMS II findings indicate the sensitivity of the detectors needs to bumped up a notch.



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chichiki123
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chichiki123
March 1, 2010 6:35 AM

maybe dark matter is just the original native stuff before the big bang….and what if the big bang didnt create space time at all?? could it just be a mere mother of hypernova’s?

Lawrence B. Crowell
Member
Lawrence B. Crowell
March 1, 2010 6:43 AM

The statistics on CDMS II are way to low to conclude that DM was detected, though the few counts obtained were suggestive.

My bet is on the neutralino, which is a condensate type of state formed by the supersymmetric partner of the photon, neutral Higgs, and Z^0, where these all have the same quantum numbers. The neutralino has a decay process, which results in Z^0 in a broken SUSY transition and photons. The PAMELA detector and Fermi spacecraft found TeV gamma photon flux from the galactic center, in some agreement with neutralino annihilations.

LC

Lawrence B. Crowell
Member
Lawrence B. Crowell
March 1, 2010 6:44 AM

@ chichiki123: Yes there are phenomenological suggests that dark matter clumping were involved with the early formation of popIII stars or “hyperstars.” Jon Hanford might weigh in on this.

LC

Aqua4U
Member
March 1, 2010 10:46 AM

So… how many more planets or dark bodies do we need to fill the gap? Is it easier to view the gravitational constant from askew?

Aqua4U
Member
March 1, 2010 11:36 AM

i.e. electromagnetic force is some 39 orders of magnitude greater than the force of gravity.

Galactic magnetic field = 0.00001 Gauss
Solar Wind = 0.00005 Gauss
Interstellar molecular cloud = 0.001 Gauss
Earth’s field at ground level = 1 Gauss
Solar surface field = 5 Gauss
Massive star typical field (pre supernova) = 100 Gauss
Toy refrigerator magnet = 100 Gauss
Sun spot field = 1000 Gauss
Jupiter magnetic field = 1000 Gauss
Magnetic Stars such as BD+54 2846 = 12,000 Gauss
White Dwarf star surfaces = 1,000,000 Gauss
Neutron star surface field = 1,000,000,000,000 Gauss
Magnetar field = 1,000,000,000,000,000 Gauss

Francois Painchaud
Member
Francois Painchaud
March 1, 2010 2:59 PM

All this mysterious “dark matter” and “dark energy” is much alike the need for “ether” at the end of the 19th century. Physics is in a crisis and needs a new Albert Einstein to come up with the equivalent of special relativity, which did away with the need for ether.

“Dark whatever” will eventually drift to the sidelines of the history of science. However, Einstein the theoretician needed Michelson and Morley the experimentalists to get a clue. So scientist must keep experimenting, until the data triggers a theoretical darkless solution from some genial brain.

Lawrence B. Crowell
Member
Lawrence B. Crowell
March 1, 2010 4:30 PM

Dark matter is not an aether. The above image of the bullet galaxy is evidence for the existence of DM by gravitational lensing. We know it is there, it is not a hypothetical things which is not observed, as was the aether. We just don’t know what it is. It is dark because if does not interact with electromagnetic radiation. We also know by various means that it is not heavily clumped, but in diffuse clouds. The problem is that as yet we do not have direct empirical data on just what it is.

LC

Greg
Member
Greg
March 1, 2010 5:34 PM
The problem with cosmology is that unlike other sciences there is no laboratory (place containing the subjects where conditions are carefully controlled) and the subjects are not within reach of the observer, although the instruments are. (i.e. your laboratory is just the place where you keep your instruments and not your subject matter.) Therefore there is always the possibility that multiple unknowns from the subject’s environment are confounding your conclusions from what evidence you managed to collect. I am sure that at some point MOND enthusiasts will place their mark on this thread. Nevertheless the case for MOND, although still a longshot imho, has become more compelling rather than less compelling over time. I for one will be… Read more »
Torbjorn Larsson OM
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Torbjorn Larsson OM
March 1, 2010 11:27 PM
to fill the gap To claim that it is a gap here is fallacious. DM is predicted by the standard cosmology, and has passed a large number of tests inside and outside of it. AFAIU the observations of the Bullet and other clusters can’t be predicted by alternatives such as MOND, for the reason that parameters differ for each case. Only matter can get around this. The other side of the story is that, just as for dark energy, there are deep and satisfying connections with other theories. Here symmetries and M theory. In fact, I guess that if DM wouldn’t be observed it would be the most interesting case. A theory that could make the same predictions… Read more »
Paul Eaton-Jones
Member
March 2, 2010 12:55 AM

My understanding [and I may be a bit wide of the mark] is that the amount of baryonic matter formed at the Big Bang was locked in right at the very beginning. Isn’t there also a relationship tied to the amounts of primordial helium produced?
Also what ever happened to the axion? In the 1980’s that was supposed to be the particle that would solve the missing mass problem.

Dilip G Banhatti
Member
March 2, 2010 2:42 AM
Dark matter may be needed for galaxian clusters’ dynamics, as Fritz Zwicky was perhaps the first to calculate in mid-1930s or so. It is not yet entirely clear if it is needed, or how much of it is needed, on lower scales. David Schramm & Gary Steigman (if I remember the 2nd name right) did a calculation to examine various scales, starting with binary galaxies & small groups through to the scale of observable universe, towards end of 1970s. Then neutrinos were the star candidates. Then came single galaxy scale into the reckoning via disk galaxy rotation curves, I guess around the same time. This is still a rather vexed issue. Although most people jump on the bandwagon… Read more »
Ignoramus
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Ignoramus
March 2, 2010 7:17 AM

“An artist’s impression of dark matter”
How appropriate for this discussion!
Someone above said “DM is predicted by the standard cosmology,”.
Wouldn’t “postulated as an explanation” be more correct?

Lawrence B. Crowell
Member
Lawrence B. Crowell
March 2, 2010 7:46 AM
Dark matter probably decoupled from ordinary luminous matter with the breaking of supersymmetry, probably with inflation or the reheating of inflation. DM then did form some sort of gravitational framework for the distribution of luminous matter. The phenomenology of this is very complicated and I am not well versed in it, so I leave deeper commentary on this to others. As for MOND, it is a reasonable methodology for modelling the phenomenology of DM with respect to galactic dynamics. However, I simply can’t take it as a real theory on some deviation from Newtonian gravity and dynamics at large scales, and very small accelerations. If it were nature frankly makes no real sense, and we would be better… Read more »
Popisfizzy
Member
March 2, 2010 8:12 AM

I’ve said it before, but god damn, conversations about dark matter would be so much easier if it were given a less enticing name, as the crazies wouldn’t gather around something they have no idea about and yap on about how it’s a lie.

SkepticTim
Member
SkepticTim
March 2, 2010 10:01 AM
The evidence presented here is pretty weak. The Italian and Chinese physicists on the DAMA Project have held out since 2000 for their claim that they are detecting dark matter. The yearly modulation could represent the Earth’s motion through a dark matter stream as it orbits the Sun. A larger DAMA/LIBRA experiment reaffirms the phenomenon, which appears as flashes in the team’s sodium iodide detector. So we can say that there is evidence for a modulation in the data at 8.2 sigma, compatible with what would be expected from some dark matter particle models in some galactic halo models. There should be results soon (or perhaps I have missed existing results) from the Large Underground Xenon detector (LUX)… Read more »
Lawrence B. Crowell
Member
Lawrence B. Crowell
March 3, 2010 4:41 AM

DAMA has reported some signal, but the statistics are still too rough to draw conclusions.

I have pondered whether some of the solid state approaches to quantum computers might work here. A small DM interaction with the nitrogen in a diamond lattice (one approach to quantum computing) might flip a quantum bit. There have to be ways to detect DM (WIMP) particles.

LC

Jon Hanford
Member
Jon Hanford
March 4, 2010 6:26 AM

Fritz Zwicky would a happier man today knowing that weak lensing (unknown to him at the time) has been used to measure up to 8 DM clumps in the Coma Galaxy Cluster, Mass segregation has been noted in the past, but this method is the most accurate available.

Ironic, to say the least! Paper at: http://arxiv.org/abs/0904.0220

Lawrence B. Crowell
Member
Lawrence B. Crowell
March 5, 2010 5:54 PM

Thanks for the reference. By weak lensing I presume this means for small Newtonian gravity.

LC

Jon Hanford
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Jon Hanford
March 5, 2010 8:33 PM

@LC, This study used weak lensing (a statistical-observational proxy based on computer scans of deep fields near massive clusters, as opposed to the more-spectacular strong lensing seen so dramatically in many galaxy clusters i.e. Abell 1689, Abell 2218 & the “Bullet Cluster”. Wikipedia has a fairly decent page on ‘strong lensing’ [ http://en.wikipedia.org/wiki/Weak_lensing ] ……

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