The Bullet Cluster's Dark Matter

What Can The (Dark) Matter Be?

Article Updated: 24 Dec , 2015

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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.



25 Responses

  1. chichiki123 says:

    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?

  2. Lawrence B. Crowell says:

    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

  3. Lawrence B. Crowell says:

    @ 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

  4. Aqua says:

    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?

  5. Aqua says:

    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

  6. Francois Painchaud says:

    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.

  7. Lawrence B. Crowell says:

    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

  8. Greg says:

    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 throughly relieved when finally direct observational evidence is discovered of dark matter and measurements are made. Until then an undercurrent of angst and frustration ever grows stronger. I hate to admit it, but the failure so far to directly detect dark matter has been in some ways crippling and has limited rapid progression of this field. Instead we see a proliferation of theories and far more questions than answers.

  9. Torbjorn Larsson OM says:

    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 is obviously not out there yet, if the above is correct.

    what if the big bang didnt create space time at all

    Depends.

    If “big bang” is the process, yes it does, because it is the ongoing expansion we see.

    If you mean specifically the convergence of wordlines you see looking back, no it doesn’t, if you take inflation seriously. Then it’s all _inflation_ way back, spawning more multiverses as it goes. Not an isolated ad hoc singularity.

    [For several reasons I hypothesize “all the way back”. I have ‘a proof’, but this comment is too small to contain it. 😀 Seriously, too long a story for here and now.]

  10. Paul Eaton-Jones says:

    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.

  11. I think this is still a very speculative area and lacking sufficient data to back up strong claims. But, broadly speaking – yes, I understand theorists think baryonic matter froze out early after the big bang and no more of it has been produced since.

    It is speculated that all the non-baryonic dark matter froze out even earlier – since it is not apparently affected by the energy densities still prevalent in the universe today (pardon the pun), while baryonic matter can still be affected by the energy densities prevalent in the universe today (e.g. baryonic matter can be ionised into plasma).

    Non-baryonic dark matter is thought to be the underlying ‘skeleton’ upon which baryonic matter settled after the big bang – which is why galaxies cluster in particular regions of the universe – and why individual galaxies can maintain their observable structure despite their fast rotation.

  12. dilip.g.banhatti says:

    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 of needing dark matter to connect mass distribution to rotation curves, there are a few independent lines of evidence to suspect that the sums haven’t been done right, but subscribed to by most people for so long that it’s now too much against the stream to say that they were wrong in the first place. I think this issue needs very urgently to be settled. I got interested in this topic when EGRET data were seen, using galaxy disk calculation package GALPROP, to imply rings of gamma ray emission in Milky Way disk, roughly cospatial with rings of OB associations found independently. While looking at the evidence available, I came across the above mentioned possibility of the sums not being done right in connecting mass distribution of disk galaxies to their rotation curves. If anyone is interested, I can quote references, which at the moment I don’t have to hand.

    As to big bang nucleosynthesis of light elements and the amount of baryonic matter, this seems to be on firm ground, as perhaps David Schramm showed first, since there’s a connection with the number of species of (light) neutrinos and the concordance of observations with (numerical) calculations. Three families of leptons (of which neutrinos are members) give the best (or only) fit, although whether they need to be Dirac type neutrinos (neutrinos & antineutrinos distinct) or Majorana type (neutrinos their own antiparticles) has probably not been examined in detail (or at least I don’t know about it).

    Hope the two comments above are helpful for progress in understanding the existence and nature of dark matter.

  13. Ignoramus says:

    “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?

  14. Lawrence B. Crowell says:

    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 off perfecting the single malt scotch than pursing cosmology and astrophysics.

    LC

  15. Popisfizzy says:

    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.

  16. SkepticTim says:

    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) at the Homestake mine in South Dakota which may help to confirm/refute/explain DAMA/LIBRA. Until we have definitive direct evidence of dark matter in the form of WIMPS, axions, etc. we should be a bit cautious about our interpretation of the indirect evidence.

  17. Lawrence B. Crowell says:

    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

  18. Jon Hanford says:

    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

  19. Lawrence B. Crowell says:

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

    LC

  20. Jon Hanford says:

    @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 ] ……

  21. Jon Hanford says:

    @LC con’t., Wiki has a somewhat useful page on ‘strong lensing’ @ http://en.wikipedia.org/wiki/Strong_lensing . Sorry for the link mixup, but these entries articulate the important differences between these very different approaches. Weak lensing mapping of galaxy clusters is already well underway(and BTW some are focused at more than DM distribution in galaxies), and the prevalence of galaxy clusters exhibiting weak lensing behaviour greatly outnumber galaxy clusters with strong gravitational arcs. Weak lensing studies are leading the way in determining DM distribution in clusters without using assumptions common to other DM mapping methods., if not for their prevalence(higher abundance). See both Wiki articles that outline what different astrophysical measurements can be obtained. 🙂

  22. Lawrence B. Crowell says:

    The reference below is worth reading, which is more from a general relativity perspective.

    http://relativity.livingreviews.org/Articles/lrr-2004-9/

    or the pdf version at

    http://relativity.livingreviews.org/Articles/lrr-2004-9/download/lrr-2004-9Color.pdf

    The categories weak and strong are based on whether there are elliptical distortions or outright rings produced. In both cases the main contributing effect is largely Newtonian. Post Newtonian effects don’t largely enter into these problems.

    I did some analysis last year, which I did not follow through on, about how general relativistic corrections amplify chaos. In other word, if you have a three body problem, where one body is a small mass (a satellite), then the motion of that body exhibits some randomness over time as measured by a Lyapunov exponent. The corrections to Newtonian mechanics from general relativity contributed to the amplification of chaotic dynamics. I started to apply this to optics, using the Poisson-like equation often employed in Einstein lensing problems. However, the issue with optics seemed a bit moot as most astrophysical problems with Einstein lenses are essentially Newtonian.

    I am intrigued by how this determines the age and frame dragging of the cosmological spacetime. This seems to potentially breathe some life back into the problem I started. The “traffic snarl” of photons arcing around is a form of “optical chaos” and the delay times measured for photons along different arcs from a source is a type of optical chaos. This chaos is then being amplified by the frame dragging of distant sources, at least as indicated by the different time delays.

    LC

  23. Lawrence B. Crowell says:

    I posted a comment in response to Jon with respect to gravitational lensing. Strangely it appears only after logging on, and there is a comment about “waiting moderation.” I reference the following paper on the spacetime physics of gravitational lenses:

    http://relativity.livingreviews.org/Articles/lrr-2004-9/download/lrr-2004-9Color.pdf

    Maybe this post will not be akin to a Cheshire Cat that appears only when logging on.

    LC

  24. DrFlimmer says:

    @ LBC

    Normally, when a comment “awaits moderation”, it will never appear publicly. Mostly this is due to two or more links you put in it. If leave the prefixes like “http://www.” aside, everything should be fine. Maybe you repost your comment?

  25. Lawrence B. Crowell says:

    I skip the first part with the references. I was unaware of this fact

    The categories weak and strong are based on whether there are elliptical distortions or outright rings produced. In both cases the main contributing effect is largely Newtonian. Post Newtonian effects don’t largely enter into these problems.

    I did some analysis last year, which I did not follow through on, about how general relativistic corrections amplify chaos. In other word, if you have a three body problem, where one body is a small mass (a satellite), then the motion of that body exhibits some randomness over time as measured by a Lyapunov exponent. The corrections to Newtonian mechanics from general relativity contributed to the amplification of chaotic dynamics. I started to apply this to optics, using the Poisson-like equation often employed in Einstein lensing problems. However, the issue with optics seemed a bit moot as most astrophysical problems with Einstein lenses are essentially Newtonian.

    I am intrigued by how this determines the age and frame dragging of the cosmological spacetime. This seems to potentially breathe some life back into the problem I started. The “traffic snarl” of photons arcing around is a form of “optical chaos” and the delay times measured for photons along different arcs from a source is a type of optical chaos. This chaos is then being amplified by the frame dragging of distant sources, at least as indicated by the different time delays.

    LC

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