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Dark Matter: what is it

  1. Oct 12, 2005 #1
    Can someone explain to me what is it or what is theorized to be dark matter. What effects does it have in regards to the cosmos.

    Thank you,
  2. jcsd
  3. Oct 12, 2005 #2


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  4. Oct 12, 2005 #3
    Thanks lisa!
  5. Oct 13, 2005 #4


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    In a universe that submits to Newtonian gravity and GR principles, dark matter is the missing mass necessary to explain: large scale structures in the universe; what holds galactic clusters together; galactic rotational curves; gravitational lensing; WMAP power spectrum, and a few other puzzling observational artifacts. Aside from that, DM is just another epicycle.
  6. Oct 15, 2005 #5


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    Thanks to people who answered my question.
    (Happy belated birthday ,if I'm not mistaking)
  7. Oct 15, 2005 #6
    How much does Dark Matter weigh

    So if anyone were to come up with a possible theory for Dark Matter, he would have to compare it with observation. So I have to wonder how much mass is needed to produce the rotation curves and the extra lenzing effects that we observe... how much more matter is needed, compared to a galaxy, to produce the constant rotation curve, and how must it be distributed to produce the effect? Thanks.
  8. Oct 15, 2005 #7
    I know this is a question that is hard to quantify .. I have absolutely no way to even start thinking about solving it.

    Say you have a diffuse mass that has quite a bit more relative motion against the mass contained in an average galaxy. What would the gravitational effect of that mass be on that galaxy as it passed it by?

    I know that problem is poorly defined .. make assumptions where you feel free to.
  9. Oct 15, 2005 #8
    Surely they had some distribution in mind in order to suggest that it flattens rotation curves, right? I even hear recently that they think there may even be some DM in the center of galaxies. What did they use for a model to even come up with the suggesting that extra mass was needed?
  10. Oct 15, 2005 #9


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    It varies, but galaxies like the Milky Way seem to require about ten times as much dark matter as luminous matter. Most of this matter is in the outer halo, however, so if you were to just look at the dark matter interior to the sun, it would make up less than half of the total -- that is, the sun's motion is probably dominated by gravitation from baryonic matter.
  11. Oct 15, 2005 #10


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    If the smaller mass (the galaxy) got close enough to the larger one, it would be tidally disrupted and you would likely see streams of stars and gas. Something like this:

    .Colliding Galaxies

    except you would only see one disrupted galaxy.
  12. Oct 15, 2005 #11


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    There are a variety of theoretical models for the dark matter profiles. A simple one that gives a flat rotation curve is the "isothermal sphere":

    [tex]\rho=\frac{v^2}{4\pi r^2G}[/tex]

    where v is the rotation speed of the galaxy and r is the distance from the center. Numerical simulations imply a profile slightly different from this (called the NFW profile):

    [tex]\rho \propto \frac{r_s}{r(1+\frac{r}{r_s})^2}[/tex]

    where r_s is a free parameter (the scale radius). It's still not clear what the best fit model is, but it's almost certainly not as simple as an isothermal sphere. It's also not clear that it should be universal (i.e. that all dark matter halos should have the same profile).

    Most models assume that there is some dark matter in the centers of galaxies (for example, the formulae I gave above), but that it's of much lower density than the luminous matter. To my knowledge, the only possible observational evidence for dark matter at the centers of galaxies is the excess gamma-radiation and the "haze" in the WMAP data near the galactic center. These are being interpreted as an annihilation signature of, perhaps, neutralinos. That's still very speculative at this point, however.
  13. Oct 15, 2005 #12
    I genuinely appreciate the help. So is the DM halo about 10 times as massive as the lumious matter?

    I'd like to ask again if there is some evidence or proof that eliminates baryonic matter, such as too much scattering by baryonic matter for any distribution?

    I'm seriously tempted to consider the energy density of the Unruh radiation applied to the acceleration due to gravity? It would seem like an easy calculation to find out. First find the energy density of this assumed Unruh radiation. This would involve an integral of Planck's density spectrum over all frequencies. I've looked at this, and I think I can find a definiate integral formula to accomplish this. This would give us an energy formula at temperature. That energy can be converted to mass, and the additional gravitational effects could be calculated from that. But you'd have to find the acceleration for the Unruh formula at a given radius from the galactic center. I suppose one could use Newton's inverse squared law as a good approximation. Then apply the equation for the Unruh temperature. Then one could construct an integral over all space of this extra mass density produced by the Unruh effect applied to acceleration due to gravity.

    I suppose this might seem like a very small effect; but that's a lot of space, and I've not done the calculation yet. Not only that, but once you have a first approximation, then you'd have to do it all over again since now you have to take into account the existence of this first approximation results. Your galaxy just acquired more mass, so it will produce more gravitational acceleration that you realized, which requires another iteration of the process. I suppose you'd have to do this 4 or 5 times to see how quickly the series converged.

    I suppose I could do it all, but I'd need help with conversion of units and some prelimiary approximation to a galaxy luminous mass distribution. Eh?
    Last edited: Oct 15, 2005
  14. Oct 16, 2005 #13


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    Yeah, see above.

    Nucleosynthesis and the cosmic microwave background both suggest that the dark matter can't be baryonic. Also, there is some evidence of dark matter that is separated from luminous matter:

    Clowe et al. 2003

    Aren't these stars in approximate free-fall? I would think that the dominant effect that made their frame non-inertial would be rotation.
  15. Oct 16, 2005 #14
    If the major contribution of DM were photons, then it would not scatter light itself. But then I suppose you'd see a microwave signature of this effect, right? If 10 times the galaxy mass were evenly distributed throughout and around the galaxy, then I wonder how hot it would be?

    In the conclusion section of the paper you cite, one finds: "Adopting big-bang nucleosynthesis limits on the mean baryonic mass of the universe excludes most of this mass from being baryons in cold, condensed structures." They do not rule out an even distribution throughout space.

    If the ZPE is the CC, then it is not scattering light since we can see distant galaxies through it. If the same ZPE is causing DM effects due to the Unruh effect applied to the acceleration of gravity, then we should not expect it to scatter light either, since it too is a ZPE effect. This would have the same low scattering cross section as WIMPs.

    That would be the Unruh effect felt by individual stars and planet, the atoms within stars, etc. I've seen calculations that show this to be very small indeed (I don't remember where I saw it). But what I'm after is the Unruh effect on space itself due to accelerated reference frames caused by gravity. Again, this would be small; but integrating over vast amounts of space (and doing multiple iterations) may have an accumulative effect of the scale of DM.

    :blushing: Oh, by the way, my thought for the day is to consider whether this Unruh effect may be the very cause of particle creation during inflation so that there is no need to suppose a Higgs boson and a false vacuum. We may already have all the physics we need. For particle creation during inflation is accompanied with very fast accelerated expansion so that horizons are small and the Unruh effect (=Hawking radiation) is more pronounced. Not only that but when the universe was small and expanding very fast, the gravitational field was more intense and perhaps this caused greater accelerated reference frames to produce a much higher Unruh effect. Though perhaps you need more of an accelerated expansion to produce accelerated reference frames with respect to others to produce this Unruh effect than you need a gravitational well. For at the beginning every frame might be at the same gravitational level so that neighboring space is not accelerating wrt other regions of space. But at least the temperature profile is there. Its expansion was accelerating very fast, its gravitational field was very strong, and the temperature was very high. Do we have a more specific temperature curve verses expansion rate or gravity strength for the inflationary epoch? If we start from a singularity and assume exponential rate of expansion, can we get a particle creation verses temperature model that ultimately match the CMB? I think that would be just a matter of how you adjust your scale factors to give how fast it is growing for a given overall size.
  16. Oct 16, 2005 #15


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    Photons can't be the dark matter. Radiation can't be bound to potential wells for long. Besides, we can see it. :smile:

    I think the point there was that we might not be able to see cold, condensed structures (in this particular system), whereas we could see diffuse gas. Nucleosythesis constraints don't depend on present-day clumping.

    Perhaps you can explain further. My understanding of the Unruh effect is that accelerating frames view the quantum vacuum as having a net temperature. Are you suggesting that the vacuum is viewing itself as having a net temperature and that space is curving as a result of the contributions to the stress-energy tensor? If so, why would the quantum vacuum not also be in free-fall?

    Again, I'm prone to question your use of the term "acceleration" here. During inflation, space itself is undergoing accelerated expansion, but spacetime is still locally Lorentzian.
  17. Oct 16, 2005 #16
    As I understand the Unruh effect, accelerating objects feel a temperature because the ZPE is not invariant wrt acceleration. It's Lorentzian and invariant wrt to velocities, but not acceleration. OK. I simply thought it was fair to apply the equivalence principle wherein there is no distinction between acceleration and gravitation. I also think that there is nothing particular to the object that is causing the temperature rise, but it is the accelerating reference frame that "feels" the temperature whether there is an object in that accelerating frame or not. Correct me if I'm wrong, but it seems that DM is distributed as though the gravity it causes around it has some sort of weight itself, right? This argue for some sort of Unruh effect. I don't believe this Unruh effect produces permanant particles (unless there is an horizon) because an accelerating observer could feel a temperature and presume collision with particles. But if he were to stop and go back at constant speed, he would never see those particles again, right? So it seems applicable only to accelerating systems of which gravity is one.

    Yes, I'm not real clear yet about all this. But it seems that if one region of space is accelerating wrt a second region, then the second would have to conclude that the first is feeling a temperature (due to the Unruh effect) so that the second should observe that the first has a higher ZPE than itself so that there must be some particles created in the second region to account for the temperature. I think it is equivalent to also think in terms of Hawking radiation. If the Universe is accelerating very fast, then there will be a much smaller cosmological event horizon which separates virtual particles out of the ZPE to create permatant particles during inflation.

    I think all this is the same as saying that the ZPE is greater in curved space than or flat space, or that curved space creates permanant particles out of the ZPE.

    Added a couple of hours later:
    Just a moment,... now I'm thinking that DM can not be any sort of permanant matter whatsoever. For if it were, then it would gravitate towards the host galaxy and eventually concentrate there. But AFAIK we don't observe older galaxies with this central concentration of DM, but all seem to have the same distribution required to flatten rotation curve and not centrally concentrated DM which would not show this same flatness.
    Last edited: Oct 16, 2005
  18. Oct 16, 2005 #17

    Thanks. I see. It would tend to tidally/asymetrically disrupt the galaxy VS symetrically pulling it apart.
  19. Oct 16, 2005 #18
    Mike2, you may find this interesting:


    Unruh's approach to gravitation and the vacuum are non-standard, and this 1993 paper (16 years after he proposed the effect named for him) is no exception.
  20. Oct 17, 2005 #19


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    I would think that an application of the equivalence principle would produce the opposite conclusion -- that the objects (or vacuum) were in an inertial frame unless they were being acted upon by a non-gravitational force. The equivalence that you speak of is, I think, that of an accelerating frame to a stationary observer in a uniform gravitational field. Remember, we are not in an inertial frame because (among other things), the surface of the earth is pushing on our feet. If that surface were not there, we would be in free-fall and this would not be equivalent to the artificially accelerating frame.

    I should think the same reasoning applies to the accelerating universe.

    I wouldn't say so. In particular, that paper I cited earlier gives an example of dark matter in apparent absence of anything else to cause gravity.

    There is a problem in which simulations have difficulty producing the exact central profile of the dark matter expected from observations, but producing flat rotation curves is not a problem in the standard CDM universe.
  21. Oct 17, 2005 #20
    My point is that if DM were permanant particles, then it would all eventually gravitate to the center and there would be none around the edges. This would occur around old stable galaxies. Is this seen in the data?
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