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Dark Matter - what's the mystery?

  1. Jan 29, 2011 #1
    Average density of universe is 9.3 x 10^-27 Kg-mass/M3. Volume of Cubic-Light-Year is 2.7 x 10^25 M3. Result is .25 Kg-mass/CLY. If Dark Matter is 5 x ordinary matter then 1.25Kg-mass of Dark Matter per CLY. What if there was a billion times more Dark Matter in a galaxy than in the space between galaxies. Then there would be 1-1/4 billion KG of Dark Matter per CLY within a galaxy. Taking that an average asteroid’s density would be about 2500Kg/M3 this would result in an asteroid of a diameter of 100M. This currently is about the lower limit of asteroids we sometimes detect in our solar system. So why is it such a big mystery as to what comprises the missing (Dark) matter?(elginz)
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  3. Jan 30, 2011 #2
  4. Jan 30, 2011 #3


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    Simply put, the data (especially data on the Cosmic Microwave Background, and elemental abundances coupled with the theory of Big Bang Nucleosynthesis) conclusively tells us that the dark matter can't be made of atoms. So the idea that it is small, non-light-emitting planets, which was earlier called the MACHO (Massive Compact Halo Objects) hypothesis, is pretty much dead. Which is why people are looking elsewhere.
  5. Jan 30, 2011 #4
    In reviewing all the "proof" for Dark Matter being nonbaryonic, I can't help getting the impression that interpretation of observations (especially from the CMB) are fitted to comply with nonbaryonic Dark Matter model.
  6. Jan 30, 2011 #5
    Just yesterday I read an article in which they had found out that there is no correlation between the formation of a black hole(ones at the galactic center) and dark matter.
    I woud have imagined the opposite since both of them happen to be invisible to us other than their gravitational effects but since DM is non-baryonic while BH seems to act normal other than objects getting closer to its event horizon.God, I need to read more into this.I have got an interview next month.
  7. Jan 31, 2011 #6


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    Can you give an example? How could a baryonic dark matter model get around the Big Bang Nucleosynthesis problem for example?
  8. Feb 1, 2011 #7


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    Last edited: Feb 1, 2011
  9. Feb 2, 2011 #8
    The average density you give is the critical density of a universe without a cosmological constant. Since the average composition of mass-energy the universe is thought to be 73% dark energy, 23% dark matter, 4.6% ordinary matter, the density of ordinary matter is only about 4.3x10^-28 Kg/m^3. A more egregious error is that a cubic light-year is about 8.5x10^47 m^3, resulting in an average mass of ~3.6x10^20 kg/ly^3 (for scale, this is 1/200th the mass of the moon per cubic light-year). It is thought that dark matter forms spherical "halos" around galaxies that are much larger than the galaxies themselves. If one assumes that the dark matter "halo" around the Milky Way is a sphere of uniform density 1 million light-years in radius with a mass of 900 billion suns (~1.8x10^42 Kg), the density would be ~4.3x10^23 Kg/ly^3 (~5.8 lunar masses per cubic light-year). The volume of a sphere centered on the Sun with a radius equal to the semi-major axis of the orbit of Pluto has a volume of about 1.0x10^-18 ly^3, so there would only be about 4.4x10^5 Kg (~440 metric tons) of dark matter within that volume (this density of dark matter is about 240 times the average for the universe). The uniform sphere of dark matter would have the effect of inducing a differential acceleration of about -1.7x10^-24 m/s^2 between points ~5.9x10^9 m closer to the galactic center than the Sun versus points ~5.9x10^9 m farther than the Sun. Needless to say, this variation is far too small to be observable with currently available instrumentation.
  10. Feb 3, 2011 #9
    Correct me if I'm wrong but I'm under the impression that we've detected the presents of dark matter with gravitational lensing? Is this right or is this just a yet to be observed candidate for experiment? If we have observed "dark matter" through gravitational lensing has there been a case where it has accounted for all of the "missing matter" in a galaxy? or has it only ever counted for some?
  11. Feb 3, 2011 #10


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    Generally speaking, and this information might be a few years old, I think the gravitational lensing experiments are restricted to entire clusters of galaxies. That is, knowing the (approximate) masses of the individual galaxies, and observing the deflection of a known light ray, we see that there is much more mass than in the individual galaxies. Thus, the information gleaned here about the dark matter content of the universe is not really helpful for determining the amount in individual galaxies (well, that's probably not true, but at least they're not exactly the same).

    So it isn't like we can say "Oh, due to a flattening rotation curve we need X amount of dark matter. And, lo and behold, a gravitational lensing event says we need X amount of dark matter to!" The two are really probing dark matter in different regimes.
  12. Feb 4, 2011 #11
    Just to add a little---Nabeshin is for the most part entirely correct, but with gravitational microlensing, it is a little bit more precise, but along with that precision comes some more ambiguity... With microlensing, you're generally dealing with very slight statistical deviations (nothing you could see with the naked eye in a telescope image). You can pick up things as small as an exoplanet, or brown dwarf, etc (locally, of course). This allows for much more precision in that you're detecting drastically smaller masses; but at the same time, these could (in some cases) be caused by molecular clouds etc.
    I have read about some cases (can't remember any off hand) in which it is quite clear that microlensing events are being caused by dark matter (e.g. nearby clusters in which the gas is fairly apparent).
    Still, I think the clearest detection is the bullet cluster---which I think was already mentioned above.
  13. Feb 4, 2011 #12
    Volume of Cubic-Light-Year is 8.47 x 1047 m3
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