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Dark Matter. Space-Time curvature. Galaxy formation

  1. Aug 14, 2014 #1
    1. Gravity is the geometric curvature of space-time caused by massive objects.
    2. Dark Matter surrounds galaxies.
    3. Dark Matter is thought to be critical in galaxy formation.
    4. The mass of Dark Matter creates curvatures in space-time around baryonic matter which forms galaxies.

    What roles do said curvatures play in the formation, shape, and rotation rates of galaxies?
     
  2. jcsd
  3. Aug 14, 2014 #2
    The notable feature of galaxy rotation is the apparent flatness of the anticipated rotation such that stars near the outer edge travel at similar speeds to stars much closer in towards the center. This creates the belief in a halo of non luminous mass to account for the lack of decreasing speed towards the edge implied by the luminous mass distribution. Originally "dark" meant non-luminous but has since become "spooky" due to lack of a better understanding of this phenomenon.
     
  4. Aug 14, 2014 #3

    marcus

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    just some partial answers.
    because over 5 times more abundant, DM gathering into clouds in early universe helped OM (ordinary) gather.

    once it has begun to gather, OM is better at carrying through with the process because it can radiate off heat as it condenses.

    but DM is critical at least in the initial phases.

    a galaxy does not have a rotation rate. It has different rotation SPEEDS at different radius.

    Many galaxies have an approximately flat speed curve. So a star that is twice as far out from center, going the same speed as us, would take twice as long to make the circuit.

    Stars have all different orbital periods depending mainly on distance from center. So there is no one rotation RATE.

    You can work out from Newton law of gravity what the density distribution has to be in order for the rotation SPEED to be roughly constant from near center out to near the edge.

    If the galaxy is embedded in a roughly spherical cloud of DM, then one can calculate that to have a flat speed curve you have to have the density of the cloud fall off as 1/R2.

    A massive central body does not produce a flat speed curve. Think about the solar system, the planet speeds fall off as 1/R.5 as you get farther out from the sun
     
  5. Aug 14, 2014 #4

    Barry911

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    Somewhat off the mark but WOW!
    The universe is increasing in complexity and this is an observable.
    Looking across our limited view of the universe, say 8 billion lys we can see galaxies as they were
    8 billion years ago. The "galactic gas" is minimally complex i.e.-H, He, Li. Galaxies closer to us say 500
    million lys have much more complex gas with all of the elements present. The time interval simply permits
    a greater number of nova type events to have occurred, and increasing atomic number and mass are
    equivalent to increasing complexity.
    So, where is the universe headed? its far beyond anthropic.
    I would appreciate any comments.

    Thanks,

    Barry911
     
    Last edited: Aug 14, 2014
  6. Aug 14, 2014 #5

    Barry911

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    Dark matter comprising about 15%(?) of the mass of the universe has been explained as
    diffuse fermions i.e.- Protons without the need for exotic particles e.g.- magnetic monopoles.
    do you folks think that this is workable?

    Barry911 (the car, not the emergency)
     
  7. Aug 15, 2014 #6

    Chalnoth

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    No..
     
  8. Aug 15, 2014 #7

    Barry911

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  9. Aug 15, 2014 #8

    marcus

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    Not workable. Here's why:
    Clouds of ordinary matter (for example neutral or ionized hydrogen) are observed and mapped. The masses of ordinary matter clouds can be estimated and is included in the estimates of ordinary matter (OM). Not enough OM mass.

    The OM clouds can be seen because OM absorbs and re-radiates light and radio waves. We can tell what the clouds are made of in many cases by what wavelengths they filter out and by their EM radiation.

    Over and above all forms of OM (including clouds) there must be about 5 times as much DM.
    DM does not absorb and radiate light. It does not enter into ordinary EM interactions.

    However the particles that comprise DM clouds can perhaps DECAY after a long period of time. the particles may not be 100% stable, so the DM clouds may emit a faint X-ray or gamma-ray signal caused by very slow radioactive decay of the DM particles.

    Radiation of 3.5 keV is in fact observed coming from where we would expect to see it if it results from DM. This does not seem to have any very good alternative explanation. It is approximately the wavelength you expect from a certain isotope of potassium, but there is no explanation why there should be so much potassium (and not other radioactive elements with different signature wavelengths). So that signal is currently under investigation.
     
  10. Aug 15, 2014 #9

    marcus

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    If anyone is interested in recent investigation about DM here is an August 2014 paper

    http://arxiv.org/abs/1408.2503
    Checking the dark matter origin of 3.53~keV line with the Milky Way center
    Alexey Boyarsky, Jeroen Franse, Dmytro Iakubovskyi, Oleg Ruchayskiy
    (Submitted on 11 Aug 2014)
    We detect a line at 3.539±0.011 keV in the deep exposure dataset of the Galactic Center region, observed with the XMM-Newton. Although it is hard to exclude completely astrophysical origin of this line in the Galactic Center data alone, the dark matter interpretation of the signal observed in Perseus galaxy cluster and Andromeda galaxy [1402.4119] and in the stacked spectra of galaxy clusters [1402.2301] is fully consistent with these data. Moreover, the Galactic Center data support this interpretation as the line is observed at the same energy and has flux consistent with the expectations about the Galactic dark matter distribution for a class of the Milky Way mass models.


    The authors refer to earlier (February 2014) papers: http://arxiv.org/abs/1402.2301
    "Detection of An Unidentified Emission Line in the Stacked X-ray spectrum of Galaxy Clusters"

    and http://arxiv.org/abs/1402.4119 "An unidentified line in X-ray spectra of the Andromeda galaxy and Perseus galaxy cluster"
     
  11. Aug 15, 2014 #10

    Chalnoth

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    In the early universe, the normal matter was a plasma. A plasma experiences pressure. Dark matter doesn't interact much at all with anything, so it doesn't experience pressure. This causes dark matter and normal matter to behave very differently in the early universe. The signature of non-interacting matter is clear as day in the Cosmic Microwave Background.
     
  12. Aug 16, 2014 #11

    Barry911

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    Thanks for the responses, however the point being made about fermionic matter was that our observing
    is density dependent. With quantum sized particles, radiation intensity (reflected) could be so low as to
    be invisible.

    Barry
     
  13. Aug 16, 2014 #12

    Barry911

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    The expansion of the universe is intrinsic to the field equations of relativity, as noted by Lemaitre and
    Friedman i.e.- a property of the mathematics alone! Is the "dark energy" requisite then necessary with
    the speculative flaton field an unnecessary redundancy?

    Barry
     
  14. Aug 16, 2014 #13

    Chalnoth

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    There is no part of that that can explain the CMB observations. In the early universe, normal matter bounces. Dark matter does not. And we very clearly see the signature of both in the CMB.

    Edit: Also, the density at the time the CMB was emitted was essentially the same everywhere. You can't have a density-dependent effect show up if there is no significant change in density!
     
  15. Aug 17, 2014 #14

    Chalnoth, could you please explain what are the two cmbr signatures?

    In this article they are now speculating that DM could be absorbing UV light..
    http://www.space.com/26795-universe-missing-ultraviolet-light.html
     
    Last edited: Aug 17, 2014
  16. Aug 17, 2014 #15

    Chalnoth

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    It stems from the angular power spectrum of the temperature anisotropies. Here's a super-short description of this bit of word salad.

    1. Temperature anisotropies: the CMB isn't perfectly uniform. Some bits are slightly hotter or slightly colder.
    2. Angular power spectrum: You can do the equivalent of a Fourier transform to get an estimate of the waves that make up the above anisotropies. You then average the amplitudes of the waves of each wavelength, no matter their direction on the sky.

    What you get is this:
    http://sci.esa.int/planck/51555-pla...ctuations-in-the-cosmic-microwave-background/

    Now, as I said above, matter that interacts with photons experiences pressure, and when said matter falls into a potential well, it will bounce back out. Matter that bounces contributes to every peak in the power spectrum. Matter that doesn't bounce only contributes to the odd peaks. So a universe that has lots of dark matter is one where the odd peaks in the CMB are significantly higher than the even peaks.

    And that's exactly what we see.

    That sounds pretty absurd to me. First, if it can absorb UV, then it can emit it. So you'd expect dark matter to be glowing in the UV range. Sounds like sensationalist garbage to me. That said, the article does contain a relevant quote:

     
  17. Aug 17, 2014 #16

    marcus

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  18. Aug 19, 2014 #17
    Chalnoth, marcus, thanks for replies. Could something as simple as the stars forming some kind of gravitationally inter-connected elastic medium along the plane of the galactic disc help explain the galaxy rotation curve anomaly?
     
    Last edited: Aug 19, 2014
  19. Aug 19, 2014 #18

    Chronos

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    Please define 'galaxy rotation curve anomaly'. Your question is unclear.
     
  20. Aug 20, 2014 #19
    I meant that the rotational velocity of stars is higher than expected for their orbits.

    http://en.wikipedia.org/wiki/Galaxy_rotation_curve

    So I wondered if the outer stars could be being dragged along to certain extent as a result of being part of a larger structure like a spiral arm? I don't know how much gravitational attraction of a spiral arm might be?

    Also I wondered if perhaps interstellar dust and particles might form a rotating medium dragging stars along with it at a higher rotational speed than we might expect?
     
    Last edited: Aug 20, 2014
  21. Aug 20, 2014 #20

    Chalnoth

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    No.

    If the only matter in galaxies was the visible matter, then the stars further out from the center could not be in stable orbits: they'd be flying off into space. If they were "dragged" by anything, they would very rapidly leave the galaxy (or at least move to a much higher orbit).

    More crucially, the way in which velocity falls off with distance from the center is completely different from what you'd expect based upon the visible matter alone (the precise rotation curve varies dramatically from galaxy to galaxy).

    More than that, we have many other reasons to believe that dark matter exists besides galaxy rotation curves. No need to make up new rationales when we have a solid one with a number of different sorts of evidence backing it, and has withstood a large number of observational tests which other proposed solutions have failed.
     
  22. Aug 20, 2014 #21

    marcus

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    Do you have a simple diagram to explain the main features of the acoustic peaks? I'm having trouble matching your even odd and bounce explanation to what I see in the power spectrum.

    Here are Lineweaver's diagrams
    http://ned.ipac.caltech.edu/level5/March03/Lineweaver/Lineweaver7_6.html
    Figure 10 is the power spectrum (which you gave a link to also)
    and Figure 11, with the long caption, is his diagrammatic explanation.
    Here is a close-up of Figure 11, but it's missing the caption so the first link is better.
    http://ned.ipac.caltech.edu/level5/March03/Lineweaver/Figures/figure11.jpg

    I can sort of follow his Figure 11, and explanation of the acoustic peaks, but I can't make it fit what you said about even odd. It seems to me that dark matter would fall together, pass through, and coast apart making the region over dense and under dense in alternation.
     
    Last edited: Aug 20, 2014
  23. Aug 20, 2014 #22

    Chalnoth

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    The second thing you need to understand the relationship between dark matter and the power spectrum is the fact that our image of the CMB is blurry: the CMB did not condense from a plasma into a gas instantly. This has the effect of suppressing the high multipole power spectrum.

    If you take this into account, then you can see what's going on by looking at the first, second, and third acoustic peaks. The first peak is by far the largest. The second peak is much smaller. The third peak is almost exactly the same height as the second.

    But remember this is on a background where higher multipoles are suppressed. If it weren't for dark matter, the first, second and third peaks would fall off in a smooth progression. The first would be largest, second would be smaller, third smaller still, etc. Instead you see every even peak a bit higher than you'd expect from the decreasing trend alone, and every odd peak a bit lower. This is less visible at very high multipoles, as the decay of the power spectrum due to the blurring I mentioned above dominates.
     
  24. Aug 20, 2014 #23

    Chalnoth

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    This site has a good diagram where they show the best-fit dark matter and normal matter only models to the CMB:
    http://ned.ipac.caltech.edu/level5/March02/Ellis5/Ellis1.html

    Note the best-fit normal matter plot has a smooth progression in height of the peaks, and doesn't fit the data at all (BDM on the plot, or baryon dark matter, which would be dark matter that is actually protons and neutrons, but we just can't see it through our telescopes).
     
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