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Dark matter?

  1. Dec 22, 2003 #1
    i heard this on some website im not sure is
    it dark matter? or was it black matter?
    the emptyness of space. i was wondering if
    anyone can explain in simple terms what this is
    about, is it just a name for the emptyness or
    is it aboit something?
  2. jcsd
  3. Dec 22, 2003 #2
    i heard that it makes up at least 90% or so of the mass in the universe
  4. Dec 22, 2003 #3


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    Current theory has it that ordinary (baryonic) matter is about 5% of the universe, non-baryonic ("dark") matter is about 25%, and the rest is dark energy. To complicate the terminology most (about 90%) of the ordinary matter is dark, that is not visible through telescopes, since it is too cold.

    If you go to arXiv (use google, if you are not familiar with it), get document astro-ph/0308418. It is a good readable detailed description of current cosmology.
    Last edited: Dec 22, 2003
  5. Dec 23, 2003 #4
    doesnt dark energy radiate like anti-gravity.
    And how do we know dark energy is ther?
  6. Dec 23, 2003 #5


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    The WMAP first-year results, together with a great deal of data from many other astronomical observations, gives:
    - 4% 'atoms' (a.k.a. baryonic matter)*
    - 23% cold dark matter (non-baryonic matter, not travelling at relativistic speeds - hence 'cold')
    - 73% dark energy.

    Neutrinos, which are 'hot dark matter', make up a negligible proportion.

    Dark matter has been detected in at least four ways:
    1) rotation curves of galaxies; the outer regions of spirals (and other types?) are rotating faster than the estimated total amount 'normal' matter we deduce from analysis of the photons we see (mostly light)
    2) gravitational lenses; distant galaxies are seen magnified and distorted as the light from them passes through the gravitational field of a closer galaxy or cluster. The amount of mass in the gravitational lens (which we work out from Einstein's GR and the image properties) is greater than we deduce from the light we can see from the foreground galaxy (or cluster)
    3) X-rays from galaxy clusters; these come from the thin gas between the galaxies. The temperature of this gas can be deduced, and if we assume a pressure equilibrium, then we can work out the amount of mass there must be in the cluster to keep the gas there. Again, this mass is far greater than that we can esimate from stars, dust, and gas (made of ordinary matter), and from black holes (e.g. at the heart of elliptical galaxies)
    4) WMAP; the cosmic microwave background (CMB) has characteristics which reflect the composition of the universe at the time the CMB was formed, ~300,000 years after the Big Bang. Cosmological models fit the observed CMB best if they contain ~23% cold dark matter, ~4% baryonic matter, etc.

    'Dark energy' was first convincingly detected in studies of distant supernovae. There studies were undertaken (among other reasons) to better determine the parameters in cosmological models. It was expected that they'd show the expansion of the universe was slowing down. In fact, they showed that it is speeding up. This gave rise to the popular characterisation that it is 'anti-gravity'. Dark energy is one of the most mysterious aspects of the universe.

    *BTW, this lends a whole new meaning to the phrase 'scum of the universe'
  7. Dec 23, 2003 #6
    wow thanks nereid, so are they still doing investigations
    on it to find more things about it? scince it's still mysterious
    when was it actually discovered?
    what's anti-gravity about?

    hope im not asking too much
    just interested
  8. Dec 23, 2003 #7


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    I don't think it's possible to ask 'too much' about this kind of thing :smile:

    Research into dark matter and dark energy are among the most active areas in astronomy today. For example, there's a proposal for a new terrestrial telescope of somewhat novel design, dubbed the 'dark matter telescope' (it'll be used for other things too). This would look for dark matter concentrations using weak gravitational lensing, in an unbiased fashion (in the sense that it will look for the signature of dark matter in images of background galaxies independent of foreground objects), and hopefully find some dark matter concentrations not associated with galaxy clusters.

    One of the objectives of the massive SDSS project is to characterise the 3D distribution of galaxies and clusters to provide a much more stringent set of statistical tests of dark matter and dark energy in various cosmological models.

    There's also a proposal for SNAP, which would be a Hubble-like space telescope with a billion-pixel camera, dedicated to finding and analysing distance (z ~> 0.5) supernovae, to nail down the time when the universe changed from expanding at a slower rate (deceleration) to greater rate (acceleration). This seems to have happened around 4 billions years ago, but the details are very sketchy so far.

    IIRC, the 'distant SN' studies were done in the mid-1990s, with firm results announced around 1997.

    The 'anti-gravity' nature of dark energy refers to the fact that it's causing the universe's expansion to accelerate, whereas the effect of gravity is to cause the rate to slow down. However, since dark energy is so poorly understood, it's a little premature to say what this 'anti-gravity' really is. There are various theories on the subject - cosmological constant, quintessence, vaccuum energy, ... all of which are fascinating in their own right - and the next five to ten years will see them cut down to size by having much better data to fit.
  9. Dec 24, 2003 #8
    Dark matter is less sweet than regular matter and is ideal for baking.
  10. Jan 1, 2004 #9
    It can be this

    We may have wrong gravity constants, or we may only be aware of a short-distance-gravity, dark matter may be just the unaccounted long-distance gravity interaction (field) that is everywhere, bacause matter is everywhere, waht if gravity behaves differently (i.e. is not a linear interaction) at different distances
  11. Jan 1, 2004 #10
    maybe you should look at the 'other' side of the boundary. I mean the other side of the spacetime membrane. Building ups on the other side will only appear as un-explanable deformations of (gravitational) spacetime, and show strange effects like repulsion and unusual attraction. That's at least one logic explanation versus 'we don't know'.
  12. Jan 1, 2004 #11
  13. Jan 3, 2004 #12


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    possibly, but not likely

    The observations which any alternative hypotheses need to account for are extensive and well characterised*.

    AFAIK, all these suggestions have been tried, and - with the exceptions of MOND and 'dark matter' - they're all ruled out by observational results.

    *they're also in the public domain (mostly), so anyone with an alternative can test their predictions quickly and (relatively) easily. Sadly, many of those with creative ideas fail to do this. There are several examples in the Theory Development board.
  14. Jan 3, 2004 #13
    Dark Matter

    Where is this Dark matter?

    Dark matter is the theoretical prediction of the Big Bang theory...the fact that they say the univers has this 'Dark matter' that can never be observed or proven indirectly or directly just shows that the theory is false and that some scientists cant accept that their life work has been a total futial attempt to prove something false

    Even Nutrinos can be proven from high enery particle accelerators...if the world/universe is made of of 90% dark matter then probability tells us that we should see this stuff everywhere...but, we dont no will we ever because it doesnt exist
  15. Jan 6, 2004 #14


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    Re: Dark Matter

    Dark matter was an important aspect of analysis of observations of the rotation curves of spiral galaxies long before it became constrained in Big Bang models - IIRC, it was more of a free parameter (to be determined by observation) than a necessity.

    This link gives an example of just how far we've come in terms of 'observing' the distribution of dark matter (a very long way indeed):

    If a significant part of dark matter is comprised of supersymmetric particles, we should get some good constraints when the LHC comes on line. Quite a lot of time on leading astronomical observatories - ground-based and in space; current and future - will be devoted to getting a better handle on the distribution and nature of dark matter.

    "Dark energy" is a different matter ( ) entirely - too soon to say how well we can expect to characterise it in the next decade or so.

    Don't forget that several decades passed between observations which convinced physicists that there are particles like neutrinos and the first direct observations of them, and even more decades before some of their key characteristics (e.g. 'rest mass') were elucidated.
  16. Jan 6, 2004 #15
    About 10 years ago, developing a detector that directly detects DM matter (i.e., DM particle being absorbed by, or interacting with the detector material) was pretty popular. I was even involved in some of the detector fabrication. I kind of lost touch with it now, and not sure what the status is.
  17. Feb 28, 2004 #16
    Dark matter is only a hypothesis

    The stars in the outer part of our galaxy move in a higher speed than what Newton's law is given. For example 60km more for our sun. This speed was proposed to be from Dark Matter that we can't see with light. The same thing happen in general relativity in which the critical density is bigger than the density we can see. So more than 90% of uuiverse matter was thought to be dark matter.

    So, I'd rather say that dark matter is only a hyperthesis such as ether before. Some new theories try to prove the dark matter to be a new interction called universal magnetic force. This force happen in all the stars in universe especially in ring galaxies and the galaxy clusters where there is hardly any gravitation from the center.
    Last edited: Feb 28, 2004
  18. Feb 28, 2004 #17


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    Welcome to Physics Forums galaphy!

    There is a resource in General Astronomy & Cosmology - the A&C reference library - which has links to an earlier discussion here on dark matter; you may find it interesting.
    Do you have any references to this new 'universal magnetic force' interaction? A few links would be much appreciated.
  19. Feb 28, 2004 #18
    Thanks Nereid for providing a useful link.
    As a Chinese, I come to the English webs for the first time. I read some article about these new theories in Chinese at the Chinese websites.among them is the System Field Theory in which it propose the universal magnetic interaction. Mostly, these new theories can't publish in the mainstream journals. But I learned that some of the articles have been published in English journals though I don't remember. I will check later and if I find it I will give a link here.
    I'm interested in astrophysics and cosmology indeed. Also general relativity, about black hole ect.

    Hope you can help me in these fields.
  20. Feb 28, 2004 #19


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    Well there are some PF members who can read Chinese (tho' I don't know if the forum is set up to display jiantizi), so don't be afraid of posting some links to a website in Chinese.
    If that's the case, I suggest that you discuss this topic in the Theory Development sub-forum (under Physics -> General Physics).
    IMHO, Physics Forums is perhaps the best interactive forum on the web to learn about these things - there are many very patient and knowledgable people who post here, and many who are only too happy to help folk such as yourself learn more. Just ask your questions (and read through the threads).
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