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Dark matter, energy and negetive gravity

  1. Apr 16, 2005 #1
    I've been reading and posting in this forum for a little while. I've noticed a few things have been mentioned about dark matter, energy, and negetive gravity. I asked my physics teacher about what dark matter was; he said it was basically protons that have negetive charges and electrons that have positive charges. We didn't discuss dark energy because he didn't remember much about it. The negetive gravity conversation briefly mentioned that for gravity there is anti-gravity which was the opposing force to gravity.

    How does the charge of protons and electrons become reversed? Is there anyway to change it back?

    What is dark energy? How does it work? Would is destroy energy? If that is what happens then what is the byproduct? How would energy be destroyed if there is a postulate that states energy can neither be created nor destroyed but can be converted?

    How do you get negetive gravity? Is there ever a case in which the negetive gravity and gravity have been equal in forces creating a condition similar to equilibrium?

    Are there any places in the universe where these exisist in large amounts?

    Lots of questions I know... :blushing: they don't need to be answered in one post, but whatever floats your boat. :smile:
  2. jcsd
  3. Apr 17, 2005 #2
    your teacher is wrong about dark matter. He has described anti-particles. we don't know what dark matter is, just as we don't know what dark energy is. dark energy is like an anti-gravity in that it causes the expansion of the universe to accelerate.
  4. Apr 17, 2005 #3


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    The dark matter that is all the rage cosmologists talk about does not radiate or noticeably interact with any other form of matter [including itself!] to any appreciable extent, aside from gravitational effects. Particles theorists thing it may consist of hitherto unobserved fundamental entities like axions. Dark energy is an attempt to explain the apparent acceleration of expansion. It is even more mysterious than dark matter [although some think they are different manifestations of the the same underlying phenomenon]. The thing your teacher was talking about, as matt noted, is anti-matter, not dark matter.
  5. Apr 18, 2005 #4
    Does dark matter do anything if it doesn't interact with anything? I thought universal acceleration wasn't explained by dark energy. How does dark energy explain the universal expansion?
  6. Apr 18, 2005 #5


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    Dark matter has the same gravitational effect as any other kind of mass, but no other observed interactions (in other words, its acts quite a bit like neutrinos, except that we know that the number of neutrios times their believed mass isn't nearly sufficient to account for dark matter). There is, in standard treatments of dark matter, a whole lot of it in many galaxies and the primary effects of dark matter are to resolve the discrepency between the movement of stars at the fringes of galaxies and their predicted movement based on gravity and observed matter alone (general relativity and Newtonian gravity are essentially equivalent in this situation). At the speed we see objects at the fringe of galaxies move, they should be flinging themselves into intergalactic space. They don't. If there is a halo of dark matter around the galaxy to give the galaxy more heft, what we observe would make sense.

    When dark matter isn't busy holding galaxies together, it bends light that passes around galaxies consistent with the predictions of general relativity for a quantity of dark matter necessary to get the dynamics of the galaxy to behave as observed. And, dark matter is also believed to influence the pattern of cosmic background radiation which is observed.

    An equation called the Friedman equation connects the expansion or contraction of the universe we would expect to see, with the density of matter-energy in the universe, given some assumptions that aren't dreadfully unreasonable and the equations of general relativity. We can observe the actual apparent accelleration of the universe more or less directly (by looking at the redshifts of distant luminous objects). We can estimate the density of stars in the universe (with star surveys and our knowledge of the mass of stars). We can estimate the density of dark matter in the universe by looking at the ratio of luminous matter to dark matter in the galaxies we have observed. This equation leaves us 70% short in matter-energy density.

    So, somebody sits around and tries to figure out how to make the numbers add up. This very bright PhD physicist said something like: "You know, if energy were distributed more or less uniformly through the entire universe at a very low energy density, our numbers would match up and the dynamics of the universe below the cosmological level wouldn't change significantly." And, an undergraduate heard this and said: "But, wait a second. We have seen lots of empty space and haven't observed this faint, uniform energy field." And, the PhD Physicist replied: "No problem, we'll call it 'dark energy' and the press will go wild." The rest is history.
    Last edited: Apr 18, 2005
  7. Apr 18, 2005 #6
    On a show I recently saw on "Scientific American" they observed that the outer rim of the galaxy was moving slower than the inner portion. In order to explain this effect they proposed that some gravitaional force must be acting on the outer rim of the galaxy to produce that effect. They claimed that dark matter produces a gravitational force even though it does not interact with normal matter or energy. It surrounds our galaxy in much greater quantities than actual matter that we are familiar with. I'm as curious as you are, misskity, to have an explanation for this.

    edit(that helps explain some things. Thanks.)
    Last edited: Apr 18, 2005
  8. Apr 18, 2005 #7


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    Now, for the advanced course on dark matter. Scientists have looked at lots of possible dark matter candidates to explain what we observe. After looking at all of them, they determined that it pretty much has to be something we haven't yet discovered yet (the buzz word is non-baryonic, which means made up of stuff other than the stuff that atomic nuclei are made out of). The leading theory is called a lambda CDM cosmology. Lambda is the Greek letter that is used for the cosmological constant in the equations of general relativity. In short, it is a number you can put in Einstein's general relativity equations, which Einstein himself was iffy about, which can be a stand in for dark energy. CDM stands for Cold Dark Matter. A major issue in Lambda CDM theory is that we haven't observed anything that is a good candidate for the CDM part. The most popular stand in is called a "WIMP". This stands for weakly interacting massive particle (good fit, eh?).

    One fairly important issue in Lambda CDM theory is how the CDM knew to go in just the right place to make the universe behave the way that it does, because CDM doesn't just appear randomly. Most of the time, it follows a quite predictable distribution patter in relation to a galaxy.

    In addition to your nice, friendly, moderate, mainstream astronomers who think that Lambda CDM is the best way to describe what we observe in the universe based on quite solid experimental evidence, there are also scary wild eyed rebel scientists (and even worse rebel laymen -- like me), we spend a great amount of time wondering if maybe the lambda CDM model is wrong and some other model might be better. Us scary people sometimes come up with ridiculous and soundly disproven ideas. Some of the favorites are: the Goddidit Theorists (who just plain don't like science and think the universe is just 6,000 years old), the Tired Light people (who think that red shift data is whacked for bad reasons), the Steady State Cosmology people (who think that there was no big bang -- the Friedman equation assume based upon reasonable evidence that there was a big bang) and the Newtonian Luddites (who think that all scientific discoveries since 1900 were full of crap).

    There are also slightly less scary people who come up with reasonable sounding ideas which are not yet widely accepted and may never garner enough attention to even be seriously considered. These people, who are sort of on probation in the scientific community, come in two groups. First, there are the alternative gravity people, and second, there are the alternative cosmology people. Alternative gravity people would like to come up with a theory that eliminates the need for dark matter by tweaking the laws of gravity, and thereby also explaining the observed data. Alternative cosmology people are usually hot to eliminate the need for dark energy, among other things.

    The two main groups of alternative gravity people are the MOND-TeVeS people, and Mannheim, who has a similar but not identical theory. There are also a few others.

    There are lots of alternative cosmologies. The main groups of alternative cosmologies are the Quasi-Steady State People, who revised out the most obvious flaws of Steady State Cosmology; the Bubble Cosmology people who think that some of the weird stuff we observe would make more sense if we assumed that the universe extended beyond the observable universe with the fringes being less dense than the center; the zero point energy people, who think that important parts of what we observe are explained by quantum fields in space similar to dark energy but different, and others. Generally speaking, mainstream physicists aren't as nice to alternative cosmology people as they are to alternative gravity people, although explaining why is way too long for one post.
  9. Apr 18, 2005 #8


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    Hi, Huckleberry (are you a fan of Doc Holliday?)! :rolleyes: Actually, our measurements of the rotation curves of typical spiral galaxies show that the outer regions of galaxies have very flat rotation curves. They rotate almost as if the stars, gas clouds, etc, are embedded on a wheel. This is not the type of behavior that physicists expected to see.

    Some physicists who are convinced in the accuracy of the gravitational model found in Einstein's General Relativity have decided that the anomalous galactic rotations can best be explained by Dark Matter. The DM must be very special. It is impossible for us to detect, so it cannot be made of Baryons, and it must be distributed in specific ways to fix the anomalous rotations in galaxies of various types. One problem with this model is: if DM is so weakly interactive, how can it assume distributions that will allow it to perfom its function?

    Other physicists have proposed variations to the mathematical models that explain the effects of gravity, and one of them (MOND) has actually made accurate predictions of the behaviour of a type of galaxy that could not even be observed until a decade after the prediction.

    There are other physicists who are searching for a mechanical basis for the gravitational force. Many of these people are folks who are working at the intersections between quantum physics and general relativity. You may find these folks working in string theory and loop quantum gravity.

    Nobody yet has described the mechanics by which gravitational attraction arises, although there have been mathematic models that model the effects of gravity with varying degrees of accuracy, including Newtonian Gravity, General Relativity, and MOND. Gravity may arise at the scales addressed by quantum physics, and that is not an easy area to model mathematically.
  10. Apr 18, 2005 #9
    So the DM surrounding these galaxies refracts the light similarly to the way light refracts in water? Maybe I'm confused and don't even know it. Just to make sure I have this right; red shifts are when the galaxies are flying away from us right? :rolleyes:
  11. Apr 18, 2005 #10
    Yeah these groups of scientists you are describing...sound kind sketchy to me. :uhh: I'm not judgeing or doubting their credibility. The way it is presented here makes them sound sketchy is all.
  12. Apr 18, 2005 #11
    These posts are turing out to be very long, partly my fault for quoting them, btu they are very hard to shorten. Can we try to cut future posts down a bit and handle one thing at a time? :redface:
  13. Apr 18, 2005 #12


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    No. Dark matter is dark. But, in accordance with general relativity, gravity affects not only matter (e.g. keeping the Sun in its path around the center of the Milky Way galaxy) but also light, because gravity is simply a change in the shape of time and space. So, for example, if star light from a distant quasar passes close to the star Alpha Centuri on the way to Earth, the path of the light will be bent.

    The gravity of dark matter in the Milky Way galaxy helps bend the space in the vicinity of the Milky Way galaxy causing the path of light that comes near to bend.

    Also, yes, redshifts are what happens to light emitting things moving away from us, and blueshifts (rarely seen, and that's a good thing) are what happens to light emitting things moving towards us.
  14. Apr 18, 2005 #13


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