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Why Dark Matter and Dark Energy?

  1. Nov 19, 2008 #1
    Hello, I'm pretty uneducated on the topic of dark matter and dark energy, but I know a little. I was just wondering why dark matter and dark energy are the more "expected to be true" theories then many others currently. I'm not suggesting that the following theories are right or wrong, I'm just wondering why people would tend to believe they're probably not the right answer.

    As far as I know one of the major reasons that dark matter is believed to exist is because from what we see from the structure of galaxies we think there should be more mass then we see. Why isn't it likely that the extra matter isn't in objects that are hard to observe like black holes, neutron stars, etc.?

    It seems dark energy is currently the most accepted idea as to why the universe is accelerating in it's expanding. Why isn't the idea that, like at extremely high speeds Newtonian physics is no longer correct and you need to use relativistic physics, at extremely long distances the effects of gravity could be different then at the distances we commonly observe.

    Thanks for your time and explanations.
    Last edited: Nov 19, 2008
  2. jcsd
  3. Nov 19, 2008 #2


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    Lensing studies pretty much ruled out big lumps of cold dark matter.
    (Gravity bends light and so some heavy compact object passing in front of a very distant bright object would make it twinkle)

    You don't need high speeds, general relatvity works at low speeds.

    Yes it is possible that GR is wrong on large scales (or at earlier times in the universe) but until someone comes up with a theory that is as neat as GR the observational astronomers are getting the blame!
  4. Nov 19, 2008 #3
    I had originally assumed this as well, but the gravitational lensing is still usually fairly hard to find. Have we inspected a galaxy close enough to be sure we're not missing a bunch of black holes and we're fairly certain of the mass of all of them? I'm not trying to say we haven't, I just don't know if we're actually that certain or not.

    Thanks for the quick reply.
  5. Nov 19, 2008 #4


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    It's a statistical thing, you just have to look at a few background galaxies - if you see no lensing events within a certain time period you can put an upper limit on the number of lensing objects. You repeat this for a background objects in random directions and you can be pretty sure of the numbers.
  6. Nov 20, 2008 #5


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    There is one independent factor. The amount of deuterium in the universe is consistent with the amount of baryonic matter (about 4% of the universe - note that only 10% of the baryonic matter is visible). Dark matter and dark energy make up the rest.
  7. Nov 21, 2008 #6
    hey now that sounds unfair. the astronomers are seeing things with their own eyes and GR is still a theory on paper...true until the next big theory.
  8. Dec 7, 2008 #7
    Yah, that's kind of what I assumed but I wasn't sure how much we really knew. I figured there was good reason for the hype. Thanks much.
  9. Dec 7, 2008 #8

    George Jones

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    Here's a little more about what mathman wrote.

    Theoretical studies of the production of chemical elements after the Big Bang, together with observations of cosmic abundances of chemical elements today, show most of the dark matter is not made of the same type of matter that make up ordinary stuff. By ordinary stuff, I mean things like people, planets, and stars, for which protons and neutrons make up the majority of their masses. This also rules out black holes that formed from the collapse of stars, as the stars were originally made of protons and neutrons. Physicists think that dark matter requires particles that have yet to be observed directly. Protons and neutrons are examples of subatomic particles called baryons, so physicists think that dark matter is non-baryonic.
    Last edited: Dec 7, 2008
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