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Dark Matter?

  1. Dec 3, 2009 #1
    My question is why do we even need dark matter to make things work ? E=mc2 so if ENERGY AND MASS are the same thing if MASS creates gravity why can energy not also create gravity can someone explain this to me? if Under general relativity any form of energy couples with space time to create the geometries that cause gravity why are we even looking for dark matter all the energy in the galaxy should be enough to make up for the lost gravity right? or am i missing something? i mean really microwaves x rays gamma rays LIGHT ITSELF i mean thats ALOT of energy so shouldn't that also be alot of gravity?
  2. jcsd
  3. Dec 3, 2009 #2


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    Energy does have a gravitational effect. Dark matter is needed since the total mass of ordinary matter plus photons is far too small to account for what is needed to hold galaxies together, etc. under current gravity theory (general relativity). Some people have proposed modifying the theory to avoid dark matter, but their ideas aren't convincing.
  4. Dec 3, 2009 #3
    E=mc2 that means a small amount of energy is equivalent to a large amount of mass tho right? and even with all that mass the model still doesnt work? they take into accoutn hawking radiation,electromagnetic and all other forms of energy?
  5. Dec 3, 2009 #4
    No, the other way around. 'Small' amount of mass is equivalent to 'large' amount of energy. But anyway, when you add all together, there is simply not enough to hold galaxies together.
  6. Dec 4, 2009 #5


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    Mass-energy equivalence leads to a lot of misconceptions. The primary one seems to be that somehow matter can be converted to energy and back again.

    This is false.

    To try to explain why, first I'll have to talk about using units where c = 1. This is to remove come confusion, and make the equations more clear. Basically, by setting c = 1, we're setting a "standard velocity". In these units, all velocities are compared against the speed of light. And the mass-energy equivalence takes on a form that makes its meaning more clear:

    [tex]E = m[/tex]

    Now, this isn't entirely correct, though, because it neglects motion. There are a few ways to add motion into the equation, but perhaps the easiest is to invoke momentum. In special relativity, we can then state that:

    [tex]E^2 = m^2 + p^2[/tex]

    Here we've added the momentum p to the equation. How is momentum related to velocity? Well, in relativistic terms, this is simple:

    [tex]v = \frac{p}{E}[/tex]

    Remember, the velocity v is a fraction of the speed of light. So we see immediately, for instance, that if the mass is equal to zero, then [tex]p = E[/tex], and therefore [tex]v = 1[/tex].

    This may seem a bit rambling. But what do the terms in this equation really mean?

    Well, momentum is clearly an energy of motion. And mass is an energy that remains even if there is no motion: mass is the energy that is somehow internal to the object in question. An object that has no mass has no internal energy, but can still have lots of energy of motion. Similarly, an object with lots of mass doesn't need to move at all to have lots of energy.

    But what it all boils down to, though, is that you can't just split off the energy from matter: there still has to be something there, with or without mass, for there to be energy. It has to either have some energy of motion, or some internal energy. Basically, our observations of dark matter show us that we have to have lots of stuff out there with very, very little momentum, and therefore lots of mass. It also can't be electrically charged (else it wouldn't be dark), and must interact only weakly with itself and normal matter (like neutrinos, but neutrinos would have too much momentum compared to their mass).
  7. Dec 4, 2009 #6


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    Chalnoth has explained the energy-mass misconception, I'll add one small comment. When we model the universe, we do include the gravitational contribution of all forms of energy, including electromagnetic radiation. It turns out that in the early universe, electromagnetic radiation was for a time the dominant component, however at the present time the energy density of radiation is very small, much smaller than 1% of the total energy density.

    So radiation is already in the calculations, its just that it has a relatively small contribution at the present time. Of course it is also the means by which we can learn about the universe (because it is what we can see), so of course it is very important, but not in a direct way.
  8. Dec 4, 2009 #7
    thanks for clearing all that up for me guys :)
  9. Dec 14, 2009 #8
    Can you list the basic properties of dark matter? How it is like to be? It is sure that can't be ordinary matter, some undiscovered (yet) elements?
  10. Dec 14, 2009 #9


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    The required properties to explain our current observations are:
    1. It must not have electric charge.
    2. It must interact at most weakly both with itself and normal matter.
    3. It must have properties such that it is produced in the right numbers in the early universe to explain the amount of it which we observe.

    It can't be ordinary matter, and since elements are made of ordinary matter, it can't be an element either.
  11. Dec 15, 2009 #10
    Thank you.

    What else did we know about DM?
    It's properties is analog with gas or solid bodies, or maybe exist just as disparate entities? Can it agglomerate in large bodies? Because you said that it interact weakly with itself I suspect that the answer for last question is "no, it can't agglomerate in large bodies".

    Is DM uniform distributed in Universe or have some connections or affinities with normal matter, so it have an analog distribution? It is static or can we say that it is affected by expansion?

    Because it is so ... enigmatic (we know little about it) how can we say that it is different by Dark Energy (an other very enigmatic thing)?
  12. Dec 15, 2009 #11
    It cant form bodies because of #2 - It must interact at most weakly both with itself
    Mathematically it is an ideal gas.
  13. Dec 15, 2009 #12


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    As Dmitry mentioned, #2 means it can't do this. Basically, galaxies are little bits of normal matter sitting in the center of huge clouds of dark matter. For the matter in the galaxies to become so compact, it had to experience friction. So the fact that dark matter is not compact means it can't experience friction. And for it to not experience friction, it can't interact with itself very strongly.
  14. Dec 15, 2009 #13
    this is some new info from a recent article i read

  15. Dec 16, 2009 #14


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    Just to be clear, when we say 'DM doesn't interact with itself' that doesn't preclude gravitational interaction. Dark matter most certainly does form 'bodies' which we call 'dark matter haloes' due to gravitational interaction. These bodies are very big, very massive and don't have very dense cores, but none the less they are bodies, and they are their existance is what drives the formation of galaxies.
  16. Dec 16, 2009 #15
    I would rather call them 'clouds', not 'bodies' as they dont have any rigidity.
  17. Dec 16, 2009 #16


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    You say potatos I say potartoes. The point is that Skolon in post #10 had taken 'DM doesn't interact' to mean that it is distributed uniformly. In fact dark matter is distributed in a very non-uniform way, and much of it exists in gravitationally bound, roughly spherical objects reffered to as dark matter haloes. Neither 'clouds' or 'bodies' are words used to normally describe them.

    By the way, in post #11 you suggest DM is mathematically an ideal gas. In fact an ideal gas in general has pressure, whereas DM has no pressure. You could say that DM is an 'ideal pressureless fluid'; that is the most minimal description you can make.
  18. Dec 16, 2009 #17
    Sorry to disagree with you.
    But it has pressure.

    DM can not interact with normal matter, so that gas can not apply force to any surfaces, so the pressure can not be measured. yet you can calculate it based on the velocity and mass of DM particles. And that pressure creates additional gravity based on GR.
  19. Dec 16, 2009 #18


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    That sort of pressure is only non-negligible if they have relativistic velocities, which they can't have to form the haloes we observe. Dark matter is thus effectively zero-pressure.
  20. Dec 16, 2009 #19


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    Dmitry, there is a technical way when you do calculations in GR that lets you define whether a fluid is pressureless of not. I am using that definition as will any textbook on the subject.

    In all GR calculation done in the presence of dark matter it is taken as a pressureless fluid. At a microphysical level this can be related to assuming a dispersion velocity of zero. All calculations to date that include a non-zero dispersion velocity (equivalent to assuming a non-zero pressure) have been found to be inconsistant with observations. Dark matter really does have to be pressureless to fit the data and a non-zero pressure is measureable.

    At some level the dispersion velocity is probably not zero, but it is very small.
  21. Dec 16, 2009 #20
    Effectively zero pressure = very low
    My disagreement was mathematical.
    It is very low, but it is not =0. The fact if particles are interacting or not is irrelevant for the calculation of a pressure.
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