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How is energy/matter density distributed through the universe?

  1. Jul 18, 2012 #1
    If you were to take a panoramic view of the sky, would the amount of electromagnetic radiation and matter/dark matter be relatively evenly distributed? Or is their a section/ sections in the sky that appear to be significantly more densely populated?
  2. jcsd
  3. Jul 18, 2012 #2


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    It would be evenly distributed.
  4. Jul 19, 2012 #3
    Good question. It depends upon the scale. Obviously there are galaxies and then there are gaps between the galaxies so it is not evenly distributed.

    One way scientists quantify this is by picking a random point in space and imagining there is a sphere centred on that point. Lets say this sphere has a radius of 1 (we'll come to units later). You then count how much mass there is inside the sphere. Then we increase the radius of the sphere to 2. The volume of a sphere goes like the radius cubed so when we go from one to two, we are increasing the volume of a sphere by a factor of

    2 cubed = 2^3 = 2x2x2 = 8

    so we then count how much mass there is in this bigger sphere. If it is 8 times what we measured when the radius was one then the matter is evenly distributed. In other words the density would be constant, remember

    density = mass / volume

    Now in real life if we imagine a sphere centred on the middle of the milky way with a radius of about 100 kpc (kpc=kilo-parsecs =1000 parsec. 1 parsec=3.2 light years) It will contain all the mass (visible and dark) of the milky way, lets say about 1 million million solar masses. If we then increase the size of our imaginary sphere to 200 kpc, the volume will increase to 8 times what it was previously. However, the total mass inside the new bigger sphere will not be 8 million million solar masses, but still 1 million million solar masses as we have already included all the milky way and there is nothing outside the milky way until you get to andromeda, which is further than 200 kpc away.

    Likewise if you started in intergalactic space, far away from anything, you would have very close to zero mass inside your sphere, but as you increase the size of your sphere, you would start to include some galaxies, so your density would then shoot up.

    Now galaxies are typically separated from each other by about 1 Mpc (Mpc= megaparsec= million parsec). So if your sphere is big enough, you shouldn't be sensitive to galaxies popping in and out of the count as you change the size of your counting sphere. In fact we find that this occurs at scales of about 100 Mpc. So if you look at a 100 Mpc sphere on the sky it will look roughly the same as another 100 Mpc sphere on the sky somewhere else.

    So on large scales (> 100 Mpc) the Universe appears to be smooth or "homogeneous".
  5. Jul 21, 2012 #4


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    I would be wiiling to assert it's not would be, it IS evenly distributed - to the best of our measurement ability.
  6. Jul 21, 2012 #5


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    Yes, that's exactly what I thought I said. My sentence was short for "If you did that experiment, you would find that it would be evenly distributed", but thanks for the emphasis.
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