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Why is matter not uniformly distributed across space time?

  1. Aug 20, 2008 #1
    Why is matter not uniformly distributed across space time??

    I was wondering why there is varying densness across space time and not uniform distribution of matter. and slighty linked to this, what is it that holds matter together, but also keeps it apart if that makes sense?? ie atoms bond together but on a larger scale move apart. it would appear that the laws of physics may vary depending on scale. is this a bad idea of a possibility or is it true??
  2. jcsd
  3. Aug 20, 2008 #2
    Re: Why is matter not uniformly distributed across space time??

    Physicists like Hoyle, Gold, and Bondi wondered the same thing. They believed in the "perfect cosmological principle," which states that the universe is homogeneous and isotropic in both space and time (meaning the density of the universe remains constant in time). The perfect cosmological principle obviously contradicts the Big Bang theory which starts out with a universe of infinite energy density and evolves (by having space expand) to the current universe (with a finite energy density). Since Hoyle and crew didn't believe that the density of the universe changed with time, they developed a rival theory called the steady state theory.

    The end of of rivalry between these two theories occurred with the discovery of the Cosmic Microwave Background (CMB), which was predicted to exist by the Big Bang theory. Since the discovery of the CMB, there has been a lot more supporting evidence for the Big Bang

    There are four forces, three of them are--for the most part*--limited to the atomic scale and responsible for holding atoms together and such. The relevant long range force on cosmological scales (~ ten million of light years) is gravity. For most of the universe's history gravity has acted to slow down the expansion of the universe, so you could say that gravity is "trying" to overcome the expansion and bring matter (i.e galaxies and clusters) closer together. Thus, for most of the universe's history nothing has been keeping matter apart.

    In the above paragraph I use the phrase "for most of the universe's history" because it has been discovered that the expansion of space has recently (about 5 billion years ago) started accelerating! The cause of this acceleration is unknown, but it can be consistently modeled by our theory of gravity, general relativity.

    Your right if you mean that some forces are stronger than other forces depending on the scale. For example, in the atom, the electromagnetic (EM) force dominates over most of it (distances of an angstrom), in the nucleus the strong force dominates of the EM force to bind the protons and neutrons together. Finally over millions of light years, the gravitational force dominates over all the other forces.

    These are excellent questions,

    * Actually there are magnetic fields that extend throughout galaxies and maybe clusters of galaxies. Still gravity is the dominant force in galaxies and on cosmological scales (~10 million light years).
  4. Aug 21, 2008 #3


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    Re: Why is matter not uniformly distributed across space time??

    The very early universe was almost perfectly homogenous - save for unavoidable and unimaginably tiny quantum fluctuations. Inflation grossly magnified these tiny fluctuations into the vast filaments of matter and voids observed in the universe today.
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