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My first post here and a question

  1. May 19, 2005 #1
    Hi gang
    I am new to this forum and I never actually studied this stuff in school. I recently just woke up one morning and had a burning desire to understand the universe. So, I bought all the college textbooks I could get about physics and astrophysics and I have just been reading and reading.

    I have a question that I wondered if someone would care to try to explain.
    Please keep the answer understandable for me, if you can. (Im only on page 150 of a 400 page book)
    Anyway, here goes:

    Supposedly, during the big bang, the explosion of matter was homogeneous and isotropic. Have we explained then, how and why "localizations of gravity" could possibly have occurred? If the gravitational constant WAS constant and the exploding, expanding matter was homogeneous and isotropic, then it doesnt seem possible that the stars and planets could have formed....
  2. jcsd
  3. May 19, 2005 #2
    I think the answer is that there was small deviations from the perfect homogeneity and isotropism, to allow matter to clump. The COBE and WMAP experiments detected such deviations in the cosmic microwave background radiation. Google like "COBE cosmology" for more info.
  4. May 20, 2005 #3
    I don't know for sure, but i would say that it was not homogenous and isotropic, as a result of the uncertainty principle. The uncertainty principle states that you can never know the position or motion of a particle to arbitrary precision, there is a minimum amount of accuracy. However this property is not just an issue with measurement, its a physical property of matter. There is a certain 'fudge factor' in the position of particles (photons, quarks, protons, etc.) and you can never get any more precise than that.

    This effect is important in a number of other cases, for one the sun could not carry out fusion reactions without it, the temperature and pressures are not high enough for particles to collide, unless you allow for 'quantum tunneling' which is basically where a particle jumps a small distance, allowed by the uncertainty principle. If it weren't for this ability of protons to 'tunnel' into each other, the temperatures in the sun would have to be 1000 times higher to allow for any fusion--we'd have no sun. I only point this out to illustrate that the uncertainty in position and motion is a real physical affect, not just a limitation on our measurement ability. This means that it could have the effect of making the early universe non-homogenous, and non-isotropic.
  5. Jun 1, 2005 #4
    evidence does not support the homogenous and isotropic idea

    one of the other biggies is the metal content of early quasars

    why the idea homogenous and isotropic is so loved I donot get

    we live in a very lumpie universe it ain't homogenous and isotropic

    deal with the facts as they are
  6. Jun 2, 2005 #5

    Its probably loved because of the idea of a big bang singularity explosion. If all the matter explodes from a singularity, then it has to explode uniformly and homogenously, according to classical theories (assuming that all information about the state of a particle is lost once it passes through an event horizon, meaning that all particles leaving an exploding singularity must come out identically. However as I said earlier quantum effects rapidly create variations. .

    This is just my understanding of what a classical theory (GR) would predict for a singularity explosion.

    Of course, QM disagrees, and because of the direct evidence of the physical reality of the uncertainty principle that is burning above our heads every day (fusion in the sun) I'm inclined to accept QMs predictions in the situation are 'more valid.'

    (Someone more knowledgeable about GR and singularity physics, feel free to correct me if i'm way off here.)
  7. Jun 2, 2005 #6


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    I read in Peebles' "Principles of Physical Cosmology" that it actually does, for scales greater than 100 Mpc.
  8. Jun 2, 2005 #7


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    I read in Peebles' "Principles of Physical Cosmology" that it actually does, for scales greater than 100 Mpc.

    Maybe in our very limited horizon, but words like homogenous, are limited
    descriptive human terminology, "good for the maths", but if the U is infinite
    how could we say this is applicable everywhere?
  9. Jun 2, 2005 #8


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    Homogeneity and isotropy are indeed supported on the largest scales. I'll refer you to my Review of Mainstream Cosmology.
  10. Jun 2, 2005 #9


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    This is as Brian Greene explains in his book as well - the Uncertainty Principle. When the early universe cooled enough, it "froze out". Any perturbations in the universe that are a result of the Uncertainty Principle were set in stone then, and as the universe expanded, these perturbations expanded too.
  11. Jun 2, 2005 #10


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    1] The key though, is that it's difficult to explain WHY it's not uniform. It is logical to see why it WOULD be, (since it started from a uniform homogenous point), but to say it isn't, requires an explanation.
    2] There is a WHOLE LOT of distance (~13Gy) between "the facts as they are" and the initial conditions - a distance that has a lot of elbow room for theories - including theories that start off homogenous and end up heterogenous.
  12. Jun 2, 2005 #11


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    You are right wolram, but I think at least we could say we are located within an homogeneous and isotropic bubble, without knowledge of its limits. On scales greater than the Hubble length the universe may be inhomogeneous as a consequence of inflation (superhorizon fluctuations).
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