Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Was the Big Bang Hot?

  1. Mar 15, 2010 #1
    To compress the observable universe into the size of a grapefruit, it had to be devoid of all motion, and therefore all heat and energy.

    Why wasn’t the heavy hunk of matter the size of a grapefruit that preceded the “Big Bang” that created our observable universe cold and without energy?

    My reasoning:
    We know that more than 99.9% of the universe is “empty” space and that everything is constantly moving: rotating, revolving, expanding, contracting—everything is in motion from the atoms in the chair I am sitting on to the earth, moon, sun, stars, and accelerated expansion of the universe. Even though atoms are more than 99.9% space and only contain a tiny amount of matter, atoms do not pass through each other because of the electrical field exerted by the spinning, revolving, rotating, matter within them. A rock that appears to be stationary is made up of atoms that are moving and producing an energy field.

    For all matter within the universe to have been compressed into the size of a grapefruit, all motion had to have ceased. Without motion there is no heat or energy.

    My conclusion is that matter must have pre-existed the expansion, and that the energy that caused the matter to begin moving and producing heat came from an external source.

    Am I missing something?
  2. jcsd
  3. Mar 15, 2010 #2


    User Avatar
    Science Advisor
    Gold Member

    The very early univese is believed to have been so hot that even subatomic particles were unable to form [i.e., it was pure energy]. The first atomic nuclei did not form until about 3 minutes after the BB. It is fairly certain the universe was much larger than a grapefruit by then.
  4. Mar 16, 2010 #3


    User Avatar
    Science Advisor

    Well, no, this reasoning is fundamentally flawed. Basically, the hot particles in region A may be all moving at extremely high velocities, tending to leave that region very rapidly, but then the hot particles in the neighboring regions will be doing the same, and so on average the inflow of hot particles from neighboring regions cancels the outflow.

    There's also the problem that your entire argument assumes a stable configuration, which never happened: our universe, as near as we can tell, was always expanding. And at very early times, this expansion was very rapid indeed.
  5. Mar 17, 2010 #4
    It wasn't matter - it was energy.

    It didn't form into matter until a long time after when conditions were cooler.
  6. Mar 17, 2010 #5


    User Avatar
    Science Advisor

    Not really. Energy does not exist separate and distinct from matter. Energy is a property of matter.

    A better way to say it would be that the average energy of particles far exceeded their rest mass energies, which meant that collisions would very frequently change the number of particles of a given type.
  7. Mar 17, 2010 #6
    From the answers to my question it appears that originally the universe was infinitely smaller and hotter consisting of pure thermal energy with sub-atomic particles moving about frantically, and that there never was a cosmic egg, or starting point for the universe. The universe cooled as thermal energy decreased through work (conversion to the nuclear forces required to create atoms???) or somehow left the system for colder areas making it possible for matter and the present universe to come into existence.

    Am I interrupting the answers correctly?
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook