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

Reasoning behind the big bang theory

  1. May 2, 2003 #1
    [SOLVED] Reasoning behind the big bang theory

    Is there are way to qualitativly explain this, or is the math beyond most people? I understand that according to this model, the universe expanded from an unimaginably tiny particle with unimaginable mass desity, and that during the first fraction of a second, the expasion was substanstial.
    The concept is hard for me to accept, it seems so unreal, but many physicists subscribe to this model, and physicists are the most resonable people, so there must be some logic behind this. What experiments were carried out that confirmed this theory? Does one require a background in modern physics to grasp this logic? I only know some classical physics myself, so maybe Im just to ig'nant to be asking this!
  2. jcsd
  3. May 2, 2003 #2


    User Avatar

    The Big Bang theory is based on a certain number of data that point to this hypothesis.

    (1) Red shifting of galaxies, increasing with distance, suggest the expansion of the universe. Extrapolate back and.... ta da!
    (2) Cosmic Microwave Background anistrophy. Basically, we can detect microwaves in space that may be explained as remnants of the BB. Further, study of the distribution of the radiation suggests a source, or major event.
    (3) Relative abundancies of certain elemenst in the universe. The large amount of lighter elements, particularly hydrogen, suggest a reasonably young universe that probably was hence created.
    (4) Logical thought that the universe must have been somehow created. Yes, pretty flimsy.

    There may be others.
  4. May 2, 2003 #3
    Will, "a theory" in science mean PROVEN hypothesis. Newtonian theory. Einstein special relativity theory. Maxwell theory. Quantum theory. Evolution theory. Big Bang theory.

    Unlike in layman terminology where word "theory" may mean anything but proven thing: "...my theory...", "Conspiracy theory", "In theory yes but in practice - no..." , "...this is just a theory", and so on.

    By the way, in Europe usually even in everyday language "theory" is refered to as verified by observation hypothesis. I don't know why in US they frequently mix theory with hypothesis.
  5. May 2, 2003 #4
    (1)Does this involve the doppler effect? Does the term red shift mean that because the object is moving away from us,( because of the expansion) the lowest visible frequency emmited from the source is percieved as having a lower frequency and not visible? If the universe was contracting, would we observe a "violet shift"

    (3)Does this imply that as the universe gets older, there will exist even heavier elements than those known?
    Last edited by a moderator: May 2, 2003
  6. May 2, 2003 #5


    User Avatar
    Science Advisor

    Not exactly, but more that as the universe gets older, the heavy elements will become more abundant as stars fuse hydrogen and supernovae make, well, everything heavier than iron, basically.
  7. May 2, 2003 #6
    I must disagree. No physics can be CONCLUSIVELY proven like a mathematical theorem. The best one can do is provide experimental data to SUPPORT the theory. But empirical data is not precise in the absolutel sense; it is possible that you may perform an experiment tomorrow which contradicts your theory.

    As for evolution theory.. Not really a theory, when it doesn't make predictions that may be experimentally verified or refuted. I believe it's true, purely on blind faith and "it sounds reasonable, a hell lot more reasonable than that bible stuff."

    Now, about the Big Bang... If we run the clock backwards, the begining of the universe resembles the creation of a sigularity inside the blackhole, which leads me to ask...
    Is the known, observable universe contained inside the event horizon of a black hole with its time axis reversed?

    (It is pointless to ask "is something really really true, in reality?" I meant are the mathematical frameworks equivalent.)
  8. May 3, 2003 #7
    That point isn't logical. It would have been more "logical" if the universe exist in time without begin or end. A "begin of time" is incomprehensible.
    Most theoretical models that try to explain the Big Bang therefore try to figure out what preceded and caused the Big Bang.
  9. May 3, 2003 #8
    Not to get of topic here, but what is the difference between "theory" and "law".
  10. May 4, 2003 #9
    "Law" was considered hip terminology a couple hundred years ago; now "theory" is in. No real difference.
  11. May 5, 2003 #10


    User Avatar
    Science Advisor
    Gold Member

    If you read Steven Weinberg's excellent book "The First Three Minutes" you will walk away with a strong understanding of the Big Bang.

    Also, you can debate theory vs. law vs. hypothesis all day long, the end result is that the Big Bang model provides a level of detail of explanation for observed phenomena which is noticably missing from its competitors (i.e. biblical creation, steady state, etc.). Anyone seeking to displace the Big Bang has some very big shoes to fill. The BB theory neatly plays back in time to when the universe was less than a second old.
  12. May 5, 2003 #11
    Yes, universe always was (and still is) a black hole.
  13. May 5, 2003 #12
    Law is what we observe (bunch of related facts). Theory is a mathematical explanation of why facts are related the way we see them.

    Say, Faraday law of e/m induction (bunch of facts about induced voltage in coils and motion of magnets around). Maxwell theory (= special relativity applied to electric charge) is the explanation.

    Or, say, facts: human and chimp genes are 98.5% identical (call it law: all primates have similar genes). Explanation: branching from common ancestor (=evolution theory).
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook