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Is the Big Bang a theory or a hypothesis?

  1. Jul 14, 2013 #1
    Hello everyone:

    I'm a doctoral student in particle physics, confused about something pretty fundamental and need your help.

    From what I know, the only evidence we have from the Big Bang is the Cosmic Microwave Background radiation (CMB), which showed up around 300k years after the hypthesised Big Bang happened.

    According to this, I see people claiming that the universe started from the size of a "proton" and expanded to that level that produced the CMB. Where did that come from? is that just simply an extrapolation? and what does make that extrapolation over 300k years valid?

    Now my question is: is there evidence for what's claimed to be true about the universe before the universe was 300k years old?

    Why can't we say that the universe would expand and crunch continuously without stop? Of course, I know that the universe is flat, but this is still not over, and the universe's expansion could be accelecrating, and then could decelerate again and go to the big crunch again, and there we'll have a new big bang when all matter crunches.

    So you see, I have some unclear issues about this topic. Could someone please clarify the story on the basis of what is proven by evidence and what is just hypothesised?

    Thanks for any efforts.
  2. jcsd
  3. Jul 14, 2013 #2


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    The big bang is a theory - and a very successful one.

    No. There are many independent observations contributing to the evidence.

    Who claims that?
    Where did what come from?
    Numerical calculations. Most of those 300k years very very boring anyway, the hot plasma of electrons and nuclei just expanded and cooled.

    See my link above. In addition, the CMB itself reveals events which happened earlier.

    That is possible, but it still includes a big bang-like event 13.7 billion years ago.

    There are no proofs in physics.
  4. Jul 15, 2013 #3
    There are “four pillars” of big bang evidence.
    1 the Hubble red shift relationship
    2 the CMB
    3 galaxy evolution
    4 the abundance of light elements

    Here is another link that may help :

    You are right to say there is no observation of the universe before the CMB was emitted about 380k after the big bang. It may be possible with future technology to detect neutrinos or gravitational waves from much earlier than 380,000 years but it’s beyond current technology.
    However particle accelerators can confirm the physics of earlier epochs and different models of the early universe (way earlier than the recombination era) make predictions for temperature variations in the CMB which we can then check against the data.
    Here is a reference that will hopefully be helpful if you want something technical
    and a lecture on CMB physics here if you want something more of a layman level:

    Its advertised about hunting for the multiverse but that just a few minutes at the end , most of its about what we can learn from the CMB.

    If you understand the term “big bang” as a model that says the universe evolved from a hot dense state then you should realise that:
    1) all the evidence supports it.
    2) nothing in the models says it wont recollapse and give a crunch followed by a bang. The models simply says there was a bang, doesn’t say anything about what happened before the bang or what will happen in the far future.

    Having said that observations at the moment don’t support a future crunch, that doesn’t mean there wasn’t a past crunch and many theories of quantum gravity imply there was. But unless you reject the idea that dark energy is a constant (which I think is not impossible) then it doesn’t look like there will be a crunch in the future. However there are respectable scientists saying there will be, here for example:
    But it is not a majority view.
    Last edited by a moderator: Sep 25, 2014
  5. Jul 15, 2013 #4


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  6. Jul 15, 2013 #5
    i think it's a hypothesis. a theory needs to be proven, how can you know it exist if you didn't exist?
  7. Jul 16, 2013 #6


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    As phil said, there is no proof in physics, just a preponderance of evidence. The big bang theory is extremely well supported by observation. It, however, does not explain, or even attempt to explain how the universe originated, only how it evolved over time from a hot, dense state. In that sense, it is merely an effective theory.
  8. Jul 16, 2013 #7


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    That really has nothing whatsoever to do with the distinction between a theory and a hypothesis.

    A theory is an explanatory framework which makes predictions about relationships between different observations. The Big Bang theory is indeed a theory because it predicts, for instance, that there will be a redshift-distance relationship and that different estimates of distance, such as how far away galaxies appear on the sky and how bright things appear, will agree.

    A hypothesis is a single, testable statement, e.g. "If I measure the redshift of a group of far-away galaxies and also measure their distances, then that redshift and distance will follow this relationship."

    Hypotheses typically stem from theories, and confirming a large number of hypotheses generally leads to people thinking the theory is likely true. A huge number of hypotheses derived from the Big Bang theory have been confirmed.
  9. Jul 17, 2013 #8
    Thank you all for the replies. They are all great, except the one from "Xyooj", which was hilarious.

    If anyone has more to say about this, please feel free to do it :)
  10. Jul 17, 2013 #9


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    That is not a hypothesis, that is a prediction of that theory.

    I like the description Wikipedia gives:
    Hypotheses can become theories, if tests confirm it.
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