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

B Confusion about the Big Bang

  1. Feb 20, 2016 #1
    On just about every thread concerning the Big Bang, beginner questions are asked which other members consider to be unfounded. Hopefully this thread can serve as a central point to help beginners understand this issue correctly.

    1. Where did the Big Bang happen and how far away is Earth from the location of the Big Bang? Shouldn't there be a location in the universe away from which all matter is traveling?

    2. If there was a singularity that exploded 13.7 billion years ago, and if the universe is infinitely large, how can matter be distributed uniformly across the universe (as the cosmology principle would dictate)? How could there be any matter 100 quadrillion light years away from the Earth if it has had only 13.7 billion years to travel?

    3. How should we envision this singularity that existed prior to the Big Bang? Was it just a pinpoint of infintely densely packed matter surrounded by a universe of otherwise empty space? Or did the explosion of this singularity actually create space and the universe and was never surrounded by anything, meaning that there was just nothing. That concept is difficult to understand.
  2. jcsd
  3. Feb 20, 2016 #2


    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper
    Gold Member
    2017 Award

    It occurred everywhere. It did not occur at a point in space, it is the beginning of the Universe itself and the expansion following is an expansion of space itself, not the expansion of matter into pre-existing space.

    Again, this is founded on your original (and unfounded) assumption that the Big Bang occurred at a single spatial point. It is an unfortunate and common misunderstanding arising from the BB being referred to as an "explosion" in popular media.

    You should not. The singularity is not really a part of the model, it is a place where the model breaks down mathematically. The Big Bang itself is mainly the theory of how the Universe evolves after that point.
  4. Feb 20, 2016 #3
    Just a correction , the big bang is not the beginning of the universe, it represents the beginning of our ability to describe the eovlution of the universe. The true age of the universe is unknown, we can say it has been expanding from a hot dense state 13.8 bio year ago, before that we dont know what was happening.
  5. Feb 21, 2016 #4
    Ok so time and space existed prior to the Big Bang? Some people are claiming that the Big Bang created time and space. If that is true, was space infinite at the very moment of the Big Bang?
  6. Feb 21, 2016 #5


    User Avatar
    Science Advisor
    Gold Member

    Big Bang theory can only tell you something about Big Bang, and not about anything that lies outside its domain of applicability.
  7. Feb 21, 2016 #6
    I'm asking about the Big Bang. Did it create space and if so, has space been infinite since the Big Bang happened?
  8. Feb 21, 2016 #7


    User Avatar
    Science Advisor
    Gold Member

    Big bang theory doesn't include a creation moment. Its applicability doesn't extend past a certain point in time. You're asking what the theory says about a hypothetical event that it doesn't have anything to say about.

    By itself, it also doesn't tell you anything about whether space is or isn't infinite - it can accommodate both, but determination of (in)finiteness is a matter of observations.
  9. Feb 22, 2016 #8


    User Avatar
    Gold Member

    You are asking about the big bang singularity, which is not the same thing as the big bang theory. As has been pointed out, we don't KNOW what was happening. "Singularity" does not mean a point, it means "the place where the model breaks down and we don't know what was going on".

    The universe may have been infinite at the beginning, in which case it is infinite now, or it may have been finite but unbounded in which case it is still finite but unbounded.
  10. Feb 23, 2016 #9
    The confusion arises because the term "big bang" is not well defined. I think its false to say the big bang theory does not include a singularity, there are different ways to define big bang. One way to define it is to say its the theory that says the universe evolved from a hot dense state. This is the definition I think i and most people on this board prefer. But some people do define it as an expansion from a singularity. This essay may help you:
  11. Feb 24, 2016 #10
    Yes, but 'the singularity' still is just a placeholder name for the state of the Universe at a point where extrapolating back in time no longer works because the math produces nonsense results such as infinite density.
    The singularity is not a 'thing' with certain properties.
  12. Feb 24, 2016 #11
    The singularity does not necessarily mean a point, it just means that space was contracted infinitely (or nearly so.) Instead of trying to imagine the big bang, which is difficult because it forces you to imagine everything coming from a point, try imagining it backwards. What happens if you take an infinite universe and compress it a trillion times? A googol times? A googolplex times? No matter how much you crush an infinite universe, it's still infinite, it's just denser.

    This is the best way to describe it: "Our whole universe was in a hot dense state..." I find that very accurate because it never says it was a point, the OBSERVABLE universe was smaller, if the universe is infinite, the universe was still then.
  13. Feb 24, 2016 #12


    User Avatar
    Gold Member

    Why is the word "necessarily" in this sentence? :smile:
  14. Mar 7, 2016 #13
    Is it sound to think of time as a series such that we just define the earliest stages of the universe to have been caused by the state of the universe just prior to a particular point of interest so all the prior states stack up linearly ( or not) closer and closer together for infinity...but still finitely ie bounded like a converging infinite series in basic math.

    That way we can have a cause for the universe ie the state if the universe just prior to an arbitary time "t" was caused by the universe at infinitesimal time t- precceding t by arbitarily small amount and so on...mathematicians have no problems with such infinite but converging series.

    Seems to get around the whole first cause and what happened before time started type conundrums.
  15. Mar 7, 2016 #14


    User Avatar
    Gold Member

    No, this seems to me to be the same mistake that is the foundation for "Zeno's Paradox".
  16. Mar 7, 2016 #15


    User Avatar
    Science Advisor

    If you are only contemplating times t where t > 0 then convergence is not an issue. The state at any time t > 0 depends only on historical states 0 < t' < t. In particular, the limit, if it exists, is irrelevant because no state evolves from the limiting state. As you say, this eliminates any need for a first cause or for anything "before time started". And, as you say, we can come up with a meaningful number for the limiting time (e.g. t=0) even though that time coordinate is not mapped in the manifold we use to describe the universe.
  17. Mar 28, 2016 #16
    How would you be able to describe BB as a point? As a part of the energy dilating out from Big bang, I would have an inside view looking out in every direction of space from the hot dense state, the same way we view the CMBR now, not an outside view of a hot dense point, like the view of our sun.
  18. Mar 28, 2016 #17
    You don't, the big bang can not be described that way neither logically nor mathematically.

    You are exactly right, if you were inside that hot dense early universe, you'd be inside what would become the CMBR. Your concept of an outside of the universe is fundamentally flawed. Here is why: our observable universe is currently about 100 billion LY across (I rounded up), so say we crush the entire universe by a factor of 100 billion. How big is the universe then? 1 light year? Nope, all of the matter of the observable universe has been crushed into a much denser space... but so did the rest of the universe that we can't see. Crush it by another factor of 100 billion, so now everything we see is crushed into a mind-blowingly dense 60 miles. How big is the universe now? Still infinite. There is no way to crush an infinite universe into an infinitesimal.

    They say to imagine the expanding universe like a balloon, but I don't like that analogy because while it demonstrates expansion, it gives the illusion of an actual physical size. It's nonsensical to discuss the universe in terms of size, at any point in the past, present, or future. Most current models assume that the volume of universe is, always has been, and forever will be, infinite.
    Last edited: Mar 28, 2016
  19. Mar 28, 2016 #18


    User Avatar
    Science Advisor
    Gold Member

    Not quite. The standard model today allows closed, flat and open geometries and observations have not ruled out a closed, finite, yet unbounded universe.
    It is just understood that it must be much larger then the observable universe, but we do not know by how much. It might however also be flat or open, in which case it is thought to be be infinite in extent.
  20. Mar 30, 2016 #19
    _Misconceptions About the Big Bang_: a very cogent and accessible paper originally published in Scientific American in the March 2005 issue. This paper is authored by Charles Lineweaver and Tamara Davis and deals with the issues discussed in this thread and many others.


  21. Mar 30, 2016 #20


    User Avatar
    Science Advisor

    Also, this Insights article might be of interest. A bit more accessible than Davis and Lineweaver.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted