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Question about Big Bang Theory

  1. Dec 29, 2013 #1
    Hello Physicists

    First time poster. To give you some background I'm an electrical engineer so i have some basic understanding of physics up through relativity/quantum. I had a thought just occur to me that isn't very easy to google an answer for so i thought I'd come to the experts here. my attempt to articulate it follows:

    I understand the basic concept of the big bang theory. red spectrum doppler shifts in light indicates all matter in the universe is moving away from eachother. logically if you were to reverse time this motion would go in the other direction forcing all matter to get closer together. What i don't understand is how we can state with certainty that if time were to reverse for long enough(~14 billion years) all this matter would converge into a single pint of infinite mass and energy.

    Is it not possible that the universe expanded from a concentration of mass with a diameter much larger than the infinitaly small point at a more recent time? For example, how do we know that the big bang wasnt actually the expansion of a sphere of matter/energy already 2 billion light years in diameter and that the expansion started 12 billion years ago instead of starting from a single point 14 billion years ago?

    any explanation would be appreciated, this has been driving me nuts all day.
  2. jcsd
  3. Dec 29, 2013 #2


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    I assume your concept of the singularity is for TV-pop-science, since no reputable physicist would say that (but it is EXACTLY what they say on TV). The singularity was of undetermined size, possibly infinite but definitely not a point.
  4. Dec 30, 2013 #3


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    Actually this is the conclusion that you draw if you just reverse the evolution of the universe based on the laws of physics that we currently know. The whole point of the singularity is that it is a very strange and unphysical situation so rather than telling us that everything was concentrated into a single point all it does is tell us that somewhere around that time those laws of physics probably no longer apply and we need to find something better to explain what is going on there. We have already made quite some progress there - using quantum mechanics and general relativity we can already "look back" to a very early stage (about ##10^{-40}## seconds, if I recall correctly) but we just don't really understand the physical laws that dominate the weird world before that that well.
  5. Dec 30, 2013 #4


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    That is a naïve calculation that assumes linearity. If you assume it is asymptotic, it only approaches zero as time approaches zero. Our equations fall apart at around t = 5.4 x E-44 seconds.
  6. Dec 30, 2013 #5
    thanks guys! happy to her my intuition was correct.

    Is is a fair inference then, that because we don't know the dimensions of the original mass/energy that exploded, we cant know for certain exactly how old the universe is?
  7. Dec 30, 2013 #6


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    No, that is absolutely not correct. The size has nothing to do with how old it is. There are several ways of determining the age of the universe and they all agree that it is about 14 billion years old (I forget the exact number but it's now good to better than 2 significant digits).
  8. Dec 30, 2013 #7


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    Not even expert testimony in a court of law is "for certain exactly". Inference is probable, and approximate.

    In this case the inference of age is supported by a number of different independent observations.
    A number of different types of evidence cross-check each other like pieces of a jigsaw puzzle and fit together.

    the mix of wavelengths in the light from old stars, telling their chemical composition
    the mix of wavelengths in the ancient light of the microwave background
    the most tested and reliably precise law of gravity we have (GR) which describes the timetable of expansion rates and how distance change interacts with matter
    the remarkably close fit the GR model gives to galaxy counts at various distances
    computer simulations of how galaxies collect and grow and take shape which AGREE with the standard model timetable.
    when we look back in time we see patterns of structure formation agreeing with how much time we estimate is needed for that to happen--we see sizes and shapes and chemical composition of galaxies that the computer sims tell us to expect to be forming at that time
    and much else, many kinds of evidence fitting together

    There is a flood of data coming in, all kinds of surveys and counts, telescopes now are even automated so they can keep on feeding the databanks without needing human control.

    The data has a REMARKABLY CLOSE FIT to the standard model, based on GR law of gravity and expansion (and other types of geometry change). Gravity and expansion come under the same law--one simple equation governs both and has been checked in a variety of situations. The 14 billion year estimate of how long expansion has been occurring is only one aspect of the standard model.

    What we observe directly, the most ancient light, goes back to around year 380,000 of the expansion and fits together with remarkable consistency with a lot of other detail.
    That is before year 1 million.
    That is before year 1 BILLION.

    So your suggestion that the real expansion might have started in YEAR 2 BILLION and the real expansion might have been going on for only 12 BILLION years is kind of like suggesting that maybe there never were any dinosaurs and a mischievous FAIRY buried all those bones in the ground to FOOL US. :biggrin::rolleyes: It is possible (nothing in science, or forensic medicine or ballistics or any such) is perfectly certain or exact) but not very likely.

    However people are still working on explaining the start of expansion. It is an active area of research. "Explosion" might be a bad mental image to have in mind. Time and the universe may extend back BEFORE the start of expansion. We cannot say. We see lots of evidence that expansion began around 14 billion years ago, but we have no clear evidence that time does not go back farther. So considerable current research is aimed at modeling what could have LED UP to the start of expansion 14 billion years ago. The start of expansion might not have been the start of the universe. More may be unfolding.
    Last edited: Dec 30, 2013
  9. Dec 30, 2013 #8
    thanks, thats what i was hoping to understand, so we have definite time for when the expansion started, but we dont currently know how much time could have elapsed before the expansion began.

    i appreciate the info. i've satisfied my nerd curiosity for the day
  10. Dec 30, 2013 #9


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    That's a good clear way to express it! Researchers have only begun to scratch the surface of that question: what could the conditions have been just prior to start of expansion? what could have been going on for some time leading up to it? Can we deduce some traces of that theorized prior process which we can ask the observation-people to look for (with their microwave antenna satellites and other kinds of receiver instruments)?

    Theorists have several different competing models of what might have led up to the start of expansion. So different predictions of what to look for have to be derived from the different models, to make it possible to shoot down some of the models by NOT finding what they predict.

    It's going to be a hard grind for these people---both theory and observation types---for the next few years. I kind of envy them though.

    If anyone has time to glance at the titles of some of the "Quantum Cosmology" papers that have appeared since 2009, here's a keyword search listing.
    It can give a rough idea of what to expect over the next 4 or 5 year period. More of the same and hopefully some inch by inch progress. Quiet, and almost completely invisible to the mass media
  11. Dec 31, 2013 #10


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    Remarkably, many loop quantum papers deal with inflation, by this replacing the inflaton field assumption by something more fundamental, perhaps one can say by a property of spacetime (on the Planck scale) itself.
    Mainstream of quantum gravity seems still string theory, in said context with the competing model called brane inflation.

    I am not very optimistic regarding progress in this research. And supposed once LQG and string theory present selfconsistent models explaining the same aspects of nature? Are they mathematically equivalent then?
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