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Thread: Big bang physics

  1. #1 Big bang physics 
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    I understand that the big bang theory is based on the observation that stars and other galaxies are moving away from us and it's thought that all matter started off in an incredibly dense small space before rapidly expanding.

    I'm not sure if I've understood this correctly but I'm under the impression that it wasn't so much of a 'bang' and rather a sudden expansion of matter.

    If that is the case then why is this theory not relevant on a quantum level? Why is it only what we see in telescopes moving away from each other and not an expansion of matter itself?


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    Brassica oleracea Strange's Avatar
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    Quote Originally Posted by HarryTheHat View Post
    I understand that the big bang theory is based on the observation that stars and other galaxies are moving away from us and it's thought that all matter started off in an incredibly dense small space before rapidly expanding.

    I'm not sure if I've understood this correctly but I'm under the impression that it wasn't so much of a 'bang' and rather a sudden expansion of matter.
    That is (almost) correct. It would be more accurate to say it is an increase of space (distance) between matter.

    If that is the case then why is this theory not relevant on a quantum level? Why is it only what we see in telescopes moving away from each other and not an expansion of matter itself?
    The expansion of space only applies where there is a homogeneous distribution of matter. This is only (approximately) true at cosmological scales, thus we only see expansion at the scale of galactic clusters.

    The other way of thinking about it is that matter is held together by electromagnetic and gravitational forces and this is "stronger" than the expansion.

    p.s. welcome to the forum


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    Thanks for your reply and the welcome sir, I'm still curious though lol.

    If everything started in this tiny dense space and expanded into what we have now, why would matter stop at the loose vibration of particles we're left with today and everything on a cosmological scale carry on with it's process of seemingly eternal expansion? Especially due to the observes similarities in the quantum and cosmological world.

    Obviously it would never be something we'd be able to perceive but Is It not possible that matter itself is also expanding to follow suit?

    I'm thinking, If you were to mark a wheel at two points, one near the centre and one near the outer edge, the outer point would travel a much greater distance than the centre point. So (if I can use this theory), could everything be moving away at the same constancy but we only have the ability to perceive that at great distances from us?

    Sorry if my diction isn't fantastic, I only half know what I'm talking about.

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  5. #4  
    Brassica oleracea Strange's Avatar
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    Quote Originally Posted by HarryTheHat View Post
    If everything started in this tiny dense space and expanded into what we have now, why would matter stop at the loose vibration of particles we're left with today
    Sorry, I don't really understand what that means. Very early in the universe subatomic particles formed as the universe cooled and then these combined to form atoms (mainly hydrogen and helium). Later, gravity caused clouds of hydrogen to collapse which formed stars amd galaxies.
    More here: Chronology of the universe - Wikipedia, the free encyclopedia

    Especially due to the observes similarities in the quantum and cosmological world.
    What similarities are you thinking of?

    Obviously it would never be something we'd be able to perceive but Is It not possible that matter itself is also expanding to follow suit?
    If matter were expanding at the same rate as the universe then we would not be able to detect the expansion of the universe as our measurements would also be changing. (Imagine trying to measure the changing length of a piece of metal with a ruler that changes length in the same way.)

    I'm thinking, If you were to mark a wheel at two points, one near the centre and one near the outer edge, the outer point would travel a much greater distance than the centre point. So (if I can use this theory), could everything be moving away at the same constancy but we only have the ability to perceive that at great distances from us?
    Firstly, the two points on you wheel are always the same distance apart, so I'm not sure how that is relevant. Also things are not moving at the "same constancy"; the further away they are, the faster they are moving (an inevitable consequence of the distance between any two points increasing by a constant scale factor).
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    I am terrible with explanations but everything in my last post was me trying to get to one point so i'll try to clarify a little.

    If matter were expanding at the same rate as the universe then we would not be able to detect the expansion of the universe as our measurements would also be changing. (Imagine trying to measure the changing length of a piece of metal with a ruler that changes length in the same way.)
    I'm more curious as to the possibility of this rather than anything definitive.

    Firstly, the two points on you wheel are always the same distance apart, so I'm not sure how that is relevant. Also things are not moving at the "same constancy"; the further away they are, the faster they are moving (an inevitable consequence of the distance between any two points increasing by a constant scale factor).
    I apologise for the terrible example, this was intended more as a reference to the observation of things at a greater distance moving further but it won't be overly important if i can get my point across.

    I'll try this again;

    If every atom in the universe was expanding at the same consistency it obviously wouldn't be something we'd notice with our human perceptions of the universe but whilst observing planetary clusters at great distances would this not give the visual effect of moving away? Or would the whole thing be relative to our perception of the situation? If you can't see any relation between matter expansion and universe expansion then is it possible for both of these theories to work simultaneously?
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    If (for the sake of argument) the expansion rate was 1%/ day then something 1 metre away would be 1.01 m tomorrow, while something 1000 km away would be 1010 tomorrow: i.e. still 106 times further away.

    On the other hand: gravity depends on mass and radius, if Earth kept the same mass then an increase in radius would reduce the surface gravity (someone jump in if that's wrong) - unless we also postulate an increasing density at the same time then we'd definitely notice...
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    Quote Originally Posted by HarryTheHat View Post
    If every atom in the universe was expanding at the same consistency it obviously wouldn't be something we'd notice with our human perceptions of the universe but whilst observing planetary clusters at great distances would this not give the visual effect of moving away? Or would the whole thing be relative to our perception of the situation? If you can't see any relation between matter expansion and universe expansion then is it possible for both of these theories to work simultaneously?
    If atoms were expanding in the same way as the universe, then we would not be able to detect any expansion as everything would be scaled the same.

    Imagine you have a metal rod and a you want to measure how much longer it gets as the temperature increases. If your ruler is made of the same metal and expands by the same amount, then you will always measure it to be the same length. If (as you are suggesting) everything expands by the same amount, then you will never be able to measure a change in length.
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    If (for the sake of argument) the expansion rate was 1%/ day then something 1 metre away would be 1.01 m tomorrow, while something 1000 km away would be 1010 tomorrow: i.e. still 106 times further away.
    This is exactly what i was looking for. Thank you so much!


    On the other hand: gravity depends on mass and radius, if Earth kept the same mass then an increase in radius would reduce the surface gravity (someone jump in if that's wrong) - unless we also postulate an increasing density at the same time then we'd definitely notice...
    Would gravity not increase? I was under the impression that bigger planets would have more gravity, atmosphere depending obviously...
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    Quote Originally Posted by HarryTheHat View Post
    Would gravity not increase? I was under the impression that bigger planets would have more gravity, atmosphere depending obviously...
    Only because bigger planets typically have more mass. If you just increase the size, the mass will not change but the distance (radius) will, therefore gravity will decrease. (And atmosphere has nothing to do with it.)
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  11. #10  
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    Quote Originally Posted by HarryTheHat View Post
    Would gravity not increase? I was under the impression that bigger planets would have more gravity, atmosphere depending obviously...
    Bigger planets have more gravity because they have more material (that's why they're bigger!)= more mass (but it does depends on the density of that mass of course).
    Think of it this way: if the radius of the Earth increased but kept the same mass then, effectively, all you're doing is moving the surface into "orbit".
    You'd be at the same distance from the Earth's centre (which is where gravity actually "acts from") as, say, the space station is now, while the Earth's mass was still the same: ergo you'd experience the same gravity as the space station currently does.
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    If it was the atoms expanding is there no way the gravitons could also expand?

    I get all the points above but this would be the same as trying to measure an expanding environment from an expanding vessel so never something we'd notice or be able to calculate
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    Quote Originally Posted by HarryTheHat View Post
    This is exactly what i was looking for. Thank you so much!
    No problem. I've always found that plugging a few basic figures in illuminates the question. It makes it less abstract.
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    Quote Originally Posted by HarryTheHat View Post
    If it was the atoms expanding is there no way the gravitons could also expand?
    Couple of problems: gravitons are hypothetical (we don't know if they exist) and, even if they did, they're the particles that mediate the force of gravity, not "cause" it. But I do get your point.
    Yeah, but that's why I said that we'd have to postulate that there's a commensurate increase in density - which would mean that some value or other in some particle or other would also have to alter along with the expansion.
    The particle would, probably, be the Higgs (guessing), and what properties would have to change I have NO idea. And even less idea on whether or not we'd be able to detect that change.
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    So there's just not enough known about this? I think I'm a little out of my depth here tbh.
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    I had a similar thought... Instead of an expanding Universe, all the matter in the Universe suddenly shrunk. It's been steadily shrinking ever since the Big Shrink event... and the shrinking is now accelerating.
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    Not sure about "not enough" known so much as "what we currently know indicates that it's not the case: we can explain observation without invoking this sort of expansion" and "we'd have to introduce numerous other factors - other than simple expansion - for it to work".
    IOW if it IS the case and we can't (ever) detect that it's so, how much does it really matter when it comes to working things out?
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    Well alot of thing don't REALLY matter, there's just compfort in truth and isn't that what it's all about?
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    Quote Originally Posted by HarryTheHat View Post
    there's just compfort in truth and isn't that what it's all about?
    Yes and no. If we can't detect it then is it "really" the truth?
    Not being able detect something (in any way) means it doesn't have any impact (whatsoever) on us - if it doesn't have impact on us, is it "real"?
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    Well yeh, i suppose you're right but there's not conclusive evidence for alot of what's believed, the big bang is a prime example of this.
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    Quote Originally Posted by HarryTheHat View Post
    If it was the atoms expanding is there no way the gravitons could also expand?

    I get all the points above but this would be the same as trying to measure an expanding environment from an expanding vessel so never something we'd notice or be able to calculate
    The universe works as it does because of the constants. If you alolow for everything to expand, but not the constants, then you would see things starting to behave differently as time went on. We would notice. The same reason works for why the expansion can't be explained by proposing that matter shrinks.

    As to expansion being only at larger scales, the basic reason is that gravity and the other forces overwhelms expansion completely at smaller scales. You only start to notice it at galactic cluster scales. The more technical reason is that due to relativistic effects, there is no expansion force at all on smaller scales. Otherwise a constant amount of work would need to be done in order to keep matter clumped together. I asked the latter question and got a very involved answer from Markus Hanke and even he had to go search for it, so it is quite technical.

    Maybe he'd grace us with a nice layman's explanation?
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    Quote Originally Posted by Neverfly View Post
    I had a similar thought... Instead of an expanding Universe, all the matter in the Universe suddenly shrunk. It's been steadily shrinking ever since the Big Shrink event... and the shrinking is now accelerating.
    The "condensing matter" universe is an old idea, and has been discussed several times on this forum. I suppose the strongest argument against it is that neither the weak nor the strong interactions scale very well.
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    Quote Originally Posted by Markus Hanke View Post
    Quote Originally Posted by Neverfly View Post
    I had a similar thought... Instead of an expanding Universe, all the matter in the Universe suddenly shrunk. It's been steadily shrinking ever since the Big Shrink event... and the shrinking is now accelerating.
    The "condensing matter" universe is an old idea, and has been discussed several times on this forum. I suppose the strongest argument against it is that neither the weak nor the strong interactions scale very well.
    <Scraps his Nobel winning paper...>

    Yeah. I knew that!
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    Quote Originally Posted by Strange View Post
    The expansion of space only applies where there is a homogeneous distribution of matter. This is only (approximately) true at cosmological scales, thus we only see expansion at the scale of galactic clusters.

    The other way of thinking about it is that matter is held together by electromagnetic and gravitational forces and this is "stronger" than the expansion.
    I'm jumping way back in the thread, but it has suddenly occurred to me that the fact that electromagnetic and nuclear forces retain their strength is not enough to keep matter from expanding; quantum mechanics is involved also. At a minimum, the argument is a lot cleaner if QM is brought into the picture.

    For concreteness, consider the hydrogen atom. The size of the atom is given by the radius of the ground state, which is gotten by solving the Schroedinger Equation. This radius a0 is given by [formatting from Wikipedia Bohr radius - Wikipedia, the free encyclopedia]



    This formula involves Planck's constant, the electron charge and mass, and the strength of electromagnetism. None of these are directly affected by the expansion of the universe, so the size of the ground state of hydrogen need not increase as a result of that expansion. As has been pointed out, if the atoms expanded proportional to the expansion of space, we would never know that there was an expansion. Therefore we know without any additional measurements that those constants are, well, constant, at least on that scale.

    I vaguely remember seeing that the specific values of these constants are pretty vital to the existence of chemical compounds as we know them. I do know that efforts have been made to verify that the values of these constants have not changed over the years, and that decent limits have been placed on the amount of change that has taken place. So the immunity of individual atoms from direct effects of the expansion of the universe is well tied down.
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