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Universe = size of grapefruit?

  1. Oct 30, 2009 #1
    I have read in many places that the universe was once smaller than an atom. However, many people here have said this is a 'cartoon' version of the big bang theory. And that the media have popularized this invalid notion...

    I now have some questions, can I have the 'true' answer (what we know now) and not the cartoon answer please :)

    1. Was the universe the size of a grapefruit at some point in the distant past? (some say the observable universe was, but if the observable universe is isotropic, then surely the whole universe is basically the same? And therefore the whole universe must have been small in the past)

    2. What is the shape of the universe?
    (I've read that it is probably flat)

    3. Is the universe infinite? (If the universe is 'space' there surely can't be an infinite amount of it? but I guess the universe is space-time, and time in theory can go on forever?)

    4. Why isn't the space between me and my bed expanding? Why isn't the space between my atoms in my body expanding?

    5. Is my body made of dark matter? What is made of dark matter? (I've read that dark matter makes up empty space, but my body has empty space etc...)

    Sorry if my questions are dumb. I would just like to get a better understanding of what the general concenus among scientists is.

  2. jcsd
  3. Oct 30, 2009 #2


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    The observable universe once had the size of a grapefruit. The total universe may be much bigger or even be infinitely big. iIf it's infinite now, it was always infinite. We simply don't know for sure, as it is not observable.
    The shape of space, as defined in cosmology, is flat. That does not mean 2D, it means 3D euclidean.
    Your questions always meant "space", not spacetime. Space can be infinite. Spacetime most likely is, as we expect the universe to be infinite in time.
    Expansion is not a physical effect, it's rather an initial condition: If the distance between you and your bed was expanding in the past, it will continue to do so. If not, not. Some, but not nearly all, call such behaviour "velocity".
    If there is either repulsive or attractive material between you and the bed, or if you and the bed gravitate, this motion will change with time.
    You can extrapolate the scenario to the whole universe, where it may be advisable to drop the "velocity"-notion and talk about expanding space instead. At least in a closed universe.
    Not if you mean cosmological DM. DM is thught to be kind of a gas, which can pervade eveything. Neither does it make up space nor is evenly distributed in space.
  4. Oct 30, 2009 #3
    I'd slightly differ with some of these questions to Ich, in that the space between you and your bed is expanding due to cosmological factors, (in a general uniform sense, ignoring the idea that we may experience less expansion being in a galaxy cluster with a local density greater than average), but your bedroom has internal binding energy. This is a key point and the reason that the universe can expand by a factor of a thousand, but the atoms contained in the universe are no larger than when they first formed from protons and electrons. The forces at work governing the minimum energy radial position for an electron to orbit a proton are no different, and if you have a little bit of expansion, then the electron may be shifted out slightly, but it'll just fall back in. The main objects that are affected by the expansion are photons, who, having no internal binding energy, are stretched mercilessly by the expansion and are now about 1000 times longer wavelength that when they began travelling at decoupling early in the universe's life.

    Also, Your body is probably not made of dark matter, though neutrinos are inside your body now and are considered hot dark matter (due to their speed), a lot of dark matter is in space. You are not dark matter because you are visible- dark matter by definition couples to gravity but weakly or not at all to electromagnetism, and a lot of it is probably just cold dust. There are some unknown ideas, that it could be super symmetric particles or heavy neutrinos, but there isn't much evidence for this. Dark energy is the one you should watch out for!
  5. Oct 30, 2009 #4


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    There are the gravitational effects I described (DE and matter between you and the bed) which provide relative acceleration. Besides that, expansion is a matter of initial conditions.
    If there is no DE, you need no force to keep the distance between you and your bed from expanding - if both start at relative rest. (I won't discuss this issue here further, as it makes a lot of people very angry and is widely regarded as a bad move.)
  6. Oct 30, 2009 #5
    Then expansion should raise temperature of matter, or not?
    Also we can build device to extrapolate energy from expansion. It should probably be larger then solar sistem to give us some resonable amounts of power.
  7. Oct 30, 2009 #6
    I don't know anything about the thermodynamics of cosmological expansion although I would agree that if something is expanding the universe, they are doing a hell of a lot of work separating galaxy clusters, unless I am missing something.
  8. Oct 30, 2009 #7
    My guess is that if expansion suddenly stops, electrons would just shrink their orbits just a little bit. I doubt that they are constantly "compensate" for expansion, if they do they would be performing work.
  9. Oct 30, 2009 #8
    They are not constantly compensating, the idea is that as space expands, the electron is displaced from equilibrium momentarily. The expansion is doing to "work", not the electron.
  10. Oct 30, 2009 #9
    But how can it be infinitely big? If the universe is infinite how can it make sense to say the universe is expanding?

    If something is infinite surely it doesn't need to expand, since it cannot get any bigger? And if it is infinite, surely we cannot know if it is expanding?!

    I read from NASA that the universe probably is flat within 2% margin of error (according to results from WMAP)

    What does it mean for the universe to be infinite? Does it mean that one could always keep adding particles?

    Even if we theoretically 'freeze' the expansion? Will it still be infinite?
    Also, does it mean there are also an infinite number of atoms in the universe? Infinite number of human beings?

    I don't understand 'infinity' whatsoever ;) It seems infinity only exists in mathematics and not in reality.

    Too many questions, sorry :P
  11. Oct 30, 2009 #10
    Think about the set {...-2,-1,0,1,2,3...}, "infinite" in size. What about the set {-4,-2,0,2,4,6...} ? Twice as large spacing between elements. How about {...-2*a(t), -a(t), 0, a(t), 2a(t), 3a(t),...} ?

    Do you agree that if we increase a over time, the set will always be infinite in size, but more distantly spaced? In cosmology a(t) is a useful factor to describe the relative sizes of the universe at different times.
  12. Oct 30, 2009 #11
    Yes, but this is a kind of concept which exists in the mind. It doesn't describe something that exists in reality.
  13. Oct 30, 2009 #12
    At this point, infinity does only exist in mathematics and not in "reality". When an infinity pops up in a theory the scientist takes it to mean the theory is incomplete. Like GR with quantum effects. A proper theory of quantum gravity would get rid of those infinities. Smooth them down to the finite. And if the universe were infinite, it would take an infinite amount of time to figure out that it was infinite. Could we ever really be sure if the universe was infinite or just really, really big?
  14. Oct 30, 2009 #13
    I don't think so. If they are instantaneously displaced back and forth then there is infinite work, if the space expands with no displacement, where is then the limit when expansion catches particles. I think that equilibrium is with expansion already accounted for, or that possible electron orbits are not exactly the same with or without expansion.

    And yes, infinite number of atoms, or infinite mass is a problem. Not for expansion, as MikeyW pointed, but for many other things.
  15. Oct 30, 2009 #14
    And that of course means, that if expansion would accelerate many orders of magnitude, that it could be capable to rip appart every atom in universe.
  16. Oct 30, 2009 #15
    Then it seems that we don't know much about the Universe :P

    I don't know why anyone would say the universe is infinite though. Since it seems infinite cannot exist in reality. Shouldn't all scientists say 'our current theories are not compatible' or something to that extent...

    And yes, I think if our technology improves to be incredibly good in the future, then we perhaps could somehow infer its approximate size.
    Last edited: Oct 30, 2009
  17. Oct 30, 2009 #16
    From what I know, scientists don't like infinities and when they pop up they are seen as an indicator of a flaw in the theory.
  18. Oct 30, 2009 #17
    Cosmological principle states that universe is, on largest scales, homogeneous and isotropic, and same for any observer.
    There are two ways to achieve that. If universe is flat on largest scale, then it must be infinite to look same for every observer.
    Other way is that it is slightly curved, and which every way you go it will look the same, but it wouldn't be infinite in content.
    From observations carried so far it appears to be flat, but there is always possibility that it is so slightly curved that we can not detect that yet.
  19. Oct 30, 2009 #18
    But this 'infinite', it seems it cannot really exist...
    Are you saying it is infinite because the expansion is faster than the speed of light? (in the flat universe)

    Surely it's a flaw in the current physical model, which is leading to this infinite
  20. Oct 30, 2009 #19
    Well I think that is why people hope that eventualy it will turn out that it is curved. If not, it would mean, among the other things that there are infinite number of me and you discusing is it, or is it not infinite.
    There is also a third possibility which is HIGHLY unlikely, and that is, that universe is flat, finite, but somehow we have special position in it, so it appears same in every direction, just for us.
  21. Oct 30, 2009 #20


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    I think almost everytime philosophers stated that something cannot exist, nature made clear that it is not philosophy. Don't tell the universe what to be.
    That said, we actually know something about the observable universe and, by definition, nothing about the unobservable. If you have a problem with infinity, think of it as finite. It doesn't matter, at least not until we get a handle on the yet unobservable, too.
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