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What is the expansion of the universe

  1. Feb 10, 2012 #1
    Just thinking of Hubble's simple expansion of the universe or galactic recession (ignoring inflation).

    How is it possible or practical to say that the universe is expanding? For something to expand, it has to expand relative to something else (if not, then inertial frames would not be relative). But if everything was expanding, then nothing would be. You could always say either the whole universe was expanding, or shrinking or neither expanding or shrinking, and the effect would be the same.

    One teacher told me that it is just the space between the galaxies that is expanding, but that made little sense, since our galaxy would soon be crushed by the space that surrounds us.
  2. jcsd
  3. Feb 10, 2012 #2


    Staff: Mentor

    When scientists say space is expanding they mean the space between stars not the stars themselves. We measure speed of expansion by measuring the redshift of objects. In every direction we look for objects outside of our galaxy we see red shifts that are proportional to the distance the object is from us. From that we conclude that things are moving away because the universe is expanding. It's statistically small that everything we measured just happens to be traveling away from us of their own volition and so conclude that no the universe must indeed be expanding.

    Conversely, we could say that all matter is shrinking but that means we are being affected by the shrinkage and that might not hold up well mathematically. As we shrink so would our measuring sticks. Hence making it appear that other objects are moving away from us. So by equivalence we choose the universe is expanding option so that we remain invariant along with our measuring sticks.
  4. Feb 10, 2012 #3

    Your teacher should have qualified her statement. It's the voids that separate galactic superclusters which are arranged as filaments and walls surrounding these voids that is expanding. The space within supereclusters themeselves or regular clusters, or solar systems or anywhere else where gravity predominates isn't expanding. The effect is that the supercluster fillaments are becoming more distant from one another regardless of their individual proper motions. The effect does not appear to be a crushing one.

    About the expansion itself generating a logical paradox, you are right, it does. Especially if we dogmatically insist that the universe is infinitely large which means it is boundless. Which creates the paradox can something which is boundless or infinite in size actually increase? If it indeed increases then it wasn't infinite to begin with because to increase means to exceed former boundaries. If the universe is finite then that leaves room for an increase into something and permits such theories as the multiunivers and the brane theories to be viable.


    To assume that whatever is mathematically representable provides indisputable proof of its existence in nature constitutes a conclusion based on a false premise. There are presently many theories involving dimensions as part of the String Theory expressed mathematically and their existence via the scientific method is as yet improvable and might well turn out to be just as bogus as other once-popular mathematically viable ideas have.

    The problem arises when we confuse potential infnity, as in a number line, with boundless tansfinite spatial infinity. Potential infinity allows for the addition of more space just as a number line allows it. However, transfinite infinity does not. http://en.wikipedia.org/wiki/Actual_infinity

    Here are some links in agreememt with this conclusion:

    Last edited: Feb 11, 2012
  5. Feb 10, 2012 #4


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    you need to read up on infinity. there is no paradox at all.

    2 x infinity = infinity
    Last edited by a moderator: Feb 11, 2012
  6. Feb 10, 2012 #5


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    It is expanding relative to itself. The easy way to see it is to understand that when we look at the universe we see that all points in the universe are moving further apart as time goes on. From our point of view no matter what direction we look in we see galaxies moving away from us. The rate at which they recede from us is increasing as we look further back in time, as things that are further away are further back in time since the speed of light is finite. Using math and models we can view another galaxies point of view and it happens to be exactly like our own. From it's point of view everything in the universe is expanding away from it. This holds true for all points in the universe. Hence, the universe itself is expanding, everything is getting further away from everything else.

    Things like stars, planets, galaxies, and etc are not increasing in size. It is space itself that is expanding. The 4 fundamental forces of nature hold these things together as space expands.

    While saying that space itself is expanding is the most correct way of thinking about it, it may be easier to just understand that all objects in the universe are getting further away from each other. If you wish to consider the matter further, then you would need to understand that without the expansion of space far away galaxies would not be able to recede from us faster than the speed of light. (Which they are doing) It is also responsible for the redshift of light, as the expansion causes photons to increase in wavelength, making them "redden" This cannot be adequately explained without the expansion of space currently. The reasons why are fairly complex and thus we have an entire field called Cosmology devoted to understanding it.
  7. Feb 11, 2012 #6
    Infinity starts (no pun intended) with the Indians and the Veda. It is a created concept. As such it is entirely open to debate what it really means, and how it "works". Do you want to take Aristotle's view, or the mathematicians? It's a matter of taste. Neither has been conclusively proved right or wrong. My personal arguement tends to revolve around the notion of being able to actually create an infinite set of numbers.
  8. Feb 11, 2012 #7


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    It is not proven; it is defined. In mathematics, we define what infinity means. If we mean something else, we must be explicit in saying so.

    I am fairly certain that the Indians and Veda had their own (non-English) word for what they were describing, and there's actually no reason to suppose it is the identical concept to what mathematics defines as infinity.
  9. Feb 11, 2012 #8

    Ken G

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    I actually don't like saying that "what's really true" is that "space itself is expanding," that's really just a picture one can use pretty effectively (if we realize as said above it's only the space on the largest scales, not "between galaxies"). The theory we use to talk about the universe is general relativity, and in general relativity there is no such thing as space by itself, the correct manifold is spacetime. However, there is a particularly convenient way to coordinatize spacetime that relies on the "cosmological principle," and this coordinatization is so convenient it is almost always used. It makes a coordinate chart that matches the local average standard of rest, averaged over all the nearby galaxy clusters. In those coordinates, simultaneity is defined as sharing the same proper age of the local standard of reference. And in those coordinates, distances between galaxy clusters increases as a function of proper age. There is no "reason" for this, other than that it is the appropriate solution to the Einstein equations about how the metric evolves that determines what distances are. We really cannot say that "space itself is expanding", because there is currently no such thing as a model of space, such that someone could say if it is expanding or not. Maybe some of these issue will be clarified once we understand what dark energy is, but we should certainly be prepared for a big surprise.
  10. Feb 16, 2012 #9
    When they say "Universe is expanding", it is just another (confusing) way to say "Matter is contracting".
    Space is not expanding.
    Too bad "expansion of the universe" is now the standard language.
  11. Feb 16, 2012 #10
    Why? (And how?)
    Clearly this so-called expansion has to somehow vary continuously, so that the distance between proton and electron in a hydrogen atom does not expand at the same rate as the distance between clusters of galaxies.
    Does the space between Earth and Moon expand?
    Between Sun and Galactic center?
    Between Sun and Andromeda?
  12. Feb 16, 2012 #11
    You should familiarize yourself with Pi.

    Although that isn't a set I spose. Just a single number, with over 10 trillion decimals places (and counting).
  13. Feb 16, 2012 #12


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    Utter nonsense
  14. Feb 16, 2012 #13
    The space between the sun and Andromeda is shrinking. If it weren't for gravity the entire universe now would be like the space between galaxies with only a margial difference in density of the particles in that medium. It is the random fluctuations in particle density in early times, evolving through gravity, that produced planets, stars and galaxies and even anomalous motions such as the Milky Way and Andromeda with respect to each other.
  15. Feb 16, 2012 #14
    Think about it.
    If two galaxies shrinks in size without moving, they will look further apart, as measured based on their own size.
    So are the galaxies shrinking or is space expanding?
    Are these two exactly equivalent ways to describe the same phenomenon?
    Maybe, or maybe not.
    In any case, it is often useful to view things from a different perspective.

  16. Feb 16, 2012 #15
    Can someone please clear this up?

    If the space is expanding between galaxies, are the random wisps of gas and molecules also expanding? If one were to fly into such space, would you see giant hydrogen atoms flying around (at some point)?

    And why is recession the same thing as expansion? Why aren't the parts of space closer to us slam into us?
  17. Feb 16, 2012 #16
    Try not to read into the stuff about matter shrinking, I believe alphachap's posts are pretty speculative and out there, and aren't based on real science. Drakkith explained it in the best lay-terms I think. What you call the stuff in between galaxies is up to you, (it's typically called space by everyone that has a grasp of astrophysics, cosmology etc), and it is that space that is expanding/growing/increasing in size.

    Here, check out this thread:

    That has some more constructive information on the subject.
    Last edited: Feb 16, 2012
  18. Feb 16, 2012 #17


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    Absolutely positively unequivacably NOT
  19. Feb 16, 2012 #18
    What if the universe isn't expanding at all? Its possible that it could be an illusion that it is
  20. Feb 16, 2012 #19


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    Atoms, trees, planets and even galaxies are not expanding. Theses things do not grow larger. Nor does the space between these things grow larger. These things are bound by forces much stronger than the expansion of the universe (though do not think of the expansion as a force).

    The "expansion" is simply a matter of the distances between things increasing, and it only occurs where gravity and other fundamental forces are essentially zero - between galaxy clusters.

    Imagine gluing a bunch of pennies to a balloon, then blowing up the balloon. You would not expect the pennies to grow larger would you? We acknowledge that the forces holding a penny together are stronger than the force created by the expansion of the balloon.

    Even if you duct taped several pennies edge-to-edge (to make a 2D molecule, planet or galaxy) the expansion of the balloon is weaker than the duct tape. Several such clusters glued to a balloon would get further apart from each other, but the clusters themselves would not come apart at the seams.
    Last edited: Feb 16, 2012
  21. Feb 16, 2012 #20
    You might as well say modern Cosmology is an illusion too. Can you go into a little more detail on why you might think this?
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