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The Expansion of the Universe

  1. May 27, 2006 #1
    My friends and I just came across a discussion of the expansion of the universe. I am new to this subject and thus have limited background knowledge to this and would like to have a few questions clarified by the professionals here :)

    Is the universe expanding? I believe my friend stated that Einstein was the one who made the claim but how did he do so? What was some of the support?

    What are its effects, ie what will happen in the future? The big crunch? :confused:

    Those are the two main questions I have as of right now. Thanks for any help :biggrin:
  2. jcsd
  3. May 28, 2006 #2


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    Expansion of the universe is a consequence of the big bang theory [BBT]. Einstein is usually credited with discovering this feature of the universe [a general relativity thing]. According to his model, the universe had a propensity to either collapse or expand. Einstein, as did most other scientists of his day, assumed the universe was infinitely old and immutable. He therefore hand-inserted a 'cosmological constant' in an attempt to preserve the universe in a perfect state of gravitational equilibrium. He later recanted this ad hoc insertion calling it 'the biggest mistake of his career' after Hubble discovered that the universe is, indeed, expanding [redshift - distance correlation].

    The future of the universe, however, is still very much in doubt. It turns out the critical density [omega] - which determines whether the universe will expand forever, or eventually collapse - is too close to exactly 1 to make a confident prediction.
  4. May 28, 2006 #3

    George Jones

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    Second question first.

    Observational evidence strongly favours a universe that expands forever. Not only that, evidence that the rate of expansion is increasing, i.e., that the expansion is accelerating, has been accumulating for almost a decade.

    These observations also have something to say about the geometry of space for the universe. The standard cosmologica modelsl allow three possible geometries for space. Space has 3 dimensions, but the possibilities are easier to visualize if we imagine that space has only 2 dimensions.

    The first possibility is that space is closed, but without boundary. Imagine 2-dimensional beings that live on the surface of a large ball. Their space is finite, but they encounter no edges as they explore space.

    It is also possible, that space is flat and infinite, like the top of an infinitely large table.

    The final possibility is that space is open an infinite, like the surface of an infinite saddle.

    As I said, space actually has 1 more dimension than these visualizations, but the ideas are the same.

    In standard cosmological models, the average density of the universe determines the geometry: above a critical density, the universe is closed; at the critical density, the universe is flat; below the critical, the universe is open.

    Observations indicate that the density of universe is very close to the critical density - too close for the observations to pin down the actual geometry.

    As above, imagine a closed universe that has two spatial dimensions, but instead of a ball, the universe is modeled by an expanding balloon, with galaxies represented by (dried) daubs of glue on the surface of the balloon. The balloon expands, but the galaxies (daubs) don't.

    Galaxy A sees galaxy B via light that travels from A to B. Represent this by drawing with a marker an undulating wave between 2 daubs. As the balloon (universe) expands, the wavelength of the the wave increases with respect to the size of the daubs (galaxies and stuff like our metre sticks in a galaxy).

    Ned Wright's website has a http://www.astro.ucla.edu/~wright/balloon0.html" [Broken].

    This actually is observed as a cosmological redshift of light from distant galaxies. It is called a redshift because red light has a larger wavelength than does blue light.
    Last edited by a moderator: May 2, 2017
  5. Jun 7, 2006 #4
    Ive got some questions, why is it stated "the universe is expanding" when it can more simpily be put as the galaxies are spreading apart. Also, if the universe has a shape their must be space outside that shape that continues infinitly. If the universe is expanding yet is finite where does it expand too. Finnaly is their a proper universaly accepted definition of the word universe, ive seen it defined as all space as a whole and also as only the matter and energy in space. The root of the word is the old english word oino which means one or basic form, so this makes me believe that it is all space both used and unused and is what my questions are founded on.
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