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A question.

  1. Jul 15, 2004 #1
    So it's possible the universe could in fact be infinite and expanding? A singularity has an finite volume but infinite density, correct? Then isn't it possible for an infinite expanding universe to have an end? I recall reading the Theory of Everything by Hawking where he talked about how if you were able to travel at the speed of light and make it to the end of the universe you'd end up on the opposite side. Would this also work for an infinite expanding universe? Also, is it possible for there to be more than one singularity?
  2. jcsd
  3. Jul 15, 2004 #2


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    yes, it is certainly possible
    and probably the simplest picture fitting the data currently available

    no, not correct
    a singularity is a region where the equations of the model fail to compute---where they get infinities instead of reasonable answers----it can be a region which is infinite in spatial extent, or it can be a single isolated point, or a line, and so on

    a singularity represents a failure of the model and is a sign it needs to be fixed, or replaced by a better model

    (the big bang singularity has been eliminated using an improved model, which growing numbers of researchers are adopting---but one can still argue about which improved model one should adopt to the classical General Relativty model that has the singularity, i.e. the failure)

    many models of the universe are possible
    an infinite universe could certainly expand for a while and then collapse
    and experience a big crunch
    but with the data they now have cosmologists are not predicting that.

    since 1998 there has been a narrowing down of the range of different
    versions of the story---mainly because of the hubble space telescope and the microwave anisotopy probe satellite----a bunch of new unprecedentedly accurate data

    cosmology has become more of an observational science and less of the old wild speculative field it used to be, where everybody including Stephen hawking had his own pet model. There is a "consensus" model now, some people call it cosmology's "standard" model. The agreement may not last, and the mainstream folks might be wrong (one always must allow for that) but there is a remarkable amount of agreement at least for the time being.

    It is time for a new popular book, but i dont know of one. the cosmology consensus does not incorporate String theory, so a new popular book about the current mainstream vision of the cosmos would probably not be by Brian Greene. And it would probably not be by Stephen Hawking either.

    Sean Carroll could write one I guess
    Eric Linder has an undergrad Cosmology textbook
    Ned Wright and some others have great intro to cosmology websites
    Lineweaver's introductory survey article is still pretty good
    Anyone who wants can look at the PF Astronomy and Cosmology sticky thread that has a collection of links to cosmology stuff online
    including some I've mentioned.

    the field has in some sense gotten a lot simpler and easier to understand since 1998 or so, but a popular book hasnt AFAIK been written
    if you want links to online articles keep asking, I or somebody will get some

    hope this is helpful
  4. Jul 15, 2004 #3
    marcus, you say an infinite universe can expand...do you mean that a universe that is infinite along some dimensions/coordinates can expand along those others in which it is finite?
  5. Jul 15, 2004 #4
    limited universe

    This does not help much but -- when people talk about what may happen or could be I try to remember the following if you take serious astronomical measurement it's only a few hundred years old , so taking the earth as center we are surrounded by a sphere a few hundred light years in radius ( mainly the closest stars ) outside of which we have received no upto date info within that recorded history -- so in a sense ANYTHING could be out there and we would not know. Considering the age of the universe ( and size ) estimates of >100 billion light years , we are reading mail THAT OLD , until someone breaks the light barrier we will never know for sure WHAT is out there , it's extent, it's conditions, or anything else.
    In a light hearted scenario Andromeda may blink out tomorrow -- I don't suppose it will -- but for sure other things will a star which has disappeared down a black hole ( and is no longer in existance ) as of the 'now' our time.
  6. Jul 15, 2004 #5


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    Take the real line and mark units on it. Now make the transformation x => 2x. All the marks are now two units apart. This one dimensional universe has expanded; it was infinite before and is now still infinite, but people (dots) living on it will see the galaxies (marks) twice as far away.
  7. Jul 15, 2004 #6
    Yeah, sorry about that. I wasn't thinking of the right kind of "expansion".
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