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Nature of the Universe

  1. Jul 2, 2009 #1
    Hi,

    There is definitely overload if you try to look this up online, and you have to pay attention to the dates the sites were made, so I thought I would ask here:

    What is the latest thinking as to the nature of the universe. Infinite? Finite? Bounded? Unbounded?

    I am still a first-year physics student, so I know the topic is years ahead of me, but I was just curious.

    Thanks!
     
  2. jcsd
  3. Jul 3, 2009 #2

    Chalnoth

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    Currently unknown, and we're not even sure if the answers to these questions are knowable.
     
  4. Jul 3, 2009 #3

    Chronos

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    Unbounded looks safe for now. Infinite or finite is a more difficult question.
     
  5. Jul 3, 2009 #4

    marcus

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    My view is this: cosmo is a mathematical science. So you know something if you have a nice math model that fits the data. What you know is whatever it tells you.

    So for instance if the spatial finite model would fit the data significantly better than the spatial infinite, then you would say that (accord. to simplest bestfit model) the U is finite.

    Beyond that, talking about what is knowable is metaphysics.

    So every time a big batch of new data is released people inspect to see whether the finite or the infinite version of the model is favored. And what the best-fit U would look like in each version. The last big release was WMAP5 in 2008. The 5th year WMAP data.
    Google "komatsu WMAP cosmology".

    The error bar for overall curvature straddles zero, with about twice as much on the positive curve side, which would mean spatial finite. The slight leaning towards positive finite is not statistically significant. So one says that the U is nearly flat and that the data is consistent with it being flat infinite, but the data is also consistent with positive curved finite (a large hypersphere).

    Komatsu et al give a 95% lower bound on the radius of that hypersphere in the finite case. It comes to slightly over 100 billion lightyears---today's distance. That is, if you could freeze expansion and send out a flash of light the flash would make the circuit and get back to you in no less than 628 billion years. As I recall this is in Table 2 on page 4 of Komatsu et al.

    That radius and that circumference are expanding currently at a rate of about 1/140 percent per million years. So if you didn't freeze expansion the flash would never make it back. That's one reason distances are commonly expressed in today's distance--now or presentday distance--with expansion frozen.

    A new spacecraft was just launched this year, to join WMAP out at the L2 lagrange point, and be an improvement on WMAP. It is called "Planck". It will probably reduce the size of the errorbar on curvature. Maybe someday, if the 95% errorbar gets narrow enough, it will all be one one side or the other of zero. Then, in the sense that one knows stuff in math science, we will know the finite/infinite answer. Or have a better idea anyway.

    The most up-to-date wide-audience outreach material on the web is from an institute in Germany called Albert Einstein Institute---part of the Max Planck Institute (MPI) group. Their outreach website is called Einstein Online. Link is in my signature at the end of the post. Their page called "A Tale of Two Big Bangs" discusses current efforts to figure out conditions around the start of expansion by improving the model so that it doesn't break down and suffer from a singularity. That's a big focus of research now: what was really there instead of the singularity that develops in the old classic model.

    If you want to know the latest thinking about that, don't google (you get old stuff). Use the stanford Spires database with a cutoff like "date > 2006" and the keyword "quantum cosmology". Currently the most highly cited models show a bounce. Like I said before, with the simplest bestfitting model (that doesn't break down/blow up just where you want to look), what you know is what the model tells you. The new models of the universe run on computer and match existing data about as well as the old classical one. They still need to be tested---predict new phenomena that can be looked for to check to see if they are wrong.

    So the answer to your questions is that obviously you are asking good questions because: even though cosmologists know a whole lot of stuff and have a whole lot of errorbars on the parameters of the universe, they still don't have definite answers to your questions. We don't know finite/infinite. No scientific reason to believe one or the other. But we may know soon so keep on the lookout. We have no scientific reason to believe that time began at the start of expansion (because some models do not break down there but keep on going back in time, despite that the old classic does break and stops being applicable.)

    Here's the Spires search for topcited quantum cosmology papers. Mostly too technical for you at this stage, but glance at abstracts to get a taste.
    http://www.slac.stanford.edu/spires/find/hep/www?rawcmd=FIND+DK+QUANTUM+COSMOLOGY+AND+DATE+%3E+2006&FORMAT=www&SEQUENCE=citecount%28d%29 [Broken]

    Here's what you get by googling "komatsu wmap cosmology"
    http://arxiv.org/abs/0803.0547
     
    Last edited by a moderator: May 4, 2017
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