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Question about the edge of the Universe

  1. Mar 2, 2009 #1
    I know that there are several theories out there about how old the Universe is and how big, etc. I also know there are theories related to Mbranes (string theory?) that state that there are "pages" of universes that sit side by side (that's the easiest way for me to think of it) and that there are millions and millions of these "pages" of universes.

    My question is what twofold: 1. What are some of the current theories on where our Universe ends and related to that, what comes after the end of our Universe? 2. What are some of the current theories on Mbrane or string theory in terms of how these Universes line up? By this I mean, do physicists think the membranes or Universes sit one on top of another on top of another, etc. or are they "pages" sitting side by side by side making a blanket of Universes, so to speak?

    I also saw a physicists this weekend who theorized that the big bang, was really two of these universal membranes hitting each other, causing a fantastic release of energy and voila' our universe was born. My question would be, how do two membranes of "nothing" collide to cause "something?"
  2. jcsd
  3. Mar 2, 2009 #2


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    The current and only realistic theory is that the universe has no edge. It's geometry is such that it is finite(probably finite), but without a boundary. It is similar to how a 2d surface can also be finite but without a boundary....for example, the surface of the earth.
  4. Mar 2, 2009 #3
    Thanks Russ.

    So if it has no boundary, then is the theory that our Universe sits alongside countless other Universes? I guess my question is and always has been since I was a youngster, "Where is the beginning and the end of space?" If we are in a "sphere" so to speak, are there millions of other spheres of Universes holding this all together?

    It can really boggle the mind when you start to hypothesize, can't it?
  5. Mar 2, 2009 #4
    So does this mean the universe is almost like a sphere of space, of nothingess? And we are just matter suspended in said nothingness? But if it is such, then what is outside this finite space? What holds the universe together? What would happen if we were to transcend this finite space?
  6. Mar 2, 2009 #5


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    These are natural questions to ask, and I think almost everyone thinks of them first when conceiving notions of the universe. Unfortunately, they are the wrong questions to ask a cosmologist. The universe is defined to be everything that is, so asking what is beyond that is a moot point, wouldn't you agree? (bracketing multiverse theories) The question is akin to the famous example of what's north of the north pole... it just doesn't have any meaning...

    I'm sorry, but science cannot answer any of these questions. I can, however. And a giant turtle holds the universe together.
  7. Mar 2, 2009 #6
    Okay, I think I understand that. So it's akin to walking around the earth, trying to find the edge. No matter how far you walk, you will just end up right back where you started. Is this what you would say?

    Funny thing is, I've actually said this before. I've also made reference to Futurama by saying that maybe the universe is supported by Bender's ***.
  8. Mar 2, 2009 #7


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    Sure, in a way. The situation's aren't really analogous unless our universe is finite unbounded but the nature of the question is really what counts: there is no logical answer, because the question is meaningless.
  9. Mar 2, 2009 #8
    I was really just curious to see what the most recent theories were as they are related to our universe. Back to the multiverse theory....are they implying that each universe lies next to each other as papers would lie on a floor? Or are they theorizing that each universe lies on top of, or beside each other as you would stack papers in a file folder?

    Just curious.
  10. Mar 2, 2009 #9


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    This is much too big of a discussion to be contained within this thread (not to mention a little off topic!). I'm sure there have been tons of posts regarding this, so just look around in the cosmology forums for a while and I'm sure someone's posted something regarding the current theories.
  11. Mar 2, 2009 #10


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    If you want the most recent then you probably don't want multiverse ideas.
    Multiverse has been declining in popularity with the experts since about 2005-2006. It's hard to pinpoint when it began to lose favor.

    If you want an outward sign, look at the big annual international String conference.
    In 2005 it was held in Toronto. Lot of string landscape multiverse talk. They even held a straw vote, by raising hands, in the main hall after a panel discussion.

    In 2008 it was held in Geneva at CERN. Only invited speakers were allowed. There were 30 invited speakers. Nobody spoke about the string landscape or multiverse ideas.

    This year it will be held in Rome. We'll see.

    My hunch is based on personal impression. I'd say it has lost a lot of the interest it once did. The clashing branes scenario got some attention around 2001-2002 but not much has happened. If ideas don't go anywhere in the reseach world, they tend to die.
    One trouble with most multiverse ideas is testability. If you cant test something----force it to make a definite quantitative new prediction that you can then see whether it's right or wrong---then it's not science.

    Maybe there is a possibility some multiverse model can be forced to predict some new phenomenon, so it's worth working on, but generally the prospects of that are not great. That makes it fantasy, something for mathematicians to play with if they want.

    I think that perspective may be one reason interest has slacked off.

    In your original post you asked:
    You probably saw Paul Steinhardt. He and Neil Turok invented the brane clash scenario back in 2001. He is still pushing it but it hasn't really caught on. Your question can't be answered except in fantasy. It depends on exotic unobserved physics. You have to assume a lot of stuff to make it go. Since it hasn't caught on (except in places like Discovery Channel i.e. pop-sci media) you might as well not ask how it is supposed to work. You will just get BS answers.

    But if you really want to pursue it, Paul Steinhardt has a website at Princeton, with an introductory essay free for download. And he has a popular book promoting the idea, that you can buy. Some title like Endless Universe. Try it on Amazon.
    Last edited: Mar 2, 2009
  12. Mar 2, 2009 #11


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    Rob, I just had an idea for you.
    If you are interested in what currently hot topics of research in this general area of conditions around the big bang, the extreme conditions, the start of expansion and so forth.

    Then you can do a keyword search of the actual professional research literature. I or anyone can show you how to get a "Top 20" recent papers on any recognized keyword topic.

    The papers come ranked by the number of times they'v been cited as reference in other research, so it's a rough measure of importance.

    Quantum gravity is a keyword category where Hawking papers used to show up when his work was popular with the experts. Also Steinhardt's papers on brane clash cosmology when that was popular with the research community.

    You can set the date parameters to get past historical periods and see what was hot. Or set it to "date > 2006" to get only 2007 and later----the really recent stuff.

    The secret is to select "sort by citation count" on the menu. I'll show you how and you can experiment around with other keywords:

    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]

    If you change what it says in the window to "and date > 1990 and date < 2000" then you will get 3 papers by Hawking in the top 20 and also papers by other famous older names like Hartle, Vilenkin, Linde, Horowitz, Veneziano.
    None of those show up in the top 20 if you change it to, for example "and date > 2004".

    The interesting thing is that experts can change their mind about what interesting---about what the promising approaches are. The research community sort of "votes with its feet" by what papers they consider useful to cite in their own work.

    If you try it out please let me know if you have any questions or problems with it.

    DK says keyword ("help" will get you more keywords to try) and you can set the date up to be any interval you want.

    Another window on what's currently going on in cosmology is the Einstein Institute's public outreach website called Einstein Online.
    I have the link in my signature in small print at the end of the post.
    Last edited by a moderator: May 4, 2017
  13. Mar 12, 2009 #12
    Thanks Marcus. This is very helpful.
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