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Cosmology Questions That Have Perplexed Me Over the Years

  1. Sep 13, 2010 #1
    Hi all, it's my first post here, and I'm glad to have found it. I'm a software engineer by trade, but I've always been immensely fascinated by cosmology, and I think it's my true passion (when I went to school, I was concerned that pursuing cosmology would not pay the bills, so the pragmatist in me decided to study computer science). Anyway, I try to get as strong a qualitative understanding on the subject matter as I can by reading popular books on the matter, and to make a long story short I've tried to keep a little list of questions I couldn't find answers for, so imagine how happy I was to stumble upon this forum! I've also included ones that I seem to have found answers for, so I'll include them as well since I'm sure I'm not the only one who thought up the question. I'll also update the answers section with information posted in the thread so future readers won't have to necessarily go through lots of posts. So without further ado:

    1) If Omega is greater than 1, that is, if there is sufficient mass such that the universe's expansion will one day slow and then reverse, what will happen to entropy? Will it continue to increase, and eventually our universe will be an ever expanding and contracting homogenous heat deathed nothing (if this is the case, then either certainly Omega is not > 1 by the anthropic principle, since the odds of us being in one of the first few "cycles" is infinitely improbable compared to us being in one of the latter cycles) or Omega is in fact > 1 but as soon as the expansion reverses, entropy reverses. Inhabitants of a contracting universe regularly drop broken eggs which hit the floor and turn into new unbroken ones. To them, the notion of a universe which tends towards disorder is as implausible to them as one which tends towards order is to us. Whereas things in our universe always decay and rot, things in their universe move from a state of chaos into one of order. Is this a logical conclusion?

    2) Our observable universe is a roughly 15 billion light year diameter sphere, but the observable universe is certainly not the entire universe (where the entire universe is defined as everything that was part of the original big bang). To us, the Earth is the center of the observable universe, and to an observer in the Andromeda galaxy, they are at the center of a slightly different observable universe (though there is considerable overlap).

    Here are two images to help illustrate what I'm trying to describe:

    If we, however, were able to teleport instantaneously to a quasar near the edge of our observable universe, say 14 billion light years away, then the earth would be on the edge of their observable universe, but they would also have a 7 billion light year radius sphere of observable universe that would share no overlap with the earth's observable universe. Given this, it would seem it is entirely possible that our observable universe could be 1/1000000000000000 of the actual universe. What ramifications would this have? Scientists currently believe that spacetime is flat, and thus our universe is infinite. They reached these conclusions by taking measurements on some of the farthest objects in our observable universe, but if our observable universe is an infinitesimal part of the entire universe, then their measurements would be like someone determining that the earth is flat by measuring from their house to their mailbox. If the observable universe is but a spec of the entire universe, then what, if anything, does that do to our understanding of the curvature of spacetime and of the age of the universe?

    3) Given that the universe is expanding and that the expansion is accelerating, at some point in the very distant future inhabitants of the milky way will assume that the milky way is the entire universe, since all other galaxies will be moving away from them so fast that no light from other galaxies will reach the milky way. Will inhabitants of the future milky way be able to accurately determine the age of their universe, and if not, wouldn't that mean that we are unable to accurately determine the age of our universe (since presumably there are plenty of galaxies more than 15 billion light years away from us that are no longer a part of our observable universe)?

    4) Is it possible to fall into a black hole? Certainly from the perspective of the person that falls into the black hole it's possible, and an extremely fast and violent death, but what about an observer far away from the event horizon, to them wouldn't my clock tick slower and slower and slower as I get closer and closer to the event horizon? And wouldn't my watch (from theirs and the rest of the universe's perspective) stop as I hit the event horizon?

    5) Why is everyone so sure that gravity travels at c? What ramifications would there be if gravity travels at either A) a finite speed faster than c, or B) infinite speed. If gravity traveled faster than c, it would mean that things outside of our observable universe could affect things inside out observable universe. Could this explain why the universe's expansion seems to be accelerating, could this explain away dark energy?

    1) So far, no good ones, seems to be an interesting thought experiment, I'd love some input.
    2) Apparently I'm not the first one to think of this (big surprise) and science is actually kind of interested in local geometry (the geometry of the observable universe) and global geometry (the geometry of the entire universe). That said, I am still unable to find anything which speaks specifically to my question.
    3) I think it's possible to determine the age of the universe without the need for distant objects, but I'm still unclear as to the correct answer to this question.
    4) http://www.xs4all.nl/~johanw/PhysFAQ/Relativity/BlackHoles/fall_in.html provides a thorough answer. I was kind of on to something. An outside observer will in fact NEVER see me hit the event horizon, but there will not be an image of me frozen near the event horizon since as things get closer to the event horizon, not only do they appear to slow down to an external observer, but they also dim. To an external observer, I will fade out into nothing long before I hit the event horizon.
    5) I did some research that seemed to talk about gravitational dampening that explained that the speed of gravity could not be infinite, though it left the possibility that it propagates faster than c. Still no definitive answer found for why gravity can't be faster than c.

    Anyway, thanks for your help with increasing my understanding physicsforums!
    Last edited: Sep 14, 2010
  2. jcsd
  3. Sep 14, 2010 #2
    1) No. First of all, it is quite clear now that big crunch is not plausible. Second, in crunching universe we would expect things to go their usual way. What would happen once when universe hits enormous density, and how exactly it would bang again is not known.

    2) Our observable universe is 46 Bly in radius. Apart from that, you are quite right. It could easily be that we are measuring curvature from "our house, to our mailbox", but since we can't see further from our mailbox, it is best we can do.

    3) Inhabitants of such future universe would have many, many observational constrains. It would appear to them that they live in static universe. Would they be able to determine age of universe? Probably yes. Fortunately (fortune or anthropic principle?) we live in times when there are plenty of evidence for us to determine age, and to track down our universe down to its birth.

    4) Answer you provided is right.

    5) Theoretical prediction of General relativity is that gravitational waves travel at C. Since GR is best theory of gravity we have, and since it gives right answers for very wide range of conditions, it is almost inevetable that it is right about that. Detecting g-waves, and measuring their speed remains holly grail of experimental physics.
  4. Sep 14, 2010 #3
    Thanks for your help! All this cosmology talk has made me decide it's been too long since I've read up, so I'm going to pick up Brian Greene's newest one tonight.

    Regarding 2), I guess there's no way for us to ever know how small a part of the universe our observable universe is. That's sort of a bummer!
  5. Sep 16, 2010 #4


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    Even if Omega>1, the universe could still be expanding forever, if there is Dark Energy.
    Entropy does not reverse.
    We don't know what replaces known physics in the case of a bounce singularity, and we don't know how the universe could start from such a low entropy state as it did. These are topics of current research.
    The infinite universe is a mathematical model. It might be infinite, or not. Most certainly it's vastly bigger than the patch we can observe.
    This has no influence on the curvature we measure here or on the age of the observable universe.
    3) Read http://genesis1.asu.edu/0308046.pdf" [Broken]. This is not comparable with our situation today: as long as we can clearly see the CMB, not much information concerning the evolution of the universe is lost.
    4) As Calimero said
    Because that's the only possibility that is consistent with physics as we know it. If you study relativity, you'll find that c is not just a speed, it's the ultimate speed.
    And even FTL gravity can't explain away Dark Energy, at least as far as I can see. Which is not very far, as in such a case almost all of physics would need to be rewritten.
    Last edited by a moderator: May 4, 2017
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