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Hypothetical Question

  1. Nov 11, 2007 #1
    Let's assume that the universe is currently about 156 billion light years across. Suppose (I know it is impossible) that we somehow devised the ability to travel much, much faster than the speed of light and were able to reach the "edge" of the expanding universe. What would happen as we continued to travel beyond this point? Would we simply reach empty space and continue to see nothing for all time thereafter, or might we possibly "see" a state of the universe that was in existence before the Big Bang?
  2. jcsd
  3. Nov 11, 2007 #2


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    Perhaps there is size but no edge. Perhaps this is like asking "Let's assume the Earth is 40 thousand km in circumference. What if I travelled 40K to the edge. What would I see?"

    Just a guess.
  4. Nov 11, 2007 #3


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    [edit: drunken math error, nevermind]
    Last edited: Nov 11, 2007
  5. Nov 11, 2007 #4


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    Funny you should ask that question as I was just contemplating that very topic - the expanding universe.

    First of all...IF we could reach the theorized outer edge of the known universe I believe many would speculate that we might simply find some of the every elusive dark matter. But "no" we would not be able to travel into it.

    Oddly enough we would be like the early explorers who thought if they went too far they would reach the edge of the flat earth and have to turn around or risk falling off.

    Personally I don't think there is an end or edge anything like that and I think there is more to this expansion theory then many are willing to admit or can grasp just yet, or maybe they just haven't taken the time to think about it. After all if we can prove the expanded universe why hasn't it change our travel time to say - the moon??

    Maybe only the smaller units within the universe are expanding as whole units?
  6. Nov 11, 2007 #5


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    why do you assume there is an edge, ColdFusion?
    in the standard cosmology picture there is no edge or boundary, and there is nothing outside the universe.

    what they call "expansion" (for lack of a better word) is simply the ON-AVERAGE increase in largescale distances

    in the model, small distances like "from here to the moon" or "the length of this yardstick" do not increase. they are determined by forces like gravity, molecular bonds, crystal spacing in metals---so they don't take part in the expansion of large distance between unconnected things

    only very large distances, between unbound things (e.g. not in orbit around each other or connected by steel rods etc) increase, and they are currently doing so at an average rate of one percent every 140 million years.

    nothing in this picture suggests to me that there is anything OUTSIDE the universe, or that it has some sort of boundary or edge. So I wonder where you get that idea? why should there be one?
  7. Nov 11, 2007 #6
    Perhaps I should have been more clear. I am referring to where the outermost remnants of the big bang exist, not a physical edge of the universe. If we travel in one direction for an infinite amount of time, what would we eventually see?
  8. Nov 11, 2007 #7

    Chris Hillman

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    Body image?

    Assuming current mainstream cosmology?

    Your "celestial sphere" would have pretty much the same appearance as you had if stayed right here for time T.

    Also, ditto tony (who was being sarcastic) and Marcus (who wasn't). Marcus's point about average appearances being the same everywhere is incorporated into my comment just above.

    Some caveats:
    • Details of where some "nearby" galaxies appear might change, as you'd expect.
    • Since this is a hypothetical, then assuming your velocity wrt the galaxies is relativistic, your "celestial sphere" would be distorted by a hyperbolic Moebius transformation; galaxies would appear somewhat crowded and blue in front of you (in your direction of motion) and somewhat sparse and red behind you.
    • Since this is a hypothetical, then assuming you are biologically immortal, if T is very long, in either case (stay at home or travel), no matter would remain within your observable universe, so you'd just see your own fingers and toes!
    • If T is even longer all matter would have decayed so you can't be physically immortal, and as we know physics trumps biology :wink:

    No cosmologists would so speculate, since you appear to express two misconceptions.
    • "dark matter" is currently thought to be associated with ordinary matter in ordinary universes (but to extend outside each galaxy in a kind of "halo").
    • the "absolute past" of an event (aka "observable universe") is not a physical "edge" you can travel to or pass beyond. The textbook by D'Inverno has a good discussion.
    Last edited: Nov 11, 2007
  9. Nov 11, 2007 #8


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    You were clear, C.F. You have a misconception about what modern cosmology says

    in the picture we get in contemporary cosmology such a place does not exist.

    the "big bang" is not what the unfortunate words might suggest. It is not an explosion of some material exploding out into empty space. There is no surrounding empty space, in the standard picture.

    what we are talking about is an expansion of SPACE ITSELF together with the matter distributed more or less uniformly in it

    this has been answered by several of us, but I will recap
    if space is finite (that is, space and the matter uniformly distributed in it) then it is analogous to a sphere surface and has no edge or boundary, and if you travel unrealistically fast you eventually come back to starting point----so you eventually see the start

    this is assuming you travel very very fast so that we can neglect the fact that space is expanding while you travel.

    if space is infinite (with matter more or less uniformly distributed in it according to the usual picture) then no matter how fast you travel the universe looks the same as it would be looking at home. but you never get back home.

    the man who made up the phrase "The Big Bang" hated the big bang idea and believed in his baby the Steady State picture. in his anger and contempt he made up a misleading phrase which caught on with journalists and has confused lay people ever since.

    the event we are talking about must not be pictured as a comic-book explosion puff-cloud in the middle of empty space.

    the event is the beginning of the observed expansion of space (that space being uniformly occupied by matter)

    about conditions at or immediately before the start of expansion, people have different models
    (a one that is increasingly common nowadays is that the expansion was preceded by a contraction, sort of a mirror image, and that when a certain critical density was reached space stopped contracting and began to expand.
    but there are several competing pictures of how the expansion started and no one clear favorite.)
    Last edited: Nov 11, 2007
  10. Nov 11, 2007 #9


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    The point you made was valid. But I was wondering where that number came from :)
  11. Nov 11, 2007 #10


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    Current models do not have any "edge" to the universe. See for instance http://map.gsfc.nasa.gov/m_uni/uni_101bb2.html

    Note that there is some confusion about this point - for instance, this is one of the fine points that the recent History Channel special on the Big Bang got wrong, deciding to popularize the Big Bang as a "primeval atom", which is not quite right.

    Now, there is a subtle issue here, in that we can't actually test current theories to see if they are right or not, because we can't travel faster than light to go see for ourselves. So the proposed experiment which is supposed to answer the question isn't even theoretically possible, muddling the waters.

    The reason that current theory does not predict any sort of edge is due to the fundamental asssumption of what is called the cosmological principle - which is the idea that the universe looks the same from everywhere. This cosmological principle is inconsistent with the idea of an edge - things look different at the edge than they do from the center.

    So the cosmological principle would say that if the universe is infinite now, it was infinite at the big bang.

    It's possible that the cosmological principle is wrong, and that the universe does have an edge, but this starts to drift off into realms of unobservable speculation.

    Note that a GR solution to a universe with an edge would be a white hole - it would have a FRW interior joined to a Schwarzschild exterior, as described in The sci.physics.faq "Is the big bang a black hole

    I will give a longish quote from the above, but I would still encourage people to read the entire original article.

    Note that with the existence of a cosmological constant / dark energy, the idea of waiting billions of years to tell if there is an edge won't necessarily work.

    So to conclude:

    Current models do not have any sort of "edge" to the universe. This implies that if the universe is infinite now, it was infinite at the time of the big bang.

    Our current models could be wrong, but it may not be possible even in principle to distinguish (with experiments that are possible in principle, (i.e. no FTL) between a finite universe with an edge sufficiently far away, and an infinite universe without an edge.

    In addition, the universe could be finite and curved - people are looking at this possibility (google for "Circles in the sky").
    Last edited: Nov 11, 2007
  12. Nov 11, 2007 #11
    I should begin by saying that I am legitimately looking for an answer...not so much posing an argument...

    If the big bang was the expansion of all space and matter, then does that mean that "nothing" lies beyond it? I have an extremley difficult time fathoming this idea of "nothingness". Can someone please explain to me the theories of what this may be? Although I do recognize that we may not have such theories yet, as it's obviously and probably beyond human perception...
  13. Nov 12, 2007 #12


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    We are already at the edge of the universe. Everything we see is 'younger' than us. No need to travel across the universe to see that. Even if ftl were possible, it would change nothing. Time would speed up in our direction of travel and run backwards behind us [we would overtake photons emitted in our past]. Once we stopped, we would still be the most ancient object in the universe - and fried to a crisp.
  14. Nov 12, 2007 #13


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    Hillary it sounds like wordplay what you are saying.

    what I have to say seems to me just simple common sense

    if the universe is everything then there is not anything else besides it.

    that is, there is nothing besides it

    so what's the problem?

    when you say beyond, in what direction are you pointing?

    there is no need for you to put quote-marks around the word nothing, so why do it?

    there is no need for you to imagine a nothing or visualize a nothing because a nothing is not anything-----it is nothing you need to understand or visualize

    so I don't understand your problem, Hillary.
  15. Nov 12, 2007 #14


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    Marcus, I think Hillary is merely puzzled and curious. We have all struggled with questions like this at some point in our lives.
  16. Nov 12, 2007 #15
    So many misconceptions...

    Firstly I'll point out that some of what I have to say is not quite the interpretation put on things by most cosmologists (although I would certainly claim that it is theoretically correct), and so if you want a more standard interpretation I'll refer you to Ned Wright's Cosmology Tutorial

    So the 156 billion light years. Assuming that the universe is infinite, not all of it is in our past light cone. As time goes on more and more of it comes into our past light cone. The boundary is known as the Particle Horizon. Note that when stuff comes into the past light cone we would see it (if we could) as it was just after the big bang. However, people tend to talk about the distance to the particle horizon as being where that stuff is now. I think that this is where the 156 billion light years comes from, but in no sense does it mark the edge of the universe - the stuff there would see surroundings much the same as we see. Note that at cosmological scales there are problems with both the idea of distance and the idea of now - you might take a look at my Cosmological Distances applet to get a feel for how these can be interpreted.

    (There's also the possibility that the 156 billion light years is the distance to something else, such as where the material that emitted the CMBR is now - I'm not sure about where the number comes from)

    Note that if there were no gravity (known as the (0,0) case, or perhaps less acccurately as the Milne universe) then there would be no particle horizon - all of the universe, (even if it were infinite) would be in our past light cone.

    So would it be possible to reach this stuff to see whether it really is the same as the stuff around us. The claim seems to be that since the space is expanding, the distance to such stuff is increasing faster than the speed of light, and so we would need to travel faster than light to reach it. This is wrong - see my article on Stretchy Space to see the problems you run into when you think in these terms. If you had a suitable transporter beam (such as that I suggest in Interstellar Travel) then the expansion of the universe would not prevent you from reaching any point in it.

    What does get in the way is that the universe is not only expanding, but that the expansion is accelerating. This means that there is a Cosmological Event Horizon. I've written about the cosmological event horizon and particle horizon in Cosmological Horizons and The Cosmological Event Horizon. Assuming that there is such a horizon then it wouldn't stop you travelling on forever, its just the stuff you would be travelling towards would be moving away from you faster and faster, so that you would never reach it (unless you could travel faster than light - and I'm working on that).

    So what if you were able to travel as fast as you liked? Well either the universe is infinite or it is finite. If it is infinite then you would never reach any sort of boundary, you would just see stuff that is more or less the same as what we see. If the universe is finite you wouldn't reach a boundary either, you would just return to your starting point eventually
  17. Nov 12, 2007 #16


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    Yes, when we were six years old and tried to imagine what Nothing was like!
    There is a whole conversation in Through the Looking-Glass about this. Whom did you see on the road coming here? I saw Nobody? Well you passed him didn't you? Oh no, I'm sure Nobody got here before i did....

    And there is that Logical Proof about Eternal Happiness and the Ham Sandwich.

    You must know it but in case not I will reconstruct.

    What is better, eternal happiness or a ham sandwich?

    quod erat demonstrandum.

    this is not the real world, it is silliness built into the patterns of the English language

    and that is what it sounded to me as if Hillary was playing, dressed up in cosmology clothes. :biggrin: whether she knew it or not.

    you know what I think the root problem is? some person (try to picture him or her) started putting quote marks around the word nothing!
    who is being quoted?
    nothing with quotes around it mystifies people and makes some of them think there is a real thing that we need to imagine or think about, masquerading behind nothing-with-quotes

    people who used quotes around the word nothing should immediately be dropped from one's acquaintance

    there is nothing besides the universe, by definition,
    so therefore there is nothing external to it that we need to try to imagine

    simple enough English if you leave off the quotes.
    Last edited: Nov 12, 2007
  18. Nov 12, 2007 #17
    Well, a lot of my questions have been answered, and misconceptions cleared up. But, Pervect, if the universe is homogeneous and isotropic on large scales according to the FRW model, how can the following, from the WMAP Cosmology page you posted, be a valid statement?
    "Because the universe has a finite age (~13.7 billion years) we can only see a finite distance out into space: ~13.7 billion light years. This is our so-called horizon. The Big Bang Model does not attempt to describe that region of space significantly beyond our horizon - space-time could well be quite different out there."

    How could it be different beyond the event horizon if we are assuming things are the same everywhere in the universe? It seems that making the statement that "space-time could well be quite different out there" implies that the FRW model might not hold, and therefore, how can we assume it holds now?

    "The Big Bang Model does not attempt to describe that region of space significantly beyond our horizon - space-time could well be quite different out there."

    This is what I was originally trying to understand. In what way would it be different out there?
  19. Nov 12, 2007 #18
    You know what? I'm going to go change my user name right now so that condescending male chauvinist pigs cannot dismiss me upon gaining the knowledge of my gender.

    In my defense, the title of this site is Physics and Math HELP...I thought I made it quite clear that I was looking for help and not trying to pose an argument. I'm sorry that my measly little question didn't measure up to your high and mighty standards, and I recognize that I am not an experienced cosmologist, but I'm interested in learning about it...so I asked you guys what the general guesses and theories are regarding the "edge" of the universe.

    ...thanks for your support!!! Hope I can return the favor sometime!! ;)
  20. Nov 12, 2007 #19


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    I only intended to suggest a gentler tone towards newbies, marcus.
  21. Nov 13, 2007 #20
    So no response to my last inquiry? Or have I been shot down as well?
  22. Nov 13, 2007 #21


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    If the FRW model holds then indeed everything beyond our visible universe would look just the same as the visible Universe does. I don't think there is any reason to think that it is otherwise.

    On the other hand, in the end we just cannot see it, and therefore can't really know. I think this is an example of where scientists generally try (or at least they should!) to be careful and conservative in what they say.

    But you are correct in suggesting that if the FRW model is universal then there is no reason to think that anything beyond the visible Universe does not conform to this model. The reason the observable universe is in any way special is simply because that is the region that we can observe to confirm the model.

    In a sense there isn't much point speculating on what the Universe is like beyond where we can see, which is why, if we are being very careful in what we say, that we can't really assert anything about the region beyond the visible Universe. Our theories certainly make predictions, but we can't confirm them.

    I hope that helps!
  23. Nov 13, 2007 #22
    I'd point out that its fairly standard in physics to do calculations on the assumption that you have an infinite system which is the same everywhere, and then to argue that real systems are large enough to make our calculations valid. In cosmology we have the extra benefit that any change in the nature of the universe outside our particle horizon shouldn't have any observable effect on what we see here.

    However, not all models claim that the universe is the same everywhere. For instance some inflationary models have us within a bubble which has stopped inflating, whilst inflation is carrying on regardless elsewhere.
  24. Nov 13, 2007 #23


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    There's no particular reason to expect things to be different "out there", but there's no guarantee that they're the same, either.

    Science can only answer questions testable by experiment. In this case, there's no experiment that we can carry out now that will shed any light on question. (If the universal expansion is accelerating, and assuming FTL travel is impossible, we may not ever be able to find out - if the universal expansion were deaccelerating, it would be theoretically possible to answer the question in billions of years, assuming that there is still some intelligence around that is interested in the answer).

    So the question is beyond the realm of science, and gets into religion and/or philosophy.
  25. Nov 13, 2007 #24
    Hillary, actually it has nothing to do with gender as many of us that have limited education in the mainstream cosmology get similar treatment from those that may feel our questions are stupid. Soon us laymen learn to just shut up and read what others post for the most part. ;)

    In defense of Marcus in particular though, he is generally VERY helpful to us in showing us the way through our murky thought process.

    This topic brings up a good point though in dealing with the unknown. It seems that any postulation or question in dealing with anything outside our observable universe is immediately met with an attitude from "those that know". But in reality, nobody knows. So I wonder why it is sometimes ok to postulate that our universe is some sort of self-contained bubble while any mention of an "edge" or "outside" to this bubble is met with scourne. Since we really don't know and probably can never know for sure, it really just comes down the the credentials of the person making the postulation.
  26. Nov 13, 2007 #25


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    My situation is that there is this fairly standardized consensus cosmology picture called LambdaCDM. I figure that before anybody proposes some alternative they should at least understand the basic features of the model that working cosmologists use every day, and observational astronomers automatically assume in analyzing their data.

    Once somebody gets the basic outlines of LCDM then, for sure since we don't actually KNOW, then let them make up whatever they want, or take a fancy to some one of the many alternative cosmologies already out there. We are all free to believe whatever we want.

    As far as the meanings of words go, my present understanding of the word Universe is very simple: it is everything. I think that is how most people here use the word. If that's what it means then I find it hard to grasp how universe could have any kind of boundary. If we start talking with everyone using different definitions of the basic words then we're apt to waste time on unreal semantic debate.

    As far as Hillary88 post goes. Here is the exchange:
    I don't think the issue here has much to do with your (BoomBoom) interest in alternative cosmologies with or without boundaries. I think this exchange concerns the philosophical sin of imputing existence to nothing by putting quotes on the word and talking about 'nothing' as if it existed in some mysterious way and was a mysterious something.
    Last edited: Nov 13, 2007
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