Strictly speaking, this is not a theory, because it can’t be tested. And the oscillating universe idea has been around for decades.
But it’s an intriguing idea, nevertheless. If true, there are mind boggling implications. Out of the many possible outcomes, a universe could come into being in the very distant future that is identical to ours. And it could happen an infinite number of times.
This is both comforting and disheartening at the same time. It’s a form of immortality gained at the price of an existence without purpose.
Think just in terms of a photon and you. The photon is a looooong way off and coming straight at you. In your reference frame it’s speed is c of course. So it will of course eventually arrive. Your speed with regard to other objects is completely irrelevant. It is always coming at you at c.
But …
Where did the photon come from? If from a quasar an appropriately long way off, it is incredibly red shifted. Any photon generated by any process we are aware of will be so red shifted that it is lost in the comic microwave background noise. And if it’s lost in the noise, then we cannot extract information from it. Go further out and, well, the term “red shift” is no longer adequate. For all practical purposes such quasars are not emitting light “at” us. Reading some of Einstein’s Gedanken experiments about Swiss trolleys might be helpful. Cones and all that happen.
The thought of the space between the photon and us expanding is nice but mucks up the relativistic issues involved.
In short: all of the light emitted at us by these very far off things will reach us. But none of these very far off things are emitting light at us. Clear?
Second thing first. The oscillating universe idea has been around a long time but this is a wholly new twist on it. The old idea presupposed a big bang back to a big crunch. This idea had problems however not the least of which was each time this happened total entropy would increase making each go-around a little harder to imagine than the last.
The new idea is based on String Theory and higher dimensions. It postulates an infinite membrane that is our universe colliding with another membrane. I’m probably missing a lot with that but read the link or Google for more detailed info. Anyway, this ‘theory’ has only been devised in the past year or two making it rather new.
As to being able to test it you are correct. We can’t do anything to see into these higher dimensions or see these other membranes. However, the theory does allow for predictions of how our universe should behave if such a thing is true. Cosmologists seem excited because this new theory because it does away with a lot of hard to explain issues found in the Standard Model. Of course, the new theory has a few holes as well that aren’t easily explained but again it is a young theory so we’ll just have to wait and see how things play out.
Well, no. The photon is approaching us at about 310[sup]8[/sup] m/s, but space is expanding between us and the photon. If it starts off far enough that the expansion of space between us and the source is greater than 310[sup]8[/sup] m/s, the distance to the photon is increasing, even though the photon is moving towards us.
Can someone explain why the total entropy must increase from cycle to cycle of a cyclic universe? And please don’t just quote the second law of thermodynamics back to me. I suspect this law must get violated as the universe contracts (not as soon as it passes its largest volume, but as it’s approaching the crunch): The entropy S = k * ln(OMEGA) where OMEGA is the number of accesible states. The number of states in a volume of space is essentially proportional to the volume, so as the volume of the universe decreases, the number of accesible states should eventually decrease as the crunch is approached. So is there some other reason reason why the entropy must increase cycle to cycle, or can someone demonstrate that the second law does in fact hold as the universe approaches a big crunch?
Original question, dismantled and rephrased in a few different variations:
a) Assuming that the light had time to get here, what is the most distant object we could conceivably see, were it to exist? (Hmm, would this be established by the highest frequency radiation possible, such that it would remain in the visible spectrum from farther away despite red-shifting? Or a ceiling on the possible concentration of radiation-producing mass?)
b) Assuming that we limit ourselves to objects kin in nature to objects that we know to exist, how far away could one of them be placed and still be visible, assuming that the light had time to get here?
c) Defining “the edge of the universe” to mean “15 billion light-years from where we stand”, what sort of object, if placed there (15 billion years ago, of course, not now), would be visible from our backyards?
d) If we posit for the sake of argument that space extends farther from us than YrsSinceBigBang * c, (i.e., ~15-20 billion light years), what formula(s) would describe an object big enough, old enough, and hot enough to be visible from here for any given distance?
Well, as already mentioned the most distant object an unaided human eye can see is the Andromeda Galaxy 2.9 million light years away (which is a helluva lot less than 15 billion light years). While the Andromeda Galaxy is actually a few billion separate objects from your perspective in your backyard you can consider it as a single object.
The only other thing you might see further with the unaided eye is a supernova which can temporarily outshine a galaxy. However, Andromeda is the closest galaxy to the Milky Way so to see a supernova farther out the exploding star would have to be in another galaxy and I don’t know if that added distance would still make the nova invisible to the human eye.
The furthest object ever seen by anything (i.e. the best telescopes) here on earth is a supernova some 10 billion light years away.
As for point ‘D’ in the post above anything beyond the 15 billion light year limit (which we are assuming here for the sake of argument) doesn’t exist as far as we are concerned. Even though something might well be there it can in no way affect us here on earth in any fashion no matter how bright it is.
I wish I could but I can’t. The link I posted earlier on a Cyclic Universe had the following to say on it but it doesn’t explain why it is so.
The more I think about it the more I’d like an answer to this as well. ZenBeam said not to spout the laws of thermodynamics back at him but that doesn’t change the facts. Entropy always increases. Period.
That’s all fine and dandy but who is to say that a Big Crunch has to obey this? Why can’t entropy be recycled as it were? I think it is taken for granted that the temperatures and pressures in a Big Bang/Crunch singularity cause the laws of physics as we understand them to utterly break down. No one really knows how things work in that state. Is there some reason for scientists to assume that the rules of entropy that apply in our universe would still apply in that singularity? Assuming a neverending cyclic universe of Big Bang/Big Crunch wouldn’t you sooner or later get a singularity near maximum entropy? An infinitely hot yet nearly completely cold at the same time? Physics has a bunch of counterintuitive pieces to it but that one boggles the mind.
The most distant source of light which can be detected is the Surface of Last Scatter, which is basically the entire contents of the Universe at the time that it became transparent. This occured at around 300,000 years after the Big Bang, which is awfully close to the beginning, from our scale. It’s currently redshifted down to the microwave background, though, so human eyes can’t see it.
As for curvature, the currently-favored models are for a flat Universe, or one so close to flat that curvature is completely negligible. There is curvature on a small scale wherever there’s mass, but the overall net effect appears to be zero curvature. Nontrivial topologies have not yet been ruled out, though. We may find out about that at the end of this summer when enough of the data are in from the MAP sattelite.
My question is, why would you want to see the edge? It’s rather dull really. There’s a small restaurant and a couple seedy looking repair shops. Tip the Hostess heavily, or the view is overwhelmingly depressing and bleak.
Of course, Inflationary Theory could be wrong but so far its done a pretty good job of explaining what cosmologists see so they’ve stuck with it. Also note that the Cyclic Universe model, if true, would also allow for the same effect. Essentially we are sitting on a membrane of infinite proportions. We could only ‘see’ as far as light can travel since the last ‘Big Bang’ (or brane collision or whatever they call it) with a whole lotta universe existing beyond our detection range.
Well, this is the part I’m questioning in a contracting universe, and not just when the universe hits the singularity.
In a finite universe of some volume and total energy, there must be a maximum possible entropy, S[sub]max[/sub]. This maximum entropy should be a function of the volume of the universe, with a larger universe having a larger maximum entropy. If a universe with entropy S contracts to a small enough volume such that S[sub]max[/sub] is less than S, then entropy has to decrease.
At least, that’s the argument someone needs to show is flawed. Three little sentences; which one’s wrong, and why?
As for the size of the universe being larger than the size of the observable universe, there were people investigating this as of several years ago (sorry, no cite). If the universe is 15 billion years old, and finite with circumference less than 30 billion lightyears, then we can see the entire universe. If it’s circumference is significantly less tha 30 billion lightyears, we’d see the same thing looking in different directions (this is what they were looking for). I think the recent discovery that the universe appears to be flat makes this a much less likely result, but I don’t think it’s been completely ruled out.
How do you get this? Entropy is a function of the volume of phase space, but that doesn’t necessarily relate to the volume of what most folks would call “space”. Entropy is a locally-defineable quantity, so the laws governing it shouldn’t depend on what the Universe is doing light-years away.
Now, Big Crunches/Bangs are potentially another matter. When you go through singularities, you can’t just ignore them locally, so it’s conceiveable that the Second Law might be violated in such circumstances.
If I have all the energy (including matter) of a universe of volume V[sub]1[/sub] confined to a smaller volume V[sub]2[/sub], but other wise at maximum entropy, the entropy is smaller than if the energy is equally spread out over the whole volume V[sub]1[/sub]. There are fewer particle states to choose from in V[sub]2[/sub] than in V[sub]1[/sub]. If I have that same energy at maximum entropy in a universe whose total volume is just V[sub]2[/sub], that should be the same maximum entropy, since you’ll have the same number of particle states to choose from.