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Parallel universes: I'm so confused

I'd just like to point out that these things are most likely untestable speculations, and thus pretty far out on the boundaries of science.
 
garys_2k said:

What I mean is, since we can see the MBR in every direction, and now we've mapped it in pretty fair detail, how can we not, in some sense, be seeing the ENTIRE universe?
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if the universe is infinite, then there are places which lie outside the light cones associated with observers on Earth. light from those places just hasn't had time to get here yet. the observed isotropy suggests that the currently observable universe is homogeneous (unless Earth happens to be located near the center of a spherically symmetric density structure). indeed, the existence of this particle horizon poses a significant problem for cosmologists: if distant (unobserved) regions of the universe are not thermodynamically coupled...why is the universe observed to be homogeneous? of course, it's always possible that previously unobserved anisotropic regions of the universe (near the moment of the big bang) will come into view at some future time.

-Gianni
 
This thread is pushing the limits of my understanding of cosmology.

I guess I had thought, based on the surface-of-a-balloon analogy, that our universe must have enough gravity, so that if you head out in any direction, you'll eventually come back to where you started (though at a later time). I based this on the isotropy of the background radiation.

But if the everything-there-is is way out there past our known universe, wouldn't something outside our known universe, a really long time ago, be visible to us (be inside our light cone) in some direction, instead of the background radiation?

I've had 1.5 beers now, so be easy on my misunderstandings.
 
DrMatt said:
I'd just like to point out that these things are most likely untestable speculations, and thus pretty far out on the boundaries of science.
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The article says they are testable, as well as falsifiable, but I must have nodded off before I got to the part where they offered support for this claim...
 
I guess I had thought, based on the surface-of-a-balloon analogy, that our universe must have enough gravity, so that if you head out in any direction, you'll eventually come back to where you started (though at a later time). I based this on the isotropy of the background radiation.
Click to expand...

Large scale curvature and isotropy are two separate issues. In the surface-of-a-balloon analogy, matter can be uniformly distributed on the surface of the balloon.
And it takes at least 10 beers to properly understand cosmology ;)

-Gianni
 
Giovanni Jensen:O
And it takes at least 10 beers to properly understand cosmology ;)
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Okay, I've had 6, so I'm almost there. So, please explain how a Universe with a definite starting point in time and with a less than infinite expansion rate, can be infinite in extent.
 
Paul C. Anagnostopoulos said:
Welcome to the forum, Giovanni!

The universe is infinite? What's with all the recent chit-chat about how it's expanding at an ever-increasing rate? I suppose it could be both.

If it's infinite, how come it has a finite age? Or is that just the part we can see?

I'm still confoozed.

~~ Paul
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Somewhere in the article (2nd page, 1st column, last paragraph) it mentions the "Island universe" model. In this, space is infinite,while the distribution of matter is in a smaller area (that has been expanding since the big bang)
 
Okay, I've had 6, so I'm almost there. So, please explain how a Universe with a definite starting point in time and with a less than infinite expansion rate, can be infinite in extent.
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the universe can start out spatially infinite (and flat) and still have infinite curvature at the instant of the big bang. remember that it's spacetime that's curved, not necessarily 3-space. the expansion rate is infinite at the initial singularity. the Friedman model with zero curvature is an example.

think of an infinite rubber sheet that starts out with infinite density everywhere.

-Gianni
 
Giovanni Jensen said:


if the universe is infinite, then there are places which lie outside the light cones associated with observers on Earth. light from those places just hasn't had time to get here yet. the observed isotropy suggests that the currently observable universe is homogeneous (unless Earth happens to be located near the center of a spherically symmetric density structure). indeed, the existence of this particle horizon poses a significant problem for cosmologists: if distant (unobserved) regions of the universe are not thermodynamically coupled...why is the universe observed to be homogeneous? of course, it's always possible that previously unobserved anisotropic regions of the universe (near the moment of the big bang) will come into view at some future time.

-Gianni
Click to expand...
Hmmm, ok, let's see if I have this right:

The MBR "horizon" (surface?) we observe, when the universe was (I think) about 380,000 years old, is probably only one part of a larger whole. In fact, the radiation we perceive is from the area of the universe we happen to be "centered" in, but an observer in another location in our universe very far from us, may be looking out at his own MBR horizon. There is nothing special about any particular observation point, nor does any observation point see the entire universe (even at that young age). All we can perceive is that part of the big bang that is near enough to us.

Is this correct?
 
Middle of page 2 - One way to do the calculation is to ask how many protons could be packed into a Hubble volume at that temperature. The answer is 10 raised to the 118 protons. Each of those particles may or may not, in fact, be present, which makes for 2 to the 10 raised to the 118 possible arrangements of protons. A box containing that many Hubble volumes exhausts all the possibilities. If you round off the numbers, such a box is about 10 to the 10 raised to 118 meters across. Beyond that box, universes--including ours--must repeat. Roughly the same number could be derived by using thermodynamic or quantum-gravitational estimates of the total information content of the universe.

Middle of page 6 - Now you are in universe A, the one in which you are reading this sentence. Now you are in universe B, the one in which you are reading this other sentence. Put differently, universe B has an observer identical to one in universe A, except with an extra instant of memories. All possible states exist at every instant, so the passage of time may be in the eye of the beholder--an idea explored in Greg Egan's 1994 science-fiction novel Permutation City and developed by physicist David Deutsch of the University of Oxford, independent physicist Julian Barbour, and others. The multiverse framework may thus prove essential to understanding the nature of time.
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(Bolds are my additions)

Do I understand this correctly? At any given instant, there is a universe somewhere that looks exactly like ours, but in the next instant, a different universe is the one that looks exactly like ours, and the first one that looked exactly like ours begins a different history than ours? Therefore, the odds are exceedingly small that your double got to look just like you and be in the same situation as you the same way you did? Therefore the experiences of you and a given double converged until **NOW** and then your experiences begin to diverge?

edited to add: Ouch.
 
Is MWI (many worlds interpretation) a scientific theory?

DrMatt said:
I'd just like to point out that these things are most likely untestable speculations, and thus pretty far out on the boundaries of science.
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Soderqvist1: A scientific theory is based on one condition, namely; the theory must have an intrinsic potential to be proven wrong. I have read the Astrobiologist Paul Davies, and Julian Brown's book, The Ghost in the Atom (issued 1986). Davies has interviewed David Deutsch, and he is enthusiastic about David Deutsch's MWI experiment with quantum computers in a near future, to test Everett's theory! I have both Julian Brown's book, The Quest for the Quantum computer, and David Deutsch's The Fabric of Reality in my home! This quote below is an abbreviated book version online!

The Quest for the Quantum Computer: Minds, Machines, and the Multiverse:
Deutsch is a physicist, winner of the 1998 Paul Dirac prize for theoretical physics and a researcher at the Center for Quantum Computation at Oxford University's Clarendon Laboratory. In the early 1980s, Deutsch's proposed experiment (described more fully in Chapter 3) sounded like the stuff of science fiction. To test the existence of multiple universes, he envisaged the construction of a thinking, conscious artificial intelligence whose memory worked "at the quantum level." Such a machine, he claimed, could be asked to conduct a crucial experiment inside its own brain and report back to us whether Deutsch was indeed right to believe in the existence of parallel universes.

Well, nearly 20 years later we have the answer because quantum computer memory is on the verge of becoming an experimental reality. Interpretations aside, it's long been known that at the atomic level waves can behave like particles, and particles have waves associated with them. A single entity such as an electron, for example, can travel along many different routes simultaneously as if it were really a spread-out phenomenon like a wave. The essential idea of quantum parallelism advanced by Deutsch was this: If an electron can explore many different routes simultaneously, then a computer should be able to calculate along many different pathways simultaneously too. http://www.simonsays.com/excerpt.cfm?isbn=0684814811

Soderqvist1: Quantum mechanics doesn't make any sense, because our commonsense stems from our daily living there a object doesn't are at many places at once, so when we read about quantum mechanics we must put our commonsense aside, and read a quantum mechanical representation and analyze its theory, if it is logically consistent, or not! :D
 
[thanks to fishbob for providing SA quote]

Maybe one of the physicists in the audience can help clear this point of confusion for me: Each of those particles may or may not, in fact, be present, which makes for 2 to the 10 raised to the 118 possible arrangements of protons. Doesn't each proton have more parameters than just "present" and "not present"? Doesn't it have a position, velocity, etc.? I can imagine that the 10^118 protons are evenly distributed throughout the volume, or that they are clumped together in planets and stars in various arrangements. I just don't see how the 2^(10^118) takes all of these possibilities into account.

I must be missing some very obvious thermodynamics "truism", because he just seems to gloss over this. Can anyone explain what I'm missing?

[edited typo]
 
Maybe I've answered my own question. This link says that the number of protons in our universe is around 10^79. I thought that the SA article was saying that the number of protons actually in our universe is around 10^118. I guess the SA article is saying that 10^118 is the maximum possible number of protons in a universe having our average temperature.

So does this mean that you could have 10^118 protons in "our little corner" of the universe, if they are packed at the "normal density" that would be found in a typical nucleus? I suppose in that case the 10^118 takes into account all of the possible positions of the protons, so you don't need to know their positions/velocities, etc.

Is this correct?
 
In this "multiple universes" model, I'd like to know what meaning "universe" is supposed to have. Otherwise, the whole business sounds like the trick of looking for the exceptionally large experimental values of 0, hoping to find one that is 1 within the bounds of experimental error--i.e., it just sorta sounds like science...
 
ChuckieR,

the argument is based on statistical mechanics of systems which are very close to thermal equilibrium. in statistical mechanics, macroscopic properties like temperature and density are determined by averaging over ensembles of possible microscopic states (possible arrangements of particles in some phase space). in thermodynamic equilibrium, every possible microstate has a nonzero probability of being realized. thus, if the universe is infinite and in thermodynamic equilibrium, then every possible microstate is realized.

of course, it seems like a bit of a jumpt to assume that the infinite universe is in thermodynamics equilibrium. the ``island universe'' is a good example of a system which is not in thermal equilibrium (at least not near the edges of the island).

-Gianni
 
All we can perceive is that part of the big bang that is near enough to us.

Is this correct?
Click to expand...

yep, that's about right. in an infinite open universe with an initial singularity, the big bang happens everywhere at once.

-Gianni
 
Giovanni Jensen said:
yep, that's about right. in an infinite open universe with an initial singularity, the big bang happens everywhere at once.-Gianni
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Wow, all these years I had in my head the image of the universe springing out from a simple, single "point" in 3D "space", and expanding outward from this point.

Maybe that was Paul's image also?

I never considered the singularity to be a "sheet" or any of these more complex "shapes" that would lead to an infinite universe.

Things don't seem so simple anymore... ain't science wonderful?
 
Giovanni said:
yep, that's about right. in an infinite open universe with an initial singularity, the big bang happens everywhere at once.
Click to expand...
Give me some way to picture this. I didn't picture the Big Bang as happening at a single point in space, Chuckie, but I pictured space-time as springing forth from the Big Bang. There is nothing "around" the universe; the expanding universe is all there is.

But now I'm supposed to picture the Big Bang as happening all around in an infinite open universe? The universe is still a product of the Big Bang, right? So I should picture an infinite Big Bang forming an infinite universe at an instant? What does it mean for the universe to be expanding then? Just that the contents are all moving away from each other within the infinity of the universe?

~~ Paul
 
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