The Universe - infinite or not ? [Archive]

russ_watters

Mar18-03, 10:43 AM

Yes... No... ;)

Is the universe infinite? Well, according to the evidence, the answer is ... we don't know. And unfortunately, this may be the best answer we'll ever get. While the curvature of the universe is something that we might get a precise fix on (the data suggest a flat universe, i.e. no spatial curvature), the global topology might never be known because of the smallness of our Hubble volume. And the global topology is just as important as the curvature is in determining the size of our universe. We can detect measure the velocity of galexies quite a long distance away. The evidence we have now certainly suggests a BB like event and a finite but boundless universe.

bogdan

Mar18-03, 10:45 AM

universe = space ?
or
universe = space-time-and-so-on ?
And...how could we find out that the universe is infinite or not ?
Give a method...even if it's impossible to be realised...

Mar18-03, 12:01 PM

Originally posted by russ_watters
No... ;)

We can detect measure the velocity of galexies quite a long distance away. The evidence we have now certainly suggests a BB like event and a finite but boundless universe.

The big bang does not say the universe is finite or infinite.

Mentat

Mar18-03, 01:59 PM

Originally posted by CJames
I often agree with you Mentat but not here. I have long suspected the universe is finite but I'm having second guesses. Anyway, I never believed it to be an absolute fact. I believe in the big bang as well. However, cosmology allows for the big bang to work even with an infinite universe. The universe simply needs to have infinite size at its creation. A singularity of infinite size AND infinite density. It's weird I know.

Well, you are entitled to your opinion. However, to say that there can be a singularity of infinite spacial extent is contradictory, and makes no sense to me.

Mentat

Mar18-03, 02:04 PM

Originally posted by Eh
The big bang does not say the universe is finite or infinite.

You know what, you bring this up a lot, but I still disagree. Sure, BB theory may not say which it is - but isn't the idea behind the BB, that the spacial dimensions themselves are expanding? If so, then the universe cannot already be infinite, because the spacial dimensions couldn't possibly get any bigger.

Mentat

Mar18-03, 02:11 PM

Originally posted by MajinVegeta
I can't agree with you more, Mentat. I would say, maybe, that the universe is on the verge of infinity?[?]

Well, for all practical purposes, it may have the potential to continue expanding infinitely. However, it will never reach infinity (if it is finite now), and so cannot really be "on the verge of infinity".

Mar18-03, 04:11 PM

Originally posted by Mentat
You know what, you bring this up a lot, but I still disagree. Sure, BB theory may not say which it is - but isn't the idea behind the BB, that the spacial dimensions themselves are expanding? If so, then the universe cannot already be infinite, because the spacial dimensions couldn't possibly get any bigger.

The important thing seems to be that expansion does not necessarily make the universe larger. This is one of the weird things you get from infinity, but it does not seem to have an inherent problem. As you noted before, each region in an infinite universe is infinitesimal compared to space as a whole. That is, each point is pretty much zero compared to the universe.

Within any region, local expansion occurs where distances between points increases. But even though local expansion is occuring within the region, the size of that region is infintely small compared to the whole universe - and always will be. There is nothing preventing local expansion from occuring, and the size of the universe never changes. Again, this is because of the nature of infinity.

RuroumiKenshin

Mar18-03, 07:52 PM

CJames

Mar19-03, 12:57 AM

I agree with Eh, although not necessarily in the way he explained it. To me it is much simpler. An infinite number is not "the biggest number possible." An infinite number, actually, has an infinite amount of room to grow. You can add, divide, multiply infinities together and you get back another infinite number. If you didn't, you'd get all sorts of paradoxes in mathematics.

Mentat, by saying an infinite universe can't get any bigger, you are saying an infinite universe has no more room to grow. You are implying that the universe has to have space to grow into. You know that is not the case.

JamesBell

Mar19-03, 04:06 AM

Olivers bells thesis 201 says that a universe can not expand without stretching and bursting causing a catastrophic wave of death and destruction of neutrinos and protons, the force of the rip will be 1.000000000000000000000000000000000000000000000000 00000000000000000000000000000000000000000000000000 00000000000000000000000000000000000000000000000000 00000000000000000000000000000000000000000000000000 00000000000000000000000000000000000000000000000000 00000000000000000000000000000000000000000000000000 00000000000000000000000000000000000000000000000000 00000000000000000000000000000000000000000000000000 000000001*10Exp

Mar19-03, 12:39 PM

Are you sure that's not just an accelerating universe?

RuroumiKenshin

Mar19-03, 09:00 PM

Originally posted by JamesBell
Olivers bells thesis 201 says that a universe can not expand without stretching and bursting causing a catastrophic wave of death and destruction of neutrinos and protons, the force of the rip will be 1.000000000000000000000000000000000000000000000000 00000000000000000000000000000000000000000000000000 00000000000000000000000000000000000000000000000000 00000000000000000000000000000000000000000000000000 00000000000000000000000000000000000000000000000000 00000000000000000000000000000000000000000000000000 00000000000000000000000000000000000000000000000000 00000000000000000000000000000000000000000000000000 000000001*10Exp

How about the big crunch?

Mentat

Mar24-03, 02:10 PM

Originally posted by CJames
I agree with Eh, although not necessarily in the way he explained it. To me it is much simpler. An infinite number is not "the biggest number possible." An infinite number, actually, has an infinite amount of room to grow. You can add, divide, multiply infinities together and you get back another infinite number. If you didn't, you'd get all sorts of paradoxes in mathematics.

Mentat, by saying an infinite universe can't get any bigger, you are saying an infinite universe has no more room to grow. You are implying that the universe has to have space to grow into. You know that is not the case.

No, I never said that it was because it didn't have room to grow into. I said that it was because it was as big as it could possibly get.

Mentat

Mar24-03, 02:15 PM

Originally posted by MajinVegeta
The universe(universe=everything) is infinite, but the spacial demensions are the ones expanding into the actual universe. It's like, what is the universe expanding into? Well, lets substitute "universe" as "subuniverse". So it's what is the subuniverse expanding into? the actual universe. I hope you understand what I mean(as I am having a hard time describing this).[a)] [:))]

This is very near (if not identical) to one of the BB theories, that people bring up quite a bit, especially in threads about the incompatibility of infinite universe with BB theory.

Mentat

Mar24-03, 02:16 PM

Originally posted by Eh
The important thing seems to be that expansion does not necessarily make the universe larger. This is one of the weird things you get from infinity, but it does not seem to have an inherent problem. As you noted before, each region in an infinite universe is infinitesimal compared to space as a whole. That is, each point is pretty much zero compared to the universe.

Within any region, local expansion occurs where distances between points increases. But even though local expansion is occuring within the region, the size of that region is infintely small compared to the whole universe - and always will be. There is nothing preventing local expansion from occuring, and the size of the universe never changes. Again, this is because of the nature of infinity.

I understand all of this, I was talking about the idea that the spacial dimensions themselves are expanding. I had thought that this is what BB theory was all about. If this were the case, then the universe could not be infinite.

Mentat

Mar24-03, 02:17 PM

CJames, infinity is not the biggest that you can get mathematically. But is the biggest that you can get physically.

Zakkur

Mar21-09, 04:08 AM

you gotta think outside the box man. like the universe can grow and grow. but like, what is it growing in? yknoe, what outside this box man, is the universe square? and outside of each wall is there an equal and opposite universe, or if its a circle, it wouldnt matter if you left te universe and looked at it because it wouldnt be there, you would be in a place where time and space didnt exist until you got there, and you would float forever thinking until you turned into your own universe, and you were the creator man. i swear i dont knoe what im talking about man. lmao.

Hesh

May6-11, 01:38 PM

The universe is not infinite nothing is naturally infinite we only see the universe as infinite because we are so small compared to it and we really dont know where it ends but it does have an end and wont expand forever because like all things it will eventually run out of energy. Space may be infinite but the universe is not.Now whether it will just stop or collapse in on itself is another thing maybe it will collapse and just create another big bang or just stop and for whats outside the universe it could be nothing or it could be another universe how do we know that there wasnt a universe when the big bang happened and that the force from the big bang didnt push that universe out of the way or push or destroy part of that universe so our universe could form but our universe could just be a galaxy or a planet in an even bigger universe if you get what i mean but yes the universe does have an end and it is not infinite.

dad3

May6-11, 02:07 PM

jposs

May6-11, 08:09 PM

The universe(universe=everything) is infinite, but the spacial demensions are the ones expanding into the actual universe. It's like, what is the universe expanding into? Well, lets substitute "universe" as "subuniverse". So it's what is the subuniverse expanding into? the actual universe. I hope you understand what I mean(as I am having a hard time describing this).[a)] [:))]

I think i do. So there is a "wall" at the edge of the universe.... say the expansion of matter is the only definition of this wall. It has more room but no stuff. The expanding universe continues on its path, I would guess dispersing at its edges more and more. But even on the outer limits of what could physically hold together we could not tell there is nothing there because there is nothing to mark its presence, other than nothing.

So say one day we could count the physical universe on any level I think we would have to call that infinite (even being quantifiable) because we have no words in any language to describe it.... Just like a Dodo first seeing things that could eat them, how could they tell there birdbrained friends that didn't see the other colony that got slaughtered anything about what happened when so many words are missing.

Well kind of went off on a different direction... sorry, it happens.

jposs

May6-11, 08:22 PM

.......but also the earth was flat at one time and every planet was manipulated by earth's gravitational force???

We think too much of ourselves. That was destruction and undeniable forces that brought us back there. And our way too "I'm better than you" mentality that keeps us there. IMnotsoHO if we weren't so good at those traits our species would be either way more primitive or as my optimism likes to think way more developed than it is today. Maybe not the same, but more.

When did a people invent the printing press? Columbus proved it was round right? Hell we didn't even know an apple falling from above you would hit you on the head until after the guy who once again proved the basics of what is was told to shut up or die. We are a bit full of ourselves, maybe not a bunch of you all, but I would be tempted to say a majority of all of us peoples.

IsometricPion

May6-11, 10:39 PM

the universe is not infinite because it is currently thought that the universe is expanding but we could have an infinite number of parallel universes.......................but also the earth was flat at one time and every planet was manipulated by earth's gravitational force??? The best model (http://en.wikipedia.org/wiki/Friedmann_equations) of the universe found so far indicates that the universe is at least extremely large, perhaps infinite (since the total density of the universe is near that which would make it flat and therefore infinite). Expansion is a matter of geometry, if one imagines a flat infinite 2-d plane on which figures are drawn, expansion of the plane would lead to every measurable distance increasing (according to rulers that do not change in size with the expansion), so the figures would enlarge into (geometrically) similar ones.So there is a "wall" at the edge of the universe....There is no edge of the universe (on this point it doesn't matter whether the universe is finite or infinite). In order to have an edge the universe would have to behave differently from what is observed (for instance, general relativity (among many other things) wouldn't work since the universe would not be representable as differentiable manifold (http://en.wikipedia.org/wiki/Smooth_manifold) near the edge).

Off topic (and irrelevant): This may be the first time I've seen a thread from 2003 necroposted (though I guess it was also posted to in 2009, but that was also a necropost, and it lead to no discussion so I don't think it counts).

dad3

May7-11, 07:02 AM

We think too much of ourselves.

Exactly

binbots

May9-11, 07:45 PM

I do not see how the universe or the multiverse can be finite. It is a all or nothing situation. Nothing can't co exist with something. If you say the universe started with the big bang then how long was this tiny super dense singularity sitting there for before the bb? If you say the universe if spacialy finite then what would happen if you reached the end? The fact that we exist means time and space are both infinite.

Chronos

May10-11, 02:36 AM

I would argue that proposal, binbots. Nothing exists amongst something all around us - like empty space between stars. It is difficult to prove the universe is either finite or infinite. Einstein surely thought it was infinite and I'm not inclined to argue with him. But, his logic was newtonian and he waffled a lot. WMAP suggests the universe is almost exactly flat - which implies it is infinite, or at least too close to call. What does that mean? A universe comprised of a finite amount of matter expanding into an endless void? An infinite amount of matter expanding into an even more endless void? The only relevant part, IMO, is the observable universe. Beyond that is the playground of mathematicians and philosophers - who remind us why our ancestors found theology appealing.

mdmaaz

May10-11, 07:48 AM

yenchin

May10-11, 08:37 AM

I believe the universe is finite. Richard Feynman's law of "alternative histories" states that each incidents happens in a different way in a different universe. If our universe was infinite, how could there be many universes. I believe that the size of a universe is finite, but the number of universes is infinite. Besides, ever since the big bang, the space between celestial objects has been increasing. Only finite things can increase in size.

The last part wasn't exactly true. If you start with the set of integer which is infinite, you map each element to twice their size i.e. n maps to 2n, then the two sets have the same cardinality, although distance between each integer is now twice as before. So when we say the universe is expanding, all we are saying is the the scale factor of the metric changes, in much the same way as "expanding" the set of integers. Thus an infinite space can expand in such a way. That said, I personally prefer a finite universe ;-)

binbots

May10-11, 06:07 PM

There is room for our infinite universe with other infinite universe. There is infinite room in infinity. I think the question is how is our universe infinite. Are we inside a larger universe etc etc? Is our own universe apart of infinite space? Or are there infinite parellel universes. Either way it is all infinite. It is the same on the small scale. No matter how small we go we will always ask: well what is that made of? The answer to the questions of the universe is that there will always be more questions forever................................

Chronos

May11-11, 02:07 AM

infinity loses meaning nestled amongst a multiverse of infinities, imo - aside from the fact the concept of infinity is unmeaningful to begin with. Everything in nature has relational meaning.

Neandethal00

May11-11, 06:08 PM

Infinity does not exist.
Let me phrase it the right way,
to humans infinity can not exist for any physical reality, because of our sensory systems.

bcrowell

May11-11, 06:24 PM

FAQ: Is the universe finite, or is it infinite?

Standard cosmological models come in two flavors, open and closed. The open type has negative spatial curvature and infinite spatial volume. The closed one has positive curvature and finite spatial volume; spatially, it is the three-dimensional analog of a sphere. Since both types are mathematically self-consistent solutions to the Einstein field equations, the finiteness or infiniteness of the universe is something that cannot be determined by solely logic but only by observation.

Current observations of the cosmic microwave background's anisotropy show that our universe is very nearly spatially flat (on the cosmological scale). If it is exactly flat, then it is a special case lying between the more general open and closed cases. The flat case has infinite volume. However, the range of uncertainty in the curvature is wide enough to be consistent with either positive or negative curvature, so right now the finiteness or infiniteness of the universe is an open question.

Sometimes people use the word "universe" when they really mean "observable universe." The observable universe is finite in volume because light has only had a finite time to travel since the Big Bang.

Chronos

May11-11, 11:36 PM

Good summary, bcrowell. An FAQ would be useful with a decent index.

ViewsofMars

May11-11, 11:50 PM

The European Space Agency (ESA) Science and Technology: Glossary

Universe
Everything that exists. The size of the observable Universe is determined by the distance light has travelled since the Universe was formed in the Big Bang, 12 - 15 billion years ago.

http://sci.esa.int/science-e/www/object/index.cfm?fobjectid=31489&fbodylongid=950

I think the ESA's glossary is quite informative.

Mutsi

May12-11, 05:30 AM

Well, for all practical purposes, it may have the potential to continue expanding infinitely. However, it will never reach infinity (if it is finite now), and so cannot really be "on the verge of infinity".

Isnt it so that Dark Matter used to be stronger then Dark Energy ? And that the universe was contracting. How is it possible to state that the universe will keep expending perhaps the strenght of DM and DE cycle.

Cosmo Novice

May12-11, 09:23 AM

This question may be the final unanswerable - obviously the OU is finite and measurable, anything outside of our OU can never be quantified. Assuming the Universe is spacially flat and homogeneous then cosmological models dictate an infinite size along its x,y,z axis.

However does time factor into this? As the second law of thermodynamics dictates the arrow of time moves only forwards then when the universe approaches the end of its life - as I understand it once matter and energy becomes seperated and diffuse to the point the universe is in final heat death? Is this plausable given infinite energy and matter states?

If something expands inifinitely then while it is not infinite at any given moment it will expand to an an infinite size over infinite time but is this relevant as cosmo models indicate the final end of the universe will be a heat death?

Chronos

May16-11, 01:33 AM

Olber's paradox refutes the infinitely old, spacious and star filled universe idea. The CMB refutes the old wives tale of infinite age all on its own.

bcrowell

May16-11, 01:50 AM

This question may be the final unanswerable - obviously the OU is finite and measurable, anything outside of our OU can never be quantified.
Anything outside of our observable universe can certainly be quantified. Just wait a while, and it will be inside our observable universe.

Assuming the Universe is spacially flat and homogeneous then cosmological models dictate an infinite size along its x,y,z axis.
We can measure the universe's spatial curvature, so why assume it?

Chronos

May16-11, 04:34 AM

How can anything outside the observable universe ever be quantified? I strongly disagree. Perhaps the source of our disagreement resides in the definition of what constitutes 'observable'.

ViewsofMars

May16-11, 01:22 PM

Mutsi brought up Dark Matter. I'd like to present the lastest about it. April 14, 2011 from the Weizmann Institute of Science:

An International team of scientists in the XENON collaboration, including several from the Weizmann Institute, announced on Thursday the results of their search for the elusive component of our universe known as dark matter. This search was conducted with greater sensitivity than ever before. After one hundred days of data collection in the XENON100 experiment, carried out deep underground at the Gran Sasso National Laboratory of the INFN, in Italy, they found no evidence for the existence of Weakly Interacting Massive Particles – or WIMPs – the leading candidates for the mysterious dark matter. The three candidate events they observed were consistent with two they expected to see from background radiation. These new results reveal the highest sensitivity reported as yet by any dark matter experiment, while placing the strongest constraints on new physics models for particles of dark matter. Weizmann Institute professors Eilam Gross, Ehud Duchovni and Amos Breskin, and research student Ofer Vitells, made significant contributions to the findings by introducing a new statistical method that both increases the search sensitivity and enables new discovery.

Any direct observation of WIMP activity would link the largest observed structures in the Universe with the world of subatomic particle physics. While such detection cannot be claimed as yet, the level of sensitivity achieved by the XENON100 experiment could be high enough to allow an actual detection in the near future. What sets XENON100 apart from competing experiments is its significantly lower background radiation, which is 100 times lower, greatly reducing the potential obscuring of any dark matter signal. The XENON100 detector, which uses 62 kg of liquid xenon as its WIMP target, and which measures tiny charges and light signals produced by predicted rare collisions between WIMPs and xenon atoms, continues its search for WIMPs. New data from the 2011 run, as well as the plan to build a much larger experiment in the coming years, promise an exciting decade in the search for the solution to one of nature's most fundamental mysteries.

Cosmological observations consistently point to a picture of our universe in which the ordinary matter we know makes up only 17% of all matter; the rest – 83% – is in an as yet unobserved form – so-called dark matter. This complies with predictions of the smallest scales; necessary extensions of the Standard Model of particle physics suggest that exotic new particles exist, and these are perfect dark matter candidates. Weakly Interacting Massive Particles (WIMPs) are thus implied in both cosmology and particle physics. An additional hint for their existence lies in the fact that the calculated abundance of such particles arising from the Big Bang matches the required amount of dark matter. The search for WIMPs is thus well-founded; a direct detection of such particles would provide the central missing piece needed to confirm this new picture of our Universe.

Please read on. . .

http://wis-wander.weizmann.ac.il/experiment-narrows-dark-matter-range

Cosmo Novice

May16-11, 05:04 PM

Anything outside of our observable universe can certainly be quantified. Just wait a while, and it will be inside our observable universe.

We can measure the universe's spatial curvature, so why assume it?

Well objects that come inside our OU will eventually be outside our OU once their recession >C. So while our OU may be growing now, at some point our OU will begin to shrink. as galaxies at the edge of our OU begin to recede >C.

Also while we can measure spatial curvature the degree of error still lends to pos,neg or flat curvature.

How can anything outside the observable universe ever be quantified? I strongly disagree. Perhaps the source of our disagreement resides in the definition of what constitutes 'observable'.

Do cosmo models not dictate an isotropic and homogenous U with only local variance - which does specifically quantify the unobservable by measuring the unobservable affect on the OU? So effectively although cosmo models are saying 'its just more of the same' then they can still safely make this quantifying assumption?

narrator

May17-11, 08:30 AM

Cosmo Novice

May17-11, 09:28 AM

Infinity question: If we headed directly into space traveling at many times the speed of light (ignoring for a moment that you can't travel that fast), maintaining exactly the same course for the whole trip, would or could we eventually find ourselves heading back to Earth?

This would depend entirely on the topology of U. If Euclidean and infinite then not as you would just travel indefintely.

If U was an n-sphere then yes.

I am sure someone can elaborate further but as this is off topic you may be better starting a fresh thread.

narrator

May17-11, 09:35 AM

I am sure someone can elaborate further but as this is off topic you may be better starting a fresh thread.

Will do, thanks ;)

George Jones

May17-11, 02:52 PM

Anything outside of our observable universe can certainly be quantified. Just wait a while, and it will be inside our observable universe.

If dark energy really is like a cosmological constant, then this isn't true for ours universe.
Well objects that come inside our OU will eventually be outside our OU once their recession >C. So while our OU may be growing now, at some point our OU will begin to shrink. as galaxies at the edge of our OU begin to recede >C.

I think that you confused the Hubble (sphere) radius with the cosmological event horizon.
For a flat universe that exponentially expands for all time, the Hubble radius is the cosmological event horizon, but (as in all universes) we never see anything cross the horizon, so we never see anything on the Hubble sphere.

In our universe, the Hubble sphere and the cosmological event horizon don't correspond, even in the distant future. If we can see galaxy A now, it will never disappear. At some future time, A will be "receding" with a speed greater than the speed of light, but, even after this time, we will see A with (exponentially) increasing redshift, and with increasing faintness. In principle, we will never lose sight of A. In fact, some stuff that we see now (for example, the CMB from the (near) the surface of last scattering) was outside the Hubble sphere when the light we now see started its journey.

Cosmo Novice

May17-11, 06:53 PM

I think that you confused the Hubble (sphere) radius with the cosmological event horizon.

What is a cosmological event horizon? What I meant was that our OU will eventually (billions of years) consist of less galaxies as once a distant galaxy receeds>C and all light emiited prior to a recession>C reaches us then we will no longer see said galaxy. Over billions of years will this not be true of all galaxies, or will clusters/superclusters stay clumped?

Is this incorrect? A little more explanation would be nice.

Thanks

Chronos

May18-11, 06:21 PM

The number of galaxies will not effectively change, merely the distance between them will increase and the CMB temperature will decrease. This is known as the 'heat death' of the universe.

George Jones

May24-11, 03:08 PM

What is a cosmological event horizon?

Consider the following two disjoint subsets of spacetime:

1) those events that we have seen, or that we will see;

2) those events that we will never see.

The cosmological event horizon is the boundary between these two subsets of spacetime.
What I meant was that our OU will eventually (billions of years) consist of less galaxies as once a distant galaxy receeds>C and all light emiited prior to a recession>C reaches us then we will no longer see said galaxy.

As I said above this isn't true. It is true that recession speeds of galaxies that we now see will eventually exceed c, but it is not true that we loose sight of a galaxy once its recession speed exceeds c. If we see a galaxy now, then we will (in principle) always see the galaxy, even when its recession speed exceeds c. It might seem that moving to a recession speed of c represents a transition from subset 1) to subset 2), but this isn't the case.

Suppose we now see galaxy A. Assume that at time t in the future, A's recession speed is greater than c, and that at this time someone in galaxy A fires a laser pulse directly at us. Even though the pulse is fired directly at us, the proper distance between us and the pulse will initially increase. After a while, however, the pulse will "turn around", and the proper distance between us and the pulse will decrease, and the pulse will reach us, i.e., we still see galaxy A.

Khashishi

May24-11, 04:49 PM

BB doesn't say if the universe is finite or infinite. The expansion of the universe is formulated as a scaling of distances, not an increase in the size of the universe. You can have a scaling of distances in a finite or infinite universe. That said, an infinite universe can't have positive curvature (and be homogeneous, isotropic). Maybe dark energy is the universe constraining itself to not become positively curved, pushing back against the pull of gravity.

Neandethal00

May24-11, 07:40 PM

Consider the following two disjoint subsets of spacetime:

Suppose we now see galaxy A. Assume that at time t in the future, A's recession speed is greater than c, and that at this time someone in galaxy A fires a laser pulse directly at us. Even though the pulse is fired directly at us, the proper distance between us and the pulse will initially increase. After a while, however, the pulse will "turn around", and the proper distance between us and the pulse will decrease, and the pulse will reach us, i.e., we still see galaxy A.

Probably not correct. I argued with a few people in another forum whether photons have inertia or not. Eventually I realized it creates more problems, specially in experimental results with light, if we assume moving frames have no effect on photons.

Which means photons of galaxies receding with FTL speed may be traveling with the galaxies with FTL speed but photons speed inside the galaxy would remain the same c.

Btw, my logical mind says galaxies are not moving at FTL speed.

George Jones

May24-11, 07:52 PM

Probably not correct.

This result can be derived from the stuff in the thread

http://www.physicsforums.com/showthread.php?t=491078.
I argued with a few people in another forum whether photons have inertia or not. Eventually I realized it creates more problems, specially in experimental results with light, if we assume moving frames have no effect on photons.

Which means photons of galaxies receding with FTL speed may be traveling with the galaxies with FTL speed but photons speed inside the galaxy would remain the same c.

Btw, my logical mind says galaxies are not moving at FTL speed.

Cosmo Novice

May27-11, 08:07 AM

As I said above this isn't true. It is true that recession speeds of galaxies that we now see will eventually exceed c, but it is not true that we loose sight of a galaxy once its recession speed exceeds c. If we see a galaxy now, then we will (in principle) always see the galaxy, even when its recession speed exceeds c. It might seem that moving to a recession speed of c represents a transition from subset 1) to subset 2), but this isn't the case.

Suppose we now see galaxy A. Assume that at time t in the future, A's recession speed is greater than c, and that at this time someone in galaxy A fires a laser pulse directly at us. Even though the pulse is fired directly at us, the proper distance between us and the pulse will initially increase. After a while, however, the pulse will "turn around", and the proper distance between us and the pulse will decrease, and the pulse will reach us, i.e., we still see galaxy A.

How will it "turn around" Can you please clarify this point.

I am in galaxy A looking at Galaxy B. At some point t in the future Galaxy B recession speed exceeds c. So surely at t the last photon ever released prior to galaxy B crossing the threshold into expansion>c is released. Once this photon gets to us would this not be the last photon we would ever see from Galaxy B?

In understand light will get redshifted, but assumed this requires expansion<c otherwise it would not be measurable.

If you can clarify this for me I would greatly appreciate this.

Lost in Space

May29-11, 11:41 AM

If something is finite it can be quantified. Einstein suggested that the universe was finite but had an infinite boundary. And if something is finite the question is, is it confined to or contained in a bigger state of finity (and so on ad infinitum), or is infinity its ultimate container?

George Jones

May30-11, 02:23 PM

I know this is very counter-intuitive, but I really did mean what I wrote in posts #52 and #55. :biggrin:
How will it "turn around" Can you please clarify this point.

Thanks for pushing me for further explanation, as this has forced me to think more conceptually about what happens.

This can happen because the Hubble constant decreases with time (more on this near the end of this post) in the standard cosmological model for our universe. Consider the following diagram:

O B A C
* * * *

* * * *
O B A C

The bottom row of asterisks represents the positions in space (proper distances) of us (O) and galaxies B, A, and C, all at the same instant of cosmic time, t_e. The top row of asterisks represents the positions in space of us (O) and galaxies B, A, and C, all at some later instant of cosmic time, t. Notice that space has "expanded" between times t_e and t.

Suppose that at time t_e: 1) galaxy A has recession speed (from us) greater than c; 2) galaxy A fires a laser pulse directed at us. Also suppose that at time t, galaxy B receives this laser pulse. In other words, the pulse was emitted from A in the bottom row and received by B in the top row. Because A's recession speed at time t_e is greater than c, the pulse fired towards us has actually moved away from us between times t_e and t.

Now, suppose that the distance from us to galaxy B at time t is the same as the distance to galaxy C at time t_e. Even though the distances are the same, the recession speed of B at time t is less than than the recession speed of C at time t_e because:

1) recession speed equals the Hubble constant multiplied by distance;

2) the value of the Hubble constant decreases between times t_e and t.

Since A's recession speed at time t_e is greater than c, and galaxy C is farther than A, galaxy C's recession speed at time t_e also is greater than c. If, however, the Hubble constant decreases enough between times t_e and t, then B's recession speed at time t can be less than c. If this is the case, then at time t (and spatial position B), the pulse is moving towards us, i.e., the pulse "turned around" at some time between times t_e and t.

If the value of the Hubble constant changes with time, what does the "constant" part of "Hubble constant" mean? It means constant in space. At time t_e, galaxies O, B, A, and C all perceive the same value for the Hubble constant. At time t, galaxies O, B, A, and C all perceive the same value for the Hubble constant. But these two values are different.

Probably some of my explanation is unclear. If so, please ask more questions.

Astralos

Jun2-11, 11:12 AM

The universe is mathematically approaching the concept of infinity, but is, and will never be, itself infinite. Nothing quantitative in the known universe can be infinite.

Cosmo Novice

Jun3-11, 08:11 AM

I know this is very counter-intuitive, but I really did mean what I wrote in posts #52 and #55. :biggrin:

Thanks for pushing me for further explanation, as this has forced me to think more conceptually about what happens.

This can happen because the Hubble constant decreases with time (more on this near the end of this post) in the standard cosmological model for our universe. Consider the following diagram:

O B A C
* * * *

* * * *
O B A C

The bottom row of asterisks represents the positions in space (proper distances) of us (O) and galaxies B, A, and C, all at the same instant of cosmic time, t_e. The top row of asterisks represents the positions in space of us (O) and galaxies B, A, and C, all at some later instant of cosmic time, t. Notice that space has "expanded" between times t_e and t.

Suppose that at time t_e: 1) galaxy A has recession speed (from us) greater than c; 2) galaxy A fires a laser pulse directed at us. Also suppose that at time t, galaxy B receives this laser pulse. In other words, the pulse was emitted from A in the bottom row and received by B in the top row. Because B's recession speed at time t_e is greater than c, the pulse fired towards us has actually moved away from us between times t_e and t.

Now, suppose that the distance from us to galaxy B at time t is the same as the distance to galaxy C at time t_e. Even though the distances are the same, the recession speed of B at time t is less than than the recession speed of C at time t_e because:

1) recession speed equals the Hubble constant multiplied by distance;

2) the value of the Hubble constant decreases between times t_e and t.

Since B's recession speed at time t_e is greater than c, galaxy C's recession speed at time t_e also is greater than c. If, however, the Hubble constant decreases enough between times t_e and t, then B's recession speed at time t can be less than c. If this is the case, then at time t (and spatial position B), the pulse is moving towards us, i.e., the pulse "turned around" at some time between times t_e and t.

If the value of the Hubble constant changes with time, what does the "constant" part of "Hubble constant" mean? It means constant in space. At time t_e, galaxies O, B, A, and C all perceive the same value for the Hubble constant. At time t, galaxies O, B, A, and C all perceive the same value for the Hubble constant. But these two values are different.

Probably some of my explanation is unclear. If so, please ask more questions.

Thankyou for taking the time to explain. This does make complete sense except one thing:

This assumes that for galaxies whose recession>c for their photons to reach us then there must be a decrease in the hubble constant. I thought the Hubble constant was the rate of acceleration of expansion and as such would always increase? I understand the constant referes to spatially constant (any given point in space will be the same constant as any other place at the same time) but am unclear whether this is increasing/decreasing.

I am a complete novice so appreciate the simpligied explanation you gave.

Thanks

George Jones

Jun3-11, 09:22 AM

This assumes that for galaxies whose recession>c for their photons to reach us then there must be a decrease in the hubble constant. I thought the Hubble constant was the rate of acceleration of expansion and as such would always increase? I understand the constant referes to spatially constant (any given point in space will be the same constant as any other place at the same time) but am unclear whether this is increasing/decreasing.

The definition of the Hubble constant H is
H = \frac{\mbox{rate at which scale increases}}{\mbox{scale of the universe}}.
The universe expands with time, so the scale of the universe increases with time. Accelerated expansion means that the rate at which the scale increases itself increases, i.e., the rate tomorrow at which the scale increases is greater than rate today at which the scale increases. If, over a given period of time, the increase in the scale of the universe is proportionately greater than the increase in the rate at which the scale increases, then the Hubble constant decreases with time (since the denominator increases faster than the numerator. Observations indicate that this true now, and that this will remain true in the future.

I might later post a specific example.

Cosmo Novice

Jun3-11, 09:30 AM

The definition of the Hubble constant H is
H = \frac{\mbox{rate at which scale increases}}{\mbox{scale of the universe}}.
The universe expands with time, so the scale of the universe increases with time. Accelerated expansion means that the rate at which the scale increases itself increases, i.e., the rate tomorrow at which the scale increases is greater than rate today at which the scale increases. If, over a given period of time, the increase in the scale of the universe is proportionately greater than the increase in the rate at which the scale increases, then the Hubble constant decreases with time (since the denominator increases faster than the numerator. Observations indicate that this true now, and that this will remain true in the future.

I might later post a specific example.

Thankyou for the explanation. Although I still find this very counter-intuitive, although I can see the logic behind galaxies whose recession>c photons still reaching us. I do not think I require a specfic example in this case but thankyou.

Ok so while I know see the logic in galaxies with recession>c light reaching us - giving certain circusmtance. Am I safe in assuming that beyond the OU current cosmological models indicate galaxies so far away and receeding so much >c that their light will never reach us?

I guess the core question I am posing is: Beyond our OU, is there a cutoff point, in terms off recession speeds>c where we will no longer recieve photons from galaxies further out than this cutoff point?

Lost in Space

Jun4-11, 09:04 AM

Aren't scales just another example of the strong anthropic principle?

Chronos

Jun4-11, 03:50 PM

Lost in Space

Jun5-11, 07:32 AM

Only to the extent necessary for the universe to be sufficiently large and ancient to permit our existence at this point in its history. Our efforts to measure scale factors is motivated by curiosity about the origins and destiny of the universe, not anthropic principles.

But if the laws of physics break down at the boundaries, aren't these scale factors merely reduced to a human perspective and, given that, are they any closer to describing reality or are they merely a reflection on what we consider important in relation to ourselves?

Fortnum

Jun5-11, 10:29 AM

Since the Big Bang took place a finite time ago, the Universe would have had to expand at an infinite rate to reach an infinite size. Unless it was already infinite at the time of the Big Bang.

bcrowell

Jun5-11, 10:33 AM

I guess the core question I am posing is: Beyond our OU, is there a cutoff point, in terms off recession speeds>c where we will no longer recieve photons from galaxies further out than this cutoff point?
If the universe is spatially infinite, then yes.

But if the laws of physics break down at the boundaries, aren't these scale factors merely reduced to a human perspective and, given that, are they any closer to describing reality or are they merely a reflection on what we consider important in relation to ourselves?
The universe doesn't have a boundary. The observable universe has a boundary. The laws of physics don't break down at the boundary of the observable universe. The boundary of the observable universe is not a place with special physical properties. It's simply the set of all points from which light has just barely had time to reach our own planet since the Big Bang. Tomorrow, that boundary will be about 3 light-days farther from us than it is today, so a certain volume of space will have become newly available to us for observation.

BTW, we have a new entry on this topic in the cosmology forum's sticky FAQ thread.

Lost in Space

Jun5-11, 10:43 AM

If the universe is spatially infinite, then yes.

The universe doesn't have a boundary. The observable universe has a boundary. The laws of physics don't break down at the boundary of the observable universe. The boundary of the observable universe is not a place with special physical properties. It's simply the set of all points from which light has just barely had time to reach our own planet since the Big Bang. Tomorrow, that boundary will be about 3 light-days farther from us than it is today, so a certain volume of space will have become newly available to us for observation.

BTW, we have a new entry on this topic in the cosmology forum's sticky FAQ thread.

Pardon my confusion, but I've been given to understand that the Big Bang is a boundary where the laws of physics break down? What about event horizons of black holes? And isn't the present an ever moving and growing boundary as well as we cannot view future events, only events in the past?

Lost in Space

Jun5-11, 10:52 AM

Since the Big Bang took place a finite time ago, the Universe would have had to expand at an infinite rate to reach an infinite size. Unless it was already infinite at the time of the Big Bang.

Is it finite, or is it just us measuring a portion of time relative to our own existence? Can time be divided into infinite pieces? In other words is there a state in which the 'finite' time we perceive since the Big Bang can be said to be 'infinite'?

Cosmo Novice

Jun5-11, 02:29 PM

If the universe is spatially infinite, then yes.

Ok. Can you please clarify.

I would suggest that even if U was spatially finite then it would probably be large enough for their to be photon emitting objects so far away that the expansion and scale factor of the intervening space would result in the photon never reaching us.

Shenstar

Jun5-11, 02:50 PM

I find an infinite universe impossible to comprehend.

thekushal276

Jun5-11, 04:55 PM

I can't agree with you more, Mentat. I would say, maybe, that the universe is on the verge of infinity?[?] i dont think that universe is infinite. logic behind this is: as we know that infinite is not a real number. that means infinite is not reality. it is our imagination. the thing which cannot be counted we refer it as INFINITE. something imaginary, not real or uncountable. but that do not means that the universe is infinite. the thing is that we are not able enough to explore it and define the size of it...

bcrowell

Jun6-11, 01:29 AM

I would suggest that even if U was spatially finite then it would probably be large enough for their to be photon emitting objects so far away that the expansion and scale factor of the intervening space would result in the photon never reaching us.
I didn't quite get that right. For a closed universe with zero cosmological constant, you get a recollapse, so nothing is ever permanently hidden from any observer. But we know that the cosmological constant isn't zero, and recollapse is ruled out.

Pardon my confusion, but I've been given to understand that the Big Bang is a boundary where the laws of physics break down? What about event horizons of black holes? And isn't the present an ever moving and growing boundary as well as we cannot view future events, only events in the past?
These are all different cases. The Big Bang is a physical singularity (not just a coordinate singularity). The event horizons of black holes are not physical singularities. The boundary between past and present isn't a uniquely defined thing in relativity. I thought you were talking about the boundary of the observable universe, which is still another thing.

bcrowell

Jun6-11, 01:31 AM

i dont think that universe is infinite. logic behind this is: as we know that infinite is not a real number. that means infinite is not reality. it is our imagination. the thing which cannot be counted we refer it as INFINITE. something imaginary, not real or uncountable. but that do not means that the universe is infinite. the thing is that we are not able enough to explore it and define the size of it...

Please read the FAQ entry "Is the universe finite, or is it infinite?" in the thread titled "Frequently Asked Questions (FAQ) about Cosmology" at the top of the cosmology forum.

Lost in Space

Jun6-11, 08:38 AM

These are all different cases. The Big Bang is a physical singularity (not just a coordinate singularity). The event horizons of black holes are not physical singularities. The boundary between past and present isn't a uniquely defined thing in relativity. I thought you were talking about the boundary of the observable universe, which is still another thing.

But aren't black holes physical singularities bounded by event horizons? And they're part of the observable universe are they not?

bcrowell

Jun6-11, 11:12 AM

But aren't black holes physical singularities bounded by event horizons? And they're part of the observable universe are they not?

The physical (non-coordinate) singularity is at the black hole's center, not at its event horizon.

Lost in Space

Jun6-11, 11:27 AM

The physical (non-coordinate) singularity is at the black hole's center, not at its event horizon.

Yes, I understand this much but surely the boundary of the black hole is its event horizon which separates us from it? Although we cannot observe the singularity at the black hole's centre, we are still aware that it's there, so isn't the event horizon in this case a boundary within the observable universe?

narrator

Jun6-11, 11:39 AM

I find an infinite universe impossible to comprehend.

I find it difficult but not impossible. We live our lives by many paradigms and when confronted with one that's different we think "contradiction" or even "contravention".

bcrowell

Jun6-11, 11:48 AM

Yes, I understand this much but surely the boundary of the black hole is its event horizon which separates us from it? Although we cannot observe the singularity at the black hole's centre, we are still aware that it's there, so isn't the event horizon in this case a boundary within the observable universe?

The boundary between the US and Canada is also a boundary within the observable universe. In your earlier post, you refer to "a boundary where the laws of physics break down:"

Pardon my confusion, but I've been given to understand that the Big Bang is a boundary where the laws of physics break down? What about event horizons of black holes? And isn't the present an ever moving and growing boundary as well as we cannot view future events, only events in the past?

When people refer to the laws of physics breaking down in GR, they usually mean singularities. You've been lumping together a lot of different things.

DavidMcC

Jun10-11, 04:34 AM

I find it difficult but not impossible. We live our lives by many paradigms and when confronted with one that's different we think "contradiction" or even "contravention".
That doesn't make an infinite big bang a physical possibility, though. The only way the universe could be infinite is if it never had a beginning, ie that its beginning was an infinite time ago. Not plausible.

Cosmo Novice

Jun10-11, 06:08 AM

That doesn't make an infinite big bang a physical possibility, though. The only way the universe could be infinite is if it never had a beginning, ie that its beginning was an infinite time ago. Not plausible.

I dont think this is necessarily true, as I understand it something does not require infinite age to be spatially infinite. If the U is infinite now, then essentially it was infinite at the moment of the BB, so you could say if the U is open and spatially flat it is temporally finite but spatially infinite.

DavidMcC

Jun10-11, 06:20 AM

narrator

Jun10-11, 09:52 AM

You may have to re-invent physics for that, Cosmo!

That depends on your understanding of physics. Even of those of us who understand everyday physics, very few understand the physics of black holes (for example).

As a simplistic example, under the right conditions, a fog appears everywhere with no starting point. Sure, the analogy breaks down if you get into the nitty gritty, but to me, it's a very rough analog of how the universe formed - one difference being that the "right conditions" were not localized.

The other aspect of this is that these theories have come from actual "science", not from untested fantasy.

DavidMcC

Jun10-11, 11:06 AM

I see a fog descending on this thread, narrator!

Neandethal00

Jun10-11, 01:44 PM

I dont think this is necessarily true, as I understand it something does not require infinite age to be spatially infinite. If the U is infinite now, then essentially it was infinite at the moment of the BB, so you could say if the U is open and spatially flat it is temporally finite but spatially infinite.

I have a growing feeling some of you are mixing up observable universe (OU) and container of the OU. OU is the region which holds galaxies , us. There may even be objects in our OU which are beyond our observation limits at this time. If the U is really expanding, it is expanding into another region, which I call container. We have absolutely ZERO knowledge about this container, which may or may not be infinite, we can only speculate about this container, our speculation would probably be light years away from 'reality'. But OU can not be infinite to humans.

My personal opinion is our 'sensory systems' are playing a big tricks on us.

bcrowell

Jun10-11, 02:25 PM

I dont think this is necessarily true, as I understand it something does not require infinite age to be spatially infinite. If the U is infinite now, then essentially it was infinite at the moment of the BB, so you could say if the U is open and spatially flat it is temporally finite but spatially infinite.

You may have to re-invent physics for that, Cosmo!

Cosmo Novice is correct. Standard spatially flat models of the universe are spatially infinite but have only existed for a finite time.

If the U is really expanding, it is expanding into another region
This is incorrect. Here is a good explanation: http://www.astro.ucla.edu/~wright/cosmology_faq.html#XIN

Ken G

Jun10-11, 03:58 PM

My own feeling is we immediately err whenever we choose to use language like "the universe is...", regardless of how we finish the sentence. It simply isn't scientific. Instead, we must ask "we gain the following benefits by modeling the universe as A, and these other benefits by modeling it as B." This language stresses two important things about science:
1) our "knowledge" is constantly changing, and
2) we have a purpose for favoring the models we favor.
So in terms of the size of the universe, we would say that we observe no spatial curvature so the model with the fewest added assumptions is one that is spatially infinite. This in no way means that the universe is infinite, nor does it make any difference if we are incredulous about it being infinite (the history of science is wall-to-wall suspension of incredulity), it just means an infinite model keeps us from having to say anything else about our model that would be completely arbitrary. It's not even a question that the universe "either is or isn't infinite", because terms like "infinite" apply to models, not to the universe itself. If we can't measure a size, even in principle, then the universe doesn't have one, neither finite nor infinite-- it is the models that are one or the other. The sole scientific statement we can make is that we have never been able to detect any curvature that would suggest a size to the universe, the rest is just model-favoring.

Neandethal00

Jun10-11, 05:48 PM

Cosmo Novice is correct. Standard spatially flat models of the universe are spatially infinite but have only existed for a finite time.

This is incorrect. Here is a good explanation: http://www.astro.ucla.edu/~wright/cosmology_faq.html#XIN

From your link:

"Everything that we measure is within the Universe, and we see no edge or boundary or center of expansion. Thus the Universe is not expanding into anything that we can see, and this is not a profitable thing to think about".

I'm sorry, but this type of statements just turn me off from main stream science.
There is a group of 'know all' scientists who have the audacity to claim 'we already know what we wanted to know'.

There is nothing wrong in saying "We DO NOT know, yet".

Ken G

Jun10-11, 06:00 PM

I think you are missing a crucial element of what he said-- he did not say we know it isn't expanding into anything, he said "this is not a profitable thing to think about." That's quite an important element of science, noticing what models are profitable to think about, and what ones are just idle speculation. In science, the proof is in the pudding-- models are good not because they are right, they are good because they actually guide our observations and allow us to make successful predictions. Too many people want science to be some kind of "oracle of truth", and they get mad when their pet theories are not getting attention. It isn't because the pet theories are wrong, it is simply because they bear no fruit. You just have to kind of deal with it, you want science to be something that it isn't, and then you blame the scientists.

apeiron

Jun10-11, 06:36 PM

As a simplistic example, under the right conditions, a fog appears everywhere with no starting point. Sure, the analogy breaks down if you get into the nitty gritty, but to me, it's a very rough analog of how the universe formed - one difference being that the "right conditions" were not localized.

This is definitely an alternative way of viewing the issue. As a phase transition. Time and space as something definite would have been born out of something far less definite. You could call it a fog, a vagueness, a pre-geometry, a perfect symmetry.

What would be "infinite" or unlimited in the fog is degrees of freedom. So there just is no issue about the size of the space that the universe emerged from, or the one it is growing into. The beginnings are defined by their lack of such dimensional organisation, and the universe by it being a state of organised, or globally constrained, dimensionality.

DavidMcC

Jun11-11, 05:05 AM

Cosmo Novice is correct. Standard spatially flat models of the universe are spatially infinite but have only existed for a finite time.

This is incorrect. Here is a good explanation: http://www.astro.ucla.edu/~wright/cosmology_faq.html#XIN

You (and UCLA) are making the same mistake as George (IMO), in equating black holes to the classical theory of them, which does not allow for an LQG BH, in which it is a mass quantum effect (just like superfluids and supeconductors, etc).
Also, flatness cannot be measured to zero error, so a large, but finite, curvature is necessarily a possibility, no matter how accurate the measurement.

narrator

Jun11-11, 08:35 AM

You (and UCLA) are making the same mistake as George (IMO), in equating black holes to the classical theory of them, which does not allow for an LQG BH, in which it is a mass quantum effect (just like superfluids and supeconductors, etc).
Also, flatness cannot be measured to zero error, so a large, but finite, curvature is necessarily a possibility, no matter how accurate the measurement.

The uncertainty principle is well established in scientific method.

bcrowell

Jun11-11, 11:35 AM

Also, flatness cannot be measured to zero error, so a large, but finite, curvature is necessarily a possibility, no matter how accurate the measurement.
Cosmological solutions with negative spatial curvature are also spatially infinite but have existed for a finite time. You simply have an issue about cosmology that you don't understand properly, as in this quote:

That doesn't make an infinite big bang a physical possibility, though. The only way the universe could be infinite is if it never had a beginning, ie that its beginning was an infinite time ago. Not plausible.

Go to the library and pull a freshman gen ed astronomy text off the shelf. Read the chapter on cosmology. This is basic stuff that you've simply gotten wrong.

You (and UCLA) are making the same mistake as George (IMO), in equating black holes to the classical theory of them, which does not allow for an LQG BH, in which it is a mass quantum effect (just like superfluids and supeconductors, etc).
The text you quoted wasn't about black holes.

It sounds like you have some ideas about black holes and cosmology that are nonstandard. Since they're nonstandard, you can't expect other people to telepathically figure out what they are when you just make vague references to "cosmic BHs" ( http://www.physicsforums.com/showpost.php?p=3347236&postcount=73 ).

If you want to discuss your ideas about black holes and cosmology, and explain why Ned Wright doesn't know what he's talking about, I suggest you start a separate thread on that in the Independent Research forum. Lay out your ideas coherently so that they can be discussed. But please do yourself a favor and step back and try to more realistically evaluate your own knowledge. There is an extreme mismatch between the elementary mistakes you're making and your assessment of your own expertise as being superior to that of professional cosmologists like Ned Wright.

Neandethal00

Jun11-11, 01:15 PM

This is definitely an alternative way of viewing the issue. As a phase transition. Time and space as something definite would have been born out of something far less definite. You could call it a fog, a vagueness, a pre-geometry, a perfect symmetry.

I can live with this foggy phase transition analogy.

Unfortunately, in no branch of sciene 'common sense reality' is replaced by 'mathematical reality' as much as it did in astronomy/astro-physics/astro-anything.

Cosmo Novice

Jun11-11, 04:38 PM

You may have to re-invent physics for that, Cosmo!

Open and spatially euclidean flat cosmological models with 0 curvature are spatially infinite and temporally finite. This models assumes that the BB began geometrically at all points in space/time and does assume spatial infinity. This is well within the laws of physics and if space was determined to have 0 curvature then this would be the current model.

dougal217

Jun11-11, 11:57 PM

The quadrupole and octupole modes of WMAP seem to have an interesting orientation along the ecliptic plane... A bounded, finite Universe with a center of gravity would account for a number of things, including the Pioneer anomaly, and wouldn't require either a hot BB, Inflation (which is in big trouble), but most of all, doesn't need a Cosmological Principle, which is unprovable

Ken G

Jun12-11, 02:17 AM

But would it instead require the tooth fairy? That's not really facetious-- the problem with forming entirely new theories to fix some of the bugs in the old one is invariably that the bugs get replaced by gorillas.

zahero_2007

Jul8-11, 05:58 AM

Infinity is an ill defined concept so the universe is not infinite

DavidMcC

Jul9-11, 04:52 AM

Zahero, infinity is well defined, but it is a pure mathematical concept, and not useful in physics, except as an approximation (since 1/infinity = 0).
The mathematical singularity of a classical black hole is a good example.

Tanelorn

Jul10-11, 07:57 AM

I understand that all finite universe possibilities have space time wrapping around around. Which is the most correct space or space time wrapping around?

bcrowell

Jul10-11, 07:53 PM

Infinity is an ill defined concept so the universe is not infinite
No, this is incorrect. We have a FAQ on this: http://www.physicsforums.com/showthread.php?t=507003

I understand that all finite universe possibilities have space time wrapping around around. Which is the most correct space or space time wrapping around?
Only space.

Primtall

Jan15-12, 07:57 PM

[QUOTE=yenchin;3293391]The last part wasn't exactly true. -

... The set of positive integers is infinite. The set of positive integers divisible by 2 is also infinite, and half the size. The set of positive integers divisible by any known prime number is infinite , and expanding (everytime a new prime number is discovered).

Neandethal00

Jan15-12, 09:07 PM

[QUOTE=yenchin;3293391]

... The set of positive integers is infinite.

No they are not infinite. They are finite but unbounded.

Added Later: Numbers are not physical entities, the universe is.

Fuzzy Logic

Jan16-12, 09:39 AM

Whether one believes the universe is infinite or not, one must believe in infinity.

If one believes that the universe is finite, then what is beyond the universe (both in time and space)? And what was before the universe?

One cannot say that there was no before or beyond because there was no time or space. The mere notion of a singularity directly contradicts that concept. The existence of a singularity implies a void of some kind that the singularity itself resided in and expanded into.

If it is insisted that all of existence was within the singularity then how could space and time expand? What did it expand into? How does one conceive expansion of something into itself? Wouldn't it be more accurate to say that all of the matter in the universe got smaller?

If one now accepts that infinity must be possible, then why is it more plausible to have an infinite void instead of an infinite universe?

revo74

Jan16-12, 11:08 AM

From my understanding we have indirect evidence of virtual particles. No one is debating whether they exist or not. What is speculative however is this notion that virtual particles could become Universes.

Assuming that Krauss is correct that our Universe was a virtual particle with the right conditions that underwent expansion and became a Universe, there is something I am confused about, which I'm hoping someone can address.

Krauss says that empty space or the vacuum of space at extremely small scales is not truly empty or "nothing". Instead there lies a brewing sea of energy where virtual particles pop in and out of existence in a fraction of a second. This suggests that this sea of energy is a feature of our Universe. It is not a separate entity. Now, it is my understanding that Krauss is claiming that our Universe spawned from this sea of energy, which suggests to me that one independent entity (our Universe) came from a separate entity (sea of energy). So does this mean that there are two entities? One which is a feature of our Universe and the other a realm of existence outside of our Universe where it came from.

Primtall

Jan16-12, 12:59 PM

[QUOTE=Primtall;3711895]

No they are not infinite. They are finite but unbounded.

Added Later: Numbers are not physical entities, the universe is.

Need to nail down some terminology : Finite to me , means that it can be measured using a Real number. Unbounded means that it has no "end". There is no real number that allows you to measure the set of positive integers thus it is infinite. The set of known prime numbers on the otherhand is Finite , as there is a definite amount of them. According to Euclid the number of prime numbers is infinite. What we have therefore is a finite set (the known primes) expanding into the infinite (the primes ). This is a good model for the expanding universe, expanding into the infinite

Primtall

Jan16-12, 02:01 PM

"Added Later: Numbers are not physical entities, the universe is"
The post related to an assertion that 'only FINITE things can expand' . Logically this isn't so, and the example of an infinite set of numbers, expanding , supports (actually proves) this point, logically.
In the mathematical universe ,logic and physically realised logic, are one and the same thing. Without getting mystical, it can be said that the the universe is a symphonic expression of the logical - but that's a different thread.

Fuzzy Logic

Jan16-12, 02:13 PM

[QUOTE=Primtall;3711895]

No they are not infinite. They are finite but unbounded.

Added Later: Numbers are not physical entities, the universe is.

Why wouldn't real numbers be infinite? How can they be both finite and unbounded? At what value does real numbers end?

Primtall

Jan16-12, 02:58 PM

Agree with you fuzzy logic. Finite things are supposed to be measurable by a real number and the set of real numbers can't be. I think the 'finite but unbounded' business relates to ants on spherical surfaces, or the way n-dimensional beings would experience n+1 dimensions ... from the perspective of a higher dimension , all of space-time might well be an ink blot on its fabric and finite in that sense.

Neandethal00

Jan16-12, 08:20 PM

Assuming that Krauss is correct that our Universe was a virtual particle with the right conditions that underwent expansion and became a Universe, there is something I am confused about, which I'm hoping someone can address.

Krauss says that empty space or the vacuum of space at extremely small scales is not truly empty or "nothing". Instead there lies a brewing sea of energy where virtual particles pop in and out of existence in a fraction of a second. This suggests that this sea of energy is a feature of our Universe. It is not a separate entity. Now, it is my understanding that Krauss is claiming that our Universe spawned from this sea of energy, which suggests to me that one independent entity (our Universe) came from a separate entity (sea of energy). So does this mean that there are two entities? One which is a feature of our Universe and the other a realm of existence outside of our Universe where it came from.

The other late night I was listening to a talk radio (guess!!) where one physicist for an hour kept babbling "the universe came from nothing, things can come from nothing", etc.
Sometimes I wonder why some simple things do not come to some brilliant minds. If something comes from a place, that place is NOT nothing. Just because we do not see or do not know yet, does not mean that place is nothing.

Krauss is right, Space is a medium. Empty space is not empty. We do not yet know the true nature and structure of this medium. Sometimes, I'm afraid we may never know. That's why to us it appears things come from nothing. Energy explanation of Krauss is a possibility.

Neandethal00

Jan16-12, 08:46 PM

[QUOTE=Neandethal00;3711971]

Why wouldn't real numbers be infinite? How can they be both finite and unbounded? At what value does real numbers end?

A few posters including me are insisting the universe is finite. It does not matter how many ways, in how many words we write this, many of you will not be able to figure out exactly where this argument comes from until you start visualizing any physical object and taking it (or its properties) to infinity. Where ever you stop, it is finite.

The post related to an assertion that 'only FINITE things can expand' Primtall.

No, the assertion is all physical things are finite.

Primtall

Jan17-12, 04:28 AM

[QUOTE=Fuzzy Logic;3712903]

A few posters including me are insisting the universe is finite. It does not matter how many ways, in how many words we write this, many of you will not be able to figure out exactly where this argument comes from until you start visualizing any physical object and taking it (or its properties) to infinity. Where ever you stop, it is finite.

Primtall.

No, the assertion is all physical things are finite.

Again, not true.
My fingernail is Infinite. It is composed out of the infinite by the imposition of an arbitrary boundary condition, in the same way that the perfect circle bounds infinity
(pi being 3.14159 .... ad infinitum). What we call the Finite is just some demarkation or
Limit imposed on the infinity of the infinitesimally small (to us). This applies equally whether you are talking about the size of the universe or the size of the coastline of Ireland (scale and Zoom being just arbitrary parameters). In the case of the expanding universe,the discussion is really about expanding Limits. Infinity is already there (your container) , and the limit of the universe as t goes to infinity , is infinity. The model of the 'set of known primes' expanding into the 'set of primes' captures the relationship quite well.

Primtall

Jan17-12, 04:48 AM

Further, there's been alot of nonsense posted about 'something' co-existing alongside 'nothing'. It's not like light and pockets of darkness. Nothing means 'nothing at all, ie. no potential , no quantum flux, no entropic chaos, no fuzz on the tv screen before the coherent signal is received, Nothing AT ALL. It really is one or the other - as apart from 'something' there is nothing. Prof. Hawkings was surely wandering when he said that m-theory solved the mystery of 'why there is something instead of nothing'. If you define 'nothing' as any of the above then yes , but you may as well go mystical and say that the material world emerges spontaneously from the non-material etheric realm. "Why is there m-theory instead of nothing ? " professor ?

minio

Jan17-12, 08:05 AM

I doubt that it could not be determined. I personally agree with theories that our universe in 3D has currently some 80 billions ly across. However if time is considered as another dimension then we cannot say if universe is finite because we have just one limit (big bang 14 bilion years ago), but not other and thus is IMHO not possible to make conclusions unless we hit the end of time. However I am not physicist nor matematician, so I might be wrong :)

Primtall

Jan17-12, 11:33 AM

Neandethal00

Jan17-12, 12:08 PM

[QUOTE=Neandethal00;3713496]

Again, not true.
My fingernail is Infinite. It is composed out of the infinite by the imposition of an arbitrary boundary condition, in the same way that the perfect circle bounds infinity
(pi being 3.14159 .... ad infinitum). What we call the Finite is just some demarkation or
Limit imposed on the infinity of the infinitesimally small (to us). This applies equally whether you are talking about the size of the universe or the size of the coastline of Ireland (scale and Zoom being just arbitrary parameters). In the case of the expanding universe,the discussion is really about expanding Limits. Infinity is already there (your container) , and the limit of the universe as t goes to infinity , is infinity. The model of the 'set of known primes' expanding into the 'set of primes' captures the relationship quite well.

Whatever.....

Numbers have no meaning until you use (physical) them.
This arguments will go no where.. Let's not waste our mental energy.

Primtall

Jan17-12, 12:33 PM

all right

minio

Jan18-12, 05:43 AM

My fingernail is Infinite. It is composed out of the infinite by the imposition of an arbitrary boundary condition, in the same way that the perfect circle bounds infinity
(pi being 3.14159 .... ad infinitum).
I am to sure if I get you completely right, but your argument stands only if the spacetime is continuous. If it is discrete, once you have boundaries you have finite size, in the sense that you have finite number of elements contained within boundaries.

Can it be though, when the thing being measured is expanding?
In case the expansion has limit where it stops than yes.

Primtall

Jan18-12, 06:57 AM

The discrete is really just a normalisation of the continuos (something to make equations and digital circuitry work - like normalising planck density). Even the samples of digital signal processing are normalisations of continuos signals. But yes, if ST was discrete and the expansion stopped, then in theory you could tot up all the blobs and all would be finite - except for within the blobs themselves where infinity would still rage ( infinitesimally small = infinitely divisible) . Whats Between the blobs though ? if the answer is 'nothing' then ST is continuos. There could well be some higher dimensional fabric between them though. Several programs running in the memory of my pc are seperated by ... the operating system i suppose ...
I don't know Minio, i think we'd have to be higher dimensional beings to know this one. The existence of a boundary satisfies the definition of 'Finite', and the fingernail or the circle are finite in that sense, but because they are infinitely divisible, in another sense , from some other level of reality, they are each an infinite universe. And 'our' level of reality is really just an arbitrary scaling parameter.

Primtall

Jan18-12, 07:23 AM

by the way, it is thought that the rate of expansion is actually increasing, so a rate of
0.007% per million years might become .008% at some time in the future :-)
This doesn't suggest convergence onto a limit as t goes to infinity, If it did then i would call it finite.

minio

Jan18-12, 07:45 AM

The question is, if time is infinite. However I agree that we would need to be "Gods" to get definite answer for infinity of our universe :)

Primtall

Jan18-12, 08:22 AM

Yes. My own belief is that the standard model of reality is backwards. We get taught that consiousness exists within Space Time , but i believe that Space Time exist within consiousness and hence the exasperating perplexity with the concept of infinity (as the free and the infinite are attributes of consiousness)

Drakkith

Jan18-12, 06:18 PM

Drakkith

Jan18-12, 06:24 PM

just logically, if 3D can be measured by a real number (80 bil in this case) then it would be finite. Can it be though, when the thing being measured is expanding ? i would have thought not but maybe there is some convoluted definition somewhere, that makes it, according to that definition, Finite.

Sure it can be measured while expanding. The key is knowing where to start and stop your measurements and accounting for the finite speed of light and the expansion over time.

Primtall

Jan18-12, 07:50 PM

Sure it can be measured while expanding. The key is knowing where to start and stop your measurements and accounting for the finite speed of light and the expansion over time.

please go on ... so you start at the BB and where do you stop ?
ca. 14 bil yrs old but radius of ca. 80 bil ly , so expansion rate > c , expansion rate actually increasing we are told. What does c have to do with it ? Do you factor in the relativity of time ? Is it finite in the sense that it converges on a limit ? Not being confrontational here, just genuinely curious.

Fuzzy Logic

Jan18-12, 08:27 PM

This whole topic is philosophy.

For all practical purposes, infinity is of no real value to us. In any given frame of reference you can say that something is finite, but without considering the whole picture you are just being ignorant and pedantic.

Primtall

Jan19-12, 03:27 AM

its all philosophy actually, even physics (hence the title phd). If you think its productive to lobotomize away infinity then knock yourself out - see how that goes for you.

minio

Jan19-12, 03:42 AM

please go on ... so you start at the BB and where do you stop ?
ca. 14 bil yrs old but radius of ca. 80 bil ly , so expansion rate > c , expansion rate actually increasing we are told. What does c have to do with it ? Do you factor in the relativity of time ? Is it finite in the sense that it converges on a limit ? Not being confrontational here, just genuinely curious.

Actually, if you consider universe a hypersphere with diameter of ca 14. bil years, then the circumference would be 80 bil ly. So to get this no expansion > c would be needed just expansion = c.

Primtall

Jan19-12, 06:52 AM

Actually, if you consider universe a hypersphere with diameter of ca 14. bil years, then the circumference would be 80 bil ly. So to get this no expansion > c would be needed just expansion = c.

so you're saying the circumference is 80 bil ly ? Do u mean the circumference or the diameter ? It seems the lower limit is a diameter of 80 bil and the expansion of ST is not bound by Einsteinian GR ?

minio

Jan19-12, 07:08 AM

Circumference. Hyphershere has 3D surface and if what we call universe would be this surface, then, if this hypersphere has diameter ca 14. bilion years, it would have circumference about 86 bilions ly which would be lenght of this "surface universe" across.

Primtall

Jan19-12, 09:15 AM

minio

Jan19-12, 09:20 AM

It was just about > c expansion. It cannot answer finite/infinite question. If time is infinite it would infinitly expand at accelerating speed. If time is finite it will stop one day.

Primtall

Jan19-12, 10:05 AM

Yeah, i guess we get into philosophy if we go near that question (whether t is finite or not). thks again, b rgds

marcus

Jan19-12, 01:29 PM

ok, a hypersphere ( with t as radius ? ). I've heard one theorist speculate that its more like a double doughnut with the expansion pouring out into one and the contraction being sucked into the other. Think i like this one better as it conforms with newton (action/reaction). But with the rate of expansion increasing i'm still perflexed about the infinite/finite question. but thks for the explanation , b rgds

The hypersphere is a common model for the case of finite spatial volume assuming overall positive curvature. A lower bound for the radius* has been estimated in one of the NASA reports ( WMAP 5th year data, Komatsu et al).

There is no simple relation between the radius and the age of the universe. It's probably not a good idea to think of the radius as a time coordinate.

Of course the U could be infinite spatial volume---this hasn't been ruled out. the way to get a numerical grip on the question is to estimate the curvature. But more refined measurement of curvature is needed. So far one can say that IF the curvature is positive and we are in the hypersphere case then with 95% certainty the curvature is LESS than a certain amount.

this corresponds to the radius of curvature being AT LEAST a certain length (with 95% certainty). And that lower bound turned out, according to Komatsu et al WMAP5 report, to be around 100 billion light years.

So by getting a quantitative handle on the curvature you get a quantitative handle on the current spatial volume of the universe in that case. IF it is finite at all, and if we are in the positive curve situation, then the radius is AT LEAST a certain amount.

It is still very iffy and preliminary. Better measurement is in progress but it could be a few years, or even many years, before we have a good grip on this curvature number, and thus on the radius in the hypersphere case. I'm clueless as to what to expect.

I like thinking about the hypersphere case but its still just a speculative exercise. I'll get a link to Komatsu et al in case you want to check it out.
Wow, I simply googled "Komatsu WMAP 5" and it came up first hit!
http://arxiv.org/abs/0803.0547
If interested, look at Table 2 on page 4 where it says "curvature radius". If the notation conventions are unfamiliar, ask.

The column to pick is WMAP+BAO+SN because that uses all the available data, not only WMAP but also galaxy counts at various distances and also supernovae. The number h can be taken to be h = 0.7. It is a way of letting people adjust according to their latest figure for the Hubble parameter. If you think the right figure is 71 km/s per Mpc then use h = 0.71. So then you see that in the positive curvature case R > 22 h-1 Gpc. That is roughly 100 billion ly. Just a lower bound, could be much larger.