Inflation theory and finite universe

[honzik]

Hi,

I have just one "simple" question:

 

Is cosmos(universe) finite or infinite? According to some articles I have read about inflation it seems that inflation theory works only with finite number of particles and finite space.

 

Suppose that inflation "spread" in time - ie. that it started somewhere (in one place or in infinite places) and subsequently (at finite speed) the effect of inflation influenced surrounding places.

 

Let's discuss the alternative that cosmos is not finite - because here we will (I think) get into some trouble:

 

a) Let's suppose that inflation was "started" at bounded volume in (infinite) space. It then follows that:

I) inflation still continues in some place in the universe (as it spreads more and more from the place where it started) - because space is infinite and the speed of inflation was (is) finite

or II) inflation ended some time ago. But in this case there has to be some (infinite) space in the universe not influenced by inflation simply because the effect of inflation didn't reach this place (inflation took finite time and spread in finite speed) - but this can imply that space is not homogenous (in places which inflation didn't reach). And maybe it can imply some other hard to explain difficulties that a better knowledged cosmologist can explain.

 

b) Let's suppose that inflation started in the whole infinite universe at approximatelly the same time. (In case it stated in the "half" of the universe - yet still infinite half - we will be at the same situation as in points aI) and aII).). But this point seems really strange - how could be inflation started at (approximatelly) the same time on the "opposite" (or better to say very very far - arbitrary far) places of the universe? How should these "opposite" places "know" that they had to start with inflation? And even if the inflation started in the whole infinite universe at the same time - there could be problem (as I nave read somewhere) with homogenity of the universe after inflation.

 

c) The most interesting (and crazy) idea: the cosmos was finite before inflation and after inflation it was infinite - which seams to me really unbelievable.

 

d) The whole inflation is just one quantum "change" (but I don't specify exactly the details) of one particle one one other quantum entity - that took place at one time all over the universe (quantum effects can show this behavior, i think).

 

Can you please tell me which of these ideas could (theoretically and reasonably) happen and which ones are only pure speculations with no theory (known up to today) behind them or that are in contradiction in some reputable theory?

 

Thank you very much for your answers.

 

Honzik

[jackson33]

honzik wrote;

honzik I have just one "simple" question:

 

Is cosmos(universe) finite or infinite?

 

 

The 'simple' answer is, no one knows. The Universe could be infinite (never ending & I would doubt) and space that our Universe sets in realistically should be infinite. Might add it will never be known...IMO.

 

Inflationary, I think your mixing up with 'expansion' and seemingly of interest to a good many people. Inflationary came to light by Fred Hoyle (1990's), others before trying to counter 'Expansion', trying to explain an alternative to the then Big Bang Theory, which in fact he named. His idea was that all it would take would be one atom of Hydrogen to form per cubic meter of space per hundred years to create the expansion (calling it inflationary or from with in) assumed by Hubble years earlier, explained as expansion from an edge.

 

http://aether.lbl.gov/www/science/inflation-beginners.html

 

All of your ideas, even others classified acceptable to science are or in part are speculative...best add... In My Opinion. However 'd' could be Incorporated into BBT.

 

Maybe this will get your post started....'honsik' an interesting name, are you from Germany? Possibly Jewish or Arabic Heritage?

[Airbrush]

I like the theory the universe was created by a collision of higher dimensions. Like two bed sheets hanging parallel and very close together on clothes lines. When a wind blows the sheets will come into contact with each other (big bang) not at one point (not at a singularity), but at various regions of indefinite sizes. Our universe is the result of a collision of our local region on one bed sheet.

[J.C.MacSwell]

I like the theory the universe was created by a collision of higher dimensions. Like two bed sheets hanging parallel and very close together on clothes lines. When a wind blows the sheets will come into contact with each other (big bang) not at one point (not at a singularity), but at various regions of indefinite sizes. Our universe is the result of a collision of our local region on one bed sheet.

 

Hmm...I'll have to sleep on that one.

[honzik]

Inflationary, I think your mixing up with 'expansion' and seemingly of interest to a good many people. Inflationary came to light by Fred Hoyle (1990's), others before trying to counter 'Expansion', trying to explain an alternative to the then Big Bang Theory, which in fact he named. His idea was that all it would take would be one atom of Hydrogen to form per cubic meter of space per hundred years to create the expansion (calling it inflationary or from with in) assumed by Hubble years earlier, explained as expansion from an edge.

 

Maybe this will get your post started....'honsik' an interesting name, are you from Germany? Possibly Jewish or Arabic Heritage?

 

As I have read some articles about inflation - it is in fact possible that inflation started even in infinite universe, but as the "inflation bubble" spread through space it only affected finite space - and the rest of the space in the universe was not affected by this "inflation bubble"? Am I right? So what was at the border of this bubble? And - had this bubble some borer? In fact - this bubble still exists and we are in it - am i right?

 

Thank you for your answers.

 

PS: I am Czech and Honzik is my diminutive name - I was just looking for some login name.

[Airbrush]

Am I right? So what was at the border of this bubble? And - had this bubble some borer? In fact - this bubble still exists and we are in it - am i right?

 

You are right. Either the universe is infinite, or it is finite, but surrounded by infinity. Beyond the border of this bubble is the unknowable.

[Martin]

You are right. Either the universe is infinite, or it is finite, but surrounded by infinity. Beyond the border of this bubble is the unknowable.

 

There's also the possibility that the universe is simply finite. Not surrounded by anything. Having no border or boundary.

 

That is mathematically simpler to deal with, so it is how cosmologists normally treat the positive curvature case.

 

Although it is an appealing image, to think of our universe as a "bubble"---as in the "eternal inflation" multiverse promoted by Andrei Linde---there is neither evidence for it, nor a plausible physical model to work with. Because so exotic, not much actual research is being done.

 

Symptomatic of this: nobody of the bubble multiverse advocates was invited to talk at Strings 2008. No talks about the "anthropic principle" as a way of selecting from the "landscape". Back in 2005 that was more popular with the researchers. Again at Strings 2009, no talks about that kind of stuff.

 

Although enormously appealing to the imagination, the "bubble-universe" bubble seems to have burst. Largely gone out of fashion with researchers. Nevertheless some very visible people still talk about it, like Andrei Linde at Stanford. Just not much in the way of papers.

 

The "brane clash" cosmology thing may also be on the way out. It was more popular back in 2003-2004 than it is today. Main proponents are Paul Steinhardt and Neil Hurok---neither have written much about it recently.

 

Steinhardt's recent papers have been aimed at showing that the extra dimensions of string theory are incompatible with inflation. He seems to have come around to accepting some type of inflation (which brane clash avoided) and his "no-go" results are seen as detrimental to string.

 

What I'm saying is at odds from the impression one could get from popular science media. I think the media still like both the clashing branes and the bubble multiverse visions very much.

 

All I can offer as evidence for what I'm saying is rather dry facts like research publication rates and citation numbers. Maybe sometime I'll trot out the stats on this. And of course nobody is obliged to follow the lead of the research community when it changes course (but it's worth keeping an eye on what the experts are interested in all the same.)

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Merged post follows:

Consecutive posts merged

Hi,

I have just one "simple" question:

 

Is cosmos(universe) finite or infinite? According to some articles I have read about inflation it seems that inflation theory works only with finite number of particles and finite space.

 

Honzik, we don't know if space is finite volume or infinite volume.

 

You got the wrong impression about inflation. There are many inflation scenarios (none based on established physics, all need some exotic un-observed mechanism).

 

Inflation can work with infinite volume. Doesn't require finite space or finite number of particles. Only certain specific inflation scenarios have finite space.

 

Inflation is plausible because it solves some interesting puzzles, like the fact that the Background temperature is nearly the same over the whole sky. But even though plausible it has not been proven that it happened. There are still alternative models to explain the same puzzles. So it is not yet shown to be necessary.

 

But with or without assuming inflation we still cannot answer if space is finite or infinite.

 

The Planck spacecraft launched in May 2009 will help answer this. It is the first European space mission aimed at studying the microwave Background.

Potentially an important advance.

 

I have to go. Back later.

[Alan McDougall]

My difficulty with an infinite eternal universe is the arrow of time would be pushed back to the infinite past, entropy could not happen and we would never had reached the moment in time we now exist in

 

A athletic competition must have a start line or the expectant spectators would wait for ever for them to pass in front of them at the finish line

[Martin]

My difficulty with an infinite eternal universe is the arrow of time would be pushed back to the infinite past, entropy could not happen and we would never had reached the moment in time we now exist in

 

A athletic competition must have a start line or the expectant spectators would wait for ever for them to pass in front of them at the finish line

 

Argument from incomprehension.

 

Some points:

* Infinite spatial volume is not logically equivalent to eternal.

Honzik is asking do we know whether spatial volume is finite or infinite.

Your talking about eternal is off topic, strictly speaking.

 

* Current research is pushing the time horizon back before the big bang and trying to model conditions leading up to it. In effect getting rid of the supposed "singularity".

That does not say anything about eternal. Just because we might pass that one barrier does not mean there will not be found another barrier. Don't jump to the conclusion of eternal just because it looks like that one barrier may be in the process of being penetrated. (And the models must be tested.)

 

* But suppose you want to consider an eternal model, extending back in time indefinitely (I think this is pre-mature but suppose you do.) The there is in principle no problem with entropy. If a single observer, with a single measure of the entropy, is possible then the entropy can just get smaller and smaller as you go back in time.

 

Think of the function e^t on the real line from -oo to +oo.

It is everywhere defined and positive and increasing. No problem.

 

Remember that the Newtonian Universe had an infinite past. And this did not bother Newton. He took that as natural. He didn't seem to think that there must be a starting line to the race. The man was not philosophically so naive.

 

And when Boltzmann delved into the concept of entropy in the 1870s and 1880s, there was no illusion of a "big bang beginning". They were still using a Newtonian Universe, with its infinite past. Boltzmann had no problem. The past consists of finite moments each of which is a finite duration back into the past. This allows a finite positive entropy to be defined at each point in the infinite past, if you can posit an observer with a consistent definition of the state space.

 

Boltzmann was philosophically sophisticated. Not only one of the greatest physicists of the 19th (along with Maxwell) but also he gave lectures on the Philosophy of Science at the University of Vienna. Cool guy.

 

Also a feminist of sorts. He fought for the right of some woman to unofficially audit courses at the university, after she was denied permission (because as a woman it didn't seem to the administration to make sense for her even to sit in lecture hall ) http://en.wikipedia.org/wiki/Ludwig_Boltzmann

Edited June 6, 2009 by Martin

[honzik]

Honzik, we don't know if space is finite volume or infinite volume.

 

You got the wrong impression about inflation. There are many inflation scenarios (none based on established physics, all need some exotic un-observed mechanism).

 

Inflation can work with infinite volume. Doesn't require finite space or finite number of particles. Only certain specific inflation scenarios have finite space.

 

 

I have the key question to understand inflation:

 

Could inflation as an event start in the whole (infinite) universe at the same time? If this is true how could the whole universe (all places in the universe) at the same time "decide" to start inflation? This seems to me unreal - How could two distant places at the same time know that they had to start some "action"?

 

Thank you for answering / oppinions.

[Martin]

...

Could inflation as an event start in the whole (infinite) universe at the same time? If this is true how could the whole universe (all places in the universe) at the same time "decide" to start inflation? This seems to me unreal - How could two distant places at the same time know that they had to start some "action"?.

 

I like your question! It's a very smart question. Let me think a few minutes. The difficulty is that there is no physical process known that could cause inflation. One has to imagine an "inflaton" field which is a kind of fairytale thing. So what can one say about what is possible or not possible for a fairytale thing that has never been observed. I will think some and then try to reply.

[ajb]

I do not think that one needs all regions in the universe to experience inflation at the same time or even the same rate.

 

All one needs is local inflation of (initially) causally connected regions. (I think)

[Martin]

I have the key question to understand inflation:

 

Could inflation as an event start in the whole (infinite) universe at the same time?...

 

Hans, it is clearly easier to imagine inflation in a finite region. This is the picture in one of the popular inflation scenarios called Eternal Inflation.

 

Your question is whether finiteness is logically forced, or not.

 

Do you know the "graceful exit" problem. The idea that inflation should all stop at the same time over a huge causal-disconnected region. This is solved by programming. The "inflaton" field is imaginary, like a fairy or unicorn, so one can imagine whatever properties one wants for it. And one can imagine that the field is programmed to decay on a definite schedule.

 

(You have seen the "slow roll" pictures of how the energy density of the "inflaton" is imagined to slowly roll down.) The inflation fantasies or, as they are called, Scenarios, have the inflation end just as the universe has been expanded by a factor of e^60. It was decided by us humans that this was the right factor for distances to expand.

 

So now all we need to worry about is that things get off to a proper start. Inflation must begin simultaneously everywhere.

 

Hans, I certainly agree that it is easier to imagine a simultaneous beginning if we are thinking of a finite region where everything is in causal contact.

 

Also I personally like finiteness in cosmology. I would be pleased if it would turn out that the Planck spacecraft launched last month eventually reports a positive curvature. The whole universe, the whole of space, then is finite volume. Basically a hypersphere. That would be the conventional interpretation of such a result from Planck.

 

But I stop short of imposing my own bias or prejudice here.

 

I can't agree with your argument that inflation must have started in a finite region in order for it to have all begun at one time. This is intuitive, but I am not fully convinced.

 

My problem is I'm skeptical that space with geometry as we are used to would even exist at that time. The classical model of the continuum (technically a differential manifold, something invented at Göttingen around 1850 by Bernhard Riemann and Carl Gauss) is probably not a good model for space at that time. And then what does "finite volume" mean? These are conditions like at the pit of a supermassive black hole. Extreme density and extreme geometry. Among other things, Heisenberg uncertainty, applied to the geometry itself (not just to particles) might come into the game.

 

It could be that the quantum geometry model of Ashtekar and others is the one that is applicable. And then space is represented by a graph. What corresponds to volume is then related to the number of nodes. A very high degree of connectivity is possible in the network. This is called the quantum regime in QG, that occurs around the time of the bounce. Quantum corrections turn out to make gravity repel instead of attract, at those very high densities. Or that model might not be applicable and yet another might apply. But it is probably not the usual intuitive geometry in either case.

 

A highly connected network can evolve into one describing ordinary 3D space with normal connectivity, or locality. This happens as a part of expansion. Only traces of abnormal connectivity may remain.

 

Ashtekar and his group have studied both the finite and the infinite case and run computer models simulating the bounce and how the expansion we see could have begun. They may prefer the finite case (the hypersphere) but they have studied both. I don't think one can rule infinite case out. I personally don't like it. But I don't think one can logically exclude it (based on what we know and don't know at present.) There is simply too much we do not know about QG (quantum geometry/gravity).

 

===============

To give you a taste of current research literature, here is a search using keywords "quantum cosmology" for the date > 2006.

http://www.slac.stanford.edu/spires/find/hep/www?rawcmd=FIND+DK+QUANTUM+COSMOLOGY+AND+DATE+%3E+2006&FORMAT=www&SEQUENCE=citecount%28d%29

In earlier times this list would be dominated by old names like Hawking, Hartle, Vilenkin, Steinhardt, Andrei Linde, perhaps even Veneziano. But there has been a shift of interest in the field. You hardly see those names, among the top papers. The dominant approach to quantum cosmology is now what is called Loop quantum cosmology. Based on representing the quantum state of geometry by a network labeled with quantum numbers. Ashtekar is a central figure. And nothing prevents this from changing again. It is a rapidly evolving field

Edited June 7, 2009 by Martin