Multiverse Evidence Explanation?

[Mordred]

Good example, I think you misunderstand me though my problem isn't so much in multiverse theories.
I can't think of any good arguments as to why the multiverse shouldn't exist. The problem I have is the tendency to explaining problems such as the information paradox by statements like

"the information isn't lost it goes to a universe without black holes"
"The wormhole opens into another universe that is why we can't find the evidence of white holes"
" that anistrophy in the CMB is evidence of an interaction between our universe and another universe"

Granted many of these postulates derive from models that are not necessarily mainstay, but their is a tendency to jump to the multiverse conclusion first as opposed to finding answers in the realm of our own universe. If it proves impossible to answer a question without a multiverse then I'm OK with it.
There is a practicality in modelling alternate universes, as one can adjust the laws to test various ideas out. However it should be clear when its specifically a modelling, as opposed to a statement of existence. Granted some of that is due to pop media but not always.

 

[Naty1]

Either it is eternal, or it is generic. If it is generic, then you have a real problem explaining why it happens once, and only once when we know that quantum mechanics is about probabilities.

This seems good logic. I have thought that either our universe is a ' custom made universe, uniquely suitable for us' or one of many universes, of all types and conditions only some of which are self sustaining and perhaps perpetuating others. Probability suggests to me the latter is more likely.

In fact, I would go so far as to say that given what we know today (which isn't much), we should strongly, strongly prefer such multiverse theories.

That might be a bit much, but I'd sure like to keep an open mind, especially given the poor record of consensus science over the ages.

 

[mfb]

especially given the poor record of consensus science over the ages.

Poor record? There are some notable exceptions, but in general I think science has been extremely successful.
Most new theories are just extensions of existing theories, modifying them in some special parts to make them more precise.

 

[Naty1]

mfb posts:

...Poor record?...Most new theories are just extensions of existing theories, modifying them in some special parts to make them more precise.

I know what you mean, but in general scientists are way too ' huffed and puffed' up about too many things...and my comment IS a bit sarcastic, but many of the 'revisions' are substantial...and I bet our views of a multiverse will be laughed at 100 years from now...I sure hope so...Here is a short list. I have omitted some famous quotes, real 'doozies' , from eminent scientists such as

"High speed train travel is impossible as air will be sucked out of people's lungs.'...

I was stunned when I tried to find ANY theory that started out, was comprehensive and correct. Even stunning relativity is bounded by quantum theory, and vice versa...
Earth is covered by a circular dome Ptolomy mistaken belief lasted 1500 years
Earth is Flat Erastothenes
Aristotelian physics Wrong for 2,000 years until Galilean physics
Earth is center of universe –Copernicus Galilelo Denied by Catholic Church
Milky way IS the universe 1920’s..Hubble showed Many galaxies
Travel over 30 MPH not possible Breathing not believed possible
Planetary oribits are circular Kepler Orbits are elliptical
Nothing left to Discover… .Wolfgang Pauli See all the stuff below….
Night sky is black due to interstellar dust Edgar Allan Poe (The writer!)
Atomic weights just a curiosity Mendelev created periodic table
Proton, neutrons elementary particles Composed of quarks
All matter came from big bang Iron and heavier stuff comes from stars
Electrons point particles More likely strings, waves
Alchemy is science Widespread transmutation not possible
Universe is common matter Vera Rubin: only 5% we know, rest we don’t!
uniform galactic rotation
Black Holes don’t exist unproven, but highly likely they do
Universe has always existed Big bang theory…universe had a start
Universe is static Hubble it's actually expanding

Space-time is continuous String , entropy, & quantum theory
spinfoam
Space has three dimensions String theory: 11 more likely (Ed Witten)
Lavoisier’s caloric theory heat is a form of energy
Thompson, Joule, Carnot
Laser fusion simple (1960) after 50 yrs, still trying
We know most mass, energy hardly know any: dark is 96% of universe

 

[marcus]

I would say the multiverse conjecture is no better as an explanation than ID.
...

That sounds fair, with emphasis on "as an explanation". If you had a theory with real testability and explanatory power but which just as a side effect also implied the existence of causally separate worlds that would seem unobjectionable since the multiverse feature would not be invoked to explain anything. Just a harmless byproduct. Or?

But there's a big difference, god is an arbitrary conjecture. In this case the multiverse is not the conjecture. It comes from inflationary cosmology...

I think statement needs some qualification, Skydive. It does not come from inflationary cosmology in general.

Here is a clean simple inflationary cosmology that does not drag any multiverse in with it:
http://arxiv.org/abs/1301.1264
I think it's preferable to the other inflation scenarios I've heard because it does not need any mysterious "quantum fluctuation" to get started, or any fine-tuning to get adequate efolds and graceful exit. No elaborate or far-fetched assumptions. No funny-shaped plateau potential to "roll down". Just the quantum corrected Friedman equation and a scalar field.

So multiverse is certainly not entailed by every inflation model. Though it is entailed by some which, in comparison with 1301.1264 look over-dressed and out-of-date. My two cents.

 

[Naty1]

Marcus:

Here is a clean simple inflationary cosmology that does not drag any multiverse in with it: Duration of inflation and conditions at the bounce as a prediction of effective isotropic loop quantum cosmolog

My oh my oh my!

So happy to see you coming around to 'bounce' ideas...I still recall when I posted a Steinhardt/Turok paper and you were...well...maybe just a tad skeptical!

[I seem to recall needing a few vodka and tonics that evening! [LOL] ]

Good to see and 'old dog learning new tricks'... much, much better than my wife says about me!

 

[mfb]

@Naty1:
Most of those things were bad science, not science, bad extrapolations or good approximations.
A quote from some scientist is not a scientific theory. There was never a theory "there is nothing left to discover", and alchemy (as an example) never had a theory "transmutation is possible", just the hope that it would be (well, it is, but not with chemical methods).

Electrons point particles More likely strings, waves

I think "More likely strings" is a very ... optimistic view for string theory. Waves and point particles are not a contradiction.Newton's laws of motion / classical mechanics: More than 300 years old, and every modern theory has this as limit of low velocities, small energies, large timescales and so on. We know that it is not exact, but it is an extremely good approximation in many setups.
Evolution: More than 150 years old, and still the most important theory in biology. More and more details were added, but the basic laws are still the same.
Ordinary matter is made out of atoms: While the idea is ~2000 years old, actual scientific experiments are newer. And the model is extremely successful.
Special relativity: More than 100 years old, survived thousands of tests without any flaw. It is the basis of every modern (fundamental) theory.
General relativity: Nearly 100 years old, survived thousands of tests. The fly-by anomaly is a bit odd, but I don't think this is new physics.
Quantum theory: Our modern world would look completely different without working applications of quantum mechanics.

By the way, your post would be easier to read with [noparse]

[/noparse]-tags around the "table".

 

[Naty1]

Most of those things were bad science, not science, bad extrapolations or good approximations.

You may think all science now is good science; I have my doubts.

But the 'good news' is we do make progress...as I think your comments imply.

 

[mfb]

Not all science, but I think the fraction improved significantly, at least in physics and related areas.

 

[Mordred]

In regards to my take on improvements that cosmology has taken, I'll tell a little story of my time on an older forum that eventually shutdown on space.com. Back in 2002, we used to get a slew of questions such as below.

1) what shape is the universe, is it flat, sphere, hyperbolic, klein bottle, donut or some other shape. the answer wasn't determined then. thank WMAP.
2) which hot big bang inflationary model is correct? unfortunately there is still debate on that but not nearly as bad.
3) which is more accurate the CDM model or the hot big bang model. unfortunately at this time CDM had numerous models, and was not nearly as fine tuned as now, also during this time DM and DE were still in hot debate.

needless to say in the past 10 years numerous advancements have come along way for which I am thankful.

 

[Garth]

@Naty1:Most of those things were bad science, not science,.

If I may add a comment; the difference between "most of those things" and science was that true science depended and depends on observation, verification and falsifiability.

"Most of those things" may have been 'bad science' but they were theoretical constructs at the time of people's limited understanding of the universe around them.

When the multiverse conjecture is able to be assessed by observation, verification and falsifiability then I will accept it as a scientific concept, otherwise I fear it may end up in the 'bad science' category.

Garth

 

[marcus]

...

When the multiverse conjecture is able to be assessed by observation, verification and falsifiability then I will accept it as a scientific concept, otherwise I fear it may end up in the 'bad science' category.

Garth

Especially since the suspected "bad science" it is not actually entailed by inflation---there is at least one simply corrected form of Friedman equation (fitting observational data) that ensures adequate inflation with high probability, without fine-tuning, and without multi-ness.
http://arxiv.org/abs/1301.1264

I would guess that we will see more non-multi models appear over the next few years, but we can say that now there is at least one. Showing that inflation is not an argument. You can have reliable robust inflation, simply, without getting multi-fied.

 

[Chalnoth]

If I may add a comment; the difference between "most of those things" and science was that true science depended and depends on observation, verification and falsifiability.

"Most of those things" may have been 'bad science' but they were theoretical constructs of people's limited understanding of the universe around them.

When the multiverse conjecture is able to be assessed by observation, verification and falsifiability then I will accept it as a scientific concept, otherwise I fear it may end up in the 'bad science' category.

Garth

The various multiverse ideas should not be understood as scientific constructions on their own, but rather as components of various theories. It makes zero sense whatsoever to disregard a theory just because it happens to include a multiverse: every single aspect of a theory is not required to be testable for it to be a good scientific theory.

Furthermore, if a particular sort of multiverse idea is used in an attempt to explain some observable phenomenon (e.g. the aforementioned value of the cosmological constant), then that explanation should be understood as a statement along the lines of, "If it turns out that the fundamental laws of physics admit this particular sort of multiverse, then this process which uses that multiverse may explain the observation."

There is absolutely nothing whatsoever about this train of thought that is remotely unusual or specific to multiverse ideas.

Finally, I'll just point out that some level of multiverse is a necessary consequence of our current understanding of the standard model of particle physics. Specifically, that electroweak symmetry breaking necessarily occurs differently in different regions of the universe. Proposed models beyond the standard model tend to only increase the variety of the potential multiverse over what we would see from the standard model alone.

 

[Chronos]

I do not perceive any objection to multiverse in principle, merely in evidence. It is alluring and seductive, but, the evidence just isn't there.

 

[Garth]

Finally, I'll just point out that some level of multiverse is a necessary consequence of our current understanding of the standard model of particle physics. Specifically, that electroweak symmetry breaking necessarily occurs differently in different regions of the universe. Proposed models beyond the standard model tend to only increase the variety of the potential multiverse over what we would see from the standard model alone.

So are you saying that mutliverse ideas are the:

theoretical constructs at the time of people's limited understanding of the universe around them.

?Garth

 

[marcus]

Finally, I'll just point out that some level of multiverse is a necessary consequence of our current understanding of the standard model of particle physics. Specifically, that electroweak symmetry breaking necessarily occurs differently in different regions of the universe...

Why do you even need the word "multiverse" which some people seem so obsessed with and use at every opportunity?

We are talking here about different regions of our universe (which you refer to as "the" universe). And you suggest the possibility that some numbers that we provisionally treat as universal constants (like the ratio of dark matter to ordinary matter) might turn out NOT to be the same everywhere. Big deal Just one thing more to understand about nature and study empirically.

Maybe in the early moments of expansion some symmetry breakings occurred at slightly different times in different places and this led to regional variety in some basic ratios and proportions.

Finding out about interesting things like that is a challenge to human ingenuity and something for us to look forward to. We have learned to count the planets around other stars and we have learned that not all stars have NINE, so the number of planets is not a universal constant.

The universe is all that exists and as time goes on we find more fascinating variety as well as regularity in it. What causes people to become enamored of a word like "multiverse", for which there seems to be no earthly need?

Let me do an experiment, Chally, and look at all the times it's used in your last post:
==quote==
The various multiverse ideas should not be understood as scientific constructions on their own, but rather as components of various theories. It makes zero sense whatsoever to disregard a theory just because it happens to include a multiverse: every single aspect of a theory is not required to be testable for it to be a good scientific theory.

Furthermore, if a particular sort of multiverse idea is used in an attempt to explain some observable phenomenon (e.g. the aforementioned value of the cosmological constant), then that explanation should be understood as a statement along the lines of, "If it turns out that the fundamental laws of physics admit this particular sort of multiverse, then this process which uses that multiverse may explain the observation."

There is absolutely nothing whatsoever about this train of thought that is remotely unusual or specific to multiverse ideas.
==endquote==

 

[skydivephil]

That sounds fair, with emphasis on "as an explanation". If you had a theory with real testability and explanatory power but which just as a side effect also implied the existence of causally separate worlds that would seem unobjectionable since the multiverse feature would not be invoked to explain anything. Just a harmless byproduct. Or?


I think statement needs some qualification, Skydive. It does not come from inflationary cosmology in general.

Here is a clean simple inflationary cosmology that does not drag any multiverse in with it:
http://arxiv.org/abs/1301.1264
I think it's preferable to the other inflation scenarios I've heard because it does not need any mysterious "quantum fluctuation" to get started, or any fine-tuning to get adequate efolds and graceful exit. No elaborate or far-fetched assumptions. No funny-shaped plateau potential to "roll down". Just the quantum corrected Friedman equation and a scalar field.

So multiverse is certainly not entailed by every inflation model. Though it is entailed by some which, in comparison with 1301.1264 look over-dressed and out-of-date. My two cents.

Hi Marcus, I agree inflation is not guaranteed to produce a multiverse and I believe I pointed that out , my previous comment was:
“I agree inflation could be wrong and the chain between inflation and eternal inflation can also be wrong. But wrong or not it is not an arbitrary conjecture.”
Nevertheless according to Guth the vast majority of inflationary models are eternal and according to Steinhadt you have to fine tune the inflaton field to get inflation to be a one time event and not eternal. Now I accept that it’s possible both of these guys may be mistaken and I think I pointed that out too. But still the point remains the same, the multiverse is the result of a calculation not an arbitrary assumption.I don’t believe thE link you gave addressed the mechanism that Guth proposed here:
http://arxiv.org/abs/hep-th/0702178
Your paper seems to only address the issue of getting inflation started, but what Guth is arguing is that once inflation starts then it never ends. Perhaps I’ve missed something and I would love it if you were to point out my mistake. But I don’t believe loop models have anything to say about whether inflation is eternal or not. I understand that it was at first thought the super inflation could replace inflation and its scalar field, but super inflation doesn’t last long enough to generate the required pertubations . The quantum corrections give a clear difference in the evolution of the universe at the time of the bounce but after a few Planck seconds, I don’t see any results that change the standard narrative needed by Guth which is at the end of inflation not at the beginning of inflation. If I'm mistaken Id love to see it.

 

[skydivephil]

I should add the I spoke to Abhay Ashtekar about this and he said it might be possible to use the wavefunction given in LQC to evolve things to the point that is relevant here and see if inflation is eternal or not eternal. But so far this has not been done.

 

[marcus]

...
Nevertheless according to Guth the vast majority of inflationary models are eternal and according to Steinhadt you have to fine tune the inflaton field to get inflation to be a one time event and not eternal. Now I accept that it’s possible both of these guys may be mistaken...

He means "inflationary models" of a particular type where inflation begins by a miracle and lasts a sufficient time by a miracle---so if you bend over backwards and say fifty hail Marys to get that to happen then nobody knows how to make it stop. And Steinhardt is also talking about inflation of a particular type. He finds it so unlikely that he has thrown it out and focused on a non-inflationary idea.

The apparent contradiction with what Barrau says arises entirely from CONTEXT. In Barrau's context you do not need any miracles or fine-tuning to have inflation start (and last a sufficient time) whenever the universe does a collapse and rebound.

And nobody is asking you to BUY this particular context. It is called "effective isotropic LHC" and is derivable from LQG assuming isotropy and Barrau and Ashtekar and friends are intellectually mature enough to reserve judgement. It is one way the world could behave. It has a very simple quantum correction of the Friedman equation which means that when you go back in time you see a bounce instead of a singularity, and because of that bounce you get inflation without miracles or fine tuning.

That does not mean effective isotropic LHC is right, it is just one possibility. Barrau himself is certainly reserving judgment and examining it critically. We have to think carefully about the assumption of ISOTROPY. He has a recent paper laying bare some of the problems surrounding that part of the picture. Conventional cosmology (not to mention usual adolescent multi-babble) is based on assuming isotropy because it makes the math really easy, but let's look at that critically now. E.g. does the bounce context involve some scrambling that restores isotropy?

So the jury is still out on understanding the start of expansion, and one interesting context to study is the context Barrau mentions in the title of his paper with Linda Linsefors
http://arxiv.org/abs/1301.1264
Duration of inflation and conditions at the bounce as a prediction of effective isotropic loop quantum cosmology
Linda Linsefors, Aurelien Barrau
(Submitted on 7 Jan 2013 (v1), last revised 3 Jun 2013 (this version, v2))
Loop quantum cosmology with a scalar field is known to be closely linked with an inflationary phase. In this article, we study probabilistic predictions for the duration of slow-roll inflation, by assuming a minimalist massive scalar field as the main content of the universe. The phase of the field in its "prebounce" oscillatory state is taken as a natural random parameter. We find that the probability for a given number of inflationary e-folds is quite sharply peaked around 145, which is consistent with the most favored minimum values. In this precise sense, a satisfactory inflation is therefore a clear prediction of loop gravity. In addition, we derive an original and stringent upper limit on the Barbero-Immirzi parameter. The general picture of inflation, superinflation, deflation, and superdeflation is also much clarified in the framework of bouncing cosmologies.
7 pages, 7 figures.

It's an important paper. You should have voted for it on the 1st quarter MIP poll
But the grownups in this game are probably going to keep their pants dry and remain noncommittal for a while (as you indicated you were in your earlier post.) Thanks for quoting my earlier post--it seems to cover what I want to say:

Here is a clean simple inflationary cosmology that does not drag any multiverse in with it:
http://arxiv.org/abs/1301.1264
I think it's preferable to the other inflation scenarios I've heard because it does not need any mysterious "quantum fluctuation" to get started, or any fine-tuning to get adequate efolds and graceful exit. No elaborate or far-fetched assumptions. No funny-shaped plateau potential to "roll down". Just the quantum corrected Friedman equation and a scalar field.

So multiverse is certainly not entailed by every inflation model. Though it is entailed by some which, in comparison with 1301.1264 look over-dressed and out-of-date. My two cents.

 

[marcus]

I should add the I spoke to Abhay Ashtekar about this and he said it might be possible to use the wavefunction given in LQC to evolve things to the point that is relevant here and see if inflation is eternal or not eternal. But so far this has not been done.

Great you discussed inflation issue with Abhay!
The equation that defines the effective isotropic context is one he came up with circa 2007!

H² = κ/3 ρ(1 - ρ/ρ_c)

This does not mean he believes it (you realize but other readers here might not). Reasonable simplification and approximation are involved. The cosmo constant Lambda could be added but is not important in early universe (it comes into play much later). This equation is just the standard Friedman that everybody uses, with the extra factor (1 - ρ/ρ_c) which has no effect except at extremely high energy density. The critical density ρ_c is on the order of Planck density, or a few percent of such.

κ is just 8πG, and is not adjustable. The critical density ρ_c is adjustable. Around 2007 Ashtekar and friends came up with this effective equation, deriving it from more complicated LQC nuts and bolts, which in the past year have been linked more securely to the main LQG theory. Around 2007 Ashtekar et al also came up with a provisional figure for ρ_c of 41% Planck density, which is still used and is what Barrau&Linsefors take as their benchmark.

The other equation that Barrau uses is that of a generic scalar field with mass m:
φ'' + 3Hφ' + mφ = 0
Simple quadratic potential.
I don't think you'd be likely to get adequate inflation from it if you didn't kick it off with the LQC bounce. So the bounce is the key to this approach.
The mass m is a second adjustable parameter, but the interesting thing Barrau Linsefors found is that the qualitative results are highly robust---they are insensitive to varying the two parameters.
Probability of getting a generous amount of inflation remains high no matter how you twiddle the knobs

My private feeling is inflation is a done deal in this type of model if you can find some mechanism that acts during bounce to restore isotropy (which might otherwise have been destroyed during collapse of the prior phase).

Both Ashtekar and Barrau will be on Pirsa VIDEO giving invited plenary talks at Loops 2013 in July and they will probably spend some time on these topics so people should stay tuned.

 

[skydivephil]

Cant wait to follow the latest development, i will try and read through the papers and try and understand better. I have to say it does seem clear to me how LQC changes things in the pre inflationary epoch and perhaps the onset of inflation itself. But what Guth argues is that once inflation starts , the inflaton field is exponentially expanding, so each half life it decays it also expands and must do so faster than it decays generating eternal inflation.
As you know I've spoken to a number of key researchers in LQC now for upcoming film (out soon I hope) and none of them have said LQC gives you inflation in a way that contradicts what Guth is saying . There is a hope that it might do, but no firm conclusion that it does.
I would like to know if you agree with each of these statements?
1 inflation has a lot of good evidence for it so its reasonable to assume some form of inflation happened, even though we are not certain.
2 many (according to Guth nearly all ) inflationary models are eternal ,
I think given these two statements there is path from current cosmology to a multiverse. Its not a certain path and anyone that says it is, should be taken down a peg or three. But neither is it pure fantasy.

 

[marcus]

2 many (according to Guth nearly all ) inflationary models are eternal ,
...

From WikiP "eternal inflation": The idea is that "inflaton" spontaneously "decays" at different times at different places. One can think of this as a clever dodge to acquire adequate efolds because regions with delayed decay dominate. So the idea of sporadic asynchronous decay is compelling to someone who has no other means of getting adequate efolds.

Barrau Linsefors two simple equations are in sharp contrast with that.
In their model there is universe-wide synchronous "decay". I wouldn't even call it "decay" because the word is associated with a random process of radioactive decay. In this case adequate inflation AND uniform halting of inflation are automatic and don't need any tuning. The entire desired process happens deterministically and is NOT eternal.

Phil, it should make you really suspicious to be told about "many" inflation models turning out eternal. First, there is no "majority rule" and second, the proliferation of many models could well indicate that particular line of investigation is SICK or has reached a dead end. The line I mean is where inflation starts by some "quantum fluctuation" and once started may only be able to "decay" by some unpredictable and equally mysterious event.

We not doing philosophy after all. This is a mathematical science and what counts is the simplest best fit math model. Speculative pink unicorn details are not interesting.
What I look for in a mathematical science is a clean simple model, deterministic as possible, that is consistent with observation.
=========================

You mention "multiverse" as a consequence of "eternal inflation". I think both those ideas have passed their shelf life. For me, "multiverse" is a non-issue. If someone shows me a clean math model that explains OUR expanding world that we live in and see around us, that's good! If it also happens to imply the coming into existence of other worlds somewhere else that, to me, is irrelevant and uninteresting. What I care about is the good fit of an elegant model, to THIS universe.
It strikes me as immature to be overly concerned with existence or non-existence of irrelevant pink unicorns. I'm sure you understand. So I am neither attracted nor repelled by accessory details like that. It just bores me when people talk about it a lot as if it mattered.

Of course 10 years ago, in 2003, there was the KKLT paper, String began its slow decline, and people like Susskind began making a desperate salvage effort by pushing Anthropic Landscape, the Big Excuse. Linde is the L in KKLT (they were a Stanford group). there was a kind of Multiverse fad in the Naughties. As long as that was going on there was a reason to dislike Multibabble because it had the ulterior motive of excusing failure of a TOE attempt.
But that is past history.
At this point the issue seems to me neutral. You either have a good model of this universe or you don't. If it HAPPENS to predict other tracts of existence, say in our past, or in our future, or effectively separate, well that is a minor detail one can simply view as neutral. Chances are some equally good model will be developed that will NOT have those particular accessory worlds, and who cares anyway? What matters with a math model is how well it works on this universe.

Frankly I don't see any valuable multi-birthing model on the horizon. "Eternal inflation" by the miracles of quantum fluctuation strikes me as a fad that has lost its fizz and gone flat. And what other brand of multi is there?

 

[Tanelorn]

Is a mulitiverse necessary to get around the fact that this universe is temporarily finite?
Or perhaps a finite universe is acceptable to us, with absolutely nothing at all ever coming before it?

 

[Chalnoth]

Is a mulitiverse necessary to get around the fact that this universe is temporarily finite?
Or perhaps a finite universe is acceptable to us, with absolutely nothing at all ever coming before it?

This is one of those things that we really don't have good information on. Without a well-evidenced model of the initial formation of our own observable universe, it's difficult to say anything at all about what that event plays in the grander scheme of things.

That said, I'm pretty sure that any reasonable model you can come up with will contain the possibility of the event happening more than just once. That is to say, I don't think it's possible to come up with a model that starts out a universe like our own and only happens once unless you explicitly assume that it only happens once.

 

[marcus]

Hi Tanelorn, your post came right after my post #82 where I was replying to Skydive, so you may have been directing your question to something I said.

Is a multiverse necessary to get around the fact that this universe is temporarily finite?

What I was discussing was the single bounce history that has been standard in Loop cosmology since around 2001. "Multiverse" means different things to different people, and some people get infatuated with the word. In Loop cosmology our classical expanding universe is preceded by a classical contracting phase, and a rebound. Is that a "multiverse"?

I don't think that corresponds to what people mean by "multiverse" but it's hard to say because the word is so vague. I would say it is one universe, that is NOT temporally finite. There is no "creation event" or beginning---simply a classical contracting phase, a rebound, with inflation, and a classical expanding phase.

The aim is not to answer philosophical questions like "why does existence exist?" Newton's universe was eternal in both directions and so is this. The aim is to have a simple elegant model, as deterministic as possible and with best possible fit to observations.

Here's the recent Loop cosmology paper that we were discussing:
==quote from an earlier post of mine, responding to Skydivephil==
So the jury is still out on understanding the start of expansion, and one interesting context to study is the context Barrau mentions in the title of his paper with Linda Linsefors
http://arxiv.org/abs/1301.1264
Duration of inflation and conditions at the bounce as a prediction of effective isotropic loop quantum cosmology
Linda Linsefors, Aurelien Barrau
(Submitted on 7 Jan 2013 (v1), last revised 3 Jun 2013 (this version, v2))
Loop quantum cosmology with a scalar field is known to be closely linked with an inflationary phase. In this article, we study probabilistic predictions for the duration of slow-roll inflation, by assuming a minimalist massive scalar field as the main content of the universe. The phase of the field in its "prebounce" oscillatory state is taken as a natural random parameter. We find that the probability for a given number of inflationary e-folds is quite sharply peaked around 145, which is consistent with the most favored minimum values. In this precise sense, a satisfactory inflation is therefore a clear prediction of loop gravity. In addition, we derive an original and stringent upper limit on the Barbero-Immirzi parameter. The general picture of inflation, superinflation, deflation, and superdeflation is also much clarified in the framework of bouncing cosmologies.
7 pages, 7 figures.

It's an important paper. You should have voted for it on the 1st quarter MIP poll
But the grownups in this game are probably going to keep their pants dry and remain noncommittal for a while (as you indicated you were in your earlier post.) Thanks for quoting my earlier post--it seems to cover what I want to say:
==endquote==

 

[Mordred]

LQC is one study of cosmology I have only a rudimentary understanding of even though I have numerous training articles on it. In a previous post in this thread I posted an LQC multiverse model that conjectures bubble multiverses from the bounce.
Have you had a chance to look it over Marcus? I would be interested in your opinions on it as your more familiar with LQC mathematics than I

 

[Tanelorn]

Thanks Marcus and Chalnoth. I just wanted to quickly mention that.

I also am not sure if a cyclical type universe is also considered a Multiverse. I suppose that each cycle is a different Universe to the last, so there are many Universes, but not occurring simultaneously. In fact this was the first Cosmology model I remember reading about, perhaps 35 years ago. I also used to listen to LP records back then and I sometimes imagined that the Universe was like a permanent recording which could be played over and over again.

 

[skydivephil]

Marcus, I agree with you that science is not done by majority vote and I’m not suggesting that because the majority of inflationary models are eternal (I’m trusting Guth on that estimate) we should say that there is a multiverse. I am not committed to a multiverse and not arguing anyone should be. I think there is more common ground between us than you imagine. All I’m saying is that the conclusion that inflation is eternal is a result of a calculation of how inflationary theorists think the inflaton field evolves. Not of someone simply dreaming up the multiverse as some ad hoc invention and so there is a difference.I agree people over use the multiverse as well, this is a good example :
http://www.scientificamerican.com/a...cations-lend-support-to-multiverse-hypothesis
If your theory doesn’t predict what we observe then there’s a problem with the theory, not the universe.
But perhaps where we differ is that I’m curious about the world and do want to know if the multiverse is a plausible idea or not, even if it has no impact on the world today. I’m curious about whether there was a previous contracting branch of the universe as well, even if it has no impact in the world today. I believe progress will be made and speculative science is part of that progress, we shouldn’t write it off as fantasy; the two are not the same. We should asses its arguments and try and progress the field. Nor should we overstate how developed speculative ideas are. I thin people are reacting against speculation being dressed up as fact and that is a good thing, but we shouldn’t go too far in the other direction and mistake a reasonable development of the theory as fantasy. I think Barrau would agree with me there:
http://workshops.aei.mpg.de/philQG/Aurelien.pdf As for eternal inflation being a dodge for adequate e folding. I was intrigued by your comment and so I reread Guth’s eternal inflation piece:
http://arxiv.org/pdf/hep-th/0702178v1.pdf
I don’t see where the argument is used to invoke eternal inflation to get adequate e folding. Maybe you can point this out to me.
I did a Google scholar search for eternal inflation date range 2003-2008: I get 668 results, 2008-2013: 1020 results.
So it seems to me interest by theorists in this result is not dying down. The string landscape maybe different but I’ve never mentioned the string landscape as a good reason to support the multiverse; I think it’s a bad reason.
I understand why those who are professionals in the field need to make a decision as to whether these lines of pursuit are worth their time. But for those of us who are simply interested onlookers, I don’t see a reason for that. We can sit on the fence and see how things progress. I trust the scientific process. If new developments in the inflationary theory show it be “non eternal” then these models will dominate, they will win. Abhay Ashtekar told me that LQC may be able to be used to tell if the inflaton field has a large enough fluctuation to generate eternal inflation but this has not been done yet.

As you know Martin Bojowald is suggesting the bounce generates a multiverse. This is not something we are putting in our upcoming LQC film as it seems too novel so far. One paper suggesting a radical result is not enough for us to be persuasive. We shall see if others publish papers agreeing or dis agreeing with his result. I think if we did include his result it would show a false picture of where the field is at, however if we were doing a project on inflation and not mentioning eternal inflation would also be misrepresenting the field.
But whilst we won’t be including Bojowald's LQC multiverse in our film I’m not going to call him a fantasist for suggesting it.

 

[Tanelorn]

Phil, Perhaps a solution would be to divide the subject in two, one being observational Cosmology and the other being speculative Cosmology. With two distinction such as this it is now easy even for a layman such as myself to distinguish between most theories.

 

[Mordred]

Observational=cosmology
speculative model cosmology=cosmogony.

The distinction is already there but seldom mentioned.

Multi-verse falls under cosmogony. At least I would think as it ties closely with universe beginnings and in some ways philosophical though not completely