Probability of stars in a multiverse

[Chalnoth]

so what properties or any physical manifestation of a universe other than the one we exist in has been tested? how did that experiment turn out?

How do you know humans descended from apes? You weren't there! Nobody has done an experiment showing an ape give birth to a human!

Arbitrary restrictions on what sorts of conclusions we can draw based upon evidence are fundamentally anti-science.

 

[Chalnoth]

I'll keep an open view on the matter. We have to be careful. I'm not keen to say that multiverse is the only interpretation for SSB. It's somewhat simple, slightly convincing notion. Simple in a sense that it is viewed directly as a possible outcome and often ignores some other basic possibilities not to mention the less unfounded nature of that idea. It 'can' be just a effect of radiative correction found in massless gauge theories were it can induce spontaneous symmetry breaking as a consequence of relationship between the masses of the scalar and vector mesons, predicting (for small coupling constants) that the scalar mesons are much lighter. (http://arxiv.org/abs/hep-th/0507214).

Well, right. For each symmetry breaking event it is possible that there was some interesting physical phenomenon that forced the symmetry breaking to happen in a very specific way. I wouldn't be horribly surprised if a few things, such as the electroweak symmetry breaking event, turned out to do this.

But I don't buy for an instant that this sort of thing can possibly explain how we go from E8xE8 down to SU(3)xSU(2)xU(1), for instance (or whatever the fundamental symmetry group happens to be).

 

[rbj]

How do you know humans descended from apes? You weren't there! Nobody has done an experiment showing an ape give birth to a human!

not answering the question.

Noth, even when the theory is there, and even when the theory makes sense, when there are holes in the evidence, you have to be prepared for surprizes. like Homo floresiensis.

you're problem is, again, that you're so enamored by a theory, versions of which are quite elegant (i think the string landscape theory of other universes is pretty elegant), that you are confusing that with evidence. i like elegance, too, but Nature doesn't necessarily give a rat's @ss about elegance.

Arbitrary restrictions on what sorts of conclusions we can draw based upon evidence are fundamentally anti-science.

that's called "strawman".

so what would have happened to special relativity if experiments like the Rossi-Hall (muon decay time is dilated) or the many that followed had turned out differently? or what would have happened to GR if Eddington's trip to measure the perihelion precession of Mercury (or the many subsequent experiments or observations supporting GR) turned out differently?

you know as well as anyone else what the meaning of "falsifiable" is and we're waiting patiently for you to describe a falsifiable physical experiment that would turn out one way if another universe exists and would turn out another way if no other universe exists.

 

[Chalnoth]

you know as well as anyone else what the meaning of "falsifiable" is

So, you think reality will just play nice and only present itself so that every single aspect of it will be neatly-falsifiable in a trivial manner?

 

[rbj]

So, you think reality will just play nice and only present itself so that every single aspect of it will be neatly-falsifiable in a trivial manner?

never said anything of the sort. so i am at a loss to why you would conclude that i think that.

please stop propping up strawmen and just answer the question posed to you.

 

[julcab12]

Well, right. For each symmetry breaking event it is possible that there was some interesting physical phenomenon that forced the symmetry breaking to happen in a very specific way.
.

We can handle that. Its not even clear whether the product of such breaking can fully constitute to a whole new set of physical laws moreover an entire zoo of universes. When a quantum particle becomes larger, the symmetry of the system as a whole becomes more unstable against small perturbations signaled by a set of noncommuting limits. It led to the particles with nonzero masses remains symmetrical but appears to be/'could be' hidden by default observation. Besides, it is pointing towards a form of indirect mechanism that hides EWSB. And one thing is certain. The specific mechanism that hides EWSB remains an open question but they have reasons to believe that a positive suspect is 'much' closer to home^^.

 

[Chalnoth]

We can handle that. Its not even clear whether the product of such breaking can fully constitute to a whole new set of physical laws moreover an entire zoo of universes.

It depends upon what you mean by, "a whole new set of physical laws." There's not really any question that having the electroweak symmetry breaking event occur somewhat differently would have impact on the properties of the weak force, and maybe also the electromagnetic force. This would likely result in different particles having different masses, and various reactions having rather different cross sections or decay times.

But that's only the tip of the iceberg: there were also symmetry breaking events earlier that transformed from a higher-order symmetry down to the standard model. We don't know what those were yet, but those may also have resulted in significant differences. I think it will be interesting to see where high energy physics goes in the next few decades, to see if we make any significant progress towards uncovering this part of the mystery.

All that said, yes, quantum mechanics guarantees that every possible result of a symmetry breaking event occurs. And even if you have trouble buying that, all you need is a universe that is larger than the typical size of a domain with a typical value of the broken symmetry, and that is trivially easy to produce in most inflation models, and perhaps even easier given that the original event that started inflation is highly unlikely to be a unique, one-off event.

 

[Chalnoth]

never said anything of the sort. so i am at a loss to why you would conclude that i think that.

please stop propping up strawmen and just answer the question posed to you.

My point is that you're making up legalistic restrictions on what sorts of things we should and should not conclude, but perhaps even worse you're jumping to the conclusion (without even bothering to argue the point) that the default assumption should be a unique universe.

But that's nonsensical: a unique universe requires more assumptions than a prolific one. Even if we knew nothing at all about physical law, but just knew a little bit about how math behaves, the default assumption should clearly be a prolific universe. To take a simple example, consider the following two situations:

1. The set of all integers with addition, negation, and multiplication operations defined. The set of all integers is closed under these operations.
2. A set of the integers from [0,5], with addition, negation, and multiplication operations defined, with the set of integers [0,5] closed under these operations.

Which of the two above requires fewer assumptions? Clearly the second: it requires all of the rules of the first set, but it also requires additional rules to determine how to deal with the fact that it only includes six numbers.

 

[rbj]

... you're jumping to the conclusion (without even bothering to argue the point) that the default assumption should be a unique universe.

citation, please.

My point is that you're making up legalistic restrictions on what sorts of things we should and should not conclude

i am making the same requirement about what it means for something to be "science" that Woit and Smolin and these guys that are fond of the phrase "Not even wrong". I'm not a big fan of Wikipedia, but i'll quote it :

http://en.wikipedia.org/wiki/Observation

The scientific method requires observations of nature to formulate and test hypotheses.[1] It consists of these steps:[2][3]

1. Asking a question about a natural phenomenon
2. Making observations of the phenomenon
3. Hypothesizing an explanation for the phenomenon
4. Predicting a logical consequence of the hypothesis
5. Testing the hypothesis by an experiment, an observational study, or a field study
6. Creating a conclusion with data gathered in the experiment

Observation plays a role in the second and fifth steps of the Scientific Method. ...

how do you plan to deal with step 5? or are you saying that at least one of these multiverse theories is immune to that requirement? that somehow your scientific theory need not be subject to falsifiable empirical testing? if it does not ever subject itself to such, then what difference does it make? (and my question is how long does it retain the status of "science"? Smolin and Woit would say the same thing about string theory, which is also nice and elegant, on paper.)

But that's nonsensical: a unique universe requires more assumptions than a prolific one.

not for theists. i don't even know that this claim is true for materialists or physicalists, and i suspect there are even those who disagree with you.

Even if we knew nothing at all about physical law, but just knew a little bit about how math behaves, the default assumption should clearly be a prolific universe.

the issue is not whether or not we know nothing about the physical law, but whether or not we know everything about it (and we don't, of course). you don't know what will be discovered or derived by our descendants 400 years into the future. but you are writing as if you do.

To take a simple example, consider the following two situations:

1. The set of all integers with addition, negation, and multiplication operations defined. The set of all integers is closed under these operations.
2. A set of the integers from [0,5], with addition, negation, and multiplication operations defined, with the set of integers [0,5] closed under these operations.

the reality regarding other universes does not give a rat's patootie about it.

Which of the two above requires fewer assumptions? Clearly the second:

i think you meant the first. but the greater reality (about the multiverse or lack thereof) still is unaffected by the argument.

 

[julcab12]

It depends upon what you mean by, "a whole new set of physical laws."

I'm referring to SSB resulting to different physical constant.

quantum mechanics guarantees that every possible result of a symmetry breaking event occurs. And even if you have trouble buying that, all you need is a universe that is larger than the typical size of a domain with a typical value of the broken symmetry, and that is trivially easy to produce in most inflation models, and perhaps even easier given that the original event that started inflation is highly unlikely to be a unique, one-off event.

Well .I don't have any problem with explicit or spontaneous symmetry breaking occurring because it did happen in nature. Some scientists can 'agree' on the multiverse theory, they can't always agree on how the multiverse actually works. I also see possibilities on mutiverse as a clear/direct consequence (The best we could think of or/ the only thing we could postulate for now) but i'll remain skeptic of multiverse until sufficient evidence is presented. The multiverse scenario right now is problematic. It can neither be verified nor falsified as per standard. Given our current universe as basis. No form of selection process can be invoked. Maintaining a plausible hypothesis, a universe generating mechanism is needed. In conclusion, without a scientifically rigorous means by which a multiverse (not SB)can exist. The concept remains a conjecture and less satisfactory. On the other hand it is 'exciting'. They've taken a great deal of speculation in attaining an answer to fine tuning but i do hope they'll find something more cohesive Or it is!? We don't know..

 

[Chronos]

I feel constrained by observational evidence - which does not yet favor the multiverse conjecture. Until that changes, I remain skeptical.

 

[Chalnoth]

citation, please.

You don't see how continually demanding to see evidence for a multiverse is, by default, assuming a unique universe?

I demand you present evidence that a unique universe is the preferred conclusion.

 

[Chalnoth]

I feel constrained by observational evidence - which does not yet favor the multiverse conjecture. Until that changes, I remain skeptical.

Why be more skeptical about a multiverse than a unique universe?

 

[rbj]

... you're jumping to the conclusion (without even bothering to argue the point) that the default assumption should be a unique universe.

citation, please.

You don't see how continually demanding to see evidence for a multiverse is, by default, assuming a unique universe?

i don't see that. it is, in fact, not the case. i do not assume a single universe and i also do not assume there are other universes out there. and settling that question (which cannot be settled for mortals) does not affect my worldview. i don't require a single universe to support my worldview, nor do i require multiple universes. i guess i cannot wrap my head around it if the number of universes is not a real integer greater than zero, unless the reality we apparently live in is really a simulation in a universeless existence (but i don't give that any credibility).

I demand you present evidence that a unique universe is the preferred conclusion.

well, i cannot. nor do i say it's the preferred conclusion.

i will say this: the universe we're in is pretty big and quite old. and we can see it pretty deeply (e.g. Hubble deep space). it could be all that there is (and there is no evidence and no hope for evidence that it's not all that there is, materially), and if that is the case, the teleological question regarding fine tuning remains. anthropic reasoning and selection bias does not cut the mustard in explaining fine tuning of a single universe from a solely materialist POV.

if the universe is one of many (some have speculated as many as $10^{10^{10000}}$), just having math that is compatible with such an existence does not, in itself, make for a mechanism for the generation of all these universes. physical law is not "stuff", but it governs the stuff that exists or emerged into existence. all adding the multiverse concept does is add another layer of turtles. from a material POV, it's still "turtles all the way down". just another step in this problem of infinite regress.

perhaps there is a multi-multiverse. that the multiverse we live in is just one of many multiverses. maybe it's even another layer deeper than that. perhaps our multiverse that has some reasonable set of common physical law (but different parameters for the different universes) is one of many multiverses where the other multiverses have a reality of magic, wizards, and pink unicorns. we don't know.

you might object and say that such a reality is ridiculous, and i might agree. but if, in order to avoid (in your mind) the teleological question, you construct the necessity of a gazillion other universes, just to answer the question for how is it that our universe seems so finely-tuned (both in fundamental constants and in initial conditions) for the existence of matter, astronomical structures, elemental diversity, or life as we understand it, and yet criticize theists as being silly for suggesting that maybe it's the consequence of design, i think that open-minded philosophers would be quite dubious of your position.

but there are both open-minded and close-minded philosophers. just as there are open-minded and close-minded comologists, physicists, electrical engineers, musicians, and parents.

 

[Chalnoth]

i don't see that. it is, in fact, not the case. i do not assume a single universe and i also do not assume there are other universes out there.

You say that, and yet you continually demand that people present evidence for one (and only one) of those options. Furthermore, you won't listen to evidence that isn't of your specific desired type.

Why do you act as if you have already assumed there is only one universe if you don't believe it?

 

[Chronos]

I think rbj is saying there is no compelling observational evidence suggesting the existence of parallel universes. To that extent, I agree. I do, however, agree there is theoretical support for this possibility.

 

[rbj]

You say that, and yet you continually demand that people present evidence for one (and only one) of those options.

Noth, if you were to consider forensics (as in debate, like being on a debate team), you're going to have to avoid strawmen (misrepresenting your opponents' position and throwing it back at them) because they will call you on it. it's quite an ineffective way to argue a point unless the other guy is really dumb.

i wasn't talking about "options", but if you insist on articulating the "options" they are:

1. there is evidence of other universes not counting the universe we exist in.

2. there is not evidence of other universes excluding the universe in which we exist.

i don't think there is any dispute regarding evidence of existence of the universe we are in. we can point telescopes out there and see evidence of such.

it started with this:

... there is about as much evidence of the existence of these other universes as there is evidence of a transcendent God or of a flying spaghetti monster.

That is completely and utterly false.

so then i respond and you respond:

and that is a matter of opinion. your opinion.

Hardly. There simply isn't any comparison. One is a solid conclusion based upon broadly-supported scientific theory (in this case the standard model of particle physics).

so i am not equating "theory" to "evidence". if you are, you have to justify the semantics (and you'll lose that argument). and i am happy to stipulate that the observational or empirical evidence for a transcendental God or flying spaghetti monster is zero (but there is evidence, not conclusive, for fine-tuning, which motivates questions). i didn't say there is less evidence of other universes than that for theism or FSM. i said "there is about as much" of which you insist "is completely and utterly false."

somehow i doubt that your position is that there is far less evidence for the other universes, and presuming that, then what you have to do is propose or describe a falsifiable material observation of something consequential of the other universe(s) and you will then have more evidence for that than there is evidence of theism or FSM. because i think we both agree that conclusive material evidence of theism or FSM is the null set, {}.

but what i continue to wonder about if you're willing to apply scientific standards (and you don't get to define them, they are defined by others: what makes for science and what is not science) to your definite claim that there is more than zero evidence for the other universes. i didn't ask for theory (and by theory, i don't mean mere hypothesis), i asked for empirical, falsifiable evidence. because it is empirical and falsifiable evidence that is lacking for theism and FSM.

string theory and theorists and Ed Witten have some of the same problem. and they are being taken to task by the likes of Smolin and Woit. and that's fine. it's what Science is about.

you see, even though it was not offered as an explanation for the negative result for Michaelson-Morley, there was otherwise no material evidence offered by Einstein for either SR or GR when they were first presented. a theory gets to hang out there for a while without evidence and, if it's a good scientific theory, it can remain as plausibly valid for some time until there comes material observational evidence that falsifies it. that's why we don't believe in the existence of aether any more because, even when we tried to, we just couldn't measure it, and we expected to be able to at least some season of the year because we had no reason to expect that the aether moves around in space with the Earth as it orbits.

but, you never answered this question:

what would have happened to special relativity if experiments like the Rossi-Hall (muon decay time is dilated) or the many that followed had turned out differently? or what would have happened to GR if Eddington's trip to measure the perihelion precession of Mercury (or the many subsequent experiments or observations supporting GR) turned out differently?

these theories of Einstein made predictions of material consequence. that something in life differs from what Newtonian physics would say and we can check on that. do the physical observations look more like what Alfred said they would or do they look more like what Isaac said they would, and they are different. they are consequently different. because science is about what makes a consequential difference. that's why science is not about invisible pink unicorns or FSM or a transcendent God, none of which submit themselves to detection or measurement.

i'm trying to get you to submit to the requirement of science that it be of empirical consequence in order for it to be science. i don't think you're quite there yet.

Furthermore, you won't listen to evidence that isn't of your specific desired type.

that science must eventually be of empirical consequence and have evidence of empirical consequence in order for it to be science, is not about anyone's "specific desired type". if SR and GR made no falsifiable predictions of material consequence, predictions that were later empirically verified, time and again, they would likely have fallen by the wayside by now. it's why the "steady state model" of the universe has fallen by the wayside. and Witten and the string theorists have to worry about that string theory won't be forever accepted as physics if it just cannot be shown to have material consequence (which can be tested).

but you seem to think that the theory and claim regarding "the assurance..., the conviction of things not seen" (where do you think that quote comes from?) need not ever make a material, consequential difference indefinitely and that it's science.

i can (and others will) assure you that such is not the case. Science must eventually be empirical. if some purported science can never be empirical, it ceases to be science and is relegated to the discipline of philosophy. that is the case whether you like it or not, whether you deny it or not.

Why do you act as if you have already assumed there is only one universe if you don't believe it?

again, i have never, ever, once said that. it's a strawman. it weakens your argument because it is evidence that while you cannot dispute what your opponent is saying then you'll dispute what your opponent is not saying.

it doesn't help you.

 

[Chalnoth]

Noth, if you were to consider forensics (as in debate, like being on a debate team), you're going to have to avoid strawmen (misrepresenting your opponents' position and throwing it back at them) because they will call you on it. it's quite an ineffective way to argue a point unless the other guy is really dumb.

If you're going to accuse me of presenting a strawman argument, you're going to have to stop proving me right again and again. You have continually compared the multiverse idea with obviously false notions like the FSM: you claim to say you don't know, your mind is open. And yet you insist on using language elsewhere that makes it absolutely, positively clear that you consider multiverse ideas to be the height of absurdity, worthy of no consideration whatsoever.

Aside from this fundamental contradiction in what you are saying, the truth is the exact opposite of the way you try to paint it. You're very much like a man who lives his entire life situated in the plains of the central United States, and then proclaims to be ludicrous any notion that there might be parts elsewhere on Earth that are very unlike those plains. This is very clear when you use the following overly-simplistic comparison above:

1. there is evidence of other universes not counting the universe we exist in.

2. there is not evidence of other universes excluding the universe in which we exist.

Because the fact of the matter is that our vision is limited by causality. We cannot see beyond the horizon. But just because we cannot see beyond the horizon this does not mean that the default assumption should be that nothing is there (or even that everything is the same as the part within our horizon).

In fact, the default should most certainly be that things are quite different far enough beyond that horizon. How different we cannot say, but it is highly unlikely that things are identical everywhere.

Furthermore, you have over-simplistically assumed that just because we are causally-limited in our vision that we cannot ever possibly obtain evidence in support of things that may lie beyond our horizon. To be fair, we will never be able to point at a specific region outside of our cosmological horizon and use that as an example. But that isn't necessary. What we do need is a coherent and well-evidenced model of high-energy physics, combined with a coherent and well-evidenced model for how universes like ours begin and evolve, that together unambiguously predict that there will be other regions with different low-energy physical laws.

In technical terms, what we need is for the vacuum to be metastable with multiple local minima, and an early-universe model which allows the universe as a whole to explore the multiple local minima. As I've pointed out, basic quantum mechanics essentially guarantees that all local minima will be explored, while the existence of spontaneous symmetry breaking makes multiple local minima highly likely. Further study of the details of the properties of the Higgs may confirm that we live in a metastable vacuum state.

 

[Mordred]

My take on multiverse is that its not only possible but also likely. Depending on how you define Universe we have examples of a multiverse even now. I know this is adding fuel to the fire but oh well.

Alternate Universe definition from Wiki.

According to a still-more-restrictive definition, the Universe is everything within our connected space-time that could have a chance to interact with us and vice versa.[citation needed] According to the general theory of relativity, some regions of space may never interact with ours even in the lifetime of the Universe, due to the finite speed of light and the ongoing expansion of space. For example, radio messages sent from Earth may never reach some regions of space, even if the Universe would live forever; space may expand faster than light can traverse it.

By that definition any region well beyond any means of interaction with our observable universe can be construed as a disconnected spacetime hence anothere universe.
Regions inside an event horizon can be construed as a separate spacetime. As we will never be able to gain any information from that region. Though the interaction of Hawking radiation and blackbody radiation could be used as a counteere argument.

Many of the models used in cosmology are open to the possibility of a multiverse. Until we have irrefutable evidence either way, Thinking that this universe is one of many or all that there is are both equally valid.

 

[rbj]

If you're going to accuse me of presenting a strawman argument, you're going to have to stop proving me right again and again.

i only take responsibility for the things that i say. i do not take responsibility for things that other persons say that i say. (nor, specifically, for what you say that i say.)

again, you cannot dispute what your opponent is saying so then you dispute what your opponent is not saying.

it doesn't help you.

You have continually compared the multiverse idea with obviously false notions like the FSM

only that we cannot measure either nor hope to ever measure either.

but if you replace "FSM" with "God", you might have some dispute with some very authoritative people about how "obviously false" the notion is.

you claim to say you don't know, your mind is open. And yet you insist on using language elsewhere that makes it absolutely, positively clear that you consider multiverse ideas to be the height of absurdity, worthy of no consideration whatsoever.

again, you cannot dispute what your opponent is saying so you dispute what your opponent is not saying.

it doesn't help you.

try to read what i wrote (most of which you don't respond to at all). and respond to what i wrote, not to what i didn't.

Aside from this fundamental contradiction in what you are saying, the truth is the exact opposite of the way you try to paint it.

so you have a special hook on "the truth"? and we should accept that based on what? your authority?

Because the fact of the matter is that our vision is limited by causality. We cannot see beyond the horizon.

might be that FSM is hiding behind that horizon. don't worry, he/she/it can't get you because that would mean that you can measure him/her/it (and you can't).

But just because we cannot see beyond the horizon this does not mean that the default assumption should be that nothing is there (or even that everything is the same as the part within our horizon).

why must there be a default assumption? why can't the position taken be that we don't know nor cannot know what is beyond the horizon?

In fact, the default should most certainly be that things are quite different far enough beyond that horizon. How different we cannot say, but it is highly unlikely that things are identical everywhere.

maybe God or FSM or Santa Claus is back there. that's quite different.

 

[rbj]

My take on multiverse is that its not only possible but also likely.

i feel approximately the same way about it as i do about the likelihood that there are E.T.'s out there on some exoplanet that supports life. i would be very surprized if somehow someone could prove that Earth can be the only planet supporting life in the Milky Way. i accept the results of the Drake equation and think that there could be presently thousands of other civilizations in the Milky Way. but distributed among 400 billion stars, the likelihood we'll get an EM transmission from any that is not drowned out by stellar radiation, that likelihood is pretty small.

but one big difference is that while it's possible (and that possibility is exciting and deserves some funding and effort) for us to detect the presence of some E.T., i have absolutely no expectation that humans will ever measure or detect either God (setting aside accounts of miracles), the Flying Spaghetti Monster, nor anything coming from another universe.

Many of the models used in cosmology are open to the possibility of a multiverse. Until we have irrefutable evidence either way, Thinking that this universe is one of many or all that there is are both equally valid.

it is for me, but not for Noth.

 

[julcab12]

Ok. Let's give a brief case of multiverse. What are the arguments and evidence for existence of a multiverse:(Study of George Ellis)

Argument 1: "There are plausibly galaxies beyond the horizon, where we can’t see then; so plausibly many different expanding universe domains where we can’t see them."

Untestable extrapolation; assumes continuity that may or may not be true. Outside where we can see, there might be (a) an FRW model, (b) chaotic inflation, (c) a closed model, (d) an island universe. No test can be done to see which is the case.

If each step in a chain of evidence is well understood and inevitable, then indirect evidence carries nearly as much weight as direct evidence. But not all the steps in this chain are inevitable.

If employed leads to the old idea of spatial homogeneity forever (`The Cosmological Principle’) rather than the multiverse of chaotic cosmology with domain walls separating phases.

CASE 1: 2 Implied by known physics that leads to chaotic inflation

The key physics (Coleman-de Luccia tunneling, the string theory landscape) is extrapolated from known and tested physics to new contexts; the extrapolation is unverified and indeed is unverifiable; it may or may not be true. The physics is hypothetical rather than tested!
Known Physics → Multiverse ??
NO!
Known Physics → Hypothetical Physics → Multiverse
Major Extrapolation
It is a great extrapolation from known physics.
This extrapolation is untestable: it may or may not be correct.

ARGUMENT 2: The String Landscape is a fantasy(scientifc standard). We actually have a
plausible landscape of minimally supersymmetric AdS4solutions of supergravity modified by an exponential superpotential. None of these solutions is accessible to world sheet
perturbation theory. If they exist as models of quantum gravity, they are defined by conformal field theories, and each is an independent quantum system, which makes no
transitions to any of the others. This landscape has nothing to do with CDL tunneling or eternal inflation.

"Points is whether one agrees or not: this analysis shows that the supposed underlying physics is certainly not well established."

ARGUMENT 2: Implied by inflation, which is justified by CBR anisotropy observations.
- it is implied by some forms of inflation but not others; inflation is not yet a well defined theory.
- Not all forms of inflation lead to chaotic inflation.
- For example inflation in small closed universes.

Two central scientific virtues are testability and explanatory power. In the cosmological context, these are often in conflict with each other.

The extreme case is multiverse proposals, where no direct observational tests of the hypothesis are possible, as the supposed other universes cannot be seen by any observations whatever, and the assumed underlying physics is also untested and indeed probably untestable.

In this context one must re-evaluate what the core of science is: can one maintain one has a genuine scientific theory when direct and indeed indirect tests of the theory are impossible? If one claims this, one is altering what one means by science.
One should be very careful before so doing. There are many other theories waiting at the door –wanting to be called science (astrology, Intelligent Design, etc).

The multiverse proposals are good empirically based philosophical proposals for the nature of what exists, but are not strictly within the domain of science because they are not testable. There is nothing wrong with empirically based philosophical explanation, indeed it is of great value, provided it is labeled for what it is. But, cosmologists should be very careful not make
methodological proposals that erode the essential nature of science in their enthusiasm to support specific theories as being scientific, for if they do so, there will very likely be
unintended consequences in other areas where the boundaries of science are in dispute.

It is dangerous to weaken the grounds of scientific proof in order to include multiverses under the mantle of `tested science’ for there are many other theories standing in the
wings that would also like to claim that mantle.

What must be done?

- Determine a viable set of criteria/ procedures for what makes a theory scientific
- Find what methods can adequately justify unobservable entities
- Apply to the multiverse case
- Apply to other contentious cases (astrology, Intelligent Design) to see how they pan out
- Put the enterprise on a solid philosophical basis.

[G F R Ellis (2006): “Issue in the Philosophy of cosmology”
In Handbook in Philosophy of Physics, Ed J Butterfield
and J Earman (Elsevier, 2006), 1183-1285.
http://arxiv.org/abs/astro-ph/0602280 ]

The multiverse: conjecture, proof, and science
George Ellis

arXiv:1208.5715

http://www.aei.mpg.de/~axkl/HermannFestProceedings/Ellis.pdf

 

[Mordred]

I thoroughly enjoyed those papers you posted. They are being added to my collection. Thanks for linking them

 

[Chronos]

George Ellis is a well known critique of the multiverse hypothesis and his underlying objection is sensible - lack of testability. It is, however, unclear if the multiverse hypothesis is truly untestable. That remains to be seen. Gurzadyan and Penrose are still promoting CCC, which is a sort of temporal variant of the multiverse idea - On CCC-predicted concentric low-variance circles in the CMB sky, http://arxiv.org/abs/1302.5162

 

[Chalnoth]

George Ellis is a well known critique of the multiverse hypothesis and his underlying objection is sensible - lack of testability. It is, however, unclear if the multiverse hypothesis is truly untestable. That remains to be seen. Gurzadyan and Penrose are still promoting CCC, which is a sort of temporal variant of the multiverse idea - On CCC-predicted concentric low-variance circles in the CMB sky, http://arxiv.org/abs/1302.5162

I generally don't think that they have anything to offer here. Their first attempt at doing this was shockingly-bad. I didn't look at this attempt in too much detail, but I don't see how it's much better.

This was a much better, and better theoretically-motivated, search:
http://arxiv.org/abs/1210.2725

They didn't find anything definitively, but they did the analysis properly. I don't know if Planck will help or not for this sort of thing, however.

The discovery of cosmic strings would be another signature of a multiverse, because cosmic strings would be direct relics of a past spontaneous symmetry breaking event.

 

[Chronos]

I agree Gurzadyan and Penrose is a stretch. Strictly an opinion, but, it appears cosmic strings are a long shot at present.

 

[Chalnoth]

I agree Gurzadyan and Penrose is a stretch. Strictly an opinion, but, it appears cosmic strings are a long shot at present.

Well, I don't know how much of a long shot they are. They are certainly very rare, given current observational evidence. There's no question about that. But it would be a very interesting discovery if we found even one in the visible universe.