You think there's a multiverse? Get real

[Garth]

I really appreciated the article in this week's New Scientist You think there's a multiverse? Get real based on Lee Smolin's new book "The Singular Universe and the Reality of Time", which is co-authored with Roberto Mangabeira Unger.

Lee has changed his tune from the days of his Cosmological Natural Selection hypothesis in which every BH spawns a new universe and a natural selection process fine tunes the physical constants to maximise the number of BHs a particular universe produces, incidentally also fine tuning the universe to be propitious for life, so there is a multitude upon multitude of universes.

Now he, with the philosopher Unger, embraces scientific realism and as he writes in the article:

Thus the multiverse theory has difficulty making any firm predictions and threatens to take us out of the realm of science. These other universes are unobservable and because chance dictates the random distribution of properties across universes, positing the existence of a multiverse does not let us deduce anything about our universe beyond what we already know. As attractive as the idea may seem, it is basically a sleight of hand, which converts an explanatory failure into an apparent explanatory success.

Smolin, rejecting the multiverse as having no predictive power, suggests we need to reject some of the principles (excess baggage) standard cosmology is built on, the scaling up of physical laws from the laboratory or solar system up to the entire universe and the 'Newtonian paradigm' (the predictions of future states from initial conditions under a set of laws).

Once we accept that we need a new paradigm to do science at the level of the universe as a whole, the next question to ask is what principle that new paradigm should be founded on...

The first is that there is just one universe. The second is that time is real and the laws of nature are not timeless but evolve. The third is that mathematics is not a description of some separate timeless, Platonic reality, but is a description of the properties of one universe.

I agree with the first and second - laws can evolve in alternative theories such as a suggested variation of G - however I find it hard not to accept the 'eternal' truths of Mathematics - Platonist as I am! I like to think 1+ 1 = 2 whether there is anybody around to think so or not.

Personally I ask the question, "Whenever we talk about a quantity, Mass, Length, Time etc., both defined here in the laboratory or applied to the distant universe, we have to also ask 'How is that quantity measured and compared with its standard unit.'h

Garth

 

[Stephen Tashi]

Is there any way to decide whether a physical quantity (in a given system of units) must always be a rational number? I don't see that assuming physical quantities can be any real number (as opposed to only a rational number or only from some other dense subset of the real numbers) has any predictive power. However, convenience and mathematical aesthetics lead to using the model of the real numbers (and the complex numbers) in physics. By analogy to that situation, the fact that a multiverse theory is not predictive doesn't rule out the fact that it might be turn out to be a good theory if the mathematics of it can be shown to be convenient.

 

[Chalnoth]

These are just the standard, baseless objections that have been answered many times before.

He seems to be defining the multiverse out of existence by saying, "There's one universe, with changing physical laws," while neglecting to notice that this guarantees that there will be different regions with different physical laws, which in turn is one kind of multiverse that many people discuss.

 

[Garth]

These are just the standard, baseless objections that have been answered many times before.

He seems to be defining the multiverse out of existence by saying, "There's one universe, with changing physical laws," while neglecting to notice that this guarantees that there will be different regions with different physical laws, which in turn is one kind of multiverse that many people discuss.

The difference being that we might be able to observe these regions with evolved physical laws and 'constants'.

Garth

 

[Chalnoth]

The difference being that we might be able to observe these regions with evolved physical laws and 'constants'.

Garth

Uhh, no. There is precisely zero difference.

To within current measurement errors, physical laws have not changed within the observable universe. And many ways that the laws can change would result in changes being phase transitions that happen suddenly rather than gradually, such that you'd never detect a change by looking at tiny variations of the laws within the observable universe.

The way you detect such different physical laws is basically identical to the multiverse theories: you don't look for the changes themselves, but for other consequences of the theory that predicts the changes that are measurable.

 

[Chronos]

I am admittedly sympathetic with Carlos Rovelli's view on multiverse theories from a 2012 interview as reported in Scientific American - re: http://blogs.scientificamerican.com...losophical-superficiality-has-harmed-physics/ :

Horgan: Do multiverse theories and quantum gravity theories deserve to be taken seriously if they cannot be falsified?

Rovelli: No.

 

[Garth]

I am admittedly sympathetic with Carlos Rovelli's view on multiverse theories from a 2012 interview as reported in Scientific American - re: http://blogs.scientificamerican.com...losophical-superficiality-has-harmed-physics/ :

Horgan: Do multiverse theories and quantum gravity theories deserve to be taken seriously if they cannot be falsified?

Rovelli: No.

I agree Chronos, and as Smolin's NS article says after talking about the unusual initial conditions of the original GR BB model:

The challenge that cosmologists face is to make sense of this specialness. One approach to this question is inflation – the hypothesis that the early universe went through a phase of exponentially fast expansion. At first, inflation seemed to do the trick. A simple version of the idea gave correct predictions for the spectrum of fluctuations in the cosmic microwave background.

But a closer look shows that we have just moved the problem further back in time. To make inflation happen at all requires us to fine-tune the initial conditions of the universe. And unless inflation is highly tuned and constrained, it leads to a runaway process of universe creation. As a result, some cosmologists suggest that there is not one universe, but an infinite number, with a huge variety of properties: the multiverse. There are an infinite number of universes in the collection that are like our universe and an infinite number that are not. But the ratio of infinity to infinity is undefined, and can be made into anything the theorist wants. Thus the multiverse theory has difficulty making any firm predictions and threatens to take us out of the realm of science.

Until observations demonstrate that such other universes do exist with high statistical significance (95% at least) then for one I am convinced that multiverse theories have taken us out of the realm of science.

Garth

 

[Chalnoth]

I agree Chronos, and as Smolin's NS article says after talking about the unusual initial conditions of the original GR BB model:

Until observations demonstrate that such other universes do exist with high statistical significance (95% at least) then for one I am convinced that multiverse theories have taken us out of the realm of science.

Garth

Would you reject any theory, then, which happens to include a multiverse as one of its predictions?

 

[marcus]

Would you reject any theory, then, which happens to include a multiverse as one of its predictions?

that's not a well-defined question. It's verbal and people fudge around with what they mean by "multiverse".
Some people are in love with the word "multiverse" and will apply it to any model of the UNIverse where some physical quantity (which might otherwise be taken to be constant) varies.
The model is what I would call a universe, operating under a set of natural laws, and predictable enough to be tested. But they point to some little "sub-law" that varies and say "look! there are different REGIONS in the universe! Therefore it is a MUUULLLTTEEEVERSE!

I would go along to a large extent with the essay in NATURE by two of the world's most prominent and respected cosmologists: Joe Silk and George Ellis. They are definite about what they mean should be regarded as outside the bounds of science. They define their terms and give their reasons.
As I recall the essay was titled something like
Defend the Integrity of Physics
Yes, here is a link:
http://www.nature.com/news/scientific-method-defend-the-integrity-of-physics-1.16535
Here are some excerpts:
==quote==
Scientific method: Defend the integrity of physics

• George Ellis
• & Joe Silk

16 December 2014
Attempts to exempt speculative theories of the Universe from experimental verification undermine science, argue George Ellis and Joe Silk.

This year, debates in physics circles took a worrying turn. Faced with difficulties in applying fundamental theories to the observed Universe, some researchers called for a change in how theoretical physics is done. They began to argue — explicitly — that if a theory is sufficiently elegant and explanatory, it need not be tested experimentally, breaking with centuries of philosophical tradition of defining scientific knowledge as empirical. We disagree. As the philosopher of science Karl Popper argued: a theory must be falsifiable to be scientific.
Chief among the 'elegance will suffice' advocates are some string theorists. Because string theory is supposedly the 'only game in town' capable of unifying the four fundamental forces, they believe that it must contain a grain of truth even though it relies on extra dimensions that we can never observe. Some cosmologists, too, are seeking to abandon experimental verification of grand hypotheses that invoke imperceptible domains ...

These unprovable hypotheses are quite different from those that relate directly to the real world and that are testable through observations — such as the standard model of particle physics and the existence of dark matter and dark energy. As we see it, theoretical physics risks becoming a no-man's-land between mathematics, physics and philosophy that does not truly meet the requirements of any.

The issue of testability has been lurking for a decade. String theory and multiverse theory have been criticized in popular books1, 2, 3 and articles, including some by one of us (G.E.)4. In March, theorist Paul Steinhardt wrote5 in this journal that the theory of inflationary cosmology is no longer scientific because it is so flexible that it can accommodate any observational result. Theorist and philosopher Richard Dawid6 and cosmologist Sean Carroll7 have countered those criticisms with a philosophical case to weaken the testability requirement for fundamental physics.
...
...

MANY MULTIVERSES
The multiverse is motivated by a puzzle: why fundamental constants of nature, such as the finestructure constant that characterizes the strength of electromagnetic interactions between particles and the cosmological constant associated with the acceleration of the expansion of the Universe, have values that lie in the small range that allows life to exist. Multiverse theory claims that there are billions of unobservable sister universes out there in which all possible values of these constants can occur. So somewhere there will be a biofriendly universe like ours, however improbable that is.

Some physicists consider that the multiverse has no challenger as an explanation of many otherwise bizarre coincidences. The low value of the cosmological constant — known to be 120 factors of 10 smaller than the value predicted by quantum field theory — is difficult to explain, for instance.
...
...
[Sean Carroll] argues that inaccessible domains can have a “dramatic effect” in our cosmic back yard, explaining why the cosmological constant is so small in the part we see. But in multiverse theory, that explanation could be given no matter what astronomers observe. All possible combinations of cosmological parameters would exist somewhere, and the theory has many variables that can be tweaked. Other theories, such as unimodular gravity, a modified version of Einstein’s general theory of relativity, can also explain why the cosmological constant is not huge7.

Some people have devised forms of multiverse theory that are susceptible to tests: physicist Leonard Susskind’s version can be falsified if negative spatial curvature of the Universe is ever demonstrated. But such a finding would prove nothing about the many other versions. Fundamentally, the multiverse explanation relies on string theory, which is as yet unverified, and on speculative mechanisms for realizing different physics in different sister universes. It is not, in our opinion, robust, let alone testable.

The manyworlds theory of quantum reality posed by physicist Hugh Everett is the ultimate quantum multiverse, where quantum probabilities affect the macroscopic. According to Everett, each of Schrödinger’s famous cats, the dead and the live, poisoned or not in its closed box by random radioactive decays, is real in its own universe. Each time you make a choice, even one as mundane as whether to go left or right, an alternative universe pops out of the quantum vacuum to accommodate the other action.

Billions of universes — and of galaxies and copies of each of us — accumulate with no possibility of communication between them or of testing their reality. But if a duplicate self exists in every multiverse domain and there are infinitely many, which is the real ‘me’ that I experience now? Is any version of oneself preferred over any other? How could ‘I’ ever know what the ‘true’ nature of real ity is if one self favours the multiverse and another does not?

In our view, cosmologists should heed mathematician David Hilbert’s warning: although infinity is needed to complete mathematics, it occurs nowhere in the physical Universe.
...
...
The consequences of overclaiming the significance of certain theories are profound — the scientific method is at stake (see go.nature.com/hh7mm6). To state that a theory is so good that its existence supplants the need for data and testing in our opinion risks misleading students and the public as to how science should be done and could open the door for pseudoscientists to claim that their ideas meet similar requirements.

What to do about it? Physicists, philosophers and other scientists should hammer out a new narrative for the scientific method that can deal with the scope of modern physics. In our view, the issue boils down to clarifying one question: what potential observational or experimental evidence is there that would persuade you that the theory is wrong and lead you to abandoning it? If there is none, it is not a scientific theory.

Such a case must be made in formal philosophical terms. A conference should be convened next year to take the first steps. People from both sides of the testability debate must be involved.

In the meantime, journal editors and publishers could assign speculative work to other research categories — such as mathematical rather than physical cosmology — according to its potential testability. And the domination of some physics departments and institutes by such activities could be rethought1,2.

The imprimatur of science should be awarded only to a theory that is testable. Only then can we defend science from attack. ■
_______________________________________
George Ellis is professor emeritus of applied mathematics at the University of Cape Town, South Africa. Joe Silk is professor of physics at the Paris Institute of Astrophysics, France, and at Johns Hopkins University in Baltimore, Maryland, USA.
e-mails: george.ellis@uct.ac.za; silk@iap.fr
==endquote==
http://en.wikipedia.org/wiki/George_F._R._Ellis

 

[Chalnoth]

It is beyond foolish to disregard an entire class of theories as being "untestable" without any consideration of whether or not they are, in fact, testable.

 

[Garth]

To within current measurement errors, physical laws have not changed within the observable universe. And many ways that the laws can change would result in changes being phase transitions that happen suddenly rather than gradually, such that you'd never detect a change by looking at tiny variations of the laws within the observable universe.

The way you detect such different physical laws is basically identical to the multiverse theories: you don't look for the changes themselves, but for other consequences of the theory that predicts the changes that are measurable.

That's the point - By saying "To within current measurement errors, physical laws have not changed within the observable universe" you are showing the idea that laws change is testable, and by your assessment falsifiable.

Now what observations "to within current measurement errors" have demonstrated the existence of other universes or more to the point could possibly falsify the existence of such a universe?

Would you reject any theory, then, which happens to include a multiverse as one of its predictions?

Of course not - I would just classify and reject that particular prediction as being "outside the bounds of science". (Thank you Marcus)

It is beyond foolish to disregard an entire class of theories as being "untestable" without any consideration of whether or not they are, in fact, testable.

I absolutely agree - therefore what are the falsifiable tests for a multiverse?

Garth

 

[Chalnoth]

Now what observations "to within current measurement errors" have demonstrated the existence of other universes or more to the point could possibly falsify the existence of such a universe?

The existence of spontaneous symmetry breaking that impacts low-energy physical laws is evidence of other "universes".

Spontaneous symmetry breaking is at the heart of the standard model of particle physics, and one of the results of said symmetry breaking is the Higgs boson, which was recently detected.

I absolutely agree - therefore what are the falsifiable tests for a multiverse?

Popperian falsifiability has not been a significant restriction of science for a very long time. More recent notions recognize that it is entirely possible for a theory to be unfalsifiable in the strict Popperian sense, while it still being possible for evidence to support the theory.

This frequently occurs, for example, with theories that have free parameters. To take a simple example, imagine a theory with a parameter that varies from 1-1000. If this parameter is between 1-5, then current experiments can detect it. If, on the other hand, the parameter is between 6-1000, then it cannot be detected. If the parameter is between 500-1000, then there is no possible way to detect evidence for the theory, even in principle. Such a theory is not falsifiable in the strict sense, but it is still possible to collect evidence that supports the theory if this parameter happens to be within the detectable range.

This is the sort of thing that people deal with with regard to a "multiverse". That the evidence is generally going to be indirect should bother nobody (our physics theories have become so abstract that evidence for them has been quite indirect for a long time now).

 

[Garth]

The existence of spontaneous symmetry breaking that impacts low-energy physical laws is evidence of other "universes".

Spontaneous symmetry breaking is at the heart of the standard model of particle physics, and one of the results of said symmetry breaking is the Higgs boson, which was recently detected.

You are confusing yourself.

If there is an ensemble of different universes in which spontaneous symmetry breaking occurs bestowing different values to physical constants and laws (resulting in the present asymmetric system in this universe) then you can explain particular (anthropic) values of those constants and laws as a selection effect. We are in this universe because we can be in no other.

So if you are saying, "spontaneous symmetry breaking ... is evidence of other "universes," then the multiverse is an a priori assumption, not a prediction, of the theory.

If you stand a box worth of pencils on their tips and let go one will end up pointing (more or less) north.

However, on the other hand, you don't need a box worth, a single pencil thus let go might end up pointing north by chance.

This universe might be a 'fluke' - how could we tell otherwise with a statistical sample of one?

Spontaneous symmetry breaking does not require a multiverse let alone provide evidence of one.

We have not yet developed a quantum gravity theory, there could be some "Beyond the standard theory" model out there that includes an explicit symmetry breaking, one that does not respect the symmetry of the equations that define why the laws and constants of physics are as they are.
(Stand a single pencil on its point and then blow on it)

therefore what are the falsifiable tests for a multiverse?

Popperian falsifiability has not been a significant restriction of science for a very long time.

Let me rephrase then, "what are the tests for a multiverse?"

Garth

 

[John_QPublic]

In the movie "The Principle" (www.ThePrincipleMovie.com), George Ellis makes the point that the multiverse has way too much explanatory power, yet is undetectable and unverifiable/unfalsifiable. Martin Selbrede states that basically cosmology is at the point where it is either God or the multiverse. If you are in southern California or Spokane, you should go see the movie.

 

[Nugatory]

In the movie "The Principle" (www.ThePrincipleMovie.com), George Ellis makes the point that the multiverse has way too much explanatory power, yet is undetectable and unverifiable/unfalsifiable. Martin Selbrede states that basically cosmology is at the point where it is either God or the multiverse. If you are in southern California or Spokane, you should go see the movie.

This might be a good note upon which to close the thread.

 

[Garth]

In the movie "The Principle" (www.ThePrincipleMovie.com), George Ellis makes the point that the multiverse has way too much explanatory power, yet is undetectable and unverifiable/unfalsifiable. Martin Selbrede states that basically cosmology is at the point where it is either God or the multiverse. If you are in southern California or Spokane, you should go see the movie.

Hi John,

I remember a 1970's paper (in Nature if I recall) by Fred Hoyle in which, after conceding the end of his infinite and eternal Steady State Theory, he is arguing the universe had to be closed (\Omega > 1) so you can have an eternal oscillating universe thus giving enough time for the immensely improbable life to form. To close the universe he needed a lot of unknown mass, which before the Cold DM days, he thought could be in the form of massive neutrinos - as there was so many of them. The only alternative he could think of was 'God did it'. (Note Fred Hoyle was the 1950's and 60's equivalent of Richard Dawkins - known more by the general public for his atheism than for his science.) I remember a conclusion of his paper that said, "Either the neutrino has mass or there is a God", which I thought at least intellectually honest (possibly admitting defeat) if a little unorthodox.However as far as the multiverse or God dilemma is concerned Smolin and Unger suggest a third way and propose a 'new paradigm'. (Don't you just love it when people use that word.;))Garth

 

[Chalnoth]

You are confusing yourself.

If there is an ensemble of different universes in which spontaneous symmetry breaking occurs bestowing different values to physical constants and laws (resulting in the present asymmetric system in this universe) then you can explain particular (anthropic) values of those constants and laws as a selection effect. We are in this universe because we can be in no other.

You don't need an ensemble. All you need is a big universe. In such a universe, the symmetry breaking will take on different values in different locations.

So if you are saying, "spontaneous symmetry breaking ... is evidence of other "universes," then the multiverse is an a priori assumption, not a prediction, of the theory.

False. The extra "universes" in this case arise from the simplest possible assumptions you can possibly make about the theory. It is necessary to add additional assumptions to get rid of the multiverse.

 

[marcus]

==quote Garth==
Smolin and Unger suggest a third way and propose a 'new paradigm'. (Don't you just love it when people use that word.;))
==endquote
Thanks Garth, for this nimble wide-ranging and entertaining discussion! Indeed there must be several "third ways". I mean lines of investigation that don't "give up" but continue to attempt to empirically explain why cosmology is the way it is---how the world comes to be this way. I wonder if Silk and Ellis will get the international community-wide conference they call for!

I quoted from the Silk Ellis call to "defend the integrity of physics" in post #9. I think "outside the bounds of science" was one of the phrases they used for what they consider dangerous trends:
disguised failure of explanation--pretended explanation on imaginary grand scale
retreat to mythology--with the elegance and patness that traditionally accompanies mythology
relaxing the rules of evidence to include elegant ways of giving up

Silk and Ellis call for a conference this year! It could be interesting if they get one started organizing, even if it doesn't actually convene in 2015. I think of it as a
"taking out the rubbish" conference.
Deciding collectively what the criteria are for empirical/testable. They are suggesting such standards be reflected in the editorial policy of physics journals and in departmental faculty makeup. It's a pleasure to hear senior people so outspoken :w

========================
Anyway there must surely be "third ways" in the making. Smolin et al "evolving laws" idea is impressive but the first order of business seems to me to be a combined quantum theory of geometry and matter (like the recent proposal of Chamseddine Connes & Mukhanov to see if GR and StdMdl can grow from a single algebraic root)
It makes sense to me that one first gets the laws (for quantum mechanics, geometry, matter) and THEN one begins to conjecture about how they might have evolved. We don't seem to have a fully unified set of laws yet. But maybe it is good what Smolin Cortes and Unger are doing. Wolfgang Wieland, a QG postdoc whose research I like very much, may have started working with Cortes and Smolin. Sorry if this post is a bit vague, basically just wanted to express appreciation.

 

[ShayanJ]

Let's look more closely to the idea of multiverse. Is it testable? Is it falsifiable? Is it useful? Is it physics? Is it about "our world"?
I don't know that much about this theory to talk about it technically and with mathematical rigour and physical precision. But I think I see some issues about it which I want to share.
Before starting, I want to quote an amusing thing from one of my professors which I think may be of use in my explanations. I had classical mechanics with him and, as you know, there's usually a discussion about motion in accelerated frames in such a course. Now when he wanted talk about the fictitious forces in such a frame, he explained it like this:
Imagine someone who has lived all of his life in a wagon and knows nothing about the outside world. Any time the wagon accelerates, he sees that all stuff moves toward one of the walls with non-zero acceleration. So he may think that "wow man, interesting, looks like there is a special kind of force, which I give it the name wagon-wall force, that from time to time accelerates these objects toward that particular wall." He then may extend his theory to answer the questions "when the force appears?" or "why only that particular wall exerts this force on objects?" And so he'll probably be OK with his own "physics" because he can explain things.
Now we're actually in such a condition. We're inside something we can't get out of. All we say is about inside this "wagon". Let's see what's the difference between an "inside-wagon" theory and an "outside-wagon" one.
All laws of physics we have now(I mean the well established ones of course), are "inside-wagon" theories. They only refer to things inside the wagon and just couldn't care less about what's outside or even whether there is anything outside or not. The important thing about such theories is that...(Not sure how to say it!)...is that we can think about them from different perspectives and study them in different experiments. I mean, we can look at them anyway we want and we can be sure there is something in nature telling us whether that perspective makes sense or not. So we understand these theories very well. And we want all our incomplete theories be like this.
But what about the "outside-wagon" theories like the multiverse? Let's just take it for granted that there can be(or even there already is) evidence for multiverse. But what kind of evidence is it? Surely it can in no way be like the evidence we have for e.g. QED. Because QED is an "inside-wagon" theory. We know from a great number of experimental and theoretical evidences we have that this should be correct. But all other universes, by definition of the multiverse, are just very much disconnected from us so the theory itself is telling us that even if it has any evidence, it should be too little. And now like that guy inside the wagon who can build his own physics without referring to outside, we always can have "inside-wagon" theories that have much more evidence and also include that little evidence of multiverse. So it seems to me any physical "inside-wagon" theory is by definition superior to multiverse.
Now this argument may seem too hand-wavy to dismiss a whole theory based on it but it actually is not that hand-wavy. I know people may say that "yeah, but that doesn't prove multiverse is wrong!". Yeah I know, but even if actually multiverse is right(which has no meaning for me), how does it make a difference to physics? Let me rephrase the question, how is it different from saying that god created the universe? Its really naive to think that only because a physicist knowing lots of fancy math is saying that, then it is physics. Because there can always be people saying that "yeah guys, we know how this universe is created, its the multiverse, and you guys don't know!...blah blah blah" and all other physicists just ignoring them because they know there should be an "inside-wagon" theory giving the solution and it doesn't matter for how long they they wait for that theory to show up, they just keep working on it because they think it should be an "inside-wagon" theory. Now doesn't this remind you anything? Let me tell you what is it. Its the same gap between people who say god created the universe and no more and people who think science can explain everything and doesn't need god.
My point is, science tries to talk about this world and "this world" is defined(inescapably) by what we can observe. So I guess we can say science is a fuzzy set. For any statement, there is an evaluation function which tells us how much this statement is part of science. And that function is the amount we can observe things about that statement. Now god is something unobservable by definition and so our function gives zero for it. This means the word "god" should appear in no statement which is intended to be called scientific. This also means science is unable to talk about god. And multiverse, our functions doesn't give zero for this but it seems to it gives a very small value. As I said, by definition, multiverse is much less observable than other theories and so its much less inside physics than other theories. So I would say, the multiverse explanation is just a little more related to science, than is the explanation "god created it".

 

[Garth]

f there is an ensemble of different universes in which spontaneous symmetry breaking occurs bestowing different values to physical constants and laws (resulting in the present asymmetric system in this universe) then you can explain particular (anthropic) values of those constants and laws as a selection effect. We are in this universe because we can be in no other.

You don't need an ensemble. All you need is a big universe. In such a universe, the symmetry breaking will take on different values at different times.

Are we taking here about "different regions with different physical laws" in the one universe (Your #3)? The words "Other universes" was yours (#12), but alright let me rephrase:- If there is an ensemble of different regions with different physical laws in which spontaneous symmetry breaking occurs... (the rest follows..)

So if you are saying, "spontaneous symmetry breaking ... is evidence of other "universes," then the multiverse is an a priori assumption, not a prediction, of the theory.

False. The extra "universes" in this case arise from the simplest possible assumptions you can possibly make about the theory. It is necessary to add additional assumptions to get rid of the multiverse.

But only if the symmetry breaking is "spontaneous" i.e. stochastic. That itself is an a priori assumption.
Explicit symmetry breaking under some constraint in a Theory of Everything may "get rid of the multiverse".
Now I haven't (yet) got a testable TOE but then neither have you.

I had an extended internet debate on the origin of the universe with someone who argued that in the BB the universe sprang out of nothing.
As there really had been no-thing, no matter, energy or physical laws in existence then there had been no conservation laws to say it couldn't happen.
A very convincing argument.
However my response was that my problem wasn't that his argument explained nothing but that it explained everything and anything and therefore it explained nothing.

I read a similar objection to the multiverse by Paul Steinhardt in an Edge article 'Theories of Anything'.

A pervasive idea in fundamental physics and cosmology that should be retired: the notion that we live in a multiverse in which the laws of physics and the properties of the cosmos vary randomly from one patch of space to another...
Among these patches, in the words of Alan Guth, "anything that can happen will happen—and it will happen infinitely many times". Hence, I refer to this concept as a Theory of Anything...
I think a priority for theorists today is to determine if inflation and string theory can be saved from devolving into a Theory of Anything and, if not, seek new ideas to replace them. Because an unfalsifiable Theory of Anything creates unfair competition for real scientific theories, leaders in the field can play an important role by speaking out—making it clear that Anything is not acceptable—to encourage talented young scientists to rise up and meet the challenge. The sooner we can retire the Theory of Anything, the sooner this important science can progress.

I cannot agree more.
Garth

 

[Chalnoth]

It absolutely disgusts me that so many people make the a priori assumption that there can be no multiverse, and then try to berate people who suggest it as a possibility.

 

[marcus]

I don't see anybody here (certainly not Silk and Ellis in the article I quoted) who rejects "multiverse" models a priori. So it is misleading and disingenuous to pretend people are doing that and fulminate against it. Silk and Ellis even mention an instance of a TESTABLE "multiverse". What they are fighting is the trend to erode empircism, relax requirements of testability.
These are world-class senior cosmologists and I took the trouble to quote some excerpts. I'd appreciate if people would have a look because I think it is an important development. Silk and Ellis want a physics/cosmology conference with people from different sides of the testability debate, to settle what the criteria are for testability and what is not testable and therefore outside science. they even call for a conference to be held this year (which might be sooner than it can be organized, I can't tell). they hope conclusions from such a conference influence journal editorial policy and departmental hiring trends.
The point is to examine, hear different sides, discuss, and hammer out appropriate standards for the community.
I will quote what I said about this in my previous post:
==quote my earlier post==
I would go along to a large extent with the essay in NATURE by two of the world's most prominent and respected cosmologists: Joe Silk and George Ellis. They are definite about what they mean should be regarded as outside the bounds of science. They define their terms and give their reasons.
As I recall the essay was titled something like
Defend the Integrity of Physics
Yes, here is a link:
http://www.nature.com/news/scientific-method-defend-the-integrity-of-physics-1.16535
Here are some excerpts:
==quote Silk Ellis essay in Nature==
Scientific method: Defend the integrity of physics
16 December 2014
Attempts to exempt speculative theories of the Universe from experimental verification undermine science, argue George Ellis and Joe Silk.

This year, debates in physics circles took a worrying turn. Faced with difficulties in applying fundamental theories to the observed Universe, some researchers called for a change in how theoretical physics is done. They began to argue — explicitly — that if a theory is sufficiently elegant and explanatory, it need not be tested experimentally, breaking with centuries of philosophical tradition of defining scientific knowledge as empirical. We disagree. As the philosopher of science Karl Popper argued: a theory must be falsifiable to be scientific.
Chief among the 'elegance will suffice' advocates are some string theorists. Because string theory is supposedly the 'only game in town' capable of unifying the four fundamental forces, they believe that it must contain a grain of truth even though it relies on extra dimensions that we can never observe. Some cosmologists, too, are seeking to abandon experimental verification of grand hypotheses that invoke imperceptible domains ...

These unprovable hypotheses are quite different from those that relate directly to the real world and that are testable through observations — such as the standard model of particle physics and the existence of dark matter and dark energy. As we see it, theoretical physics risks becoming a no-man's-land between mathematics, physics and philosophy that does not truly meet the requirements of any.

The issue of testability has been lurking for a decade. String theory and multiverse theory have been criticized in popular books1, 2, 3 and articles, including some by one of us (G.E.)4. In March, theorist Paul Steinhardt wrote5 in this journal that the theory of inflationary cosmology is no longer scientific because it is so flexible that it can accommodate any observational result. Theorist and philosopher Richard Dawid6 and cosmologist Sean Carroll7 have countered those criticisms with a philosophical case to weaken the testability requirement for fundamental physics.
...
...

MANY MULTIVERSES
The multiverse is motivated by a puzzle: why fundamental constants of nature, such as the finestructure constant that characterizes the strength of electromagnetic interactions between particles and the cosmological constant associated with the acceleration of the expansion of the Universe, have values that lie in the small range that allows life to exist. Multiverse theory claims that there are billions of unobservable sister universes out there in which all possible values of these constants can occur. So somewhere there will be a biofriendly universe like ours, however improbable that is.

Some physicists consider that the multiverse has no challenger as an explanation of many otherwise bizarre coincidences. The low value of the cosmological constant — known to be 120 factors of 10 smaller than the value predicted by quantum field theory — is difficult to explain, for instance.
...
...
[Sean Carroll] argues that inaccessible domains can have a “dramatic effect” in our cosmic back yard, explaining why the cosmological constant is so small in the part we see. But in multiverse theory, that explanation could be given no matter what astronomers observe. All possible combinations of cosmological parameters would exist somewhere, and the theory has many variables that can be tweaked. Other theories, such as unimodular gravity, a modified version of Einstein’s general theory of relativity, can also explain why the cosmological constant is not huge7.

Some people have devised forms of multiverse theory that are susceptible to tests: physicist Leonard Susskind’s version can be falsified if negative spatial curvature of the Universe is ever demonstrated. But such a finding would prove nothing about the many other versions. Fundamentally, the multiverse explanation relies on string theory, which is as yet unverified, and on speculative mechanisms for realizing different physics in different sister universes. It is not, in our opinion, robust, let alone testable.

The manyworlds theory of quantum reality posed by physicist Hugh Everett is the ultimate quantum multiverse, where quantum probabilities affect the macroscopic. According to Everett, each of Schrödinger’s famous cats, the dead and the live, poisoned or not in its closed box by random radioactive decays, is real in its own universe. Each time you make a choice, even one as mundane as whether to go left or right, an alternative universe pops out of the quantum vacuum to accommodate the other action.

Billions of universes — and of galaxies and copies of each of us — accumulate with no possibility of communication between them or of testing their reality. But if a duplicate self exists in every multiverse domain and there are infinitely many, which is the real ‘me’ that I experience now? Is any version of oneself preferred over any other? How could ‘I’ ever know what the ‘true’ nature of real ity is if one self favours the multiverse and another does not?

In our view, cosmologists should heed mathematician David Hilbert’s warning: although infinity is needed to complete mathematics, it occurs nowhere in the physical Universe.
...
...
The consequences of overclaiming the significance of certain theories are profound — the scientific method is at stake (see go.nature.com/hh7mm6). To state that a theory is so good that its existence supplants the need for data and testing in our opinion risks misleading students and the public as to how science should be done and could open the door for pseudoscientists to claim that their ideas meet similar requirements.

What to do about it? Physicists, philosophers and other scientists should hammer out a new narrative for the scientific method that can deal with the scope of modern physics. In our view, the issue boils down to clarifying one question: what potential observational or experimental evidence is there that would persuade you that the theory is wrong and lead you to abandoning it? If there is none, it is not a scientific theory.

Such a case must be made in formal philosophical terms. A conference should be convened next year to take the first steps. People from both sides of the testability debate must be involved.

In the meantime, journal editors and publishers could assign speculative work to other research categories — such as mathematical rather than physical cosmology — according to its potential testability. And the domination of some physics departments and institutes by such activities could be rethought1,2.

The imprimatur of science should be awarded only to a theory that is testable. Only then can we defend science from attack. ■
_______________________________________
George Ellis is professor emeritus of applied mathematics at the University of Cape Town, South Africa. Joe Silk is professor of physics at the Paris Institute of Astrophysics, France, and at Johns Hopkins University in Baltimore, Maryland, USA.
e-mails: george.ellis@uct.ac.za; silk@iap.fr
==endquote from Nature==
http://en.wikipedia.org/wiki/George_F._R._Ellis
==endquote from my earlier post==

 

[Chronos]

An unfalsifiable theory is not science. It might be philosophy, even theosophy, but, not science. Science is empirical. That is arguably a weakness, I think it is an imperative.

 

[Chalnoth]

When you effectively tell people that they can't even *speculate* about multiverse ideas, yes, you are saying that you aren't willing to consider any theory which contains a multiverse. It's disgusting.

 

[ShayanJ]

When you effectively tell people that they can't even *speculate* about multiverse ideas, yes, you are saying that you aren't willing to consider any theory which contains a multiverse. It's disgusting.

Of course anyone is free to speculate about anything s\he wants. Otherwise we all would hate pure mathematicians, right?
The point is, as I said in my last post, it makes no difference to accept or not to accept multiverse. Its like QM's shut up and calculate. People say "yeah, we shut up and calculate but we also sometimes don't shut up!", now Bohr and Heisenberg may say "no guys, you should shut up" and the answer is just "yeah, you're right, its OK, we shut up but not always!". What I want to say is, people are free to speculate about multiverse and think it is the right theory, but it makes no difference that others accept it or not, anyway they'll continue to find other theories and , as I explained, any such theory will have more evidence than multiverse and this is inescapable.
You see, I'm only telling there is a big difference between multiverse and e.g. the standard model. People can forever resist accepting multiverse with no harm, but anyone reluctant to accept the SM, will be abandoned academically.
And this somehow defines how science should progress. Any theory should be somehow that the majority of the experts agree on it so that we have a mainstream path. Otherwise it becomes like philosophy that anyone is allowed to say anything and anyone can accept or reject anything. So the very fact that so many experts can simply ignore multiverse, is a philosophical problem for it which rules it out from physics to philosophy. I mean it can be correct, but not in a physical sense, but only in a philosophical sense. So if multiverse is going to remain in physics, it should change that. If it does, then its OK. Otherwise it will be philosophy, not physics.

 

[Chalnoth]

Then why are you hating on speculative models?

 

[Haelfix]

These threads always degenerate into a bunch of philosophy blah blah blah. There are also frequently very puzzling positions that I believe muddies the water considerably.

Look, cosmologists are intelligent people. The reason why many believe in multiverse theories, is that they were LED to that conclusion by the preponderance of the evidence and the available theoretical models. Indeed there is a rather intricate chain of reasoning involving the evidence for the existence of inflation (CMB data, etc), the space of theories within the inflationary framework, the physics of spontaneous symmetry breaking in the presence of gravity (the Coleman-DeLucca analysis), the metastability of the electroweak vacuum and the smallness of the cosmological constant. If someone doesn't understand what or why these assorted experimental facts mean for the case for multiverse, then they haven't thought about this properly and they ought to think about it harder.

In particular, the notion that the status quo will remain forever is just completely wrong. It will be decided by data as per usual. For instance, the case for a multiverse is drastically modified (I would say put in the hospital), if the following data sets (prior to this week) turn out to converge: Planck, WMAP, BAO and BICEP. The four of them together implied a value of the spectral tilt and r, that was simply inconsistent with inflation. Therefore a lot of the chain of reasoning necessary for the multiverse would collapse and scientists will have to come up with something else. However, (and this has been happening for 30 years now) if the data continues to favor inflationary scenarios that involves chaotic or eternal inflation, then people will continue to favor multiverse models. My question to the naysayers is, how could this be any other way?

 

[Garth]

It absolutely disgusts me that so many people make the a priori assumption that there can be no multiverse, and then try to berate people who suggest it as a possibility.

Then why are you hating on speculative models?

I am not making the a priori assumption that there can be no multiverse - there may be as many other universes as you want - the point is we cannot see them. (Unless somebody travels through a BH into one and returns to tell, then the hypothesis will enter the realm of observed science.)

I don't hate speculative models - I have spent a lifetime trying to develop one, however I have also tried to see whether it can be tested and falsified. (In its last incarnation it was!)

Think of the Intelligent Design argument:

Q. Why is the gravitational constant 'just right'? Why is the Cosmological constant so little? etc. etc. -

A. God did it She made it 'just right'!

The problem with this answer is twofold - it is bad science - once you think you have the answer you stop looking for the real one. Also it is bad theology - as soon as somebody else finds the answer then your god, the god-of-the-gaps disappears a little more.

Now Q. Why is the gravitational constant 'just right'? Why is the Cosmological constant so little? etc. etc. -

A. The multiverse did it! With an a priori assumption that the symmetry breaking process is stochastic and therefore there are a large/infinite number (ensemble) of other regions/patches/universes where the values of these constants are different then we must be in this universe (or this part of the universe) where they are 'just right'.

What's the difference between the two answers?

Actually I might go further and say that as 'One unobservable God' is a simpler answer than a 'large/infinite number of other unobservable regions/patches/universes' then by Occam's razor it is the better answer and more scientific! ;)Perhaps looking for a 'third way' is not so "disgusting"?

Garth

 

[Larry Pendarvis]

It is beyond foolish to disregard an entire class of theories as being "untestable" without any consideration of whether or not they are, in fact, testable.

So the theory that a class of theories is untestable... is that theory testable?

 

[Larry Pendarvis]

"... They began to argue — explicitly — that if a theory is sufficiently elegant and explanatory, it need not be tested experimentally, breaking with centuries of philosophical tradition of defining scientific knowledge as empirical.We disagree."

It seems to me that the difference between an explanatory theory and a predictive theory is just history. In a low-density region of a galaxy where scientists have not yet observed gravity, string theory would predict gravity and that could be subsequently confirmed by experiment.

 

[stevendaryl]

I don't read Smolin as saying that multiverses are impossible. I just read him as saying that there isn't much we can learn from thinking about multiverses. He's really just saying that we should focus on the one universe that we can observe. So it seems to me more like his statement about what's worth spending our intellectual resources and time on.

 

[Garth]

I don't read Smolin as saying that multiverses are impossible. I just read him as saying that there isn't much we can learn from thinking about multiverses. He's really just saying that we should focus on the one universe that we can observe. So it seems to me more like his statement about what's worth spending our intellectual resources and time on.

Absolutley right.

Garth

 

[marcus]

I don't read Smolin as saying that multiverses are impossible. I just read him as saying that there isn't much we can learn from thinking about multiverses. He's really just saying that we should focus on the one universe that we can observe. So it seems to me more like his statement about what's worth spending our intellectual resources and time on.

Yes! The aim is to empirically explain and anticipate THIS nature and this cosmos that we live in, and experience, and are a part of.

 

[marcus]

Steven, thanks for the clear statement! This may be obvious and not need saying, but I think there is a special issue about the past, because we don't live in it and experiment with it or directly experience it. Some models of the past CAN be tested by what they tell us we may anticipate from future observations. IOW to fall within the bounds of science a model of the past must make unambiguous predictions about what we can observe by looking more closely at the present.

We had an example of this recently with the BICEP vs Planck controversy over possible polarization swirls in the CMB. Different early universe models (versions of inflation and no-inflation bounce starts) predict different amounts of swirl---different "tensor-to-scalar" ratios. Saying, in effect, if you look more closely at the CMB and measure polarization as well as temperature fluctuations you will see such-and-such.

A model of the past can't be too flexible as to what it leads us to anticipate, otherwise it's mythology, not science. This is the key to Paul Steinhardt's critique of cosmic inflation. He was one of a panel four cosmologists chosen to discuss the state of inflation scenarios at the December 2014 Paris conference on the Planck mission results.
He gave a really interesting talk (archived on video) which was then followed by the panel discussion. Here are links to conference videos:
Steinhardt's critical review:
http://webcast.in2p3.fr/videos-introduction-_critical_review_of_inflation
The panel debate ( Steinhardt, Mukhanov, Linde, Brandenberger) that followed:
http://webcast.in2p3.fr/videos-debate_theoretical_problems_way_forward

A bit of discussion here at PF:
https://www.physicsforums.com/threa...on-inflation-alts-and-spirited-debate.788685/

 

[Garth]

Of course the support for the multiverse hypothesis gets worse as there is now a problem with the evidence supporting inflation.

The first prediction of the inflation hypothesis, which was the universe should be approximately flat, seems to have been borne out by the CMB, although other models may also give a flat universe, even the old and discarded Steady State Model.

The second piece of evidence was the prediction of B Mode polarisation in the CMB due to gravity waves from inflation and apparently found in the BICEP2 polarisation data, loudly proclaimed in March last year. However it had now been shown by Plank that what they actually found was dust - more than they had expected
Constraint on the primordial gravitational waves from the joint analysis of BICEP2 and Planck HFI 353 GHz dust polarization data

We make a joint analysis of BICEP2 and recently released Planck HFI 353 GHz dust polarization data, and find that there is no evidence for the primordial gravitational waves

I was at a lecture back in November by http://www-astro.physics.ox.ac.uk/~Dunkley/Home.html of Oxford University who is working on the problem. As I said in this thread Was the announcement that BICEP2 has detected Inflation Gravity Waves premature? She is now working on a balloon experiment to be launched in two years time to see if there is an Inflation Gravity Wave signal hidden behind the dust signal detected by BICEP2.
Planck intermediate results. XXX. The angular power spectrum of polarized dust emission at intermediate and high Galactic latitudes

Finally, we investigate the level of dust polarization in the BICEP2 experiment field. Extrapolation of the Planck 353GHz data to 150GHz gives a dust power ℓ(ℓ+1)C^BB_ℓ/(2π) of 1.32×10⁻²μK²CMB over the 40<ℓ<120 range; the statistical uncertainty is ±0.29 and there is an additional uncertainty (+0.28,-0.24) from the extrapolation, both in the same units. This is the same magnitude as reported by BICEP2 over this ℓ range, which highlights the need for assessment of the polarized dust signal even in the cleanest windows of the sky.

Garth

 

[Haelfix]

In the 80s, before people knew much about the details of the CMB, Inflation and in particular chaotic inflation and eternal inflation were invented and made several model independant predictions. They predicted superhorizon correlations between different parts of the sky, they predicted flatness, they predicted the shape of the adiabatic perturbations to be of a very specific type (nearly but not quite scale invariant). They predicted the lack of a vector perturbation, the prediction of a nearly but not quite value of 1 for ns and so on and so forth (the last model independant prediction is gravitational waves, which might be strong enough to actually see).

Of course what happened is that every single one of those predictions turned out to fit experiment, and in the meantime, people figured out new models (withot the multiverse) that happened to also fit the observation (keep in mind you are essentially matching a few experimental parameters to theories that have infinitely many possible couplings and forms, so its not a surprise that you can find an adhoc model that fits observation with some particular chosen property).

Now, 30 years later, we get to hear about how inflationary versions of the multiverse is not falsifiable, completely ignoring the history of what came before. But the point is the multiverse was a generic consequence of theories that have already been extremely well tested. This isn't something that is being pulled out of a hat. If there is a better idea that comes around without the multiverse, then it seems clear that people will adopt that instead.

However what's completely crazy, is the weird notion that we should simply excise all possible models that happen to predict a multiverse as a byproduct, simply b/c a long dead philosopher doesn't think that that's science.

 

[Larry Pendarvis]

In the 80s, before people knew much about the details of the CMB, Inflation and in particular chaotic inflation and eternal inflation were invented and made several model independant predictions. They predicted superhorizon correlations between different parts of the sky, they predicted flatness, they predicted the shape of the adiabatic perturbations to be of a very specific type (nearly but not quite scale invariant). They predicted the lack of a vector perturbation, the prediction of a nearly but not quite value of 1 for ns and so on and so forth (the last model independant prediction is gravitational waves, which might be strong enough to actually see).

Of course what happened is that every single one of those predictions turned out to fit experiment, and in the meantime, people figured out new models (withot the multiverse) that happened to also fit the observation (keep in mind you are essentially matching a few experimental parameters to theories that have infinitely many possible couplings and forms, so its not a surprise that you can find an adhoc model that fits observation with some particular chosen property).

Now, 30 years later, we get to hear about how inflationary versions of the multiverse is not falsifiable, completely ignoring the history of what came before. But the point is the multiverse was a generic consequence of theories that have already been extremely well tested. This isn't something that is being pulled out of a hat. If there is a better idea that comes around without the multiverse, then it seems clear that people will adopt that instead.

However what's completely crazy, is the weird notion that we should simply excise all possible models that happen to predict a multiverse as a byproduct, simply b/c a long dead philosopher doesn't think that that's science.

Out of the (possibly) infinite number of universes, is there a universe in the multiverse where the physics does not point to a multiverse?

 

[Chronos]

I don't recall anyone objecting to empirical tests. All new ideas, such as GR, are met with suspicion until unambiguous empirical predictions are offered and confirmed. A theory that happens to predict a multiverse is insufficient unless that aspect of it meets the litmus test of occam's razor.

 

[MattRob]

The existence of spontaneous symmetry breaking that impacts low-energy physical laws is evidence of other "universes".

Spontaneous symmetry breaking is at the heart of the standard model of particle physics, and one of the results of said symmetry breaking is the Higgs boson, which was recently detected.Popperian falsifiability has not been a significant restriction of science for a very long time. More recent notions recognize that it is entirely possible for a theory to be unfalsifiable in the strict Popperian sense, while it still being possible for evidence to support the theory.

This frequently occurs, for example, with theories that have free parameters. To take a simple example, imagine a theory with a parameter that varies from 1-1000. If this parameter is between 1-5, then current experiments can detect it. If, on the other hand, the parameter is between 6-1000, then it cannot be detected. If the parameter is between 500-1000, then there is no possible way to detect evidence for the theory, even in principle. Such a theory is not falsifiable in the strict sense, but it is still possible to collect evidence that supports the theory if this parameter happens to be within the detectable range.

This is the sort of thing that people deal with with regard to a "multiverse". That the evidence is generally going to be indirect should bother nobody (our physics theories have become so abstract that evidence for them has been quite indirect for a long time now).

My favorite post in this thread.

If you want to answer if something is physics, then it's probably best to step back and ask what physics is. Physics is not philosophy. While I love and respect philosophy, it's a separate field, just as any kind of engineering is. But the thing is, physics can be very closely tied in with philosophy, so I think some types of "philosophical musings" ought to be entertained with caution, as they could lead to testable predictions.

Also, it's important to specify what kind of multiverse we're talking about, here, because each of them are very different.

If a multiverse comes as an inevitable consequence of a theory that's been experimentally confirmed based on other predictions, then it's definitely a part of physics. By way of metaphor; you can't accept GR's testable predictions of phenomenon in the solar system, and then reject its prediction of Schwarzschild solutions. Likewise, if we were to detect quantum black holes, or discover that gravity fails to follow the inverse-square law at small lengths and follows higher-power inverse laws (as per predictions of string theory), then that's also evidence for everything the theory entails, such as a brane multiverse model. If there are simpler explanations for experimental results, then they ought to be very seriously considered, certainly, but it's also worth noting that string theory made the prediction first. (But certainly the better example in this paragraph is the first, since the simplest explanation of solar system phenomenon also inevitably leads to the prediction of black holes)

Perhaps it's also important to remember that theories are only part of a process to uncover more of the mysteries of the universe, than solidified statements of truth.

So I'd say multiverses - even completely inaccessible ones - are entirely within the domain of physics if they come as an inevitable consequence of well-established theories. For example, one could possibly argue that the space within a Schwarzschild radius could be considered another universe, because it is completely inaccessible (albiet, one-way) to us. This does not, however, mean that we reject the notion of a singularity, or the prediction that there is space inside a black hole.

I think more to the point of what a lot of people are getting at, though, is generally ideas that aren't testable but have explanatory power and "elegance." Just like how astronomers sometimes use a non-physical "celestial sphere" model to make certain predictions and math easier, these non-testable theories may have a place if they aid progress in making other testable predictions - so long as we remember that they're not necessarily physical if they're only heuristic tools.

 

[marcus]

So I'd say multiverses - even completely inaccessible ones - are entirely within the domain of physics if they come as an inevitable consequence of well-established theories. ...

Hi MattRob, big "if" :) People will tell you that eternal inflation (with its proliferation of different expanding regions) is inevitable consequence of inflation, but google what's between brackets [inflation w/o self-reproduction] and you get:
http://arxiv.org/abs/1409.2335
Inflation without Selfreproduction
Viatcheslav Mukhanov
(Submitted on 8 Sep 2014)
We find a rather unique extension of inflationary scenario which avoids selfreproduction and thus resolves the problems of multiverse, predictability and initial conditions. In this theory the amplitude of the cosmological perturbations is expressed entirely in terms of the total duration of inflation.
11 pages.

http://arxiv.org/abs/1412.2518
Inflation without self-reproduction in F(R) gravity
Shin'ichi Nojiri, Sergei D. Odintsov
(Submitted on 8 Dec 2014)
We investigate inflation in frames of two classes of F(R) gravity and check its consistency with Planck data. It is shown that F(R) inflation without self-reproduction may be constructed in close analogy with the corresponding scalar example proposed by Mukhanov for the resolution the problems of multiverse, predictability and initial conditions.
6 pages

V. Mukhanov is one of the world's top cosmologists---look him up in Wikipedia. He has been a major figure in the successful development and testing of inflation models. He sees the multiple proliferation of inflation as a problem (involving the weakening of predictability) so he has addressed that and shown that it is not a necessary consequence. You can have inflation without "multiverse".

People seem to gloss over this and talk as if it were true that "multiverse" is an inevitable consequence of inflation. But I think it is good to be skeptical about that.
We don't yet know the whole story. And we really don't know if inflation itself is necessary! It is one way the early universe may have achieved (near) flatness and sameness in all directions and an even spectrum of fluctuations. But some interesting other mechanisms achieving the same results have been proposed.

 

[marcus]

Since we just turned a page, I'll bring forward a post I want to refer to:

So I'd say multiverses - even completely inaccessible ones - are entirely within the domain of physics if they come as an inevitable consequence of well-established theories...

Hi MattRob, big "if" :) People will tell you that eternal inflation (with its proliferation of different expanding regions) is inevitable consequence of inflation, but google what's between brackets [inflation w/o self-reproduction] and you get:
http://arxiv.org/abs/1409.2335
Inflation without Selfreproduction
Viatcheslav Mukhanov
(Submitted on 8 Sep 2014)
We find a rather unique extension of inflationary scenario which avoids selfreproduction and thus resolves the problems of multiverse, predictability and initial conditions. In this theory the amplitude of the cosmological perturbations is expressed entirely in terms of the total duration of inflation.
11 pages.

http://arxiv.org/abs/1412.2518
Inflation without self-reproduction in F(R) gravity
Shin'ichi Nojiri, Sergei D. Odintsov
(Submitted on 8 Dec 2014)
We investigate inflation in frames of two classes of F(R) gravity and check its consistency with Planck data. It is shown that F(R) inflation without self-reproduction may be constructed in close analogy with the corresponding scalar example proposed by Mukhanov for the resolution the problems of multiverse, predictability and initial conditions.
6 pages

V. Mukhanov is one of the world's top cosmologists---look him up in Wikipedia. He has been a major figure in the successful development and testing of inflation models. He sees the multiple proliferation of inflation as a problem (involving the weakening of predictability) so he has addressed that and shown that it is not a necessary consequence. You can have inflation without "multiverse".

People seem to gloss over this and talk as if it were true that "multiverse" is an inevitable consequence of inflation. But I think it is good to be skeptical about that.
We don't yet know the whole story. And we really don't know if inflation itself is necessary! It is one way the early universe may have achieved (near) flatness and sameness in all directions and an even spectrum of fluctuations. But some interesting other mechanisms achieving the same results have been proposed.

So I'd say multiverses - even completely inaccessible ones - are entirely within the domain of physics if they come as an inevitable consequence of well-established theories...

Again MattRob, you may be thinking of "multiverse" as an "inevitable consequence" of "inflation". But is inflation a "well-established theory"? We need to be careful. I think changes are in the air. Paul Steinhardt of Princeton is also one of the world's top cosmologists and he has actually been one of the major figures in developing inflation ideas and models. He has also worked on alternatives to inflation. Here is what he wrote in Nature in June 2014:
http://www.nature.com/news/big-bang-blunder-bursts-the-multiverse-bubble-1.15346
==quote Steinhardt Big Bang Blunder Bursts the Multiverse Bubble in Nature, June 2014==
...
...
The BICEP2 incident has also revealed a truth about inflationary theory. The common view is that it is a highly predictive theory. If that was the case and the detection of gravitational waves was the ‘smoking gun’ proof of inflation, one would think that non-detection means that the theory fails. Such is the nature of normal science. Yet some proponents of inflation who celebrated the BICEP2 announcement already insist that the theory is equally valid whether or not gravitational waves are detected. How is this possible?

The answer given by proponents is alarming: the inflationary paradigm is so flexible that it is immune to experimental and observational tests. First, inflation is driven by a hypothetical scalar field, the inflaton, which has properties that can be adjusted to produce effectively any outcome. Second, inflation does not end with a universe with uniform properties, but almost inevitably leads to a multiverse with an infinite number of bubbles, in which the cosmic and physical properties vary from bubble to bubble. The part of the multiverse that we observe corresponds to a piece of just one such bubble. Scanning over all possible bubbles in the multiverse, everything that can physically happen does happen an infinite number of times. No experiment can rule out a theory that allows for all possible outcomes. Hence, the paradigm of inflation is unfalsifiable.

This may seem confusing given the hundreds of theoretical papers on the predictions of this or that inflationary model. What these papers typically fail to acknowledge is that they ignore the multiverse and that, even with this unjustified choice, there exists a spectrum of other models which produce all manner of diverse cosmological outcomes. Taking this into account, it is clear that the inflationary paradigm is fundamentally untestable, and hence scientifically meaningless.

Cosmology is an extraordinary science at an extraordinary time. Advances, including the search for gravitational waves, will continue to be made and it will be exciting to see what is discovered in the coming years. With these future results in hand, the challenge for theorists will be to identify a truly explanatory and predictive scientific paradigm describing the origin, evolution and future of the Universe.
==endquote==

This is someone who, like Mukhanov, has done important work involved with inflation in the past. And in what I think was the most important cosmology conference of 2014 (the December Paris conference on the Planck mission results) both Mukhanov and Steinhardt were chosen to be on the panel discussing the inflation paradigm (and alternatives to it).

Has "inflation" been experimentally or observationally confirmed?

 

[marcus]

If a multiverse comes as an inevitable consequence of a theory that's been experimentally confirmed based on other predictions, then it's definitely a part of physics...

MattRob, the argument supporting multiverse notions is typically that they are "inevitable consequences" of some inflation scenario like "eternal inflation". But on what basis can one say that inflation is experimentally confirmed? Or observationally?
Another of the world's top cosmologists is Robert Brandenberger and he and his group at McGill have been working on a type of bounce cosmology that tries to obviate inflation. Get rid of the unphysical "singularity" at the start of expansion of the standard cosmic model and also achieve the early universe features which inflation was hypothesized to provide.
Yi-Fu Cai is a postdoc at McGill who has worked closely with Brandenberger and I would suggest checking out a recent paper of his.
Google [LambdaCDM bounce] to get:
http://arxiv.org/abs/1412.2914
A ΛCDM bounce scenario
Yi-Fu Cai, Edward Wilson-Ewing
(Submitted on 9 Dec 2014)
We study a contracting universe composed of cold dark matter and radiation, and with a positive cosmological constant. As is well known from standard cosmological perturbation theory, under the assumption of initial quantum vacuum fluctuations the Fourier modes of the comoving curvature perturbation that exit the (sound) Hubble radius in such a contracting universe at a time of matter-domination will be nearly scale-invariant. Furthermore, the modes that exit the (sound) Hubble radius when the effective equation of state is slightly negative due to the cosmological constant will have a slight red tilt, in agreement with observations. We assume that loop quantum cosmology captures the correct high-curvature dynamics of the space-time, and this ensures that the big-bang singularity is resolved and is replaced by a bounce. We calculate the evolution of the perturbations through the bounce and find that they remain nearly scale-invariant. We also show that the amplitude of the scalar perturbations in this cosmology depends on a combination of the sound speed of cold dark matter, the Hubble rate in the contracting branch at the time of equality of the energy densities of cold dark matter and radiation, and the curvature scale that the loop quantum cosmology bounce occurs at. Finally, for a small sound speed of cold dark matter, this scenario predicts a small tensor-to-scalar ratio.
14 pages, 8 figures

 

[MattRob]

Hi MattRob, big "if" :) People will tell you that eternal inflation (with its proliferation of different expanding regions) is inevitable consequence of inflation, but google what's between brackets [inflation w/o self-reproduction] and you get:
http://arxiv.org/abs/1409.2335
Inflation without Selfreproduction
Viatcheslav Mukhanov
(Submitted on 8 Sep 2014)
We find a rather unique extension of inflationary scenario which avoids selfreproduction and thus resolves the problems of multiverse, predictability and initial conditions. In this theory the amplitude of the cosmological perturbations is expressed entirely in terms of the total duration of inflation.
11 pages.

http://arxiv.org/abs/1412.2518
Inflation without self-reproduction in F(R) gravity
Shin'ichi Nojiri, Sergei D. Odintsov
(Submitted on 8 Dec 2014)
We investigate inflation in frames of two classes of F(R) gravity and check its consistency with Planck data. It is shown that F(R) inflation without self-reproduction may be constructed in close analogy with the corresponding scalar example proposed by Mukhanov for the resolution the problems of multiverse, predictability and initial conditions.
6 pages

V. Mukhanov is one of the world's top cosmologists---look him up in Wikipedia. He has been a major figure in the successful development and testing of inflation models. He sees the multiple proliferation of inflation as a problem (involving the weakening of predictability) so he has addressed that and shown that it is not a necessary consequence. You can have inflation without "multiverse".

People seem to gloss over this and talk as if it were true that "multiverse" is an inevitable consequence of inflation. But I think it is good to be skeptical about that.
We don't yet know the whole story. And we really don't know if inflation itself is necessary! It is one way the early universe may have achieved (near) flatness and sameness in all directions and an even spectrum of fluctuations. But some interesting other mechanisms achieving the same results have been proposed.

Hello, Marcus. Thanks for the detailed replies, but a few things do somewhat bother me based off of what I'm understanding: Just because you can have inflation without a multiverse, doesn't necessarily mean that a model without a multiverse is true. I understand that there's lots of problematic implications that spring from a multiverse theory, but I don't think that quite justifies adding a constraint that prevents self-replication just because self-replication brings up sticky issues. In a similar way, I'm sure if GR were formulated correctly, it could be made to match experiments and observations of the early twentieth century, but given a condition that prevents the runaway collapse of stars into black holes, and thus avoid the sticky issues of a singularity and inescapable event horizon. But nature seems to have little regard for avoiding difficult problems.

And true, inflation isn't a sealed deal, but it is still one of the best cosmological models I'm aware of - though honestly I'm not aware of very many (on that note, your later mention of the renewed interest in bounce cosmology is very interesting).

Scanning over all possible bubbles in the multiverse, everything that can physically happen does happen an infinite number of times.

While true, an easy way to get around this problem is to work with probabilities instead. In QM, technically just about anything is possible - but impractically improbable. True, working with infinities is a very sticky problem - but if the universe is endless then we're practically working with infinities already. A workable/heuristic solution might just be to only work within a certain system.

MattRob, the argument supporting multiverse notions is typically that they are "inevitable consequences" of some inflation scenario like "eternal inflation". But on what basis can one say that inflation is experimentally confirmed? Or observationally?
Another of the world's top cosmologists is Robert Brandenberger and he and his group at McGill have been working on a type of bounce cosmology that tries to obviate inflation. Get rid of the unphysical "singularity" at the start of expansion of the standard cosmic model and also achieve the early universe features which inflation was hypothesized to provide.
Yi-Fu Cai is a postdoc at McGill who has worked closely with Brandenberger and I would suggest checking out a recent paper of his.
Google [LambdaCDM bounce] to get:
http://arxiv.org/abs/1412.2914
A ΛCDM bounce scenario
Yi-Fu Cai, Edward Wilson-Ewing
(Submitted on 9 Dec 2014)
We study a contracting universe composed of cold dark matter and radiation, and with a positive cosmological constant. As is well known from standard cosmological perturbation theory, under the assumption of initial quantum vacuum fluctuations the Fourier modes of the comoving curvature perturbation that exit the (sound) Hubble radius in such a contracting universe at a time of matter-domination will be nearly scale-invariant. Furthermore, the modes that exit the (sound) Hubble radius when the effective equation of state is slightly negative due to the cosmological constant will have a slight red tilt, in agreement with observations. We assume that loop quantum cosmology captures the correct high-curvature dynamics of the space-time, and this ensures that the big-bang singularity is resolved and is replaced by a bounce. We calculate the evolution of the perturbations through the bounce and find that they remain nearly scale-invariant. We also show that the amplitude of the scalar perturbations in this cosmology depends on a combination of the sound speed of cold dark matter, the Hubble rate in the contracting branch at the time of equality of the energy densities of cold dark matter and radiation, and the curvature scale that the loop quantum cosmology bounce occurs at. Finally, for a small sound speed of cold dark matter, this scenario predicts a small tensor-to-scalar ratio.
14 pages, 8 figures

As I put in parenthesis earlier, this is very interesting. I'll be sure to read it soon before commenting on it, specifically, any more than this.

I will concede that the point you've made for inflationary cosmology, as a whole, is very persuasive. But, my point really wasn't about inflationary cosmology's implications on a multiverse scenario, so much as it was some commentary on the issue of multiverses more generally.

You certainly made a compelling point on multiverses with different physical laws being inherently unfalsifiable, but I would add a detail to this point more generally, rather than applying strictly to current inflationary cosmological models: I'm not entirely sure that it's fair to call them completely untestable. If they can, given some conditions, such as those we observe in our universe, make testable predictions, then they're testable. If tests yield a negative result - but a free parameter can be modified to make it match data - then it is, indeed, unfalsifiable, but not necessarily untestable/unconfirmable.

On a very general, note, though, it is rather worrisome that as a general trend theories become harder and harder to test. On a somewhat related note, in the early twentieth century it was embarrassing to posit the existence of neutrinos, iirc, because they had not yet been observed and could not be with the technology of the time. Now it seems rather commonplace to suggest fields and particles exist that have not yet been observed.

Perhaps I should note, though, that I am just an undergraduate, so most of my understanding of these things is largely informal, so apologies if it's too lacking.

 

[Haelfix]

You certainly made a compelling point on multiverses with different physical laws being inherently unfalsifiable

This is not quite true, as I shall explain. Once you accept the type I multiverse (in Tegmarks classification) through a certain chain of reasoning, the jump to the type II multiverse requires just one additional assumption: Namely that there are fundamental scalars in the universe that have metastable ground states. Well, we now know of at least one candidate that qualifies (the Higgs particle) and many models of beyond the standard model physics spit out new particles that have exactly these properties. From that point on, well understood and conservative physics describes the decay of the false vacuum from spontaneous symmetry breaking, and the emergence of new bubbles with different physics.

How does one test such a thing? Well you go out, and you 'create' the particle(s) that cause the problem. Of course one issue is that we don't know at what energy scales these guys live in, but for instance, when we measure the Higgs particle, we actually can make enough measurements to pin down whether the vacuum stays radiatively stable or not. Interestingly enough, detailed measurement of the mass of the Higgs as well as the mass of the Top Quark (and a few other precision observables) have tentatively set the electroweak vacuum to be metastable, at about the age of our universe). *If* this holds up (and there are additional assumptions about btsm physics in there), it will mean that with half life on the order of the age of the universe, a Higgs particle somewhere in our universe will eventually decay, with explosive particle production and all sorts of dramatic effects (for us) that will create bubble nucleation and the end of the universe as a consequence. This is one reason why the case for the multiverse is somewhat enhanced by the discovery of the detailed properties of the Higgs.

So anyway my point is that it is falsifiable, in principle, just of course nearly impossible to do in practise unless you get lucky (which perhaps we already have with the Higgs).

 

[Garth]

So the test is: that if the universe dissolves in a sea of fire then "there are fundamental scalars in the universe that have metastable ground states" and therefore the multiverse exists.

As this process of transitioning to a lower energy state, this “vacuum decay,” would create a bubble of a new vacuum state that would expand outward at the speed of light then we wouldn’t have any warning until we were obliterated.

The hypothesis "the multiverse exists" would have been proven true but only by the destruction of the whole universe and when there was no-one left to appreciate the fact.

Some test!

Actually gargantuan energy levels are probably necessary for this transition to occur and we are (probably?!?) safe yet.

But it illustrates the problem of testability. We are building theories about energy levels far beyond testable limits (particle colliders bigger than the whole Earth etc.) and the fact that we can construct a theory that is mathematically consistent is no guarantee that that is or was the way the universe actually works/ed.

Garth

 

[jz92wjaz]

I have always liked the idea of a Multiverse because you don't have to explain why there is one, and just one universe, as opposed to an infinite number that continue to naturally occur. I personally think there are many universes. Even so, as cool of an idea as it is, I agree that it's not science until we can adequately test it, and that we should strive to solve problems by looking extensively within our own Universe, as if the things outside of it (whether or not they exist) don't impact us.

 

[Haelfix]

So the test is: that if the universe dissolves in a sea of fire
Garth

Well that's certainly one way lol, but its more the creation of that particle in the lab and the study of its properties. Then one would know provided we had a big enough detector.

It's the same thing with the inflaton (another presumed scalar). If you actually could create one in your solar system sized particle accelerator, you could measure the properties of the potential, and pin down what type of class of inflationary physics it corresponds too. Just like that, you would know whether there was a type I multiverse (b/c you would know whether it was in the class of eternal or chaotic inflation).

 

[Garth]

Well that's certainly one way lol, but its more the creation of that particle in the lab and the study of its properties. Then one would know provided we had a big enough detector.

It's the same thing with the inflaton (another presumed scalar). If you actually could create one in your solar system sized particle accelerator, you could measure the properties of the potential, and pin down what type of class of inflationary physics it corresponds too. Just like that, you would know whether there was a type I multiverse (b/c you would know whether it was in the class of eternal or chaotic inflation).

Yes, the argument is not about whether other universes exist or not - I can imagine as many as you like and 'somewhere over the rainbow' they might all exist. The argument is about the nature of science and how science progresses.

Going back to my #28 argument - I wasn't trying to be rude in comparing the multiverse hypothesis with Intelligent Design but to emphasise something Paul Steinhardt was saying ... an unfalsifiable Theory of Anything creates unfair competition for real scientific theories.

One reason the Intelligent Design argument is wrong is because once you take that route there is no motivation to find the real reason to explain a natural phenomena.

Equally the reason the Multiverse argument is wrong is because in Inflation we have a theory in which, amongst the infinite number of patches/universes, in the words of Alan Guth, "anything that can happen will happen—and it will happen infinitely many times". So not only is there no predictive power but also, satisfied that we have explained 'everything' by the multiverse, we have no motivation to find the real reasons for the natural phenomena that lay within fundamental physics and the origin of this universe..

Let me illustrate again with that old chestnut "Spontaneous symmetry breaking".
The word "spontaneous" implicitly assumes a stochastic process, it's random, and the reason a physical law/constant takes a particular form is that there is an ensemble of universes across which the process takes place so that particular value can occur in at least one - our own. "Spontaneous" is an a priori assumption that generates a hypothetical multiverse.

It doesn't have to be so - in a stochastic process you can get a particular value just by luck, a 'fluke'; unfortunately science cannot do much with that - but then the universe is also in an observable sample of one and I do know physicists who stop there, "It's a fluke, get over it and move onto something more worthwhile."

However the inquisitive nature of the human species is strong and some of us want to find out deep reasons why the universe is as it is. And in this case it involves treating the symmetry breaking process as being explicitly caused by some extra terms in the Lagrangian of the system. Then the furtherance of the scientific process involves working out what those terms were and how they might be tested for.

Garth

 

[Haelfix]

So no only is there no predictive power but also, satisfied that we have explained 'everything' by the multiverse, we have no motivation to find the real reasons for the natural phenomena that lay within fundamental physics and the origin of this universe..

However the inquisitive nature of the human species is strong and some of us want to find out deep reasons why the universe is as it is. And in this case it involves treating the symmetry breaking process as being explicitly caused by some extra terms in the Lagrangian of the system. Then the furtherance of the scientific process involves working out what those terms were and how they might be tested for.
Garth

I was sort of hoping to emphasize that contrary to your claims that there is predictive power by theories that involve the multiverse as a byproduct. That they frequently do make very specific physical predictions that *are* observable and that already have been observed and that could have been falsified many times in the past.

As for the second paragraph. Sorry, I don't follow the argument or the claim. Are you really saying that you don't approve of spontaneous symmetry breaking as a physical process, and instead need to look for explicit symmetry breaking?

 

[marcus]

Anyway to recap what I pointed to in posts #40, 41, 42 and elsewhere, what we are seeing is some world-class experts in cosmology (George Ellis, Joe Silk, Slava Mukhanov, Paul Steinhardt, Robert Brandenberger,...) explain why there is no compelling scientific case for "multiverse".
1. "multiverse" thinking rests mainly on the mistaken idea that INFLATION necessarily must run hog-wild and generate a huge mulitiplicity of separate expanding regions.
But Mukhanov says no: you can have inflation without "selfreproduction" (posts #40 and 41)
2. Furthermore the inflationary paradigm may itself be too FLEXIBLE to be properly tested, says Steinhardt. It seems able to adapt to any set of observations that comes along, suggesting that its status as science is shaky and that it may be time to develop and study more predictive alternatives that achieve the same good results. (Steinhardt's Nature article "Big Bang Blunder Bursts Multiverse Bubble", post #41)
3. Finally, Robert Brandenberger and others including a close collaborator Yi-Fu Cai, have been developing bounce cosmologies which do not need inflation in order to achieve the desired results of near flatness, sameness in all directions, and evenness of fluctuation spectrum. There is much recent work along these lines so I will simply take as an example a paper by Cai&Wilson-Ewing which was titled "LambdaCDM Bounce". Essentially it takes standard cosmic model, puts it through collapse, rebound, expansion, and checks that the results are compatible with current observations. (post #42).