You think there's a multiverse? Get real

[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).

 

[marcus]

To be clear, I wouldn't say these people are arguing AGAINST the multiverse idea. If you like the idea you can believe in it, if you want. It is just not a scientific idea.
They are explaining why there is no compelling scientific case for it. And there are alternatives which appear compatible with observation and which may not be so "flexible". :)

 

[MattRob]

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

If I recall correctly, the amount of energy required for this to occur exceeds the amount of energy density that would form an event horizon in our cosmos' current state, so perhaps we're permanently safe from this because even if it did happen as a statistical fluke of QM's randomness, then it'd contain itself in a (at least microscopic) black hole which would quickly evaporate away? Heh, that's an almost Hitchhiker's Guide kinda idea. Universe-destroying phenomenon occur in intergalactic space on a regular basis. Fortunately, they're quickly contained in black holes every time and quickly vanish by Hawking radiation without a trace.

Anyway to recap what I pointed to in posts #41, 42, 43 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" (post #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 up, suggesting that its status as science is shaky and it may be time to develop and study alternatives that achieve the same good results. (Steinhardt's Nature article "Big Bang Blunder Bursts Multiverse Bubble", post #42)
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 #43).

Once again, I don't see why "do not need" should be a strong argument for something. It seems somewhat arbitrary to add a constraint that universes cannot reproduce. Also, once again, yes, anything that can happen will happen an infinite number of times, but that's also technically true in an infinite spatial expanse within our universe; yet that does little to muddle physics. Also, given the sheer size of the visible universe as contrasted to fundamental particles, extremely unlikely things can happen with QM quite easily; but once again this is non-issue because we still respect events' respective likelyhoods, rather than their possibility.

[...]
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 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.. [...]

Because of the probabilistic rather than deterministic nature of QM, the same thing could actually be said of it. Could I quantum tunnel through the floor at any given moment? Technically, yes, but the odds are so slim it's truly non-issue. Likewise, yes, there is a universe where everything just happens by insane chance, but that's really non-issue because it's so unlikely.

Furthermore, if our universe is infinite, then out there there's an infinite number of identical copies of me falling through the floor due to quantum tunneling; but I'm really not bothered because I know there's more(?) who aren't, and by probabilities, I'm really much safer from that threat than, say, poor dietary habits.

All that being said, I do agree on the principle that scientific theories need to be testable to be science, with a cautionary note that sometimes non-testable, or even simply wrong heuristic models may sometimes still be helpful in making other discoveries (such as Galilean approximations, or interpretations of QM). And furthermore, sometimes theoretical advances simply outpace observational capabilities. The schwarzchild solution existed long before we ever had the tools to discover AGN, probe their natures, or discover Sagittarius A. While it's a very rare occurance for theory to outpace observation with success, it is, nonetheless, possible, and perhaps even necessary given the increasing difficulty of yielding experimental results.

On that note,

[...]

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

I remain somewhat hopeful that they'll find new and clever ways to experiment, much akin to interferometry making radio studies of the cosmos possible. Simply put, you don't know what you don't know, so hopefully some ingenious solutions will come along to get the effect of colliders bigger than Earth without needing actual colliders that large.

 

[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.

In the past thirty years or so I have worked through countless versions of Inflation theory, each making different predictions. In the words of Paul Steinhardt

Over the entire multiverse, there are infinitely many distinct patches. 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......

Inflation, once started, runs eternally and produces a multiverse of pockets whose properties vary over every conceivable possibility—flat and non-flat; smooth and non-smooth; scale-invariant and not scale-invariant; etc. Despite laudable efforts by many theorists to save the theory, there is no solid reason known today why inflation should cause our observable universe to be in a pocket with the smoothness and other very simple properties we observe. A continuum of other conditions is equally possible. In string theory, a similar explosion of possibilities has occurred, driven by attempts to explain the 1998 discovery of the accelerated expansion of the universe. The acceleration is thought to be due to positive vacuum energy, an energy associated with empty space. Instead of predicting a unique possibility for the vacuum state of the universe and particles and fields that inhabit it, our current understanding of string theory is that there is a complex landscape of vacuum states corresponding to exponentially different kinds of particles and different physical laws. The set of vacuum space contains so many possibilities that, surely, it is claimed, one will include the right amount of vacuum energy and the right kinds of particles and fields. Mix the inflation and string theory, and the unpredictability multiplies. Now every combination of macrophysical and microphysical possibilities can occur.

The only prediction that Inflation has made is that the universe should be nearly flat. It seems to be extraordinarily flat, and I expect there is some version of the theory out there that can accommodate this. Other theories also produce a flat universe, so the prediction is not unique to inflation.

As PS said all the other 'predictions' were really cases of model fitting - with a continuum of possible models to fit from.

And now the prediction of a gravitational wave signature in the CMB is in the balance. The prediction was of a B-mode polarisation signal of strength 'X', but ~2'X' was observed. Planck has shown that in that area of sky the B-mode polarisation signal from dust is ~2'X'+/-error bars.

There is still a possibility that a primordial gravitational wave signal might be hidden in the error bars and it is that which is being investigated now; it will take two years to get the experiment together.

Whatever that balloon experiment finds it will mean that if there is a primordial gravitational wave signal hidden in there it will be far smaller than the one predicted, but hey! we can find another version of inflation that will 'predict' that.

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?

I was saying three things:

1. The presumption that the process was 'spontaneous', that it was a stochastic process, implies a large/infinite ensemble of possible universes - the multiverse, which has been fed into the theory as an a priori assumption.
2. We don't know if the symmetry breaking was spontaneous - there might be other physical factors in some yet to be found TOE that make the process one of Explicit symmetry breaking.
3. We will never find that TOE if we have satisfied ourselves that we have already found the answer in an untestable and infinite possibility multiverse 'theory of anything'.

So yes, in this cosmological case I don't approve of spontaneous symmetry breaking as a physical process because it is unscientific and it short circuits further scientific investigation. Again as Paul Steinhardt said, "Because an unfalsifiable Theory of Anything creates unfair competition for real scientific theories".

Thank you Marcus for the link to PS's Nature article, which also emphasises what I say about the BICEP2 results.

I repeat Marcus' quote from PS with just one line: The answer given by proponents is alarming: the inflationary paradigm is so flexible that it is immune to experimental and observational tests.

Garth

 

[mitchell porter]

My 2 cents:

We should distinguish between:

a theory which makes plenty of testable predictions, and also has the untestable implication that there is a multiverse

a theory which posits a multiverse and obtains a testable implication from thisA theory in the first category might be... standard model plus chaotic inflation. The evidence for chaotic inflation would come from standard cosmological observations, but along with implications for CMB etc, the theory would imply the existence of other inflating regions that are eternally out of reach to us. (That is the weakest sort of "multiverse" because they are in our space-time, but inaccessible.)

A theory in the second category might be... some "theory" (guesswork) about the string-theory landscape, plus eternal inflation. In the previous example, the physics in every inflationary region is standard model, standard model, standard model. In this case, there is a guess that e.g. the extra dimensions curl up differently in different inflationary regions, so there will be a different effective physics; and then one would contrive to predict things about beyond-standard-model physics (and retrodict things about standard-model physics) on the basis of e.g. reasoning about what kind of effective physics is most common in the landscape.

 

[Haelfix]

A few comments about the word games being played here, and Steinhart's 'argument'.

It is important here to distinguish between an idea and a model (I choose to not use the word 'theory' here, as it is being overloaded). The inflationary paradigm is an idea, a specific instantiation of that idea is a model, like say m^2 phi^2. Models are relatively easy to falsify as they always make very specific predictions, ideas on the other hand, are very difficult to kill off and sometimes impossible.

Another example of this is the idea that there exists particles beyond those catalogued in the standard model of particle physics. A specific instantiation of that idea, is say a new Dirac Fermion with a certain mass and set of couplings.

Before we go claiming that we should only deal in 'scientific' ideas, consider that the latter example is completely unfalsifiable, even in principle, yet it is the very thing that every working physicist spends most of their life contemplating.

So... One of the many problems with Steinhart's arguments, is that he claims that the inflationary paradigm is unfalsifiable, and then gives as an alternative, a specific model. You see, it's completely disengenous, b/c he knows perfectly well that his more general idea, the 'ekyprotic universe' is very much as hard to falsify as inflation is. This holds for all of the alternative ideas, like the bounce paradigm, and so forth. Further all of these 'alternative ideas' have only made retrodictions, inflation was the first to actually make a nontrivial prediction that stood the test of experiment.

Now, the notion that inflation predicts anything you want with a multiverse is just not correct. Here's what Steinhart is talking about. It turns out that a very specific set of inflationary models that happen to inflate eternally, will, if you tweak things in a very specific way (eg by making a manifestly *wrong* choice of measure amongst other things) will break many of the assumptions that keep the mathematics under control (slow roll,a proper phase of reheating, etc). This then leads to regions of space that have bizarre physics with high probability.

So instead of simply throwing those solutions out, he chooses to keep them and then use them as a representative of the whole idea.

An analogy is worth pointing out. It would be as if someone told you that the idea of General Relativity is not scientific, b/c a certain solution (say the Godel Universe) violates cause and effect, and can give you anything you want. Therefore GR can't make predictions! It's just not a reasonable claim.

Anyway, the right way to proceed is to keep working on all of these ideas, gathering data, and then seeing which models survive. It is perfectly plausible that a model like m^2 phi^2 turns out to be the only one left standing at some point in the future (with more refinement of the r-ns plane), at which point people should be prepared to accept that there will be more 'believers' in notions like the multiverse and that this is, as it should be. People will have simply updated their Bayesian priors, exactly as science is always done.

 

[Garth]

It is perfectly plausible that a model like m^2 phi^2 turns out to be the only one left standing at some point in the future (with more refinement of the r-ns plane), at which point people should be prepared to accept that there will be more 'believers' in notions like the multiverse and that this is, as it should be.

Now I wonder why you say that? It sounds like you are already a 'believer' in the inflation paradigm. But science isn't about believing, it's about observing, testing and verifying.

Garth

 

[nikkkom]

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)

I wouldn't call this model "Multiverse", as even the decay to stable vacuum and a giant fireball are occurring in the same space-time. What is "multiversy" about that?

I think that we should decide whether theories are testable or not individually, on a per-theory basis. Not just slap "all Multiverse theories are non-testable" sticker over all of them. Even some theories with multiple disjoint space-times predict "leakage" between them.

 

[Garth]

I think that we should decide whether theories are testable or not individually, on a per-theory basis. Not just slap "all Multiverse theories are non-testable" sticker over all of them. Even some theories with multiple disjoint space-times predict "leakage" between them.

If this "leakage" can be and is detected, or if we find a way of 'seeing' through the 'singularity' at the heart of a black hole or through the 'singularity' at the heart of the Big Bang and see another universe beyond then the situation changes completely. The multiverse becomes part of observed science.

Until then we cannot count as prediction a result which is one such amongst many thrown up by a spectrum of alternative models. A detection of such a result is simply a matter of model selection.

If we have a huge number of models to select from then the detection of one particular value, predicted by one particular model, may be almost inevitable, ("the inflationary paradigm is so flexible that it is immune to experimental and observational tests" )(my #53); it says nothing about the verisimilitude of the cognitive framework that produced those models.

Garth

 

[Dotini]

I stumbled across a paper that might be relevant to your discussion. Hard for me to say if its any good or not.
http://arxiv.org/abs/1501.01919
https://medium.com/the-physics-arxi...n-to-tear-modern-cosmology-apart-d334a7fcfdb6

 

[Haelfix]

Now I wonder why you say that? It sounds like you are already a 'believer' in the inflation paradigm. But science isn't about believing, it's about observing, testing and verifying.

I completely agree. For instance, if you had asked me this question 6 months ago, I would have said that based on the polarization data based on BICEP, that a model like m^2 phi ^2 was looking pretty good, amongst all the competitors (for both theoretical and experimental reasons) and that the multiverse hypothesis was looking perfectly plausible. Based on the data that was received a week ago from the Planck teams, the data has changed and hence my bayesian prior has updated as well. m^2 phi ^2 is now slightly disfavored and I've updated my belief back to small field 'quantum gravity' models that produce inflatons with much smaller values of r. Some of these have multiverses, and some of them its unclear. Consequently my 'belief' in the multiverse hypothesis is a little lower.

This is always slightly personalized, b/c some people know a little more about one theory than others, and so have different priors, but this is roughly the process every physicist undertakes at some time or another.

My point was that this process is completely insensitive to philosophy. I wasn't going to penalize m^2 phi ^2 bc it had a type i multiverse as a byproduct, regardless about whether it fit into someone's idiosyncratic definition of 'science'.