Does multiverse really explain fine-tuning?

[abc5]

Hello everyone,

I am not sure about this issue and this is the reason why I am asking this question. I can write many sentences on this topic, but I will try to make my point as short is possible.

I will suppose you are familiar with multiverse idea and anthropic principle..

My question is: Let's suppose there are billions and billions of other universes, does that solve problem of fine-tuning? Wouldn't there still have to be some fundamental laws which creates universes in multiverse- in other words, wouldn't there have to be "multiverse generator" which would have to be fine-tuned "for creating other universes with different constants"... Am I missing something, or with multiverse theory problem is not solved, just pushed one step further?

 

[mfb]

Wouldn't there still have to be some fundamental laws which creates universes in multiverse- in other words, wouldn't there have to be "multiverse generator" which would have to be fine-tuned "for creating other universes with different constants"

There is no fine-tuning involved in this part, because the creation of universes can be very general. You don't need to be a good darts player to hit random spots on the wall with each attempt, but you have to be good to hit the bullseye with one attempt.

 

[atyy]

One of the technical issues here is called the "measure problem".
https://www.quantamagazine.org/20141103-in-a-multiverse-what-are-the-odds/

 

[abc5]

But you still need a mechanism which would throw the arrows... If I get this right, the mechanism is eternal inflation, which will produce the universes forever-infinite number of universes..

But what if there is no infinite number of universes? What if there is no such thing as eternal inflation, and at one point, inflation have to stop... Let's suppose there is finite number of universes, and if there is no enough universes to cover "fine-tuning of our universe" then the multiverse theory, would not solve the problem of fine-tuning.. Is that correct?
I am not challenging anything or anybody, as you can see, english is not my native language( I am sorry for my grammar mistakes) and I am not physicist... But there are some ideas, which would make eternal inflation impossible, so because of that I am asking the question...
https://www.newscientist.com/article/mg22229692-600-quantum-twist-could-kill-off-the-multiverse/

 

[William White]

The Anthropic Principle seems neater.fine-tuning is a semi-religious statement. We are a product of the universe, so we should not be suprised that the universe looks the way it does.

 

[atyy]

fine-tuning is a semi-religious statement. We are a product of the universe, so we should not be suprised that the universe looks the way it does.

But it is a good religion. Its philosophy is that all our theories are wrong, and not the final theory. If we knew the true final theory, then we wouldn't be surprised if it is fine tuned, since the true ultimate reality is simply what it is.

One technical way of stating this religion is via Wilsonian renormalization.

 

[Orodruin]

A number of posts have been deleted, please keep to the original topic given by the OP about fine tuning and the multiverse.

 

[mfb]

But you still need a mechanism which would throw the arrows

Right, but this mechanism does not have to be fine-tuned. It just has to exist.

 

[Chalnoth]

Hello everyone,

I am not sure about this issue and this is the reason why I am asking this question. I can write many sentences on this topic, but I will try to make my point as short is possible.

I will suppose you are familiar with multiverse idea and anthropic principle..

My question is: Let's suppose there are billions and billions of other universes, does that solve problem of fine-tuning? Wouldn't there still have to be some fundamental laws which creates universes in multiverse- in other words, wouldn't there have to be "multiverse generator" which would have to be fine-tuned "for creating other universes with different constants"... Am I missing something, or with multiverse theory problem is not solved, just pushed one step further?

Yes. Multiverse theories typically have a mechanism for the production of a large number of regions like the one we live in. Eternal inflation is one such mechanism that has been proposed.

Also, multiverse theories can only explain fine tuning if that fine tuning is necessary for any observers to exist. That's often not very easy to demonstrate.

 

[mfb]

Also, multiverse theories can only explain fine tuning if that fine tuning is necessary for any observers to exist. That's often not very easy to demonstrate.

If there is no fine-tuning necessary there is no need to explain fine-tuning.

 

[Hornbein]

Hello everyone,

I am not sure about this issue and this is the reason why I am asking this question. I can write many sentences on this topic, but I will try to make my point as short is possible.

I will suppose you are familiar with multiverse idea and anthropic principle..

My question is: Let's suppose there are billions and billions of other universes, does that solve problem of fine-tuning? Wouldn't there still have to be some fundamental laws which creates universes in multiverse- in other words, wouldn't there have to be "multiverse generator" which would have to be fine-tuned "for creating other universes with different constants"... Am I missing something, or with multiverse theory problem is not solved, just pushed one step further?

You are right, the multiverse just pushes the problem one step further. That's what scientists do. It keeps them busy and off of drugs.

More specifically, it obviates fine tuning of physical constants, but it doesn't explain how the whole situation came to be in the first place. But who knows? You've got to take these things one step at a time.

 

[palmer eldtrich]

The multiverse generator might be eternal inflation or the many worlds of Qm, or maybe a cyclic universe with different constants in each cycle. Or maybe one universe with varying constants in it. i don't see why these have to be fine tuned. They may or may not be true.

 

[Chronos]

The multiverse does one thing very well, it explains observations that appear random or improbable, like the values of physical constants. Under most multiverse theories, all possible values for physical constants, and combinations thereof, are realized somewhere in the multiverse. We just happen to reside in one that has the particular values we measure. This yields a satisfyingly high probability [100%] a universe like ours exists - reducing our reality to an anomalous statistic and disincentivizes scientific inquiry .

 

[Drakkith]

The multiverse does one thing very well, it explains observations that appear random or improbable, like the values of physical constants. Under most multiverse theories, all possible values for physical constants, and combinations thereof, are realized somewhere in the multiverse. We just happen to reside in one that has the particular values we measure. This yields a satisfyingly high probability [100%] a universe like ours exists - reducing our reality to an anomalous statistic and disincentivizes scientific inquiry .

Sounds pretty boring.

 

[Garth]

The multiverse does one thing very well, it explains observations that appear random or improbable, like the values of physical constants. Under most multiverse theories, all possible values for physical constants, and combinations thereof, are realized somewhere in the multiverse. We just happen to reside in one that has the particular values we measure. This yields a satisfyingly high probability [100%] a universe like ours exists - reducing our reality to an anomalous statistic and disincentivizes scientific inquiry .

Sounds pretty boring.

I always say that a theory that can explain anything and everything explains nothing - boring indeed.

Garth

 

[stevendaryl]

I always say that a theory that can explain anything and everything explains nothing - boring indeed.

Garth

On the other hand, it seems that we should be open to the possibility that some facts may have no non-boring explanation.

 

[Chalnoth]

If there is no fine-tuning necessary there is no need to explain fine-tuning.

It is certainly possible for apparent fine tuning to be explained by as-yet-unknown physical laws creating relationships between different parameters. But there's also no reason to believe such explanations exist if the apparent fine tuning is necessary for any observers to exist.

We may eventually be forced to conclude that there is no explanation for such apparent fine tuning except that the fundamental laws are prolific and certain pieces of apparent fine tuning are necessary for observers.

 

[Haelfix]

On the other hand, it seems that we should be open to the possibility that some facts may have no non-boring explanation.

Indeed the question "why are planetary orbits fine tuned for life" is answered in just such a manner. There is no deeper mechanism at play for selecting Earth like planets in the Goldilocks zone. As best we can tell, it's purely a matter of having large ensembles and the anthropic principle.

 

[Chronos]

The mulitverse 'expains' everything. It eliminates the need for scientiific inquiry. It is little better than a rartional version of theology, You merely replace your deity of choice with probabilities.

 

[Chalnoth]

The mulitverse 'expains' everything. It eliminates the need for scientiific inquiry. It is little better than a rartional version of theology, You merely replace your deity of choice with probability.

This is completely false and you should be aware of that fact. Many specific multiverse models have testable predictions. Some are incomplete or poorly constructed and make no predictions, but that shouldn't poison the entire paradigm.

It's really sad that so many dismiss multiverse models out of hand simply because they make them uncomfortable.

 

[Garth]

This is completely false and you should be aware of that fact. Many specific multiverse models have testable predictions. Some are incomplete or poorly constructed and make no predictions, but that shouldn't poison the entire paradigm.

Such as the CMB Cold Spot or The 'Dark' Flow? And yes the evidence has not stood up in the Planck data and so those models have proved to be falsifiable.

However, on the other hand such data could have been just a cosmological non-gaussian distribution of matter.

If hard evidence of other universes should be discovered, such as (being highly speculative) a future gravity wave telescope seeing 'through' the 'singularity' at the heart of a black hole - or the big bang - and image other universes beyond, then the multiverse will enter the realm of hard observed science. Until such time of incontrovertible evidence it remains in the realm of interesting speculation.

t's really sad that so many dismiss multiverse models out of hand simply because they make them uncomfortable.

I'm not uncomfortable with the hypothesis of the multiverse - if God/Nature (using 'God' in the "God does not play dice" sense) made one universe He/She/It can make as many as they want.

I just think it makes bad science.

Steinhardt: My concern was that the multiverse is a ‘theory of anything’, a proposal that allows all possible cosmological outcomes (smooth or not smooth, curved or flat, etc.) and, consequently, is not subject to empirical tests. Some claim that superstring theory allows exponentially many (or perhaps infinitely many) possibilities for the fundamental laws (masses of particles, types of forces, etc.) and that there is no guiding principle to determine which set of physical laws is more probable. The sets of laws comprise what is called the “string landscape.”

Combine the inflationary multiverse with the string landscape, and now one has a ‘supertheory of anything’: both the cosmological properties and the microphysical properties of the universe are accidental and unpredictable.

As we understand superstring theory better, I truly hope we find that there are sound reasons why the physical laws we observe are naturally selected. Superstring theory, combined with an improved cosmological picture, may then lead to a powerfully explanatory and predictive theory.

(Emphasis mine)

Garth

 

[Chalnoth]

I'm not uncomfortable with the hypothesis of the multiverse - if God/Nature (using 'God' in the "God does not play dice sense") made one universe He/She/It can make as many as they want.

I just think it makes bad science. (Emphasis mine)

Just because it's tricky to get right doesn't mean it's bad science. It just means it's tricky to get right.

Especially when you consider that the standard model unambiguously predicts a multiverse (specifically with regards to the Cabibo angle varying from region to region, which impacts the behavior of the weak nuclear force), and so far GUT's predict an even more diverse multiverse.

 

[Garth]

Especially when you consider that the standard model unambiguously predicts a multiverse (specifically with regards to the Cabibo angle varying from region to region, which impacts the behavior of the weak nuclear force), and so far GUT's predict an even more diverse multiverse.

In every theory where the parameters, such as the Cabibo angle, have no generally accepted theory that explains why the measured values are what they are, the standard response is to hypothesise that they are determined at some early stage by a stochastic spontaneous symmetry breaking.

By using a stochastic process as a hypothesis the approach has to assume that there is an ensemble of other universes, where in other universes the symmetry breaking process results in different values of the parameter in question.

Such an assumption, that the parameters are simply the outcome of random chance, short circuits any investigation into any possible physical cause, an explicit symmetry breaking, that has determined those particular values and thus tends to dissuade further research for such causes.

The multiverse is therefore an a priori assumption of such an approach, not an unambiguous prediction, and one that may actually prevent further discovery.

Garth

 

[Chalnoth]

In every theory where the parameters, such as the Cabibo angle, have no generally accepted theory that explains why the measured values are what they are, the standard response is to hypothesise that they are determined at some early stage by a stochastic spontaneous symmetry breaking.

By using a stochastic process as a hypothesis the approach has to assume that there is an ensemble of other universes, where in other universes the symmetry breaking process results in different values of the parameter in question.

Such an assumption, that the parameters are simply the outcome of random chance, short circuits any investigation into any possible physical cause, an explicit symmetry breaking, that has determined those particular values and thus tends to dissuade further research for such causes.

The multiverse is therefore an a priori assumption of such an approach, not an unambiguous prediction, and one that may actually prevent further discovery.

Nope. The multiverse is the absence of an assumption. Explicit symmetry breaking requires adding additional assumptions to the theory. With spontaneous symmetry breaking, one need only postulate a higher-order symmetry, such as SO(10). You then tend to get a number of local minima within its interaction potential, some of which have very long lifetimes.

Explicit symmetry breaking requires that you have SO(10) plus an additional rule that determines how the symmetry is broken.

Also, Garth, it is people like yourself that are attempting to dissuade further research. Even if some particular symmetries are broken spontaneously, then there remains the possibility that some other symmetries are broken explicitly. It makes precisely zero sense to discourage any investigation into spontaneous symmetry breaking because you're uncomfortable with the multiverse.

 

[Garth]

Nope. The multiverse is the absence of an assumption.

That statement itself is quite an assumption!

It makes precisely zero sense to discourage any investigation into spontaneous symmetry breaking because you're uncomfortable with the multiverse.

As I said, I am very comfortable with the hypothesis of a multiverse - but in order for it to be other than just a hypothesis I waiting for you to show me one of these other universes.

Garth

 

[palmer eldtrich]

That statement itself is quite an assumption!As I said, I am very comfortable with the hypothesis of a multiverse - but in order for it to be other than just a hypothesis I waiting for you to show me one of these other universes.

Garth

I can't show the inside of a black hole either or Hawking radiation, so do you assume there is no inside of black hole and no such things as Hawking radiation?

 

[Garth]

I can't show the inside of a black hole either or Hawking radiation, so do you assume there is no inside of black hole and no such things as Hawking radiation?

Of course not.

There is plenty of evidence that BH's themselves exist (Cygnus X-1, M87 etc.) with their appropriate event horizons and therefore it is a reasonable assumption that those horizons have interiors.

As to what happens in those interiors we are reliant on the theoretical extrapolation of the well tested theory of GR. But in doing so we are extending the theory well beyond the lower curvature regimes in which the theory has been tested and so have to be wary. A quantum-gravity theory modification of GR might predict a different interior.

It will be interesting to see what such a Q-G theory will say about Hawking radiation, which to date has not been observed.

Remember

“I would rather have questions that can't be answered than answers that can't be questioned.”― Richard Feynman

[PLAIN]http://www.goodreads.com/author/show/1429989.Richard_Feynman[/PLAIN]

Garth

 

[palmer eldtrich]

The only observational evidence you have for black holes is their external influence on their surroudings. You have no evidence they have an interior. You agree that Hawking radiation has not been observed. So why shouldn't we say that the idea that black holes have interiors or emit Hawking radiation is Sci Fi fantasy or theology or whatever other derogatory term you want to throw.
Clearly there is a difference between believing in black hole interiors and Hawking radiation and theology. The former is well motivated by physics we understand, the latter isn't. in other words we can have some trust in the predictions of a theory beyond what we can observe if we can confirm its predictions for things we can observe. That trust won't be as high as the things we observe but it would not be zero either.
In the case of the multiverse , it is not a hypothesis but a prediction of inflation; sort of .
The evidence for inflation is not zero but its not as good as black holes. The theoretical motivation is that according to Guth and Steinhardt, eternal inflation is a generic property of inflation. Its possible inflation didnt happen, also that we have misunderstood inflation and its not really eternal. But at the moment i would say it looks like from the current evidence and theorertical understanding we have that inflation did happen and it is eternal.
I think the problem is that people think science should give us clear answers , but why should it? Maybe the universe isn't that kind. I think the reality is that confidence in statements should a spectrum, not a black and white answer. Sometimes our confidence enables us to make statements with no reasonable doubt , other times there are plenty of doubts but that's not the same as believing in theology or fantasy. Other times belief is just fantasy. i think the multiverse is somewhere along this spectrum. it is not a confirmed fact but to call it fantasy or theology is just silly .

 

[Garth]

Other times belief is just fantasy. i think the multiverse is somewehre along this spectrum. it is not a confirmed fact but to call it fantasy or theology is just silly .

I agree - except that I never called it "fantasy or theology" - the word I used was 'hypothesis'.

But I do also agree with Steinhardt about Inflation and the multiverse it begets:

Since 1983, it has become clear that inflation is very flexible (parameters can be adjusted to give any result) and generically leads to a multiverse consisting of patches in which any outcome is possible. Imagine a scientific theory that was designed to explain and predict but ends up allowing literally any conceivable possibility without any rule about what is more likely. What good is it? It rules out nothing and can never be put to a real test.

Garth

 

[palmer eldtrich]

I understand you didnt call it fantasy or theology but many people have, including on this forum. I am glad we agree that our confidence in ideas ranges along a spectrum and that the multiverse is neither confirmed scientific fact nor unfounded fantasy. Steinhardt I think contradicts himself because he says you can discriminate between his ekpyrotic model and inflation by observing the gravitational wave spectrum. but if that's true then how does he justify that inflation can't be tested?
Maybe you can make models of inflation that didnt have Omega =1 and ns =.96 etc but I am not convinced they would be taken them so seriously. The fact is inflation is the best model we have right now for structure formation. Maybe a better model like Ekpyrosis will replace it one day, but so far that hasnt happened. As we understand inflaiton at the moment it looks like it generates a mulitverse. So i think we have good scientific reasons to believe in a multiverse. But clearly that belief should not be at the same level that we believe in black holes. The whole argument is based on a false notion that science should give us definitive answers to questions. I wish the world worked that way, but alas it doesnt.

 

[Garth]

Steinhardt I think contradicts himself because he says you can discriminate between his ekpyrotic model and inflation by observing the gravitational wave spectrum. but if that's true then how does he justify that inflation can't be tested?

The Ekpyrotic universe model predicts no primordial gravitational waves detectable by BICEP2, whereas Inflation in many guises did. If such g-w's had been found that would have falsified Steinhardt's model.

In fact they weren't discovered as we know, BICEP2 picked up a signal from dust and whether there is a weaker g-w signal hidden in there has yet to be determined.

So it might seem that the Ekpyrotic model has won over the Inflationary one, however Inflation is so flexible that one can simply constrain the parameters fed into the theory and produce a weaker or zero prediction of the g-w signal and thus save the theory. Actually we all do this with our theories - it's just that inflation is more flexible than most.

So i think we have good scientific reasons to believe in a multiverse. But clearly that belief should not be at the same level that we believe in black holes. The whole argument is based on a false notion that science should give us definitive answers to questions. I wish the world worked that way, but alas it doesnt.

Indeed, especially when we are scratching away at the boundaries of what is observable.

Garth

 

[Chalnoth]

That statement itself is quite an assumption!

Why do you think this?

If you take the standard model at face-value using the minimal assumptions required to explain the available evidence, then you have a model which unambiguously predicts a multiverse. You can, of course, add assumptions to get rid of the multiverse, but then you're just making unwarranted assumptions without evidence to support them. Maybe you can get rid of the multiverse (in this case, different low-energy physical laws in different locations), but I find that prospect highly unlikely.

Fine tuning that is required for any observers to exist is in itself powerful evidence of fine tuning. The picture here is that if we imagine the full possible configuration space of the universe as given by our current state of knowledge as a sheet of paper, and we then draw a dot to represent the part of that configuration space where life is possible. Those that want to get rid of the multiverse tend to argue that the rest of the paper isn't real: it's just an illusion due to our lack of knowledge, and the real space of possibilities aren't much larger than that small dot.

But the question that I ask is: why should that be the case? Why should the total space of possibilities tailor itself perfectly for life to exist?

I think most theorists expect that a significant fraction of that space of possibilities is impossible, and that knowledge of the correct GUT will demonstrate this. But I don't think there's any reasonable argument to be had that it won't remain much, much larger than the part of the space where life is possible. The cosmological constant is good example here: independent of our current observations, it is impossible for any structure formation if the magnitude of the cosmological constant were more than about $10^{-120}$ in natural units. For a long time, most theorists believed that there must, therefore, be some kind of symmetry that sets the cosmological constant to zero. But that has proven very difficult (for discussion, Sean Carroll described some of the difficulties in his book here).

Granted, the fact that it's hard to write down theories which predict a small or zero cosmological constant doesn't necessarily mean that it can't be true. But there's just no reason to prefer a unique universe as a default assumption. The simplest assumptions, given current knowledge, lead to some kind of multiverse.

 

[palmer eldtrich]

The Ekpyrotic universe model predicts no primordial gravitational waves detectable by BICEP2, whereas Inflation in many guises did. If such g-w's had been found that would have falsified Steinhardt's model.

In fact they weren't discovered as we know, BICEP2 picked up a signal from dust and whether there is a weaker g-w signal hidden in there has yet to be determined.

So it might seem that the Ekpyrotic model has won over the Inflationary one, however Inflation is so flexible that one can simply constrain the parameters fed into the theory and produce a weaker or zero prediction of the g-w signal and thus save the theory. Actually we all do this with our theories - it's just that inflation is more flexible than most.Indeed, especially when we are scratching away at the boundaries of what is observable.

Garth

That's like saying Mars rovers have shown theories that life is unique on Earth have won over theories that imply life in the universe is not unique to Earth. Technically true but hardly impressive. It's hard to find life outside of Earth even if its there. Its hard to find gravity waves even if they are there. I spoke to a number of people that worked on Planck and they said they had very little expectation of finding the b mode ( although they hoped they might get lucky ) so not finding it was not really a big shock and I don't think you should read too much into it.

 

[Chronos]

The multiverse hypothesis is filled with paradoxes, as deep and varied as the number of metastable vacuum states.

 

[Chronos]

Under a multiverse scenario, such as that proposed by Tegmark, you cannot forbid causal connections between different 'universes'. So there must surely be other universes that are observationally accessible to us. So, to quote Fermi, where are they?

 

[ShayanJ]

One of the technical issues here is called the "measure problem".
https://www.quantamagazine.org/20141103-in-a-multiverse-what-are-the-odds/

I don't understand this part:

Proponents of the multiverse idea must show that, among the rare universes that support life, ours is statistically typical.

Why? I don't think that's necessary. Our universe doesn't need to be typical. In an infinite number of universes with all possible values of the constants, its enough to have one that supports life and we can be in that one. Why does it need to be typical?

 

[ShayanJ]

And about accepting or rejecting the multiverse hypothesis. Its not that there can be two types of physicists:proponents of multiverse, opponents of multiverse. There are actually three: proponents of multiverse that accept it as a final answer, proponents of multiverse that this hypothesis makes sense to them but still don't stop searching, opponents of multiverse.
I think the serious objection(from Garth and others) is to the first group. And chalnoth's objection should be toward the third group. But I think most people are actually of the second group, which seems a very reasonable viewpoint to me. I mean, the multiverse hypothesis makes sense, but that doesn't mean we should stop searching. We continue our search till either there is good experimental evidence for multiverse or we find another solution.

 

[Garth]

Nope. The multiverse is the absence of an assumption.

That statement itself is quite an assumption!

Why do you think this?

Because we see just one universe and a sky full of a microwave background that we try to explain with incomplete physics.

When we have a quantum gravity theory and know what the Inflaton, the Dark Matter particle and Dark Energy are then we may know what we are talking about.

Until then we hypothesise and make untested (and perhaps untestable) assumptions.

Garth

 

[newjerseyrunner]

Under a multiverse scenario, such as that proposed by Tegmark, you cannot forbid causal connections between different 'universes'. So there must surely be other universes that are observationally accessible to us. So, to quote Fermi, where are they?

Perhaps in the microwave background radiation, perhaps gravity is weak because it seeps into other universes, maybe dark energy appears to be coming from no where because it's leaking from another universe. Until data contradicts a possibility, that possibility must be considered valid, no matter how unlikely.

Aren't there many different types of multiverse theory? I've heard theories where our universe being a small pocket where the laws of physics are what they are, but as you get further away the just sort of change, we just happen to be in a pocket where everything looks the same. I've also heard theories that our universe may exist within a black hole of a bigger universe and fine tuning is explained by evolution, natural selection favors universes that make a lot of black holes and evolve through recursion. Along lines of the recursion, I've heard ideas in my own field that natural selection prefers universes that produce intelligent life and life is somehow responsible for making more universes (simulation hypothesis.)

 

[Chalnoth]

That's like saying Mars rovers have shown theories that life is unique on Earth have won over theories that imply life in the universe is not unique to Earth. Technically true but hardly impressive. It's hard to find life outside of Earth even if its there. Its hard to find gravity waves even if they are there. I spoke to a number of people that worked on Planck and they said they had very little expectation of finding the b mode ( although they hoped they might get lucky ) so not finding it was not really a big shock and I don't think you should read too much into it.

Yup. Simulations put Planck's 2-sigma detection limit right at the edge of the feasible range for the B-mode signal, so that even if Planck had detected it, it likely wouldn't have been a very significant detection.

 

[Chalnoth]

Perhaps in the microwave background radiation, perhaps gravity is weak because it seeps into other universes, maybe dark energy appears to be coming from no where because it's leaking from another universe. Until data contradicts a possibility, that possibility must be considered valid, no matter how unlikely.

I don't think so. There are an infinite number of possibilities to investigate, so it's very worthwhile to have some ideas of the kinds of things to investigate even before there is any evidence to guide the way.

A pretty good rule of thumb that is frequently used is Occam's Razor: examine the number of postulates of the theories, and the one with fewer postulates should be investigated first. This is one reason why string theory remains popular among a number of theorists: it has very few degrees of freedom in the base theory, and it predicts quantum gravity. It's hard to say how likely it is that this theory describes the world around us, but this is solid ground to at least consider the theory worth investigation.

On the other end of things, there are a number of theoretical ideas that need a number of complicated mechanisms added to them in order for them to work out. For example, if you have a theory that includes tachyons, then any interaction at all between those tachyons and normal matter results in instability (basically, the tachyons cause the universe to explode and never stop exploding).

Aren't there many different types of multiverse theory? I've heard theories where our universe being a small pocket where the laws of physics are what they are, but as you get further away the just sort of change, we just happen to be in a pocket where everything looks the same. I've also heard theories that our universe may exist within a black hole of a bigger universe and fine tuning is explained by evolution, natural selection favors universes that make a lot of black holes and evolve through recursion. Along lines of the recursion, I've heard ideas in my own field that natural selection prefers universes that produce intelligent life and life is somehow responsible for making more universes (simulation hypothesis.)

Yes. Max Tegmark produced a concise classification, as well as proposing his own new multiverse concept:
http://space.mit.edu/home/tegmark/PDF/multiverse_sciam.pdf

Basically, they boil down to:
1. Different initial conditions.
2. Different low-energy physical laws.
3. Different quantum realizations.
4. Different mathematical structures (this is the one that Tegmark proposed).

 

[mfb]

Under a multiverse scenario, such as that proposed by Tegmark, you cannot forbid causal connections between different 'universes'. So there must surely be other universes that are observationally accessible to us. So, to quote Fermi, where are they?

Why? I cannot forbid that your door bell rings while you are reading this post. Does it imply it has to happen?

 

[Chalnoth]

Why? I cannot forbid that your door bell rings while you are reading this post. Does it imply it has to happen?

It seems to be the case in quantum mechanics that that which is not expressly forbidden necessarily occurs.

However, there are two facts that limit this:
1. With a complex theory, it isn't always clear exactly what is and isn't possible within that theory. It isn't as simple as, "I can't think of a way in which this theory prevents it, therefore it must occur." You have to do more work to show that it really is possible within the theory.

2. Just because something is possible within the theory doesn't mean it's at all likely. If you're going to attempt to tie it to observation, you have to actually show that the effect of interest is expected to be observed.

 

[mfb]

It seems to be the case in quantum mechanics that that which is not expressly forbidden necessarily occurs.

Only with MWI. And not even there in every world.

2. Just because something is possible within the theory doesn't mean it's at all likely. If you're going to attempt to tie it to observation, you have to actually show that the effect of interest is expected to be observed.

Exactly, and that's what I was missing in your post.

 

[Chalnoth]

Only with MWI. And not even there in every world.

I was talking about Gell-Mann's Totalitarian Principle, described here:
https://en.wikipedia.org/wiki/Totalitarian_principle

 

[rootone]

Does this mean that everything that is not expressly forbidden is compulsory?

 

[Chalnoth]

Does this mean that everything that is not expressly forbidden is compulsory?

This rule seems to be accurate when calculating the probability of various interaction outcomes in quantum field theory. Some might argue that this isn't "real", it's just a mathematical tool that is used to calculate those probabilities, but it's at least suggestive.

 

[Chronos]

I'm nor suggesting logic trumps the scientific method, but, I do suggest science without logic is a mess.

 

[Haelfix]

The Ekpyrotic universe model predicts no primordial gravitational waves detectable by BICEP2, whereas Inflation in many guises did. If such g-w's had been found that would have falsified Steinhardt's model.

In fact they weren't discovered as we know, BICEP2 picked up a signal from dust and whether there is a weaker g-w signal hidden in there has yet to be determined.

So it might seem that the Ekpyrotic model has won over the Inflationary one, however Inflation is so flexible that one can simply constrain the parameters fed into the theory and produce a weaker or zero prediction of the g-w signal and thus save the theory. Actually we all do this with our theories - it's just that inflation is more flexible than most.
Garth

I explained last time in the other thread about this subject that this was an attribution fallacy. Essentially comparing apples to oranges. After Bicep2, Neil Turok was busy writing down Ekyrprotic models that fit the new values of r. Of course that if you compare a specific model (the simplest version of the Ekyprotic universe) with an idea (all possible inflationary models), you will find that the former is more predictive than the latter.

What really matters in this business is model independent predictions. Inflationary theory has 5 or 6 nontrivial predictions, and every single one of them (other than a g-w spectrum) has been verified. This includes very specific shapes of the CMB, which everyone in the field thought was crazy talk when it was first proposed. Of course what happened was all new ideas were tempered on these new constraints after experiment picked them out and not surprisingly there is still a large amount of wiggle room and we are still left with a multiplicity of options that have to be tested/constrained and eventually thrown out (and it is conceivable that we might never be able to throw out all possibilities).

What is true, is that when you analyze in details what's left standing, the existence of something like a multiverse is 'generic'. This is particularly powerful, considering that most of the best (for different phenomenological reasons) and simplest models that are left in this group typically do include multiverses as an outcome.

 

[Garth]

What is true, is that when you analyze in details what's left standing, the existence of something like a multiverse is 'generic'. This is particularly powerful, considering that most of the best (for different phenomenological reasons) and simplest models that are left in this group typically do include multiverses as an outcome.

Except that outcome has arisen from incomplete physics which does not have a quantum gravity theory and that does not know what the Inflaton or Dark Matter, or Dark Energy are.

The extrapolation of theory into the infinite landscape of string and inflation hypotheses may be nothing more than the outcome of our ignorance.

The problem, as I have said several times, is this approach may actually dissuade further investigation into any actual physical causes of the phenomena that may be there to be found - and to be found in this universe, the only one that we have actually observed.

Garth