Multiverse Cosmology: Exploring Physicists' Views

[marcus]

...
My personal worry would be that if eternal inflation and LQC are both true, and I recognise this is a big if, that might remove the bounce from detectability. Do you agree this is correct or have I misunderstood?

I think you are right about that. Can't speak with much confidence about the such inflation scenarios so I have to emphasize that I just suspect that's right, without feeling sure.

In practical research community terms the main issue is what was the immediate cause of the big bang or as I prefer to say the immediate cause of the start of expansion.

Regarding that question "a random quantum fluctuation in some unknown physics" and "LQG bounce" are two competing answers. In a practical, immediate problem sense, I mean.

LQG is comparatively mundane unexotic and I expect in the near/medium term they'll be looking for signs of it having happened. If they see signs then research time and money will go more into studying that, with less interest in eternal inflation.

On the other hand if they don't see signs of bounce having happened then I would expect interest in Loop cosmology and perhaps LQG as a whole to wane. With more active interest in some random quantum fluctuation as a trigger for expansion.

 

[marcus]

If you accept what Ashtekar says about the enhanced liklihood of sufficient ordinary inflation in the Loop case then the usual support for "Eternal Inflation" is flawed. There is much less reason to be interested in it, beyond inherent fantasy-appeal of the the grandiose vision.

Specifically, sufficient means 60 e-folds--to produce the observed uniformity. The usual argument is basically one of desperation: "we can't think of any normal physics mechanism for inflation to get started, and then continue 60 e-folds, and then stop!" But Ashtekar can. (Loop is comparatively mundane, you quantize and go with established cosmology. I might say it is no more than a "hop" of faith.)

Once you posit some exotic leap-of-faith mechanisms you see stuff happening like eternal inflation. Support for eternal inflation depends on not being able to think of any other way that an adequate inflation episode could have started and then turned off.

It looks to me like some of these guys with exotic brane-clash and multiverse ideas have vested interest in ignoring simple answers--which threaten the raison d'être for some unnecessarily elaborate pet constructs.

You saw what happened at the end of Neil Turok's talk---we discussed this earlier. I'll get the link in case anyone else wants to check it out. It was the opening talk at a Perimeter conference he and some other people organized on "Challenges for Early Universe Cosmology"
http://pirsa.org/11070044/
Overview of the Challenges
Neil Turok
12/07/2011 - 9:00 am

The video lasts 1 hour 10 minutes and Elena's comment about the Ashtekar Sloan work starts right about 1 hour 7 minutes. You can of course drag the button to just hear the last 3 minutes. But the talk is interesting overall--especially the comments from the audience---Leonard Susskind, Sean Carroll etc etc. IIRC this starts around minute 55.

At this point Turok has his summary slide up. The challenges he identifies are:
Singularity
Tuning [in particular to get inflation which continues long enough and then turns off]
Reliance on anthropics
Measure
Several audience comments stressed challenges related to entropy: "2nd law" paradoxes.

I'm not suggesting one should take this talk as actually authoritative/representative about early universe cosmology. Neil, Lenny, Sean, Lindei, Vilenkin...etc are vocal but they are not currently writing a lot of papers or getting cited very much. I think it is a subcommunity which may be feeling a bit on the defensive at present. The interesting part is to see what arguments they have organized to justify this collection of ideas.

 

[eloheim]

Aren't there some Anthropic Principle-related suggestions for the Multiverse? Like one would expect the values of any natural "constants" vital to intelligent life to be only barely deviant from typical. Basically, assuming that a universe fit for life will need to be more picky about them than otherwise, finding we live in a "1-in-a-trillion" universe when there are a "million-in-a-trillion" other ways (universes) that allow for life would make our theories on such things seem very unlikely to be true.

Does anyone have a good understanding of if we have any indications on this for our values (universe), or even any notions of what kind of values we should be looking at?

 

[Deuterium2H]

Please keep religious discussion, either pro or con, out of all posts in this thread, and out of all posts in the science forums at Physics Forums.

Hi George,

Not saying you don't...but I hope you are just as strong in your advice when it comes to the opposite scenario. For example, Hawking's fervent insistence that our Universe does not need or require a "Creator", is just as much a metaphysical and philosophical position (and as such, is outside the bounds of "Science) as those who posit some sort of Intelligent Creator behind it all.

In either case, we are stepping outside "Science", and what can be tested and potentially falsified. If Religion has no place in the discussion of the origin of the Universe, neither does the speculative metaphysics of a "cyclic" universe, or even a "multiverse" for that matter, IMHO.

 

[jdm1001]

Sorry everyone, I had not intention to create a discussion on determining the what the un-caused, cause is. It is absolutely imperative that faith in science must be supported by observations and known facts. All I wanted to do is show how the study of the un-caused cause and the study of the unknown are rapidly merging into the one in the same.

 

[juanrga]

I really know nothing whatsoever about cosmology although I find it very interesting. It seems that a lot of physicists nowadays reckon there's a multiverse. I don't particularly want there to be a multiverse. WHAT DO YOU FOLKS THINK. Also, if there was a multiverse, would the laws of physics be the same for all of the universes? And would every single possibility be actually occurring in some universe out there? Like, in one universe am I being cut into bits from the toes up without anaesthetic and then having my body regenerated by some piece of advanced technology and then being cut up again OVER AND OVER FOREVER? All the while having faeces smeared in my face?

Multiverse is a non-scientific hypothesis.

 

[Chalnoth]

Multiverse is a non-scientific hypothesis.

Completely and utterly false. Try again.

 

[juanrga]

Completely and utterly false. Try again.

Multiverse is a non-scientific hypothesis

http://en.wikipedia.org/wiki/Multiverse#Criticisms

 

[Chalnoth]

Multiverse is a non-scientific hypothesis

http://en.wikipedia.org/wiki/Multiverse#Criticisms

Noting that some people say that doesn't actually make it true. Back in here in reality, it is very much a scientific hypothesis. For example:
http://arxiv.org/abs/0901.0007

 

[juanrga]

Noting that some people say that doesn't actually make it true. Back in here in reality, it is very much a scientific hypothesis. For example:
http://arxiv.org/abs/0901.0007

My original statement remains unchanged

 

[otherme]

Hi, I'm new here and don't pretend to understand very much of what's being said but physics fascinates me. Multiverse theory fascinates me. And, as with all sciences, don't you start with a postulate then go about proving or disproving that postulate. Something like multverse and M theory would be, I would imagine, very difficult to prove one way or the other. From my understanding, universes are close to, connected and sometimes intertwine each other - or so the theory goes. I should say - a theory goes, as there are so many. Like I said, I don't understand much but find it all fascinating. I feel there is something more than what we see with our eyes.

 

[rbj]

I agree with a Multiverse of observable universes if that is the right word. I have trouble with them having different laws of Physics,

i think the idea is that they might have different fundamental constants than our universe.

in my opinion, the concept of the Multiverse was cooked up so that the (weak) Anthropic principle would be able to explain away any teleological argument about the existence of God. if there are many, many other universes, some might be life friendly and some not. and it's an example of selection bias that we find ourselves in a universe that is life friendly.

 

[rbj]

Completely and utterly false. Try again.

So when you show me an experiment that will test and falsify the existence of some other universe, I'll show you an experiment that will do the same regarding God. Wanna see my God-measuring device?

Chalnoth, your belief, and that is all it is, is unscientific.

 

[Chalnoth]

i think the idea is that they might have different fundamental constants than our universe.

in my opinion, the concept of the Multiverse was cooked up so that the (weak) Anthropic principle would be able to explain away any teleological argument about the existence of God. if there are many, many other universes, some might be life friendly and some not. and it's an example of selection bias that we find ourselves in a universe that is life friendly.

The only way you could possibly come to this conclusion would be through complete and utter ignorance of the scientific discussion surrounding the multiverse. Put simply, no god ever has or ever will come into it, because no scientist worth their salt considers a god as a reasonable hypothesis, or has done so for quite a long time. There are strong epistemological reasons for this which I won't go into, but suffice it to say that it is fundamentally impossible to make forward progress in science by using a god hypothesis (no matter which god you're talking about).

Instead, the argument has always been between two camps within the theoretical physics community. On the one side, we have physicists who think that the natural laws we observe must be derivable from some fundamental theory. This has, for much of the history of physics, been the majority view. However, recent work in developing grand-unified theories has put doubt on this view, to the point that high-energy theorists are becoming increasingly convinced that it is just not feasible. And so physicists are increasingly moving away from the idea of a fundamental theory from which everything we observe inevitably follows and towards a fundamental theory which is prolific. A prolific fundamental theory explains everything we observe by stating that many things happen, and life occurs where it can occur, with most of the universe being uninhabitable.

None of this has ever referenced any sort of god, either before or after.

 

[Chronos]

Science seeks a causal relationship between properties and evolution of the universe. God is a first principle proposition and neither science or mathematics is the right tool for dealing with first principles.

 

[skydivephil]

i think the idea is that they might have different fundamental constants than our universe.

in my opinion, the concept of the Multiverse was cooked up so that the (weak) Anthropic principle would be able to explain away any teleological argument about the existence of God. if there are many, many other universes, some might be life friendly and some not. and it's an example of selection bias that we find ourselves in a universe that is life friendly.

The history of science is not really a question of your opinion. It is possible to go and look up things and see where certain ideas came from.
There are different definitions of the multiverse. I would sugggest the two most popular ideas are the many worlds interpretation of Qm and the inflationary multiverse.
The first was invented to deal with the problems of the Copenhagen interpretation of QM, nothing to do with anthropics. The second came from inflation. Inflation was desgined to solve one problem and one problem only, that's the magnetic monopole problem, again nothing to do with anthropics. Later it was realized it solved other problems and as the theory was developed it was argued the inevtiable consequence of inflation was a multiverse. If you would like to read about this I suggest reading Alan Guths book "The Inflationary Universe", there is also a new biography of Hugh Everett which woudl enable you to understand the motivation of the first type. They were not invented as a way of dealing with arguments for god, that just not histroically accurate at all.

 

[pebbleanrock]

Science seeks a causal relationship between properties and evolution of the universe. God is a first principle proposition and neither science or mathematics is the right tool for dealing with first principles.

I read a book explaining the creation of the universe. It said the universe started its journey as a tiny empty space surrounded by a sea of energy at absolute zero. The energy was in the form of straight strings before it had decayed into the circular stringed form of the particles of this universe. In describing the space /matter or energy relationship of the centre of a black hole, Einstein said it was infinity + infinity + infinity and was accused of a mistake. While a black hole has not reached this state, it does describe the sea of energy or the state the black hole energy is trying to entropize to. This is best envisaged as a super BEC [ Bohr Einstein Condensate ] where an almost infinite quanta of straight string fits into the same space. The energy, under pressure has arced into the tiny empty space and raised the temperature of the nearby sea. As this almost infinitely deep straight string area decays into circular string quantum they have their own piece of space. Part of the residue of each matter/antimatter annihilation would be 2 quanta of new space. A multitude of waves is moving through the energy sea. These differing waves of straight string then decay into different energy amounts thus producing the vast array of different particles. As well as the annihilation, other particles combine to form hydrogen and helium. The book is the Bible and seems to have been incorrectly read for thousands of years. My full essay can be found at <pebbleanrock.org>. comment?

 

[cephron]

I read a book explaining the creation of the universe. ... The book is the Bible and seems to have been incorrectly read for thousands of years. My full essay can be found at <pebbleanrock.org>. comment?

The bible is simply not a scientific textbook. The important points made in Genesis are about who God is and what his relationship is to man and creation, not the details of the mechanistic process by which it all happened. To try to turn the Bible into a science textbook is worse than useless.

I think Chronos had it right. Science makes models to explain objective, repeatable observations. Science simply does not make any claims about God, because God doesn't interact with the world in an objective, repeatable basis.

 

[pebbleanrock]

The bible is simply not a scientific textbook. The important points made in Genesis are about who God is and what his relationship is to man and creation, not the details of the mechanistic process by which it all happened. To try to turn the Bible into a science textbook is worse than useless.

I think Chronos had it right. Science makes models to explain objective, repeatable observations. Science simply does not make any claims about God, because God doesn't interact with the world in an objective, repeatable basis.

You do not know this . You have been told by somebody. 'circle of the earth' THEN they thought "its round." hang the Earth on nothing"then they thought we're just floating there" "Write on tablet of your heart" heart transplant scientists find in 1990's that heart nerves support memory. Entropy,"the heavens will wear out like a garment" all these written 3500 years ago. Now its opened up again.

 

[cephron]

You do not know this . You have been told by somebody.

Which part? That God doesn't interact with the world in an objective, repeatable basis?

Technically, you're right--perhaps in the future someone will identify an objective, repeatable observation that somehow can only be explained by God existing. But such a thing has not yet been discovered--if it had, it would be trivial to objectively prove that God exists. If you can do that, I'm all ears (although you'd have to find a different forum to do it, this one is about science). But anyway, until that happens, science has nothing to say about God.

Edit: Exegesis wars are fun, but this forum is not the place for them. The fundamental problem with using the bible as a science textbook, though, is that you'll only be able to selectively interpret (and sometimes horrendously stretch) the bible to match what science has already told us. Good luck using the bible to make any scientific predictions that science hasn't already made...

Edit 2: Sorry for sounding harsh, but there won't be much sympathy here for using the bible for "science". I wanted to at least explain why it doesn't work, but this is actually off-topic from the thread. If you want to continue this discussion, you should start a new thread. Probably not in the cosmology section, though.

 

[Chronos]

Science tells you as much about God as eating meat loaf tells you about the chef. Theism is deliberately excluded from science because it has zero utility in modeling the universe. Worse yet, it tempts us to concede the match before we even comprehend the rules of the game - which is, at best, unconstructive.

 

[cephron]

Well, to be fair, it's playing an entirely different game altogether. Its goals are different from science's; its definition of "constructive" as well. You can't really get an objective definition of "constructive" (in this context) without dipping into philosophy or something. But I would agree that it has no scientific utility. I like your meatloaf analogy. ;)

 

[Chalnoth]

Its goals are different from science's;

In a way. But only in that the goal of science is to discover the true nature of reality, while the goal of religion, where it interacts with science, is to convince people that the religion offers the secrets to the true nature of reality.

 

[rbj]

Science tells you as much about God as eating meat loaf tells you about the chef. Theism is deliberately excluded from science because it has zero utility in modeling the universe. Worse yet, it tempts us to concede the match before we even comprehend the rules of the game - which is, at best, unconstructive.

everything you say here is agreeable to me, Chronos.

i see that they deleted some of the personal attack posts, but not all:

The only way you could possibly come to this conclusion would be through complete and utter ignorance of the scientific discussion surrounding the multiverse.

i didn't suggest to delete any posts, particularly my response to Chalnoth's arrogant attack post, only that they lock the thread. oh well, who's to judge the judgment of the admins.

so Chalnoth is allowed to point to others and accuse them of "complete and utter ignorance of the scientific discussion surrounding the multiverse" when, in fact, he/she has no idea what the other's level of ignorance is.

so, i'll ask again, is the hypothesis of other universes a testable, falsifiable hypothesis? if Chalnoth says it is, i'll continue to ask him to define such an experiment or something that would be measured or experienced differently if other universes existed than if they did not.

it's a very similar challenge made by respectable skeptics like Michael Shermer: "Here's the deal, there is no conflict between science and religion as long as the God you believe in doesn't do anything."

i agree with Shermer on this. i just want to hold the same standard of falsifiability to the belief in other universes.

some time is allowed between the development of a theory and when its falsifiability is tested. the aether was proposed long before the Michaelson-Morley experiment (which shown that, if the aether existed, it didn't seem to have any effect on anything, including when we would have expected it to have some effect). GR was proposed a few years before Eddington traveled south to measure the shift in the perihelion precession of Mercury (which seemed to support the prediction from GR).

string theory and M-theory are nice little theories. one elegant explanation for the source and existence of other universes is that they result from other solutions to the same brane equations (that i will certainly admit i do not understand in any depth). these different solutions can result in different universes that may have different fundamental constants (and i mean the dimensionless ones, like those that John Baez has enumerated), some sets of fundamental constants will result in matter forming and stars living long enough for life to evolve enough to ask the question "how is it that we are here?" some combinations of these constants will not, and those universes will go by utterly unbeheld.

it's nice and elegant, but it's not a falsifiable theory. we can't test it. it's about the same as Michael Shermer's concept of God.

now, if Chalnoth might choose to be a little more humble about this (maybe take after the example of Chronos), i would be interested in what he/she has to say. but if his/answer is a sweeping and arrogant "Completely and utterly false. Try again." or "The only way you could possibly come to this conclusion would be through complete and utter ignorance ...", then i am less interested in whatever he/she has to say.

and i would like it if the sysops here would be more even handed in deleting or retaining contentious posts.

 

[Chalnoth]

rbj, when you come into a thread and launch into a series of baseless accusations that have nothing whatsoever to do with any of the actual scientific discussion on the topic, you should expect people to get a little annoyed at you.

And yes, I stand by my statement that you are horribly ignorant of the scientific discussion on this matter, because the epithets you have thrown out have nothing whatsoever to do with the actual scientific discussion. An accusation of ignorance, by the way, does not necessarily impinge upon your character. We are all ignorant of a great many things. What's wrong with you is you feel the need to pontificate about things which you know nothing about. Such hubris is not going to make people happy with you.

 

[rbj]

we'll let your words speak for themselves and we'll let my words speak for themselves. trouble is, some of mine were deleted, so they don't speak for themselves anymore.

other people can judge who is displaying hubris, and who is engaging in the topic.

 

[marcus]

So when you show me an experiment that will test and falsify the existence of some other universe, I'll show you an experiment that will do the same regarding God. Wanna see my God-measuring device?

Chalnoth, your belief, and that is all it is, is unscientific.

RBJ, I agree with what I think your first paragraph is saying. Physical existence of some other universe (place with fundamentally different laws) is not testable hypothesis, so is not scientific as we traditionally use the word.

I don't know that Chalnoth BELIEVES in a physical multiverse. If he did, that would not be a scientific belief, in my view. But I don't know his personal beliefs so let's leave the personal issue out.

I suspect your historical theory about the roughly 2003-2008 multiverse buzz is wrong, there were probably several conscious/unconscious motives at work. Certainly in popular books and discussion we saw people urging Multiversery as a way to thwart the Theists. But that could have been just a rhetorical ploy, not the original reason for "cooking up".

So I think your second paragraph in the next passage is probably mistaken, and Chalnoth's historical account probably a more accurate description of what was happening at least among a vocal minority of theorists.

i think the idea is that they might have different fundamental constants than our universe.

in my opinion, the concept of the Multiverse was cooked up so that the (weak) Anthropic principle would be able to explain away any teleological argument about the existence of God. if there are many, many other universes, some might be life friendly and some not. and it's an example of selection bias that we find ourselves in a universe that is life friendly.

My point is simply that it is naive to assume that our present (inadequate) idea of physical law is a good place to stop and declare "Multiverse"

So far it looks like we are in an endless series of explanations and there is always a next one and it is never the last. We figure out the fundamental laws and then we realize that we need to explain why we have these laws. So we scratch our heads and figure out the laws-of-the-laws---the explanation that is one layer deeper. And then we realize...

And as we continue to do this, our (mathematical) language evolves, and our concepts develop explanatory power. Our species learns, our language grows.

It would be naive, at this point, to stop the process and say "God". To declare that something unexplainable chose the laws for us in their present form and stop trying to explain.

It would be equally naive, and in some sense equivalent, to say "Multiverse" and declare that the present imperfect laws are not explainable by a deeper process that we can try to understand. To give up. The quest for ever-deeper fundamental theory is over. The entirety of what we have now is a random lawless accident. To stop looking for a deeper explanation. (which will never be final).

"God" and "Multiverse" are simply two (nearly equivalent) ways of saying give up and shut up---stop trying to explain how it happens to be the way it is. And they are naive. And arrogant, because they assume that our mathematical language and explanatory tools have reached a final state of evolution making further deeper explanation impossible. We don't know this. We have no evidence that they have. This is why both ways of arbitrarily ending the discussion are not scientific.

Anyway that's my take on the situation.

It's not as much of a big deal now. As far as I can tell among scientists themselves the "Multiverse" buzz has died down considerably since around 2008. Eventually I think the same will happen in the popular books and media. Greene's "Hidden Reality" and Hawking's "Grand Design" books will be forgotten and the public will move on too.

 

[Mark M]

My point is simply that it is naive to assume that our present (inadequate) idea of physical law is a good place to stop and declare "Multiverse"

So far it looks like we are in an endless series of explanations and there is always a next one and it is never the last. We figure out the fundamental laws and then we realize that we need to explain why we have these laws. So we scratch our heads and figure out the laws-of-the-laws---the explanation that is one layer deeper. And then we realize...

And as we continue to do this, our (mathematical) language evolves, and our concepts develop explanatory power. Our species learns, our language grows.

It would be naive, at this point, to stop the process and say "God". To declare that something unexplainable chose the laws for us in their present form and stop trying to explain.

It would be equally naive, and in some sense equivalent, to say "Multiverse" and declare that the present imperfect laws are not explainable by a deeper process that we can try to understand. To give up. The quest for ever-deeper fundamental theory is over. The entirety of what we have now is a random lawless accident. To stop looking for a deeper explanation. (which will never be final).

"God" and "Multiverse" are simply two (nearly equivalent) ways of saying give up and shut up---stop trying to explain how it happens to be the way it is. And they are naive. And arrogant, because they assume that our mathematical language and explanatory tools have reached a final state of evolution making further deeper explanation impossible. We don't know this. We have no evidence that they have. This is why both ways of arbitrarily ending the discussion are not scientific.

Anyway that's my take on the situation.

It's not as much of a big deal now. As far as I can tell among scientists themselves the "Multiverse" buzz has died down considerably since around 2008. Eventually I think the same will happen in the popular books and media. Greene's "Hidden Reality" and Hawking's "Grand Design" books will be forgotten and the public will move on too.

Wow, excellently put. Even though there may or may not be some kind of multiverse, I don't know why people so quickly assume that the answer is either "Multiverse" or "Intelligent Design".

Hopefully, once we have a very well developed theory of everything, it may show our Universe is the way it is because this is simply the only way it could have been.

 

[Chalnoth]

I don't know that Chalnoth BELIEVES in a physical multiverse. If he did, that would not be a scientific belief, in my view. But I don't know his personal beliefs so let's leave the personal issue out.

I think belief is just the wrong way to think about this altogether. Either it's true or it isn't, and what we believe has no bearing whatsoever on the truth. All that we can do is gather evidence and from the evidence deduce what is likely and is not likely to be true.

So I deliberately choose not to say that I believe anything at all about the nature of the universe. There are merely degrees of confidence.

My point is simply that it is naive to assume that our present (inadequate) idea of physical law is a good place to stop and declare "Multiverse"

It's this sort of emotional objection to multiverse ideas that I dislike the most among scientists (as opposed to bad arguments from non-scientists, which can be far, far worse...). There are two things to point out:

1. Perhaps the question is a bad question altogether. To draw an analogy with another science, biology, for a long time scientists tried to answer the question, "What was the intention of this?" in regards to some feature of life. For example, what is the purpose of a bird's wing, or the red color of blood? For some things about life, these answers at least seem obvious. For others they aren't that obvious. And for still other things about life, the question is downright perplexing (e.g. what is the purpose of the appendix?). But what Darwin showed, and what has been confirmed by centuries of observation and experimentation, was that this was simply a bad question to ask in the first place: there is neither intention nor purpose to life, and we simply cannot understand how life acts until we first understand this.

If we do find that the answer to a particular question is indeed that it is a bad question in the first place, then we must accept that. And, in fact, recognizing that it was a bad question can open the door for a cornucopia of new possibilities for understanding better how the universe works.

2. It is often simply false that a multiverse idea is remotely similar to "giving up". Imagine, for example, the cosmological constant. Here is a very simple model for the cosmological constant which could be tested experimentally. Imagine that we have a theory which predicts that the cosmological constant can take on a huge number of values, but not every value. Let's say that the values within a factor of two of the measured density fraction, for example, are 0.332, 0.654, 0.736, 0.991, 1.02, 1.35. This hypothetical theory predicts it could be one of these, or some other number much larger or smaller, but it cannot ever be anything in between. If we go out and measure and find that the cosmological constant is 0.710, then we have falsified the theory. Alternatively, if we go out and find that it is precisely 0.736, then we gain confidence that this is, in fact, the right explanation.

So simply throwing out multiverse ideas before we even get started because you don't like them is being exceedingly premature. If this were merely a matter of getting annoyed at one particular multiverse proponent's arguments, then that's fine. There are many bad arguments thrown out for many different kinds of things. But I strenuously object to the tarring of an entire class of ideas with nasty epithets simply because you don't like them.Anyway, my personal take here is that it is extraordinarily likely, given what we know today about cosmology, quantum mechanics, and high-energy physics, that we live in a multiverse.

From cosmology, we can arrive at the conclusion that the whole universe is likely to be far, far larger than our observable part of it from a multitude of angles. For instance, grand unified theories predict the existence of heavy, stable magnetic monopoles which would, in the classical big bang theory, vastly outnumber the normal matter that we know and love. This is because in the big bang theory, heavier things precipitate earlier-on, and the earlier something is produced, the more of them are produced. But trivial observation demonstrates that magnetic monopoles are at least exceedingly rare in our observable universe, if they exist at all. A simple solution to this is inflation, where an exceedingly rapid expansion rate early-on, when these magnetic monopoles would have been produced, spreads the monopoles so far across space that we'd be lucky to have even a single one within our observable universe.

There are many other arguments that you can make, from different directions, but in the end it is very, very likely that the whole universe is much, much bigger than our observable portion of it. This leads to a multiverse of perhaps the simplest and most likely sort.

Then, from quantum mechanics, the wave function of quantum mechanics unambiguously predicts a multiverse of a multitude of outcomes from a single interaction. We can get rid of this multiverse if we simply assume it is not there, but at best this adds no predictive power to the theory. At worst it removes predictive power. So the quantum multiverse is extraordinarily likely. As a side comment, a number of cosmologists are looking into the idea that "the universe is big" multiverse and the quantum multiverse are one and the same thing.

From the high-energy physics side of things, we are finding that some aspects of high-energy physics that we experience are due to accidents in the past, known as spontaneous symmetry breaking events. If we combine these accidental events with the apparent fact that the universe is much, much larger than the part of it we can observe, we arrive at the conclusion that in parts of the universe far beyond our horizon, the laws of physics are likely to be rather different. We don't yet know exactly how different things can be, as we don't know all of the spontaneous symmetry breaking events that occurred. But from what we do know, it is very likely that they are different if we go far enough away.

This leads us to what is perhaps the most important aspect of the multiverse: that it is rather likely that there are some aspects of reality which are accidental rather than inevitable. Some people don't like this, and try to come up with rationalizations to avoid it. But I have yet to see anybody come up with a rational argument against it.

 

[Chronos]

We have long harbored the suspicion our universe is what it is because it is the only way it could be. The problem is we do not know all there is to know about the workings of the universe. So until we 'know' what the universe 'is', we cannot assert this is the only way it could be. In part, this is an argument for determinism - that events are preordained by initial conditions and partial derivatives, which only allow fixed, predictable outcomes. This perspective was rather wildly popular among physicists until quantum theory earned respectability, and wiped the smug, deterministic certainty off their faces.

The problem with 'many worlds/multiverses' ,IMO, is the cure is worse than the disease. The hypothesis is not only unproven, but, widely regarded [even by proponents] as unprovable. We have no observational access to 'alternative realities', so until such a time, it is untestable. We can, and have, developed mathematical models that allow for, or at least do not forbid alternative realities, but, mathematical models can and do yield results that have no known physical manifestations in our universe. Of course you can always claim such results only apply in alternative universes, but, this is unlikely to sway your physics professor.

 

[Mark M]

The problem with 'many worlds/multiverses' ,IMO, is the cure is worse than the disease.

Yeah, I agree. Even though the multiverse would be an excellent explanation of particles values and other particulars, it sort of defeats the purpose of science - like when the Church argued with Galileo, saying it was pointless to think about things such as the Heliocentric model because "God could have made any way he wanted".

Similarly, in the some multiverse theories, you can blame every particular of the universe on the Anthropic Principle.

Fortunately, most multiverse theories that include such things, like Many Worlds, are seeming less likely. If there is a multiverse, it seems like it would be more along the lines of the pocket universes of chaotic inflation.

 

[Chalnoth]

The problem with 'many worlds/multiverses' ,IMO, is the cure is worse than the disease. The hypothesis is not only unproven, but, widely regarded [even by proponents] as unprovable.

The issue here is that multiverse ideas do not exist in a vacuum. They are but one component of larger models. Confirm the other components of these models (which are often measurable), and we gain confidence for the multiverse component.

There is no scientific theory which we require every single conceivable measurement be performed before we accept the theory is true. We merely wait until the evidence is strong. We do not, for example, feel the need to wait until we have a fossil for every point in the family tree that stretches between humans and chimpanzees. In fact, we can be pretty darned sure it is fundamentally impossible for us to ever find that many fossils. And yet, the evidence that there was such a lineage, despite not having direct evidence of every step, is absolutely overwhelming.

So it is patently absurd to require that every component of a scientific theory be confirmed. We should, instead, do what is always done in science: perform tests where they can be performed, and judge competing theories against one another based upon how they match up to the existing evidence and their own internal complexity. There is no reason whatsoever to latch onto one specific aspect of a theory or model which can't be directly confirmed. Just focus on the testable components.

For the many worlds interpretation of quantum mechanics, for example, we should focus our attention at the boundary of wavefunction collapse, which is very much observable. And absolutely essential to understand in detail for quantum computing.

 

[sahmgeek]

Then, from quantum mechanics, the wave function of quantum mechanics unambiguously predicts a multiverse of a multitude of outcomes from a single interaction. We can get rid of this multiverse if we simply assume it is not there, but at best this adds no predictive power to the theory. At worst it removes predictive power. So the quantum multiverse is extraordinarily likely. As a side comment, a number of cosmologists are looking into the idea that "the universe is big" multiverse and the quantum multiverse are one and the same thing.

How, in the name of science, does one jump from a range of possible (not actual) outcomes at the quantum level to actualized, yet unobserved multiple worlds? i want to call this hogwash, but instead will suggest that it is a very VERY liberal interpretation.

 

[rbj]

I don't know that Chalnoth BELIEVES in a physical multiverse. If he did, that would not be a scientific belief, in my view. But I don't know his personal beliefs so let's leave the personal issue out.

well, let's see what he has to say:

I think belief is just the wrong way to think about this altogether. Either it's true or it isn't, and what we believe has no bearing whatsoever on the truth. All that we can do is gather evidence and from the evidence deduce what is likely and is not likely to be true.

So I deliberately choose not to say that I believe anything at all about the nature of the universe. There are merely degrees of confidence.
...
Anyway, my personal take here is that it is extraordinarily likely, given what we know today about cosmology, quantum mechanics, and high-energy physics, that we live in a multiverse.

"personal take" ≠ "belief" ?

well, it's my personal take that this is lawyer-speak. Chalnoth believes we live in a multiverse, something that has absolutely no measurable nor experiential effect on any of us and has no promise to have such. it is not testable nor falsifiable.

my only issue is that if someone else were to use the same language but substitute "God" in for "Multiverse" then we would hear from a sysop:

Please keep religious discussion, either pro or con, out of all posts in this thread, and out of all posts in the science forums at Physics Forums.

despite his denial, Chalnoth believes in other universes. and he says so using other language. and since he has as much chance of verifying or falsifying such belief as he would for gnomes or fairies or gremlins, it is not a scientific belief. and when i point that out, he descends upon me for attacking him and for hubris when the attacking and hubris is all his own doing.

"God" and "Multiverse" are simply two (nearly equivalent) ways of saying give up and shut up---stop trying to explain how it happens to be the way it is. And they are naive. And arrogant, ... not scientific.

yes.

 

[rbj]

Wow, excellently put. Even though there may or may not be some kind of multiverse, I don't know why people so quickly assume that the answer is either "Multiverse" or "Intelligent Design".

it's because of the observations often labeled as Fine-tuned Universe.

if our Universe is the sole universe in reality, and if its existence is a once-in-an-eternity event never to happen again or somewhere else, then the fact that some of the approximately 26 dimensionless fundamental constants are within the Goldilocks range they are in, is remarkable.

one explanation that makes this less remarkable is that of many other universes that likely differ from ours slightly. some of these universes may have fundamental constants that are conducive to the development of matter, so that stars don't burn themselves out prematurely before life can evolve on small rocky planets like Earth. but many more of these universes would likely not and those universes will not have any life emerging and evolving to the point where they might ask questions like "how can we be so lucky to live in this life-friendly universe?" it's simple selection bias. this is a simple application of the Weak Anthropic Principle.

it's similar to the Dicke observation that the Universe is about 10 billion years old, about what it has to be for beings like us to emerge and evolve to ask such an existential question. but the Universe gets to experiment with a variety of ages. once it was 10 million years old, and there wasn't anyone around to look out into the night sky and think about this. and once it was 1 billion years old and there still wasn't anyone around to ponder such. and someday it will be a trillion years old and no one will be around to be observing anything.

so, for a reality when all possible values of some parameter gets to be tried out, the WAP simply turns the question around. instead of asking "How can it be that we are so lucky to have a Universe at just the right age that we can exist?", the WAP turns it around and asks "At what age or ages of the Universe can we expect life to emerge and evolve?" to which the answer is about 10 billion years.

but about the fundamental constants; for a single and sole Universe that didn't get to try out other variations for those constants, the question for how we could be so lucky for them to be "just right" remains remarkable. but if Reality can take many, maybe even an infinite number of stabs at defining the parameters of a universe, then it's only in the universes that have parameter sets that are life-friendly that any life will emerge and evolve to the point where they can ask the big questions.

so, for a single and sole Universe, there remains the big teleological question which can lead some people to a teleological explanation (some might label it "Intelligent Design" but this does not necessarily mean what the Discovery Institute calls ID). but a reality of many, many universes that emerge with different random fundamental parameters and initial conditions allows the Weak Anthropic Principle to obviate any teleological explanation.

Hopefully, once we have a very well developed theory of everything, it may show our Universe is the way it is because this is simply the only way it could have been.

i think history has shown that as physics (and the other sciences, we have deep questions about what is consciousness and qualia and whether or not we are philosophical zombies) answers questions, even more new questions are unearthed. i think the Theory of Everything is really a Pie in the Sky.

we'll get a very well developed theory of everything when we get world peace, justice for all, and truthful and uncorrupted leaders. don't count on it ever happening.