# Universe or Multiverse?

1. May 2, 2007

### Chronos

Defining what is/can be included in a description of what constitutes the universe is a frequent topic of interest. Most of us generally agree it consists of 'all that is possible', but have difficulty achieving a consensus on what is vs is not 'possible'. While I tend to steer clear of purely philosophical approaches, I am attracted to efforts to marry logic with mathematical structure. So [after much adieu], I thought some similarly afflicted souls might find this interesting:

http://www.anthropic-principle.com/preprints/spacetime.pdf.

Nick Bostrom has written some very engaging and well grounded philosophy of science stuff over the past decade. He has achieved some degree of repectability in more traditional circles having coauthored a couple of papers on Arxiv with some familiar names:

http://arxiv.org/abs/astro-ph/0512204
Title: How unlikely is a doomsday catastrophe?
Authors: Max Tegmark (MIT), Nick Bostrom (Oxford)

http://arxiv.org/abs/gr-qc/9906042
Title: Cosmological Constant and the Final Anthropic Hypothesis
Authors: Milan M. Cirkovic, Nick Bostrom

I'm also fascinated by Lucien Hardy's work on causaloids, so take that into consideration.

2. May 2, 2007

### marcus

I disagree on two levels. I'm skeptical that most of us (posters at PF) would say the universe is 'all that is possible'
and I personally don't think of the universe that way.

I think of it as the one particular possibility (out of a range of possibilities, presumably) that we have.

==================
The name of your thread reminds me of the title of a new book, "Universe or Multiverse", that is going on sale next month
and that we were discussing here:

There are some 25 contributors, each of whom wrote one chapter. According to the Cambridge University Press webpage on the book, chapter 20 is
by Lee Smolin and discusses his CNS (cosmic natural selection) conjecture---something that I know you keep track of, as I do.

Last edited: May 2, 2007
3. May 2, 2007

### marcus

there still are interesting questions of definition

1. we can't see the past or the future, but we can infer things about them using laws which we test in the present

2. we can't see beyond a certain horizon, but similarly we can infer stuff about the rest of the universe using laws which we test on earth and with astronomical observation.

Both those inferences involve the assumption that the laws were and will be the same and are the same beyond the horizon, so you could turn the definition around and say that the universe is the domain of the particular set of natural laws that we verify in other words
it is where these laws hold

So for me the universe is not every possibility because there appear to be other possible versions of the laws of nature, at least with different values of the coefficients.
==================

Smolin's CNS picture is a MULTIverse picture because it allows the fundamental constants of nature (like 1/137) to change at the pit of a black hole where a new tract of spacetime sprouts off.

the terms I am using are not sharply defined but I tend to think of OUR universe as the inferred domain of our set of natural laws with our set of fundamental constants.

maybe other universes (with a different number for 1/137) exist and maybe they don't

In any case, my universe is not all possibilities, it is the PARTICULAR possibility which we got----that we know largely by inference based on what we observe in our presentday vicinity.

it is a beautiful beautiful enigma, the most beautiful I know, like a person that you know well but never fully know (no matter how long you live with them)
===============

I looked at Bostrom's writing some years ago, but was not drawn to it. BTW he has contributed chapter 24 of the new book ("U or M") coming out next month
===============

I am glad you are intrigued by Lucien Hardy! That is a different kettle of fish!
I hope you pursue his causaloid idea and can explain some of what he's doing to the rest of us.

Last edited: May 2, 2007
4. May 3, 2007

### island

Finite or infinite? Don't assume that this is a given, because it is not.

5. May 3, 2007

### cesiumfrog

Marcus, are you saying that if a fundamental constant is very different in one particular galaxy, then you do not consider that galaxy to be part of our universe? And you will consider it to "become" part of our universe at a later time, if we formulate new laws that successfully describe and explain that variation of a former fundamental "constant"?

6. May 3, 2007

### Wallace

To me it's pretty clear on one level, the definition of 'the Universe' (capital U) is that it is all that exists. The very concept of something existing outside the Universe is logical contradiction.

However, what can (and certainly has) changed is our understanding of the expanse of the Universe. Once upon a time what we now know as the Solar system was thought to encompass all that exists (at least in a physical sense, religion aside), later we realized we could infer the distance to nearby stars and then clusters and mapped the structure of what we then thought of as 'the Universe'. Then we noticed what were dubbed 'spiral nebulae' that turned out to be distant objects that were similar entities to what we had previously thought of as the Universe. Our view changed and we renamed the local structure the Milky way instead of the Universe.

We now have a picture of the Big Bang and have essentially seen as far as we can (via the CMB) into the observable Universe. If we are ever able to confirm some theory of how Big Bang type events can occur, or separate space-time regions budding off from black holes, I think it would be wrong to call this a 'multi-verse', we would simply rename the space-time region we can observe (the Observaverse? maybe the Cosmos with a capital C?) and our concept of what 'the Universe' encompasses would be modified.

As to whether we could ever confirm that these regions exist is a separate question that it difficult to answer. I would suspect not, but history is littered with overly bold statements about what we can or cannot ever know that look comical in hindsight!

7. May 3, 2007

### marcus

That is too speculative. Give me a specfic case where one of the fundamental physical constants is different in some other galaxy.

If I saw good observational evidence that one of the parameters of the Standard Model was NOT constant throughout space and time, I would be extremely delighted.

that would mean there was new physics---a deeper theory---to discover. The "ex-constant" parameter would be shown to NOT be a constant and theorists would look for some model (presumably involving fundamental constants, as theories tend to ) which would govern the observed changes in that parameter.

For example, the Hubble parameter has changed by many orders of magnitude over the life of the universe. It is not a fundamental constant. At one time it was called a constant, which was a misnomer. The Friedmann equation governs how the Hubble parameter changes.

But I doubt that you can come up with convincing evidence that anything on today's list of fundamental constants has changed either over space or over time. If you could, that would of course be great! It wouldn't change what I think of as the universe, though.

Last edited: May 3, 2007
8. May 3, 2007

### marcus

OK, I will think about that.
It's a question about the best way to steer the development of language.

Your idea is that we shouldn't allow the word "multiverse" to creep into the language. ALL that exists, that we know about or infer using models that we can check by local observation, should be called the universe plain and simple.

From an English language standpoint, I like that. I never liked the word "multiverse"---a jazzy plastic sound like one gets from the advertising industry and sales departments.

So we keep the grand noble word Universe and maybe later we infer that the universe may have several different spacetime regions---if the bigbang was a bounce then there would, for instance, be the contracting region before the bounce.

And we include those other regions in the Universe, and never bother with the gimmicky word "multiverse". I see the stylistic point of that. The word's in better taste.

Last edited: May 3, 2007
9. May 4, 2007

### Chronos

10. May 4, 2007

### Chronos

Mixed messages, marcus. Defining what constitutes a 'universe' is not a trivial effort. Even the most BI versions assume something that resembles a background . . . unless you resort to a 'something from nothing' posture. Current versions of 'lambda' appear to imply 'space' may extend beyond our hubble bubble. A disturbing concept, IMO.

Last edited: May 4, 2007
11. May 8, 2007

### ukmicky

Cheers lads my favorite subject with lots of good links to keep me going .

12. May 10, 2007

### Chronos

Perhaps a different question to consider. Does 'space' extend beyond our hubble volume? I resist that notion because it confers 'space' the property of being more 'universal' than the matter residing in our hubble volume. Are there other hubble volumes containing matter whose presence can only be deduced by the curvature it imposes upon the curvature of our portion of metaspace? Not a very background independent concept IMO. Is this to say that matter residing in other hubble volumes, while forbidden to interact with matter in my hubble volume, is free to affect the shape of my spacetime? I remain fond and am reluctant to part with certain toy models of reality - like causality and thermodynamics.

13. May 10, 2007

[...So we keep the grand noble word Universe and maybe later we infer that the universe may have several different spacetime regions---if the bigbang was a bounce then there would, for instance, be the contracting region before the bounce.]

I agree with this, marcus. However, the universe is defined as all the particles and energy that exist and the spacetime in which all events occur, which implies that all events occur in one spacetime.

14. May 10, 2007

### wilgory

Whatever happened to the term "continuum".

If universe means "everything that is", then multiverse would imply "multiple everythings" which is contradictory.

We may never know if there is anything beyond our space-time continuum. If there is, then it should be considered part of the universe.

Multicontinuums is one possibility,however I do like the sound of multihubblebubbles.

I also have been wondering why the phrase "unified field theory" was replaced by "theory of everything".

I don't have a problem understanding what people mean by terms like "multiverse", but I see a lot of discussion about this on the forums I visit.

It would be nice if there was an authority that defined the terms used to discuss the universe.

15. May 10, 2007

### Garth

wilgory

Universe = A continuum.
Multiverse = Many separate continuums.
Unified Theory = A theory unifying the electromagnetic and weak forces: sudsequently discovered by Salam, Glashow & Weinberg's as the electro-weak theory.
Grand Unified Theory = Theory unifying the electromagnetic, weak and strong forces.
Theory of Everything = Theory unifying the electromagnetic, weak, strong and gravitational forces.

Chronos: the Copernican Principle would suggest that in a large enough universe 'space' would extend beyond our Hubble volume, even if we can never observe it.

Garth

Last edited: May 10, 2007
16. May 10, 2007

So what is the origin of the multiple continuums that we now acknowledge? Several "Little Bangs?" One "Big Bang"?

17. May 10, 2007

### marcus

the language situation is confused.
I would only speak of ONE continuum, although it could have several regions separated by bounces.
there is a serious problem what to call things!

I personally wouldn't acknowledge the existence of multiple continua. I also doubt there is a consensus on that in any of the fields of research I watch.
Even among string theorists there is a split, with some expressing fierce resistance to acknowledging the "landscape of string theory" as something real rather than just a menu of theoretical possibilities.
Cosmologists seem also far from unanimously adopting "eternal inflation" with its multiple "bubble universes"
(working mainstream cosmologists often just ignore it---they work with the continuum they know, which may be the only one there is)

in non-string QG you basically have one continuum, although some researchers have found they can probe back before the big bang.
GR breaks down, but quantizing GR leads to more rugged models which do not break down. So they can be run back in time to a prior collapsing phase.
I guess one can think of that as a different continuum if one wants, but the researchers themselves, who are running the model, act like it is part of our continuum, just going further back in the past---their model evolves smoothly and (wave-function) deterministically thru the point where the old model broke down and had a "singularity" so why not?

WAIT, I DIDN'T SEE THIS UNTIL JUST NOW. It was something I said about "several different spacetime regions" that led to confusion. I'll try to clarify.
Thanks for this, madphysics! I believe I see eye-to-eye with you on this. None of us can choose how language evolves and what meanings words take on. Language is like a big animal with a life of its own. But if I could express a preference I would say that I hope Universe and Spacetime Continuum keep their old meanings. And I will just go on thinking of there being ONE universe and one continuum.

But in the context of Loop Quantum Cosmology (LQC) I would acknowledge that this one continuum could be "pinched" at places so that it could appear to be organized into several or many spacetime regions. So that perhaps in some cases a black hole collapse continues through a bounce to a re-expanding region. Perhaps time-evolution does not simply stop, at the pit of a black hole. Perhaps time-evolution extends back to before the bigbang into a contracting phase of some kind. I dont want to try to imagine the branchy monster of the whole shebang. Only tentatively consider what might be happening at our own apparent singularities---briefly prior to our BB, briefly and very provisionally after some of our BH.

I want to consider it all to be the same continuum---but punctuated by some events of very high (Planck-level) density and temperature---places where the continuum becomes "un-classical".

Ashtekar's group has found from their computer modeling of bounces that the bounce tends to occur when the density reaches a level of about 80 percent of Planck. this could be wrong, it is just what they get by running the models in various cases.

I'm reluctant to yield any ground to the idea of multiplicity (being a staunch Unity freak) but what else can you do with these awkward singularities. Either one just has to write a question mark over each black hole and at the onset of expansion (bang-singularity), or one has to allow for a bounce and a possible continuation, which sort of looks like a new region of the same continuum. where is that ???? smiley. yes. here:

Last edited: May 10, 2007
18. May 10, 2007

### setAI

All Universes are Cheaper Than Just One

In general, computing all evolutions of all universes is much cheaper in terms of information requirements than computing just one particular, arbitrarily chosen evolution. Why? Because the Great Programmer's algorithm that systematically enumerates and runs all universes (with all imaginable types of physical laws, wave functions, noise etc.) is very short (although it takes time). On the other hand, computing just one particular universe's evolution (with, say, one particular instance of noise), without computing the others, tends to be very expensive, because almost all individual universes are incompressible, as has been shown above. More is less!

Many worlds

Suppose there is true (incompressible) noise in state transitions of our particular world evolution. The noise conveys additional information besides the one for initial state and physical laws. But from the Great Programmer's point of view, almost no extra information (nor, equivalently, a random generator) is required. Instead of computing just one of the many possible evolutions of a probabilistic universe with fixed laws but random noise of a certain (e.g., Gaussian) type, the Great Programmer's simple program computes them all. An automatic by-product of the Great Programmer's set-up is the well-known many worlds hypothesis'', ©Everett III. According to it, whenever our universe's quantum mechanics allows for alternative next paths, all are taken and the world splits into separate universes. From the Great Programmer's view, however, there are no real splits -- there are just a bunch of different algorithms which yield identical results for some time, until they start computing different outputs corresponding to different noise in different universes.

From an esthetical point of view that favors simple explanations of everything, a set-up in which all possible universes are computed instead of just ours is more attractive. It is simpler.

Juergen Schmidhuber
from http://arxiv.org/abs/quant-ph/9904050

19. May 12, 2007

### RussT

20. May 12, 2007

### marcus

go to arxiv.org and look up L Smolin.
One paper is "Scientific Alternatives to the Anthropic Principle"
http://arxiv.org/abs/hep-th/0407213

he also has a book about it called "Lives of the Cosmos"---that would have the most accessible lengthiest explanations.

he also has some 1993-1995 papers about it

Here is his most recent about it:
http://arxiv.org/abs/hep-th/0612185
The status of cosmological natural selection

that would have REFERENCES at the end to earlier papers that explain in more detail

the arxiv.org search engine is your friend
http://arxiv.org/find
type smolin in the "author" blank and type things like "natural selection" in the title or abstract blanks

all the arxiv papers are free for PDF download, it's a great resource

Last edited: May 12, 2007