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Probability of stars in a multiverse

by jimjohnson
Tags: galaxies, multiverse, stars
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rbj
#55
Feb22-13, 11:07 PM
P: 2,251
Quote Quote by Chalnoth View Post
If you're going to accuse me of presenting a strawman argument, you're going to have to stop proving me right again and again.
i only take responsibility for the things that i say. i do not take responsibility for things that other persons say that i say. (nor, specifically, for what you say that i say.)

again, you cannot dispute what your opponent is saying so then you dispute what your opponent is not saying.

it doesn't help you.

You have continually compared the multiverse idea with obviously false notions like the FSM
only that we cannot measure either nor hope to ever measure either.

but if you replace "FSM" with "God", you might have some dispute with some very authoritative people about how "obviously false" the notion is.

you claim to say you don't know, your mind is open. And yet you insist on using language elsewhere that makes it absolutely, positively clear that you consider multiverse ideas to be the height of absurdity, worthy of no consideration whatsoever.
again, you cannot dispute what your opponent is saying so you dispute what your opponent is not saying.

it doesn't help you.

try to read what i wrote (most of which you don't respond to at all). and respond to what i wrote, not to what i didn't.


Aside from this fundamental contradiction in what you are saying, the truth is the exact opposite of the way you try to paint it.
so you have a special hook on "the truth"? and we should accept that based on what? your authority?

Because the fact of the matter is that our vision is limited by causality. We cannot see beyond the horizon.
might be that FSM is hiding behind that horizon. don't worry, he/she/it can't get you because that would mean that you can measure him/her/it (and you can't).



But just because we cannot see beyond the horizon this does not mean that the default assumption should be that nothing is there (or even that everything is the same as the part within our horizon).
why must there be a default assumption? why can't the position taken be that we don't know nor cannot know what is beyond the horizon?

In fact, the default should most certainly be that things are quite different far enough beyond that horizon. How different we cannot say, but it is highly unlikely that things are identical everywhere.
maybe God or FSM or Santa Claus is back there. that's quite different.
rbj
#56
Feb22-13, 11:35 PM
P: 2,251
Quote Quote by Mordred View Post
My take on multiverse is that its not only possible but also likely.
i feel approximately the same way about it as i do about the likelihood that there are E.T.'s out there on some exoplanet that supports life. i would be very surprized if somehow someone could prove that Earth can be the only planet supporting life in the Milky Way. i accept the results of the Drake equation and think that there could be presently thousands of other civilizations in the Milky Way. but distributed among 400 billion stars, the likelihood we'll get an EM transmission from any that is not drowned out by stellar radiation, that likelihood is pretty small.

but one big difference is that while it's possible (and that possibility is exciting and deserves some funding and effort) for us to detect the presence of some E.T., i have absolutely no expectation that humans will ever measure or detect either God (setting aside accounts of miracles), the Flying Spaghetti Monster, nor anything coming from another universe.

Many of the models used in cosmology are open to the possibility of a multiverse. Until we have irrefutable evidence either way, Thinking that this universe is one of many or all that there is are both equally valid.
it is for me, but not for Noth.
julcab12
#57
Feb23-13, 06:34 AM
P: 153
Ok. Let's give a brief case of multiverse. What are the arguments and evidence for existence of a multiverse:(Study of George Ellis)

Argument 1: "There are plausibly galaxies beyond the horizon, where we can’t see then; so plausibly many different expanding universe domains where we can’t see them."

Untestable extrapolation; assumes continuity that may or may not be true. Outside where we can see, there might be (a) an FRW model, (b) chaotic inflation, (c) a closed model, (d) an island universe. No test can be done to see which is the case.

If each step in a chain of evidence is well understood and inevitable, then indirect evidence carries nearly as much weight as direct evidence. But not all the steps in this chain are inevitable.

If employed leads to the old idea of spatial homogeneity forever (`The Cosmological Principle’) rather than the multiverse of chaotic cosmology with domain walls separating phases.

CASE 1: 2 Implied by known physics that leads to chaotic inflation

The key physics (Coleman-de Luccia tunneling, the string theory landscape) is extrapolated from known and tested physics to new contexts; the extrapolation is unverified and indeed is unverifiable; it may or may not be true. The physics is hypothetical rather than tested!
Known Physics → Multiverse ??
NO!
Known Physics → Hypothetical Physics → Multiverse
Major Extrapolation
It is a great extrapolation from known physics.
This extrapolation is untestable: it may or may not be correct.

ARGUMENT 2: The String Landscape is a fantasy(scientifc standard). We actually have a
plausible landscape of minimally supersymmetric AdS4solutions of supergravity modified by an exponential superpotential. None of these solutions is accessible to world sheet
perturbation theory. If they exist as models of quantum gravity, they are defined by conformal field theories, and each is an independent quantum system, which makes no
transitions to any of the others. This landscape has nothing to do with CDL tunneling or eternal inflation.

"Points is whether one agrees or not: this analysis shows that the supposed underlying physics is certainly not well established."

ARGUMENT 2: Implied by inflation, which is justified by CBR anisotropy observations.
- it is implied by some forms of inflation but not others; inflation is not yet a well defined theory.
- Not all forms of inflation lead to chaotic inflation.
- For example inflation in small closed universes.

Two central scientific virtues are testability and explanatory power. In the cosmological context, these are often in conflict with each other.

The extreme case is multiverse proposals, where no direct observational tests of the hypothesis are possible, as the supposed other universes cannot be seen by any observations whatever, and the assumed underlying physics is also untested and indeed probably untestable.

In this context one must re-evaluate what the core of science is: can one maintain one has a genuine scientific theory when direct and indeed indirect tests of the theory are impossible? If one claims this, one is altering what one means by science.
One should be very careful before so doing. There are many other theories waiting at the door –wanting to be called science (astrology, Intelligent Design, etc).

The multiverse proposals are good empirically based philosophical proposals for the nature of what exists, but are not strictly within the domain of science because they are not testable. There is nothing wrong with empirically based philosophical explanation, indeed it is of great value, provided it is labeled for what it is. But, cosmologists should be very careful not make
methodological proposals that erode the essential nature of science in their enthusiasm to support specific theories as being scientific, for if they do so, there will very likely be
unintended consequences in other areas where the boundaries of science are in dispute.

It is dangerous to weaken the grounds of scientific proof in order to include multiverses under the mantle of `tested science’ for there are many other theories standing in the
wings that would also like to claim that mantle.

What must be done?

- Determine a viable set of criteria/ procedures for what makes a theory scientific
- Find what methods can adequately justify unobservable entities
- Apply to the multiverse case
- Apply to other contentious cases (astrology, Intelligent Design) to see how they pan out
- Put the enterprise on a solid philosophical basis.

[G F R Ellis (2006): “Issue in the Philosophy of cosmology”
In Handbook in Philosophy of Physics, Ed J Butterfield
and J Earman (Elsevier, 2006), 1183-1285.
http://arxiv.org/abs/astro-ph/0602280 ]

The multiverse: conjecture, proof, and science
George Ellis

arXiv:1208.5715

http://www.aei.mpg.de/~axkl/HermannF...ings/Ellis.pdf
Mordred
#58
Feb23-13, 12:01 PM
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P: 1,857
I thoroughly enjoyed those papers you posted. They are being added to my collection. Thanks for linking them
Chronos
#59
Feb23-13, 07:21 PM
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George Ellis is a well known critique of the multiverse hypothesis and his underlying objection is sensible - lack of testability. It is, however, unclear if the multiverse hypothesis is truly untestable. That remains to be seen. Gurzadyan and Penrose are still promoting CCC, which is a sort of temporal variant of the multiverse idea - On CCC-predicted concentric low-variance circles in the CMB sky, http://arxiv.org/abs/1302.5162
Chalnoth
#60
Feb23-13, 10:25 PM
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Quote Quote by Chronos View Post
George Ellis is a well known critique of the multiverse hypothesis and his underlying objection is sensible - lack of testability. It is, however, unclear if the multiverse hypothesis is truly untestable. That remains to be seen. Gurzadyan and Penrose are still promoting CCC, which is a sort of temporal variant of the multiverse idea - On CCC-predicted concentric low-variance circles in the CMB sky, http://arxiv.org/abs/1302.5162
I generally don't think that they have anything to offer here. Their first attempt at doing this was shockingly-bad. I didn't look at this attempt in too much detail, but I don't see how it's much better.

This was a much better, and better theoretically-motivated, search:
http://arxiv.org/abs/1210.2725

They didn't find anything definitively, but they did the analysis properly. I don't know if Planck will help or not for this sort of thing, however.

The discovery of cosmic strings would be another signature of a multiverse, because cosmic strings would be direct relics of a past spontaneous symmetry breaking event.
Chronos
#61
Feb24-13, 12:03 AM
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I agree Gurzadyan and Penrose is a stretch. Strictly an opinion, but, it appears cosmic strings are a long shot at present.
Chalnoth
#62
Feb24-13, 05:50 AM
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Quote Quote by Chronos View Post
I agree Gurzadyan and Penrose is a stretch. Strictly an opinion, but, it appears cosmic strings are a long shot at present.
Well, I don't know how much of a long shot they are. They are certainly very rare, given current observational evidence. There's no question about that. But it would be a very interesting discovery if we found even one in the visible universe.


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