# Probability of stars in a multiverse

by jimjohnson
Tags: galaxies, multiverse, stars
 P: 84 Having recently read two books (Just Six Numbers by Martin Rees and The Fallacy of Fine-Tuning by Victor Stenger) I am confused on one issue. Assuming a multiverse, what is the probability that stars and galaxies would form in a universe. Stenger would say it is high but Rees would say it is low. Based on the clarity of arguments, my vote would be with Rees. What do others think?
 P: 1 In any system where the individual parts still act on each other with gravity it would only be a matter of time. Especially when you know how the early stars were formed.
 Sci Advisor PF Gold P: 9,182 I'm no fan of multiverse theories, but, assuming it is correct, we still have no clue how probable structure formation [stars, galaxies, etc.] may be. Assuming the fundamental constants of nature are free to arbitrarily choose any value at the beginning of any given universe, I agree the probability appears vanishingly small. But, using the infinitude of alternate universes as an excuse to dismiss the odd coincidence our universe happens to be just right for structure formation sounds a little hand wavy to me.
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## Probability of stars in a multiverse

I agree with your comment but wanted to clarify the authors positions. Of the six numbers in Rees' book, all appear to be critical for the formation of stars and galaxies: N - the ratio of gravitation and electromagnetic forces (1039); ε -the percent of energy released in hydrogen to helium conversion (0.7%); Ω - the ratio of actual density to critical density (0.3); λ -the cosmological constant (0.7); Q - proportion of galaxy rest mass needed to disperse galaxies (10-5); and D - the number of dimensions (3). The first two are basic forces. The second two are relate to energy and expansion. The last two are properties of space. Rees says that changing any one of these independently would not produce stars as we know them. Stenger says changing two or more may produce a stable environment because of the way they interact. Analysis appears to be very technical physics. Anyway, something to think about.
 Sci Advisor PF Gold P: 9,182 My suspicion is the fundamental constants of nature are somehow intimately related, not free to arbitrarily assume random values. On that basis, I would hazard to guess structure formation is more likely than not in most 'multiverses'.
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 Stenger says changing two or more may produce a stable environment because of the way they interact.
I have the Reese book. I have not seen Stenger's view.....which seems interesting.

Either our universe is 'one a kind never to be repeated' or we are in just one of perhaps an infinite number of universes, some habitable, some not. A related view which I have seen and find appealing is that from the universes which are born only those universes that can evolve do so, and of those, only the ones that can lead to new universes survive in the long run. If you can't have babies, your species dies out.
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 Quote by Chronos My suspicion is the fundamental constants of nature are somehow intimately related, not free to arbitrarily assume random values. On that basis, I would hazard to guess structure formation is more likely than not in most 'multiverses'.
There's no real reason to suspect that. And everything we learn about the universe is pushing us in the other direction entirely. Though granted it is true that what we know now about the necessary physics is far too little to make a strong determination, what little we do know seems to be pushing in the direction of no such relationship that makes structure formation (or life) likely.

Also, we can infer nothing whatsoever about the likelihood of structure formation from the fact that we observe it: the probability that intelligent observers will observe structure formation is precisely equal to one, because without structure formation there can be no intelligent observers in the first place.

As far as what fraction of the universe beyond our cosmological horizon has structure formation? Well, we don't know. I suspect that fraction is very small indeed, based upon the very little that we do know about the topic.
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Chalnoth:
 As far as what fraction of the universe beyond our cosmological horizon has structure formation? Well, we don't know. I suspect that fraction is very small indeed, based upon the very little that we do know about the topic.
I get the first two sentences, but not the last.... Why might that be? Too young to have evolved much??
 P: 16 "Multiverse" theories are realy speculative atm. There is no way to tell if something is going to happen when you don't know anything about it. Structure formulation would depend on the laws of physics. If you believe that the laws of physics and all the constants except the cosmological constant are the same in the "multiverse" then our universe would be exception because if you change the cosmological constant by a little it affects the structure formulation in the universe by a lot. However if you believe that some other constants or laws of physics are not the same then you can't say that because you can achieve other combination of values of the constants such that the universe has galaxies and stars even if it has much different cosmological constant.
 P: 121 First, Let's put the speculation on one side and 'ASSUME' it is true. As per chaotic inflation theory. Multiverse tend to stop stretching in some region. Different bubbles may experience different spontaneous symmetry breaking resulting in different properties such as different physical constants. Acc. to WIKI. "Linde and Vanchurin calculated the number of these universes to be on the scale of 10^10^10,000,000. I don't know the probability of stars forming due to fact that we can't be sure of the variation of constants/factors for star formation as mentioned by Chronos.
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 Quote by Naty1 Chalnoth: I get the first two sentences, but not the last.... Why might that be? Too young to have evolved much??
With slightly stronger gravity relative to the other forces, you get nothing but black holes. With slightly weaker gravity or a slightly larger cosmological constant no structures form at all. We don't know how likely these things are, but if these numbers vary much at all, star-bearing regions are probably extremely rare.
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 Quote by Sayajin "Multiverse" theories are realy speculative atm. There is no way to tell if something is going to happen when you don't know anything about it. Structure formulation would depend on the laws of physics. If you believe that the laws of physics and all the constants except the cosmological constant are the same in the "multiverse" then our universe would be exception because if you change the cosmological constant by a little it affects the structure formulation in the universe by a lot. However if you believe that some other constants or laws of physics are not the same then you can't say that because you can achieve other combination of values of the constants such that the universe has galaxies and stars even if it has much different cosmological constant.
This is why proper comparisons use only dimensionless numbers that aren't prone to these problems.
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 Quote by Sayajin "Multiverse" theories are realy speculative atm. There is no way to tell if something is going to happen when you don't know anything about it. Structure formulation would depend on the laws of physics. If you believe that the laws of physics and all the constants except the cosmological constant are the same in the "multiverse" then our universe would be exception because if you change the cosmological constant by a little it affects the structure formulation in the universe by a lot. However if you believe that some other constants or laws of physics are not the same then you can't say that because you can achieve other combination of values of the constants such that the universe has galaxies and stars even if it has much different cosmological constant.
 Quote by Chalnoth This is why proper comparisons use only dimensionless numbers that aren't prone to these problems.
i realize why we need to consider only dimensionless values for the fundamental constants of the universe to compare to other possibilities. but i do not understand why the need to consider dimensionless numbers has anything to do with what Sayajin wrote. he/she said nothing about units or the like.

but i do sorta object to saying that the Cosmological Constant is 0.7 . it is not 0.7 .

also, i thought that the whole idea of the concept of multiple universes is so that even if it is unlikely for some universal parameter to take on some necessary value for structure to form and eventually life that is intelligent enough to behold that structure, even if that is highly unlikely, it is not remarkable that we see such structure and such values for those universal parameters in the universe we behold. it's called selection bias, or specifically in this case, the Anthropic principle.
 Sci Advisor PF Gold P: 9,182 Im no more a fan of the anthropic principle than the multiverse conjecture. Both go to great [and rather fantastic] lengths to explain the universe we observe - and resist any observational constraints. I think there must be a simpler, less 'finely tuned' expanation.
 P: 84 I found two references addressing opposite views on the probability that stars/galaxies form. In the first, page 151 in The Hidden Realities, Greene quotes Weinberg on his argument for galaxy formation based on the value of the cosmological constant. His conclusion is that if it were a few hundred times larger there would be no galaxies. Using analogy he concludes that if E124 universes existed in a multiverse, then one like ours would be likely. The second reference is from Stenger's book page 227 and is based on the Principle of Mediocrity: " This implies that when we use physics to compute the possible range of a parameter, the value of that parameter should not be at the edges of that range but somewhere in the mediocre in-between." Not sure we will ever know.
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 Quote by Chronos Im no more a fan of the anthropic principle than the multiverse conjecture. Both go to great [and rather fantastic] lengths to explain the universe we observe - and resist any observational constraints.
The weak anthropic principle is necessarily true. It makes as much sense to object to it as to object to the statement that 2+2=4. To not take it into account when considering questions of, "Why these laws?" is foolish: it's a selection effect that must be considered to have a chance at arriving at the correct answer.

 Quote by Chronos I think there must be a simpler, less 'finely tuned' expanation.
Why?
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 Quote by rbj i realize why we need to consider only dimensionless values for the fundamental constants of the universe to compare to other possibilities. but i do not understand why the need to consider dimensionless numbers has anything to do with what Sayajin wrote. he/she said nothing about units or the like.
No, it actually does. For example, the reason why you don't get nothing but black holes now is not because of the absolute value of G, but rather of G's strength relative to the other forces: as long as gravity is weak enough that there is a regime where stable compact matter can exist without forming a black hole, we can have structure.

Gravity's strength compared to the other forces is around $10^{-40}$. If it were around $10^{-39}$ or so weaker, it would overwhelm the other forces and we'd have nothing but black holes.

 Quote by rbj but i do sorta object to saying that the Cosmological Constant is 0.7 . it is not 0.7 .
Sure. The 0.7 number is based on convenience, and is particular to our period of time. A more sensible number is the one based on its ratio compared to the Planck scale, approximately $10^{-120}$. If the dark energy were much higher, around $10^{-119}$ or so, the universe would expand too rapidly and no structures could form.

Anyway, my point is that if we take the space of all possible laws that we know of, the space in which structures can form only comprises a teeny, tiny subset of that space. Now, it is possible that there are details of the universe that we don't know which disallow or disfavor certain parts of the whole space. It's also possible that the fundamental laws of physics are richer and more varied than we currently imagine, leading to a much larger parameter space than a naive analysis based on the standard model would predict. Either way, work in high energy physics seems to be pushing that there probably isn't anything that really makes the low-energy laws we see the preferred ones in any sense.
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 Quote by Chalnoth No, it actually does. For example, the reason why you don't get nothing but black holes now is not because of the absolute value of G, but rather of G's strength relative to the other forces: as long as gravity is weak enough that there is a regime where stable compact matter can exist without forming a black hole, we can have structure. Gravity's strength compared to the other forces is around $10^{-40}$. If it were around $10^{-39}$ or so weaker, it would overwhelm the other forces and we'd have nothing but black holes. Sure. The 0.7 number is based on convenience, and is particular to our period of time. A more sensible number is the one based on its ratio compared to the Planck scale, approximately $10^{-120}$. If the dark energy were much higher, around $10^{-119}$ or so, the universe would expand too rapidly and no structures could form.
i think actually we agree on substance and are differing with semantics.

i would say that there is no comparison of $G$ (or $c$ or $\hbar$ or $\epsilon_0$) to any other situation. they're all just 1. including $G$ (actually, i think it's $4 \pi G = 1$).

what makes gravity so much weaker, from the POV of subatomic particles, is that the electric charge of any of these particles (if charged) is in the ballpark of the Planck charge, or the rationalized Planck charge, where $e = \sqrt{4 \pi \alpha}$ while the masses of any of these particles is far, far less than the Planck mass, like something like 10-19 or something like that.

i do not believe that there is some intrinsic parameter of free space that is $G$. the parameter $G$ is only a manifestation of the units we choose to measure things.

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