Predictions and tests of multiverse theories

Don N. Page

Institute for Theoretical Physics, University of Alberta

23.1 Multiverse explanations for fine-tuning

Many of the physical parameters of the observed part of the Universe, whether constants of nature or cosmological boundary conditions, seem fine-tuned for life and us [1-4]. There are three common explanations for this. One is that there is a 'Fine-Tuner' who providentially selected the physical parameters so that we can be here. Another is that it is just a coincidence that the parameters turned out to have the right values for us to be here. A third is that the observed Universe is only a small part of a much vaster Universe or multiverse or megaverse or holocosm (my own neologism for the whole), and that the physical parameters are not the same everywhere but take values permitting us in our part.

These three explanations are not necessarily mutually exclusive. For example, combining a Fine-Tuner with coincidence but without a multiverse, perhaps the Universe was providentially created by a God who had a preference for a particularly elegant single universe which only coincidentally gave values for the physical parameters that allowed us to exist. Or, for a Fine-Tuner with a multiverse but without coincidences, perhaps God providentially created a multiverse for the purpose of definitely creating us somewhere within it. Or, for coincidence and a multiverse without a Fine-Tuner, if the Universe were not providentially created, it might be a multiverse that has some parts suitable for us just coincidentally. Or, it might even be that all three explanations are mutually true, say if God providentially created a multiverse for reasons other than having us within it, and yet it was a coincidence that this multiverse did contain us.

On the other hand, it seems conceivable (in the sense that I do not see any obvious logical contradiction) that the Universe is determined by some sort of blind necessity that requires both our own existence and a single

Universe or Multiverse?, ed. Bernard Carr. Published by Cambridge University Press. © Cambridge University Press 2007.

world with a single set of physical parameters. In this case, the Universe is not providential (in the sense of being foreseen by any God), but nor is our existence coincidental.

Thus, logically, I do not see that we can prove that any combination of the three explanations is either correct or incorrect. However, it does seem a bit implausible that none of these explanations is at least partially correct, and it also seems rather implausible that the large number of fine-tunings that have been noticed are mere coincidences.

I should perhaps at this point put my metaphysical cards on the table and say that - as an evangelical Christian - I do believe the Universe was providentially created by God, and that - as a quantum cosmologist with a sympathy toward the Everett 'many worlds' version of quantum theory - I also strongly suspect that the Universe is a multiverse, with different parts having different values of the physical parameters. It seems plausible to me that - in a quantum theory with no arbitrary collapses of the wave-function -God might prefer an elegant physical theory (perhaps string/M-theory with no adjustable dimensionless parameters) that would lead to a multiverse that nevertheless has been created providentially by God with the purpose of having life and us somewhere within it.

Although personally I have less confidence in string/M-theory than in either providence or the multiverse, nevertheless string/M-theory is very attractive. It does seem to be the best current candidate for a dynamical theory of the Universe (i.e. for its evolution, if not its state), and it does strongly appear to suggest a multiverse. Since string/M-theory has no adjustable dimensionless constants, if it predicted just a single set of parameters, it would seem very surprising if these parameters came out right for our existence. Thus, if string/M-theory - or some alternative with no adjustable dimensionless constants - were correct, it would seem much more plausible that it would lead to a multiverse, with different parts of the Universe having different physical parameters.

Indeed, string theorists [5-15] have argued that string/M-theory leads to an immense multiverse or landscape of different values of physical parameters and 'constants of nature'. It is not yet known whether the range of values can include the physical parameters that allow life, such as those within our part of the Universe, but that does seem at least plausible with the enormous range suggested in the string landscape or 'stringscape'.

One objection that is often raised against the multiverse is that it is unobservable. Of course, this depends on how the multiverse is defined. One definition would be the existence of different parts, where some physical parameters are different, but this just shifts the arbitrariness to the choice of this set of physical parameters. Obviously if some quantity which varies with position (such as energy density) were included in the set of physical parameters, then even what we can see could be considered a multiverse. But if we just include the so-called 'constants of nature', such as the fine structure constant and various other coupling constants and the mass ratios of the various elementary particles, then what we can observe directly seems to consist of a single universe. Indeed it would be rather natural - if ad hoc -to define a multiverse with respect to the physical parameters that have no observable variation within the part we can directly see. In this case, the multiverse becomes unobservable, and it becomes an open question whether parts of the Universe we cannot see have different values of these constants. Many would argue that it is a purely metaphysical concept that has no place in science.

However, in science we need not restrict our entities to be observable -we just want the simplest theory, whether using observable or unobservable entities, to explain and predict what is observable. One cannot test scientifically a theory that makes predictions about what is unobservable, but one can test a theory that makes use of unobservable entities to explain and predict the observable ones. Therefore, if we find a multiverse theory that is simpler and more explanatory and predictive of what is observed than the best single-universe theory, then the multiverse theory should be preferred. The success of such a multiverse theory itself would then give credence to the existence of the unobservable multiverse.

Another objection that is often raised against multiverse theories is that, naively, they can 'explain' anything and predict nothing, so that they cannot be tested and considered scientific. The idea is that if a multiverse gives all possible physical parameters or other conditions somewhere within the multiverse, then the parameters and conditions we observe will exist somewhere. Hence what we observe is 'explained' at least somewhere. On the other hand, if every possibility exists, then we cannot predict any non-trivial restriction on what might be observed. If a theory makes no non-trivial predictions, then it cannot be tested against observations, and it can hardly be considered scientific.

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