The multiverse as a scientific concept — part II

multiverseby Coel Hellier

[The first part of this essay can be found here.]


The comparison of cosmological models with high-quality and detailed observations of the early universe has led to the “inflationary” version of the Big Bang. This hypothesizes that, very early in the first second after the initial quantum fluctuation, only about 10-35 seconds later, our universe started an episode of exceptionally rapid exponential expansion, growing by a huge factor of about 1030 in only a tiny fraction of a second. This “inflation” was proposed in order to explain several otherwise puzzling features of the universe, including: (1) the similarity of the universe in opposite directions in the sky; (2) the exceptional smoothness of the cosmic microwave background (CMB); (3) the fact that the universe appears to have a very close balance between the total amount of matter and energy and the expansion rate, thus giving space a near-zero “curvature” on the largest scales; and (4) the absence of heavy particles such as magnetic monopoles that would otherwise be expected to be seen.

Thus inflation is motivated by strong empirical evidence [7]. Further, after inflation had been proposed, it was used to predict the spectrum of fluctuations expected in the CMB. These temperature fluctuations originate as quantum fluctuations in the inflating field, later to be frozen into the CMB, and the inflationary model gives specific predictions of their form and power spectrum. These predictions have now been compared to the better-and-better data from the WMAP and Planck satellites, and several ground-based experiments, and the result is again an exceptional agreement, giving strong support for inflation.

Further still, the inflationary model makes specific predictions about gravitational waves generated by the rapid inflationary episode. Recently the BICEP2 experiment has reported detecting these gravitational waves, imprinted on the CMB just as predicted, which would be an additional strong confirmation of inflation. These results are new and need confirming, but at a minimum they show that the inflationary model can be directly tested. Thus the inflationary Big Bang model is robust, mainstream cosmology. The mechanism driving inflation is, however, less understood.

Modern physics explains all known interactions in terms of four forces (gravity, electromagnetism, and the strong and weak nuclear forces), and at the exceptionally high temperatures in the very early Big Bang all four forces are expected to have existed in a “symmetric” state where they all acted similarly. As the universe expanded and cooled the symmetry was broken and the four forces developed the different characteristics that we see today. This process would have been analogous to the “phase transition” from a liquid (hot) state to a crystalline (cold) state. Such a phase transition has a “latent heat of crystallization” given out during the transition. It is, though, possible for material to get stuck in a “supercooled” state where it hasn’t yet made the transition, and thus has extra energy than expected for its temperature. It is this energy — thought to be associated with the phase transition that breaks up the strong and electroweak forces — that is thought to drive the ultra-rapid expansion of the inflationary era.

The tricky bit of inflationary models is then getting the universe to drop out of the “supercooled” inflationary state (rather than being stuck in that state forever), and thus give its energy into the hot Big Bang that then produces our universe. In order to get such models to work theorists have developed a scenario in which a quantum fluctuation can cause a limited region to drop out of the inflationary state, forming an expanding bubble of normal-state universe.

Thus, overall, we have our universe originating as a quantum fluctuation in the quantum-gravity era, at a scale of 10-43 seconds, leading to the exponentially expanding inflationary-state, followed by quantum fluctuations within the inflationary state, at a scale of 10-35 seconds, leading to bubbles of normal-state universe. However, in such a scenario, the inflationary-state stuff surrounding the bubble will be expanding vastly faster than the normal-state bubble, and thus the size of the inflationary-state regions continue to increase, even as bubbles continually drop out into the normal state.

The result is called “eternal inflation,” a “Swiss cheese” mixture in which bubbles of normal universe are continually forming out of a surrounding and exponentially expanding inflationary state. One of those bubbles would be our universe, and that bubble would now have expanded to vastly larger than our observable horizon. Thus, the only things we can now see are in our bubble, our normal-state universe.

This is a multiverse scenario. It says that somewhere beyond our observable horizon there is a transition, beyond which is supercooled, inflationary-state stuff. And in that rapidly expanding vastness are other bubbles, other universes, like ours, but now separated from us by unfathomable distances. If you don’t like the idea of a multiverse extending vastly beyond our observable horizon, or consider it to be unscientific, then realize that conventional cosmological models extend to infinity in much the same way. The stuff beyond our observable horizon is real, it is just a long distance away. There is no reason to declare such stuff not “real” just because of the finite value of the speed of light, which means that we humans can never receive information from those regions (especially since the location of that observable horizon depends entirely on where you are looking from, and it also continually recedes as you look at it).

The only sensible alternative to this multiverse idea is that our universe extends vastly beyond our observable horizon (to infinity?) and is normal-state all the way. Is that really preferable? If you do prefer normal-state all the way, then you have a problem in constructing an inflationary model that correctly makes the transition from the inflationary state to a normal state everywhere, especially given that any “transition front” would have a speed limited by c.

As it is, we have strong observational and theoretical arguments that lead us to an eternal-inflation model of the Big Bang, and that eternal-inflation produces a multiverse. It is currently rather difficult to produce a model of the Big Bang that works, and that explains the observations, and which does not automatically produce a multiverse. Admittedly we cannot observe those other universes, the other normal-state bubbles continually forming in the inflationary-state multiverse, but good, sensible and scientific reasons lead us to conclude that they likely exist. And, as stated early on, it is not necessary to empirically validate every prediction of a theory in order to have good confidence in that theory. To accept a theory and its implications all one needs is to have validated some of the predictions of the theory, and to have established that overall the theory does a better job than any alternative that we know of. As a comparison, no-one disputes the validity and scientific status of laws of gravity, that do an excellent job of predicting the times of future solar eclipses, just because we cannot verify those eclipse timings indefinitely into the future; and similarly it is not grounds to reject a model as unscientific just because we cannot verify its predictions indefinitely into the far distance.

Are the “physical constants” constant?

Now let’s ask a further question. Are those other bubble-universes in the multiverse just like ours? In what ways might they be different? How much scope is there for the bubbles to differ?

Above I used the analogy of a bubble dropping out of the inflationary state being akin to a liquid freezing. Consider a snowflake freezing in a high-up cloud. The freezing of each snowflake complies with the same underlying laws of physics, and yet each snowflake is different, with a related but distinct pattern. This tells us that some aspects of what we see are local “accidents,” variations allowed by the underlying laws but contingent on local circumstance.

The transition from inflationary state to normal state is thought to be due to the fundamental forces changing from a “symmetric” state, where they acted similarly, to a broken-symmetry state where they have different strengths. Further, physical “constants” such as the masses of particles and the values of electromagnetic charges are much the same thing as the strengths of the forces, since essentially they are all telling us how particles interact with each other. Thus we can ask whether the values of the masses and charges of particles and the strengths of forces are dictated by the fundamental physical laws, or whether they are local accidents, dependent on the local contingency of symmetry breaking early on in the Big Bang.

We don’t know the answer to that, but if it is the latter then we would expect each bubble universe to be different in the same way that each snowflake is different, and thus to have different physical constants. Note that — somewhat counter-intuitively — the latter suggestion is more parsimonious. The correct interpretation of Occam’s razor is in terms of the information content needed to specify a model [8]. If you have to explicitly specify the couple-of-dozen fundamental constants of the standard model of particle physics, that takes a lot of information. It takes less information to say that values for the constants are strewn around at random. After all, no-one claims that every snowflake having a different pattern is unlikely under Occam’s razor, since everyone accepts that the individual patterns are accidents, variations allowed by deeper-level rules.

As for falsifiability, it would be unscientific to add information to a model that had no observational motivation or consequences, and thus was unfalsifiable. However, if we are simply extrapolating from an observationally motivated model, or even reducing the information content of our model, while ensuring that the remaining information is observationally motivated, then that is scientific, even if not all of the implications of the model can be tested.

Considering this idea one quickly arrives at the obvious point that we human observers could only find ourselves in a bubble that had parameters suitable to have produced us, and it may be that the vast majority of such bubbles would be too alien to support or produce us. This is an entirely normal way of thinking. No-one nowadays supposes that there is some mechanism that ensures that an Earth-size planet gets placed in an orbit at the right distance from its star to allow it to have liquid water; we now know that extra-solar planets have a huge variety of orbits. We, of course, find ourselves on a planet suitable for us, but likely there are vast numbers of similar but uninhabited planets where the conditions are not right for life.

Similarly, no-one since Darwin would argue that we find polar bears in the Arctic and camels in deserts because that was carefully and deliberately arranged; rather, we understand that the local fauna is the contingent product of the local environment — a statement that applies just as much to the multiverse.

If you still find that line of reasoning unpalatable, consider that Steven Weinberg used it to predict that the value of the “dark energy” parameter in our universe would be small but non-zero. This prediction was made a decade before the observational detection of dark energy, at a time when most cosmologists assumed there was no dark energy. Then it was found, with a value in line with Weinberg’s prediction, but which is vastly smaller than given by attempts at calculating it from fundamental physics [9].

Weinberg’s verified prediction is currently the best explanation we have of the amount of dark energy in our universe. At this point, with an infinite extent of expanding inflationary-state stuff, dotted with island-universe bubbles, with each universe having different values for the physical constants, we have a full-blown multiverse of the sort to give critics a fit of the vapors. Yet, everything above is solid scientific reasoning, motivated and supported by observational evidence, and with already-demonstrated predictive power. That does not mean that it is true or proven, but it is a fully scientific concept and, regardless of the critics, the scenario is becoming increasingly accepted and mainstream among physicists.

A last remark: if the above is correct then in all likelihood we are near the middle of such a bubble and not near its edge. Yet there is some possibility that we are near an edge, and if we are then there would be nothing to stop us observing it if we looked far enough into the distant universe. In principle, then, the scenario is directly verifiable [10].


Coel Hellier is a Professor of Astrophysics at Keele University in the UK. In addition to teaching physics, astrophysics, and maths he searches for exoplanets. He currently runs the WASP-South transit search, finding planets by looking for small dips in the light of stars caused when a planet transits in front of the star. Earlier in his research career Coel studied binary stars that were exchanging material, leading up to his book about Cataclysmic Variable Stars.

[7] For an account of inflation and its observational motivations read this wiki page or this pdf of a chapter of Max Tegmark’s book Our Mathematical Universe.

[8] See this article of mine for a defense of Occam’s razor as a scientific concept.

[9] Naive calculations of the amount of dark energy expected give values about 10120 too big, perhaps the biggest error in the history of physics! Why is it that the actual value is 10120 times smaller than expected?

[10] And of course cosmologists are already looking for possible effects of “colliding bubbles”, which might be visible in the CMB. See this post for an example.


Categories: essay

Tags: , , , , ,

171 replies

  1. Dear Everyone,

    I’ve been mulling over the responses to my article and thinking about how people react to the multiverse idea. It seems to me that it is comes down to whether one regards the multiverse as an outlandish idea that really should only be considered given strong evidence, or whether one considers it be really quite unexceptional, and therefore a sensible idea to explore given only moderate pointers. Let’s break this down onto three elements:

    1) Stuff beyond our observable horizon, from where we can never get empirical verification. Well, there really is no alternative. There is no way of having a scientific model (whether multiverse or not) that does not continue past the observable horizon. Anyone not accepting that is quickly going to land themselves in far worse scientific and philosophical difficulties then simply extrapolating models beyond the observable horizon.

    This might be an issue for anyone taking a strict line on empirical verification and falsification, and thus wanting to declare any statement about anything beyond the observable horizon to be unscientific and thus metaphysical speculation, but most scientists will disagree and hold that such extrapolation is mundane and unexceptional (and that those taking a very narrow view on empirical verification and falsification are wrong), in exactly the same way that extrapolating models backward and forward in time is commonplace and unexceptional.

    2) The idea that distant regions (beyond the observable horizon) may be in a different “phase”. Well, *if* inflation holds, then this is merely a rather mundane and technical issue of the phase-transition from the inflationary phase to our current state. If we think about how a lake freezes, it never freezes uniformly everywhere at once, some parts get frozen while other parts are still liquid. That sort of thing holds throughout physics. Thus if we do have a transition from an inflationary state or our current state, then it is almost inevitable that there will be or will have been different regions of the universe in different states at the same time. Again, the basic idea there is as unexpectional as a partially frozen lake. The more outlandish idea would be that this does not happen.

    3) Some physical constants may vary. Over history, science has been a process of discovering which aspects of our environment are local contingent accidents and which are fundamental. For example, the strength of gravitational acceleration on Earth’s surface and the value for atmospheric pressure are local contingent accidents. We currently presume that things like the electron mass are not in that category, but are instead fundamental.

    But, it is hugely presumptuous to suppose that we know enough about physics to know for sure that something is in the “fundamental” category and not in the “local contingent accident” category. This fact is that we don’t know what determines the electron mass and the couple-of-dozen constants of the standard model. We simply can’t say whether they are fundamental and thus must have the same value everywhere in space and time.

    Given that, it is entirely reasonable to ask the question. It is the same sort of question we always ask when we don’t know. We don’t understand why the electron has that mass, so we ask, does something require it to have that mass, or could it be different? It would be remiss not to ask that.

    Given all of the above I don’t see anything outlandish about the “eternal inflation” multiverse idea. It is exactly the sort of thing we should be thinking about as we try to understand these things further. To try to disallow this on philosophical or aesthetic grounds seems peculiar. Of course we should apply appropriate skepticism when considering these ideas — as always in science — but it seems to me that the stance of “no you should not consider these possibilities” is the unscientific one.


  2. Hi Robin,

    Sorry, it’s just hard for me to get a handle on what you’re asking, but I think I do now:

    You are talking about in the context of a given universe, on Tegmark’s account of inflation, how can there ever actually be an infinite number of universes, since certain parts of the universe are created before others?

    In that case, the answer, as I understand it, is that simultaneity is not a concept that makes sense. We cannot ask how many universes there are right now, because “right now” is not well defined.

    Yes, Tegmark’s account seems to privilege certain parts of the universe as created before others, and it only seems that they are the same age to observers within the universe, who as they travel at light speed from an objectively “older” part of the universe to an objectively “younger” part have the universe continuously created before them so that by the time they get to their destination it looks just like it was always there.

    However, and this is just a suspicion, I think that there may be more ways of looking at an inflationary universe. My instinct rebels against the idea that any reference frames are really privileged, and my hunch is that we can take any part of the universe as our starting point for Tegmark’s explanation, such that there really are no objectively older parts of the universe. It would just depend on your frame of reference. This is very counter-intuitive and possibly incorrect, but keeping in mind how time and space can be warped on general relativity it seems plausible to me. This would perhaps also make nonsense of the idea of a linear flow of time outside of the perspective of a given universe.

    So, in my view, the way to think about it is to see the multiverse as one big timeless structure containing an infinity of universe. Time really only makes sense within particular universes and only then from specific reference frames. I hope that helps, though I suspect it probably won’t!


  3. Edit: in the second paragraph “how can there ever actually be an infinite number of *universes*” should read “how can there ever actually be an infinite number of *galaxies*”


  4. Coel,
    How many people are saying that we shouldn’t think about these sorts of things? I think most, like me, are fine with it, providing we label it as speculative. Whether you call it science, metaphysics, or whatever, I think the main thing to communicate is that these are possibilities. Someday, we may be able to move them into established fact. But until then, we should be honest about what we’re talking about.


  5. Coel,
    I think you’re completely missing the point, this is not at all about the multiverse being “outlandish”, it’s about whether it’s legitimate science at all, or not much different than theology. First of all, an important distinction is whether we’re talking about other universes with the same or different physics. My own reaction to the general idea that whatever is responsible for the Big Bang also could generate other Big Bangs and other disconnected versions of our universe with the same physics is that it’s perfectly plausible, not outlandish, but also not very interesting, since it answers none of the questions I’m interested in about where the laws of physics come from. The evidence for specific models that do this though is quite tenuous, and being heavily overhyped, which is a different problem.

    The more serious issue is the issue of models that produce different laws of physics, with the implication that these are environmental, so efforts to understand them are misguided. As you yourself admit, the models for which there is (tenuous) evidence say nothing about this, they tell us nothing at all about the parameters of the Standard Model. But what multiverse proponents are doing is making public claims to have evidence that the laws of physics are environmental. The problem is not that these claims are outlandish but that they’re wrong. There is no such legitimate evidence. The arguments being made for this are not legitimate, evidence-based science. What you and others are doing are finding yourself caught in defending kinds of arguments that can’t ever be tested, and the implications of doing this are serious. As Massimo is pointing out to you, in defense of dubious ideas, you’re already publicly trashing our understanding of evolutionary theory and the important distinctions there between it and religion. This is a very dangerous path, and people have headed down it for reasons (frustration with lack of progress on hard problems, unwillingness to admit that their ideas, e.g. string theory, don’t work) that are all too human, but scientifically unethical.


  6. Hold on, where is Coel trashing our understanding of evolutionary theory? You’re talking about his disagreement with Gould’s essay? Is Dawkins therefore also trashing our understanding of evolutionary theory where he makes basically the same points?


  7. I really wish people would stop referring to Dawkins as an evolutionary biologist. He is a (brilliant) science popularizer, but – unlike Gould, who continued to publish peer reviewed papers through the end of his career – Dawkins has stopped doing active research back in the ’70s. As a result, he has actually lost touch with the cutting edge, as evidenced by his dismissal, in a footnote of one of his recent books, of the entire field of epigenetic inheritance, of which he manifestly knows nothing at all.


  8. Hi Peter,

    My own reaction to the general idea that whatever is responsible for the Big Bang also could generate other Big Bangs and other disconnected versions of our universe with the same physics is that it’s perfectly plausible, not outlandish, but also not very interesting, since it answers none of the questions I’m interested in about where the laws of physics come from.

    OK, though I come to this from the perspective of an astronomer who is interested in Big Bang cosmology in its own right, so that sort of multiverse does interest me, even if it is unrelated to the issue of where the laws of physics come from.

    As Massimo is pointing out to you, in defense of dubious ideas, you’re already publicly trashing our understanding of evolutionary theory and the important distinctions there between it and religion.

    Am I really? How? I do agree with you that we need to be honest about what is and is not supported by evidence. I don’t agree that it is clear that we can never test ideas of the constants of physics being environmental. I think it’s a valid thing to consider and that, over time, it may be the case that it can be tested.


  9. I know he is not an evolutionary biologist. But you cannot accuse him of being someone who is ignorant of evolution (epigentics notwithstanding). I fully accept that Gould has much more credibility as an evolutionary biologist, but if Dawkins makes the same mistake that Coel and I make then at least we’re in pretty good company so it’s not down to simple ignorance or a commitment to defend dubious ideas.

    In any case, to say that Coel is trashing our understanding of evolutionary biology is going way too far. There is a dispute on which is the correct way of defending the ToE against the charge that evolution is a tautology. That’s a reasonably minor point, surely.


  10. DM,
    you are ignoring the substance of Peter Woit’s reply to Coel.
    You should rather reply to the substance and not chase a peripheral remark.


  11. Coel can defend himself against the substance of the reply. I picked on that one comment because it was particularly egregious in my eyes, and also because it could also be taken to apply to me since I joined Coel in his disagreement with Massimo.


  12. Coel,
    Sure, you can speculate about the idea that the laws of physics are environmental, but when you do that publicly you need to make it clear that you don’t have a viable testable model that does this. Hoping that someday you might have one is different and doesn’t count.

    About evolutionary theory, I’m no expert and defer to those that are. It just seems to me that when challenged that your argument is a tautology, to go for “hey, the evolutionary theorists do this too” is a really bad move.


  13. It is a bad move, and it doesn’t reflect what evolutionary theorists actually do (pace Dawkins), as I tried to explain.


  14. Hi Massimo,

    it doesn’t reflect what evolutionary theorists actually do (pace Dawkins), as I tried to explain.

    And I fully accept that, but It doesn’t reflect what cosmologists do either.

    Recall that we were not comparing inflationary theory to the theory of evolution but the anthropic principle to survival of the fittest. These are in my view both tautologies with implications inspiring more profound ideas which are not tautological (multiverses and evolutionary theory).


  15. DM, I still maintain we should stick to the substantive issues and not dilute the debate with side issues.


  16. Hi Peter,

    you can speculate about the idea that the laws of physics are environmental, but when you do that publicly you need to make it clear that you don’t have a viable testable model that does this.

    I do agree with you that we should all be clear and honest about these things. I might also agree that some could be criticised as being remiss on that (at some point I suspect I’m going to upset DM by declaring that Tegmark Level 4 and the MUH is utterly silly).

    About evolutionary theory, […] It just seems to me that when challenged that your argument is a tautology, to go for “hey, the evolutionary theorists do this too” is a really bad move.

    I don’t think that evolutionary theory is at all tautological. I do think that one particular phrase (“survival of the fittest”) that is made as a commentary on evolution is tautological. That in no way makes evolutionary theory tautological. I think this because it seems to me that the only sensible definition of “fitness” makes that phrase tautological.

    I can quote others who agree, e.g. here is Allen Orr: “In the crudest terms, fitness involves the ability of organisms— or, more rarely, populations or species— to survive and reproduce in the environment in which they find themselves” Surely Allen Orr is a decent enough cite?

    I’m still baffled as to how else one could define “fitness” (and indeed until this thread I considered this to be accepted by everyone nowadays).


  17. I really don’t think we need to rehash that discussion, I just think that putting the weak cosmological “principle” on the same level as the theory of natural selection is very strange. The latter is an actual scientific theory, the former is a trivial tautology.


  18. Hi Massimo,

    I know I must be annoying you, and I’m sorry. I genuinely don’t want to rehash it, and if you hadn’t articulated my position in a way I do not recognise I would not feel the need to clarify.

    , I just think that putting the weak cosmological “principle” on the same level as the theory of natural selection is very strange.

    I agree! This is not what I am doing. To attempt to clarify, just in case this has all been one big misunderstanding, I am putting the weak anthropic principle on the same level as “the survival of the fittest”, and not at all on the same level as the theory of natural selection.


  19. (at some point I suspect I’m going to upset DM by declaring that Tegmark Level 4 and the MUH is utterly silly)

    Don’t worry, I have every confidence in your ability to restrain yourself from making such a regrettable statement.


  20. So, in my view, the way to think about it is to see the multiverse as one big timeless structure containing an infinity of universe. Time really only makes sense within particular universes and only then from specific reference frames.

    I agree and this is why I was talking about causal sequence rather than time, which is what I think Tegmark means.

    For example the inflationary period is clearly “before’ any galaxies start to form in a causal sense.

    In any case it doesn’t matter, Tegmarks statement that inflation creates infinitely many galaxies “by going on forever” is probably inaccurate as it would be inaccurate to say that your apple analogy creates infinitely many rotten apples by “going on forever” (even if it did go on forever).

    I have found that I can never understand these things until I can grasp the actual mathematics, and that is a while away for me at the moment 🙂

    In any case it seems clear to me that nothing can create infinitely many of something by “going on forever”.


%d bloggers like this: