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Have cosmologists lost their minds in the multiverse?

The recent BICEP2 observations – of swirls in the polarisation of the cosmic microwave background – have been proclaimed as many things, from evidence of the Big Bang and gravitational waves to something…

If there’s a multiverse out there can we see it? Flickr/Maciek Bielec, CC BY-NC-SA

The recent BICEP2 observations – of swirls in the polarisation of the cosmic microwave background – have been proclaimed as many things, from evidence of the Big Bang and gravitational waves to something strange called the multiverse.

The multiverse theory is that our universe is but one of a vast, variegated ensemble of other universes. We don’t know how many pieces there are to the multiverse but estimates suggest there many be squillions of them.

But (if they exist) there has not been enough time since our cosmic beginning for light from these other universes to reach us. They are beyond our cosmic horizon and thus in principle unobservable.

How, then, can cosmologists say they have seen evidence of them?

Seeing the unobservable

Unobservable entities aren’t necessarily out-of-bounds for science. For example, protons and neutrons are made of subatomic particles called quarks. While they cannot be observed directly, their existence and properties are inferred from the way particles behave when smashed together.

But there is no such luxury with the multiverse. No signals from from other universes have or will ever bother our telescopes.

While there is some debate about what actually makes a scientific theory, we should at least ask if the multiverse theory is testable? Does it make predictions that we can test in a laboratory or with our telescopes?

The answer is yes, but perhaps not as you’d expect. And the exploration of the multiverse theory involves some very complex, and very controversial, ideas.

The mark of the generator

If your multiverse theory generates its universes via some physical process, then that process may leave its fingerprints on this universe. This is what BICEP2 might have seen.

A LC-130 aircraft passing the NSF South Pole station Dark Sector which houses the BICEP2 telescope (centre). Steffen Richter, Harvard University, CC BY

Cosmologists think that in its earliest stages, the universe underwent an extraordinarily rapid expansion, known as inflation. In many versions of inflation, gravitational waves leave an imprint in fossil radiation, recently observed as characteristic swirls in this ancient light; a successful prediction of inflation.

In some versions of inflation, the process that causes our universe to inflate is expected to produce huge numbers of other universes. Evidence for inflation isn’t exactly direct evidence for the multiverse, but it’s a start.

A known generator

We cannot see the creation of other universes, but if we have evidence for the physics that powers the universe generator then we have another piece of the puzzle.

In particular, a multiverse theory that requires only well-tested physics such as gravity and quantum fields is preferable to one that requires new physics, or requires extrapolating known physics to situations where we expect them to break down.

Inflation’s scorecard is mixed: some of the underlying physics is known, some is hypothetical, and some worry that it skirts close to (or perhaps into) the quantum gravity regime, where all tested physical theories break down.

Observing our universe in the ensemble

Let’s think about prediction with a simple example. Alice predicts that a certain factory makes 99% red widgets, 1% blue. Bob predicts the opposite: 99% blue and 1% red.

A packet arrives from the factory and they open it to find a red widget – whose theory is correct? Neither theory is certainly false, but the evidence clearly favours Alice.

A multiverse theory will (by definition) predict the statistical properties of its universes. We can then ask whether our universe is the kind of universe one would expect to observe.

The more unusual our universe is, the more likely it is that a different multiverse theory would better explain our universe. And if our universe is just too weird for the vast majority of multiverse theories, then the whole idea of a multiverse comes under question.

It is thus relevant to ask: how typical is our universe of the set of possible universes?

There is one way in which our universe is highly unusual: it contains life. If our laws of nature were only slightly different then our universe would look and behave quite differently: atoms would fall apart, or the universe would have expanded so fast that stars and galaxies could not form.

Most cosmological scenarios would have left our universe stone-cold dead, devoid of life (as explained in the video below).

Life in a fine-tuned universe.

The multiverse can handle this. The probability of observing a particular type of universe depends on that universe first creating observers. We are not just passive observers, setting up our equipment and taking measurements of the universe at our leisure. We are products of this universe.

While universes with observers may be highly unusual in the entire multiverse, they will obviously be the norm for observed universes. And so, the life-permitting nature of our universe can be counted as a successful prediction of the multiverse. (Prediction in the logical, rather than chronological sense.)

Revenge of the Boltzmann Brains

Or can it? We’ve assumed that the most likely way for a universe to make observers is via suitable laws and biological evolution, as in our universe. Such a universe is probably extremely unusual in the multiverse. But what if just any old universe could get lucky and fluke a few observers?

Quantum mechanics, the same physics that predicts the inflationary fluctuations in the cosmic microwave background, seen by BICEP2, also predicts that there is an extremely tiny probability of a fully-formed brain spontaneously popping out of “empty” space. Given enough time and space this vanishingly improbable event will occur.

While such freak observers, known as Boltzmann Brains, would be massively outnumbered by biological observers in our universe, they could be common in the almost unending time and space of the entire multiverse.

In that case, the fact that we are not that kind of observer is like seeing the red widget – it is evidence against a multiverse theory that says we should expect to be freaky observers. The multiverse is not just testable; it might even fail.

Ifs and buts

At the moment, there are too many ifs and maybes in this story.

Observations do not uniquely favour inflation though the BICEP2 results are an impressive step in this direction. It is a matter of some debate whether inflation naturally generates a multiverse.

Further, many multiverse theories struggle to predict anything, so clearly there is much much more to be done.

But positing the multiverse is not, as claimed by some, the end of science. It may be the start of the biggest scientific adventure of all.

Join the conversation

40 Comments sorted by

  1. Rick Ryals

    logged in via Facebook

    "Have cosmologists lost their minds in the multiverse?"

    Absolutely, it's only a "dilemma" because cutting edge theory doesn't allow for the obvious classical solution to the problem.

    Flat, barely expanding universe maximizes work since energy has the maximum amount of time to do work before it goes inert. It is an energy conservation law, duh.

    Simple-stupid and obvious to all who cannot see the forest for the trees...

    It's a cosmological principle, just like the anthropic principle is "the" alleged" selection principle, possibly even related via the entropic efficiency.

    A near perfectly balanced structure that is perched precariously between diametrically opposing runaway tendencies in order to maintain said entropic efficiency in an accelerating universe.

    The Classical Hierarchy Solution

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  2. Ead Roberts

    Human

    "While universes with observers may be highly unusual in the entire multiverse, they will obviously be the norm for observed universes. And so, the life-permitting nature of our universe can be counted as a successful prediction of the multiverse."
    Tosh.
    Ever heard of a circular argument...? We observe the universe and propose a multiverse which entails there will be life in observed universes which we are, thus supporting the multiverse notion.
    People think. Some people think about unicorns. Unicorns have magical properties which make some people think of them, which some people do, therefore providing evidence that unicorns are real.

    If this is what passes for logic amongst multiverse cosmologists they might need to start begging money, not just questions... ;-)

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    1. Geraint Lewis

      Professor of Astrophysics at University of Sydney

      In reply to Ead Roberts

      I think you missed the point. People will propose mechanisms to generate a multiverse. If this mechanism cannot produce universes like our own, the mechanism is in trouble.

      Unicorns are not based in science.

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    2. Ead Roberts

      Human

      In reply to Geraint Lewis

      I think missing the point is in the eye of the person missing a point. My point is that the reasoning put forward is fallacious. Science is not supposed to be based on fallacious thinking...

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    3. Geraint Lewis

      Professor of Astrophysics at University of Sydney

      In reply to Ead Roberts

      I disagree.

      Science will consider multiple versions of the multiverse (scientific, that makes predictions for the kinds of universe that can exist in the multiverse).

      If a particular multiverse does not predict the existence of universes like the one we inhabit, we can effectively reduce confidence in that particular multiverse theory.

      Our existence can constrain scientific theories of the multiverse.

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    4. Ead Roberts

      Human

      In reply to Geraint Lewis

      Let's agree that a theory that predicts we don't exist is hardly a candidate for serious consideration :-) I'm not sure this adds anything meaningful to the discussion however. Certainly not to the underlying question begging contained in the way you have expressed it in the article.
      Unfortunately naive scientific realism is a dominant force even amongst cosmologists :-) This is why we end up with 96% of the universe (not to mention the potentially infinitely many multiverses) being made of 'stuff' we don't understand to fill in the gaps in the things we think we do understand.

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    5. Luke Barnes

      Super Science Research Fellow at University of Sydney

      In reply to Ead Roberts

      "a theory that predicts we don't exist is hardly a candidate for serious consideration"

      Right. And a theory that predicts that we are unlikely to observe what we observe is similarly in trouble. The multiverse, despite appearances, might be ruled out in such a way.

      I don't see the connection between naive scientific realism and dark matter and dark energy. Even the instrumentalist has to do something to her equations beyond the "ordinary matter and ordinary gravity" terms to account for observations of the accelerating expansion of the universe.

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    6. Ead Roberts

      Human

      In reply to Luke Barnes

      The connection is this: the 'observations' of there not being enough mass to explain galactic phenomena and the accelerating inflation of the universe are both inexplainable by the received cosmological theories. So rather than seeing the problem with the theories, instead the problem is seen to be with reality! The equations say there must be more mass and energy, so let's pretend there is. And let's adjust this quantity to suit any new 'observations' we happen to make. Never mind that this relegates…

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    7. Luke Barnes

      Super Science Research Fellow at University of Sydney

      In reply to Ead Roberts

      "So rather than seeing the problem with the theories, instead the problem is seen to be with reality! "

      Nope. The problem is with our theories. Specifically, our understanding of gravity and our census of the "known" universe does not fit with the way the universe is behaving. There is too little "known" matter to hold galaxies together, and no "known" form of energy will cause the expansion of the universe to accelerate.

      This is similar to the discovery of Neptune. Uranus was observed to move in a way that "gravity + known matter in the solar system" failed to predict, so there is some problem with our prediction. We needed to postulate a new planet - and then went looking for it.

      Also, energy is not conserved in cosmology. http://www.preposterousuniverse.com/blog/2010/02/22/energy-is-not-conserved/

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    8. Ead Roberts

      Human

      In reply to Luke Barnes

      Fascinating slippage there Luke. At one moment you concede the problem is with theory, then slip into the realism I alluded to and start saying that there is too little known matter to hold galaxies together and no known energy to cause accelerated inflation. This is precisely how dark matter and dark energy have come to be spoken of as 'real things' out in the universe, rather than human concepts attempting to plug theoretical holes. The same goes for multiverse theories. I believe this slippage…

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  3. Joe Gartner

    Eating Cake

    So lets get this straight... rather than confront that the statistically improbable universe we inhabit could be just that - real and a fluke - cosmolgists would rather explain this away with a multiverse that conveniently can't be directly observed; and, what's worse, are quite comfortable with brains spontaneously popping into existence.

    So spontaneously appearing brains are not weirdly improbable but the universe, as it stands, is weirdly improbable ....

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    1. Joe Gartner

      Eating Cake

      In reply to Luke Barnes

      I read the article, and thanks for the advice, but you beg the question in the article.
      You state that presence of a observers in the universe is evidence for the multiverse. Presence of observers in this universe does provides evidence for this universe, it only provides evidence of other universes if you cannot accept the statistical improbability of this universe..

      Boltzman brains only support or refute multiverses if you already accept that there are multiverses... otherwise the presence…

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    2. Mensur Omerbashich (HM King of Bosnia)

      Titular King of Bosnia and All of Illyria at Bosnian Royal Family, PO Box 1, Sarajevo Bosnia

      In reply to Geraint Lewis

      Mach was an erudite. According to original (ancient) definition, an atom is "the smallest part of matter". We named "atom" the first thing Boltzmann et al. thought was small enough to obey certain rules. But then we continued finding smaller things than the official "atom", didn't we. And continued...

      So who is to say that we have found the (Greek) atom? As a psychologist who knew human psyche inside-out without a need to resort to (a) Boltzmann brain, Mach's claim still seems unchallenged…

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    3. Luke Barnes

      Super Science Research Fellow at University of Sydney

      In reply to Joe Gartner

      "Improbability" is hypothesis dependent. A Royal flush is improbable if I'm dealing fairly but not as improbable if I'm cheating. The statistical improbability of this universe relative to what hypothesis? If it's the hypothesis that the constants of our universe were set by chance (or whatever) then there's no reason to just accept the low likelihood of our universe given that hypothesis. Maybe it's true, but we will only know that if we compare it to other hypotheses. The multiverse is such a competitor…

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    4. Joe Gartner

      Eating Cake

      In reply to Luke Barnes

      thanks for the reply, Luke.
      I understand your point but you still 'beg the question' in the article. I am not being pointlessly captious here - but you assert that the presence of an observer in this universe is evidence of the mutliverse... because a multiverse theory predicts that some universes will have observers and most won't. But the presence of observers only provides evidence of the multiverse if one first accepts the hypothesis, otherwise the more plausible explanation is that observers…

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  4. Fred Pribac

    logged in via email @internode.on.net

    Maybe cosmologists have lost their MOND in the multiverse.

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  5. Tony Smith

    logged in via Twitter

    Dreadful headline, but always a useful topic.

    I come to this from a couple of angles: the first history & philosophy of science focusing into philosophy of complexity; the second a long and deep comparison of cellular automata rule spaces which speaks to the notion of local (bubble by bubble) variation in the physics.

    Every time a step expansion of our horizon has been discovered, the discovery has been resisted, be it celestial spheres to solar system and galaxy or galaxy to expanding universe…

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  6. Antony Eagle

    Senior Lecturer in Philosophy at University of Adelaide

    The way that the multiverse hypothesis gets confirmed by evidence isn't actually particularly novel. Almost never is scientific inference deductive; it's almost always an inference to the most likely explanation of the observed evidence. In the case of the multiverse, the observed evidence is that some universe contains life (in particular, ours). And, assuming the multiverse hypothesis is set up in the right way — in particular, if it meets the condition that if some description of a universe is…

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    1. Anne-Marie O'Neill

      domestic

      In reply to Antony Eagle

      Thanks, Antony. Use of the term multiverse is already a biased gambit, as 'verse' points to some sort of spatiotemporal manifold. ('vertere' = turn) It might be less partisan to suppose an 'existence generator' affords every logically possible set of axioms and primitives, an infinitesimally small proportion of which amount to any sort of plenum.

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    2. Ead Roberts

      Human

      In reply to Anne-Marie O'Neill

      I always thought universe came from sanskrit - translating roughly to 'one song' based on the original notion of reality being premised on a sound (many Christians refer to it as a Word).

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    3. Luke Barnes

      Super Science Research Fellow at University of Sydney

      In reply to Antony Eagle

      "But what sense of 'luck' could this be?"

      Objective Bayesian probabilities. Such probabilities are not about physical processes or frequencies, but about the degree to which propositions imply other propositions. A good summary can be found here: http://arxiv.org/abs/0908.3212 .

      So we ask: Suppose all I knew were the laws of nature as we understand them, but without the values of the constants, and that single universe exists that is described by these laws. What support does this give to the proposition that life exists? Given the life-tuning of the constants, we should consider it very unlikely that life exists. There is no need to appeal to probabilistic processes or physical propensities or frequencies or meta-laws under which the constants actually vary.

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    4. Antony Eagle

      Senior Lecturer in Philosophy at University of Adelaide

      In reply to Luke Barnes

      Two points. (1) Objective Bayesianism might be attractive if there was any plausible way of uniquely determining a priori degrees of entailment. Unfortunately, all of the recipes proposed (MaxEnt and Solomonoff-inspired algorithmic probability prominent amongst them) involve substantive empirical presuppositions about the structure of permissible priors. (The paper you linked to is a nice justification of the standard probability axioms as generalisations of conjunction and disjunction, but of course…

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    5. Luke Barnes

      Super Science Research Fellow at University of Sydney

      In reply to Antony Eagle

      "substantive empirical presuppositions about the structure of permissible priors." The problem if prior probabilities is difficult but that doesn't mean that the whole enterprise fails. The goal is to do the best we can with the information we have. (Do you have a link to a paper discussing the empirical presuppositions of MaxEnt and Solomonov). The problem of prior probabilities is not unique to fine-tuning.

      "if I don't permit the constants to vary freely ...". Wrong question. My prior information…

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    6. Antony Eagle

      Senior Lecturer in Philosophy at University of Adelaide

      In reply to Luke Barnes

      My favourite paper on problems for objective Bayesianism (old but still incisive): Teddy Seidenfeld (1979), 'Why I am not an objective Bayesian', Theory and Decision 11 (4):413-440, doi: 10.1007/BF00139451

      On the second point, what I still don't quite see is why I ought to suspend what I know about the constants of nature, and yet not suspend what I know about the form of the laws, in thinking about what the probabilities for different possibilities are. Of course we're going to have to bracket…

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    7. Luke Barnes

      Super Science Research Fellow at University of Sydney

      In reply to Antony Eagle

      * First article seems to be a problem with MaxEnt rather than Bayesianism per se, though I'll have to give it a read. Of course, if the Bayesian approach can't justify it's priors then the whole thing falls over.

      * I use the constants of nature as a particularly well studied and perhaps more easily understood example. But in principle we'd want to consider varying the forms of the equations as well. We can draw some tentative conclusions along these lines. See my review article (http://arxiv.org/abs/1112.4647

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  7. Mensur Omerbashich (HM King of Bosnia)

    Titular King of Bosnia and All of Illyria at Bosnian Royal Family, PO Box 1, Sarajevo Bosnia

    "While there is some debate about what actually makes a scientific theory, we should at least ask if the multiverse theory is testable? (...) The answer is yes, but perhaps not as you’d expect. And the exploration of the multiverse theory involves some very complex, and very controversial, ideas."

    Not so controversially -- and in not such a complex manner at all (involving an absolute generalization of resonant gravity effects) -- but Multiverse (of Tegmark's Type II) was proven mathematically…

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  8. Nick Holmes

    retired environmental scientist

    This is an area where ordinary language isn't good enough. For instance, what's this business of light from other universes being beyond our cosmic horizon? Surely, by definition, multiverses are separate, so there's nothing for light to travel through to our universe. Language problems.

    The notion of Boltzmann brains seems suspiciously like the monkeys and a manuscript for Hamlet. Yeah, in an infinite series of universes anything can happen that is allowed by the laws of physics (supposing that…

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    1. Nick Holmes

      retired environmental scientist

      In reply to Geraint Lewis

      Thanks, Geraint. Nice diagram and makes sense even to a non-cosmologist like me. But my point was really about the possibility of transfer of information between universes in the multiverse. I have been assuming that each such universe is "bounded" by non-space and non-time (with the corollary that no information can be transferred between universes) and that our particular universe is that originating in the Big Bang. Have I got that wrong: was the BB was the origination of all universes in the…

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    2. Mensur Omerbashich (HM King of Bosnia)

      Titular King of Bosnia and All of Illyria at Bosnian Royal Family, PO Box 1, Sarajevo Bosnia

      In reply to Nick Holmes

      I said Newton defined his own physics (not gravity) while using certain units, and in doing so he also attached them units to a constant as if it too were a physical quantity. This appeared to yours truly as a silly move, but was undoubtedly the only practical solution at all for Newton to close his physics.

      So that was perfectly fine, as everyone is entitled to think up his/her own ways (even throne claimants, mind you), and have those ways accepted for as long as it made sense mathematically-physically.

      Newton's mechanics turned out to be largely coincidental to that of our known universe. Little did he know that some day some pretender would throw his eye beyond Newton's horizon, and look at things strictly the way they are, without ever making any compromise or resorting to stand-alone thought experiments.

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  9. Greg Field

    logged in via Facebook

    Dear authors, just to clarify, when you use the term "multiverse theory", should this be "multiverse hypothesis"? Or are you stating that multiverse theory is a FACT, like gravitational, heliocentric or evolutionary theories are FACT?
    (Please excuse my limited, unlearned understanding of what seems to be a fascinating subject) :)

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    1. Luke Barnes

      Super Science Research Fellow at University of Sydney

      In reply to Greg Field

      We're using theory and hypothesis interchangeably. They are distinguished from observations. ("Facts" is too loaded).

      To call something a theory is to imply nothing about how much evidential support it has. For the theory of gravity - loads of support. For the multiverse theory - maybe some, stay tuned.

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  10. Edward Binns

    Host of Ethics Seminars, MBA/CPA

    I wonder if Dr Barnes and Dr Lewis will allow me to barge into this conversation, even though I've forgotten most of my trigonometry that I learned in school decades ago.

    The cover article of the October, 2008, Scientific American was written by Martin Bojowald and was called "Big Bang or Big Bounce?: New Theory on the Universe's Birth." To oversimplify, the article postulated that the universe goes through a cycle of bang-bounce-bang-bounce-bang-bounce... every 30 billion years or so. That's…

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    1. Luke Barnes

      Super Science Research Fellow at University of Sydney

      In reply to Edward Binns

      That is one model for the multiverse. One could arrange the universes one after the other in time. The model would have to postulate not just a bouncing mechanism, but a means by which the constants of nature are randomised (in some sense) at each bounce - otherwise each universe would be the same.

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  11. Comment removed by moderator.