Is Our Universe One of Many? Here’s How We Can Find Out

By Eugene Lim, King's College London | September 3, 2015 12:33 pm

multiverse

The existence of parallel universes may seem like something cooked up by science fiction writers, with little relevance to modern theoretical physics. But the idea that we live in a “multiverse” made up of an infinite number of parallel universes has long been considered a scientific possibility – although it is still a matter of vigorous debate among physicists. The race is now on to find a way to test the theory, including searching the sky for signs of collisions with other universes.

It is important to keep in mind that the multiverse view is not actually a theory, it is rather a consequence of our current understanding of theoretical physics. This distinction is crucial. We have not waved our hands and said: “Let there be a multiverse.” Instead the idea that the universe is perhaps one of infinitely many is derived from current theories like quantum mechanics and string theory.

Quantum Implications

You may have heard the thought experiment of Schrödinger’s cat, a spooky animal who lives in a closed box. The act of opening the box allows us to follow one of the possible future histories of our cat, including one in which it is both dead and alive. The reason this seems so impossible is simply because our human intuition is not familiar with it.

But it is entirely possible according to the strange rules of quantum mechanics. The reason that this can happen is that the space of possibilities in quantum mechanics is huge. Mathematically, a quantum mechanical state is a sum (or superposition) of all possible states. In the case of the Schrödinger’s cat, the cat is the superposition of “dead” and “alive” states.

But how do we interpret this to make any practical sense at all? One popular way is to think of all these possibilities as bookkeeping devices so that the only “objectively true” cat state is the one we observe. However, one can just as well choose to accept that all these possibilities are true, and that they exist in different universes of a multiverse.

The String Landscape

String theory is one of our most, if not the most, promising avenue to be able to unify quantum mechanics and gravity. This is notoriously hard because gravitational force is so difficult to describe on small scales like those of atoms and subatomic particles – which is the science of quantum mechanics. But string theory, which states that all fundamental particles are made up of one-dimensional strings, can describe all known forces of nature at once: gravity, electromagnetism and the nuclear forces.

However, for string theory to work mathematically, it requires at least ten physical dimensions. Since we can only observe four dimensions: height, width, depth (all spatial) and time (temporal), the extra dimensions of string theory must therefore be hidden somehow if it is to be correct. To be able to use the theory to explain the physical phenomena we see, these extra dimensions have to be “compactified” by being curled up in such a way that they are too small to be seen. Perhaps for each point in our large four dimensions, there exists six extra indistinguishable directions?

A problem, or some would say, a feature, of string theory is that there are many ways of doing this compactification –10500 possibilities is one number usually touted about. Each of these compactifications will result in a universe with different physical laws – such as different masses of electrons and different constants of gravity. However there are also vigorous objections to the methodology of compactification, so the issue is not quite settled.

But given this, the obvious question is: which of these landscape of possibilities do we live in? String theory itself does not provide a mechanism to predict that, which makes it useless as we can’t test it. But fortunately, an idea from our study of early universe cosmology has turned this bug into a feature.

The cosmic microwave background. Scoured for gravitational waves and signs of collisions with other universes. NASA / WMAP Science Team/wikimedia

The cosmic microwave background. Scoured for gravitational waves and signs of collisions with other universes. Credit: NASA / WMAP Science Team

The Early Universe

During the very early universe, the universe underwent a period of accelerated expansion called inflation. Inflation was invoked originally to explain why the current observational universe is almost uniform in temperature. However, the theory also predicted a spectrum of temperature fluctuations around this equilibrium which was later confirmed by several spacecraft such as Cosmic Background Explorer, Wilkinson Microwave Anisotropy Probe and the PLANCK spacecraft.

While the exact details of the theory are still being hotly debated, inflation is widely accepted by physicists. However, a consequence of this theory is that there must be other parts of the universe that are still accelerating. However, due to the quantum fluctuations of space-time, some parts of the universe never actually reach the end state of inflation. This means that the universe is, at least according to our current understanding, eternally inflating. Some parts can therefore end up becoming other universes, which could become other universes, etc. This mechanism generates a infinite number of universes.

By combining this scenario with string theory, there is a possibility that each of these universes possesses a different compactification of the extra dimensions and hence has different physical laws.

Testing the Theory

The universes predicted by string theory and inflation live in the same physical space (unlike the many universes of quantum mechanics which live in a mathematical space), meaning they can overlap or collide. Indeed, they inevitably must collide, leaving possible signatures in the cosmic sky which we can try to search for.

The exact details of the signatures depends intimately on the models – ranging from cold or hot spots in the cosmic microwave background to anomalous voids in the distribution of galaxies. Nevertheless, since collisions with other universes must occur in a particular direction, a general expectation is that any signatures will break the uniformity of our observable universe.

These signatures are actively being pursued by scientists. Some are looking for it directly through imprints in the cosmic microwave background, the afterglow of the Big Bang. However, no such signatures are yet to be seen. Others are looking for indirect support such as gravitational waves, which are ripples in space-time as massive objects pass through. Such waves could directly prove the existence of inflation, which ultimately strengthens the support for the multiverse theory.

Whether we will ever be able to prove their existence is hard to predict. But given the massive implications of such a finding it should definitely be worth the search.

The Conversation

This article was originally published on The Conversation.

Top image by Jaswe/ Shutterstock

CATEGORIZED UNDER: Space & Physics, Top Posts
MORE ABOUT: the Universe
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  • John C

    The problem I (and many physicists and cosmologists) have with the multi-verse theory is it seems to be an all too convenient explanation for the extraordinarily unlikely physical values of our universe that have been discovered. Not to mention the fact that other universes are be definition forever closed off from our own and thus beyond the means of proving their existence scientifically. That makes the multi-verse a philosophical, not a scientific concept.

    It would be much more scientific to stick with what we can prove, which is that there is just one universe. And then try to explain how exactly the very odd physical values we observe arose.

    • ziff

      But to prove that there is only one universe, you have to disprove the existence of others, so same problem.
      I agree that at the moment it may be more philosophical, but there is good historical precedent. At one point, we thought there was only one sun- we didn’t have the technology to prove otherwise. Then we figured out our sun is one of billions in our galaxy, but there was only one galaxy. And then in turn we figured out there are billions of galaxies, etc.
      So in that light, maybe some day we’ll actually detect the existence of other universes (or not).
      It then raises another fun question- what’s beyond the multiverse? Heh.

      • http://www.mazepath.com/uncleal/qz4.htm Uncle Al

        “But to prove that there is only one universe, you have to disprove the existence of others” No problem, hence my comment, below.

      • Kevin A

        Infinite universes leave no room for anything else beyond.

        • Doug Huffman

          Your conception beggars infinity.

          An infinite space XOR time realizes all possibilities in all variations. We are unique in only this infinitesimal.

          • stefan

            Yep, this variations is an extra dimension we are blind to; running upon the surface of single manifestation like cockroach upon the surface of ball … even QM math screams the truth, majority “scientists” are reluctant to see the apparent… unfortunately (or fortunately) science attracts people with naturally low IQ…

        • stefan

          Why not, the infinities of higher power allows to have hierarchy among infinities…

    • Kevin A

      one, in of itself, is evidence of 2 or more.
      look around you…where else in nature do you see only one of a type of anything?

    • Kevin A

      They are only extraordinary and weird if you perceive them that way. How is something ought to exist extraordinary?

    • Courtney

      “Imagination is more important than knowledge. For knowledge is limited to all we now know and understand, while imagination embraces the entire world, and all there will ever be to know and understand” – Einstein

      • Luiz Claudio S Querido

        Does this mean we should believe the imaginary is real?

  • OWilson

    Opening a box to find a cat that is both dead and alive, is just a paradox, one of many, resulting from our limited mathematical logic applied to what we know of QM.. The set of sets (Russell) is another, an infinity of infinities, some greater, some lesser, is yet another.
    Postulating an infinite number of universes is the result of mathematical reasoning, and it works, as does the Big Bang, as does Inflation when the Big Bang suddenly “speeds up” at a very convenient time. :) as does Dark Matter, Dark Energy, all “invoked” to explain what we see, (as did the Ptolemy’s infamous epicycles. He mathematically explained how Mars could appear to be going forward and backward.)
    Mathematics can model every possible event, if all the input is valid.
    Where it fails, is when you must insert assumptions, rather than empirically observed facts. This applies to all but the most simple and contained chaotic system.
    The computer that can successfully model a chaotic state needs an infinity of input, as well as complete quantification of an initial state. That lies in the realm of fortune telling.
    It is yet beyond our capability to produce such a machine.
    So string theory, multiverses, cats both dead and alive, memes and other interesting speculations, are like the poster below says, venturing into the philosophical. Whatever gets the physicists through the night!

  • http://www.mazepath.com/uncleal/qz4.htm Uncle Al

    1) The multiverse exists.
    2) It interacts here.
    3) Interaction adds degrees of freedom to thermodynamics.
    4) Spectrometry.
    5) No multiverse.

    Again!

    1) 45 years of quantum gravitation and SUSY are empirically sterile.
    2) Boson photon vacuum rules fail toward fermionic quark matter. Parity violations, symmetry breakings, chiral anomalies, baryogenesis, Chern-Simons repair of Einstein-Hilbert action are vacuum trace chiral anisotropy toward hadrons.
    3) Opposite shoes within a vacuum left foot have different energies.
    4) Blow a cryogenic molecular beam of racemic D_3-4-oxatrishomocubane (rigid cage molecule, 8 chiral centers of 11 skeletal atoms, big dipole moment, facile gram synthesis) through a chirped-pulse FT microwave spectrometer. If rotational spectra (temperatures) of the racemate’s enantiomers are not degenerate, vacuum chirality toward matter is measured.
    5) Physics is quantitatively repaired.

    LOOK

  • http://network.nature.com/profile/U9E0B7EFA A. A. Aiya-Oba

    Cosmos is eternal entanglement (equator) of self-contradiction, absolute unity of relative infinite units, oneness of pairness.-Aiya-Oba (Natural philosopher and discoverer of Nature’s absolute logic).

    • http://www.mazepath.com/uncleal/qz4.htm Uncle Al

      Clap one hand, record it video and audio, play it back.

      (3,472,073)^7 + (4,627,011)^7 = (4,710,868)^7, falsifying Fermat’s Last Theorem. Go ahead, multiply it out…or demonstrate 7 + 1 does not equal 2.

      • Kevin A

        Actually it does equal 2 PAIRS of 4.

        • http://www.mazepath.com/uncleal/qz4.htm Uncle Al

          2^3, now you have three. Irrelevant. A fool tries to multiply it out. A wise man goes modulo-7 for the last digit

      • http://network.nature.com/profile/U9E0B7EFA A. A. Aiya-Oba

        Congratulations Uncle Al.- Aiya-Oba (Discoverer of Nature’s absolute logic and state).

  • http://www.librarything.com/profile/Bretzky1 Brett Champion

    What’s more likely: that there are in fact multiple universes in existence, or that the theories upon which that likelihood is based are wrong?

  • Veganomics

    “it is rather a consequence of our current understanding of theoretical physics.” This is just string theory propaganda, plain and simple. The whole idea that there is experimental evidence for all of this is just a big fat lie; it just isn’t true. Now, someday, yes, we might discover something, this is always true. Chances are it will be a new particle, a new theory, a new adventure, and we can look forward to this. In this universe.

  • Kevin A

    I agree the universe is not “uni”
    Nothing in nature can exist in such a state. Look around…where do you see 1 of any type of anything? There are subtle differentiations of those of the same type…but never just one of anything

  • Mike Richardson

    Fascinating thought experiment at the present, but perhaps one of the most profound discoveries in science if it can be proven. One of the gravity wave detector instruments referenced in the article, LIGO, is here in Louisiana near the town of Livingston. It’s recently undergone an upgrade to increase it’s sensitivity and give it a better chance of detecting gravitational waves. This should give a better understanding of black hole formation, neutron star mergers, and other massive distortions of space time, in addition to the detection of possible signatures from parallel universes. Though some may question the pursuit of such knowledge, apart from the intangible joy of discovery and learning more about how our universe (and yes, possibly others) works, there are bound to be many currently unforeseen benefits from this expanding pool of cosmic knowledge. After all, one hundred years ago, who knew how useful Einstein’s Theories of Relativity and atomic theory would be?

  • http://network.nature.com/profile/U9E0B7EFA A. Aiya-Oba

    Existence of our universe is rock evidence of existence of multiverse.

  • http://network.nature.com/profile/U9E0B7EFA A. Aiya-Oba

    Unity of infinity, equator (entanglement ) of self-contradiction, eternal oneness of pairness relativity, is Nature’s absolute logic and state, the self-creator of All in all (Cosmos).

  • Peter Geery

    Of course we live in a Multiverse, the celestial, the terrestrial, the infernal, purgatory… : )

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