Scientists Are Searching for Collisions Between Different

8/18/2019 Scientists Are Searching for Collisions Between Different

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Scientists are searching for collisions between different ‘universe bubbles’ in the cosmicmicrowave background. Geralt

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The existence of parallel universes may seem like something cooked up by sciencefiction 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 haslong 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 thetheory, 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 israther a consequence of our current understanding of theoretical physics. Thisdistinction is crucial. We have not waved our hands and said: “Let there be amultiverse”. Instead the idea that the universe is perhaps one of infinitely many isderived from current theories like quantum mechanics and string theory .

The many-worlds interpretation

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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 thepossible 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 notfamiliar with it.

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

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

Miaaaaultiverse Robert Couse-Baker/Flickr, CC BY-SA

The string landscape

String theory is one of our most, if not the most promising avenue to be ableto unify quantum mechanics and gravity . This is notoriously hard becausegravitational force is so difficult to describe on small scales like those of atoms and

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subatomic particles – which is the science of quantum mechanics. But stringtheory, which states that all fundamental particles are made up of one-dimensionalstrings, can describe all known forces of nature at once: gravity, electromagnetismand the nuclear forces.

However, for string theory to work mathematically, it requires at least ten physicaldimensions. Since we can only observe four dimensions: height, width, depth (allspatial) 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 thephysical 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 eachpoint in our large four dimensions, there exists six extra indistinguishabledirections?

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 usuallytouted about. Each of these compactifications will result in a universe with

different physical laws – such as different masses of electrons and differentconstants of gravity. However there are also vigorous objections to themethodology 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, whichmakes it useless as we can’t test it. But fortunately, an idea from our study of earlyuniverse cosmology has turned this bug into a feature.

The early universe

During the very early universe, just after the Big Bang, the universe underwent

a period of accelerated expansion called inflation. Inflation was invoked originallyto explain why the current observational universe is almost uniform intemperature. However, the theory also predicted a spectrum of temperaturefluctuations around this equilibrium which was later confirmed by severalspacecraft such as Cosmic Background Explorer, Wilkinson Microwave AnisotropyProbe and the PLANCK spacecraft.

While the exact details of the theory are still being hotly debated, inflation is widelyaccepted by physicists. However, a consequence of this theory is that there must beother parts of the universe that are still accelerating. However, due to the quantumfluctuations 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 currentunderstanding, eternally inflating. Some parts can therefore end up becomingother universes, which could become other universes etc. This mechanismgenerates a infinite number of universes.

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

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The cosmic microwave background. Scoured for gravitational waves and signs of collisions with other

universes. NASA / WMAP Science Team/wikimedia

Testing the theory

The universes predicted by string theory and inflation live in the same physicalspace (unlike the many universes of quantum mechanics which live in amathematical space), they can overlap or collide. Indeed, they inevitably mustcollide, 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 inthe distribution of galaxies. Nevertheless, since collisions with other universes mustoccur 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 itdirectly through imprints in the cosmic microwave background, the afterglow ofthe Big Bang. However, no such signatures are yet to be seen. Others are lookingfor indirect support such as gravitational waves, which are ripples in space-time asmassive objects pass through. Such waves could directly prove the existence ofinflation, which ultimately strengthens the support for the multiverse theory.

Whether we will ever be able to prove their existence is hard to predict. But giventhe massive implications of such a finding it should definitely be worth the search
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Scientists Are Searching for Collisions Between Different