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 — 10^500 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 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.

This article was originally published on

**The Conversation.**
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Physics