The idea of a multiverse is pretty familiar
from sci-fi, but how much of it is science
and how much is fiction? There are a few different
theories in physics around the multiverse.
Let’s take a look at the ones that aren’t
completely out of this world.
Number one:  The Cosmological Multiverse
The easiest one to explain 
is called the cosmological multiverse.
The idea here is that the universe
expanded at a crazy fast speed
in a fraction of a second after the big bang.
As this happened
there were quantum fluctuations that caused
separate bubble universes to pop into existence.
Each one then started inflating itself and
creating more bubbles. These universes
were no longer causally connected with one
another – so they were free to develop in
different ways. Even with different laws of
physics, and maybe even extra dimensions.
Number two.
String theory is one way physicists have attempted
to unite the universe under one set of very
complicated rules. However, it requires some
serious theoretical reimagining of reality
to make it work and it predicts a frankly
ridiculous number of universes, maybe 10 to
the 500 or more, all with slightly different
physical parameters. The calculations make
sense in theory, but it’s notoriously difficult
to test these ideas in reality.
Number three.
And then there’s the quantum multiverse.
This idea was put forward by physicist Hugh
Everett, who came up with the “many worlds”
interpretation of quantum physics. Everett’s
theory is that quantum effects cause the universe
to constantly split. It could mean that decisions
we make in this universe have implications
for other versions of ourselves living in
parallel worlds.
Number 4.
Physicists recently think they may have spotted
evidence for a parallel universe going backwards
in time. It’s pretty tentative but here’s
the idea. The big bang might have actually created
two different universes – one made mostly matter
– that’s us – and one made 
mostly antimatter. If this theory is correct,
it predicts that there should be a new typr of particle
called a right-handed neutrino. Now two observations
from an experiment in Antarctica may have
seen one.
Alternative explanations have been put forward
to account for these observations, but all
of them have been ruled out. What’s left
is a theory suggesting a topsy-turvy universe
was created in the same big bang as our own
and exists in parallel with it. A mirror world.
In this mirror world, positive is negative,
left is right, up is down and time runs backwards.
It could be the most mind-melting idea ever
to have emerged from the Antarctic ice – but
it might just be true.
So if a single vast, ancient and mysterious
universe isn’t enough for you, there are
various theories and even some evidence that
there are multiple universes out there. Unfortunately
it looks like we won’t ever be able to communicate
between universes, let alone visit them.
What do you think – are there other versions
of you out there? Let us know in the comments
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