A brief History of Time: From Big Bang to Black Holes

Eternal inflation and the multiverse
The developments described above mean that in the last two decades,
inflation has been transformed from speculation into a cornerstone of
modern cosmology. But not everyone likes its conclusions, especially since
we now believe inflation likely gives rise to a vast number of universes,
known collectively as a multiverse.
As I mentioned above, inflation predicts that the universe will be nearly,
but not perfectly, uniform. The deviations from uniformity are imposed by
quantum mechanics, and have now been precisely characterized from
observations of the cosmic microwave background.
The very same quantum mechanical effect can give rise to the multiverse.
Inflation is driven by a strange type of energy that has antigravitational
properties; on average, the amount of this energy decreases as inflation
proceeds, until there is no longer enough and the accelerated expansion

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ends. But, in some regions of space-time, quantum fluctuations temporarily
reverse the overall trend. Such regions gain more energy and consequently
inflate for longer.
In 1986, the Russian–American physicist Andrei Linde calculated that, if
inflation starts at a sufficiently high energy, there will always be some place
where the fluctuations win: the energy remains high, and inflation continues
eternally. But there will be other places where the fluctuations lose, and the
expected trend of decreasing energy takes hold. Such patches become entire
individual universes such as our own. If we could zoom out far enough, we
would see countless other universes, separated by Linde’s regions of the
multiverse that are continuing to inflate.
Eternal inflation and the no boundary proposal together predict that our
universe is not unique. Instead, from the quantum fuzz at the big bang many
different universes emerge, possibly with different local laws of physics and
chemistry. We may not live in the most probable of all universes. Rather, we
live in one where the conditions are favorable for complexity and the
development of life. Even though we cannot go from one universe to
another, the successful predictions of the theory for observations in our
universe provide support for the world view predicted by the no boundary
proposal.
For a long time, many physicists brushed these arguments to one side.
The idea of a multiverse makes some people queasy, and they would rather
assume inflation to take place at a lower energy, so sidestepping Linde’s
argument. However, the latest observations from the Planck satellite make
this escapology trick look increasingly implausible.