Andrei Linde is Professor
of Physics at Stanford University and the author of several important theories,
including the inflationary
universe theory, the inflationary multiverse theory and the
theory of eternal
inflation. These theories taken together envisage a multiverse with an
eternally growing fractal structure in which universes continuously arise, with
different laws of low energy physics operating in each of them.

In a 2009 paper (1) Linde and Vanchurin attempted to calculate how many universes there are in a multiverse. The initial calculation came to 10^10^10^7, a very large number and one which they admit could be wrong by many orders of magnitude. But they quickly point out that a multiverse has no meaning without an observer, if correctly viewed by quantum physics.

“One of the implications of this
result is that one can talk about the evolution of the universe only with
respect to an observer. In the limit when the mass of the observer vanishes,
the rest of the universe freezes in time. In this sense, the number of distinct
observable histories of the universe is bounded from above by the total number
of histories that can be recorded by a given observer. And this number is
finite.”

Their calculation takes into
account the total amount of information that can possibly be absorbed by a
human brain during its lifetime, which they deem to be about 10^16 bits, “which
means that a typical human brain can have about 10^10^16 different
configurations, which means that a human observer may distinguish no more than
10^10^16 different universes.” From this number they hope to calculate the probability
of a universe in which the laws of physics would allow intelligent life to
evolve, the so-called anthropic
principle.

So how many parallel universes are
there in a multiverse? As many as your brain can fathom.