  The discovery of accelerating expansion of the universe has led us to take
the dramatic view that our universe may be one of the many universes in which
low energy physical laws take different forms: the multiverse. I explain
why/how this view is supported both observationally and theoretically,
especially by string theory and eternal inflation. I then describe how quantum
mechanics plays a crucial role in understanding the multiverse, even at the
largest distance scales. The resulting picture leads to a revolutionary change
of our view of spacetime and gravity, and completely unifies the paradigm of
the eternally inflating multiverse with the many worlds interpretation of
quantum mechanics. The picture also provides a solution to a long-standing
problem in eternal inflation, called the measure problem, which I briefly
describe.
