  I propose that if the universe was born as a baby universe on the other side
of the event horizon of a black hole existing in a parent universe, then the
corresponding white hole provides the absolute inertial frame of reference in
the universe. The principle of relativity then allows to construct an infinity
of other inertial frames. Consequently, this scenario could give the origin of
inertia and complete Einstein's general theory of relativity by making it
consistent with Mach's principle.
