  Following the spirit of the equivalence principle, we take a step further to
recognize the free fall of the observer as a method to eliminate causes that
would lead the perceived vacuum to change its original state. Thus, it is
expected that the vacuum should be in a rigid Minkowski state or be uniformly
expanding. By carefully investigating the impact on measurement caused by the
expansion, we clarify the exact meaning of the uniformly expanding vacuum and
find that this proposal may be able to explain the current observations of an
accelerating universe.
