  Independent studies by different authors have proposed that classicality may
be induced in quantum objects by cosmological constraints presented by an
expanding universe of finite extent in space-time. Cosmological effects on a
quantum system can be explored in one approach by considering an object at rest
in space with a universal Hubble expansion taking place away from it, and
developing a Schroedinger type governing differential equation incorporating an
intrinsic expansion speed. Wave function solutions to this governing equation
exhibit pronounced central localization. The extent of concentration of
probability depends on mass; objects with small masses tend to behave in a
delocalized manner as ordinary quantum objects do in a static space, while
quantum objects with large masses are concentrated into much smaller regions.
To develop a criterion for classicality, we consider that if the size of the
localized region of concentrated probability density is larger than the size of
the corresponding extended object, then quantum behavior could be expected;
whereas if the region of high probability density for the location of the
center of mass is smaller than the size of the object, the object would behave
in a more classical manner. The resultant size threshold for classicality
accords with results of other studies examining these issues based on
uncertainty relations and wave packets. This size threshold is informative for
the case of compact extended objects and, as the constraint applies to the
center of mass of the system, does not lead to inconsistencies for quantum
correlations between distant entangled quantum objects. While local decoherence
may lead to classicality under a variety of conditions, cosmologically induced
classicality would appear to cause fundamental limitations on quantum behavior
in our universe.
