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From Wikipedia:
In mathematics, a ball is the space bounded by a sphere. It may be a closed ball
(including the boundary points that constitute the sphere) or an open ball (excluding
them).
We propose that some equations concerning the “balls”, can be related with various
parameters of some cosmological models as the “Multiverse” and the “Eternal
Inflation” linked to it, which provides that space is divided into bubbles or patches
whose properties differ from patch to patch and spanning all physical possibilities.
In 1983, it was shown that inflation could be eternal, leading to a multiverse in which
space is broken up into bubbles or patches whose properties differ from patch to
patch spanning all physical possibilities.
When the false vacuum decays, the lower-energy true vacuum forms through a
process known as bubble nucleation. In this process, instanton effects cause a
bubble containing the true vacuum to appear. The walls of the bubble (or domain
walls) have a positive surface tension, as energy is expended as the fields roll over
the potential barrier to the true vacuum.