Some theories propose that there are more universes.
In one of them it is said that there are parallel universes, due to inflations after the big bang that created them.
But in the two-slit experiment is also known the multiverses theorie. The probability wave is not collapsing but each possibility is creating a universe on his own.
So they both speak about other universes. But is there a link between them? Or are that just completely different things/theories?
The inflationary theory you mention is probably eternal inflation. In this theory there is just one universe but different parts of it are causally disconnected i.e. the different parts cannot affect each other in any way. Whether these constitute a multiverse comes down to terminology. In principle there is a continuous spacelike straight line that links any two points in the multiverse, so I would not describe this as separate universes.
The theory you mention in connection to the two slits experiment is the many worlds interpretation of quantum mechanics. However the physical reality of the many worlds is a contentious issue. So once again whether this really counts as multiple universes is a matter of opinion.
