Existence of multiple universe? In this youtube video, Prof. Michio Kaku mentions there could be multiple universes and if there are, then there could be a worm hole to some other universe.

My question is: We are not sure if the Multiverse exists, so how can we believe or have imagined that worm holes could exist and create a path to another universe?
We don't know. The Multiverse Theory is really just a hypothethis; sure, it makes sense, but it cannot be scientifically proven. We really have no reason to doubt that this is our only universe. Wormholes are just as hypothetical, since they would require infinite energy or negative energy to be created, neither of which we can get. Also, keep in mind that String Theory is, for the most part, unprovable. It cannot be confirmed through scientific experiments and is mostly guesswork.

Also, it will take trillions of years before the Universe gets too cold for life, and we will be long gone by then. However way you put it, it's unlikely that humans will ever reach other habitable planets before we go extinct, should our planet be destroyed. The closest potentially-habitable planet is nearly 480 lightyears from us – as in, it would take us 480 years to get there, if we travelled at the fastest speed in the Universe. We surely won't live to see the end of the Universe.