Brian Greene's the Hidden Reality and Michio Kaku's Parallel Worlds fascinates me, it really gives a
grounding in the science and gets you excited about the possibilities different way a multiverse could
exist.
-----
I will start by giving the explanation for the universe's expansion from a purely scientific perspective
then I will consider the possibility and problems of the proposition that the universe is expanding by the
spontaneous creation of parallel and/or multiverses. Astronomers until recently believed that the universes
expansion was or will eventually slow down at which point the gravitational force of the universe's matter
would cause it to collapse on itself. However through careful study of distant galaxies scientists discovered
(by means of the Doppler shift in the light of distance galaxies which indicated that they were moving farther
away form us at an increased rate) that it was in fact speeding up.
----
For this reason they proposed the concept of dark energy or a cosmological constant to account for the
invisible energy that overcomes the gravitational pull of the universe. Now concerning parallel and/or multi
verses popping into existence you seem to have a different definition universe. Universe is everything in
existence so it will make little sense to speak of a universe within a universe. Even if you adopt the conven-
tional parallel/multiverse theory which describes multiple isolated universes operating by different laws, it
still will make no sense to state that the coming into existence of other universes completely isolated from
ours has any impact on its expansion.
----
So multiverse seems to me a highly speculative and pure philosophical (remains a philosophy). it is compelling
but not conclusive, YET it gives a pleasing occupation for a thinkers mind.