Another universe might lurk only millimeters away from our universe,
but we wouldn't know it because it exists on its own membrane separated
from our membrane in some extra spatial dimension. Matter on the other
membrane would be invisible but could exert a gravitational effect and
would, in fact, constitute the "dark matter" for which astrophysicists
have sought for some years.
In a recent paper Paul Steinhardt (Princeton) and Neil Turok (Cambridge)
propose that the structure in our universe may well have come about
in the collision of two such membrane universes. All the historical
events in the life of our cosmos--initial big bang, subsequent expansion
of galaxies, even the currently observed accelerated expansion phase,
and finally a contraction into a "big crunch"--would be played
out in a recurring drama.
This cyclic cosmology (an extension of Steinhardt's "ekpyrosis"
theory; see Update
535) uses all the latest tools of string theory, accounts for the
"dark energy" supposedly firing cosmic acceleration, and would
have no need for an ad-hoc "inflationary" phase appended
to the standard big bang model to explain such cosmological features
such as the horizon problem (why the extreme edges of the visible universe
seem to be at the same temperature). (Sciencexpress,
25 April, soon to be in Science.)