We do not yet know what happened around the time of the big bang,
but cosmology offers many ideas for possible scenarios. This leads to two
important and complementary task: on the one hand to determine which one of
these scenarios is favored by observations, and on the other to see which
ones can be realized in fundamental theory, e.g. in string theory. The
mechanism of eternal inflation leads to a further conceptual leap: not only
might different cosmologies be possible in theory, they might in fact come
to co-exist in space and time. In this case, a new question emerges: which
one of this multitude of diverse universes are we most likely to inhabit?
