rationalskepticism.org

S. W. Hawking and Thomas Hertog wrote:In particle physics, one usually computes S-matrix ele-
ments. This is useful to predict the outcome of laboratory
experiments, where one prepares the initial state and mea-
sures the final state. It could be viewed as a bottom-up
approach to physics, in which one evolves forward in time
a particular initial state of the system. The predictivity of
this approach arises from and relies upon the fact that one
has control over the initial state, and that experiments can
be repeated many times to gain statistically significant
results.

But cosmology poses questions of a very different char-
acter. In our past there is an epoch of the early universe
when quantum gravity was important. The remnants of this
early phase are all around us. The central problem in
cosmology is to understand why these remnants are what
they are, and how the distinctive features of our universe
emerged from the big bang. Clearly it is not an S-matrix
that is the relevant observable1 for these predictions, since
we live in the middle of this particular experiment.
Furthermore, we have no control over the initial state of
the universe, and there is certainly no opportunity for
observing multiple copies of the universe.

In fact if one does adopt a bottom-up approach to
cosmology, one is immediately led to an essentially clas-
sical framework, in which one loses all ability to explain
cosmology’s central question — why our universe is the
way it is. In particular a bottom-up approach to cosmology
either requires one to postulate an initial state of the uni-
verse that is carefully fine-tuned [10] —as if prescribed by
an outside agency — or it requires one to invoke the notion
of eternal inflation [11], which prevents one from predict-
ing what a typical observer would see.

Here we put forward a different approach to cosmology
in the string landscape, based not on the classical idea of a
single history for the universe but on the quantum sum over
histories [12]. We argue that the quantum origin of the
universe naturally leads to a framework for cosmology
where amplitudes for alternative histories of the universe
are computed with boundary conditions at late times only.
We thus envision a set of alternative universes in the land-
scape, with amplitudes given by the no boundary path
integral [13].

The measure on the landscape provided by no boundary
initial conditions allows one to derive predictions for ob-
servations. This is done by evaluating probabilities for
alternative histories that obey a set of constraints at late
times. The constraints provide information that is supple-
mentary to the fundamental laws and act as a selection
principle. In particular, they select the subclass of histories
that contribute to the amplitude of interest. ...