Chapter 3

2

The case for an accelerated expansion of the universe is in fact so strong that it has made headlines in the popular press, e.g. [#!Ti01!#] and [#!Fa01!#].

3

In fact it is the dilemma of string/M-theory, that there is no underlying principle which aids in the understanding of the theory or equivalently with the words of Greene in [#!Gr99!#]: `as we look into the next stage in the development of string theory, finding its ``principle of inevitability'' -- that underlying idea from which the whole theory necessarily springs forth -- is of the highest priority'. Also recall Yoneya's words quoted in note [#!ch1hm6!#]. In effect string/M-theory is up to now `only' a mathematical formalism. In the often quoted words of Witten, string/M-theory `is a part of 21 $^{\text{st}}$ century physics that fell by chance into the 20 $^{\text{th}}$ century'; [#!Gr99!#]. Although, there is a glimmer on the horizon indicating that such a principle may exist. See footnote on page .

However, the biggest problem of string/M-theory is probably that to date there is absolutely no sign of experimental verification (or falsification) of its predictions. Hence there are no experimental guide lines directing or restricting the theory. Even supersymmetry faces this dilemma of maybe only being a mathematically consistent way out of conceptual problems with no relevance to reality. However, a small hint at the fact that supersymmetry may not be a totally alien concept to nature comes from calculations of the spectrum of heavy atoms, where standard theoretical models fail and a supersymmetrical approach^B.6 gives the observed energy levels; [#!Jo99!#].