It was originally hoped that M-theory would predict all the constants of Nature uniquely. However, recent developments suggest that this is not the case and that the number of compactifications could be enormous (e.g. 10500
), each one corresponding to a different vacuum state and a different set of constants (Bousso and Polchinksi 2000
). This is sometimes described as the “string landscape” scenario. Each solution is associated with a different minimum of the vacuum energy and corresponds to a different universe, so the values of the physical constants would be contingent
on which one we happen to occupy (Susskind 2005
). A crucial feature of the string landscape proposal is that the vacuum energy would be manifested as what is termed a cosmological constant. This is an extra term in the field equations of general relativity, originally introduced by Einstein to make the universe static. One of the most exciting recent developments in cosmology has been the discovery from observations of distant supernovae that the expansion of the universe is accelerating. This suggests that the density of the universe is dominated by some form of “dark energy” and this is most naturally interpreted as a cosmological constant. It is this discovery that has attracted so many string theorists to the subject.