String theory landscape
The string theory landscape refers to the collection of possible false vacua in string theory, together comprising a collective "landscape" of choices of parameters governing compactifications.
The term "landscape" comes from the notion of a fitness landscape in evolutionary biology. It was first applied to cosmology by Lee Smolin in his book. It was first used in the context of string theory by Susskind.
In string theory the number of false vacua is thought to be somewhere between 1010 to 10500. The large number of possibilities arises from choices of Calabi–Yau manifolds and choices generalized magnetic fluxes over varioushomology cycles.
If there is no structure in the space of vacua, the problem of finding one with a sufficiently small cosmological constant is NP complete.
This is a version of the subset sum problem.
Fine-tuning of constants like the cosmological constant or the Higgs boson mass are usually assumed to occur for precise physical reasons as opposed to taking their particular values at random. That is, these values should be uniquely consistent with underlying physical laws.
The number of theoretically allowed configurations has prompted suggestions that this is not the case, and that many different vacua are physically realized. The anthropic principle proposes that fundamental constants may have the values they have because such values are necessary for life (and hence intelligent observers to measure the constants). The anthropic landscape thus refers to the collection of those portions of the landscape that are suitable for supporting intelligent life.
In order to implement this idea in a concrete physical theory, it is necessary to postulate a multiverse in which fundamental physical parameters can take different values. This has been realized in the context of eternal inflation.
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