Swampland (physics)

In physics, the term swampland refers to effective low-energy physical theories which are not compatible with string theory, in contrast to the so-called "string theory landscape" of compatible theories. In other words, the swampland is the set of consistent-looking theories with no consistent ultraviolet completion in string theory.

Developments in string theory suggest that the string theory landscape of false vacua is vast, so it is natural to ask if the landscape is as vast as allowed by consistent-looking effective field theories. Some authors, such as Cumrun Vafa,^[1] suggest that is not the case and that the swampland is in fact much larger than the string theory landscape.

One of the principles of the Swampland program is that there are a finite number of quantum gravity backgrounds ... More precisely if we fix a cutoff we expect a finite number of string vacua. This in particular suggests that there is an upper bound on N the number of massless or light species modes allowed in a consistent quantum gravity theory. A heuristic explanation of this may have been that if there were no upper bound on N then the entropy of a fixed size black hole which should be at least N (as the black hole can be made up of such constituents) and this would lead to a contradiction if there were no bound on N. However, this reasoning is not quite correct.
In a quantum theory of gravity we expect that any effective theory breaks down for energies E ≥ M_P, where quantum gravitational effects become strong and invalidate the low energy effective theory. Therefore in any effective theory including quantum gravity we expect to have a cutoff Λ ≤ M_P. In some cases we expect Λ ∼ M_P. This is the case for example for M-theory in 11 dimensions. On the other hand there are also cases where Λ ≪ M_P. For example for 10d string theory the cutoff is given by the string scale Λ ∼ M ∼ g_S^1/4M_P which for g_s ≪ 1 is far below the Planck scale.^[2]

Swampland conjectures[]

The swampland conjectures are a set of conjectured criteria for theories in the string theory landscape. Some proposed swampland criteria:^[3]

• If there is a charge symmetry, that symmetry has to be a gauge symmetry, not a global one, and at least one charged particle must have a mass in Planck units less than the gauge coupling strength. However, not all charged particles are necessarily light. The same applies to magnetic monopoles as well. This criterion would imply the weak gravity conjecture.
• The sign of some higher order terms in the effective action is constrained by the absence of superluminal propagation.
• There are finitely many types of massless particles.

References[]

External links[]

• Lecture by Cumrun Vafa, String Landscape and the Swampland, March 2018

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