In other words, the Swampland is the set of consistent-looking theories with no consistent ultraviolet completion with the addition of gravity.
The Swampland program aims to delineate the theories of quantum gravity by identifying the universal principles shared among all theories compatible with gravitational UV completion.
In quantum gravity, a local structure of observables is emergent rather than fundamental.
Moreover, in quantum gravity, it is believed that different spacetime topologies can contribute to the gravitational path integral, which suggests that spacetime emerges due to one saddle being more dominant.
Moreover, in quantum gravity, UV and IR are closely related.
In addition to general arguments based on black hole physics, developments in string theory also suggests that there are universal principles shared among all the theories in the string landscape.
[2][3][4] The criteria are often motivated by black hole physics, universal patterns in string theory, and non-trivial self-consistencies among each other.
The original motivation for the conjecture goes back to black holes.
Therefore, it is believed that the process of black hole formation and evaporation violates any conservation, which is not protected by gauge symmetry.
Therefore, it is believed that higher-form global symmetries are also excluded from quantum gravity.
Cobordism conjecture states that there must be a dynamical process which connects the two backgrounds to each other.
In other words, there must exist a domain wall in the lower-dimensional theory which separates the two backgrounds.
[8] This conjecture is universally satisfied in string theory, but is also motivated by black hole physics.
Since the exponential of entropy counts the number of states, the non-zero entropy of black holes suggests that for sufficiently high charges, any charge is realized by at least one black hole state.
The completeness of spectrum hypothesis is closely related to the no global symmetry conjecture.
[10] The weak gravity conjecture postulates that every black hole must decay unless it is protected by supersymmetry.
gauge symmetry, there is an upper bound on the charge of the black holes with a given mass.
However, whether or not a black hole with a charge and a mass that exactly satisfies the extremality condition exists depends on the quantum theory.
Weak gravity conjecture can be generalized to higher-form gauge symmetries.
The generalization postulates that for any higher-form gauge symmetry, there exists a brane which has a charge-to-mass ratio that exceeds the charge-to-mass ratio of the extremal branes.
String dualities have played a crucial role in developing the modern understanding of string theory by providing a non-perturbative window into UV physics.
An example of this is T-duality, where there are two dual descriptions to understand a string theory with an internal geometry of a circle.
However, each perturbative description becomes valid in a different regime of the parameter space.
The circle's radius manifests itself as a scalar field in the lower dimensional theory.
The new description includes a tower of light states corresponding to the Kaluza-Klein (KK) particles.
T-duality is the statement that there exists an alternative description which captures these light winding states as KK particles.
Note that in the absence of a string, there is no reason to believe any states should become light in the limit where the size of the circle goes to zero.
If one takes the vacuum expectation value of the scalar fields to infinity, there exists a tower of light and weakly coupled states whose mass in Planck units goes to zero.
Moreover, the mass of the particles depends on the canonical distance travelled in the moduli space
[12] In other words, the leading tower of states can either be understood via dimensional reduction of a higher dimensional theory (just like the example provided above) or as excitations of a weakly coupled string.