FQxI’s Eduardo Guendelman, a physicist at Ben-Gurion University of the Negev, in Israel, has published a new paper in The European Physical Journal C, which shows a certain exotic variety of string theories may be more conducive to describing our real universe compared with more conventional versions of string theory.
In the early 21st century, string theorists realised that string theory's equations give rise to a mind-boggling number of possible solutions–10500 different vacua, corresponding to a near infinite number of possible forms that a universe could take in a vast string theory landscape. Soon after it was realised that this landscape is surrounded by a so-called “swampland” of superficially viable-looking physical quantum field theories that turn out to be incompatible with quantum gravity, on closer inspection.
To delineate the landscape from the swampland, it was proposed that theories in the landscape must obey certain “swampland constraints.” The problem is that when conventional string theories satisfy these constraints, physicists find that they cannot easily reproduce inflation–the short burst of rapid expansion that our early universe is believed to have undergone–or dark energy, which is thought to be accelerating the growth of our universe today. “The more conventional string theories are very unfriendly to inflation, in particular to 'slow-roll scenarios,' even the existence of de Sitter space as a vacuum of the theory–the vacuum of our actual universe–which is the basis not only of inflation, but also of dark energy,” says Guendelman. “The swampland constraints are making cosmology impossible or almost impossible for the practical cosmologist because the real universe appears to be firmly in the Swampland of the conventional string theory.”
All strings have some tension, but in most conventional models the value of this tension is added in by hand, arbitrarily. Guendelman has been examining models in which the tension arises dynamically. His new paper describes the formulation of such a theory and shows that due to the dynamical nature of the tension, the swampland constraints are greatly weakened. This is because calculations deriving the constraints are related to the size of the so-called 'Planck scale'–thought to correspond to the smallest possible size of anything in the universe, including a string. But because the Planck scale is itself related to the string tension, in these models, the Planck scale itself becomes dynamical, says Guendelman. "In the regime where the dynamical tension, and therefore also the Planck scale, becomes very big, the constraints become irrelevant or very weak," says Guendelman. “So dynamical tension string theory is friendly to inflation and dark energy.”
Image credit: Adriana Makridou, "A Brief Introduction to String Theory Landscape and Swampland."