Who Are Some Prominent String Theorists?

Cosmology: does string theory lead to the swampland?

Many of those speculations about the marshland have prevailed against attack, and some today "have very solid theoretical foundations," says Hirosi Ooguri, a theoretical physicist at the California Institute of Technology and one of Vafa's first collaborators in the marshland. The weak gravity conjecture, for example, has accumulated so much evidence that it is believed to be universal. So it could apply regardless of whether string theory has anything to do with reality.

The border of the string landscape

But where exactly does the landscape end and where does the marshland begin? In answering this question, physicists fall back on decades of experience in the construction of string universes. The biggest challenge here is that string theory predicts the existence of ten space-time dimensions, i.e. six more than the three space dimensions and one time dimension that can be seen in our universe. It is commonly assumed by experts that the six extra dimensions at each point are rolled up so small that they cannot be observed.

The string landscape then arises from all the different ways in which these six additional dimensions can be knotted in different spacetime geometries. However, as Vafa and others have found, certain principles seem to apply here, favoring certain arrangements. For example, the coiled dimensions typically want to contract inward, while fields like that of electromagnetism tend to push everything apart.

In many of the known configurations of the six dimensions, from which simple and stable universes emerge, a negative, i.e. attractive vacuum energy is necessary to compensate for these effects, so you end up in an anti-de-sitter universe. A repulsive vacuum energy as we humans observe it in our cosmos, on the other hand, represents a great challenge for the world builders.

The KKLT paper by Kachru, Renata Kallosh, Andrei Linde and Sandip Trivedi suggested ways and means by which vacuums could still be constructed with positive and constant energy. The constructs involved, however, are complex, and over the years scientists have recognized that the tricks would potentially boost instabilities. Kachru says he has "no serious doubts" that stable de-sitter cosms can still be built with these aids. However, many researchers now see it differently and look skeptically at the statements of the KKLT scenario.

Impetus to act

Meanwhile, Vafa considers a concerted search for definitely stable de-sitter universes in the string landscape to be long overdue. With the assumption from his paper, he primarily aims to advance the topic. In his view, string theorists have so far lacked the drive to seriously consider whether their work can describe our world. Instead, they took the position that our universe was already hidden somewhere in the landscape, after all it was huge. "Most of the community is still on the side of the De-Sitter constructions," he said. The belief is widespread that there must be such worlds with positive vacuum energy in string theory, after all, we ourselves lived in one.

What is certain is that the American-Iranian physicist's assumption has stimulated the research community to act. Researchers like Wrase are looking for stable de-sitter counterexamples, while others play around with little-explored models of the quintessence. Vafa, on the other hand, wants to know whether his suspicion will hold up. "If we work towards evidence for or against it, we will certainly make progress," he says.