Another Big Problem For String Theory

A diagram of string theory dualities. Yellow arrows indicate S-duality. Blue arrows indicate T-duality. These dualities may be combined to obtain equivalences of any of the five theories with M-theory (Image and caption via Wikipedia).

A schematic illustration of the relationship between M-theory, the five superstring theories, and eleven-dimensional supergravity. The shaded region represents a family of different physical scenarios that are possible in M-theory. In certain limiting cases corresponding to the cusps, it is natural to describe the physics using one of the six theories labeled there (from the same source).

Another new paper has largely established that one popular to theorize about class of string theories (Type IIB) has no variation ("vacua") that are consistent with our observations that the universe has a positive cosmological constant (technically speaking "de Sitter space"), rendering fifteen years of research attempting to apply this class of string theories to the real world a waste.

There are several other classes of string theories, so all hope for string theory is not lost yet. But, even more concerning for sting theorists is the possibility that since all of the different classes of string theories have deep connections to each other though S-dualities and T-dualities, that bind them to a common M-theory, that this result might be later proved to hold for all versions of sting theory, effectively falsifying the entire string theory enterprise.

Another recent paper, which I blogged previously, has made similar claims, and explores the impact of this result on non-Type IIB string theories for which there has not been as much research regarding the question that seems to have made it possible to falsify Type IIB string theory.

This is on top of some other long standing problems with string theory, such as the non-detection of any experimental evidence for supersymmetry (which is an element of almost all classes of string theory), the non-detection of more the four dimensions of space-time (which is an element of almost all versions of string theory), and the lack of any identified string theory vacua that correspond to the Standard Model and general relativity compatible universe in which we live.

String theorists are hoping that it might be possible to solve the lack of deSitter vacua with quintessence, i.e. by putting something into the vacuum rather than changing the nature of the vacuum itself.

This also leaves me wondering about questions like: how different phenomenologically from each other are (1) classical general relativity from (2) a spin-0 massless graviton in flat Minkowski space (with all of the properties apart from spin-2 of a standard spin-2 massless graviton) from (3) a spin-2 massless graviton in flat Minkowski space from (4) a spin-2 massless graviton in de Sitter space. My intuition is that options (2) and (3) are much less different phenomenologically from (1) and (4) than is conventionally assumed.

There is a short anthology of the literature on this new problem at this site, which I reproduce in the pertinent part below the fold (without reformatting).

On the other hand, discussion casting doubt on the existence of a large number of de Sitter spacetimeperturbative string theory vacua includes the following:

• Tom Banks, The Top 10500$10^{500}$ Reasons Not to Believe in the Landscape (arXiv:1208.5715)
• Brennan-Carta-Vafa 17
• Ulf Danielsson, Thomas Van Riet, What if string theory has no de Sitter vacua? (arXiv:1804.01120)

Implications of the possible non-existence of de Sitter vacua in string theory are explored in

• Georges Obied, Hirosi Ooguri, Lev Spodyneiko, Cumrun Vafa, De Sitter Space and the Swampland(arXiv:1806.08362)
• Prateek Agrawal, Georges Obied, Paul Steinhardt, Cumrun Vafa, On the Cosmological Implications of the String Swampland (arXiv:1806.09718)
• Cumrun Vafa, Cosmology and the String Swampland, talk at Strings 2018 (pdf slides, recording)

But then:

• Jakob Moritz, Ander Retolaza, Alexander Westphal, Towards de Sitter from 10D, Phys. Rev. D 97, 046010 (2018) (arXiv:1707.08678)

See also

• Savdeep Sethi, Supersymmetry Breaking by Fluxes (arXiv:1709.03554)