# String theory in turmoil over dark energy and the Higgs field

Last June a group of string theorists proposed a conjecture showing that string theory contradicts the current interpretation of dark energy. A new study now shows that the conjecture itself is incompatible with the existence of the Higgs particle, creating a lot of heated discussion in the world of fundamental physics.

Formalized in the 1970’s, string theory is a theoretical framework of physics in which the fundamental building blocks of reality are understood to be *strings* rather than the point-like particles we are familiar with. The theory is compatible with traditional particle physics and with its interpretation of matter being made of particles with mass, charge etc., as in string theory these properties are determined by the vibrational state of the strings. What is novel in string theory is that it attempts to tackle the unsolved problem of quantum gravity by proposing that, like other fields, the gravitational field is quantized, i.e. made of discrete units or quanta called gravitons (a vibrational state of the string), each carrying a unit of gravitational force. In this context, string theory is thus a self-contained mathematical model which aims to be a theory of everything describing all fundamental forces and forms of matter in one consistent framework.

Last June a group of string theorists from Harvard and Caltech published a conjecture advancing that string theory is fundamentally incompatible with our current understanding of dark energy. This is problematic as in our current model dark energy makes ~70% of the total energy in the universe and is responsible for its observed expansion at an accelerating pace.

Up to the proposal of the conjecture, dark energy seemed to be accounted for in string theory with the *apple in a fruit bowl* analogy. String theory assumes that there are particles that can be described as fields and have a state of minimal energy—like an apple in a bowl. The apple always lies at the bottom of the bowl in its state of minimum energy. Everywhere else inside the bowl its energy is higher and energy needs to be spent to move it from the bottom. The apple however is not at complete zero energy as the bowl can be on the ground, or on a table—at a higher energy but still at its minimum inside the bowl. “In string theory there are fields which could explain dark energy in a similar way—locally, they are in a state of minimal energy, but still their energy has a value greater than zero,” explains Timm Wrase, from the Institute for Theoretical Physics, Vienna University of Technology. “So these fields would provide the so-called dark energy, with which we could explain the accelerated expansion of the universe.”

However last June, world-renowned string theorist Cumrun Vafa from Harvard University published an article which suggested that such “bowl-shaped” fields of positive energy are not possible in string theory.” If that is true,” Wrase says “the accelerated expansion of the universe, as we have imagined it so far, is not possible. The accelerated expansion would then have to be described by a field with quite different properties, like a tilted plane on which a ball rolls downhill, losing potential energy.”

The controversy pushed Timm Wrase to dig into the issue and find that if Cumrun Vafa’s conjecture was true it would prohibit certain types of fields which we already know to exist, such as the Higgs field. Wrase’s results, which sparked some turmoil in the string theory community, have been recently peer reviewed and published in the journal Physical Review. “This controversy is a good thing for string theory,” Timm Wrase commented. “Suddenly, a lot of people have completely new ideas which nobody has thought about before.” Wrase and his team are now investigating which fields are allowed in string theory and at which points they violate Vafa’s conjecture. “Maybe that leads us to exciting new insights into the nature of dark energy—that would be a great success,” Wrase concludes.

#### Carlo Bradac

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