🪢 This new approach to string theory could finally explain our Universe

String theory has long been presented as physicists' best candidate for describing the fundamental nature of the Universe. Yet, at the beginning of the 21st century, it became apparent that most versions of reality described by its equations do not match observations of our own Universe.

The predictions of conventional string theory are incompatible with the observation of dark energy, which appears to accelerate the expansion of our Universe. They also do not align with viable theories of quantum gravity, instead predicting a vast 'swampland' of impossible universes.



A new analysis by Eduardo Guendelman, a physicist at Ben-Gurion University of the Negev in Israel, shows that an exotic subset of string models could offer an escape from this swampland. In these models, string tension is generated dynamically.

In the 2000s, string theorists realized that the theory's equations do not describe a single universe but a staggering number of 10⁵⁰⁰ possible solutions. Each of these solutions corresponds to a universe with its own particles and forces, creating what is called the 'landscape' of string theory.

In 2005, it was discovered that this landscape is surrounded by a 'swampland' of solutions that, while appearing viable, are actually incompatible with any functional quantum gravity theory. To distinguish the landscape from the swampland, 'swampland constraints' were proposed.

Guendelman published a paper in The European Physical Journal C showing that a certain exotic subset of string theories might better describe our actual universe. In these models, string tension and the Planck scale become dynamic, weakening the swampland constraints.

What is string theory?

String theory is a theoretical approach in physics that attempts to describe elementary particles as tiny vibrating strings. These strings can vibrate at different frequencies, with each frequency corresponding to a different particle.

This theory aims to unify the four fundamental forces of nature: gravity, electromagnetism, the strong nuclear force, and the weak nuclear force. It promises a coherent description of the Universe at both the quantum and cosmological scales.

However, string theory requires the existence of additional spatial dimensions beyond the three we know. These extra dimensions are compactified, meaning they are curled up on themselves at scales so small they escape our direct perception.

Despite its mathematical elegance, string theory has yet to be confirmed by experimental evidence. It remains one of the most promising yet speculative avenues in theoretical physics.

Why does dark energy pose a problem for string theory?

Dark energy is a hypothetical form of energy that would explain the accelerating expansion of the Universe. Its existence is inferred from astronomical observations, but its exact nature remains a mystery.

In the framework of conventional string theory, dark energy poses a problem because it seems incompatible with 'swampland constraints'. These constraints limit the types of universes the theory can describe consistently with quantum gravity.

Conventional string theory models predict that dark energy should be much weaker than what is observed, or even nonexistent. This places our Universe in the 'swampland' of theoretically impossible solutions.

Guendelman's work suggests that models with dynamic string tension could circumvent this problem. By making the Planck scale dynamic, these models weaken the constraints and open the door to a coherent description of dark energy.