String Theory Advances Cosmological Constant Bound, Predicting Shorter de-Sitter Lifetimes

The fundamental nature of gravity at extremely high energies remains a central question in theoretical physics, and understanding the limits of Einstein’s theory is crucial to resolving this. Ahmad Moradpouri from Sharif University of Technology, along with colleagues, investigates the ‘species scale’, the energy level at which modifications to general relativity become important, and its implications for the evolution of the universe. This research explores a new version of the ‘TCC conjecture’, termed the ‘refined TCC’, which links this species scale to a fundamental cutoff and examines whether string-like vibrations maintain their characteristics even as the universe expands. The team’s findings, derived from analysing cosmological solutions within various string theories, suggest a limited lifespan for accelerating universes and, significantly, indicate that the initial big bang singularity may be an impossibility, potentially supporting alternative cosmological models like string gas cosmology where spacetime remains consistently well-behaved.

The investigation centres on whether stringy modes remain stringy and never exceed the Hubble horizon, predicting a shorter lifetime for de-Sitter spacetime and imposing a bound on the cosmological constant. Researchers present supporting evidence for this refined conjecture from cosmological solutions derived from different compactifications of various string theories, suggesting that both the original and refined conjectures are incompatible with the big bang.

Swampland Criteria and Dark Dimension Consistency

This paper explores the Swampland Program in string theory, identifying criteria to distinguish consistent string theory solutions from those that are physically unrealistic. A central concept is Trans-Planckian Censorship, a conjecture preventing the breakdown of effective field theory at extremely high energies by proposing that some physical process will invalidate the theory before inconsistencies arise. The research also connects to the idea of a Dark Dimension, a proposed extra spatial dimension potentially explaining dark matter and dark energy. The paper references the Distance Conjecture, which links the distance between stable universes in the string landscape to the number of species present, and the de Sitter Conjecture, suggesting that true de Sitter space is unstable in string theory.

The importance of the species scale is emphasized, as a large number of species can lead to a breakdown of effective field theory at lower energies. Scientists are exploring how the species scale is constrained by Swampland criteria. The team delves into the mechanisms of trans-Planckian censorship, suggesting that the creation of particles associated with the dark dimension can trigger the breakdown of effective field theory. They investigate how the properties of dark matter particles are constrained by Swampland criteria and censorship. A significant portion of the work involves mathematical derivations and solutions related to string theory, cosmology, and effective field theory, analysing scalar fields, black holes, and other relevant objects.

The research also references singularity theorems from general relativity, exploring how these might be modified or avoided in string theory, and connects these ideas to inflationary cosmology, exploring how Swampland criteria and censorship might constrain inflationary models. Specific techniques include analysing the O(16) x O(16) Heterotic String model, exponential potentials, and carefully counting species in string theory solutions. The paper aims to narrow down the string landscape, address cosmological puzzles, develop a deeper understanding of quantum gravity, and push the boundaries of mathematical rigor in string theory. This paper presents a sophisticated investigation into the interplay between the Swampland Program, trans-Planckian censorship, and the dark dimension, arguing that Swampland criteria and the need to avoid inconsistencies impose strong constraints on string theory solutions and the nature of dark matter.

Refined Conjecture Limits De Sitter Lifespan

Scientists have established a refined version of the Trans-species Conjecture, a principle governing quantum fluctuations in string theory, and demonstrated its implications for cosmology and the nature of spacetime. This work centres on the species scale, the energy level at which corrections to Einstein’s theory of gravity become significant, and explores whether stringy modes maintain their characteristics without exceeding the Hubble horizon. The refined conjecture predicts a shorter lifespan for de-Sitter spacetime, a universe undergoing accelerated expansion, and imposes a specific limit on the cosmological constant. Experiments reveal that cosmological solutions derived from various string theory compactifications consistently satisfy the refined conjecture, providing strong support for its validity.

The team demonstrated this across scenarios involving one modulus and extending to more complex compactifications with two moduli. Measurements confirm that the species scale, in the weak coupling limit, closely corresponds to the string mass scale, and in the strong coupling limit, relates directly to the eleven-dimensional Planck mass. Results demonstrate that the Trans-Planckian Censorship Conjecture and its refined variant are incompatible with the existence of a big-bang singularity, suggesting that the very beginning of the universe must have been fundamentally different than previously thought. The refined conjecture places an upper bound on the lifetime of de-Sitter spacetime, and analysis reveals a constraint on the Hubble parameter, setting an upper limit on the cosmological constant and offering a potential explanation for its unexpectedly small value.

This research suggests that the string length is a fundamental scale governing the universe, and that spacetime must remain regular at all times. This research presents a refined version of the Trans-Planckian Censorship Conjecture, building upon earlier work seeking to understand the limits of effective field theories when coupled with gravity. Scientists investigated how a lower energy cutoff, the species scale, impacts string theory and cosmology, proposing that this scale plays a crucial role in determining the lifespan of de-Sitter spacetime and setting a bound on the cosmological constant. The findings suggest a fundamental incompatibility between the original and refined conjectures and the existence of a big-bang singularity, leading to the conclusion that a singularity must be avoided in any consistent theory of quantum gravity. The researchers demonstrate that scenarios like string gas cosmology maintain a regular spacetime background, avoiding the problematic singularity.