From infinite to infinitesimal: Using the Universe as a dataset to probe Casimir corrections to the vacuum energy from fields inhabiting the dark dimension

Promptly after high-resolution experiments harbinger the field of precision cosmology low- and high-redshift observations abruptly gave rise to a tension in the measurement of the present-day expansion rate of the Universe (H_0) and the clustering of matter (S_8). The statistically significant discrepancies between the locally measured values of H_0 and S_8 and the ones inferred from observations of the cosmic microwave background assuming the canonical Λ cold dark matter (CDM) cosmological model have become a new cornerstone of theoretical physics. Λ_sCDM is one of the many beyond Standard Model setups that have been proposed to simultaneously resolve the cosmological tensions. This setup relies on an empirical conjecture, which postulates that Λ switched sign (from negative to positive) at a critical redshift z_c ∼ 2. We reexamine a stringy model that can describe the transition in the vacuum energy hypothesized in Λ_sCDM. The model makes use of the Casimir forces driven by fields inhabiting the incredible bulk of the dark dimension scenario. Unlike the Λ_sCDM setup the model deviates from ΛCDM in the early universe due to the existence of relativistic neutrino-like species. Using the Boltzmann solver CLASS in combination with MontePython we confront predictions of the stringy model to experimental data (from the Planck mission, Pantheon+ supernova type Ia, BAO, and KiDS-1000). We show that the string-inspired model provides a satisfactory fit to the data and can resolve the cosmological tensions.