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 (H₀) and the clustering of matter (S₈). The statistically significant discrepancies between the locally measured values of H₀ and S₈ 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. ΛₛCDM 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 zc ~ 2. We reexamine a stringy model that can describe the transition in the vacuum energy hypothesized in ΛₛCDM. The model makes use of the Casimir forces driven by fields inhabiting the incredible bulk of the dark dimension scenario. Unlike the ΛₛCDM 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.