Can an anti-de Sitter vacuum in the dark energy sector explain <i>JWST</i> high-redshift galaxy and reionization observations?

ABSTRACT The unexpectedly large abundance of UV-bright galaxies at $z\gt 10$ discovered by the James Webb Space Telescope poses a significant challenge to the standard Lambda cold dark matter ($\Lambda$CDM) cosmology. This work tests whether modifying the cosmological background, and thereby the growth of structures, can resolve this tension without invoking significant evolution in the astrophysical properties of early galaxies. We investigate an alternative framework featuring an anti-de Sitter vacuum in the dark energy sector, which naturally arises in quantum gravity models like string theory and can enhance early structure formation. Using a self-consistent semi-analytical model that couples galaxy evolution with reionization, we confront this scenario with a wide range of observations. We show that while a model tailored to fit the high-z UV luminosity functions appears promising, it is in strong tension with cosmological constraints from the cosmic microwave background and other low-redshift probes. Conversely, models within this framework that satisfy these constraints provide only a modest boost to structure formation and fail to reproduce the observed galaxy abundances at $z\gt 10$. Although these models remain consistent with the cosmic reionization history, we find that this class of cosmological modifications is insufficient on its own to explain the galaxy excess. Our study underscores the importance of holistic testing for any beyond-$\Lambda$CDM proposal; apparent success in one observational regime does not guarantee overall viability. By demonstrating the limitations of a purely cosmological solution, our results strengthen the case that evolving astrophysical properties are a necessary ingredient for solving the challenge of early galaxy formation.