Axions Across Spacetimes: from Holography to Cosmology through the Landscape

Objective

Axions are pseudoscalar particles described by a periodic field, so that a shift symmetry protects their masses. They play a central role in many extensions of the Standard Model, with implications for quantum gravity, cosmology and experimental physics. As such, axion physics is ideally situated at the crossroad of fundamental theory, particle phenomenology, and experiments. Despite their growing relevance, a first-principles explanation for the presence of axions is still lacking. ACROSS aims to take crucial steps towards a deeper understanding of axions, their interplay with the background spacetimes and the necessity of their presence as an inevitable consequence for consistency, if any. Can we argue on general and model-independent grounds that an axion should exist in our Universe? Given the significant experimental effort underway to detect axions, answering this question is both timely and essential. This project will integrate tools from quantum field theory in curved spacetimes, holography and string compactifications to investigate the nature of axions. Special emphasis will be placed on their behaviour within the AdS/CFT correspondence and in de Sitter spacetime. The findings will be contextualized within the landscape of string theory vacua, and broader quantum gravity considerations, ultimately informing both theoretical and experimental communities, offering guidance for ongoing axion searches. ACROSS will be hosted by Harvard University (outgoing phase), with a return phase at Scuola Normale Superiore in Pisa. The Fellowship will support advanced training and establish international collaboration, laying the foundation for a lasting European research programme on axions in curved backgrounds. By bridging multiple disciplines, ACROSS will deepen our understanding of axions as fundamental elements in theoretical physics and provide a novel framework for assessing their presence—or absence—in quantum gravity and, ultimately, in our Universe.