ESO White Paper on Intensity Interferometry: Cosmology, Fundamental Physics, Quantum Optics

TL;DR

The paper addresses the need for calibration-free geometric probes of cosmic expansion and dark-matter microphysics, plus a new frontier in quantum astrophysics. It proposes intensity interferometry (II) as a robust, atmosphere-insensitive method to achieve sub‑milliarcsecond resolution with long baselines by measuring second-order coherence. It outlines concrete cosmology gains, including $D_A = R/ heta$ measurements from SN ejecta and AGN BLR, offering a pathway to cross-check the distance ladder and potentially inform the $H_0$ tension. It also details a DM substructure program via astrometric weak lensing and microarcsecond differential astrometry, aiming to detect halos down to $10^{-6}$–$10^{-2} M_\odot$. Finally, it sketches a quantum-astrophysics program exploring nonclassical light in astrophysical lines and a long‑term, fiber‑fed survey facility for intensity-correlation measurements.

Abstract

In this whitepaper, we outline how recent technological advances and ongoing developments open qualitatively new science opportunities in cosmology, fundamental physics, and quantum astrophysics. First, intensity interferometry can contribute to one of the most foundational observables in cosmology: the expansion rate of the Universe. Its angular resolution allows it to resolve the angular extent of extragalactic objects such as supernovae or quasars; combined with a physical scale local to the source, this yields an angular diameter distance and hence a 'Hubble diagram'. Second, the nature of dark matter can be probed via the astrometric lensing signatures of tiny dark matter halos. Third, intensity interferometry gives direct access to second-order coherence properties of astrophysical emission, opening a window onto genuinely quantum aspects of astrophysical light.