Intensity Interferometry Reveals New Ways to Measure the Universe’s Expansion Rate

Scientists are increasingly exploring the potential of intensity interferometry to address fundamental questions in cosmology, fundamental physics, and astrophysics. Robin Kaiser, William Guerin, and Farrokh Vakili, from Universit e Cote d’Azur and the CNRS Institut de Physique de Nice, lead a collaborative effort involving researchers from Universit e Grenoble Alpes and the IPAG, the Czech Technical University in Prague and the Institute of Physics of the Czech Academy of Sciences, and Lund University’s Division of Astrophysics. Working with colleagues at these institutions, alongside contributions from Stefan Funk, Prasenjit Saha, and others, the team detail how recent technological advances promise to unlock new observational capabilities. This research is significant because it demonstrates how intensity interferometry can refine measurements of the Universe’s expansion rate, probe the nature of dark matter through astrometric lensing, and provide unique insights into the coherence properties of astrophysical light, potentially revolutionising our understanding of the cosmos.

Scientists are pioneering a revival of intensity interferometry, a technique that exploits the subtle correlations in light to achieve unprecedented resolution in astronomical observations. This work details how advancements in detector technology and computational power are unlocking new scientific opportunities across cosmology, fundamental physics, and quantum astrophysics.

By measuring the second-order coherence of light, researchers are bypassing limitations imposed by atmospheric turbulence and optical path differences that plague conventional telescopes. The result is a pathway to sub-milliarcsecond resolution and precise differential astrometry, allowing for detailed studies of distant objects and phenomena. This innovative approach promises to refine our understanding of the Universe’s expansion rate, a persistent challenge in modern cosmology.

Intensity interferometry offers a unique, calibration-independent method for measuring distances to celestial objects like supernovae and quasars, sidestepping systematic errors. By directly resolving angular scales at extremely high resolution, scientists can construct a “Hubble diagram”, a crucial tool for determining cosmic distances and the Hubble constant

Beyond cosmology, this research opens a new window onto the elusive nature of dark matter. The technique enables the detection of astrometric lensing signatures caused by tiny dark matter halos, providing a geometric probe of dark matter microphysics. These halos subtly deflect light from background sources, creating measurable distortions that can reveal their properties and distribution.

Furthermore, intensity interferometry provides direct access to the quantum properties of astrophysical light, allowing for investigations into the fundamental nature of light emission from celestial sources. Modern instruments, built upon proof-of-principle demonstrations at observatories like VERITAS and MAGIC, are now capable of routine measurements, expanding the community and accelerating the pace of discovery.

Recent progress includes measurements of spectroscopic binaries, offering parallax-independent distance calculations, and extending observations to fainter systems within the Magellanic Clouds. This work envisions a future where intensity interferometry can resolve the morphology and distance of supernovae, measure the angular sizes of red clump stars, and even probe the broad-line emission regions of active galactic nuclei with unprecedented precision, potentially resolving the ongoing Hubble tension. The technique also promises to map three-dimensional stellar motions across the Galaxy, providing insights into the Galactic potential and the distribution of dark matter.

Reconstructing cosmic distances and mapping dark matter haloes via photon correlations

Intensity interferometry, a technique employing arrays of telescopes to measure the correlation of light fluctuations, underpins this research into cosmology, fundamental physics, and astrophysics. Rather than directly imaging objects, it reconstructs information from the statistical properties of detected photons, offering unique capabilities for resolving fine details and probing faint signals.

This approach circumvents the limitations imposed by diffraction, allowing for angular resolutions unattainable with single telescopes of comparable size. Specifically, the work leverages this technique to determine angular diameter distances to extragalactic objects like supernovae and quasars, constructing a ‘Hubble diagram’ to refine measurements of the Universe’s expansion rate.

To probe the nature of dark matter, the study implements astrometric lensing, meticulously tracking the minute deflections of background sources caused by the gravitational influence of tiny dark matter halos. These deflections manifest as accelerations of individual images or correlated proper-motion patterns, detectable through precise astrometry. The methodology benefits from existing data releases from the Gaia mission, while also anticipating gains from the microarcsecond-class differential astrometry achievable with intensity interferometry on a smaller, brighter sample of sources.

This allows for mapping three-dimensional stellar motions across the Galaxy, directly probing the Galactic potential and dark matter halo. Furthermore, the research explores the potential of intensity interferometry to access second-order coherence properties of astrophysical emission, a realm of light behaviour not fully captured by traditional spectroscopic methods.

By focusing on strongly lensed quasars, where multiple images are magnified by intervening galaxies, the study proposes measuring the evolving separation vectors between image pairs to isolate signals from dark matter halos. A forward model developed for the associated power spectral density indicates that differential angular acceleration between image pairs is the most informative observable, potentially revealing halos with masses as low as 10−6 to 10−2 M⊙, far below current detection limits. This innovative application of intensity interferometry promises to unlock new insights into the kinetic decoupling scale and transfer function cut-offs shaped by particle microphysics.

Detecting low-mass dark matter haloes with astrometric lensing and intensity interferometry

Dark matter halo masses between 10−6 and 10−2 M⊙ are now potentially within reach of detection via astrometric lensing signatures observed over multi-year periods. This sensitivity stems from the potential for microarcsecond-precision differential astrometry using bright, quadruply-imaged systems like B1422+231. Stellar microlensing noise, a potential confounding factor, can be effectively mitigated through analysis of image-shape variability and photometric data, leaving a bounded residual acceleration floor for more accurate measurements.

The research demonstrates access to second-order coherence properties of astrophysical emission, opening new avenues for understanding the nature of light from celestial sources. Intensity interferometry (II) is rapidly advancing due to improvements in detector technology, enabling observations of significantly fainter objects than previously possible.

These advancements are being incorporated into several international efforts, including the Multi-Aperture Spectroscopic Telescope (MAST), QUASAR, and the Large Fibre Array Spectroscopic Telescope (LFAST). Extended-path schemes are being developed to broaden the effective field of view of II from the diffraction-limited coherence patch to the atmospheric isoplanatic angle.

This modification retains angular resolution and light-centroiding performance while enabling arcsecond-scale differential astrometry with microarcsecond-level precision. Such precision is crucial for probing the kinetic-decoupling scale and transfer-function cut-offs shaped by particle microphysics related to dark matter. The implementation of second-order coherence measurements closely follows the Hanbury Brown-Twiss technique but requires only a single telescope, simplifying the instrumentation. Optical forces, previously restricted to repulsive radiation pressure, are now harnessed for precise wavefront control.