GLOW

Science

The science case

Gravitational waves and fast radio bursts emit coherent radiation that preserves diffraction and interference patterns. That single property opens a unique regime of gravitational lensing, right at the moment the instruments are ready to exploit it.

A propagating wave is an opportunity to understand all elements of gravitational lensing: the sources, the lenses, and gravity itself. Through new methodologies, GLOW will illuminate some of the universe's darkest secrets: sources beyond the detector horizon, the small-scale distribution of dark matter, and the properties of gravity and dark energy.

A binary black hole at left, a galaxy lens at center and the Earth at right. An arrow traces a gravitational wave from the source, bending as it passes the lens, on to the observer.
Distant and strong-field sources Magnified black holes at z > 3, and sources near supermassive black holes. Dark matter & small scales Halos and compact objects below 106 M☉, where baryons no longer trace the mass. Testing gravity and dark energy Distortions on propagating waves that can't be produced in Einstein's theory.
Wave optics Diffraction, interference and plasma lensing: the theory, and the algorithms to compute it. Microlensing Stars and remnants in the lensing galaxy, treated collectively. Data analysis Waveform models, population analyses, and simulation-based inference.
A wave leaves a distant binary, travels through a lens and reaches us, carrying information about its journey.

Click on the elements above to know more about the science goals (top row) and the methodologies (bottom row).

A new frontier for gravitational lensing

Light traveling through the Universe is deflected and magnified by gravitational fields, occasionally producing multiple images of the same object.[1] Gravitational lensing began as a prediction thought unlikely to be observed,[2] and became a most productive tool for astronomy: multiply imaged quasars, cosmography, maps of dark matter inside lens galaxies[3] , images of supermassive black holes[4] , exoplanets found by microlensing[5] . It has now been seen across the electromagnetic spectrum, and each source and spectral range brings its own strengths.

Two new kinds of source are about to join that list, and they behave differently from anything lensed before.

The uniqueness of coherent sources

Gravitational waves and fast radio bursts are phase coherent. Where light from a distant galaxy arrives as an incoherent sum and the lens can only magnify or split it, a coherent signal carries its phase intact, and the lens distorts the wavefront itself. The result is a frequency-dependent (or chromatic) distortion: diffraction and interference.[6] The distortions not only reveal that lensing occurred, they also carry detailed information about the lens.

Two conditions have to hold together here. The signal must be phase coherent, and the source must also be effectively point-like: one large enough that different parts of it are lensed differently smears out its own interference pattern. A galaxy fails on both counts. A merging black-hole binary, smaller than any scale the lens imprints on it, passes on both.

A diffraction pattern in visible light. A circular hole 200 µm across, lit by one colour at a time: the rings contract as the wavelength falls, the first dark ring sitting at 1.22 λ/D, while the core brightens as 1/λ². The very long wavelengths of fast radio bursts (≳ 4 cm, so ≲ 7.5 GHz) and gravitational waves (≳ 150 km, so ≲ 2 kHz) bring diffraction phenomena to a new scale.

What gravitational waves add

Gravitational waves from compact binary coalescences can be modeled from first principles, allowing us to disentangle lensing effects from source properties. Moreover they probe gravity directly and propagate cleanly, allowing interferometers to monitor the entire sky.

What fast radio bursts add

Fast radio bursts are short and powerful extragalactic transients[7, 8] with millions of recorded cycles. They have precise sky localization, and their interaction with ionized matter (plasma lensing[9] ) turns them into a probe of the baryons on very small scales.

Because diffraction is sensitive to structure comparable in size to the wavelength, coherent sources reach lens masses and scales that image-based lensing cannot resolve. This is what makes them a probe of dark matter on sub-galactic scales, potentially reaching planetary-scale objects for fast radio bursts: scales far too small and too dark to access any other way.

Why now

The observational landscape is evolving fast. LIGO–Virgo–KAGRA recently reported the discovery of GW231123, a compelling candidate for a lensed gravitational wave.[10] Soon, their network will deliver thousands of detections from compact binary mergers.[11] Radio surveys (CHIME, DSA-2000, CHORD, BURSTT) are increasing the number of observed fast radio bursts dramatically.[12, 13, 14] A first discovery of lensed gravitational waves and fast radio bursts is expected in the coming years.[15, 16, 17] GLOW will develop the tools needed to interpret these observations.

What follows is a shift from discovery to population: detector upgrades, the next generation of ground-based observatories (Einstein Telescope, Cosmic Explorer)[18, 19] , and the SKA survey will elevate these numbers by orders of magnitude. LISA, scheduled to launch in 2035[20] , opens the low-frequency band and a different set of lensing signatures. GLOW will help us prepare for lensing observations with these exciting facilities.

What GLOW will deliver

The project serves three overarching goals:

New horizons
Discover lensed gravitational waves, including sources at high redshift and in the vicinity of supermassive black holes
Fundamental physics
Probe dark-matter objects on elusive scales, the nature of gravity, and theories of dark energy
Readiness
Have the framework in place for upcoming fast-radio-burst surveys, next-generation detectors and multi-messenger follow-up

Methodological developments

  • WP1 · Theory: numerical methods for microlensing and plasma diffraction
  • WP2 · Discovery: microlensed waveforms, population analysis and accelerated parameter estimation
  • WP3 · Fundamental physics: effective description of dark-matter diffraction and gravitational-wave propagation

The project builds on several years of work on wave-optics lensing, which led to the first public GLoW code[21] , which already computes diffraction by general matter distributions fast enough to perform systematic analyses.

References

Primary sources for this section. Work by the project itself is on the publications page.

  1. P. Schneider, J. Ehlers, E. E. Falco, Gravitational Lenses Astronomy and Astrophysics Library, Springer (1992)
  2. T. Sauer, A brief history of gravitational lensing Einstein Online
  3. Planck Collaboration, N. Aghanim et al., Planck 2018 results. VIII. Gravitational lensing Astron. Astrophys. 641, A8 (2020)
  4. Event Horizon Telescope Collaboration, K. Akiyama et al., First M87 Event Horizon Telescope results. I. The shadow of the supermassive black hole Astrophys. J. Lett. 875, L1 (2019)
  5. B. S. Gaudi, Microlensing surveys for exoplanets Ann. Rev. Astron. Astrophys. 50, 411 (2012)
  6. C. Leung et al., Wave mechanics, interference, and decoherence in strong gravitational lensing (2023)
  7. E. Petroff, J. W. T. Hessels, D. R. Lorimer, Fast radio bursts at the dawn of the 2020s Astron. Astrophys. Rev. 30, 2 (2022)
  8. CHIME/FRB Collaboration, M. Amiri et al., The first CHIME/FRB fast radio burst catalog Astrophys. J. Supp. 257, 59 (2021)
  9. J. M. Cordes et al., Lensing of fast radio bursts by plasma structures in host galaxies Astrophys. J. 842, 35 (2017)
  10. LIGO Scientific, Virgo and KAGRA Collaborations, A. G. Abac et al., GWTC-4.0: searches for gravitational-wave lensing signatures arXiv preprint (2025)
  11. LIGO Scientific Collaboration, LIGO–Virgo–KAGRA observing plan Observing capabilities documentation
  12. G. Hallinan et al., The DSA-2000 — a radio survey camera Bull. Am. Astron. Soc. 51, 255 (2019)
  13. K. Vanderlinde et al., The Canadian Hydrogen Observatory and Radio-transient Detector (CHORD) Canadian Long Range Plan white paper (2019)
  14. H.-H. Lin et al., BURSTT: Bustling Universe Radio Survey Telescope in Taiwan Publ. Astron. Soc. Pac. 134, 094106 (2022)
  15. A. R. A. C. Wierda, E. Wempe, O. A. Hannuksela, L. V. E. Koopmans, C. Van Den Broeck, Beyond the detector horizon: forecasting gravitational-wave strong lensing Astrophys. J. 921, 154 (2021)
  16. G. P. Smith et al., Discovering gravitationally lensed gravitational waves: predicted rates, candidate selection, and localization with the Vera Rubin Observatory Mon. Not. Roy. Astron. Soc. 520, 702 (2023)
  17. L. Connor, V. Ravi, Stellar prospects for FRB gravitational lensing Mon. Not. Roy. Astron. Soc. 521, 4024 (2023)
  18. M. Maggiore et al., Science case for the Einstein Telescope JCAP 03, 050 (2020)
  19. M. Evans et al., A horizon study for Cosmic Explorer: science, observatories, and community (2021)
  20. LISA Collaboration, M. Colpi et al., LISA definition study report (2024)
  21. H. Villarrubia-Rojo, S. Savastano, M. Zumalacárregui et al., GLoW: novel methods for wave-optics phenomena in gravitational lensing Phys. Rev. D 111, 103539 (2025)