Strongly Lensed Gravitational Waves Offer New Way to Measure the Cosmic Dipole
The universe looks almost the same in every direction, almost. A subtle asymmetry known as the cosmic dipole reveals that Earth is moving through the cosmos at about 370 kilometers per second relative to the cosmic microwave background (CMB). But for years, measurements of this dipole from the CMB and from counts of distant radio galaxies have disagreed, raising questions about whether our standard cosmological picture is missing something fundamental.
A new study by Anson Chen and Jun Zhang, posted on the arXiv preprint server in July 2026, proposes an entirely independent way to settle the question: use strongly lensed gravitational waves (GWs) linked to galaxy surveys. The work forecasts that next-generation detectors could measure the cosmic dipole with enough precision within a decade to distinguish between competing interpretations.
The Cosmic Dipole Anomaly
The CMB dipole is the largest anisotropy in the microwave sky, and it is straightforwardly interpreted as a Doppler shift caused by the Solar System’s motion through the universe. The standard cosmological model predicts that the dipole seen in the distribution of matter, for example, the number counts of radio galaxies and quasars, should match the CMB dipole in both direction and magnitude, after accounting for the same kinematic effects.
It does not. Measurements from radio galaxy catalogs consistently find a dipole two to five times larger than what the CMB implies. A 2025 colloquium paper in Reviews of Modern Physics by Secrest, von Hausegger, Rameez, Mohayaee, and Sarkar placed the discrepancy at over 5 sigma significance, calling it a serious challenge to the foundation of modern cosmology.
The disagreement could mean the universe is not truly isotropic on large scales, violating the Cosmological Principle that underpins the standard Lambda-CDM model. Or it could point to subtle systematics in the radio data. What cosmologists need is a completely independent probe with different sources of systematic error.
Gravitational Waves Enter the Picture
Gravitational wave astronomy has already revolutionized the study of black holes and neutron stars. Now researchers are asking whether it can help with cosmology. Earlier work by Mastrogiovanni et al. in 2022 showed that the number count dipole of ordinary GW detections from binary mergers could be measured by the future Einstein Telescope (ET) and Cosmic Explorer (CE), but required millions of events, potentially a decade or more of observation.
Chen and Zhang, both at the University of Chinese Academy of Sciences, take a different approach. Instead of counting ordinary GW events, they focus on the rarer but more informative subset: gravitational waves that have been strongly lensed by intervening galaxies.
When a massive galaxy sits between Earth and a merging black hole binary, its gravity bends spacetime and splits the GW signal into multiple copies, each arriving at a slightly different time. These multiply imaged events carry rich information. From the waveform alone, one can infer the luminosity distances to both the lens and the source. By matching these systems to host galaxies identified in optical galaxy surveys like the Vera C. Rubin Observatory’s Legacy Survey of Space and Time, the redshifts of both the lens and source become known.
This combination of distances and redshifts is exquisitely sensitive to the cosmic dipole. The dipole imprints itself on the inferred luminosity distances, the angular diameter distances in the lens model, and the observed number count of events as a function of sky position. By modeling all these effects simultaneously, a statistical sample of lensed GW events can constrain the dipole magnitude and direction alongside standard cosmological parameters like the Hubble constant.
Forecasts for Next-Generation Detectors
The study simulated realistic observations with the planned third-generation GW observatories: the European Einstein Telescope and the U.S.-based Cosmic Explorer, operating together as a global network. Using the Singular Isothermal Sphere lens model and accounting for roughly 70 percent of strongly lensed events being doubly imaged, Chen and Zhang ran thousands of mock data realizations.
The results are promising. With 10 years of ET-CE observations, the dipole magnitude g could be constrained to g = (2.45 +1.53 -1.28) x 10^{-3} in the most optimistic scenario. This level of precision is sufficient to detect a dipole consistent with either the CMB or the larger radio galaxy count values.
The authors found that combining constraints from doubly lensed events with those from triply and quadruply lensed events, which allow more precise lens model reconstruction, significantly sharpens the measurement. Even in less optimistic scenarios, the method provides a meaningful independent cross-check.
“Although challenging, strongly lensed GWs offer a novel approach to measuring the cosmic dipole, providing an independent consistency test with different systematics from electromagnetic probes,” the authors write.
A New Window on Cosmic Anisotropy
Strong gravitational lensing of gravitational waves has not yet been definitively observed, but theory predicts that ET and CE should detect dozens of such events per year. Methods for identifying them are steadily improving. A 2025 study by Liu and Liao showed that incorporating positional priors from the Euclid galaxy lens catalog could boost the confidence of lensing identification by an order of magnitude.
The Chen and Zhang study adds another reason to be excited about lensed GWs: they are not just tools for measuring the Hubble constant or testing general relativity. They also offer a gravitational handle on one of cosmology’s most persistent puzzles.
If the dipole measured through lensed GWs matches the CMB value, it would strengthen the case that the radio galaxy dipole is inflated by unknown systematic effects. If it matches the larger radio dipole instead, it would suggest that the CMB interpretation itself needs revision, perhaps pointing to a genuinely anisotropic universe or a more exotic origin for the CMB dipole.
Either way, the answer may come not from more powerful telescopes looking at more galaxies, but from the subtle echoes of colliding black holes, bent by gravity and split into multiple images, carrying a message about the motion of everything through the cosmos.
Reference: Anson Chen and Jun Zhang, “Prospect of Measuring the Cosmic Dipole by Associating Strongly Lensed Gravitational Waves with Galaxy Surveys,” arXiv:2605.19476 (2026).
Related: N. Secrest et al., “Colloquium: The Cosmic Dipole Anomaly,” Rev. Mod. Phys. 97, 041001 (2025), arXiv:2505.23526.
Coverage note: This article was written based on the preprint arXiv:2605.19476 and supplementary searches for news coverage and related literature. No media outlets have yet covered this specific paper at the time of writing.