⚛️ general relativity

First measurement of the Hubble constant from gravitational wave-galaxy cross-correlations

This paper presents the first measurement of the Hubble constant ( $H_0 = 67^{+18}_{-15}$ km s $^{-1}$ Mpc $^{-1}$ ) and a constraint on the gravitational wave bias by detecting a cross-correlation peak between gravitational wave events and galaxy distributions using the "Peak Sirens" method.

Original authors: Isabela Santiago de Matos, Charles Dalang, Tessa Baker, Raul Abramo, João Ferri, Miguel Quartin

Published 2026-08-18

📖 4 min read🧠 Deep dive

CC BY 4.0

Original authors: Isabela Santiago de Matos, Charles Dalang, Tessa Baker, Raul Abramo, João Ferri, Miguel Quartin

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

For decades, astronomers have been trying to measure the speed at which our universe is expanding, a value known as the Hubble constant. This number is fundamental to understanding the age, size, and ultimate fate of the cosmos. To find it, scientists traditionally rely on two main types of cosmic rulers: the light from exploding stars and the faint afterglow of the Big Bang. However, these two methods have recently begun to disagree with each other, creating a tension that suggests our current understanding of physics might be incomplete. In the last decade, a new tool has emerged to help solve this puzzle: gravitational waves. These are ripples in the fabric of space-time caused by violent cosmic events, such as the collision of black holes or neutron stars. Unlike light, which can be dimmed or scattered by dust, gravitational waves travel through the universe almost undisturbed, carrying a direct and precise record of the distance to their source. The challenge has always been that while these waves tell us exactly how far away an event happened, they do not reveal how fast that event is moving away from us, a piece of information usually found by looking at the color of the light from the host galaxy.

A team of researchers has now taken a significant step toward bridging this gap by using a new technique to link gravitational waves directly to the galaxies that host them. Instead of trying to find the specific galaxy for a single event, which is often difficult when the location of the wave is fuzzy, the team looked at the statistical relationship between many gravitational wave events and the distribution of millions of galaxies across the sky. They focused on two specific gravitational wave detections from a recent catalog of observations: one from a collision involving a black hole and a smaller object, and another from a famous collision of two neutron stars that was also seen with telescopes. By comparing the three-dimensional map of where these waves came from with the map of where galaxies are located, the researchers searched for a specific pattern. They reasoned that if the waves and the galaxies are truly connected, there should be a distinct peak in their correlation at the exact distance where the galaxies' redshifts match the waves' distances. This method, which they call "Peak Sirens," allows them to measure the expansion rate of the universe without needing to know the precise redshift of the host galaxy for every single event.

The team successfully detected this correlation signal with a high degree of statistical confidence, marking the first time this specific method has been observed in real data. The signal was driven primarily by the best-localized event in their dataset, a collision that occurred roughly 255 million light-years away, with a smaller contribution from the closer neutron star collision. By analyzing the position of this correlation peak, the researchers calculated a value for the Hubble constant of 67 kilometers per second per megaparsec, with an uncertainty range that spans from 52 to 85. While this measurement is not yet precise enough to settle the debate between the different expansion rates found by other methods, it is consistent with the values derived from the cosmic microwave background and other dark siren studies. More importantly, the study demonstrated that this technique works even with a very small number of gravitational wave events, proving that the method is viable for future, more precise measurements as more detectors come online.

Beyond measuring the expansion rate, this work also provided the first observational constraint on a property called the gravitational wave bias. This number describes how strongly the sources of gravitational waves trace the underlying distribution of matter in the universe. The researchers found that this bias is less than 4.3, a result that helps refine our understanding of where these violent cosmic collisions are likely to occur relative to the galaxies we see. The study relied on a massive catalog of galaxies and sophisticated computer simulations to model the expected patterns, ensuring that the detected signal was not just random noise. The authors noted that while their current data is limited by the small number of well-localized gravitational wave events available, the technique is robust and does not rely on complex assumptions about the masses of the colliding objects. As more gravitational wave events are detected in the coming years, this method is expected to become a powerful new way to map the universe and test the laws of gravity, offering a fresh perspective on one of the most enduring mysteries in modern cosmology.

Technical Summary: First Measurement of the Hubble Constant from Gravitational Wave-Galaxy Cross-Correlations

Problem and Motivation
Gravitational wave (GW) observations from compact binary coalescences (CBCs) provide direct measurements of luminosity distance ( $D_L$ ) but lack intrinsic redshift ( $z$ ) information. Recovering redshift is essential for constraining cosmological parameters, particularly the Hubble constant ( $H_0$ ). While "bright siren" methods (using electromagnetic counterparts) and "dark siren" methods (statistically associating GW events with galaxy catalogs based on mass distributions and merger rates) are established, they face limitations regarding astrophysical modeling dependencies and the scarcity of confirmed counterparts. This paper addresses the need for a technique that bypasses external distance calibrations and minimizes reliance on assumptions about the CBC mass distribution and merger rate.

Methodology: The Peak Sirens Method
The authors apply the "Peak Sirens" method, which utilizes the 3D angular cross-correlation between GW sources and galaxies. The core physical premise is that GW sources reside within galaxies; therefore, the spatial correlation between GW events and galaxy distributions peaks when the redshift bins of the galaxy catalog align with the luminosity distance bins of the GW sources.

The technical implementation involves:

Data Construction:
- Galaxy Map: Using the GLADE+ catalog (K-band), the authors construct a pixelated number count map ( $N_g$ ) using HEALPix ($nside=32$). The radial direction is decomposed into redshift bins ( $z_i$ ), with finer binning ( $\Delta z \approx 0.0033$ ) for $z < 0.02$ to capture nearby events.
- GW Map: Using the GWTC-3.0 catalog, a 3D probability map ( $N_{gw}$ ) is constructed by integrating the posterior distributions of GW events over luminosity distance bins ( $D_{Lj}$ ).
Cross-Spectrum Analysis: The authors compute the angular cross-spectrum $C_\ell(z_i, D_{Lj})$ between the galaxy and GW maps. Unlike standard dark siren approaches, this method does not rely on auto-correlations or assumptions about the CBC mass function.
Theoretical Modeling: Theoretical cross-spectra are modeled using simulations where GW sources are randomly injected into galaxies within a fiducial cosmology (Planck 2018). The method assumes a linear GW bias ( $b_{gw}$ ) and models the peak position in the $(z, D_L)$ plane. The position of the peak is sensitive to $H_0$ , while the amplitude is sensitive to the GW bias.
Statistical Inference: A Gaussian likelihood is used to compare observed cross-spectra against theoretical predictions. The analysis focuses on the dominant parameters: $H_0$ and $b_{gw}$ .

Key Contributions and Results
The paper reports the first detection of the GW-galaxy cross-correlation signal and the first measurement of $H_0$ using this specific technique.

Data Selection: The analysis utilizes two GW events from GWTC-3.0:
- GW190814: A black-hole/low-mass compact object merger, noted as the best-localized event in the catalog ( $\Omega_{90} = 18.5$ deg $^2$ ).
- GW170817: The known bright siren (BNS merger). Crucially, the authors treat this as a dark siren in this analysis, not using its known host galaxy redshift, to demonstrate the method's viability without external EM redshift data.
Detection Significance:
- Using GW190814 alone, the cross-correlation peak is detected at 3.4 $\sigma$ significance.
- Including GW170817 (without its redshift) raises the significance to 5.9 $\sigma$ .
Cosmological Constraints:
- Hubble Constant ( $H_0$ ): The combined analysis yields $H_0 = 67^{+18}_{-15}$ km s $^{-1}$ Mpc $^{-1}$ (68% CI). This result is consistent with other dark siren measurements and Planck CMB data, though with larger uncertainties due to the small sample size.
- GW Bias ( $b_{gw}$ ): The study provides the first observational constraint on the GW bias, finding $b_{gw} < 4.3$ at 95% confidence interval (CI). This parameter quantifies how strongly GW sources trace the underlying dark matter field relative to the galaxy sample.
Methodological Validation: The authors verify that the signal is robust against shot noise and that the peak position is a model-independent cosmic standard ruler, insensitive to the precise shape of the matter power spectrum.

Significance and Claims
The paper positions this work as a foundational step ("the first stone") for a new class of cosmological probes. Key claims regarding significance include:

Independence from Astrophysical Modeling: Unlike standard dark siren analyses, Peak Sirens do not require assumptions about the CBC mass distribution or merger rate, reducing astrophysical entanglement.
Direct Probe of Bias: The method offers a unique avenue to constrain the GW bias ( $b_{gw}$ ), a parameter inaccessible to current dark siren techniques, which is critical for understanding compact object formation mechanisms.
Feasibility with Limited Data: The detection of a 5.9 $\sigma$ signal using only two GW events demonstrates that the technique is viable even with sparse data, provided the events are well-localized and the galaxy catalog is sufficiently deep.
Future Potential: The authors note that while current precision is limited by the small number of events and the depth of the GLADE+ catalog, the method scales with the square root of the number of GW events. They project that with ~100 well-localized events, $H_0$ could be measured to ~4% precision. The technique is also highlighted as a promising tool for testing modified gravity theories where GW luminosity distances deviate from standard electromagnetic distances.

The authors remain modest regarding the current precision, acknowledging that the result is not yet competitive with state-of-the-art CMB or standard candle measurements, but emphasize its compatibility with existing constraints and its potential for future refinement with 3rd-generation GW detectors and deeper galaxy surveys (e.g., DESI, GAIA).

Technical Summary: First Measurement of the Hubble Constant from Gravitational Wave-Galaxy Cross-Correlations

More like this