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Measurement of the Hubble constant with high-energy neutrinos

This paper presents the first observational realization of a new neutrino-based distance ladder method using high-energy astrophysical neutrinos from 12 Seyfert galaxies to measure the Hubble constant, yielding a result of $H_0 = 49^{+40}_{-30}\,\mathrm{km\,s^{-1}\,Mpc^{-1}}$ that is consistent with existing determinations while demonstrating a novel approach free from electromagnetic propagation systematics.

Original authors: Gonzalo Herrera, Nicholas Kamp, Carlos A. Argüelles

Published 2026-08-04

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CC BY 4.0

Original authors: Gonzalo Herrera, Nicholas Kamp, Carlos A. Argüelles

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

Imagine the Universe as a giant, expanding balloon. For decades, scientists have been trying to measure exactly how fast this balloon is inflating. This speed is called the Hubble constant ( $H_0$ ), and knowing it is crucial because it tells us the age of the Universe and how it will end. But here's the tricky part: to measure the speed of expansion, you first need to know the exact distance to faraway objects. Measuring cosmic distances is like trying to guess how far away a friend is in a dark room just by how loud their voice sounds. Usually, we use light (photons) as our voice. But light is a fickle messenger; it gets blocked by dust, absorbed by gas, and scattered by clouds, making it hard to know how loud the voice actually started. This has led to a massive disagreement in the physics world: one group of scientists, using the oldest light in the Universe, says the balloon is expanding at one speed, while another group, using exploding stars, says it's faster. This is known as the "Hubble tension," and it's one of the biggest mysteries in modern physics.

Now, imagine a messenger that doesn't care about dust, gas, or darkness. Enter the neutrino: a tiny, ghost-like particle that can zip through entire planets without bumping into anything. Because they travel unattenuated, neutrinos are perfect for measuring cosmic distances without the "static" that messes up light. In this new study, a team of researchers decided to try a bold experiment: use high-energy neutrinos from active galaxies as a new ruler to measure the Universe's expansion. They didn't just simulate this; they looked at real data from the IceCube Neutrino Observatory, which sits buried in the ice of Antarctica, waiting for these ghostly particles to pass through.

The team focused on 12 specific galaxies called Seyfert galaxies. These are cosmic powerhouses with supermassive black holes at their centers, surrounded by a hot, glowing "corona" of plasma. The researchers used a clever trick based on a relationship between two types of energy these galaxies emit: X-rays and neutrinos. Think of the corona like a kitchen. The X-rays are the heat coming from the stove, and the neutrinos are the smell of the food cooking. The scientists hypothesized that if you know how hot the stove is (the X-ray brightness), you can predict how strong the smell should be (the neutrino brightness). If the smell is weaker or stronger than expected for a given heat, it tells you how far away the kitchen is.

Using data from IceCube's 14-year collection, the team found that for these 12 galaxies, the relationship between the X-ray heat and the neutrino smell isn't a simple 1-to-1 match. Instead, they found that as the galaxies get brighter in X-rays, the neutrino signal doesn't grow as fast as some theories predicted. Specifically, they measured a slope value of $\beta = 0.67^{+0.16}_{-0.25}$ . This result suggests that the "kitchen" isn't perfectly efficient; the neutrino production becomes less efficient as the galaxy gets brighter, ruling out a specific "calorimetric" limit where the efficiency would be perfect (a value of $\beta = 1$ ) at about 2 standard deviations of certainty.

With this new "neutrino ruler" calibrated, they calculated the Hubble constant. Their result is $H_0 = 49^{+40}_{-30}$ km s $^{-1}$ Mpc $^{-1}$ . While this number has a large margin of error (meaning the true value could be anywhere from 19 to 89), it is a groundbreaking first step. It shows that neutrinos can indeed be used as a new tool for cosmology, free from the dust and gas that confuse light-based measurements. The result sits comfortably between the two conflicting values from previous studies, suggesting that neutrinos might eventually help solve the Hubble tension, but for now, the measurement is more of a proof-of-concept than a final answer. The authors emphasize that while the uncertainty is large, the method is solid and offers a fresh, systematic-free way to look at the expanding Universe.

Technical Summary: Measurement of the Hubble constant with high-energy neutrinos

Problem Statement
Determining the scale of the Universe and resolving the "Hubble tension"—the $\sim 5\sigma$ discrepancy between early-Universe (Planck CMB) and late-Universe (Cepheid–supernova) determinations of the Hubble constant ( $H_0$ )—is a central challenge in modern cosmology. Nearly all existing distance probes rely on photons, which are subject to propagation systematics including dust extinction, intergalactic absorption, scattering, and complex radiative-transfer modeling. These effects introduce uncertainties that are difficult to disentangle from intrinsic source properties. A distance measurement utilizing a messenger that traverses the Universe unattenuated would bypass these electromagnetic systematics entirely.

Methodology
This work introduces a new rung on the cosmic distance ladder utilizing high-energy astrophysical neutrinos from point sources as "standardizable candles." The method relies on the physical correlation between neutrino luminosity ( $L_\nu$ ) and X-ray luminosity ( $L_X$ ) in the coronae of Seyfert galaxies, described by the power-law relation:
$L_\nu = \kappa L_X^\beta$
where $\kappa$ is a normalization constant and $\beta$ is a slope encoding corona physics.

The analysis utilizes a sample of 12 X-ray-selected Seyfert galaxies identified in IceCube's 14-year public point-source release. The methodology proceeds as follows:

Anchoring: The ladder is anchored by a non-redshift distance to NGC 1068, derived from a combination of Cepheid variables and the Tip of the Red Giant Branch (TRGB) ( $10.93 \pm 0.74$ Mpc). This breaks the degeneracy between the normalization $\kappa$ and $H_0$ .
Data Extraction: Neutrino signal counts ( $\hat{n}_s$ ) and uncertainties are extracted for each source using the SkyLLH unbinned likelihood framework applied to 14-year IceCube data. X-ray fluxes ( $F_X$ ) are taken from the BAT AGN Spectroscopic Survey (BASS), corrected for absorption.
Likelihood Construction: The authors construct a Gaussian likelihood in luminosity space. The predicted neutrino signal depends on the Hubble constant through the luminosity distance $d_L(z, H_0)$ . Specifically, the expected signal scales as $d_L^{2(\beta-1)}$ . If $\beta = 1$ (the calorimetric limit), the distance dependence cancels, rendering the method blind to $H_0$ . Therefore, a non-unity slope is required for the measurement.
Statistical Inference: A Markov Chain Monte Carlo (MCMC) analysis is performed over the parameter space $\{H_0, \beta, d_{1068}\}$ . The normalization $\kappa$ is profiled out analytically at each likelihood evaluation. The analysis accounts for peculiar velocities and CMB-frame redshift corrections, particularly for low-redshift sources.

Key Results
Using the 12-source sample, the authors report the first measurement of the Hubble constant using high-energy astrophysical neutrinos:
$H_0 = 49^{+40}_{-30} \text{ km s}^{-1} \text{ Mpc}^{-1} \quad (68\% \text{ credible interval})$
The analysis simultaneously yields the first population-level measurement of the neutrino–X-ray luminosity slope:
$\beta = 0.67^{+0.16}_{-0.25} \quad (68\% \text{ credible interval})$

Key Findings and Physics Implications

Sub-calorimetric Slope: The data prefer a sub-linear slope ( $\beta < 1$ ), excluding the calorimetric limit ( $\beta = 1$ ) at approximately $2\sigma$ . This suggests that in these Seyfert coronae, neutrino production becomes less efficient per unit of X-ray luminosity as the source brightens. The authors suggest this may be due to synchrotron cooling of secondary pions/muons or Bethe–Heitler pair production losses in dense, magnetized coronae.
Consistency with Existing Probes: The measured $H_0$ is consistent with both the Planck CMB value ( $67.4 \pm 0.5$ ) and the SH0ES Cepheid–supernova value ( $73.0 \pm 1.0$ ) within $1\sigma$ , though the large uncertainty prevents this measurement from currently resolving the Hubble tension.
Systematic Freedom: The measurement is free of electromagnetic propagation systematics (extinction, scattering). The primary uncertainties arise from the per-source counting statistics of the neutrino data and the intrinsic scatter in the $L_\nu$ – $L_X$ relation (absorbed into the X-ray flux uncertainty).

Significance and Claims
The paper claims to demonstrate the viability of high-energy neutrinos as a novel cosmographical probe. Its primary significance lies in establishing a new rung of the cosmic distance ladder where photons are only required at the local anchor (NGC 1068) and never along the propagation path to the observer. While the current precision is limited by the small sample size and IceCube's effective area, the authors project that the upcoming IceCube-Gen2 observatory, with an $\sim 8\times$ increase in effective area, could reduce the uncertainty on $H_0$ to $\sim 16\text{--}20 \text{ km s}^{-1} \text{ Mpc}^{-1}$ with $\sim 50$ resolved sources. The work establishes that neutrinos can provide independent distance constraints that are immune to the dust and absorption systematics limiting all other current distance probes.

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