🔭 astrophysics

The DESI DR1 Peculiar Velocity Survey: global zero-point and $H_0$ constraints

This paper utilizes the Dark Energy Spectroscopic Instrument's first data release to calibrate the zero-point of the Fundamental Plane and Tully-Fisher relations, yielding a precise measurement of the Hubble constant ( $H_0 = 73.7 \pm 1.1$ km s $^{-1}$ Mpc $^{-1}$ ) that offers a robust alternative to supernovae-based determinations while highlighting the potential for future percent-level accuracy.

Original authors: A. Carr, C. Howlett, A. J. Amsellem, Tamara M. Davis, K. Said, D. Parkinson, A. Palmese, J. Aguilar, S. Ahlen, J. Bautista, S. BenZvi, D. Bianchi, C. Blake, D. Brooks, T. Claybaugh, A. Cuceu, A. de la

Published 2026-06-19

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: A. Carr, C. Howlett, A. J. Amsellem, Tamara M. Davis, K. Said, D. Parkinson, A. Palmese, J. Aguilar, S. Ahlen, J. Bautista, S. BenZvi, D. Bianchi, C. Blake, D. Brooks, T. Claybaugh, A. Cuceu, A. de la Macorra, P. Doel, K. Douglass, S. Ferraro, J. E. Forero-Romero, E. Gaztañaga, S. Gontcho A Gontcho, G. Gutierrez, H. K. Herrera-Alcantar, K. Honscheid, D. Huterer, M. Ishak, R. Joyce, A. G. Kim, D. Kirkby, A. Kremin, O. Lahav, C. Lamman, M. Landriau, L. Le Guillou, M. E. Levi, M. Manera, A. Meisner, R. Miquel, J. Moustakas, S. Nadathur, W. J. Percival, F. Prada, I. Pérez-Ràfols, F. Qin, C. Ross, G. Rossi, E. Sanchez, D. Schlegel, H. Seo, D. Sprayberry, G. Tarlé, R. J. Turner, B. A. Weaver, P. Zarrouk, R. Zhou, H. Zou

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

The Big Picture: Measuring the Universe's Speed

Imagine the Universe is a giant balloon being blown up. Astronomers want to know exactly how fast that balloon is inflating right now. This speed is called the Hubble Constant ( $H_0$ ).

To measure this, you need two things for a distant object:

How fast it's moving away from us (easy to measure by looking at its color shift).
How far away it actually is (very hard to measure).

This paper is about the Dark Energy Spectroscopic Instrument (DESI), a massive telescope project that just released its first batch of data (DR1). The team used this data to measure the distances to over 100,000 galaxies. By combining these new distance measurements with the speed data, they calculated the Universe's expansion rate with incredible precision.

The Problem: The "Ruler" is Missing

The paper explains that while DESI is great at measuring relative distances (like saying "Galaxy A is twice as far as Galaxy B"), it doesn't know the absolute scale (like saying "Galaxy A is 100 million light-years away").

Think of it like this: You have a very accurate tape measure, but you don't know if the "inch" marks on it are actually inches, or if they are centimeters, or if they are inches that have stretched out. You need to calibrate the tape measure against a known standard.

In astronomy, this calibration is called the "Zero-Point." Without it, your map of the Universe is distorted.

The Solution: Using "Standard Candles" and "Standard Rulers"

To fix the tape measure, the astronomers used two types of cosmic objects that act as known benchmarks:

Spiral Galaxies (The Tully-Fisher Relation): These act like Standard Candles. If you know how bright a spiral galaxy should be based on how fast it spins, you can tell how far away it is by how dim it looks to us.
Elliptical Galaxies (The Fundamental Plane): These act like Standard Rulers. If you know how big a galaxy should be based on how its stars move, you can tell how far away it is by how small it looks.

DESI measured over 100,000 of these galaxies. However, they still needed to anchor these measurements to a known distance to get the final number.

The Innovation: The "Group Chat" Trick

The biggest challenge was that the "anchors" (galaxies with known distances, like those hosting supernovae) were very rare in the DESI data. It was like trying to calibrate a ruler using only one or two known marks.

The team came up with a clever workaround: Galaxy Groups.

The Analogy: Imagine you are trying to find the average height of people in a city, but you only know the exact height of a few specific people. Instead of just measuring those few, you look at their neighborhoods. If you know the height of one person in a neighborhood, you assume everyone in that neighborhood is roughly at the same distance from the center of the city.
The Result: By grouping galaxies together, the team increased their number of "anchors" by a factor of 10. Instead of just matching a few galaxies, they matched entire clusters. This allowed them to calibrate their "tape measure" much more accurately.

The Results: How Fast is the Universe Expanding?

After calibrating their measurements using a famous catalog of exploding stars (Type Ia Supernovae) as the ultimate reference, the team calculated the expansion rate:

The Number: 73.7 km/s per Megaparsec.
- Translation: For every 3.26 million light-years you go further away, the Universe is expanding an extra 73.7 kilometers per second faster.
The Precision: This is the most precise measurement of its kind using this specific method. The statistical error is tiny (only 0.06), meaning the measurement is extremely tight.
The Catch: The biggest uncertainty comes from the "calibration" itself (the fact that the supernovae they used to calibrate also have their own small errors). The total uncertainty is about 1.1.

Why This Matters

Consistency: Their result (73.7) agrees very well with other recent measurements that use supernovae (like the SH0ES project), which is a big deal because it confirms those earlier findings.
A New Tool: This proves that DESI can measure the expansion of the Universe independently. It's not just relying on the old "distance ladder" methods; it's building a new, massive ladder with over 100,000 rungs.
Future Potential: The paper notes that as DESI continues to observe more of the sky, they will be able to use even more direct anchors (like specific types of stars called Cepheids) to reduce the uncertainty even further, potentially reaching a "percent-level" accuracy.

Summary

The authors took a massive new dataset of 100,000 galaxies, used a clever "grouping" trick to find enough reference points to calibrate their measurements, and determined that the Universe is expanding at 73.7 km/s/Mpc. This confirms previous high-speed measurements and sets the stage for even more precise cosmology in the future.

Technical Summary: The DESI DR1 Peculiar Velocity Survey: Global Zero-Point and $H_0$ Constraints

Problem Statement
While galaxy redshift surveys provide precise spectroscopic redshifts, they cannot distinguish between cosmological recession velocities and local peculiar velocities (PVs) induced by gravitational inhomogeneities. Accurate distance measurements are required to break this degeneracy, enabling tests of the Hubble–Lemaître law and studies of structure growth. Previous peculiar velocity (PV) samples have been limited in size compared to redshift surveys, leading to large statistical uncertainties in derived cosmological parameters. Specifically, the Dark Energy Spectroscopic Instrument (DESI) Early Data Release (EDR) Fundamental Plane (FP) measurement of the Hubble constant ( $H_0$ ) suffered from significant statistical errors due to reliance on a single calibration anchor (the Coma cluster). This paper addresses the need for a larger, more robust calibration of the DESI PV sample to measure $H_0$ with reduced statistical uncertainty and to investigate systematic limits in absolute calibration.

Methodology
The authors utilize the DESI Data Release 1 (DR1) Peculiar Velocity Survey, which includes over 100,000 galaxies with relative distance measurements derived from the Tully-Fisher (TF) relation for spiral galaxies and the Fundamental Plane (FP) relation for elliptical galaxies.

Sample Construction: The analysis combines 96,758 FP galaxies and 7,277 TF galaxies (after applying a redshift cut of $z < 0.1$ ).
Group-Based Calibration: To overcome the small overlap between the DESI PV sample and external distance calibrators, the authors employ a group catalog derived from four external surveys (2dFGRS, 6dFGS, 2MRS, and SDSS; Lim et al. 2017). By assuming all members of a galaxy group share the same distance, they effectively double the number of available calibrators.
Zero-Point Determination: The TF and FP relations provide relative distances. To anchor these to an absolute scale, the authors calculate a global zero-point by comparing the DESI log-distance ratios ( $\eta$ $η$ ) against external distance ladders.
- Fiducial Calibrator: Type Ia supernovae (SNe Ia) from the SH0ES/Pantheon+ sample.
- Alternative Calibrators: Surface Brightness Fluctuation (SBF) distances, masers, Cepheids, and the Tip of the Red Giant Branch (TRGB).
- Coma Cluster: Used as a specific test case, calibrated via both SBF and SNe Ia.
Systematic Testing: The authors perform extensive tests on systematics, including variations in grouping algorithms, FP/TF catalog selection cuts (e.g., spiral contamination, quality cuts), redshift corrections (using $z_{CMB}$ vs. velocity-field reconstructed $z_{HD}$ ), and the application of group-based corrections to the weighted mean.
Cosmological Fitting: The calibrated distances are fitted to a Hubble diagram using a Taylor series expansion of the luminosity distance in a flat $\Lambda$ CDM universe. The fitting utilizes the dynesty nested sampling algorithm via bilby, incorporating a full covariance matrix that accounts for statistical errors, zero-point uncertainties, and systematic floors.

Key Contributions

Scale: This work presents the largest PV sample to date, utilizing over 100,000 galaxies, a significant increase over previous catalogs.
Methodological Improvement: The introduction of a group-matching technique significantly increases the number of calibrator overlaps (e.g., increasing SN Ia host overlaps from ~45 to ~571 galaxies), thereby reducing statistical uncertainty.
Systematic Characterization: The paper provides a detailed error budget, distinguishing between statistical uncertainty, zero-point uncertainty (treated as an irreducible floor), and systematic uncertainties arising from the analysis pipeline.
Calibration Comparison: It offers a direct comparison of $H_0$ constraints derived from multiple independent distance ladder rungs (SNe Ia, SBF, Masers, Coma cluster) within the same DESI dataset.

Results

Fiducial Measurement: Calibrating to the SH0ES/Pantheon+ SNe Ia, the authors measure:
$H_0 = 73.7 \pm 0.06 \text{ (stat.)} \pm 1.1 \text{ (syst.)} \text{ km s}^{-1} \text{ Mpc}^{-1}$
The statistical uncertainty is drastically reduced compared to the EDR FP result ($0.35$ km s $^{-1}$ Mpc $^{-1}$ ), approaching the precision expected from the sample size increase ( $\propto 1/\sqrt{N}$ ).
Alternative Calibrators:
- SBF: Yields $H_0 = 74.1 \pm 0.06 \text{ (stat.)} \pm 1.1 \text{ (syst.)}$ , consistent with the SN Ia result.
- Combined SN+SBF: Provides the tightest constraint, $H_0 = 73.9 \pm 0.06 \text{ (stat.)} \pm 1.0 \text{ (syst.)}$ , though the authors note the systematic uncertainty may be underestimated due to a lack of full SBF covariance data.
- Coma Cluster: Calibrating solely to the Coma cluster yields higher values ( $H_0 \approx 77.4-77.6$ ), suggesting that single-cluster calibration may introduce bias compared to a multi-anchor approach.
- Masers: With only two overlapping galaxies, the result is $H_0 = 72.6 \pm 5.6$ , demonstrating the current limitation of low overlap with primary calibrators.
Systematics: The total systematic uncertainty is estimated at $1.1$ km s $^{-1}$ Mpc $^{-1}$ , dominated by the uncertainty inherited from the SH0ES distance ladder. The internal systematics of the DESI PV analysis (grouping, selection cuts) contribute a smaller $\sim 0.29$ km s $^{-1}$ Mpc $^{-1}$ .
$q_0$ Constraints: When allowing the deceleration parameter $q_0$ to vary, the fit yields $q_0 = -0.56^{+0.01}_{-0.02}$ , though the authors caution that this is heavily influenced by the prior and the limited redshift range of the PV sample.

Significance and Claims
The paper claims that the DESI DR1 PV survey provides a measurement of $H_0$ with statistical precision superior to previous PV-based measurements and competitive with the SH0ES result, while maintaining consistency with the SH0ES/Pantheon+ value. The authors assert that their method demonstrates the viability of using the TF and FP relations as an alternative "Hubble flow" sample for $H_0$ measurements.

However, the authors remain modest regarding the ultimate precision. They explicitly state that the current measurement is limited by the "irreducible" systematic uncertainty inherited from the three-rung distance ladder (Cepheids $\to$ SNe Ia). They claim that future data releases with larger samples and direct overlap with primary calibrators (Cepheids, TRGB) will allow the uncertainty to decrease toward the percent level, potentially providing an independent check on the Hubble tension. The paper concludes that while supernovae currently remain the best calibrator due to statistical power and sky overlap, the DESI PV survey is a robust and growing alternative to the traditional supernova distance ladder.