New DESI Data Shifts the Local Value of the Hubble Constant ✦
Analysis of baryon acoustic oscillations (BAO) from DESI data reveals evidence for dark energy evolution. Using w0wa models favored by the DESI data reduces the local Hubble constant H0 by up to 2.5 km/s/Mpc relative to the ΛCDM estimate. When constraints from cosmic microwave background (CMB) and type Ia supernovae (SNIa) data are applied, the shift narrows: for DESI+CMB the reduction is 1.1 ± 0.38 km/s/Mpc, and for DESI+CMB+SNIa it is 0.5 ± 0.1 km/s/Mpc. The dependence of local H0 measurements on the cosmological background and the weak constraints on cosmology at low redshifts indicate the importance of this effect for the Hubble tension problem.
Context
Since Edwin Hubble discovered the universe's expansion, the Hubble constant has become a key number in cosmology. Its exact value, however, remains contentious: the discrepancy between local measurements and inferences from the cosmic microwave background (discovered by Arno Penzias), which carries the imprint of the first moments after the Big Bang, is known as the Hubble tension. It was traditionally assumed that local measurements are insensitive to the details of background cosmology, but growing precision (approaches developed by Adam Riess and others) is forcing a rethink of this assumption. New expansion data from spectroscopy of millions of galaxies by the DESI instrument, pointing to the evolution of dark energy, could substantially alter the picture.
Methods
The authors used data from the Hubble Distance Network (H0DN)—a combination of local distance measurements up to redshifts z ~ 0.1—along with open-source code to extract H0. To assess the impact of the cosmological model, they varied the dark energy parameters: in wCDM and w0wa models, where w is the equation-of-state parameter and wa its evolution. The matter density ΩM includes dark matter and ordinary matter. Using Markov chains from a joint analysis of DESI DR2 BAO, CMB data, and type Ia supernovae, they weighted H0 values according to model likelihoods.
Results
With a fixed ΛCDM and matter density ΩM=0.3, the local H0 = 73.50 km/s/Mpc. Varying ΩM within reasonable limits changes H0 by only ±0.29 km/s/Mpc, but more significant shifts occur when dark energy evolves. In the wCDM model with w far from -1, H0 can drop by 2.7 km/s/Mpc. In the DESI-preferred w0wa model with hints of w0 = -1 - wa/3, the H0 shift can reach 2.5 km/s/Mpc. A joint analysis with CMB and SNIa narrows the allowed range: for DESI+CMB+SNIa, H0 = 73.03 ± 0.81(stat.) ± 0.11(model) km/s/Mpc, which is 0.5 km/s/Mpc lower than the baseline; for DESI+CMB, the reduction is 1.1 ± 0.38 km/s/Mpc. Thus, accounting for possible dark energy evolution lowers the local H0 value.
Implications
It is shown that changes in the background cosmology at low redshifts can significantly impact local measurements of the Hubble constant—an effect previously overlooked. This not only adds systematic uncertainty to direct measurements but also demonstrates that the Hubble tension could be partially alleviated by modifications to the late universe, not just the early one. DESI's results underline that the standard ΛCDM model of expansion may be incomplete.
Future development
If the DESI data hold up, all local H0 measurements will need to be re-evaluated taking into account dynamic dark energy. Future surveys like Euclid and Roman will provide more precise BAO and supernova data, allowing tighter constraints on the w0wa model. Moreover, there will be a need to study physically motivated dark energy models, such as freezing scalar fields, rather than just phenomenological parameterizations.
Impact
This research will impact the field of cosmological standard rulers, the calibration of local distances via supernovae and Cepheids, as well as the interpretation of the cosmic microwave background.
Next steps
Next steps include reanalyzing all local distance datasets within w0wa models, and developing methods for jointly accounting for cosmological uncertainties when measuring H0.
Key open problems
This work is directly linked to the problem of the Hubble tension—the mismatch between H0 values from the early and late universe—and to the mystery of dark energy's nature, especially its possible evolution over time.
🎯 Over almost a century of measuring the Hubble constant, its value has evolved from the original 500 km/s/Mpc (obtained by Edwin Hubble himself) to today’s 73 with around 1% precision. Yet, the mystery of the Hubble tension remains unsolved.
Key numbers
- Baseline H0 value (ΛCDM): 73.50 km/s/Mpc
- Maximum H0 shift from w0wa model: 2.5 km/s/Mpc
- H0 reduction for DESI+CMB+SNIa: 0.5 km/s/Mpc
- Statistical error of H0DN: ±0.81 km/s/Mpc
- Redshift of local measurements: z ~ 0.1