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The DESI results impact the local determination of $H_0$

This paper demonstrates that incorporating DESI's evidence for evolving dark energy into local Hubble constant ( $H_0$ ) measurements reduces the inferred value by up to $2.5\,\mathrm{km\,s^{-1}Mpc^{-1}}$ compared to standard $\Lambda$ CDM assumptions, a shift that is mitigated but still significant when combined with CMB and SNIa data, thereby highlighting the impact of background cosmology assumptions on the Hubble tension.

Original authors: Michael S. Turner, Dragan Huterer

Published 2026-06-05

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: Michael S. Turner, Dragan Huterer

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 is a giant, expanding balloon. For decades, cosmologists have been trying to measure exactly how fast this balloon is inflating right now. This speed is called the Hubble Constant ( $H_0$ ).

Here is the problem: When we measure the speed by looking at nearby stars and galaxies (the "local" method), we get one number. When we look back at the baby picture of the universe (the Cosmic Microwave Background) and use our best theories to predict what the speed should be today, we get a slightly different, slower number. This mismatch is known as the Hubble Tension. It's like two different speedometers in the same car giving you different readings, and nobody knows which one is right.

This paper, written by Michael Turner and Dragan Huterer, suggests that the "local" speedometer might be slightly off because we've been using the wrong map to read it.

The Map and the Terrain

To measure the local speed of the universe, astronomers look at how far away things are and how fast they are moving away. To figure out "how far," they need a map of the universe's history.

For a long time, everyone used the same map, called $\Lambda$ CDM. It's a very simple, standard map that assumes the universe's expansion has been smooth and predictable, driven by a constant force called Dark Energy. Using this map, the local measurements say the universe is expanding at 73.5 units of speed.

However, a new, very powerful telescope instrument called DESI (Dark Energy Spectroscopic Instrument) recently looked at the universe and found something strange. The data suggests that Dark Energy isn't constant; it might be changing over time, like a car that accelerates or decelerates. DESI prefers a more complex map (called $w_0w_a$ CDM) that allows for this change.

The "Wrong Map" Effect

The authors of this paper asked a simple question: What happens to our local speed measurement if we stop using the simple, old map and start using the new, complex map that DESI likes?

They ran the numbers using the same local data (the same stars and galaxies) but swapped the map. Here is what they found:

The Shift: When they used the new DESI-favored map, the calculated speed of the universe dropped.
The Magnitude:
- If they only used the new DESI data, the speed dropped by as much as 2.5 units.
- If they combined the new DESI data with other cosmic data (like the baby picture of the universe and supernova explosions), the speed dropped by about 1.1 units.

A Creative Analogy: The GPS and the Road

Think of the local measurement of the Hubble Constant like a GPS trying to tell you your speed.

The Old Way ( $\Lambda$ CDM): You tell the GPS, "Assume the road is perfectly straight and flat." The GPS calculates your speed based on that assumption and says, "You are going 73.5 mph."
The New Way (DESI): The DESI instrument looks at the road ahead and says, "Actually, the road has a slight hill and a curve right here."
The Result: When you tell the GPS to recalculate your speed taking that hill and curve into account, it says, "Oh, if the road is curved, your actual speed is closer to 72.4 mph."

The paper argues that the "local" measurement of the universe's speed isn't a fixed number; it depends heavily on the shape of the "road" (the cosmology) we assume exists at low distances.

Why This Matters for the "Tension"

The Hubble Tension exists because the "Baby Picture" method says the speed is 68, and the "Local" method says it's 73.5. That's a big gap.

This paper suggests that the "Local" method might be overestimating the speed because it's using an oversimplified map. If the universe is actually behaving like the new DESI map suggests (with changing Dark Energy), the local speed isn't 73.5; it's actually closer to 72.4 or even lower.

This doesn't solve the tension completely (72.4 is still higher than 68), but it narrows the gap. It tells us that the "local" measurement isn't as independent of our theories as we thought. The value we get depends on the background story we tell about the universe.

The Bottom Line

The authors conclude that we cannot treat the local measurement of the universe's speed as a standalone fact. It is deeply tied to our assumptions about how the universe works at low distances.

If Dark Energy is constant: The speed is ~73.5.
If Dark Energy is evolving (as DESI suggests): The speed drops to ~72.4 or lower.

This means the "Hubble Tension" might not just be a measurement error or a need for new physics in the early universe; it might also be a sign that our understanding of the universe's recent history (the last few billion years) needs a serious update. The "speedometer" is working fine, but the "map" it's reading might need to be redrawn.

Technical Summary: The Impact of DESI Results on Local Hubble Constant Determinations

Problem Statement
The paper addresses the "Hubble tension," the persistent discrepancy between the local, distance-ladder determination of the Hubble constant ( $H_0 \approx 73.50 \pm 0.81$ km s $^{-1}$ Mpc $^{-1}$ ) and the indirect determination derived from Cosmic Microwave Background (CMB) data assuming the standard $\Lambda$ CDM model ( $H_0 \approx 68 \pm 0.5$ km s $^{-1}$ Mpc $^{-1}$ ). While much attention has been paid to "early universe" modifications (e.g., early dark energy) to resolve this tension, the authors investigate the dependence of local $H_0$ determinations on the assumed background cosmology at low redshifts ( $z \lesssim 0.1$ ). Specifically, they examine how recent Dark Energy Spectroscopic Instrument (DESI) results, which suggest evolving dark energy, alter the local $H_0$ value when analyzed under non- $\Lambda$ CDM models.

Methodology
The authors utilize the Hubble Distance Network (H0DN) dataset, a combination of low-redshift measurements ( $z \simeq 0 - 0.1$ ) used to derive the local Hubble constant. They employ a computer code that analyzes these measurements and their covariances to extract $H_0$ under varying cosmological assumptions.

The study proceeds in three stages:

Baseline: Establishing $H_0$ using the standard $\Lambda$ CDM model with $\Omega_M = 0.3$ .
Parameter Variation: Testing the sensitivity of local $H_0$ to variations in matter density ( $\Omega_M$ ) and the dark energy equation of state ( $w$ ) within the $w$ CDM framework.
DESI-Constrained Models: Analyzing the impact of the $w_0w_a$ $w_{0} w_{a}$ CDM model, where the dark energy equation of state evolves as $w(a) = w_0 + w_a(1-a)$ $w (a) = w_{0} + w_{a} (1 - a)$ . The authors sample cosmological parameters ( $\Omega_M, w_0, w_a$ $Ω_{M}, w_{0}, w_{a}$ ) from Markov chains constrained by:
- DESI Data Release 2 (DR2) BAO data alone.
- DESI + CMB data.
- DESI + CMB + Type Ia Supernova (SNIa) data.

They calculate the weighted average of the Hubble constant derived from the H0DN analysis for models consistent with these constraints, comparing the results to the baseline $\Lambda$ CDM value.

Key Contributions and Results

Cosmological Dependence of Local $H_0$ : The authors demonstrate that local determinations of $H_0$ are not independent of the background cosmology, even at low redshifts where the Hubble Law ( $d_L \approx cz/H_0$ ) is often assumed to be sufficient. While the dependence on $\Omega_M$ is small ( $\Delta H_0 \approx \pm 0.29$ km s $^{-1}$ Mpc $^{-1}$ ), the dependence on the dark energy equation of state is significant.
Impact of DESI Results: The DESI DR2 BAO data prefer a contour in the $w_0$ $w_{0}$ – $w_a$ $w_{a}$ plane with a degeneracy axis of $w_0 = -1 - w_a/3$ $w_{0} = - 1 - w_{a} /3$ (for $w_a < 0$ $w_{a} < 0$ ), indicating evidence for evolving dark energy. When the H0DN data is analyzed using models consistent with these DESI contours:
- Assuming DESI DR2 alone, the inferred $H_0$ can be as much as 2.5 km s $^{-1}$ Mpc $^{-1}$ lower than the $\Lambda$ CDM baseline.
- When constrained by DESI + CMB, the downward shift is $1.1 \pm 0.38$ km s $^{-1}$ Mpc $^{-1}$ , resulting in $H_0 = 72.44 \pm 0.81 \pm 0.38$ km s $^{-1}$ Mpc $^{-1}$ .
- When further constrained by DESI + CMB + SNIa, the shift is $0.5 \pm 0.1$ km s $^{-1}$ Mpc $^{-1}$ , resulting in $H_0 = 73.03 \pm 0.81 \pm 0.11$ km s $^{-1}$ Mpc $^{-1}$ .
Expansion Rate Features: The DESI-preferred models feature a dark energy component peaked around $z \simeq 0.5$ , which creates a corresponding feature in the expansion rate history. This evolution significantly alters the luminosity distance integral used to derive $H_0$ from low-redshift data.
CMB Connection: The authors note that moving along the DESI degeneracy direction also affects the CMB determination of $H_0$ . Specifically, the CMB-inferred $H_0$ decreases as $(1 - w_a)^{-0.044}$ , a trend that tends to maintain the tension rather than resolve it.

Significance and Claims
The paper claims that the background cosmology at low redshift is not well constrained by current data, and this lack of constraint introduces a significant systematic uncertainty into local $H_0$ determinations. The primary significance of the work is the demonstration that changes to the low-redshift cosmological background, consistent with recent DESI observations, can significantly shift the locally determined value of the Hubble constant.

The authors argue that resolving the Hubble tension may require considering physics at both high and low redshifts, as well as the interplay between them. They caution that while $w_0w_a$ CDM models consistent with DESI data are useful for probing deviations from $\Lambda$ at low redshifts, they may exhibit unphysical behavior over a large range of redshifts and should not be treated as a complete alternative cosmology without further validation. Ultimately, the work highlights that the "local" measurement of $H_0$ is more model-dependent than previously assumed, adding a layer of complexity to the Hubble tension debate.