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"SNe Ia twins" in the Hubble flow, and the determination of H0

By utilizing a carefully selected sample of 12 high-accuracy "SNe Ia twins" in the Hubble flow anchored by both Cepheid and JAGB distance measurements, this study derives a Hubble constant of approximately 72.38 km s⁻¹ Mpc⁻¹, thereby confirming the reality of the Hubble tension.

Original authors: Pilar Ruiz-Lapuente, Antonio Quintana-Estellés, Jonay I. González Hernández, Andrea Pastorello

Published 2026-04-22

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CC BY 4.0

Original authors: Pilar Ruiz-Lapuente, Antonio Quintana-Estellés, Jonay I. González Hernández, Andrea Pastorello

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 Cosmic Speed Limit: How "Twin" Supernovae Are Solving a 100-Year-Old Mystery

Imagine you are trying to measure the speed of a car driving away from you. You know how bright the headlights should be, and you measure how dim they look. From that, you can guess how far away the car is. If you know how long it's been driving, you can calculate its speed.

For decades, astronomers have been trying to measure the speed of the universe's expansion (the Hubble Constant, or $H_0$ ). But they have a massive problem: The universe is giving them two different answers.

The "Baby Picture" Method: Looking at the Cosmic Microwave Background (the afterglow of the Big Bang), the universe seems to be expanding at about 67 km/s per Megaparsec.
The "Adult Picture" Method: Looking at nearby exploding stars (Supernovae) and measuring their distance with "rulers" like Cepheid stars, the universe seems to be expanding at about 73 km/s per Megaparsec.

This difference is so big that it's like measuring a car's speed and getting 60 mph from the dashboard, but 85 mph from the radar gun. Physicists call this the "Hubble Tension." Something is wrong with our understanding of physics, or our measuring tools are broken.

The New Approach: Finding "Twins"

The authors of this paper (Ruiz-Lapuente and colleagues) decided to stop guessing and start comparing. They introduced a method using "SNe Ia Twins."

Think of Type Ia supernovae (exploding stars) as identical twins. They are born from the same type of white dwarf star and explode with almost the exact same amount of energy. Because they are so similar, if you see two of them, you can compare them directly.

The Old Way (The Ladder):
Traditionally, astronomers build a "distance ladder."

Rung 1: Measure the distance to a nearby star using geometry (like holding your thumb up to judge distance).
Rung 2: Use that nearby star to calibrate a "standard candle" (a Cepheid star) in a slightly farther galaxy.
Rung 3: Use that Cepheid to calibrate a Supernova in a faraway galaxy.
The Problem: Every time you climb a rung, you might introduce a tiny error. If the ladder is crooked, your final measurement is wrong.

The New Way (The Twin Method):
Instead of climbing a ladder, the authors found a nearby twin and a faraway twin that are practically identical in their DNA (their spectra, or light signatures).

They found a "local" supernova (like SN 2011fe or SN 2013aa) that they know the distance to very precisely.
They found a "distant" supernova in the Hubble flow that looks exactly like the local one.
Because the twins are identical, any difference in how bright they look is purely due to distance, not because one is naturally dimmer or redder.

It's like having two identical lightbulbs. You know one is 10 feet away. You see the other one across the room. Because you know they are the exact same bulb, you can calculate the distance across the room just by comparing how dim the second one looks. No ladder needed.

The "Twin" Discovery

The team selected 12 distant supernovae and matched them with 5 "anchor" supernovae nearby. They used a super-computer (MCMC) to compare the light spectra of the twins, phase by phase, like comparing two movies frame-by-frame.

The Results:

They calculated the expansion rate of the universe to be 72.38 km/s/Mpc.
This number is very close to the "Adult Picture" (73) and far away from the "Baby Picture" (67).
Conclusion: The Hubble Tension is real. It's not a mistake in the math; the universe really is expanding faster than the early-universe models predict.

Why Did the Old Methods Get It Wrong?

The paper also acts as a detective story, explaining why previous measurements sometimes gave weird results.

The authors found that in the big catalogs of supernovae (like Pantheon+), some "twins" were being misidentified.

The Dusty Room: Some supernovae were behind clouds of cosmic dust. The old methods tried to correct for this dust but often underestimated how much dust was there.
The Analogy: Imagine looking at a red car through a dirty window. If you think the window is only slightly dirty, you might think the car is naturally red. But if the window is actually very dirty, the car might be white!
Because they underestimated the dust, they thought the stars were dimmer than they really were, which made them think the stars were farther away than they actually were. This skewed the speed calculation.

The "Twin Method" is better at spotting this dust because it compares the entire spectrum (the full rainbow of light) of the twins, not just a few data points. It can tell the difference between a star that is naturally red and a star that is just dusty.

The Final Verdict

By using these "twins for life," the authors confirmed that:

The universe is expanding at a rate of roughly 72.4 km/s/Mpc.
The "Baby Picture" (CMB) and the "Adult Picture" (Supernovae) are genuinely disagreeing.
This means our current model of the universe (the $\Lambda$ CDM model) might be missing a piece of the puzzle—perhaps "New Physics" in the early universe that we haven't discovered yet.

In short: The universe is expanding faster than we thought, and by finding the perfect "twins" among the exploding stars, we finally have a ruler precise enough to prove it. The mystery isn't a measurement error; it's a clue to a deeper secret about how our universe works.

1. The Problem: The Hubble Tension and Calibration Issues

The paper addresses the persistent Hubble Tension, the significant discrepancy (at the $5.7\sigma$ to $6\sigma$ level) between the value of the Hubble constant ( $H_0$ ) derived from the Cosmic Microwave Background (CMB) by the Planck collaboration ( $67.4 \pm 0.5$ km s $^{-1}$ Mpc $^{-1}$ ) and the value derived from the local Universe using Type Ia Supernovae (SNe Ia) calibrated with Cepheid variables (SH0ES project: $73.3 \pm 0.9$ km s $^{-1}$ Mpc $^{-1}$ ).

The authors identify specific limitations in the standard "three-rung" distance ladder method used by major collaborations (Pantheon+ and the Carnegie-Chicago Hubble Program, CCHP):

Parameterization Failures: The standard light-curve parameterizations (SALT2 for Pantheon+, polynomial fits for CCHP) often fail to accurately model the color and reddening of SNe Ia, particularly for those with high host-galaxy extinction. This leads to inaccurate distance moduli and individual $H_0$ values that deviate significantly from the mean.
Anchor Discrepancies: While some anchor galaxies show agreement between methods (Cepheids vs. TRGB vs. JAGB), others (like M66 or NGC 5643) show fluctuations or divergences depending on the collaboration and method used, introducing systematic uncertainties.
Selection Bias: The standard method relies on modeling a diverse family of SNe Ia, which can introduce biases if specific subsamples are not representative.

2. Methodology: The "SNe Ia Twins" Approach

The authors propose a direct, single-step method to bridge the local distance scale and the Hubble flow, bypassing the need for a fiducial absolute magnitude calibration of the entire SN Ia population.

Concept of "Twins for Life": Instead of relying solely on light-curve stretch ( $s$ ) and color ( $c$ ), the method identifies SNe Ia that are spectral twins. These are supernovae that share identical spectral features (pseudo-equivalent widths of specific lines like Si II $\lambda$ 5972 and $\lambda$ 6355, and pseudo-continuum shapes) throughout their evolution, not just similar decline rates.
The Process:
1. Selection: Identify nearby "anchor" SNe Ia with well-determined distances (via Cepheids, TRGB, or JAGB) and find their spectral twins in the Hubble flow ( $z > 0.015$ ).
2. Spectral Comparison: Compare the spectra of the Hubble flow SN with its twin at multiple phases (early and nebular).
3. MCMC Analysis: Use Markov Chain Monte Carlo (MCMC) techniques (via the emcee package) to optimize the likelihood function. The model fits the flux differences between the observed and reference spectra to determine:
  - The relative distance modulus ( $\Delta \mu$ ).
  - The relative reddening difference ( $\Delta E(B-V)$ ).
4. Anchoring: The method uses a small, carefully vetted sample of 4 anchor galaxies (M101, NGC 5643, NGC 7250, M66) where distances determined by SH0ES (Cepheids) and CCHP (JAGB/TRGB) converge.
Subtypes: The study covers both Core Normal (CN) and Broad Line (BL) subtypes. BL subtypes (like SN 1989B) are particularly useful as anchors for high-velocity Hubble flow SNe.

3. Key Contributions

Direct Distance Measurement: The method measures distances from $\sim 6$ Mpc to $\sim 300$ Mpc in a single step, validating the three-rung method's output while avoiding the accumulation of errors from intermediate rungs.
Spectral Reddening Determination: By comparing spectra directly, the method provides a more robust determination of host-galaxy reddening ( $E(B-V)$ ) than light-curve fitting alone, identifying cases where standard parameterizations underestimate extinction.
Cross-Validation of Calibrators: The study successfully cross-validates distances using both Cepheid (SH0ES) and JAGB (CCHP) anchors, demonstrating that despite different methodologies, they converge on similar $H_0$ values when applied to a consistent sample of twins.
Identification of Systematic Errors: The paper pinpoints specific SNe Ia in the Pantheon+ and CCHP samples (e.g., SN 2007A, SN 2007ca) where color parameters ( $c$ or $B-V$ ) were underestimated, leading to erroneous individual $H_0$ values (e.g., $<65$ km s $^{-1}$ Mpc $^{-1}$ ).

4. Results

The study analyzed a sample of 12 SNe Ia in the Hubble flow, paired with 5 nearby anchors (SN 2011fe, SN 2013aa, SN 2017cbv, SN 2013dy, SN 1989B).

Hubble Constant Values:
- Cepheid Anchored (SH0ES): $H_0 = 72.56 \pm 1.54 \text{ (stat)} \pm 1.33 \text{ (sys)}$ km s $^{-1}$ Mpc $^{-1}$ .
- JAGB Anchored (CCHP): $H_0 = 72.20 \pm 1.53 \text{ (stat)} \pm 1.33 \text{ (sys)}$ km s $^{-1}$ Mpc $^{-1}$ .
- Combined Mean: $H_0 = 72.38 \pm 1.54 \text{ (stat)} \pm 1.33 \text{ (sys)}$ km s $^{-1}$ Mpc $^{-1}$ .
Agreement with Standard Methods: The mean $H_0$ derived from the twin method is consistent with the SH0ES and CCHP averages when using Cepheids, confirming the validity of the three-rung ladder for the bulk of well-modeled SNe.
Discrepancies Explained: The authors show that the outliers in existing compilations (SNe yielding $H_0 \approx 60-65$ km s $^{-1}$ Mpc $^{-1}$ ) are caused by underestimates of reddening in the color parameters of the light-curve fits. The twin method corrects these by directly measuring the spectral flux differences.
Hubble Diagram: The resulting Hubble diagram (Figure 16) shows a tight correlation with an intercept corresponding to $H_0 \approx 72.8$ km s $^{-1}$ Mpc $^{-1}$ .

5. Significance and Conclusion

Confirmation of the Hubble Tension: The results confirm that the Hubble tension is real. The value of $\sim 72.4$ km s $^{-1}$ Mpc $^{-1}$ is robustly incompatible with the Planck CMB value of $67.4$ km s $^{-1}$ Mpc $^{-1}$ .
Methodological Validation: The "SNe Ia twins" approach validates the distance scale derived by Pantheon+ and CCHP for the majority of SNe, proving that the three-rung method works when applied to a clean sample.
Diagnostic Tool: The method serves as a powerful diagnostic tool to identify and correct systematic errors in large SN compilations, specifically regarding reddening and light-curve parameterization.
Implications for Cosmology: Since the tension persists even with a direct, single-step measurement using independent anchors (Cepheids and JAGB), the authors conclude that the discrepancy likely requires new physics in the early Universe (beyond the standard $\Lambda$ CDM model) rather than being an artifact of local distance measurement errors.

In summary, this paper provides a rigorous, spectrum-based verification of the local expansion rate, isolating systematic errors in previous analyses and reinforcing the reality of the Hubble Tension.