🔭 astrophysics

EMPRESS. XV. A New Determination of the Primordial Helium Abundance Suggesting a Moderately Low $Y_\mathrm{P}$ Value

Using Subaru near-infrared spectroscopy of 29 galaxies, including 14 extremely metal-poor systems, this study determines a moderately low primordial helium abundance of $Y_\mathrm{P} = 0.2402^{+0.0040}_{-0.0040}$ , which aligns with recent EMPG-based and ACT CMB constraints but suggests a mild tension with the Standard Model that may indicate nonzero lepton asymmetry.

Original authors: Hiroto Yanagisawa, Masami Ouchi, Akinori Matsumoto, Masahiro Kawasaki, Kai Murai, Kimihiko Nakajima, Kazunori Kohri, Yuma Sugahara, Kentaro Nagamine, Ichi Tanaka, Ji Hoon Kim, Yoshiaki Ono, Minami Nak

Published 2026-04-06

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: Hiroto Yanagisawa, Masami Ouchi, Akinori Matsumoto, Masahiro Kawasaki, Kai Murai, Kimihiko Nakajima, Kazunori Kohri, Yuma Sugahara, Kentaro Nagamine, Ichi Tanaka, Ji Hoon Kim, Yoshiaki Ono, Minami Nakane, Keita Fukushima, Yuichi Harikane, Yutaka Hirai, Yuki Isobe, Haruka Kusakabe, Masato Onodera, Michael Rauch, Hidenobu Yajima

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, cosmic kitchen that opened its doors just moments after the Big Bang. In those first few minutes, the chefs (the fundamental forces of nature) cooked up the very first ingredients: mostly hydrogen, a little bit of helium, and a tiny pinch of lithium.

This paper is like a team of cosmic chefs and food critics trying to figure out the original recipe. Specifically, they are trying to measure exactly how much helium was in that first batch of "primordial soup."

Here is the story of their investigation, broken down into simple concepts:

1. The Mystery of the "Missing" Helium

For decades, astronomers have been trying to measure the amount of helium created in the Big Bang (called $Y_P$ ). Think of this like trying to taste a soup to see how much salt was added before the chef started adding vegetables and spices later on.

The problem is that most galaxies we can see today are like "stew" that has been cooking for billions of years. They have added so many heavy elements (like oxygen and carbon) that it's hard to tell what the original helium-to-hydrogen ratio was. To get the true "original recipe," scientists need to find galaxies that are like freshly baked bread—extremely young and almost entirely made of the original ingredients. These are called Extremely Metal-Poor Galaxies (EMPGs).

2. The New Ingredients: Subaru's "Super-Eyes"

In the past, scientists had to guess the original recipe by looking at "stew" (metal-rich galaxies) and mathematically subtracting the extra ingredients they added over time. This is like trying to guess the original salt level in a soup by tasting a bowl that's been sitting out for a week; it's risky and prone to error.

This new study, led by Hiroto Yanagisawa and a large team, used the Subaru Telescope in Hawaii. They used special infrared cameras (SWIMS and MOIRCS) that act like super-powered night-vision goggles. These goggles allowed them to see a specific type of light (the He I 10830 line) that is invisible to regular telescopes.

The Analogy: Imagine trying to find a specific person in a crowded room. Regular telescopes see the whole crowd. The Subaru's infrared goggles can see a specific glowing badge on that person's shirt, allowing the team to count them accurately even in a dense crowd.

They observed 29 new galaxies, 14 of which were these rare, "freshly baked" EMPGs. This is a huge improvement over previous studies, which only had a handful of these rare galaxies.

3. Solving the "Temperature vs. Density" Puzzle

To measure the helium, the team had to solve a tricky physics puzzle. Inside these galaxies, the gas is hot and dense, but the heat and the density affect the light in a way that makes them look similar (a "degeneracy"). It's like trying to tell if a room is hot because the heater is on, or because there are too many people in it.

The Solution: The Subaru telescope's ability to see that specific infrared light (He I 10830) acted like a thermometer that also counts people. It broke the tie, allowing the team to know exactly how hot and how dense the gas was, leading to a much more accurate helium count.

4. The Result: A Lower Number

After crunching the numbers for their 29 new galaxies plus 58 older ones (a total of 67), they found a new value for the primordial helium abundance: 0.2402.

The Twist: This number is slightly lower than what most previous studies found. It's like if everyone in the world agreed a cake had 200 grams of sugar, but this new, more precise taste test says it actually has 180 grams.

This lower number is interesting because it matches:

Recent measurements from the Atacama Cosmology Telescope (ACT) looking at the Cosmic Microwave Background (the "afterglow" of the Big Bang).
A few other recent studies that also found lower numbers.

However, it is still a bit lower than the "Standard Model" prediction (the theoretical recipe we thought was correct).

5. The Cosmic Implication: A "Leaky" Universe?

Why does a slightly lower helium number matter?

In the Big Bang, the amount of helium created depends on how fast the universe was expanding in its first few minutes.

Standard Theory: The universe expanded at a specific, predictable speed.
The Tension: The new, lower helium number suggests the universe might have expanded slower than we thought, or that something else was happening.

The authors suggest a fascinating possibility: Lepton Asymmetry.

The Analogy: Imagine the universe as a balanced scale. On one side, you have matter; on the other, antimatter. Usually, we think they were perfectly balanced. But this result suggests there might have been a tiny, invisible weight on the scale (a "lepton asymmetry") that tipped the balance just enough to change how the helium was cooked.

If this is true, it could help solve another massive mystery in physics called the "Hubble Tension" (where different ways of measuring the universe's expansion rate give different answers). A "leaky" or asymmetric universe could be the missing piece of the puzzle.

Summary

This paper is a major step forward in cosmic archaeology. By using the Subaru telescope to find the universe's "freshest" ingredients, the team has refined the measurement of the Big Bang's helium recipe.

Their finding of a slightly lower helium value hints that our current understanding of the early universe might need a small tweak—perhaps involving a hidden imbalance between particles and antiparticles. While it's not a final proof yet, it's a strong clue that the universe's first moments were more complex and interesting than we previously imagined.

The Bottom Line: We are getting a clearer picture of the universe's birth, and that picture is starting to look a little different than the one we drew before.

1. Problem Statement

The determination of the primordial helium abundance ( $Y_P$ ) is critical for testing Big Bang Nucleosynthesis (BBN) and constraining cosmological parameters, particularly the effective number of neutrino species ( $N_{\text{eff}}$ ).

The Hubble Tension: There is a $\sim 5\sigma$ tension between the Hubble constant ( $H_0$ ) derived from the Cosmic Microwave Background (CMB) and local distance ladder measurements. A higher $N_{\text{eff}}$ (greater than the standard model value of 3.044) could increase the early universe expansion rate, potentially alleviating this tension.
The $Y_P$ - $N_{\text{eff}}$ Link: $Y_P$ is sensitive to $N_{\text{eff}}$ . A higher expansion rate (higher $N_{\text{eff}}$ ) leads to a higher neutron-to-proton ratio, resulting in more helium production.
Observational Challenges: Previous determinations of $Y_P$ $Y_{P}$ suffer from:
1. Lack of Extremely Metal-Poor Galaxies (EMPGs): Most samples rely on metal-poor galaxies (MPGs; $0.1-0.4 Z_\odot$ ), requiring significant extrapolation to zero metallicity to find the primordial value.
2. Systematic Uncertainties: Degeneracies between electron temperature ( $T_e$ ) and density ( $n_e$ ) in photoionization modeling, and uncertainties in atomic collision strengths for hydrogen lines.
3. Statistical Limitations: Small sample sizes of EMPGs (typically $<10$ ) lead to large statistical errors.

2. Methodology

The authors present a new analysis based on the EMPRESS (Extremely Metal-Poor Representatives Explored by the Subaru Survey) 3D project.

A. Observational Data

New Sample: Near-infrared (NIR) spectroscopy of 29 galaxies using the Subaru Telescope (SWIMS and MOIRCS instruments).
- Target: 14 are EMPGs (metallicity $< 0.1 Z_\odot$ ).
- Key Line: Detection of the He I $\lambda 10830$ Å emission line. This line is crucial for breaking the $T_e$ - $n_e$ degeneracy because its intensity is highly sensitive to electron density.
Literature Sample: Combined with 58 galaxies from previous studies (Hsyu et al. 2020; Matsumoto et al. 2022) that have robust optical and NIR data.
Total Final Sample: After rigorous selection criteria, a final sample of 67 galaxies was used for the $Y_P$ determination.

B. Data Analysis and Modeling

Flux Measurement: Gaussian fitting of emission lines (H I, He I, [O II], [O III], [S II], [S III], etc.) to derive flux ratios and equivalent widths.
Photoionization Modeling (YMCMC):
- Used the YMCMC code (Hsyu et al. 2020) to model the physical conditions of the H II regions.
- Parameters: Electron temperature ( $T_e$ ), density ( $n_e$ ), reddening ( $c(H\beta)$ ), stellar absorption, and neutral helium fraction.
- Breakthrough: The inclusion of He I $\lambda 10830$ allows the model to constrain $n_e$ independently, resolving the degeneracy that plagues optical-only studies.
Selection Criteria: Galaxies were excluded if:
- YMCMC failed to reproduce observed line ratios within $2\sigma$ .
- Neutral helium ( $y^0$ ) contribution was significant (based on the radiation softness parameter $\eta_{\text{rds}}$ ).
- Nitrogen-to-oxygen ratios indicated chemical enrichment deviating from primordial trends ( $\log(\text{N/O}) > -1.0$ ).

C. Derivation of $Y_P$

Linear Extrapolation: The helium-to-hydrogen number ratio ( $y = \text{He/H}$ ) was plotted against oxygen abundance ( $\text{O/H}$ ).
Regression: A linear fit $y = y_P + \frac{dy}{d(\text{O/H})}(\text{O/H})$ was performed using a maximum likelihood estimator that accounts for intrinsic scatter ( $\sigma_{\text{int}}$ ).
Systematic Checks: The authors tested different atomic data sets (CHIANTI vs. standard), prior distributions (allowing negative physical parameters to avoid bias), and slope assumptions (fixed slope vs. free slope).

3. Key Contributions

Largest EMPG Sample: This study significantly increases the number of EMPGs with reliable He I $\lambda 10830$ detections (14 new EMPGs), reducing the reliance on extrapolation from metal-rich galaxies.
Breaking Degeneracy: The systematic use of the He I $\lambda 10830$ line in a large sample provides the most robust constraints on electron density to date, minimizing systematic errors in $T_e$ and $n_e$ determination.
Comprehensive Systematic Assessment: The paper rigorously tests the impact of atomic data choices, prior distributions in MCMC, and chemical evolution models on the final $Y_P$ value.

4. Results

A. Primordial Helium Abundance ( $Y_P$ )

Fiducial Result: Using the full sample (EMPGs + MPGs) with standard priors:
$Y_P = 0.2402 \pm 0.0040$
(Corresponding to number abundance $y_P = 0.0790 \pm 0.0017$ ).
Comparison:
- This value is $\sim 1\sigma$ lower than most previous observational estimates (e.g., Izotov et al. 2014, Hsyu et al. 2020) and the Standard BBN (SBBN) prediction ( $Y_P \approx 0.2467$ ).
- It is consistent with the recent low- $Y_P$ result from Matsumoto et al. (2022) and the CMB constraint from the Atacama Cosmology Telescope (ACT).
Robustness: Variations in atomic data, priors, and slope assumptions shift the result by only $\sim 0.0017$ , which is within the statistical uncertainty and does not alter the conclusion of a "moderately low" $Y_P$ .

B. Cosmological Implications

Combining the new $Y_P$ with the primordial deuterium measurement $(D/H)_P = (2.527 \pm 0.030) \times 10^{-5}$ :

Standard Model Tension:
- Assuming standard physics ( $\xi_e = 0$ ), the result implies:
  $N_{\text{eff}} = 2.54^{+0.20}_{-0.25}$
- This is in mild tension ( $\sim 2\sigma$ ) with the standard value $N_{\text{eff}} = 3.044$ and the Planck CMB constraint.
Lepton Asymmetry Solution:
- To resolve the tension, the authors allow for a non-zero electron neutrino degeneracy parameter ( $\xi_e$ ), representing lepton asymmetry.
- The best-fit parameters become:
  $N_{\text{eff}} = 3.23^{+0.20}_{-0.26}, \quad \xi_e = 0.05^{+0.02}_{-0.03}$
- This suggests a $\sim 2\sigma$ evidence for lepton asymmetry, which would increase the expansion rate (raising $N_{\text{eff}}$ ) and lower the required $Y_P$ to match observations, potentially alleviating the Hubble tension.

5. Significance

Cosmology: The result provides independent evidence supporting a "low" primordial helium abundance, which challenges the standard BBN prediction if interpreted strictly within the standard model.
New Physics: The findings hint at the existence of lepton asymmetry in the early universe. If confirmed, this could provide a mechanism to reconcile the $H_0$ tension by increasing $N_{\text{eff}}$ without violating other cosmological constraints.
Future Directions: The paper emphasizes that while the statistical significance is promising ( $\sim 2\sigma$ ), further observations of EMPGs and improved assessments of atomic data systematics are essential to confirm the lepton asymmetry hypothesis and resolve the Hubble tension definitively.

In summary, this paper leverages Subaru's NIR capabilities to refine the measurement of primordial helium, finding a value that is lower than historical averages and consistent with emerging CMB data, thereby pointing toward potential new physics in the form of lepton asymmetry.