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Astrophysics

Fireworks at Cosmic Dawn: relieving BAO-CMB tensions with the Pop III.1 Flash

Yash Aggarwal, Christopher Cain, Garett Lopez, Hy Trac, Anson D'Aloisio, Philip Tanedo, Jonathan C. Tan

Featured June 21, 2026

Original Paper

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Simply

An early burst of supermassive stars lights up the universe, helping solve a cosmic puzzle about how much matter exists, while still fitting other clues about when the universe became clear.

In depth
The paper addresses cosmological tensions by proposing an early reionization phase driven by supermassive Pop III.1 stars. This "flash ionization" at high redshift increases the cosmic optical depth to reconcile CMB and BAO data. Crucially, the Pop III.1 stars exhibit self-regulating feedback, which limits their spatial clustering and reduces the patchy kinetic Sunyaev-Zel'dovich (pkSZ) signal, thus satisfying observational constraints from both pkSZ and Ly forest data.

Key Takeaways

  • 1
    An early Pop III.1 flash ionization phase can achieve a high cosmic optical depth () needed to resolve BAO-CMB tensions.
  • 2
    Self-regulating feedback in Pop III.1 stars imposes a minimum source separation, leading to weakly clustered ionized regions that suppress the patchy kinetic Sunyaev-Zel'dovich (pkSZ) signal.
  • 3
    The proposed non-monotonic reionization history, combining early Pop III.1 flash with later Pop II galaxy-driven reionization, is consistent with both Ly$\alpha$ forest (late end) and pkSZ (low power) observational constraints.

Conceptual Flow

HIGH LEVEL
1
Methodology: Early Ionization with Self-Regulation

Special early stars turn on, but they stop other stars from forming too close, making the universe light up smoothly.

Early Stars
Self-Regulation
Create Ionized Bubbles
Weakly Clustered Ionization
2
Results: High Optical Depth, Low pkSZ Signal

This smooth early lighting, combined with later galaxy light, makes the universe opaque enough but without too much patchy light, matching observations.

Weakly Clustered Ionization
Later Galaxy Ionization
Combine Effects
High Optical Depth
Low Patchy Signal