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Accepted · first revision · record

Title: What is the 21-cm fluctuation power during cosmic dawn and reionization?
Plain: Radio arrays try to map how the hydrogen 21-cm signal varies from place to place in the early universe. Bright radio emission from our own galaxy is about 100,000 times stronger and must be removed.
Precise: Detect the dimensionless 21-cm power spectrum $\Delta^2_{21}(k, z)$ at $k \sim 0.1 \text{ to } 0.5\,h/\mathrm{Mpc}$ for $z = 6 \text{ to } 25$, separating Galactic and extragalactic foregrounds. An answer is a detection with signal-to-noise above 5 in at least one redshift bin, giving the timing of X-ray heating and reionization.
Settled by: HERA full-array or SKA-Low power-spectrum detection confirmed by an independent array.
Kind: well-posed
Answer type: value
References: (empty)
Literature status: open
Status note: HERA Phase I (2022) and Phase II (2025) set upper limits without a detection.
Posed since: (empty)