What is the 21-cm fluctuation power during cosmic dawn and reionization?
In plain words
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 statement
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.
What would settle it
HERA full-array or SKA-Low power-spectrum detection confirmed by an independent array.
Status in the literature
Unverified note: it names recent work and was partly written without a live literature search; it awaits a check by an editor.
HERA Phase I (2022) and Phase II (2025) set upper limits without a detection.
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