The correlation spectrum measured at a BEC sonic horizon was thermal at the temperature predicted by Hawking's formula.
Assessment
The evidence favors the claim, but the examination is incomplete or the evidence is indirect.
In 2019 the Technion group of Jeff Steinhauer reported measuring the spectrum of Hawking-like phonon radiation in a Bose-Einstein condensate with a sonic horizon (Muñoz de Nova, Golubkov, Kolobov and Steinhauer, Nature 569, 688). A spectrum extracted from thousands of repeated measurements of density correlations across the horizon, building on the earlier observation that correlated phonon pairs straddle the horizon, agreed well with a Planck distribution, and the fitted temperature matched the Hawking temperature implied by the horizon's measured surface gravity, as analogue gravity theory predicts. A 2021 follow-up found the emission to be stationary in time, supporting the reading of the spectrum as steady emission rather than a transient of horizon formation.
The result is credible but its strongest reading is disputed. A subsequent theoretical analysis showed that models which accurately reproduce the measured correlations predict significant departures from an exactly thermal spectrum, meaning the data establish consistency with a thermal spectrum at the predicted temperature within the measured frequency band and experimental precision, not exact thermality. The measurement therefore stands as good approximate confirmation of Hawking's temperature formula in an analogue system, while the finer question of exact thermality would require higher precision and a wider frequency band to resolve.
Full reasoning: the evidence and decisions behind this verdict
The primary source is Muñoz de Nova, Golubkov, Kolobov and Steinhauer, "Observation of thermal Hawking radiation and its temperature in an analogue black hole", Nature 569, 688 (2019), arxiv.org/abs/1809.00913. The correlation spectrum was extracted from the density-density correlation function across the sonic horizon, and the paper reports it agrees well with a thermal spectrum with temperature given by the analogue surface gravity (predicted Hawking temperature 0.351(4) nK from the measured flow and sound-speed profiles), with a grey-body factor taken as near unity for the linear-dispersion regime probed. This rests on the prior, well-replicated observation that the experiments observed correlated phonon pairs across the horizon, which stands verified.
The main published challenge is Isoard and Pavloff, Phys. Rev. Lett. 124, 060401 (2020), arxiv.org/abs/1909.02509: a model including evanescent channels and zero modes accurately reproduces the measured correlation function yet exhibits significant departures from thermality and complicates the extraction of a single Hawking temperature. Recorded as the departure-from-thermality subclaim, this does not dispute the data; it shows the data underdetermine exact thermality. That gap is why the status is supported rather than verified: the affirmative reading (thermal at the predicted temperature) fits the data well, but so does a mildly non-thermal spectrum.
Corroboration: the 2021 follow-up (Kolobov et al., Nature Physics 17, 362) found the spontaneous emission stationary in time, weighing against transient or artifact explanations of the 2019 spectrum. The separate dispute over whether the signal is vacuum-seeded or amplified classical noise concerns the origin of the radiation, not the measured shape of the spectrum, and so is carried on the claims that depend on this one rather than here.
What would change the conclusion: a replication with wider frequency coverage and higher precision either confirming Planckian shape (toward verified) or resolving the Isoard-Pavloff deviations (toward contested or contradicted as stated); or a demonstrated flaw in the surface-gravity determination or the windowing used to extract the spectrum. Credence 0.7 reflects that the claim is very likely true as an approximate, within-precision statement, and uncertain as a statement of exact thermality.
Decomposition
How this claim breaks down: each argument is stated as it runs, with its subclaims linked inline. ↗︎ opens a subclaim; the map shows how they fit together.
Because correlated phonon pairs were observed across the sonic horizon, a spectrum could be extracted from roughly 6,000 repeated measurements of the cross-horizon density correlations, and it fit a Planck distribution whose temperature matched the value implied by the independently measured flow and sound-speed profiles, given that analogue gravity predicts thermal Hawking-like phonon emission at a temperature set by the horizon's surface gravity. The finding that the emission observed in the 2021 follow-up was stationary in time corroborates reading the spectrum as steady Hawking-like emission rather than a transient of horizon formation.
Because theoretical models that reproduce the 2019 correlation data predict significant departures from a thermal spectrum, the measured correlations cannot discriminate between exactly thermal radiation and mildly non-thermal radiation, so the fit to a Planck form at a single Hawking temperature may reflect the limited frequency band and precision of the measurement rather than established thermality.
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