JWST found black holes at high redshift that appeared impossibly massive — billions of solar masses assembled within the first billion years of the universe. The standard accretion models struggled to grow them that fast. Theorists proposed exotic formation channels: direct-collapse black holes from pristine gas, primordial seeds, mergers of intermediate-mass precursors. The mass was the problem, and the solutions were creative.
Trinca, Lupi, Haardt, and Madau (arXiv 2602.22305, February 2026) propose that the black holes aren't overmassive. They're under-observed. The masses were overestimated because the X-ray emission was missing.
Black hole mass estimates from broad emission lines alone are degenerate: a massive black hole accreting slowly produces the same broad lines as a less massive black hole accreting quickly. X-ray luminosity breaks the degeneracy — it correlates with accretion rate and provides an independent handle on the mass. But deep Chandra observations of these high-redshift AGN show X-ray non-detections. The standard interpretation: the black holes are so massive that their accretion rates are low (sub-Eddington), making the X-ray faint relative to the optical/UV emission. Low accretion rate implies high mass.
The alternative: the black holes are low-mass (10⁶–10⁷ solar masses) accreting at super-Eddington rates. At super-Eddington accretion, the accretion flow puffs up into a geometrically thick structure with a narrow funnel along the rotation axis. The X-ray-producing corona — the hot, magnetically confined plasma above the disk — is trapped inside this funnel. The funnel walls shadow the corona, confining its radiation to a narrow beam. The corona is also radiatively over-cooled by the intense UV/optical radiation flooding the funnel from the thick disk walls. The result: suppressed X-ray emission, not from low accretion, but from geometric hiding.
The Markov Chain Monte Carlo analysis of the spectral data reveals bimodal solutions. One mode: massive black holes accreting slowly. The other: low-mass black holes accreting super-Eddington, with suppressed X-rays. The second solution is physically preferred — it's consistent with local galaxy scaling relations and doesn't require exotic formation mechanisms.
The black holes aren't too big for the universe. The universe is just too opaque to see them clearly. The mystery was in the measurement, not the mass.