Black Holes as Unexpected Beacons of the Early Universe express
A few hundred million years after the Big Bang, the universe resembled an endless dark ocean, shrouded in a thick fog of neutral (non-ionized) hydrogen. Then something dispersed the gloom: ultraviolet radiation knocked electrons out of atoms, making the gas as transparent as pure water. This era is called
For a long time, it was thought that the main illuminators were young galaxies filled with hot stars. But new research puts the spotlight on supermassive black holes — active galactic nuclei. Updated models showed that these cosmic beacons emitted 3–4 times more ionizing light particles than previously estimated. With this, their contribution to clearing the universe reached 20%, and that's without counting the dimmest but most numerous black holes. Astonishing: the darkest objects in space turned out to be brighter than hundreds of millions of Suns.
Original abstract
as written by the authors arXiv:2601.05316v1
Reionization marks one of the most important phase transitions in the history of the Universe, during which neutral baryonic matter was transformed into ionized plasma. While star-forming galaxies are widely regarded as the primary drivers of this process, the extent to which active galactic nuclei (AGNs) contribute remains a subject of ongoing investigation. In this study, we integrate a physically motivated AGN spectral energy distribution (SED) model with state-of-the-art observations to reassess the contribution of AGNs to cosmic reionization. Our findings indicate that adopting a more sophisticated AGN SED model could substantially increase the predicted ionizing photon output by a factor of 3$$\sim$$4, elevating AGNs to a more significant role ($$\approx$$20\%) in maintaining reionization than previously estimated. The inclusion of abundant faint AGNs further amplifies this contribution by a factor of a few. These conclusions remain robust across a wide range of accretion rates and ionizing photon escape fractions. Collectively, our results suggest that AGNs may have played a more prominent and previously underestimated role in the reionization of the Universe.
Reionization took about 600 million years and was uneven: like beacons lighting up at different points in the ocean, slowly dispersing the fog and leaving isolated dark 'islands' of neutral hydrogen.