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A spinning black hole acts like a cosmic generator: its rotation pumps energy into an invisible scalar cloud — a prime candidate for dark matter. A scalar field is like temperature: at every point in space it's just a single number, with no arrows or vectors. The cloud grows until it overflows. Then it collapses, shooting out a narrow beam of radio emission — a natural laser that lasts milliseconds but shines brighter than an entire galaxy. This radio pulse carries the imprint of everything around it: gas, dust, stars, so astronomers use it to probe the most warped regions of spacetime — an idea pioneered by Robert Dicke. From the flash's characteristics, you can even tell which type of dark matter birthed it. The big surprise: such flashes could be the only direct window into the Universe's dark sector, invisible to ordinary telescopes.

Original abstract

as written by the authors arXiv:2601.11988

Scalar-tensor theories predict fundamental scalar fields of considerable interest in astrophysics and cosmology. We investigate the superradiant instability of scalar clouds around Kerr black holes, showing that stimulated decay generates detectable electromagnetic signals. The growth of the superradiant scalar cloud differs from that of other bosonic fields and depends sensitively on the matter distribution surrounding the black hole, which originates from the scalar-matter coupling realized by the chameleon mechanism in modified gravity theories. In non-uniform matter distributions, stimulated emission from scalar clouds offers an observational signature that distinguishes fundamental scalars from other light bosonic fields.