Do black holes exist? Observations at the finest resolution so far indicate that only gross deviations in the behaviour of gravity from that predicted by general relativity can invalidate the case that they do. It is believed that the centre of essentially every galaxy, including our own, plays host to a supermassive black hole. In a small fraction of galaxies, large quantities of gas rain down into these giant black holes, causing the black hole to grow while releasing enough energy within the central few light hours of the galaxy to outshine all of the galaxy's stars thousands of times over. This is more than a mere cosmic firework show; the energy released as the black hole grows can shape or even shut off the processes by which the galaxy itself forms. In other words, supermassive black holes may well be the safety valve that regulates galaxy formation, preventing galaxies from growing too big too fast. But although they are rapidly becoming a standard part of our model of how galaxies form and evolve, it is important to step back and ask just how strong is the case that these monster black holes actually exist.
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