Observational evidence for the existence of black holes at the centers of galaxies is quite compelling. Here are some key pieces of evidence:
Stellar Orbits: Astronomers have studied the motion of stars in the central regions of galaxies, including our own Milky Way. By tracking the orbits of these stars over long periods of time, they have found that some stars are moving at extremely high speeds around a compact, invisible object at the center. The only known explanation for such high velocities is the presence of a supermassive black hole.
Gas and Dust Accretion: Many galaxies exhibit strong radio emissions originating from their central regions. These emissions come from the interaction of gas and dust falling into the central black hole. As the material spirals inwards, it becomes heated and emits radiation across a wide range of wavelengths. These observations support the presence of a supermassive black hole actively accreting matter.
X-ray Emissions: Some galaxies exhibit intense X-ray emissions from their centers. These emissions arise from the extremely hot gas in the accretion disk around the black hole. The X-ray emissions are consistent with the properties expected from black hole accretion processes.
Quasars: Quasars are incredibly bright, distant objects thought to be powered by the accretion of material onto supermassive black holes. They emit vast amounts of radiation, including visible light, X-rays, and radio waves. The detection of quasars at the centers of galaxies provides strong evidence for the presence of supermassive black holes.
Gravitational Waves: In 2015, the Laser Interferometer Gravitational-Wave Observatory (LIGO) detected gravitational waves for the first time. These waves were produced by the merger of two stellar-mass black holes. This groundbreaking discovery confirmed the existence of black holes and demonstrated the validity of Einstein's general theory of relativity, which predicts the existence of black holes.
While direct imaging of a supermassive black hole's event horizon is still a significant challenge, the cumulative evidence from various observational techniques strongly supports the existence of black holes at the centers of galaxies.