Black Hole Shadow as a Test of General Relativity: Quadratic Gravity

Understand

Observations of the black hole shadow of supermassive black holes, such as Sagittarius A* at the center of our Milky Way galaxy, allow us to study the properties of black holes and the nature of strong-field gravity.

• According to the Kerr hypothesis, isolated, stationary, and axisymmetric astrophysical black holes are described by the Kerr metric.
• The Kerr hypothesis holds in General Relativity and in some modified gravity theories, but there are others in which it is violated.
• In principle, black hole shadow observations can be used to determine if the Kerr metric is the correct description for black holes, and in turn, they could be used to place constraints on modified gravity theories that do not admit the Kerr solution.