Astrophysical Black Holes: What Can They Tell Us About Gravity?

Ramesh Narayan

Astrophysicists have discovered two varieties of black holes in the universe: (i) Stellar-mass black holes with masses in the range 5 to 20 solar masses, (ii) Supermassive black holes with masses in the range million to several billion solar masses. Observations of these objects have the potential to constrain theories of gravity. Progress is being made on two fronts:

(1) Over the last decade, considerable evidence has accumulated that astrophysical black holes have event horizons. This evidence rules out theories of gravity that forbid horizons. It also potentially constrains the existence of naked singularities.

(2) While black hole masses have been successfully measured for many years, it is only recently that spin measurements have finally become feasible. This field is still in its infancy. However, it has the potential to provide tests of the No-Hair Theorem.