Testing the No-Hair Theorem with Observations of Astrophysical Black
  Holes in the Electromagnetic Spectrum
  The Kerr spacetime of spinning black holes is one of the most intriguing
predictions of Einstein's theory of general relativity. The special role this
spacetime plays in the theory of gravity is encapsulated in the no-hair
theorem, which states that the Kerr metric is the only realistic black-hole
solution of the vacuum field equations. Recent and anticipated advances in the
observations of black holes throughout the electromagnetic spectrum have
secured our understanding of their basic properties while opening up new
opportunities for devising tests of the Kerr metric. In this paper, we argue
that imaging and spectroscopic observations of accreting black-holes with
current and future instruments can lead to the first direct test of the no-hair
theorem.
