Strong-field tests of general relativity with black holes

7 problems

• Do post-merger gravitational-wave echoes reveal structure at the horizon scale?

  Open

  If a black hole had a reflecting surface just outside where its horizon should be, part of the ringing would bounce back and forth and produce delayed repeated pulses called echoes. Finding or excluding them tests whether horizons are what general relativity says.

• Does the quadrupole moment of supermassive black holes equal $-J^{2}/(M c^{2})$?

  Open

  A Kerr black hole's shape is fixed by its mass and spin, so its flattening, measured by the quadrupole moment, must take one exact value. Small bodies spiraling into massive black holes, to be observed by the LISA space detector, can measure it.

• Detect the nonlinear gravitational-wave memory of black hole mergers

  Open

  General relativity predicts that a passing gravitational wave leaves detector mirrors permanently displaced, the memory effect, because gravitational waves themselves act as a source of gravity. The effect is small and has not been measured.

• Do remnant black hole ringdown tones match Kerr at percent precision?

  Partially resolved

  A newly merged black hole rings like a bell at frequencies fixed by its mass and spin if it is a Kerr black hole. Measuring two or more tones in one event tests this directly.

• When does black hole ringdown become a sum of linear quasinormal modes?

  Contested

  Fits that include higher overtones often start near the peak of the signal, but near the peak nonlinear effects may mimic overtones. Where linear ringing begins determines which modes can be claimed as detected.

• Does the photon ring of M87* have the shape Kerr predicts?

  Open

  Light that circles a black hole several times forms a thin bright ring whose shape depends only on the black hole's mass, spin and our viewing angle. Measuring it would test the Kerr geometry directly but requires radio telescopes in space.

• Are quadratic quasinormal modes present in observed ringdowns?

  Partially resolved

  General relativity is nonlinear, so two ringing tones of a black hole should combine into weaker extra tones at the sum of their frequencies. Detecting them would test the nonlinear part of Einstein's equations.