What are gravitational waves, and how were they first directly detected?
A: Oscillations in the electromagnetic field; first detected by the Hubble Space Telescope.
B: Ripples in the curvature of spacetime produced by accelerated masses; first directly detected by the LIGO observatory.
C: Disturbances in the dark matter field; first detected by the Large Hadron Collider.
D: Fluctuations in the density of the cosmic microwave background; first detected by the Planck satellite.