Gravitational waves: ripples in spacetime and music of the spheres

LIGO opens a new window to look further into the universe by hearing the chirp of gravitational waves (GWs). We provide a concise introduction of GW solution of the linearized field equation of general relativity (GR), the quadrupole radiation formula, the strength estimate of GWs, and the inspiral-merge-ringdown waveforms of binary black holes simulated by numerical relativity. Finally we present an outline of GW astronomy and cosmology by reciting the predictions and conjectures proposed by Kip Thorne concerning GWs and the related subjects.