What kind of waves are ripples in spacetime predicted by general relativity?

Ripples in spacetime predicted by general relativity are gravitational waves. They are traveling disturbances in spacetime curvature, generated when massive objects undergo changing, especially asymmetric, accelerations.

Albert Einstein predicted gravitational waves in 1916, shortly after publishing general relativity. Unlike sound waves, they do not need air or another material medium. They can travel through the vacuum of space at the speed of light and carry energy away from their sources.

Powerful sources include merging black holes, merging neutron stars, and some supernova processes. The waves stretch and compress distances by an extraordinarily small amount as they pass. Laser interferometers measure this change by comparing the lengths of perpendicular arms.

Gravitational waves were first detected directly in 2015 by the LIGO collaboration, from a binary black-hole merger. They are not radio waves: radio waves are electromagnetic radiation, while gravitational waves are changes in spacetime geometry itself.