Final answer:
Gravitational waves are ripples in spacetime caused by the movement of massive objects, and they have been observed both indirectly and directly.
Indirect evidence for gravitational waves has been observed through changes in the orbit of binary neutron star systems. In 2015, direct detection of gravitational waves was achieved by the LIGO observatory through the detection of the merger of two black holes.
Step-by-step explanation:
Gravitational waves are ripples in spacetime that propagate at the speed of light and are predicted by general relativity.
They are caused by the movement of massive objects, such as binary neutron star systems. While indirect evidence for gravitational waves has been observed through changes in the orbit of such systems, direct detection has also been achieved.
In 2015, the Laser Interferometer Gravitational Wave Observatory (LIGO) detected the signal produced by the merger of two stellar-mass black holes, providing the first direct observation of gravitational waves.
This discovery opened a new window to study the universe and confirmed the existence of gravitational waves, as predicted by Einstein's theory of general relativity.