Gravitational waves cause the length of one 2.5 mile long arm to decrease very slightly, while the length other arm increases.
To be able to detect gravity waves, LIGO must be able to measure changes in length of order 1 part in 1016 - about 1/10 the size of a proton - quite a technical feat.
LISA, a proposed space based gravity wave detector would detect gravity waves from supermassive black hole mergers, and a gravitational analogue of the CMB.
COSMOS to HUMANITY, Spring 2008
Lecture 2 , slide 9