Chapter 3.2 Why should we care about Gravitational Waves

Gravitational wave astronomy could expand our knowledge of the cosmos dramatically. For starters, gravitational waves, though weakening with distance, are thought to be unchanged by any material they pass through and, therefore, should carry signals unaltered across the vast reaches of space. By comparison, electromagnetic radiation tends to be modified by intervening matter.
Aside from demonstrating the existence of black holes and revealing a wealth of data on supernovae and neutron stars, gravitational wave observations could also provide an independent means of estimating cosmological distances and help further our understanding of how the universe came to be the way it looks today and of its ultimate fate. Gravitational waves might unveil phenomena never considered before. Nature is smarter than any theorist trying to imagine or calculate what might be out there!

What is gravitational wave
What are Gravitational Waves?
Predicted in Einstein's General Theory of Relativity, gravitational waves are disturbances in the curvature of spacetime caused by the motions of matter. Propagating at (or near) the speed of light, gravitational waves do not travel "through" spacetime as such -- the fabric of spacetime itself is oscillating. Though gravitational waves pass straight through matter, their strength weakens proportionally to the distance traveled from the source. A gravitational wave arriving on Earth will alternately stretch and shrink distances, though on an incredibly small scale -- by a factor of for very strong sources. That's roughly equivalent to measuring a change the size of an atom in the distance from the Sun to Earth!
No wonder these waves are so hard to detect.