I am not a specialist, but following the [not so] recent news about the detection of gravitational waves from what is thought to be a merger of 2 black holes about 1 billion light years away involving a combined mass only 60 times the sun, I couldn't help but wonder..
How much of the universe is gravitational waves?
The facts LIGO is expected to hear these gravitational waves more frequently, combined with significant mass loss (on the order of 5%) each of these events represents, raises questions like At what rate is the cosmos dissipating matter as gravitational waves? How much of universe's mass has been converted to gravitational waves since the Big Bang? Answers to such questions would no doubt depend on the average number of times black holes merge together to attain a given mass. Over cosmological time scales, this churning might add up.
Does non-linear superposition admit standing gravitational waves?
Speaking of cosmological scales, if the universe is humming with these, what happens when 2 or more gravitational waves meet? Linear superposition does not work here, since GR is non-linear. My cursory search for the topic turned up little. I was wondering whether under some configurations such wave-wave interactions can yield standing waves, the kind of effects that might bear on dark matter and dark energy. For a standing wave, here, I imagine any wave-wave effect that propagates at subluminal speeds should suffice.
Information conservation perspective
Finally, I wonder how much (if any) of the information buried in merging black holes is radiated out again as gravitational waves. I don't see this much discussed in the context of the black hole information loss problem. (If the information content of the black hole is proportional to its surface area, and the stable, post-merger surface area is less than the sum of the pre-merger surface areas, my thinking goes, then maybe some of that information had to escape as gravitational waves?)