You might enjoy this crazy 1998 paper, which I just found, which adds a bit of heft to the the "not much" guess above. It seeks to examine gravitatonal waves s

You might enjoy this crazy 1998 paper, which I just found, which adds a bit of heft to the the "not much" guess above. It seeks to examine gravitatonal waves shed by a 2-galaxy cluster. "pessimistic point of view justified by the combination of great numerical difficulties (three-dimensional calculations) together with very disappointing small values expected for the gravitational-wave luminosity, the amplitude, etc" "In brief, our simulated cluster produces changes in the relative distance of the order of 10-22—detectable with current technology—in a short period of 4 days." (LIGO 2015 was about the same relative length change (h), but with a frequency of 35 - 250 Hz. I'm not sure how catching part of a gravitational wave of ~ 10^-17 Hz could be caught by a LIGO-like detector, since only higher derivatives of h are useful for identifying gravitational radiation). http://iopscience.iop.org/article/10.1086/311446/fulltext/985163.text.html http://iopscience.iop.org/article/10.1086/311446/fulltext/98... (but of course for cooling we're more interested in the luminosity, which is small compared to that of an ordinary galaxy, but a bit more interesting compared to the blackbody radiation of a galactic-mass diffuse cloud of atomic hydrogen, although that's only considering the normal-matter member of the binary, rather than the total energy leaving the region of spacetime the binary is in.)