When two supermassive black holes ( SMBHS ) approach within 1 - 10 mpc , gravitational wave ( GW ) losses begin to dominate the evolution of the binary , pushing the system to merge in a relatively small time . During this final inspiral regime , the system will emit copious energy in GWs , which should be directly detectable by pulsar timing arrays and space - based interferometers . At the same time , any gas or stars in the immediate vicinity of the merging 5MBHs can get heated and produce bright electromagnetic ( EM ) counterparts to the GW signals . We present here a number of possible mechanisms by which simultaneous EM and GW signals will yield valuable new information about galaxy evolution , accretion disk dynamics , and fundamental physics in the most extreme gravitational fields .