Supermassive blackholes in galactic centers are by statistical arguments expected to fuse in the collisions of galaxies so often that the generated gravitational radiation produces a detectable hum. This should produce a background hum which should be seen as a jitter for the arrival times of photons of radiation from pulsars. This jitter is same for all pulsars and therefore is expected to be detectable as kind of "hum" defined by gravitational radiation at low frequencies. The frequencies happen to be audible frequencies. For the past decade, scientists with the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) collaboration tried to detect this constant "hum" of low-frequency gravitational waves (see this). The outcome is negative and one should explain why this is the case.
I do not know how much evidence there exists for nearby collisions of galaxies in which fusion of galactic supermassive blackholes really take place. What would TGD suggest?
- In TGD Universe galaxies could be like pearls in necklace carrying dark magnetic energy identifiable as dark matter. This explains galactic rotation curves correctly 1/ρ force in plane orthogonal to the long cosmic string (in TGD sense) defining the necklace gives constant velocity spectrum plus free motion along string: this prediction distinguishes TGD from the competing models. Halo is not spherical since stars are in free motion along cosmic string. The galactic dark matter is identified as dark energy in turn identifiable as magnetic energy of long cosmic string. There is a considerable evidence for these necklaces and this model is one of the oldest parts of TGD inspired astrophysics and cosmology.
- Galaxies as vehicles moving along cosmic highways defined by long cosmic strings is more dynamical metaphor than pearls in necklace and better in recent context. The dominating interaction would be the gravitational interaction keeping the galaxy at highway and might make fusion of galactic blackholes a rare process.
- The gravitational interaction between galaxies at separate highways passing near each other would be secondary interaction and galaxies would pass each other without anything dramatic occurring.
- If the highways intersect each other the galaxies could collide with each other if the timing is correct but this would be a rare event. This is like two vehicles arriving a crossing simultaneously. In fact, I wrote for a couple of years ago about the possibility that Milky Way could have resulted as the intersection of two cosmic highways (or as a result of cosmic traffic accident).
- If the galaxies are moving in opposite directions along the same highway, the situation changes and a fusion of galactic nuclei in head on collision is unavoidable. It is difficult to say how often this kind of events occur: it could occur that galaxies have after sufficiently many collisions "learned" to move in the same direction and define analog of hydrodynamical flow. A cosmic flow has been observed in "too" long scales and could correspond to a coherent flow along cosmic string.
For a summary of earlier postings see Latest progress in TGD.