Tens of thousands of white-dwarf binaries are expected to be detected, along with binaries involving neutron stars and black holes in various combinations. GAIA’s catalogue will still, in 2028, be the principal optical reference for these observations, and we can expect dozens or more binaries in that catalogue to be observed by LISA. LISA will identify many more, the nearest of which can then be followed up with the James Webb Telescope (JWST). These observations will lead to improved understanding of interactions, mass transfer, and double white dwarfs as supernova progenitors.
By 2028, advanced LIGO (aLIGO) and partners will have good statistics on the population of relativistic compact binaries out to Gpc distances, and LISA’s complete census of that population in the Galaxy will allow us to compare our Galaxy with the cosmological norm, a comparison that is very difficult to make with any other stellar population, and which will tell us much about the history of our Galaxy.
If binaries of 100 solar mass black holes exist, then LISA and the Einstein Telescope (ET) could make joint observations of a few merging systems with comparable sensitivity; the systems would move into the ET band in a period of a few years. Combining these observations would improve the angular positions over those which ET could measure.
In the next few years, eRosita will study tidal disruptions of stars out to redshifts ~ 1 and look for massive black holes, although in a much higher-mass regime than LISA.
The Event Horizon Telescope (EHT) should test general relativity and probe the horizon of Sgr A*, the massive black hole at the centre of the Milky Way. EHT should also explore the shape and properties of the accretion flow onto Sgr A* and measure its spin.
Within the next few years, GRAVITY may be able to observe the orbits around Sgr A*of currently unknown stars with periods of just 1-2 years and test general relativity and infer the hidden distribution of the dark population of objects around Sgr A* and hence study the different models for mass-segregation in galactic nuclei.