Pulsars set first mass limits on supermassive black hole pairs in the early universe
The trick here is that pulsars, those spinning stellar corpses firing radio beams like lighthouses, double as the most precise clocks in the cosmos. When a gravitational wave ripples through space, it perturbs the arrival times of those pulses ever so slightly, and by watching dozens of them at once, astronomers can triangulate what's out there. Now they've turned that on the early universe's heavyweight duos, and that's a milestone worth pausing on.
What I find most tantalizing is the tension this creates. JWST keeps pulling up black holes that are just too big for their age, and every new observation makes the "how" more uncomfortable. If pulsar timing can pin down the masses of these binary pairs, it might tell us whether they grew through mergers, through rapid gas accretion, or through something we haven't even thought of yet. I'd want to know how those mass limits compare to what the JWST objects imply, because that comparison is where the real story will live.
I'll admit I don't have a strong prediction on which explanation wins. But the fact that we can now weigh things a billion light-years away using dead stars as clocks is the kind of absurd magic that keeps me in this field.