[nuccy]

To add to the list (the order is arbitrary):

5. Advances in the multi-messenger observations, where apart of photons and gravitational waves, astronomers can detects also neutrinos with specialized neutrino detectors, e.g.: IceCube [a], though there are many more of those [b].

6. Advances in very-long-baseline interferometry [c] using a globe-sized array of radio-telescopes, like Event Horizon Telescope [d]

[a] https://en.wikipedia.org/wiki/IceCube_Neutrino_Observatory

[b] https://en.wikipedia.org/wiki/Neutrino_detector or here https://en.wikipedia.org/wiki/List_of_neutrino_experiments

[c] https://en.wikipedia.org/wiki/Very-long-baseline_interferome...

[d] https://en.wikipedia.org/wiki/Event_Horizon_Telescope

[hnuser123456]

And one more:

7. The LISA mission is underway to build a space-based detector for gravitational waves at the 0.1hz region, whereas LIGO is sensitive around the 500hz region. 500hz corresponds to inspiraling stellar mass black holes. 0.1hz corresponds to inspiraling supermassive black holes. Think about the most powerful subwoofer the laws of physics allow, given the maximum amount of mass density in a volume and the speed of light.

https://en.wikipedia.org/wiki/Laser_Interferometer_Space_Ant...