A tale of two black holes: Sgr A* and M87*
Sera Markoff
Black holes are one of the most exotic consequences of Einstein’s General Relativity, yet they are also very common, ranging from stellar remnants up to billions of times more massive than our sun. Despite their reputation as cosmic vacuum cleaners, they actually drive extremely complicated astrophysical systems that can drastically influence their surroundings. In particular, via their powerful outflows, black holes help shape the way the Universe looks today...but not at all times. Black holes undergo cycles of activity, so to understand their role over cosmological timescales we need to understand not only how they power such outflows from just outside their event horizons, but also what drives their cyclic behavior. Thanks to the global very long baseline interferometry (VLBI) project the Event Horizon Telescope (EHT), we have now directly imaged the event horizon region for two nearby supermassive black holes: Sgr A* in our own Galactic center, and M87* in the Virgo cluster of galaxies. After a brief review about EHT and our key results so far, I will put them into the context of our greater understanding of black hole activity, with emphasis on the gains made by combining EHT observations with those from other multi-wavelength/multi-messenger facilities. I will also give some perspective on what is (literally) on the horizon in the coming decade.