Ah, the stately hot air balloon. Not really useful for all that much except for having a grand old time floating around the skies, it’s still a marvel of simple physics. Fill a giant balloon with air. Heat up the air. The balloon goes up. What could be more simple than that?
Not much, actually.
The physics behind a hot air balloon is buoyancy. When heated, the air inside the balloon becomes less dense than the surrounding atmosphere. Less dense things placed inside of more dense things float, and hence the hot air balloon rises, like an ice cube floating in a glass of water.
To make this floating thing actually happen, there’s of course a lot of cool microphysics going on. The air molecules constantly strike the outside of the balloon in all directions, while the molecules inside the balloon are doing the same thing. Almost magically, all these interactions and bombardments nearly cancel each other out, leaving a slight pushing force that resists gravity and raises the balloon.
This buoyancy happens across the universe on scales small and great. From ice cubes to icebergs, from hot air balloons to some of the largest bubbles in the universe, blown by giant black holes into massive clusters of galaxies.
Clusters of galaxies themselves are some of the largest things in the cosmos, home to a teeming hive of over a thousand individual galaxies spread out over millions of cubic lightyears. And threaded throughout that enormous volume is a hot, thin gas, known delightfully as the intracluster medium.
At the center of most galaxy clusters sits a single massive galaxy, and inside that galaxy sits a gigantic black hole. A real whopper, topping out at millions or even billions of times the mass of the sun. This black hole feeds and eats on the surrounding gas, and as that gas swirls in it compresses and heats up, driving incredibly strong electric and magnetic fields in the maelstrom.
Some of the gas passes into the black hole, never to be seen by the universe again. But some gas whips around the outer surface of the black hole, riding lines of magnetism, spinning up to the poles and blasting out in enormous jets that reach out tens of thousands of lightyears, far beyond the confines of its host galaxy.
As the jet pierces the intracluster medium, it eventually slows down, cools down, and stops, filling the surrounding space with the hot plasma ejected at the base of the jet from near the black hole. Over the course of hundreds of thousands of years, this plasma accumulates, eventually forming a bubble thousands of lightyears across.
This bubble, hotter and less dense than its surroundings, then detaches from the jet and rises into the galaxy cluster before losing coherence and disrupting.
A giant, cosmic hot air balloon.
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