Tomorrow I'm giving a lecture about cosmology in astro 101, and a lecture about the Fermi Paradox in astrobiology. These lectures are both TOTALLY WILD and I am so looking forward to completely blowing my students' minds.

The cosmology lecture ends with theories about the end of the universe. My summary is based on a talk I saw by @AstroKatie last year (who wrote a whole book about this topic! "The End of Everything")

There are 4 possible ways the Universe could end:

1. The Big Crunch:

The expansion of the Universe could eventually slow, stop, and then the Universe would begin contracting until it repeats the Big Bang in reverse.

It's not just galaxies getting compressed together, but radiation too! We would get roasted by the Cosmic Microwave Background+light from all stars ever (whoa!)

Everything ends in nuclear fire.

2. The Big Freeze/Heat Death:

The Universe could continue expanding as it is now. Galaxies will move away from us faster and faster.

Stars will continue to form for a few trillion years, but eventually all the gas will be used up and all the stars will die without any new star formation. Black holes will continue to accrete any matter around (white dwarfs, black dwarfs, planets), until they are the only objects left in the Universe. Eventually they will evaporate. It's cold, dark, boring.

3. The Big Rip:

If the Universe is dominated by "phantom dark energy" that effectively gets stronger as densities get lower, the Universe will unravel from largest to smallest structures.

Galaxies, stars, planets, and finally even atoms will be ripped apart.

4. Vacuum Decay: We *could* be living in a Universe where the Higgs field isn't in its preferred state. (The Higgs field controls basically all of physics).

It could suddenly shift via quantum tunnelling and change *all of physics* and destroy everything in the Universe. This bubble of absolute destruction would radiate outward through the Universe at the speed of light, so we wouldn’t see it coming. Everything would just end, apparently instantaneously.

Current physics suggests option #2 is most likely. But one of these ends WILL ACTUALLY HAPPEN someday!

(In trillions of years or more, don't stress!)

So, the other mind-blowing lecture I'm doing today is about the Fermi Paradox. It's the very simple question posed by physicist Enrico Fermi: if the Universe has been around for billions of years longer than the Earth, presumably there are many life-bearing planets that have a huge head-start on us.

"So where is everybody?"

(Important cringy historical note: Fermi was one of the Manhattan Project scientists who built the first atomic bomb, making is possible to quickly wipe ourselves out 😬)

Possible solutions to the paradox:

1. We are alone.

We can't rule this out, Earth is still the only place in the Universe we know FOR SURE has life. It is possible that we are the first technologically advanced civilization that has arisen in the Universe (of course, then we have the logical problem of being special - never a good answer in astronomy)