Hi prof. Linder. I'm a student. I was in that class. It was nice lecture ~~
Anyway, I have a question. (I was so shy that I couldn't ask you..) You said Universe's temperature is 3K. Am I right? Then, you said, as universe expends more and more the temperatrue is lower. right? then, What means "Freezeout"? I mean, Does that mean just when times pass, all things will stop bivrating in cold condition? I don't understand that exactly... It is so abstract to me. and if there are my misunderstandings so far, tell me about it.

Thank you for asking the questions.  The average temperature of the Universe in "empty space", far from any star or galaxy, is 2.7 K today.  When the Universe has expanded in the future to be 2 times as big as it is today, the temperature will be only 1.35 K.  The word "freezeout" though does not mean only the temperature of the Universe - it means that the interactions of particles stops.  Remember in the early universe there were many interactions, and this gave equilibrium, or a balance, between creations and annihilations of particles.  Once the energy is lower (temperature is lower), and the universe is less dense, the particles can no longer interact (or even find each other to collide), and the interaction stops - this is called freezeout.  I hope this helps you.

Thank you for your detail answer.