Ask Ethan #64: What happens to matter as the Universe expands? (Synopsis)

“The trees that are slow to grow bear the best fruit.” –Molière

So if the Universe is expanding and cooling, what does that mean for the matter in it? Sure, it’s easy to visualize how radiation cools: it has a wavelength, space expands, and so as the wavelength gets stretched, the energy drops.

Image credit: James Imamura of University of Oregon, viahttp://hendrix2.uoregon.edu/~imamura/123cs/lecture-5/lecture-5.html.
Image credit: James Imamura of University of Oregon, viahttp://hendrix2.uoregon.edu/~imamura/123cs/lecture-5/lecture-5.html.

But the energy must have dropped for matter as well, otherwise it wouldn’t have lost enough kinetic energy to become gravitationally bound into gas clumps, stars and galaxies! And yet, those things very much exist! What’s the resolution to this?

Image credit: Paul Hooper at Spirit Design, with Mat Pieri and Gongbo Zhao, ICG.
Image credit: Paul Hooper at Spirit Design, with Mat Pieri and Gongbo Zhao, ICG.

Find out — and find out who our first contest winner is — on this week’s Ask Ethan!