Preposterous Universe

Friday, March 05, 2004
Peppermint Dark Energy

As advertised, I was on Science Friday this afternoon with Adam Riess and Richard Ellis. (Listen here; registration required.)

We talked mostly about dark energy, the mysterious stuff that is smoothly distributed through space, constant (or nearly so) as a function of time, and makes up 70% of the energy in the universe. It's mysterious for several reasons. For one thing, the leading candidate for dark energy is vacuum energy, which is just a kind of energy that is perfectly constant throughout space and time; but our estimates of how big the vacuum energy should be are larger than the observed amount of dark energy by one hundred and twenty orders of magnitude (a one followed by 120 zeros). For another, the amount of dark energy is comparable to the amount of matter in the universe (the other 30%), even though they change dramatically with respect to each other as the universe expands.

An issue that arose during the discussion was whether dark energy worked against gravity. It's important to understand that dark energy is not a new "force," but a new kind of "stuff," that creates its own gravitational field. The funny thing is that this gravitational field pushes things apart, rather than pulling them together. But the force communicating the push is just gravity as Einstein figured it out -- a manifestation of the curvature of spacetime.

We also talked about crazy new ideas, which are certainly worth considering. (It was during the course of this discussion that I extemporaneously introduced the concept of "peppermint-flavored dark energy," although I'm not sure that will catch on.) Crazy ideas range from some energy source that changes slowly, but is nevertheless dynamical, all the way to tossing out Einstein's general relativity and invoking new behaviors for gravity. People have tried all sorts of things, and should definitely keep trying, but so far the vanilla-flavored dark energy remains the model to beat.

Ideas on culture, science, politics.
Sean Carroll

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