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Dark Energy [Q&A]


Karthik asks, what is Dark Energy?

To put it succinctly: I don’t know. At this point, it is just a name given to something that we don’t really understand. We know from looking at the brightness of Type Ia supernovae (which have very predictable intensities, so their apparent brightness is a good tracker of distance) that the Universe used to expand slower at earlier times. Which means that it is expanding faster now than it used to. It is possible to account for this via a term called the Cosmological Constant in Einstein’s gravitational field equations. Dark Energy is what is supposed to cause a non-zero cosmological constant.

Here are a couple of nice lectures, given at a fairly popular level, that wrestle with the implications of the observations and current theoretical thinking. First, a talk by Rocky Kolb of the University of Chicago, given at the Harvard-Smithsonian Center for Astrophysics in Feb 2008: “Taking Sides on the Dark Energy Issue” (the talk is available as a RealVideo stream). The second is a talk by Michael Turner (also of UChicago) given in Feb 2011 at the Cosmic Origins lecture series at the University of Arizona: “Dark Matter, Dark Energy and Inflation: The Big Mysteries of Cosmology” (it is also available via iTunesU in the iTMS).