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Friday, October 9, 2009

Dark Energy

Sean Carroll over on Cosmic Variance posted a video of Lloyd Knox playing the "Dark Energy Song" a few days ago. If you don't feel like clicking on the link, I'll show it again here:



The lyrics can be found here. Prof. Knox apparently uses this song to illustrate the concept of dark energy to his astronomy class. In fact, that was one of his students who filmed it and put it up on the Tube. For those of you who don't immerse themselves in astronomy every day, all day, dark energy is a device used to explain the apparent acceleration in the expansion of the universe. It accounts for approximately 70% of the mass-energy in the universe, but no one actually knows what it is.

I think Prof. Knox's song is great (I would even go so far as to say it's AWESOME!!!), but I have problems with dark energy. I mean, there is still a problem in that we don't know what it is. The leading theory is that it is a cosmological constant (otherwise known as Einstein's greatest blunder, but that's a story for another day), that is, it is a constant energy density that permeates the vacuum of space. The main problem with this is that the estimated value of the energy density of the vacuum as determined by cosmologist is 10^120 times smaller than the value as determined by particle physicists... and that's a BIG difference.

My main beef with it, however, is more philosophical. It seems to me that adding an invisible and undetectable energy in order to explain the acceleration of the universe's expansion to be a little silly. Sure it explains the results ok, but so did epicycles in terms of planetary retrograde motion. It seems to me that a better explanation would be an alternate theory of gravity. A more recent Cosmic Variance post talks about preliminary evidence that Einstein's theory of general relativity doesn't work to explain large scale clustering. Of course, there aren't yet any alternate theories of gravity that explain the universe any better than general relativity, but that doesn't mean that there won't be.

I guess the point of all this is that we don't know everything about the universe, and in my opinion, if observations don't fit the theory, the theory needs revising. Don't get me wrong... I love general relativity because of its elegance, but I think there are still serious problems with our way of using it to explain phenomena in the universe.