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Dark Energy Is Still, Uh, Whatever the Hell Dark Energy Is

OK, you can exhale. "New measurements":http://www.nasa.gov/home/hqnews/2011/mar/HQ_11-073_Hubble_Dark_Energy.html taken by the Hubble space telescope have ruled out one of a couple of alternate explanations of dark energy, and it appears that our...

OK, you can exhale. New measurements taken by the Hubble space telescope have ruled out one of a couple of alternate explanations of dark energy, and it appears that our conception of dark energy as a massive, universe-filling repulsive force is in the clear. (When science gets an idea of how something explains something else, it rigorously tests all possible alternatives, no matter how perfect that something may be as an explanation.)

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Let’s step back. Late last century, something pretty disconcerting was discovered: the universe is expanding, and not just expanding but accelerating in its expansion. It should be shrinking, or at least slowing down due to the gravitational attraction between stuff in the universe. We can tell that it’s expanding based on the “red shift” we encounter when looking at extremely distant, extremely bright supernovae.

More: we can tell some things about a supernova based on its brightness, but we can tell it’s moving away because the waves of light that it’s sending our way become affected such that that light shifts towards red. OK? One of the earlier alternate explanations was that the shift is coming from cosmic dust interfering with the light. Well, that didn’t work because we realized that the universe shouldn’t have always been accelerating, only since its density got so low as to where repulsive energy could overpower gravitational energy.

So, there was a point where gravity dominated and the repulsive dark energy force would have been about eight times weaker. And, at that point in history, the red shift should be different. Guess what: it was. And so dust was ruled out.

This other alternate suggests that our corner of the universe is emptier than other places. And with less stuff, there’s less gravity holding things together. So, it just looks like it’s expanding. We can test this too—the illusion breaks down at a certain expansion rate. That is, if things are moving outward too fast, we can’t account for it locally. It only works if the expansion rate is 60 kilometers per second per megaparsec or less (a megaparsec is about 3.26 light-years) The new Hubble measurements have the expansion rate at about 74 0 kilometers per second per megaparsec.

Science being science, that’s still not enough to totally rule out a local void explanation, and opponents have suggested a “void within a void” that might work. But dark energy looks like a more likely candidate, again. Of course, we still don’t really know what dark energy is, but I’ll get more into that in a forthcoming post.

Reach this writer at michaelb@motherboard.tv.