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Does Dark Energy Exist?

John on September 30, 2008 at 1:23 pm

From Live Science:

Earth may be trapped in an abnormal bubble of space-time that is particularly void of matter. Scientists say this condition could account for the apparent acceleration of the universe’s expansion, for which dark energy currently is the leading explanation.

Dark energy is the name given to the hypothetical force that could be drawing all the stuff in the universe outward at an ever-increasing rate. Current thinking is that 74 percent of the universe could be made up of this exotic dark energy, with another 21 percent being dark matter, and normal matter comprising the remaining 5 percent.

Until now, there has been no good way to choose between dark energy or the void explanation, but a new study outlines a potential test of the bubble scenario.

If we were in an unusually sparse area of the universe, then things could look farther away than they really are and there would be no need to rely on dark energy as an explanation for certain astronomical observations.

“If we lived in a very large under-density, then the space-time itself wouldn’t be accelerating,” said researcher Timothy Clifton of Oxford University in England. “It would just be that the observations, if interpreted in the usual way, would look like they were.”

So the advantage of the void theory is that it removes the need for dark energy, which at this point no one can really explain. Dark energy is simply that force which is causing the observed expansion. The problem with the void alternative doesn’t come from observation at all but from fundamental assumptions about our place in the cosmos:

One problem with the void idea, though, is that it negates a principle that has reined in astronomy for more than 450 years: namely, that our place in the universe isn’t special. When Nicholas Copernicus argued that it made much more sense for the Earth to be revolving around the sun than vice versa, it revolutionized science. Since then, most theories have to pass the Copernican test. If they require our planet to be unique, or our position to be exalted, the ideas often seem unlikely.

“This idea that we live in a void would really be a statement that we live in a special place,” Clifton told SPACE.com. “The regular cosmological model is based on the idea that where we live is a typical place in the universe. This would be a contradiction to the Copernican principle.”

Among other things, the Copernican principle is a fault-line of sorts between scientists like Richard Dawkins, who use it to argue for atheism and theistic scientists like Guillermo Gonzalez who argued in his book Privileged Planet that our place in the universe is already in violation of the Copernican principle.

This isn’t to say that proving dark energy exists would prove Dawkins right, or that proving we live in a void proves Gonzalez right. In either case, we would still be dealing with probability arguments. However there is no doubt that finding we lived in a relatively peaceful bubble inside a larger universe would do damage to the Copernican principle in general and any arguments that rely on it.

Personally, the void theory seems more appealing to me. That’s not because it mitigates atheistic arguments but because it provides a simpler explanation that doesn’t (to my knowledge) require any new physics. It also might make more sense of this strange behavior, also reported by Live Science recently:

Patches of matter in the universe seem to be moving at very high speeds and in a uniform direction that can’t be explained by any of the known gravitational forces in the observable universe. Astronomers are calling the phenomenon “dark flow.”

[...]

The scientists deduced that whatever is driving the movements of the clusters must lie beyond the known universe…

In these regions, space-time might be very different, and likely doesn’t contain stars and galaxies (which only formed because of the particular density pattern of mass in our bubble). It could include giant, massive structures much larger than anything in our own observable universe. These structures are what researchers suspect are tugging on the galaxy clusters, causing the dark flow.

The “dark flow” results seem to suggest that there’s some big stuff going on outside the observable universe and that, by comparison, we’re living a sheltered existence. Is that observation compatible with the void theory? It would seem so, but I’d like to ask a couple physicists before committing to that bit of speculation.

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