In the April issue of Scientific American, there is an article by Timothy Clifton and Pedro Ferreira pondering if dark energy really exists, or if we perhaps live in the middle of a billions of light-years wide void. A void is a region of space where there is less matter than average, therefore the force of gravity would be less than on average, so the expansion of the Universe would appear to be faster than we would expect, which is just what we see with dark energy. (For what I think is a simple and honest explanation of dark energy, listen to or read the May 12 episode of the 365 Days of Astronomy podcast.)
The authors of the article do what I think is a very good thing: they present a pretty balanced review of both sides of the argument for and against giant voids. One of the arguments against a giant void is that it would appear to violate the Copernican principle.
The Copernican principle is one of the primary pillars of the science of astronomy. It says simply that we do not occupy a privileged location in the Universe. As you might guess, the principle comes out of the Copernican view of the Solar System, that the Earth is not the center of the Solar System, the sun is. In other words, we are not the center of the Universe, but rather we are just a small part in the larger whole.
The Copernican principle has been invoked many times. When astronomers first started to make maps of the Milky Way galaxy, the Sun seemed to be very close to the center (see, for example, this map by famed astronomer William Herschel). This raised alarm bells among some astronomers, because the Copernican principle said we shouldn't be someplace special, and the center of the entire galaxy would certainly seem like a special place, especially since we didn't know that other galaxies existed. This issue was resolved when Harlow Shapley, a great early 20th-century astronomer, used globular clusters to show that we were not at the center of the Milky Way, but way off to one side.
Barely had that crisis been resolved when a new one arose. Astronomer Edwin Hubble discovered that almost every galaxy in the Universe was moving away from the Milky Way, and the further away it was, the faster it was moving. This would seem to put the Milky Way at the center of the Universe, which is again against the Copernican principle! This conundrum was solved when it was recognized that the entire Universe was expanding. In an expanding Universe, no matter where you are, it looks like every other thing is moving away from you. (This is one of those concepts that is hard to grasp; the time-tested "raisin bread" analogy seems to work best, though since many people these days have never seen a loaf of bread rise, a balloon analogy also helps.)
So, back to the voluminous void from the article. For the void explanation of dark energy to work, the Milky Way would need to be located at almost the exact center of a gigantic void. How big? The void would need to be a large fraction of the observable universe! So, we would need to be near the center of a structure that is a good fraction the size of the observable Universe. That doesn't sit well with the Copernican principle. This argument alone doesn't rule out a giant void (the authors admit to many other problems with the void idea), but it makes it hard to swallow.
But I do think we need to be careful in applying the Copernican principle. The principle can be taken too far. For example, we see lots of stars near the center of our galaxy. If any of those stars harbor life-bearing planets, then any intelligent creatures on those planets will find themselves at the center of a galaxy. It's only when they discover other galaxies that they would realize they are not in a special place.
Likewise, if there is a giant void in our Universe, some galaxy will likely be near the middle of it. And if there is life on a planet around a star in that galaxy, they will find themselves in a fairly special place in the Universe.
In summary, the Copernican principle is one of the primary philosophical underpinnings of the science of astronomy. It is not physical law in and of itself, but it does require us to use an abundance of caution around any hypothesis that requires us to live in a special place of the Universe.
The more I think about it, the more I've been imagining the Copernican principle as the Robot from Lost In Space, running around and saying, "Danger! Danger, Will Robinson!" When Copernicus speaks, we should all listen.
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