The cosmic microwave background radiation (CMB) is the gift that just keeps on giving. It played an instrumental role in turfing cosmology out of the realms of philosophy and into the realm of quantitative science. Now, researchers are using it to test a principle that's long been tacitly accepted—the idea that we live no place special.
The problem is that we live on exactly one planet and that gives us exactly one point of view when observing the universe. This means that almost any observation can be explained by a set of appropriately tailored mathematical constructs, provided that the Earth sits at the center of some location that isn't typical of the rest of the universe. The approach of astronomers (and cosmologists) has been to assert that we have no reason to believe that we are special, therefore we are not. Consequently, any explanation that we derive from our observational data should apply no matter where we are. Which just leaves us one big question: is the assertion correct?
This is where the CMB can help us. The CMB provides us with our only view of the very early universe and, if we were someplace special, this would show up as an aberration in the smoothness of the CMB. The CMB is very smooth, so we already know we aren't that special. Nevertheless, the relatively recent observation that the rate at which the universe is expanding is increasing—the cause of which is gathered under the catch-all name, dark energy—has reopened the question because this inflation could still be explained by local conditions.
Previous work had shown that the Earth would need to sit near the center of a void (compared to the rest of the universe) to replicate astronomical observations. However, such a void would cause a change in the CMB. First, the relative motion of ionized gases would cause a Doppler shift (this changes the color of the CMB). Second, some CMB light that had originated from within the void would have reflected off the surface of the void. This should also be detectable as a change in the intensity of the CMB from a constant distance.
After making calculations and constraining them with the observed rate at which the universe is expanding, the researchers were able to show that the largest possible void size was 2.5Gpa. However, they could not model a void that was also compatible with the CMB data, meaning that the existence of such a void is extremely unlikely. They hasten to point out, however, that there may be some very special configurations that might not show up in their tests.
If you are asked where you live, the most likely answer is:"no place special."
Physical Review Letters, 2008, DOI: 10.1103/PhysRevLett.100.191302