Pedagoguery

In 1998, the world of cosmology was shaken to its foundations when observations of distant supernovae seemed to indicate that the expansion of the universe was accelerating. This seemed to indicate that most of the universe was made up of a mysterious “dark energy” whose existence was not even hinted at by standard physics. Recently, though, a potential alternative to dark energy has arisen. The catch is that it requires us to relinquish a long-held principle: the Copernican principle.

Copernicus was the first scientist to openly postulate that we were not special by removing the Earth from its place of prominence at the center of the universe. Further refinement of that idea has moved us further and further from the center of things, until we fully recognize that the place we occupy in the universe is nothing particularly special. In fact, this view has been particularly powerful. It has allowed us to extrapolate our local conditions out into the universe at large. Today, we call this the cosmological principle, which states that we do not live in a special place in the universe.

However, what if it is wrong? The supernovae observations could also be explained if we live near the center of a void of gigantic proportions. By void, I do not mean a totally empty space, for clearly that is not the case. I simply mean a region that has a significantly lower density than average, say half or a third. If this void occupies most of the observable universe, and we are located very near the center, than all of our current observations would be explained without the need for dark energy.

How would it work? Well, the universe would be expanding, and the expansion would be slowing, but it would not be slowing at the same rate everywhere. The expansion would slow faster in the denser parts of the universe, whereas it would slow more slowly in the voids. If a supernova were to explode early in the universe in one of the denser portions, by the time the light reached us in the void, it would have had to travel farther, and thus have been stretched out more due to the expansion of space, than it otherwise would have been, making it dimmer and redder. This is precisely what we observe.

The biggest problem with this hypothesis is its unlikelihood. Currently, the observed matter distribution in the universe is very well described by invoking quantum fluctuations that were greatly magnified during inflation. Using that scenario, the chances of a giant void of the size needed to explain the supernova results would be one in 10¹⁰⁰, a staggeringly small number. This is somewhat offset by the fact that such voids would expand more quickly than the rest of the universe, and as such, an observer would potentially find himself in one with rather more likelihood than you would think. It is possible that while this hypothesis does violate the cosmological principle by putting us in a special place, it does not violate the principle of mediocrity, which states that we are typical observers.

How can we distinguish between the cases of the giant void and dark energy? There are several ways. First of all, the chances of us being in the exact center of the void are extremely small. As a result, if the void model were correct, we would expect the observed rate of expansion to vary depending on the direction in which a supernova was observed. Secondly, galaxy clusters reflect some light, acting as a weird sort of mirror. By observing the reflected light, we should be able to view our own cosmic neighborhood, and thus see if we live in such a void. A third way is to observe how galaxies and clusters evolve in different areas. Such evolution is somewhat dependent on the local density. If we find fluctuations consistent with different local densities, it would be support for this hypothesis.

Finally, the cosmic microwave background could provide hints. If the void hypothesis is correct, the microwave background would be hotter in one direction and cooler in the opposite, called a dipole. We do in fact observe this, but it is usually attributed to our motion relative to the background. In addition, certain features would line up, also something which is observed. Finally, a preferred direction would result in large scale motion of galaxies and clusters, which some have claimed to observe, but which is controversial.

There are a number of instruments and plans to gather the data that would distinguish between the hypotheses, and developments over the next decade should be very interesting.

Next time, color vision.