Pedagoguery

There is strong evidence that the expansion of the universe is expanding. The reason for this fact is uncertain, but the leading candidate for the explanation is dark energy. The question then becomes, what is dark energy? That is a question that is much more difficult to answer.

The main question about dark energy is does it vary over space and time. If so, then it takes the form of a cosmological constant, as described in general relativity. If not, it is called quintessence. Those names are simply labels – scientists do not know the physical basis for either phenomenon. But observations can help us distinguish between them. All we have to do is to observe how the expansion of the universe has changed over time.

There are several ways in which to observe the expansion of the universe over time. The first uses the same mechanism that discovered the acceleration to begin with – Type 1a supernovae. Type 1a supernovae result when a white dwarf acquires enough mass to push it over the Chandrasekhar limit, disrupting the electron degeneracy pressure that holds it up against gravitational collapse. This results in a runaway nuclear reaction that blows the star apart. Because of this, it is believed to be a good “standard candle”, in that all Type 1a supernovae would have the same intrinsic brightness, and thus by measuring that brightness, you can tell how far away it is. This is a good mechanism, but it rests on several untested assumptions. Does a white dwarf composed primarily of helium act the same way as one composed primarily of carbon and oxygen? How does a potential asymmetry in the nuclear reaction affect the brightness of the supernova? These uncertainties mean that this mechanism by itself is not sufficient to do the job.

A second mechanism relies on sound. In the very early universe, the hot gas that filled the universe was suffused with sound waves. These sound waves became amplified during the inflation epoch and resulted in areas that were over- or under-dense with density peaks starting at around 436,000 light years apart.. As the universe continued to expand, the areas of overdensity tended to be where galaxies and galaxy clusters formed. By mapping the clustering of galaxies over time, we can track how these baryon acoustic oscillations evolved over time.

A third mechanism deals with the size of galaxy clusters. The more slowly the universe expands, the larger galaxy clusters can grow. So by observing the growth of galaxy clusters over time, we can get a handle on how the expansion of the universe has changed over time. There are a couple of ways this particular observation can take place. The first is by observing gravitational lensing of galaxies. The second is to observe the effect that the hot, x-ray emitting gas that clusters are embedded in affects the cosmic background radiation.

All of these observations are designed to yield a parameter called the equation of state of dark energy. If the value of this number is precisely -1, then dark energy is a cosmological constant. Any other value and it is quintessence. So far, observations of all three mechanisms have yielded a value from -0.85 to -1.1, which is not good enough to give us a definitive answer, but the results are tantalizing, since they neatly bracket the magic value of -1. More observation is needed, however.

Next time, naked singularities.