Once Before Time - Martin Bojowald [75]
This process and the structure of black holes will occupy us in the next chapter. Here we are interested merely in the fact that not all the starry matter participates in the collapse, a significant part being blown out into space. The released energy can be seen as a huge explosion, a supernova. One subclass of supernovae, called “type Ia,” has the advantage of having very special properties: Precisely observing the color distribution of their emitted light, for instance by decomposing it with a prism, allows reliable conclusions about the total amount of energy spewed out. From this, in turn, one can derive the supernova’s distance from us using two basic facts: The explosion should appear darker if it is farther away; and the more energy emitted, the brighter it should be. Its visible brightness thus depends on the distance and the total energy. Since we directly measure the brightness and determine the total energy from the color distribution, the distance can be inferred. For normal stars, without any information as to how brightly they are burning, this would be impossible. Their distances can often be found only indirectly, for instance by comparison with other objects in their neighborhood, such as those supernovae. For this reason, supernovae are designated standard candles, a kind of normalized lighthouse for the exploration of the universe by astronomers.
Moreover, supernovae shine very intensely as a result of the violence of the underlying event, making them visible from afar. Owing to the finite speed of light, they allow us to see traces of the universe a long time ago, at the time when their light had to start on its journey in order to reach us now. The expansion history of the universe, in turn, can be traced back by determining not only the distance of the supernovae but also—again from the colors of their light—their velocity caused by the expanding universe. (Additional contributions to the velocity are made by gravitational attraction to masses in their neighborhood. Since supernovae, like their progenitor stars, are situated in galaxies that tend to rotate around a center, this motion adds to the supernova velocity. But these contributions are usually small and, moreover, are distributed randomly compared to the systematic expansion velocity; they do not play a role when a large class of supernovae is considered.)
The distance thus provides the supernova’s explosion time before now; the color distribution, the cosmic expansion rate back then. In this way, a relationship results: expansion rate depending on time, a function of exactly the form obtained by solving Einstein’s equations for a given energy composition in the universe. Comparing a large class of solution curves with the observed piece of the expansion-time relationship allows one to see which composition best agrees with the recent past of our universe. This constitutes another method to determine cosmological parameters, independent of those measurements discussed in the preceding two sections. Here one has to see single stars, in contrast to whole galaxies in their mappings or even the skywide microwave radiation in its cosmic background. Supernovae, accordingly, allow only a determination of the most recent part of the universe’s history. By referring to vastly different times, the main contributions in the whole of all obtained data—cosmic microwaves, galaxy maps, supernovae—are independent of one another; checking whether they agree in details is an immense consistency test.
Consistency is confirmed indeed, and it holds a surprise. We saw that details of the heavenly intensity distribution of microwaves can be explained only if there is a very special, dark contribution to the energy of the universe. In recent times, such a contribution must have implied an acceleration of the cosmic expansion. The recent past is just the domain of supernova observations, and the expansion history they provide shows the same acceleration. Such an energy form is thus seriously to be reckoned with, even though a theoretical