Cosmology

After the Second World War, the science of nuclear physics developed to a point at which it was clear that nuclear reactions would have taken place during the early phases of the big bang. Again scientists ran the motion picture backwards through an era of nuclear physics, attempting to predict what elements should have been produced during the early history of the universe. Their predictions were then compared to the elemental abundances of the oldest stars and the agreement was amazingly good.

The detailed calculations depended critically on whether the calculated model was for an open or closed universe. If the universe were open, then the era of nuclear reactions would not last long enough to produce elements heavier than hydrogen and helium. In such models some deuterium is formed, but the amount is extremely sensitive to the initial density of matter. Since deuterium tends to be destroyed in stars, the current measured value places a lower limit on the initial amount made in the big bang. The best present estimates of primordial deuterium suggest that there is not enough matter in the universe to stop its expansion at any time in the future.

In the event of an open universe, scientists are pretty clear what the future holds. In 1997, researchers from the University of Michigan released a detailed projection of the four phases of the universe, including the ultimate end in the Dark Era, some 10¹⁰⁰ years hence. Currently, the universe is in the Stelliferous Era, dominated by high-energy stars and filled with galaxies. Some 1000 trillion years from now, the universe will enter the Degenerate Era. Stars will have burned down into degenerate husks that can no longer support hydrogen burning reactions, and will exist as white dwarfs, red dwarfs, brown dwarfs, or neutron stars; some massive stars will have collapsed into black holes, which will consume the other star relics.

Next, the universe will progress into the Black Hole Era, about 100 trillion trillion trillion years from the present. At that time, black holes will have swallowed up the remaining bodies in the universe and will gradually leak radiation themselves, essentially evaporating away over trillions of years. Finally, the universe will reach the Dark Era, in which no matter will exist, only a soup of elementary particles like electrons, positrons, neutrinos, and other exotic particles.

In 1998, astronomers studying a certain group of supernovas discovered that the older objects were receding at a speed about the same as the younger objects. According to the theory of a closed universe, the expansion of the universe should slow down as it ages, and older supernovas should be receding more rapidly than the younger supernovas. The fact that observations have shown the opposite has led scientists to believe that the universe is either open or flat.

Cosmologists still battle over the exact nature of the universe. Most scientists agree that the age of the universe ranges between 13 and 15 billion years. The exact age, for instance, is a matter of great controversy between rival research teams at Carnegie Observatories and the Space Telescope Science Institute. When researchers recently used an analysis of polarized light to show that the universe is not isotropic, i.e., not the same in all directions, their findings were disputed almost as soon as they were published.