Genesis: The origin of the universe

National Forum, Winter 1996 by Wersinger, J-M

General Relativity presents us with a universe that is far removed from our everyday experience of the world. For starters, space and time are not absolutes, independent of the matter and energy in the universe. Rather, they are very much a part of the universe, affected by the presence of matter and en (The famous equation E=mc sup 2 informs us that matter and energy are the same thing: matter is a condensed form of energy. From now on the word matter will cover both matter and energy.) General Relativity teaches us that matter curves space and warps time. The denser the matter, the more it both curves space and slows time in its surroundings. This curvature of space affects the motion of objects and consequently appears as a force acting on these objects, the force of gravity. In General Relativity the force of gravity is an effect of geometry, the geometry that matter gives to space.

There is actually an overall curvature of space for the universe as a whole that depends on the total amount of matter present in it. If enough matter is present in the universe, the curvature is said to be positive; the universe is closed and finite. The fate of such a universe is a Big Crunch, a universe that collapses back on itself -- a victory for gravity. If not enough matter exists in the universe, the curvature is said to be negative; the universe is open and infinite. It will expand forever -- gravity loses and expansion wins. The infinitely narrow boundary case between the open and the closed universe is called the flat universe. It is neither open, nor closed, but it sits uncomfortably on the unstable thin line between these two cases. These three geometrical possibilities play an essential role in the understanding of the origin and the fate of the universe.

Running the history of the universe backwards in time, we conclude that in the past galaxies were closer to each other. The equation of General Relativity shows that between 10 and 20 billion years ago all the matter in the universe was concentrated in a state of infinite density and temperature, called a singularity. After the first fraction of a second in the history of the universe, the universe that we now see was less than the size of a proton. A tremendous explosion must have occurred to blast the universe apart, the Big Bang. In the process of expanding, the universe has been thinning, cooling, and slowing down owing to the effect of gravity that matter exerts on itself. This is the essence of the Big Bang model.

At first, the scientific community was very reluctant to accept the idea of a birth of the universe. The discovery in 1964. of a pervasive radiation filling the universe, the Cosmic Background Radiation (CBR), by Arno Penzias and Robert Wilson of the Bell Telephone Laboratories, provided the Big Bang model with the observational support it needed.

The CBR is a relic of a time when the universe was much hotter. At the time of its emission the CBR filled the universe with a powerful visible glow. As the fabric of the universe stretched, so did the wavelength of the CBR, which went from a visible, all-pervasive, powerful glow to the currently measured very cold microwave background radiation. Before the emission of the CBR, the universe was opaque, and radiation could not make its way through the soup of fast-moving electrically charged particles filling the universe. As the universe cooled, electrons managed to combine with positively charged nuclei, and stable atoms were formed. Thus, about 100,000 years after the explosion, radiation and matter decoupled, and the universe became transparent. This time is also called the radiation barrier. Careful all-sky analysis of the CBR reveals its extreme smoothness in all directions. At the 100,000-year mark, the universe was uniform (the same everywhere) and isotropic (the same in every direction).