Big Bang Cosmology
Currently, the most widely accepted scientific view of the history of the Universe is based on various models of the Big Bang theory. This is because the Universe seems to be expanding, meaning simply that the space between the galaxies and quasars is increasing with time. A retro-projection back in time points to a Universal Singularity, a single point where spacetime did not exist and from which the
The idea that the Universe has not been around forever and actually had a beginning is fairly new, having emerged as a rather disturbing revelation in the first half of this century. The term "Big Bang" was coined by the cosmologist Fred Hoyle who used it to refer to a theory that to him seemed absurd. Years before, when Einstein applied his relativistic equations to cosmological models, the possibility of universal expansion appeared as a natural consequence, but he assumed it to be a nonsensical solution, so introduced a cosmological constant into his equations to stabilize the model. He considered this to be his greatest blunder once the physical evidence for the expansion began to emerge.
This evidence first began to appear to astronomer Edwin Hubble during the late 1920's. For years, he studied the motion of galaxies and used the Doppler effect to judge their motion. He soon discovered that the light from essentially every galaxy was red-shifted -- they all appeared to be moving away from us. This was amazing enough, but over time a definite pattern emerged. The most distant galaxies had the greatest red-shift -- the farther away a galaxy was, the faster it appeared to be receding. Hubble's Law was formulated, in which the velocity of recession is directly proportional to the distance from the observer. The most obvious explanation was that we live in an expanding universe which had a singular point of creation in the distant past. This idea was not easy to accept at the time, and when Hubble's initial estimates placed an improbably young age on the Universe, it was easily dismissed. Gradually, the data was refined, Hubble's Constant was adjusted, and by the 1950's the Big Bang had become a much more plausible theory.
Not until 1965 did more direct evidence surface. It appeared suddenly and convincingly compared to the gradual formulation and acceptance of the red-shift evidence, yet it was every bit as unexpected. Engineers Arno Penzias and Robert Wilson, while conducting tests of a microwave receiver for Bell Laboratories, discovered the presence of a background noise that was the same no matter where they pointed their directional antenna. They are credited with the discovery of an actual remnant of the Big Bang itself. Since then, the cosmic microwave background radiation has been found to be isotropic (the same in all directions) to a very high degree, and is thus considered to be a cooled remnant of an early cosmological state in which the Universe was very nearly homogeneous. It glows in the microwave region of the electromagnetic spectrum at about 2.7°K, just barely above absolute zero.
The Big Bang Universe can be modeled pictorially as an expanding sphere, or more precisely a hypersphere, since the entire volume of three-dimensional space is represented by the two-dimensional plane of the spherical surface. The singularity of the Big Bang is the central point within the sphere, and time is represented simply by the expansion of the sphere away from the center. As the size of the hypersphere increases, galaxies and quasars on its surface become farther apart -- the space between them increases. An observer anywhere in the universe would observe the very same red-shift relationship that surprised Hubble.
Modern cosmology does not concern itself with what came before the singularity of the Big Bang other than to say that it transcends physical knowledge, just as the singularity itself does. In fact, as we project the Universe back in time to that point, physics breaks down before we reach the singularity, at what is called the Planck time, a tiny fraction of a second after the creation event, at t=10-44 second. Scientific cosmology is primarily concerned with the earliest stages after the Planck time, because this is the most interesting physics, and is exactly the same physics dealt with in quantum field theory.
Cosmology is also concerned with the future evolution of the Universe, which presents three distinctly different possibilities. The Universe will either continue to expand forever, in which case it will eventually become very cold and lifeless; or it will slow its expansion more and more until it stops forever in a state of perfect equilibrium, which is unlikely because that would be very unstable; or it will slow its expansion to a stop and then begin to collapse in a "Big Crunch", returning to the singularity. The latter scenario provides for the possibility of an immediate rebirth into another big bang, an entirely new spacetime manifold, and thus also provides for the possibility of an infinite series of expansions and collapses, which solves the problem of what came before our Big Bang. This once again provides us with an unending Cosmos, and even though our particular cycle of spacetime history will end the way it began, the concept of the Universal Singularity itself is a realization of a timeless state.
As an expression of one of these three possible futures, a big bang universe will have a relativistic geometry associated with it, a geometry which is independent of the hyperspherical model mentioned above. A universe that expands forever is called hyperbolic, and is referred to as an open universe, one that stops expanding and reaches equilibrium is called flat, and one that eventually recollapses is called spherical, and is referred to as a closed universe. What decides which future we will experience is the balance between the rate of expansion and the amount of matter in the Universe. If there is enough mass in the Universe to gravitationally overcome the energy of expansion, the Universe will eventually recollapse, and like a "cosmic reset", an entirely new spacetime history may be born in another big bang.
Currently, cosmologists do not have enough information to know exactly which future will apply to us. The most recent measurements, however, indicate that the rate of expansion appears to be actually increasing. This pushes the age estimate toward the older end of the range, to about 15 billion years, and pretty well establishes that a continuous expansion is the fate of our spacetime -- we live in an open universe. There is simply not nearly enough mass to our Universe to slow its expansion. Even the possible "dark matter" candidates, such as the exotic particles, will not be enough according to the recent estimates. The neutrino, for example, is a tiny and very fast quantum which, so far, has no detectable mass. There are literally billions of them passing through our bodies every second, and if it is found to have even the tiniest amount of mass, it was once thought that it could go a long way to close the Universe all by itself. The recent study, however, seems to place little hope in its chances of having enough mass to be of much help.
The philosophically disturbing thing about an open universe is that, ultimately, the universe becomes meaningless. It is doomed to become so spread-out, cold, and non-interactive, that taken to the infinite extreme it becomes conceptually nonexistent. That's strange enough in itself, and just what "nonexistence" means from our perspective of existence can generate lively philosophical discussion, but when we include in our pondering the fact that it happened after a spacetime which just happened to include the incredible history of the human story, and probably many others, the idea of a meaningless universal future is at the same time stunning and numbing. We can, for now at least, rest assured that any such condition as a big crunch or big freeze is a long way off. After all, our sun will expand into a red giant some 5 billion years from now, long before our Universe ends, and even this event is much too far away for us to worry about other than for cosmological and philosophical speculations.
Our present Universe seems to be perfectly designed for life, at least for human life and other DNA-based bioforms. This is called the Anthropic Cosmological Principle. If any one of the physical laws were even slightly different than it is, life would not have developed.
A closed universe provides an infinite series of universes to overcome the probabilities against the formation of life. With an unending series of spacetimes, the formation of life is essentially assured. Since the expansion and recollapse continues forever, the Cosmos will eventually get it just right, and naturally, that would be the one we would find ourselves in, wondering about the strangeness and beauty of such a thing.
In an open universe, it would seem that the Universe had to actually have been designed with a special life-forming "cosmic code" from the start -- either that or we were just incredibly lucky to have beaten the odds. Quantum cosmology, however, allows for all the infinite possibilities of the bouncing series of spacetimes; except now the "other universes" exist simultaneously with ours (parallel universes, many worlds theory); and we would quite naturally find ourselves in one which supports bioform life, and again, wondering about the strangeness and beauty of such a thing.
"Blessed is he who shall stand at the beginning, and he shall know the end and he shall not taste death." -- St. Thomas