The Big Bang Picture of The Universe

"The big bang," says Joseph Silk in his treatise The Big Bang, "is the modern version of creation" (Silk, 1989, p. 1). Silk, a professor of astronomy with the University of California at Berkeley, knits together most of the major issues in modern cosmology—the study of the large-scale structure and evolution of the universe—within the framework of the signal event most astrophysicists now believe gave birth to the universe around 15 billion years ago. Any such universal theory will rest on a fundamental cosmological principle, and Silk traces the roots of the Big Bang theory to Copernicus, who in 1543 placed the Sun at the center of the universe, displacing Earth from its long-held pivotal position. With this displacement of Earth from a preferred point in space came the recognition that our vantage point for regarding the universe is not central or in any way significant. Since observations show the arrangement of galaxies in space to vary little, regardless of direction or distance, scientists believe the universe is isotropic. This regularity is then imposed over two other basic observations: everything seems to be moving away from everything else, and the objects farthest removed from Earth are moving away proportionally faster. Taken together, these phenomena invite time into the universe and allow theorists to "run the film backwards" and deduce from their observations of current phenomena how the universe probably originated, in order to have evolved into what is manifest today. From this line of thinking, it follows that the universe began from a single point of infinitely dense mass, which underwent an unimaginable expansion some 10 billion to 20 billion years ago.

Although the astronomical data currently available favor a Big Bang cosmology, predictions based on the Big Bang model are continually examined. Perhaps the most convincing single test was accomplished in 1965, when radio astronomers Arno Penzias and Robert Wilson, using a highly sensitive receiver designed at their Bell Laboratories facility in New Jersey, detected what has come to be called the cosmic microwave background radiation. Spread uniformly across the celestial sky, this sea of microwaves represents the reverberating echo of the primordial explosion, a find that later garnered the Nobel Prize for Penzias and Wilson. Subsequent studies of this background radiation show it to be uniform to better than 1 part in 10,000. Such isotropy could arise only if the source were at the farthest reaches of the universe. Moreover, measurements of the radiation taken at different wavelengths, says Silk, indicate that it "originates from a state of perfect equilibrium: when matter and radiation are in equilibrium with one another, the temperatures of both must be identical" (Silk, 1989, p. 84).

Cosmologists call the point at the very beginning of time a singularity. Before it, classical gravitational physics can say or prove nothing, leaving all speculation to the metaphysicians. Big Bang theory encompasses a series of events that occurred thereafter, which conform to two continuing constraints: first, the laws of physics, which are believed to be universal, and second, data from observations that are continually probing farther in space, and therefore further back in time toward the event itself. This series of events cosmologists can "date," using either lookback time from the present or cosmic time. When measuring time forwards, singularity (the moment of creation) represents time zero on the cosmic calendar.

The events occurring during the first second have been hypothesized in great subatomic detail—and necessarily so—for at that point the temperature and pressure were greater than the forces that bind particles together into atoms. As time ensues, space expands, and matter thus thins out; therefore, temperature and pressure decrease. This "freezing" permits the forces that generate reactions between particles to accomplish their effects, in essence leading to the manufacture of a more complex universe. Complexity then becomes entwined with evolution, and eventually Earth and the universe reach their present state. Where the state of universal evolution has reached, cosmologists can only speculate. Since their observations rely on time machine data that only go backwards, they extrapolate according to the laws of physics into the future.

At the moment of creation, all four forces were indistinguishable; the most advanced theories in physics currently suggest that the forces were united as one ancestral force. But as the universe cooled and evolved, each force emerged on its own. At 10-43 second, gravity first separated, precipitating reactions between particles at unimaginably intense pressures and temperatures. At 10-36 second, the strong and the electroweak forces uncoupled, which in turn permitted the fundamental particles of nature, quarks and leptons, to take on their own identity. At 10-10 second, the four forces of the universe were at last distinct; as the weak force separated from the electromagnetic force, conditions deintensified to the point where they can be simulated by today's particle physicists exploring how matter behaves in giant particle accelerators. At 10-5 second, the quarks coalesced to form the elementary particles—called baryons—and the strong forces that bind the nucleus together came to dominate. By 1 second after singularity, the weak nuclear force allowed the decay of free neutrons into protons and the leptons, or light particles, the electrons, and neutrinos. Within 3 minutes, the protons and neutrons could join to form the light nuclei that even today make up the bulk of the universe, primarily hydrogen and helium. These nuclei joined electrons to form atoms within the next 1 million years, and only thereafter, through the evolution of particular stars, were these elements combined into the heavier elements described in the periodic table.

From the earliest moment of this scenario, the universe was expanding outward. The use of the term Big Bang unfortunately suggests the metaphor of an explosion on Earth, where mass particles hurtle outward from a central source, soon to be counteracted by the comparatively massive effects of the planet's gravity. In the universe according to Einstein, however, it is the fabric of space itself that is expanding—carrying with it all that matter and energy that emerged during creation's first microseconds, as described above. Gravity assumes a central role, according to Einstein's conception of the universe as an infinite fabric that evolves in a four-dimensional world called space-time. The mass that may be conceptualized as part of the fabric of space-time has a distorting effect on its shape, ultimately curving it. Einstein's ideas came to transform the face of science and the universe, but other astronomers and cosmologists, using general relativity as a touchstone, were the progenitors of the Big Bang conception that has come to define the modern view.