Big Bang - the Story of the Universe

The Story of the Universe

"How did we get here?" This is a question that has puzzled humans for thousands of years. From the earliest times in human history, people have been proposing answers to this eternal question. The same awe that drives us to question today has spurred countless theories from humans of all walks of life. Until recently, these theories have all been religious in nature, placing credit squarely on the good whims of a deity or deities. In the past century, though, scientists have been venturing their own ideas and making significant strides toward answering this age-old query.

The foremost of these explanations is the "Big Bang" theory. It states that our universe came from a tiny, extremely dense and hot state about 13.7 billion years ago. How this theory developed is important to understanding its validity.

Our ability as humans to observe the heavens has expanded exponentially since Galileo first peered through his then-revolutionary telescope. Since then our knowledge of what is beyond our Earth has grown and consequently changed dramatically. We realized that Earth is not the center of our solar system. We then found that our Sun is not the center of our galaxy. Eventually we even learned that our galaxy is nothing special, being only one of the more than one hundred billion galaxies like it in the observable universe. And we observed that most of those galaxies are actually moving away from us. Early on, the significance of this discovery was not realized. It was not until 1927, when a Roman Catholic priest named Georges Lemaitre derived the Friedmann-Lemaitre-Robertson-Walker equations from Albert Einstein's equations of general relativity, that the first glimmerings of a "big bang" theory arose.

The Friedmann equations relate various cosmological parameters within the context of general relativity, the breakthrough scientific discovery Albert Einstein made in the early twentieth century that changed forever our views of gravity, space, and time. They were derived from the Einstein field equations. These ten field equations set out the mathematical basis on which Einstein hung his claim that the fundamental force of gravitation is the curvature of space-time by matter and energy. From this several assumptions of symmetry appropriate for a cosmological model are made. (Symmetry in physics refers to something that remains unchanged under different sets of circumstances.) Applying this relatively simple formula to a fluid with a given equation that describes the state of the fluid under a given set of physical conditions will yield the time evolution and geometry of the universe as a function of the fluid density. It was then deduced, based on the nature of the other galaxies' movements that the universe must have come from the "explosion" of a "primeval atom."

In 1919 Edwin Hubble, an American astronomer, arrived at Mount Wilson Observatory in Los Angeles County, California. It was there that he observed the redshift of galaxies and in 1929, along with Milton L. Humason, formulated the empirical Redshift Distance Law of galaxies, or Hubble's law. Coupled with the Cosmological Principle, which states that the Universe is homogeneous and isotropic (independent of direction), it can be inferred that the Universe is expanding. This is opposite what Einstein assumed was true. He had proposed a static universe, and after Hubble's discovery, he relented and called his former idea his "biggest blunder."

Now there were two opposing camps. Those who sided with Lemaitre's Big Bang theory or with a British astronomer named Fred Hoyle. Hoyle asserted a "steady state" universe that was eternal. He explained the red shift by admitting that, yes, galaxies were shifting apart, but new galaxies were being created between them in some kind of "creation field." This kept the Universe from changing. Ironically, Hoyle actually gave Lemaitre's idea its name in a BBC program broadcast on March 28, 1949, when he called it "this big bang idea."

One consequence of the Big Bang theory is a cosmic microwave background throughout the Universe. George Gamow, Ralph Alpher, and Robert Herman predicted this in 1948. This was not widely discussed until the early 1960s, when the idea was picked up by Robert Dicke and Yakov Zel'dovich. Some of Dicke's colleagues at Princeton University, David Todd Wilkinson and Peter Roll, began building their own Dicke radiometer in 1964 to observe this phenomenon. Nearby, in Holmdel, New Jersey, two fellows by the names of Arno Penzias and Robert Woodrow Wilson at Bell Telephone Laboratories were doing much the same thing, except they planned to use their radiometer for radio astronomy and satellite communication experiments. Once they had completed several experiments, their instrument had an excess 3.5 Kelvin antenna temperature they could not explain. They decided to contact Dicke, and upon further inspection discovered that the temperature was a result of the microwave background.

Steady-staters were ready with the defense that the microwave background was a result of scattered starlight from far-off galaxies. However, during the 1970s , it was established by the scientific community that the microwave background was actually a remnant of the Big Bang. This is for several technical reasons that won't be delved into here, but the basic reason was that new measurements at a range of frequencies showed that the microwave background was a thermal, black body (something that absorbs all electromagnetic radiation that it encounters) spectrum, a result the steady state model could not produce.

And so the Big Bang model became the premier explanation for the beginning and expansion of the Universe. Most theoretical cosmology work since then has been extensions and refinements of the Big Bang theory.

What does the theory state? Well, it gives us the age of the Universe. Based on observations made by the Wilkinson Microwave Anisotropy Probe, which measures variations in the cosmic microwave background, the universe is about 13.7 billion years old. This conclusion is reached by using the location of the first acoustic peak, or disturbance, in the CMB power spectrum to determine the size of the universe at recombination (the point at which photons began to appear independently of matter and compose the CMB). The time it takes light to travel to this surface yields a fairly good age for the universe.

The Big Bang theory also gives us an idea of how the universe evolved. The history of this evolution is divided into many steps. The very early universe is poorly understood because no accelerator experiments