The two basic principles that lie at the heart of the Big Bang theory are General Relativity and the Cosmological Principle.

Relativity

Before the Big Bang and evolution were widely accepted, the commonly accepted Western view was that the earth played a central part in the Creation story. It was even accepted that the earth is the center around which all other heavenly bodies revolve. This geocentric (earth-centered) view was challenged by Copernicus, who proposed the heliocentric view that the earth moves around the sun.

Today, it is believed that the earth is a relatively insignificant planet, revolving around a relatively insignificant star—the sun—which in turn circles around an ordinary galaxy—the Milky Way. The Milky Way in turn circles around a relatively minor cluster of galaxies called the Local Cluster, which is like a speck in the vastness of the universe. This is called the "Mediocrity Principle.”

A Michelson interferometer.
Source: Falcorian on Wikimedia Commons. https://commons.wikimedia.org/wiki/File:Michelson_Interferometer.jpg...

The difference between a heliocentric theory and a geocentric theory is motion. A number of experiments have been conducted in order to measure the absolute motion of the earth through space. In 1881 Albert A. Michelson, together with his colleague Edward Morley, designed and built an apparatus called an interferometer to measure this motion.ⁱ

Since it was shown that light acted also as a wave, it was assumed that, just like sound waves have to travel through some form of material, so space must contain some form of material through which light traveled. Nobody knew what the medium was, so they called it "aether," and their experiment was designed to measure the earth’s speed through this aether.

They expected that the rotation speed of the earth at the latitude where the experiment was conducted would be equal to the speed of earth through the aether. The two scientists intended to turn their instrument until they found a maximum fringe shift produced by light traveling in two perpendicular directions. The position of maximum shift would show in which direction the earth was moving and the size of the fringe shift would show how fast it was moving.

Much to their surprise, the answer turned out to be zero. No matter how the earth moved, the aether "ocean" it moved in always moved with it so that the earth was constantly in still "water." This means that the earth in fact is not moving through the universe as earlier suggested.

In 1905, Albert Einstein faced up to the contradictions. He reformatted a theory that had been proposed by Dutch Physicist Hendrick Lorentz, and published his version of the theory of relativity, which provided a mathematical solution to the problem, thus divorcing it from the physical observations. His equation did not require an aether. The equation e = mc2 was also predicted by relativity. This video helps clarify Einstein's theory of relativity:

The Cosmological Principle

The cosmological principle, in its simplest form, states that the universe looks the same from every location within it. In an expanding universe, in which the rate of expansion increases linearly with distance, the universe should always look the same from any location within it.

Accepting this principle overcomes the appearance that everything is moving away from the earth as predicted by redshift. However, there is no way to test the validity of this principle, because given the vastness of space, we cannot go far enough away to check the validity of the assumption.

In fact, from all observations that are possible, the opposite seems to be true. Wherever we have been in space, and wherever we have looked, the principle seems to be violated. There is no uniformity, but rather magnificent variety.

Wherever we look within the solar system—the galaxies, the super clusters, and the gas clouds—the heavens speak of anything but a boring uniformity. There is a varied distribution of matter, which contradicts the very essence of the principle. And if the principle is in trouble, then the Big Bang is in trouble.

The view that the universe must be uniform is central to the Big Bang theory, and Einstein and other scientists who sought to apply the theory of general relativity to the universe assumed this to be the case. But the cosmological principle is on very shaky ground, especially since modern technology, telescopes, and space exploration opens up substantially more of the universe than was known in Einstein’s day. The great chains of galaxies that curl around vast regions of empty space called "voids" suggest, ironically, the void nature of the cosmological principle.

The astrophysicists of today are not unaware of this quandary. The professor of Astrophysics at Nottingham University admits what happens if a uniform universe is disproven:

The foundations of the big bang models would crumble away. We’d be left with no explanation for the big bang, or galaxy formation, or the distribution of galaxies in the universe.ⁱⁱ

If this is the case, do Hubble's physical observations withstand enough scrutiny to uphold the Big Bang theory? Find out in the next article.