Big Bang Theory of the Universe: Definition & Overview

Big Bang Theory of the Universe: Definition & Overview
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  • 0:02 Definition
  • 1:05 Oscillating Universe Theory
  • 2:07 Steady State Theory
  • 3:09 Big Bang Details
  • 5:52 Evidence
  • 8:03 Lesson Summary
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Lesson Transcript
Instructor: Richard Cardenas

Richard Cardenas has taught Physics for 15 years. He has a Ph.D. in Physics with a focus on Biological Physics.

In this lesson, you'll learn about the most current theory on how the universe began, the Big Bang Theory. You'll learn about the theories about the origin of the universe that came before the theory as well as the events, discoveries, and scientists involved in the formulation of it.

Definition of the Big Bang Theory

As the song goes, 'The whole universe was in a hot dense state, then nearly fourteen billion years ago expansion started. . .' Before The Big Bang Theory was a popular television show, it actually referred to the theory that talks about how the universe began.

The Big Bang theory tells us that the entire universe was once condensed in the form of a primeval atom, or a dense mass. Then, between 10 billion to 20 billion years ago (the exact timing is not currently locked down), a gigantic explosion caused the universe to expand. The dense mass, or primeval atom, expanded very rapidly, spewing out all matter and energy in the vast void. As the universe expanded, it cooled down, and as it cooled down, the elements started to form: stars were born from the simplest elements, and the stars formed galaxies. Therefore, the explosion of a tiny seed of matter and energy formed into the universe that we know today.

Other Theories about the Universe's Origin

Before we look at the Big Bang theory in more detail, let us take a look at two other theories that were believed to explain the origin of the universe.

The Oscillating Universe Theory

The Oscillating Universe, or Cyclic theory, is a name given to a variety of theories that speculated that the universe undergoes a cycle of expansion and contraction. Scientists in the 1920s, including Albert Einstein, considered this theory as the theory that best explains the origin of the universe. According to this theory, when the Big Bang happened the first time, the universe expanded from a tiny speck. The universe expands up to a point, then it stops expanding and begins to contract. When this universe is compacted into a speck of the right size, it explodes again, and the whole process repeats. If this theory is correct, we have no way of knowing which universe we live in. We can be in the initial universe that emerged from a tiny speck, or we can be in any one of the other universes that exploded after the universe contracted back to a tiny speck. Think of the universe as a spring. If you compress a spring and release it, the spring expands then contracts, then expands again. The spring oscillates between a complete compression and a complete expansion.

Steady State Theory

The Steady State theory was proposed in 1948 by Hermann Bondi, Thomas Gold, and Fred Hoyle. According to this theory, the universe was, the universe is, and the universe always will be. This theory proposes that the universe is eternal and had no beginning and will have no end. It is expanding, but at a very uniform or steady rate that looks the same on a large scale. But if the universe was expanding, then the density of the universe must decrease. If the density decreases, how can it look the same? Proponents of this theory revised the theory to include the statement that matter is continually being created out of nothing, and as a result, the density of the universe remains constant as it expands.

Steady State

This figure is an illustration of the steady state theory. Notice that the contents of the universe look the same even as it expands. It is like baking a cake. The cake is just the dough heated up to a point of expansion. The baked cake and the dough are the same material, except the cake is the expanded version of the dough.

The Big Bang Theory Details

Georges LeMaitre was a Jesuit priest from Belgium that first suggested that the universe began from a primordial atom. However, it was British astronomer Fred Hoyle who coined the phrase the Big Bang. The Big Bang theory was based on the observation that the galaxies are moving away from each other. But how did astronomers know that the galaxies were moving away from each other? The answer came from something called the Doppler effect, which is the changing frequency of a wave depending on the direction it's moving. You may be familiar with the Doppler effect for sound waves. If you are standing on the sidewalk and an ambulance approaches you with its siren on, the sound you hear has a higher pitch (or higher frequency) than if the ambulance was not moving. Similarly, the sound you hear as the ambulance passes and moves away from you has a lower pitch (or lower frequency). This is called the Doppler effect.

Light is also considered to be a wave, just like sound is a wave. Therefore, light can also undergo a Doppler shift. But instead of looking at different pitches of sound, we have what is called a red shift or a blue shift. This figure illustrates the Doppler effect for light.

Doppler Light Effect

In the figure, the sun is blue shifted when it is moving toward the observer or approaching the observer, and the sun is red shifted if it is moving away from the observer, or receding.

Astronomers use spectroscopy to look at the composition of a star. Spectroscopy is simple the process of passing light from a distant object, like a star or a galaxy, through a diffraction grating. A diffraction grating is a device that is used to split light into its components. This means that any light passed through a grating will be separated into its component colors. When astronomers used a grating to look at the spectra of stars or galaxies, they noticed that they were all red-shifted. The figure illustrates how we observe a red shift. A laboratory baseline image is taken then compared to the spectra of objects with different distances from the earth.

Red and Blue Shift

You can clearly see how the lines shown in the baseline laboratory spectrum get shifted out toward the red more as we move further away from the earth. This observation was key to the formulation of the Big Bang theory.

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