Evidence for the Big Bang Theory: Background Radiation, Red-Shift and Expansion

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  • 0:07 The Big Bang Theory
  • 1:52 Universe Expansion and…
  • 4:24 Cosmic Background Radiation
  • 5:31 Elements in the Universe
  • 6:10 Lesson Summary
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Lesson Transcript
Instructor: Amy Meyers

Amy holds a Master of Science. She has taught science at the high school and college levels.

Discover what evidence exists to support the Big Bang theory of the birth of the universe. Learn how cosmic background radiation, the red shift of light and the ongoing expansion of the universe led scientists to believe that the universe was started with the Big Bang.

The Big Bang Theory

Billions of years ago - more than 14 billion years - there was nothing: no matter, no energy, no space. Scientists needed a way to explain how everything began, where it came from, what happened. Although there are several different theories about the origins of the universe, the Big Bang theory is the prevalent one because no other model is as good at explaining everything in the universe. The Big Bang theory isn't perfect, but it's the best we've got.

Contrary to popular belief, the Big Bang was not an explosion
Big Bang Was Not an Explosion

Scientists believe our universe comes from a singularity. Scientists don't actually know where this singularity came from or exactly what it is, but they do know that a singularity is an infinitely small, hot area of infinite pressure and density. These singularities defy our known laws of physics because they didn't appear in space. Rather, space began inside them. Before the singularity, nothing existed. There was no space, no time, no matter, no energy.

One of the misconceptions about the Big Bang is that it was an explosion, like with fire and sound and, well, kind of like a bomb. It wasn't. Instead, it was probably more like a balloon being blown up - a really tiny balloon. A balloon starts small and expands outward at roughly the same speed all around. Our universe is that balloon.

When scientists first propose a hypothesis to explain a phenomenon, they expect to find certain evidence to support it and turn their hypothesis into a theory. There are several main pieces of evidence that support the Big Bang theory. One is the fact that the universe is expanding, proven with something called red shift. The second is something called cosmic microwave background radiation. The third is the abundance of different elements in the universe.

Universe Expansion and Red Shift

The red-shift of light is evidence that the universe is expanding

The evidence that the universe is expanding comes with something called the red shift of light. Light travels to Earth from other galaxies. As the light from that galaxy gets closer to Earth, the distance between Earth and the galaxy increases, which causes the wavelength of that light to get longer.

This is similar to the Doppler effect of sound waves, which also happens with light waves. I'm sure you've heard it for yourself in real life: as an ambulance approaches you, the siren pitch seems to increase, and as it moves away from you, the siren pitch decreases. This is due to how the sound waves increase or decrease depending on the movement of the object emitting the waves relative to the person hearing them. As the ambulance approaches you, the sound waves get closer together and shorter, which results in a higher pitch. As the ambulance moves away from you, the sound waves stretch out and get longer, which results in a lower pitch. If everything in the universe is moving apart from everything else, then those light waves should move further apart and get longer. Longer wavelengths of visible light are red, and that is why it is called the red shift.

This red shift of light gives scientists information on the speed and direction that a star is moving. This in turn tells scientists that stars are not only moving away from us, but they are also moving away from each other.

Edwin Hubble studied galaxies in the 1920s and proved that everything in the universe is expanding
Edwin Hubble

In the 1920s, Edwin Hubble used the 100-inch Hooker telescope in Southern California to study the Andromeda Galaxy. He took pictures of faint galaxies and studied them. Eventually, he was able to calculate the velocity for the galaxy and show that everything is moving away from everything else and thus expanding. If that's true, then scientists figured that the universe was previously much smaller than it is now. All of this evidence was the first observational support for the Big Bang theory that Georges Lemaitre first proposed in 1927.

If the universe is currently growing, then the universe was smaller in the past. There must have been some point in time when the universe was half its current size. Then there must have been a time when it was half that size. Think about this: if the universe has grown to currently be a size 1, sometime in the past it was a size 1/2, and before that 1/4, and before that 1/8, 1/16, 1/32, 1/64, etc. If you keep doing the fractions, you will get down to an infinitesimally small size and, eventually, nothing. (Yes, I know you can't really get down to nothing, but as I mentioned, the singularity from which the universe comes defies the laws of physics.)

Cosmic Background Radiation

The next piece of evidence that supports the Big Bang theory is the finding of cosmic microwave background radiation (CMB). With a normal telescope, all you can see in between stars is darkness, but if you have a very sensitive radio telescope, you can see more. Cosmic microwave background radiation (CMB) is the faint background radiation that astronomers pick up with sensitive radio telescopes.

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