The Universe - What is the Big Bang?

Recall how previously you learned that using the Doppler Effect will help determine if a red shift or a blue shift is occurring with respect to stars. This activity is designed to help you see how the red shift provides the opportunity to model the Big Bang Theory as it relates to the red shift.


  • Large latex balloon
  • Permanent marker
  1. Blow up the balloon until it is about 10 centimeters (4 inches) in diameter.
    • Do not tie the end.
    • Have a partner mark six dots scattered around the surface of the balloon.
    • Label one of dot as "MW," to represent the Milky Way Galaxy.
    • The other dots represent other galaxies throughout the universe.
  2. Without letting any air out of the balloon, blow more air into the balloon until the diameter is approximately 5 cm larger.
    • Observe the change in the location of the dots relative to each other.
  3. Repeat the process, this time blowing up the balloon so that it is 5 cm larger in diameter.
  4. Now blow up the balloon so that it is almost as full as it can go. DO NOT POP IT. Repeat your observations.


  1. If you were standing at the point labeled "MW," what kind of shift would you expect to see in the spectra of the dots representing other galaxies?
  2. Why are color shifts important in understanding what is happening to the universe?
  3. Imagine that you didn't see the balloon start expanding; but you then witnessed the balloon continue to expand, what could you infer about the size of the balloon before you began watching?
  4. How does the expansion of the balloon relate to the expansion of the universe?

Hubble's explanation of the expanding universe.
When Hubble noticed all of the red shifts compared to the very few blue shifts, he realized that the universe is expanding. Just as you did during the balloon analysis, scientists can now point to what the universe must have looked like in the past if we are currently watching it expand. Remember our loaf of raisin bread. If you walked into the kitchen and observed that the bread had been rising, what could you say about what the loaf looked like 15 or 20 minutes ago? The same thing happens with the universe. As we watch the universe get larger, we can say that at some point in time, the matter and energy that forms the universe must have been much closer together. What could have sent all of that material flying outward into space? Only an explosion beyond anything we have ever seen could have done that.

This explosion is known as the big bang, and is generally referred to as the beginning of the universe. At a certain time in the past, all matter and energy was found in one spot. Over 13 billion years ago, the big bang sent that matter and energy outward, and the universe began. If the universe is truly expanding, then that means that at some point in time, all of the matter must have been together, just as in the balloon analysis.

The Big Bang Theory
The Big Bang theory is the most widely accepted cosmological explanation of how the universe formed. According to the Big Bang theory, the universe began about 13.7 billion years ago. Everything that is now in the universe was squeezed into a very small volume. Imagine the entire known universe compressed into a single, hot, chaotic mass. An explosive expansion — a big bang — caused the universe to start growing rapidly. All the matter and energy in the universe, and even space itself, came out of this expansion.

After the Big Bang
In the first few moments after the Big Bang, the universe was unimaginably hot and dense. As the universe expanded, it became less dense and began to cool. After only a few seconds, protons, neutrons, and electrons could form. After a few minutes, those subatomic particles came together to create hydrogen. Energy in the universe was great enough to initiate nuclear fusion and hydrogen nuclei were fused into helium nuclei. The first neutral atoms that included electrons did not form until about 380,000 years later. The matter in the early universe was not smoothly distributed across space. Dense clumps of matter held close together by gravity were spread around. Eventually, these clumps formed countless trillions of stars, billions of galaxies, and other structures that now form most of the visible mass of the universe.

After the origin of the Big Bang theory, many astronomers still thought the universe was static (or unmoving). An important line of evidence for the Big Bang was discovered in 1964. In a static universe, the space between objects should have no heat at all; the temperature should measure 0 K (Kelvin is an absolute temperature scale). But two researchers at Bell Laboratories used a microwave receiver to learn that the background radiation in the universe is not 0 K, but 3 K (Figure below). This tiny amount of heat is known as cosmic background radiation (CBR).  Cosmic background radiation is left over energy from the Big Bang.   The presence of CBR is evidence that supports the Big Bang Theory.

This image shows the cosmic background radiation in the universe as observed from Earth and Space Probes.

How we know about the early universe :

History of the Universe, part 2:

Source: Open Education Group Textbooks - Earth Science