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15.10: Big Bang

Difficulty Level: At Grade Created by: CK-12
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How did everything begin in the Big Bang?

If the universe is expanding, the next logical thought is that in the past it had to have been smaller. A point even. The time when the universe began is the explosion known as the Big Bang.

The Big Bang Theory

Timeline of the Big Bang and the expansion of the Universe.

The Big Bang theory is the most widely accepted cosmological explanation of how the universe formed. If we start at the present and go back into the past, the universe is contracting — getting smaller and smaller. What is the end result of a contracting universe?

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 all of the known universe in a single, hot, chaotic mass. An enormous explosion — a big bang — caused the universe to start expanding rapidly. All the matter and energy in the universe, and even space itself, came out of this explosion.

What came before the Big Bang? There is no way for scientists to know since there is no remaining evidence.

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.

If you look at an image of galaxies at the far edge of what we can see, you are looking at great distances. But you are also looking across a different type of distance. What do those far away galaxies represent? Because it takes so long for light from so far away to reach us, you are also looking back in time (Figure below).

Images from very far away show what the universe was like not too long after the Big Bang.

Background Radiation

After the origin of the Big Bang hypothesis, many astronomers still thought the universe was static. Nearly all came around when 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 left over from the Big Bang. Since nearly all astronomers now accept the Big Bang hypothesis, what is it usually referred to as?

Background radiation in the universe was good evidence for the Big Bang theory.

An explanation of the Big Bang: http://dvice.com/archives/2009/08/big-bang-animat.php.

How we know about the early universe: http://www.youtube.com/watch?v=uihNu9Icaeo&feature=channel.

"History of the Universe," part 2: http://www.youtube.com/watch?v=bK6_p5a-Hbo&feature=channel.

"The Evidence for the Big Bang in 10 Little Minutes" provides a great deal of scientific evidence for the Big Bang (2g): http://www.youtube.com/watch?v=uyCkADmNdNo (10:10).

KQED: Nobel Laureate George Smoot and the Origin of the Universe

George Smoot, a scientist at Lawrence Berkeley National Lab, shared the 2006 Nobel Prize in Physics for his work on the origin of the universe. Using background radiation detected by the Cosmic Background Explorer Satellite (COBE), Smoot was able to make a picture of the universe when it was 12 hours old. Learn more at: http://science.kqed.org/quest/video/nobel-laureate-george-smoot-and-the-origin-of-the-universe/


  • The Big Bang theory states that the universe began as a point and expanded outward.
  • No one can know what came before the Big Bang because there is no remaining evidence.
  • The tiny bit of background radiation in the universe is energy remaining from the Big Bang.


Use this resource to answer the questions that follow.


1. Explain the Big Bang Theory.

2. Who suggested the Big Bang Theory?

3. Who discovered cosmic microwave radiation?

4. What does cosmic microwave radiation indicate?

5. What questions are not answered with the Big Bang Theory?


1. How is the idea that the universe started in a big bang a logical extension from a fact?

2. What evidence is there that the universe began in a big bang?

3. What happened in the first minutes after the Big Bang?

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Difficulty Level:
At Grade

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Date Created:
Feb 24, 2012
Last Modified:
Dec 15, 2016
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