Big Bang vs Steady State

How did the universe begin?

For much of the 20th century, scientists argued between two big ideas. The Big Bang theory says the universe began from a tiny, hot, dense point and has been expanding ever since. The Steady State theory said the universe has always existed, staying much the same over time. This module compares the two, and looks at the evidence that finally decided between them.

Key terms

Four ideas run through the whole debate:

What does the Big Bang theory say?

According to the Big Bang theory, how did the universe begin?

• It began from a tiny, hot, dense point and has been expanding ever since
• It has always existed and never changes at all
• It stays the same size, with matter created to fill the gaps
• It started very large and has been shrinking

Big Bang and Steady State terms

Two rival theories

Both theories agree the universe is expanding, but they disagree about its beginning.

Evidence for the Big Bang

Select the TWO pieces of evidence that support the Big Bang theory.

• Red-shift, showing that distant galaxies are moving away
• Cosmic microwave background radiation from all directions
• Objects falling when you drop them on Earth
• The daily rise and fall of the sea tides

What settled the debate?

Here is the key grade-9 point: both theories can explain red-shift and an expanding universe, so red-shift alone did not decide between them. The deciding evidence was the CMB radiation. The Big Bang predicts it as the leftover heat of the early universe, but the Steady State theory cannot explain it. Its discovery made the Big Bang the accepted model.

Complete the picture

The _____ theory says the universe began from a hot, dense point and has been expanding. The rival _____ theory said the universe has always existed. Light from distant galaxies shows _____, evidence of an expanding universe. Both theories can explain that, so the evidence that settled the debate was the _____ radiation. Because only one theory can explain it, the accepted model is now supported by more _____.

Spot the correct statements

Tap the TWO statements that are correct.

• The Big Bang theory says the universe began from a hot, dense point.
• The Steady State theory can fully explain the CMB radiation.
• Red-shift shows that distant galaxies are moving away from us.
• Red-shift on its own proves the Big Bang and rules out Steady State.
• The universe is getting smaller over time.

How the debate was settled

Put the reasoning that led scientists to accept the Big Bang in order.

• Astronomers observe red-shift: distant galaxies are moving away
• This shows that the universe is expanding
• Both the Big Bang and the Steady State theory can explain expansion
• Scientists then discover the CMB radiation coming from all directions
• Only the Big Bang explains the CMB, so it becomes the accepted model

Why was the CMB decisive?

Why did the discovery of CMB radiation settle the debate in favour of the Big Bang?

• Both theories explain red-shift, but only the Big Bang predicts the CMB
• The CMB signal is much louder than red-shift
• The Steady State theory predicted the CMB first
• The CMB has nothing to do with either theory

You are the scientist

Reason through the evidence for each situation.

• A galaxy far away shows a large red-shift. What does this tell you?
• A colleague says red-shift on its own proves the Big Bang. Are they right?
• Faint microwave radiation is found coming evenly from every direction. Which theory does this support?

Explain the accepted model

Explain why the Big Bang theory, rather than the Steady State theory, is the currently accepted model of the universe.

• State what each theory says about the origin of the universe
• Explain what red-shift shows, and why it does not decide between the two theories
• Explain what the CMB radiation is and why only the Big Bang can account for it
• Conclude why the Big Bang is the accepted model