The Big Bang, Dark Matter and Dark Energy

How did the Universe begin?

Redshift shows that galaxies are rushing apart: the Universe is expanding. Rewind that expansion and everything was once packed together. So how did it all begin? This module weighs two theories, meets the evidence that settled the question, and explores the dark matter and dark energy that make up most of the cosmos.

Two theories of the Universe

Both theories were built to explain the expanding Universe, but they disagree about its beginning. Evidence has since made the Big Bang the accepted theory.

A shared starting point

The Big Bang and Steady State theories were both built to explain which observation?

• That the Universe is expanding
• That the Sun rises in the east
• That the Moon shows phases
• That stars appear to twinkle

The evidence, in four words

Four observations point to the Big Bang:

Match each observation to what it shows

Which are Big Bang evidence?

Select the TWO observations that provide evidence for the Big Bang.

• A microwave afterglow coming from every direction
• Quasars found only in the distant, early Universe
• The daily rising and setting of the Sun
• The monthly phases of the Moon

The clinching evidence: the CMB

The Big Bang predicts a faint glow left over from when the young Universe cooled. It is found coming from every direction as microwaves, at about 2.7 K above absolute zero. The Steady State theory predicts no such afterglow, so the cosmic microwave background is the strongest single piece of evidence for the Big Bang.

The history of the Universe

Put these stages of the Universe into order, earliest first.

• The Big Bang: the Universe begins hot and dense
• The Universe expands and cools rapidly
• The cosmic microwave background is released
• The first stars and galaxies form
• Galaxies spread out to today, still expanding

Cosmology in words

The _____ theory says the Universe began about 13.8 billion years ago and has expanded ever since. Its strongest evidence is the cosmic microwave _____, a faint glow from every direction. Most of the cosmos is invisible: unseen dark _____ is felt only through gravity, while dark _____ drives the expansion to speed up. Neither can be seen because they give out no _____.

Finding the invisible

Dark matter gives out no light, so how do astronomers know it is there?

• From its gravitational pull on stars and galaxies
• From the light it shines into telescopes
• From the sound it makes in space
• From radio messages it sends out

Match each part of the cosmos to what it is

Big Bang evidence

Tap the TWO statements that are TRUE.

• The cosmic microwave background is evidence for the Big Bang, not the Steady State theory.
• Dark matter is detected through its gravity, not through its light.
• Quasars are spread evenly through the nearby Universe today.
• Dark energy is slowing the expansion of the Universe down.
• Most of the Universe is made of ordinary matter like stars and planets.

What best explains it?

Decide what each observation points to.

• A faint microwave glow is found coming from every direction in the sky. Which theory does this support?
• A galaxy spins so fast that its visible stars alone could not hold it together by gravity. What best explains this?
• Observations show the expansion of the Universe is speeding up over time. What is thought to cause this?

Evidence and the dark Universe

Explain how astronomers know the Big Bang happened, and why most of the Universe is described as dark. Refer to at least two pieces of evidence for the Big Bang, and to dark matter and dark energy.

• Describe two observations that support the Big Bang
• Explain why the cosmic microwave background is such strong evidence
• Explain what dark matter is and how it is detected
• Explain what dark energy does to the expansion of the Universe