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16.5 The Beginning and Future of the Universe

GCSE Physics · AQA · Chapter 16: Space

A free GCSE Physics lesson for AQA (8463) from OBA School: the Big Bang theory, cosmic microwave background radiation, dark matter, dark energy and the future of the universe (physics only). Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

  • Describe the Big Bang theory of the origin of the universe
  • Explain how red-shift and the cosmic microwave background radiation (CMBR) support the Big Bang theory, and why CMBR rules out the Steady State theory
  • Explain what dark matter and dark energy are and the evidence that they exist
  • Describe how the future of the universe depends on its density, and why the expansion is thought to be accelerating
  • Explain how scientific models of the universe change when new evidence is found

Key words

  • Big Bang theory: The universe began about 13.8 billion years ago from a tiny, extremely hot and dense region and has been expanding ever since.
  • Steady State theory: An old rival theory: the universe has always existed and new matter is made as it expands, so it always looks the same.
  • CMBR: Cosmic microwave background radiation: microwaves reaching us from every direction, left over from the early universe.
  • Dark matter: Matter that does not give out or absorb light, so it cannot be seen. It is detected by the effect of its gravity.
  • Dark energy: An unknown form of energy thought to be making the expansion of the universe speed up.
  • Density of the universe: The mass per unit volume of the universe. It helps decide whether the expansion will ever stop.

The Big Bang theory

Red-shift shows that the universe is expanding. Run the expansion backwards in time and everything gets closer together, denser and hotter. The Big Bang theory says that:

  • the universe began about 13.8 billion years ago from a very small, extremely hot and dense region;
  • space and time began then – the Big Bang was not an explosion into empty space;
  • the universe has been expanding and cooling ever since. First hydrogen and helium formed, then stars and galaxies.

Worked example: estimating the age of the universe

A galaxy is 1.9 × 10²⁵ m away and is moving away at 4.2 × 10⁷ m/s. Assume it has always moved at this speed.
time = distance ÷ speed = 1.9 × 10²⁵ ÷ 4.2 × 10⁷ = 4.5 × 10¹⁷ s
in years: 4.5 × 10¹⁷ ÷ 3.15 × 10⁷ = 1.4 × 10¹⁰ years

Answer: about 14 billion years – the time since it was next to us

Two rival theories

For many years the Steady State theory was a strong rival. It said the universe has no beginning: as galaxies move apart, new matter appears between them, so the universe always looks the same.

Both theories can explain red-shift, so red-shift alone could not decide between them. Scientists needed a prediction that one theory made and the other did not.

  • Red-shift: distant galaxies moving away; Big Bang theory explains it; Steady State theory explains it
  • Further galaxies have bigger red-shifts; Big Bang theory explains it; Steady State theory explains it
  • CMBR from every direction; Big Bang theory predicted it and explains it; Steady State theory cannot explain it

Cosmic microwave background radiation

  • If the universe began hot and dense, it was filled with high-energy, short-wavelength electromagnetic radiation.
  • As space expanded, this radiation was stretched to longer and longer wavelengths. It should now be microwave radiation, reaching us from every direction.
  • In 1965 scientists detected exactly this: weak microwaves coming from all directions in space, almost the same everywhere.
  • Only the Big Bang theory can explain the CMBR, so most scientists then accepted it. Satellites have since mapped the CMBR in great detail.

Exam tip: Red-shift shows the universe is expanding. CMBR shows it had a hot, dense beginning. Use both for the Big Bang.

Dark matter and dark energy

Dark matter. Stars in the outer parts of a galaxy orbit its centre much faster than expected. If the visible stars and gas were all the mass there is, the outer stars would move more slowly, and fast ones would fly off. There must be extra, unseen mass whose gravity holds the galaxy together.

Dark energy. Since 1998, observations of supernovae in very distant galaxies have shown that the distant galaxies are moving apart faster and faster: the expansion is accelerating. Gravity is attractive, so it would slow the expansion down. Scientists think an unknown dark energy is causing the acceleration.

Key fact: Nobody yet knows what dark matter or dark energy actually are. Together they seem to make up about 95% of the universe.

The future of the universe

  • If the density of the universe is greater than a certain value, gravity will eventually stop the expansion and pull everything back together: a Big Crunch.
  • If the density is less than this value, the universe will expand forever, cooling as stars die out.
  • Observations of an accelerating expansion suggest the universe will expand forever.

How science works. The Big Bang theory replaced the Steady State theory because of new evidence (CMBR). Discovering the accelerating expansion changed ideas again. Better telescopes and detectors give new data, and scientific models are tested and changed to fit it.

Common mistakes

  • Saying “the universe started with a big explosion” and nothing more. Describe a tiny, extremely hot, dense region expanding (and cooling).
  • Saying red-shift proves the Big Bang. Red-shift shows expansion; the CMBR is the evidence only the Big Bang can explain.
  • Saying the CMBR comes from the Sun or from stars. It comes from all directions and was left over from the early universe.
  • Mixing up dark matter (unseen mass, found by its gravity) and dark energy (unknown cause of the accelerating expansion).

Revision questions

Try these, then check your answers below each one.

Question 1. Which observation can be explained only by the Big Bang theory? (Choose from: cosmic microwave background radiation, red-shift of distant galaxies, the Sun being a main-sequence star, the existence of black holes.)

Answer: cosmic microwave background radiation

Working: CMBR. Both theories explain red-shift, but only the Big Bang theory predicted and explains the CMBR.

Question 2. True or false? The cosmic microwave background radiation comes mainly from the Sun.

Answer: False

Working: False. It comes from every direction in space and is radiation left over from the early universe, stretched to microwave wavelengths.

Question 3. What would happen if the density of the universe were greater than a certain value? (Choose from: gravity would stop the expansion and pull everything back: a Big Crunch, the universe would expand faster and faster, the universe would stay exactly the same size from the start, new matter would be created.)

Answer: gravity would stop the expansion and pull everything back: a Big Crunch

Working: Above that density, gravity would eventually stop the expansion and reverse it: a Big Crunch. Current observations of accelerating expansion suggest it will expand forever.

Related lessons

Continue in the full course

This lesson is section 16.5 (AGCPH1605) of our free GCSE Physics course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Physics course.

 

 

 

 

 

 

 

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