7.2.1Energy – Forces Doing Work
Theories of the Universe
5 min read 5 parts Lesson 9 of 14 Unit quiz
Section 1 of 5The Big Bang Theory
Origin
- About 14 billion years ago the Universe began from a tiny region that was extremely hot and dense.
Expansion
- Space has been expanding ever since, carrying galaxies away from each other.
Cooling
- As the Universe expanded it cooled, allowing matter, stars and galaxies to form.
Balloon model
- Dots on an inflating balloon move apart from each other, with the surface standing for expanding space.
Density
- The density of matter and galaxies decreases as the Universe gets bigger.
Section 2 of 5The Steady State Theory
The Steady State Theory
Main idea
- The Universe has always looked more or less the same and has no beginning or end.
Expansion
- It accepted that galaxies move apart, but said new matter forms in the gaps to keep the density constant.
Popularity
- It was a serious rival to the Big Bang until the mid-1960s.
Problem
- It could not explain new evidence, in particular the microwave radiation found coming from all directions.
| Feature | Big Bang | Steady State |
|---|---|---|
| Beginning | Yes, an initial hot dense state | No beginning |
| Density over time | Decreases | Stays constant |
| Today's status | Accepted | Rejected |
Section 3 of 5How Scientists Decide
1
Observations show that distant galaxies move away from us, which both theories explain.
2
Scientists look for evidence that only one theory can account for.
3
Cosmic microwave background radiation was found to come from all parts of the sky, as the Big Bang predicts.
4
The Steady State theory had no explanation for this radiation, so support for it fell away.
5
Theories remain open to change, and any new evidence can lead to the model being improved or replaced.
Section 4 of 5Features of the Big Bang Model
Evidence 1
- Galaxies are moving apart, with the more distant ones moving faster.
Evidence 2
- Cosmic microwave background radiation exists everywhere in space.
Prediction
- Because the Universe was once hotter and denser, leftover radiation should fill space today.
Unanswered questions
- The model does not explain what existed before the Big Bang or what caused it.
Section 5 of 5Exam Checklist & Tips
7.2.1 – Key Points for the Exam
- Big Bang theory The theory that the Universe began from a very hot, dense point and has been expanding since.
- Steady State theory A rejected theory that the Universe has always been the same on a large scale.
- Expansion The increasing distance between galaxies as space itself stretches.
- Universe Everything that exists, including all matter, energy and space.
- Evidence Observations or measurements that support or contradict a theory.
Common Mistakes
- Describing the Big Bang as an explosion in space; space itself expanded.
- Saying that galaxies are expanding; the space between galaxies increases.
- Forgetting that the Steady State theory also accepted expansion but needed new matter.
- Giving the age of the Universe in millions instead of billions of years.