The Big Bang Theory
This lesson covers the Big Bang theory and its supporting evidence — as required by the Edexcel GCSE Physics specification (1PH0), Topic 7: Astronomy. This is a Paper 2 topic. You need to understand what the Big Bang theory states, know the two key pieces of evidence (red-shift and cosmic microwave background radiation), and understand why the CMBR is the strongest evidence.
What Is the Big Bang Theory?
The Big Bang theory states that:
- The universe began from an extremely hot, dense point (sometimes called a singularity) approximately 13.8 billion years ago.
- Since then, the universe has been expanding and cooling.
- All matter, energy, space and even time itself were created in the Big Bang.
- The expansion continues today — and is accelerating.
Key Points
- The Big Bang was NOT an explosion in space — it was the rapid expansion of space itself.
- There was no "outside" the Big Bang — the Big Bang created all of space.
- Before the Big Bang, there was no "before" in the conventional sense — time itself began with the Big Bang.
Exam Tip: Be careful with your language. The Big Bang was not an explosion of matter into existing empty space. It was the beginning of space itself. Examiners will distinguish between these two ideas.
Evidence for the Big Bang Theory
There are two main pieces of evidence that support the Big Bang theory. Both are required by the Edexcel specification.
Evidence 1: Red-Shift of Galaxies
The Observation
As covered in previous lessons:
- Almost all galaxies are red-shifted — they are moving away from us.
- The further away a galaxy is, the greater its red-shift (the faster it is receding).
- This relationship (Hubble's law: v = Hd) shows the universe is expanding uniformly.
How This Supports the Big Bang
- If the universe is expanding now, then in the past it must have been smaller and denser.
- Rewinding the expansion all the way back leads to a point where all matter and energy were concentrated in an incredibly small, hot, dense state.
- This is consistent with the Big Bang model — the universe started from a tiny point and has been expanding ever since.
Exam Tip: Red-shift tells us the universe is expanding. The Big Bang theory explains WHY it is expanding — because everything started from a single point and has been moving apart ever since.
Evidence 2: Cosmic Microwave Background Radiation (CMBR)
The Discovery
In 1965, two American astronomers — Arno Penzias and Robert Wilson — were working with a radio antenna at Bell Telephone Laboratories in New Jersey. They detected a faint microwave radiation coming from every direction in the sky, at all times of day and night, regardless of where they pointed their antenna.
At first, they thought it was interference (they even cleaned pigeon droppings from the antenna!). But the signal persisted. They eventually realised they had discovered the cosmic microwave background radiation (CMBR).
What Is the CMBR?
The CMBR is:
- A faint glow of microwave radiation that fills the entire universe, coming from every direction equally.
- It has a temperature of approximately 2.7 K (2.7 degrees above absolute zero, or about −270.5 °C).
- It is almost perfectly uniform — the same in every direction, with only tiny variations (about 1 part in 100,000).
How the CMBR Supports the Big Bang
According to the Big Bang theory:
- Shortly after the Big Bang, the universe was extremely hot — filled with high-energy radiation (short wavelength, high frequency).
- As the universe expanded, this radiation was stretched — its wavelength increased.
- Over 13.8 billion years of expansion, the radiation has been stretched from very short wavelengths (gamma rays) to microwave wavelengths.
- The CMBR we detect today is this remnant radiation from the early universe, cooled and stretched by the expansion of space.
Why CMBR Is the Strongest Evidence
The CMBR is considered the strongest evidence for the Big Bang because:
- It was predicted by the Big Bang theory before it was observed (George Gamow and others predicted in the 1940s that if the Big Bang occurred, there should be leftover radiation at approximately 5 K — the actual value of 2.7 K is remarkably close).
- It comes from every direction equally — consistent with the Big Bang happening everywhere at once (not at a single point in space).
- Its temperature (2.7 K) matches exactly what is predicted by calculations based on the Big Bang model and 13.8 billion years of expansion.
- No other model has successfully explained the existence and properties of the CMBR.
Exam Tip: CMBR is the most commonly examined piece of evidence for the Big Bang. You must be able to explain: (1) what it is (faint microwave radiation coming from all directions), (2) who discovered it (Penzias and Wilson, 1965), (3) what caused it (remnant heat from the Big Bang, stretched to microwaves by expansion), and (4) why it supports the Big Bang theory.
The Steady State Theory — The Alternative
Before the CMBR was discovered, there was a competing theory called the steady state theory:
| Feature | Big Bang Theory | Steady State Theory |
|---|
| Beginning | Universe had a definite beginning ~13.8 billion years ago | Universe has always existed — no beginning |
| Change | Universe is evolving — was hotter and denser in the past | Universe looks the same at all times — matter is continuously created to fill expanding space |
| CMBR | Predicted and explained as remnant radiation from the hot early universe | Cannot explain the CMBR |
| Status | Accepted by the vast majority of scientists | Largely abandoned after CMBR discovery |
Why the Steady State Theory Was Rejected
- The steady state theory cannot explain the existence of the CMBR.
- If the universe has always looked the same, there is no reason for a uniform background radiation to exist.
- The discovery of the CMBR in 1965 effectively ruled out the steady state theory and confirmed the Big Bang as the accepted model.
Exam Tip: You may be asked to explain why the Big Bang theory is now accepted over the steady state theory. The key answer is the CMBR — the Big Bang theory predicts it, the steady state theory cannot explain it.
Timeline of the Universe