IGCSE Physics / Cosmology
The Big Bang theory
Edexcel International GCSE Physics (4PH1), chapter 28: Cosmology, specification point 8.13P. Free lesson.
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The Big Bang theory describes how the universe began in an extremely small, hot and dense state, then expanded and cooled. Its history and evidence help explain how the universe has changed over time.
How the universe began
The universe began about 13.8 billion years ago. Everything was in an extremely small region that was hot and dense. Dense means a huge mass packed into a tiny space. The Big Bang theory describes a small, hot, dense beginning, followed by expansion and cooling. Calling it a big explosion misses what the theory says. The key idea is that the universe began in this state and has been expanding ever since.
How the universe changed over time
As the universe expanded, it cooled. Once it was cool enough, particles and then the simplest atoms formed, mostly hydrogen and helium. Gravity pulled gas together to form stars, and stars into galaxies. This took much longer than the first stages. The universe is still expanding and cooling today, and galaxies are still moving apart.
Why stars did not form straight away
The early universe was too hot for atoms, let alone stars. It had to expand and cool first. Then gravity slowly pulled gas together, allowing stars and galaxies to form. The order matters: expansion and cooling came before atoms, stars and galaxies.
Evidence for the Big Bang theory
Two main pieces of evidence come from light and radiation reaching us from space. Astronomers measure them using telescopes. Red-shift: light from distant galaxies is stretched towards red. This shows that the galaxies are moving away, so the universe is expanding. Cosmic microwave background (CMB): microwaves reach us from every direction. They are leftover radiation from the hot early universe.