The Big Bang
The universe expanded from a hot dense state 13.8 billion years ago.
About this topic
The Big Bang theory states that the universe began about 13.8 billion years ago from an extremely hot, dense state, and has been expanding and cooling ever since. Evidence includes the redshift of galaxies (Hubble's law), the cosmic microwave background and the abundances of hydrogen and helium.
The first atoms formed about 380,000 years after the start; stars and galaxies formed later from gravity. The Big Bang was an expansion of space itself, not an explosion into space. Dark matter and dark energy shape its evolution.
Hubble's law says galaxies move away at a speed proportional to their distance, v = H0 times d, with H0 of about 70 km per second per megaparsec. A galaxy 100 megaparsecs away therefore recedes at about 7,000 km/s. The light is stretched to longer, redder wavelengths because space itself expands, a cosmological redshift. Running the expansion backward gives an age of about 13.8 billion years.
The cosmic microwave background is a faint glow at 2.7 K that fills the sky, the leftover light released when atoms formed, and it was predicted before it was found accidentally by Penzias and Wilson in 1965. In the first few minutes nuclear fusion produced about 75 percent hydrogen and 25 percent helium by mass, matching observations. The most common misunderstanding is picturing the Big Bang as an explosion at a point in an existing empty space; it occurred everywhere at once. Another is thinking that galaxies fly through space like shrapnel; instead, space between them grows. Also, the Big Bang theory describes what happened after the beginning, not what caused it.
Key ideas
- Cosmic microwave background
- Redshift of galaxies
- Expansion continues
Quick check
Use cases: where The Big Bang helps
- Homework: review the key idea behind The Big Bang before you start a problem set.
- Exam revision: use the quick check to test whether you can recall The Big Bang without notes.
- Visual learning: play with the interactive lab to see how The Big Bang changes when you adjust its inputs.
- Teaching a friend: explain The Big Bang in your own words, then compare your version with the key ideas.
Earth science helps with weather forecasts, natural hazards, energy and climate planning.
Frequently asked questions
What is The Big Bang?
The universe expanded from a hot dense state 13.8 billion years ago.
What will I learn in The Big Bang?
Cosmic microwave background. Redshift of galaxies. Expansion continues.
Is The Big Bang part of Earth & Space?
Yes. It is one of 60 Earth & Space topics on LearnViz HighSchoolEdu, with 3 key ideas, a live animation and a quick check.
Does the The Big Bang lab work on a phone or tablet?
Yes. The animation resizes to fit your screen and supports touch, so you can drag, tap and slide with your finger.
How should I study The Big Bang?
Read the summary and key ideas first, play with the interactive lab to see the idea in motion, then answer the quick check. If you get a question wrong, go back to the key idea it tests.
Do I need an account, and is my progress saved?
No account is needed. Marking a topic as learned is saved in your own browser, so it stays on that device and is not sent to a server.