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Astronomy
The Big Bang Theory
How the Universe Began
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The Big Bang Theory explains how the universe began about 13.8 billion years ago in an extremely hot, dense state and has been expanding and cooling ever since. It is the leading scientific model for the history of the universe because it connects observations from galaxies, radiation, and the chemical elements. The idea matters because it helps scientists reconstruct the past and predict how cosmic structures formed. It does not describe an explosion into empty space, but the expansion of space itself.
Key Facts
- Age of the universe: about 13.8 billion years.
- Hubble's law: v = H0d, where farther galaxies recede faster on average.
- Cosmic redshift means light from distant galaxies is stretched to longer wavelengths as space expands.
- The cosmic microwave background is leftover radiation from the early universe, now cooled to about 2.7 K.
- Early universe nucleosynthesis produced mostly hydrogen and helium, with small amounts of deuterium and lithium.
- Scale factor relation: 1 + z = a_now/a_then, where z is redshift and a is the size scale of the universe.
Vocabulary
- Big Bang Theory
- The scientific model that describes the universe expanding and cooling from a very hot, dense early state.
- Cosmic microwave background
- Faint microwave radiation from the early universe that fills space in every direction.
- Redshift
- The stretching of light to longer wavelengths, often caused by the expansion of space between galaxies.
- Hubble's law
- The observation that more distant galaxies generally move away from us faster.
- Nucleosynthesis
- The formation of atomic nuclei, including the production of light elements in the early universe.
Common Mistakes to Avoid
- Calling the Big Bang an explosion in space is wrong because the model describes the expansion of space itself, not matter flying outward from one central point.
- Thinking galaxies expand internally with the universe is wrong because gravity holds galaxies, solar systems, and atoms together while the large-scale distances between galaxy clusters increase.
- Assuming redshift always means ordinary motion through space is wrong because cosmological redshift is mainly caused by space stretching while light travels.
- Saying the cosmic microwave background comes from stars is wrong because it is leftover radiation from the hot early universe, released long before the first stars formed.
Practice Questions
- 1 Using Hubble's law v = H0d with H0 = 70 km/s/Mpc, find the recession speed of a galaxy 200 Mpc away.
- 2 A distant galaxy has redshift z = 3. Using 1 + z = a_now/a_then, what fraction of today's scale factor did the universe have when the light was emitted?
- 3 Explain why the Big Bang Theory says there is no center of expansion inside the universe, even though distant galaxies appear to be moving away from us.