Why did the universe start cooling down?

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Right after the big bang, ALL matter was superheated to a plasma state at trillions of degrees. Bodies tend to stay in the same state they are, unless they are affected by an external force. If there was no “space” or other form of cold mass to transmit heat to, why did the superheated matter start to cool down?

I’m thinking it may be related to energy released when particles started to merge/combine with others to form atoms, molecules, etc., but I’m sure someone here has a better explanation.

In: Planetary Science

4 Answers

Anonymous 0 Comments

Right after the big bang, the universe was expanding extremely rapidly. Hot gases cool as they expand. It took about 380,000 years for the universe to expand enough and cool enough that atoms began to form as electrons joined nuclei. Once the gas was not ionized it became transparent and the universe as we know it began to emerge.

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