Avian’s Blog

This topic came up in an IRC channel the other day. How does the universe compare to a cup of tea? How long after the big bang did it take the universe to cool through the temperatures, characteristic of the popular hot beverage?

Fortunately both tea and the early universe are fairly well understood these days.

In the case of tea, the cup cools down approximately exponentially. Time constant depends on the thermal resistance between the drink and the ambient, its mass and specific heat. Fortunately, a dedicated research unit provides empirical data on the subject.

As far as the universe is concerned, that part of its cooling is described by the Friedmann equation for the matter-dominated era.

As you can see from the graph above, the universe took slightly longer than your five o'clock tea to cool down.

However, unlike the cup of tea, the universe at that age wasn't terribly interesting. All the fun sub-atomic business had finished long ago and darkness was filled more or less evenly with neutral hydrogen and helium.

It will take more than 100 million years for the first stars to light up and another 10-something billion years before an odd biped gets the idea of throwing a fistful of leaves into boiling water.