Georgina Deans, of Rangi Ruru School, asks :-

What is the temperature of the universe?

Saskia Besier, an astronomy graduate student at the University of Canterbury, responded.

It is not often in astronomy that we can answer questions with exact reasons or precise numbers. This is one of the few that we can! The temperature of the universe is 2.7 degrees Kelvin or minus 270.3 C which is very cold.

The temperature of the universe is one of the most important things we know in astronomy. The universe might only be minus 270.3 C but it still had to get this 'heat' from somewhere. A very long time ago the universe was much hotter than it is now. Since then, the universe has aged and grown a lot bigger and because of that the temperature has dropped.

The change from a hot, dense and small universe to the cold, empty and large one that we have now is predicted by the 'Big Bang' model. By measuring the temperature of the universe today and using the model then we can work out how hot it was in the past. Originally the temperature must have been about 10 C - much hotter than the inside of our sun and so hot that even atoms couldn't exist: the universe was just a soup of loose electrons, protons and neutrons.

To measure the temperature of the universe we can't use thermometers because it is too cold. Instead, we take a picture of the universe. On earth, things that are warm emit radiation which we mostly call 'infra-red' radiation. Humans can't seen infrared but we have designed special instruments (things like 'night vision goggles' and 'heat-seeking' devices) that can. Something as cold as the universe still emits radiation but this radiation is 'microwave' - exactly the same sort of radiation that is used to cook food! By using detectors that respond to microwaves we can take a picture of the universe and work out how hot it is.

I hope this helps answer your question. The temperature of the universe is one of the most important reasons the 'Big Bang' model may right. Just knowing what it is has been one of the most important scientific discoveries ever - it was a very clever question to ask!