|MadSci Network: Astronomy|
Thank you for your question. It is quite understandable to be confused when one hears an astrophysicist talk about the temperature of the Universe, or that the Universe was hotter in the past and has cooled with time. People's common concept of temperature is in relation to the warmth of the air surrounding them. When someone tells you that a room is 70° F, what they are saying is that the air in the room is at that temperature. What does that mean?
Well, temperature is a macroscopic measure of energy. When you measure the temperature of the air you are measuring the average energy of all the particles that make up the air (mostly molecules of Nitrogen, Oxygen, and Carbon Dioxide). Likewise, if you measure the temperature of a rock you are measuring the average energy of the particles that make up that rock. The more energetic the particles the higher the temperature.
When astronomers talk about the cooling of the Universe they are talking about the temperature of the particles that make up the Universe. A common particle that astronomers measure is the photon (the single unit of electro-magnetic radiation, i.e., light). Light permeates the entire Universe and when we measure the average energy of light left-over from the ancient Universe (so not the newer light generated by stars and such) we can determine its average energy and hence measure a temperature. The light we see left-over from the ancient Universe is in the form of microwaves and has a temperature of 3K or just 3 degrees Kelvin above absolute zero. In in a time shortly after the Big Bang this temperature was much much higher.
So it is not the vacuum's temperature that astronomers talk about when they mention the temperature of the Universe. Strangely enough a vacuum could be assigned a temperature. Quantum mechanics tells us that a vacuum is not quite a vacuum, but that it is a foam of particles and anti-particles coming in and out of existence faster than can be observed. All this quantum activity leads to an empty volume of space having an energy. So, if it has an energy a temperature could be assigned to the vacuum. But again, this is not what astronomers mean when they talk about taking the Universe's temperature.
I hope that helps. Thanks again for your question.
Graduate Student in Astronomy, at UC Berkeley
[Moderator's Note: For more on this cosmic microwave background radiation, see the MAP website.]
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