I heard a discussion about a method to determine the age of the universe based on the cooling rate of the background microwave radiation. It said that the current temperature is 3 degrees Kelvin. I was wondering how “temperature” could be attributed to microwave radiation. I thought temperature measured the kinetic energy of atoms bouncing around. I also thought that temperature did not have any meaning in a vacuum. So when they talk about the temperature of background microwave radiation, do they mean it in the traditional thermometer way or in a different way?
Good question. There are a couple different aspects to the answer.
-
The radiation can be considered a “gas of photons”. In this way, the temperature is related to the energy of the photon gas. (A photon is a “particle of light energy” - a unit of electromagnetic radiation.)
-
The spectrum of the microwave background is very exactly that of a “black body”. “Spectrum” means, loosely, the different wavelengths (“colors”, if we could see them) of the radiation. More precisely, make a graph of the energy at a particular wavelength as a function of the wavelength. The resulting curve is called the “spectrum”. A “black body” is an ideal object that absorbs all of the radiation that falls on it. (Real objects only absorb some radiation, the rest is reflected.) It is possible to calculate the spectrum of radiation from a black body at a given temperature, and the observed spectrum of microwave background matches that for a black body at 2.7 K.
-
Black body radiation is also called “cavity radiation” since you can create a nearly perfect black body by making a box (the cavity) and putting a small hole in the side. Any radiation falling on the hole will bounce around inside the box until it is absorbed, so the hole can be considered a black body. If the radiation inside the box is in equilibrium with the walls of the box (energy can be exchanged freely between them), then the temperature of the box will be the same as the temperature of the black body radiation spectrum.
-
Finally, we can think of the universe as a huge cavity or box. The radiation is at a certain temperature that can be determined by the spectrum. There are of course no “walls” in the universe. But at an earlier time the radiation was in equilibrium with the matter in the universe, at a much higher temperature. Since that time, the universe has been cooling down, and the radiation temperature has been dropping.
Hope this helps.
A black body will give off radiation based upon its temperature. You can determine the temperature of a body based upon its radiation signature. The background radiation is the signature of a black body 2.7 degrees. An object cooler than the background radiation will absorb more than it gives off and will warm up (if you can call 2.7 degrees warm).
To add to the answers above, another way of thinking about it is to imagine dropping some atoms into this “bath” of radiation. These atoms will interact with the radiation, either being sped up or slowed down until they’re moving with some average speed. The temperature of the radiation is just that of a gas with the atoms bounding around at that speed. It’s a bit like dropping a small object into a large bath of water: it heats up or cools down to the temperature of the bath. So you could, somewhat indirectly, define the temperature of water as the temperature a small object dropped in winds up at.
Astrophysicists are trying to concentrate the cosmic background radiation, effectively doubling its “temperature,” and apply it to domestic food preparation.
This is known as Six Degrees of Kelvin Baking.
Doc, this is some sort of crypto-campaign for the :yuk: smilie, right? 