Characters in science fiction movies tell us that space is cold, but how cold? It turns out that the universe does have a temperature. When we take a thermometer to space we find it to be a chilly 2.73 degrees Kelvin. For comparison, the 98.6 degrees Fahrenheit that characterizes our own body temperature would be 310.2 degrees on the Kelvin scale.
The universes’ temperature was first measured by Penzias and Wilson in 1964. They detected ‘noise’ in their radio receivers and eventually tracked it down to be the ambient temperature of the universe. We now call this the “cosmic microwave background” radiation, as this temperature is best measured at microwave frequencies. At this time in history it was not yet known whether the universe was expanding from a hot dense state, called the hot Big Bang model, or whether the universe has more or less always existed as is in a steady state. The discovery of the cosmic microwave background radiation is significant as it provides evidence for the hot Big Bang model. It also led to the Nobel Prize award in 1978.
How did this cosmic microwave background radiation come about? Well, we think the universe started out very dense and hot. It expanded very rapidly at first, and then after that continued to expand and cool with time. Just as normal bomb on Earth will be extremely hot at first, and then cool down over time, the hot Big Bang was also extremely hot, except it has not cooled down completely!
If this picture is correct, then when we look out to large distances, and hence to longer look-back times, the universe’s ambient temperature should be higher than 2.73 degrees Kelvin. Indeed we have been able to measure the universe’s temperature out to large distances. We find that the temperature increases with redshift as expected for an object that has been expanding and cooling for 13.7 billion years. Interestingly, this radiation is measured to be the same temperature good to one part in 100,000 in all directions of space. In the next installment will explore how tiny differences in the temperature form the likely seeds of the growth of the first stars and galaxies.
