The average temperature of the Universe is very low – about three degrees above absolute zero. But is it cold in space? To measure the temperature of deep space, there must be matter, because that’s how we define temperature. The temperature of the room you are currently in is determined by the average movement of all the air molecules in the room. The more energy they have, the faster they fly and the higher the temperature. If you touch a really hot object, its atoms and molecules will vibrate violently, giving it a very high temperature.
There is not much matter in interstellar space. The average density of the universe is about one hydrogen atom per cubic meter. This makes it very difficult to determine the temperature of the matter in interstellar space. But space itself is saturated with something else, an endless sea of radiation that is very, very cold.
This radiation comes from stars, galaxies, and so on, but by far the largest source of radiation in the universe is cosmic microwave radiation (or CMB radiation). The CMB originated when the universe was about 380,000 years old. At that time, our cosmos was about a million times smaller than it is today, and was in a state of hot, dense plasma. As the universe expanded and cooled, it became neutral, emitting radiation with a temperature of about 10,000 Kelvin, the same as at the surface of the Sun.
This radiation makes up over 99.999% of all radiation left in space. Since its appearance, our Universe has expanded, which diluted that same radiation, lowering its temperature. In addition, the cosmic expansion is spreading into the light itself, moving it to longer and cooler wavelengths.
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Is it that cold in space? The combined action of the expansion reduced the temperature of the CMB to about three degrees above absolute zero. This means that if you were in interstellar space, your body would cool and cool and cool to almost absolute zero. But it wouldn’t be able to reach that temperature because the cosmic microwave background radiation would warm you up by transferring its energy into your body. So you wouldn’t reach absolute zero, but would come into equilibrium with the CMB, and that’s how we define the temperature of interstellar space.
