Why is it so cold in space

How cold is the universe?

In order to answer the question about the temperature of the universe, we first have to take a look at what temperature actually is. In physics, temperature is defined as a measure of the mean energy per degree of freedom of a particle system. For simple gases, the temperature indicates the movement of the gas molecules: the faster the molecules move, the higher the temperature. If we look at the universe as an absolute vacuum, then it has no temperature at all. Because in an absolute vacuum there are no moving particles - consequently the temperature is no longer defined there.

But we don't want to make it that easy for ourselves. Because from the earthly point of view there is a perfect vacuum in space. But it is not completely free of matter. In near-Earth space, where most satellites orbit, the remaining density of the atmosphere is even several hundred quadrillion atoms per cubic centimeter. On the surface of the moon, the density is still around one billion atoms per cubic centimeter. And in interstellar space we find, on average, one hydrogen atom per cubic centimeter.

Of course, this thin gas also has a temperature. The temperature of the terrestrial high atmosphere rises again above 100 kilometers and reaches a value of around 1400 Kelvin in near-Earth space. In galaxy clusters, the intergalactic gas often reaches temperatures of several million Kelvin. Astronomers find the lowest temperatures in the interior of dark molecular clouds, where temperatures are often only a few tens of Kelvin.

But there is not only matter in space. The cosmos is evenly filled by the so-called background radiation, a holdover from the Big Bang, the hot phase of the creation of the universe. The spectral energy distribution of the background radiation corresponds exactly to the radiation of a black body with a temperature of 2.7 Kelvin. Often these 2.7 Kelvin are referred to as the temperature of space far from all sources of radiation.

Cosmic background radiation

However, this is misleading. On the one hand, no temperature can be assigned to the room itself - see above. On the other hand, the temperature of matter in today's cosmos is independent of the temperature of the radiation. Even if no stars or galaxies had formed in the cosmos, i.e. the gas had been distributed completely evenly in space, the temperature of the gas would not correspond to the temperature of the background radiation. In fact, in this case, the gas would be around 0.03 Kelvin, considerably colder than the background radiation.

Finally, we shouldn't forget that radiation and ordinary matter make up only a small part of the total mass in the universe: around 80 percent of the mass consists of dark matter, a mysterious substance made up of previously unknown elementary particles. Dark matter can also be assigned a temperature that is independent of the temperatures of normal matter and radiation. Most cosmologists today favor the cold dark matter model. “Cold” means that the dark matter particles move non-relativistically, that is, at speeds that are significantly lower than the speed of light. The exact speed of the particles is not yet known - and so we do not know the exact temperature of dark matter: It can be a few hundred or a thousand Kelvin. A British team of researchers determined the temperature of dark matter to be 10,000 Kelvin from their study of dwarf galaxies in 2006, but this value is still controversial.