How hot is dark matter

Interstellar gas: It's getting hotter and hotter in space

The temperature in space is difficult to grasp with our everyday conception of hot and cold on earth. Because the density of matter in space is usually extremely low, there is nothing that would heat an astronaut's body - consequently it is cold almost everywhere. However, space, especially the space around a galaxy, is not completely empty: It is filled with the interstellar medium, which consists of hydrogen and dust, among other things. An average temperature can be derived from the kinetic energy of the few particles in it - and this in turn is extremely high by earthly standards. It is around two million degrees Celsius.

As reported by a team of astronomers headed by Yi-Kuan Chiang from Ohio State University in Columbus, this was not always the case. In the early days of the universe eleven billion years ago, the average temperature measured in this way was around 200,000 degrees Celsius. In their contribution to the journal “The Astrophysical Journey”, the scientists also provide an explanation for the phenomenon. According to this, the finely distributed matter heats up because the gas is attracted by the force of gravity of the galaxies and compressed in the process.

"This attraction is very strong," says Chiang in a press release on the study, "so strong that more and more gas is compressed and heated up by shock waves." The attraction of the so-called dark matter halos, large, spherical accumulations of invisible dark matter that surrounds practically all galaxies. It is also responsible for the fact that over the billions of years the galaxies come together to form large-scale structures, such as galaxy clusters. These gigantic structures then drive the heating, the researchers found.

How "hot" the interstellar gas is is revealed by a subtle local change in the cosmic microwave background. Chiang and team evaluated data from the Planck space telescope and related the mean temperature measured at specific locations to the distance of the object. This can be found out through its redshift. The more an object's light is shifted to red wavelengths, the further away it is. The further away it is, the older it is. And as the current measurements show: the older it is, the cooler it is.