As I’ve mentioned before, the Universe is bathed in something called the Cosmic Microwave Background (CMB), which can be thought of as an echo of the Big Bang. This is currently at a temperature of 2.7 Kelvin, and is cooling slowly as the Universe expands. So I was interested to see a story a few months ago about a place that is even colder than this. How can this be? Well, if somewhere is shielded from the CMB and undergoes adiabatic expansion (when no heat is supplied to the system), then it is possible to naturally get to a lower temperature than the all-pervasive CMB. This seems to be just what has happened in the story here.
The nebula in question, dubbed the Boomerang Nebula because of its appearance, has been observed by the Atacama Large Millimetre Array (ALMA), an array of radio dishes high in the desert of Atacama where the air is exceptionally dry allowing millimetre astronomy to be conducted.
Like most areas of star formation, the central parts of the Boomerang Nebula are shrouded in gas and dust. This means that to observe what is going on at the centres of such nebulae we need to look at longer (far infrared and millimetre) wavelengths to see through the dust, as the dust effectively absorbs light at visible wavelengths. The dust also shields the inner parts of the nebula from starlight, meaning it can get extremely cold inside the nebula. But, in order to get colder than the CMB requires some adiabatic expansion to have taken place, and astronomers have calculated that the inner parts of the Boomerang Nebula are at a temperature of just 1 Kelvin! There are colder places in the Universe, for example in many of our astronomical instruments, where we routinely cool detectors to a few thousandths of a Kelvin. However, it would seem the Boomerang Nebula currently has the record for the coldest naturally occurring place in the Universe.