In a vacuum of ultra deep space with virtually 1 atom per cubic meter density, how do you determine the temperature of

Question: In a vacuum of ultra deep space with virtually 1 atom per cubic meter density, how do you determine the temperature of the vacuum to be 3 kelvin given such low matter density?
- Keywords:
  deep,
  density,
  space,
  temperature
Asked by anon-244775 to Ondrej, Jordan, Eleanor, Ed, Christine, Alice on 18 Mar 2020.
- Edward Banks answered on 18 Mar 2020:
  
  This is a lovely question! You’re quite right that without any particles around, we can’t really take a temperature reading in the standard way, by measuring the kinetic energy of the particles. Instead, we have to turn to radiation- even though there’s no particles, there is still radiation, particularly in the form of the cosmic microwave background. The CMB is present everywhere in the universe as a remnant of the big bang, and still has enough energy to heat things up to about 3 kelvin. Although this will be getting colder over time!
- Eleanor Jones answered on 18 Mar 2020:
  
  As Ed says, in a vacuum, we rely on the cosmic microwave background for our measurements since there are no particles and EM radiation can travel through the vacuum.

Question: In a vacuum of ultra deep space with virtually 1 atom per cubic meter density, how do you determine the temperature of the vacuum to be 3 kelvin given such low matter density?