How Can We Understand the Concept of Universe's Temperature?

[pixel01]

The universe is mostly empty, how can we understand the concept of temperature of the universe?

Thanks

 

[Vanadium 50]

Why is "mostly empty" a problem? A tank of gas at 1 psi is also "mostly empty", yet it has a temperature.

The bigger problem to me seems to be that the universe is not in thermodynamic equilibrium, so it doesn't have a single temperature. The best you can hope to talk about is an average.

 

[pixel01]

Why is "mostly empty" a problem? A tank of gas at 1 psi is also "mostly empty", yet it has a temperature.

The bigger problem to me seems to be that the universe is not in thermodynamic equilibrium, so it doesn't have a single temperature. The best you can hope to talk about is an average.

Thank you.
So the average temperature of the universe must be calculated based on the temperatures of all the stars, planets, satellites etc.. while considering the vastness of the universe?

 

[Naty1]

Pixel...suggest you read about cosmic microwave background radiation..it reflects the early universe was a lot hotter and today measures 2.7 degrees Kelvin...on cosmological scales it is extraordinarily homogenous...wikipedia as an accessible source...but as noted there are hot spots like stars (our sun is one ) and cold spots (like black holes)...