Confusion about the thermal interpretation's account of measurement

[vanhees71]

Within the thermal interpretation it is considered to be in a mixed state. Indeed, the experimental record tells us that the universe has a positive temperature, whereas no temperature can be assigned to a pure state.

What is considered to be in a mixed state? The entire universe? Well, why not?

 

[A. Neumaier]

What is considered to be in a mixed state? The entire universe?

yes, its state was under discussion. Assigning to the universe a temperature distribution (microwave background) means that the state of the universe is approximately a local equilibrium state and hence a mixed state.

 

[vanhees71]

Well, the microwave background temperature is a very much coarse grained observable. It's about the large-scale "average" structure of the universe, described by an FLRW metric using the above considered assumption of the cosmological principle to extrapolate it to "the entire (observable) universe". In this sense it's a paradigmatic example for the concept to consider coarse-graining to understand the classical behavior of macroscopic observables of a many-body system.