The Coldest Thermometer in Physics: Temperature, Entropy, and Energy of the Present Vacuum's Horizon

A vacuum-dominated universe has a horizon, and the horizon has a temperature and an entropy. This paper computes all three for the present vacuum from a Hubble constant of 67.4 km/s/Mpc: a horizon radius of 1.37 x 10^26 metres, a Gibbons-Hawking temperature of 2.66 x 10^-30 kelvin, and an entropy of 2.27 x 10^122 units of Boltzmann's constant. The vacuum energy enclosed, 5.69 x 10^69 joules, divided by the entropy, agrees with the thermal energy k_B T to within a factor of 0.68, verifying the horizon identity in the script's arithmetic. Comparison with inflation-scale vacua shows the same formulae spanning the whole hierarchy of vacuum densities. The paper is a computation from stated assumptions, not a measurement.