The Hubble Sphere as an Extremal Reissner-Nordström Black Hole Carnot Engine Operating at the CMB Temperature of: T_cmb=Sqrt(T_max*T_min)≈ 2.725K

Haug [1] has recently suggested that the extremal solution to the Reissner-Nordström metric can be used for black hole cosmology. While the Bekenstein-Hawking entropy is related to the surface area in a Schwarzschild black hole, an extremal black hole has zero net entropy. We will demonstrate how the extremal universe Hubble sphere is likely a Carnot engine and how this helps us derive a formula for the CMB temperature now T_cmb=Sqrt(T_max*T_min)≈ 2.725K. This unlike the Lambda-CDM model, which cannot predict the CMB value at present.