This is addressed in the paper. In the second paragraph of the third section, labeled Discussion: Thermal gradients are needed for life. These can be supplied

This is addressed in the paper. In the second paragraph of the third section, labeled Discussion: Thermal gradients are needed for life. These can be supplied by geological variations on the surface of rocky planets. Examples for sources of free energy are geothermal energy powered by the planet’s gravitational binding energy at formation and radioactive energy from unstable elements produced by the earliest supernova. These internal heat sources (in addition to possible heating by a nearby star), may have kept planets warm even without the CMB, extending the habitable epoch from z ∼ 100 to later times.