Because the far-future evolution of stars depends on physical questions which are poorly understood, such as the nature of dark matter and the possibility and rate of proton decay, it is not known precisely how long it will take white dwarfs to cool to blackness. Barrow and Tipler estimate that it would take 1015 years for a white dwarf to cool to 5 K; however, if weakly interacting massive particles (WIMPs) exist, it is possible that interactions with these particles will keep some white dwarfs much warmer than this for approximately 1025 years. If protons are not stable, white dwarfs will also be kept warm by energy released from proton decay. For a hypothetical proton lifetime of 1037 years, Adams and Laughlin calculate that proton decay will raise the effective surface temperature of an old one-solar-mass white dwarf to approximately 0.06 K. Although cold, this is thought to be hotter than the cosmic background radiation temperature 1037 years in the future.