Why would a mind prefer the dark?
Erasing one bit leaves behind heat of at least k × T × ln 2, where T is the temperature of the surroundings. The temperature sits right there in the formula, so lowering it lowers the price in direct proportion. The reasoning is on the page about the cost of forgetting.
Guess first Compare the cost of forgetting one bit in a 300 K room with the cost in empty space in the year 2540. How many times cheaper is space?
About 110 times cheaper. The leftover glow of the Big Bang sits at 2.7 K in 2540, and at that temperature a bit costs 2.6 × 10−23 joules to forget, against 2.9 × 10−21 in a warm room.
The Vigil's radiators sit at 30 K. At the Landauer limit, a petawatt supports 3.5 × 1036 erasures a second, ten times what the same power supports at 300 K. The gain is per joule, not per second: a fixed radiator sheds less heat the colder it runs, so more erasures a second need more radiator area.
The universe also cools as it expands. When it is 100 billion years older, the background will have fallen to 0.0081 K, and forgetting will cost correspondingly less. In 351 billion years the background will be colder than Vesper's Hawking radiation, making Vesper warmer than the background.
If dark energy remains a cosmological constant, horizon radiation sets a minimum temperature of 2.2 × 10−30 K and a corresponding minimum erasure cost. The page on the cooling universe explains this limit, and Why they might be asleep examines whether a patient civilization would wait for lower temperatures.