Unless they can figure out reversible computing to the point their computer doesn't really need any power, they also have to contend with the Landauer limit, so

Unless they can figure out reversible computing to the point their computer doesn't really need any power, they also have to contend with the Landauer limit, so counting to 2^255 (at current cosmic microwave background temperature) would need about 2^255 k 3kelvin ln(2) / c^2 = ~9 million solar masses of fuel (assuming perfect effeciency).