:Note Hawking's preposition. It's "to", not "by". Also note that the "on Earth" seems to be your personal interpretation - it's really quite hard to heat anything to 2.73K on Earth (because you won't find anything that needs heating). Also note that your (or Hawking's?) math is a bit off, -271.3°C is only 1.88K. Since the CMB is roughly 2.73K, it would heat a pizza in otherwise empty space to -270.43°C (unless my math is off, which is a very real possibility at any time, but especially when I lecture about other peoples shortcomings).