The radiation left over from the Big Bang is the same as that in your microwave oven but very much less powerful. It would heat your pizza only to minus 271.3*C - not much good for defrosting the pizza, let alone cooking it.

Hawking uses this comparison while explaining the cosmic microwave background (CMB)—the faint “afterglow” of the Big Bang that now fills the universe. In popular lectures and books aimed at non-specialists, he often reached for everyday analogies to convey scale: the CMB is literally microwave radiation, but at a temperature of about 2.7 K above absolute zero, far colder and weaker than a kitchen microwave. The “pizza” line is a characteristic Hawking-style joke meant to anchor an abstract cosmological fact (the universe’s residual thermal radiation) in a familiar domestic image, emphasizing just how cold and dilute the present-day universe is.

Hawking uses a homely comparison—microwave ovens and pizza—to make the cosmic microwave background (CMB) intuitively graspable. The “radiation left over from the Big Bang” is real electromagnetic radiation, but today it is extremely weak and cold, corresponding to a temperature only a few degrees above absolute zero. By joking that it would warm a pizza only to about −271.3°C, he translates an abstract cosmological fact (the Universe’s residual thermal glow) into everyday experience. The quip also underscores a key point in modern cosmology: the Universe has expanded and cooled dramatically since the Big Bang, leaving behind a faint, nearly uniform background that serves as crucial evidence for the Big Bang model.