Answer
$3\times10^{-13}m $
Work Step by Step
Use Wien’s law, equation 27–2.
From Fig. 33–29, the temperature 1.0 s after the universe’s birth was about $10^{10}K$.
$$\lambda_p =\frac{2.90\times10^{-3}m \cdot K}{T}$$
$$\lambda_p =\frac{2.90\times10^{-3}m \cdot K}{10^{10}K}=3\times10^{-13}m$$
This is in the gamma ray region of the EM spectrum.
