Off-peak Posted on 2012-06-06 by Dag-Erling Smørgrav I love XKCD. I love this panel. I even bought the T-shirt. But in all these years, am I really the only one to have noticed that the comic and the T-shirt are wrong? The peak energy density of the cosmic microwave background radiation occurs at 160.2 GHz, not 160.4 GHz as the shirt says.