© NASA, COBE.
The cosmic microwave background, which is thermal radiation left over from roughly 390,000 years after the Big Bang, has a nearly perfect blackbody spectrum. The shape of the curve and location of the peak indicate that the blackbody temperature of the CMB is 2.725 K. The shape of this universal curve of the blackbody spectrum posed a profound dilemma for theoretical physics. To explain the shape, Max Planck introduced his quantum hypothesis. Because he could not motivate his hypothesis, he found it difficult to believe his own theory. He used data taken by some colleagues who achieved incredible experimental expertise. The data shown here, taken by a satellite-borne radiometer called COBE, is so accurate that the error bars of the individual points all lie within the width of the plotted curve. (Unit: 5)