The Temperature of the Universe Today
Measurements of the cosmic microwave background radiation allow us to determine the temperature of the universe today. The brightness of the relic radiation is measured as a function of the radio frequency. To an excellent approximation it is described by a thermal of blackbody distribution with a temperature of T=2.735 degrees above absolute zero. This is a dramatic and direct confirmation of one of the predictions of the Hot Big Bang model.
The COBE satellite measured the spectrum of the cosmic microwave background in 1990, showing remarkable agreement between theory and experiment. The diagram below shows the results plotted in waves per centimeter versus intensity. The theoretical best fit curve (the solid line) is indistinguishable from the experimental data points (the point-size is greater than the experimental errors).