The universe has been expanding ever since the Big Bang, gradually stretching the wavelength of the CMB to an average of a couple millimeters, or several thousand times longer than the visible radiation emitted by the typical star.
由於大爆炸後，宇宙一直在膨脹擴張，目前已將背景輻射的平均波長延伸至數個毫米，約較一般星星發出的可見光波長長了數千倍。