As the universe has expanded, the wavelength of this ancient light has grown with it. It currently stands at several millimeters and is invisible to the naked eye. In fact, we can only observe it with specially built radio telescopes based either on the Earth or in space.
這種古老光的波長，隨著宇宙不斷地膨脹擴張而拉長，目前平均波長約為數個毫米，是人類肉眼無法看到的「微波」波段，所以只能透過特製的電波望遠鏡或衛星觀測。