Soon after the Big Bang the whole universe was a hot, glowing fog. As space expanded, that light was stretched to longer and longer wavelengths and cooled, until today it arrives from every direction as faint microwaves at about 2.7 K. That uniform glow is the cosmic microwave background, the strongest evidence for the Big Bang. Stretch the expansion and watch the light cool out of the visible and into the microwave band.
Pick how much expansion is needed, then tune the slider until the temperature reads the target.