Short answers, pulled from the story.
Radiative cooling is the process by which a body loses heat by emitting thermal radiation. Every physical body spontaneously emits electromagnetic radiation; materials that radiate in the 8-13 micrometre infrared window, where the atmosphere is transparent, can shed heat directly into the cold void of space.
In 2014, researchers developed the first daytime radiative cooler using a multi-layer thermal photonic structure. It achieved surface temperatures 5 degrees Celsius below ambient air under direct sunlight, making passive cooling in daylight scientifically feasible for the first time.
In India, shallow ceramic trays filled with water and insulated below with hay were placed under a clear night sky; radiation upward overcame convective warming and froze the water. In Iran, large flat reflection pools were built on insulative beds and surrounded by high walls that blocked warm convective air currents.
Paintable porous polymer coatings achieve solar reflectances of 0.96-0.99 and thermal emittance of 0.97, keeping surfaces 6 degrees Celsius below ambient with cooling powers of 96 watts per square metre. Silvered polymer films reported in 2015 stayed 11 degrees Celsius cooler than commercial white paints under a mid-summer sun.
The James Webb Space Telescope uses a large reflective sunshield to permanently block radiation from the Sun, Earth, and Moon. The telescope structure, held in constant shadow, cools purely by radiation to an operating temperature of roughly 50 K.
Experimental hydronic systems integrating daytime radiative cooling surfaces have achieved seasonal energy efficiency ratios up to 35 times higher than conventional air conditioning in modelled cases for Madrid and Rome. A full-scale test in Arganda del Rey, Spain, reduced indoor temperatures by up to 8.7 K while maintaining comfort under outdoor temperatures approaching 40 degrees Celsius.