As explained on page 442 in “Quasars and Light, the Eddington Limit, Galaxies and Stars, Angular Momentum, and Gravity Waves,” on Day 1, matter was so close together that powerful gravitational forces began forming quasars. Thus, the universe was filled with extreme light.
Black-Body Radiation. A black body is defined as one which absorbs all incoming radiation. The light emitted from such a body is black-body radiation. When light reflects enough times off surrounding matter—as it would early in the creation week, before the much smaller universe was stretched out—everything reached a common temperature and the space between that matter was filled with black-body radiation.1 Days later, after the universe was stretched out, temperatures dropped. [Stretched out space is discussed beginning on page 439: “Why Is the Universe Expanding?”]
This black-body radiation was discovered in 1965 and is called the cosmic microwave background (CMB) radiation. Its temperature today corresponds to a very cold 2.73 kelvins (- 454.76°F).
What did this light look like on Day 1, before the Sun, Moon, and stars were made on Day 4 and before the heavens were stretched out? Blinding light reached Earth from all directions. After the universe was stretched out, light arrived primarily from the direction of the nearest quasar which would soon be at the center of our galaxy. (That quasar has since devoured all nearby matter and is now the black hole with 4.3 million solar masses at the center of our Milky Way galaxy.) Shadows then appeared; light was separated from darkness.
And God separated the light from darkness. (Genesis 1:4b)
Earth, rotating since its creation on Day 1, then experienced day-night cycles even before the Sun was created on Day 4. Today, thousands of years later the CMB reaching Earth is uniformly spread out over the entire sky. This is because black-body radiation uniformly filled the much smaller universe before the heavens were stretched out. Since Day 4, the Sun has been Earth’s dominant light source.