Second Evening—Expansion
As the first morning drew to a close the light became less intense and redder, like a fire that is going out. Almost all the high-energy rays were broken up by that time, and the particles collided with each other less and less frenetically. After 380 000 years, when the temperature had dropped to only 3 000º C (5 000º F), the nuclei could at last capture and hold the free electrons and form the first atoms. The scattering of the light diminished, as it does when a morning fog dissipates. The last light traveled freely in all directions, from every place toward every other place in the universe. Expansion made the universe dark and transparent, open to our inspection, and the second evening began.
The second great epoch of darkness began when the expansion had cooled the gas that formed at the end of the first morning to the point where it no longer emitted light. Galaxies and stars as we know them now were as yet unformed, and there was nothing else to shine. In the darkness the entire universe was expanding. All regions were separating from each other. All regions were also expanding, but the denser regions did not expand as rapidly as the rarefied regions. Eventually gravity overcame the tendency of the dense regions to expand, and they began to contract in on themselves.
The pressure forced the expansion of the universe, and the expansion cooled it. The expansion also cooled the light. This means that the expansion stretched out the light waves longer and longer until the light waves became heat waves. Heat waves are invisible, dark to our eyes. Further cooling over the course of thousands of millions of years stretched the heat waves into millimeter waves, then microwaves, and finally into waves a television set can detect.
The second great epoch of darkness began when the expansion had cooled the gas that formed at the end of the first morning to the point where it no longer emitted light. Galaxies and stars as we know them now were as yet unformed, and there was nothing else to shine. In the darkness the entire universe was expanding. All regions were separating from each other. All regions were also expanding, but the denser regions did not expand as rapidly as the rarefied regions. Eventually gravity overcame the tendency of the dense regions to expand, and they began to contract in on themselves.
The pressure forced the expansion of the universe, and the expansion cooled it. The expansion also cooled the light. This means that the expansion stretched out the light waves longer and longer until the light waves became heat waves. Heat waves are invisible, dark to our eyes. Further cooling over the course of thousands of millions of years stretched the heat waves into millimeter waves, then microwaves, and finally into waves a television set can detect.
