c. 13.7 Billion BCE

Recombination Era

The universe’s early years were a time of intense heat, pressure, and radiation. All of space was bathed in the primordial light of highly ionized atoms and subatomic particles, interacting, colliding, decaying, and recombining at temperatures of millions of degrees. This period in cosmic history is often referred to as the radiation era. By the time the universe was about 10,000 years old, the expansion of space and the decay of many energetic particles had cooled the cosmos to “only” about 12,000 kelvins (kelvins, or K, are a measure of the temperature above absolute zero). This was an important threshold, because as the universe continued to cool, the total energy from heat and ionizing radiation became less than the total so-called rest mass energy of matter itself, embodied in physicist Albert Einstein’s famous equation E = mc². Still, for hundreds of thousands of years longer, the universe was essentially just an opaque, dense, high-energy soup of constantly colliding ionized protons and electrons. But as the expansion and cooling continued, radiation energy continued to decrease as compared to the rest of mass energy.

By about 400,000 years after the Big Bang, the temperature had dropped to only a few thousand kelvins—low enough to allow electrons to be captured (deionized) into stable hydrogen atoms and for multiple hydrogen nuclei to form the universe’s first molecules: hydrogen gas, or H². This period in the universe’s early history is known as the recombination era.

The cool thing about recombination is that it allowed the universe’s remaining radiation—mostly high-energy photons and other subatomic particles—to decouple from matter and thus to finally travel, relatively unimpeded, through space. The universe grew colder and darker over the next few hundred million years, a time which cosmologists have dubbed “the dark ages.” The residual 3-kelvin glow of the early universe’s joyously freed radiation energy, known as the Cosmic Microwave Background, can still be detected today.

NASA’s Wilkinson Microwave Anisotropy Probe (WMAP) satellite generated this sky map of the residual heat left over after the early universe’s initial expansion. The small fluctuations in temperature seen here—only a few hundred-millionths of a degree—acted as the seeds for the first stars and galaxies in the universe.