The era of the universe called the Dark Ages is as mysterious as its name implies.
According to theoretical models, shortly after the big bang the universe was filled with a glowing plasma, or ionized gas. As the universe cooled and expanded, electrons (negatively charged particles) and protons (positively charged particles) began to bind together to form neutral (uncharged) hydrogen atoms. As the last of the light from the big bang spread out from the once-dense mass of the universe—now about 378,000 years old—the universe would have been a dark place, with no new sources of light to illuminate its fog of cooling, neutral hydrogen gas.
Some of that gas would have begun coalescing into dense clumps, pulled together by gravity. As these clumps grew larger and denser, they would become stars, and eventually the large groupings of stars and gas known as galaxies. Slowly, light would begin to shine again in the universe. Eventually, as the early stars grew in number and brightness, they would have emitted enough ultraviolet radiation to reionize the hydrogen, removing the electrons from their bonded protons and neutrons, clearing the hydrogen fog.
This time period is known as the Epoch of Reionization. Exactly when this occurred is one of the questions the James Webb Space Telescope will help answer, by looking for glimpses of very distant objects, like quasars, still dimmed by neutral hydrogen gas. Webb’s infrared instruments have the ability to see light from extremely distant objects that has had to travel for billions of years to reach us, which means it will allow us to view some of the universe’s first objects.
Much remains to be uncovered about the time of reionization. Astronomers’ understanding is that the universe right after the big bang would have consisted of hydrogen, helium, and a small amount of lithium. But the stars we see today also contain heavier elements—elements that are created inside stars. So how did those first stars form from such limited ingredients?
Webb can also show us how early galaxies formed from those first clumps of stars. The universe’s first stars, believed to be 30 to 300 times as massive as our sun and millions of times as bright, would have burned for only a few million years before dying in tremendous explosions known as supernovae. These explosions spewed the recently manufactured chemical elements of stars outward into the universe before the expiring stars collapsed into black holes or dim, cinder-like cores.
Astronomers suspect that the black holes that were formed from the collapse of the earliest stars that violently shredded the gas and stars around them, releasing light so bright that they would be classified not as a black holes but as a mini-quasars (black holes are, counterintuitively, surrounded by some of the most luminous regions in the universe, dense concentrations of stars and liberated energy). The mini-quasars, in turn, may have grown and merged to become the huge black holes found in the centers of present-day galaxies. Webb will try to find and understand these supernovae and mini-quasars to put theories of early universe formation to the test.
Webb will show us whether the first galaxies formed along filaments and webs of dark matter, as expected, and when this formation took place. Right now we know the first galaxies formed anywhere from 378,000 years to 400 million years after the big bang. Many models have been created to explain which era gave rise to galaxies; with Webb, astronomers will be able pinpoint the precise time period.