Webb and the Infancy of the Universe
Shortly after the Big Bang, the universe was filled with a glowing plasma, or ionized gas. As the universe cooled and expanded, electrons and protons began to bind together to form neutral hydrogen atoms. As the last of the light from the Big Bang escaped, the universe — now about 378,000 years old — would have been a dark place, with no 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, galaxies. Slowly, light would begin to shine again in the universe. Eventually, as the early stars grew in numbers and brightness, they would have emitted enough ultraviolet radiation to "reionize" the hydrogen, removing the electrons from their bonded protons and neutrons.
Webb, with its ability to see light from extremely distant objects that has had to travel for billions of years to reach us, will see some of the universe's first objects. As Webb observes light that's traveled from the far reaches of the cosmos, it captures images of distant stars and forming galaxies as they were in the earliest stages of the universe.
Astronomers know the universe became reionized because when they look out in space and back in time at the light of very distant quasars — incredibly bright objects thought to be powered by supermassive black holes at the centers of galaxies — they don't see the dimming of their light that would occur through a fog of neutral hydrogen gas. They find clouds of hydrogen, but almost no detectable clouds of neutral hydrogen drifting between galaxies, meaning the gas was at some point reionized. Exactly when this occurred is one of the questions Webb will help answer, by looking for glimpses of very distant objects, like quasars, still dimmed by neutral hydrogen gas.
Much remains to be uncovered about the time of reionization. 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 will not be able to see the very first stars of the Dark Ages, but it'll witness the generation immediately following, and analyze the kinds of materials they contain.
Webb will 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, or "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.
Scientists suspect the black holes born from the explosion of the earliest stars devoured gas and stars around them, becoming the extremely bright objects called "mini-quasars." 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. 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, but Webb will pinpoint the precise time period.