What Did the Young Universe Look Like?

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.

The Great Photon Escape

Our universe was born in what has become known as the big bang. For hundreds of thousands of years afterward, the universe was a sea of hot protons and electrons moving around each other. Eventually they cooled enough to let the positive and negative forces attract each other, and the joining of the particles released lots of energy in the form of light. That initial light spread throughout the universe, but no new light followed, and the universe entered what is known as the “Dark Ages.” The cosmic Dark Ages lasted for millions of years until the first stars burst to life and began to illuminate the universe. CREDIT: Produced by the Space Telescope Science Institute’s Office of Public Outreach (STScI); Narration: Alia Shawkat; Writers: Joel Green and Vanessa Thomas; Designer: Marc Lussier; Special thanks to Greg Bacon, George Becker, Mia Bovill, Bonnie Eisenhamer, John Godfrey, Adam Green, Mikey, Hussein Jirdeh, Jason Kalirai, Courtney Kivowitz, Brandon Lawton, Katelin Mora, Bonnie Meinke, Denise Smith, Stephanie Smith, Massimo Stiavelli, Frank Summers, and Tracy Vogel.

Matter in the early universe slowly coalesced into larger structures, from molecules and clouds of molecular gas to stars and eventually galaxies. Radiation from these early cosmic objects would eventually begin the time of the universe known as "reionization." Credit: NASA/Goddard Space Flight Center and the Advanced Visualization Laboratory at the National Center for Supercomputing Applications

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.

Cosmic Conundrums

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?

Hubble Ultra Deep Field
The Hubble Ultra Deep Field is a look back in space and time that captures an estimated 10,000 galaxies in various stages of evolution, back to within 500 million years of the Big Bang. Webb's infrared vision will allow it to reach back even farther, to see the very first stars and galaxies.

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.

Last Updated: May 31, 2018

Keywords: Cosmology Universe Age/Size

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