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Webb Science: The End of the Dark Ages: First Light and Reionization

Until around 400 million years after the Big Bang, the Universe was a very dark place.  There were no stars, and there were no galaxies. Scientists would like to unravel the story of exactly what happened after the Big Bang.  The James Webb Space Telescope will pierce this veil of mystery and reveal the story of the formation of the first stars and galaxies in the Universe.

hydrogen atom

As the Universe expanded after its origin in a Big Bang, the hot soup of fundamental particles (such as free protons and electrons) started to cool down.  This allowed electrons and protons to pair up and form "neutral hydrogen atoms," (i.e. hydrogen atoms with one electron and one proton).  This process of pairing up is called "Recombination" and it occurred about 400,000 years after the Big Bang.  As the free electrons were now bound to protons, light could travel freely since it was no longer stopped by frequent scattering off the free electrons.

The Universe went from being opaque to transparent at this point, and "the era of recombination" is the earliest point in our cosmic history to which we can look back with any form of light.  This is what we see as the Cosmic Microwave Background today with satellites like the Cosmic Microwave Background Explorer (COBE) and the Wilkinson Microwave Anisotropy Probe (WMAP). At right is an illustration of the timeline of the universe.

timeline of the universe
Credit: WMAP

Another change occurred after the first stars formed.  Theory predicts that the first stars were 30 to 300 times as massive as our Sun and millions of times as bright, burning for only a few million years before exploding as supernovae.  The energetic ultraviolet light from these first stars was capable of splitting hydrogen atoms back into electrons and protons (or ionizing them). Observations of the spectra of distant quasars tell us that this occurred when the Universe was almost a billion years old.

That era, when the universe was a billion years old, is known as “the epoch of reionization.” It refers to the point when most of the neutral hydrogen was destroyed by the increasing radiation from the first massive stars.  Reionization is an important phenomenon in our Universe’s history as it presents one of the few means by which we can (indirectly) study these earliest stars.  But scientists do not know exactly when the first stars formed and when this reionization process started to occur. 

The emergence of these first stars marks the end of the "Dark Ages" in cosmic history, a period characterized by the absence of discrete sources of light. Understanding these first sources is critical, since they greatly influenced the formation of later objects such as galaxies.  The first sources of light act as seeds for the later formation of larger objects.

The Hubble Ultra Deep Field
Credit: Robert Williams and the Hubble Deep Field Team (STScI) and NASA

Additionally, the first stars that exploded as supernovae might have collapsed further to form black holes. The black holes started to swallow gas and other stars to become objects known as “mini-quasars,” which grew and merged to become the huge black holes now found at the centers of nearly all massive galaxies.

Webb will address several key questions to help us unravel the story of the formation of structures in the Universe such as: When and how did reionization occur?; What sources caused reionization?; What are the first galaxies?

To find the first galaxies, Webb will make ultra-deep near-infrared surveys of the Universe, and follow up with low-resolution spectroscopy and mid-infrared photometry (the measurement of the intensity of an astronomical object's electromagnetic radiation). To study reionization, high resolution near-infrared spectroscopy will be needed.

Below is a video that talks about how the James Webb Space Telescope's ability to look farther into space than ever before will bring newborn galaxies into view. (Visit our science visualization page for more versions of this video.)