Soon, we may be able to peer back in time and “see the first light in the universe.”

NASA has been using this bold promise to generate buzz for the 2018 James Webb Space Telescope (JWST). But, the rather-romantic phrasing of one of the JWST’s primary mission goals belies a very real, groundbreaking capability of the instrument pegged as the successor to the iconic Hubble Space Telescope.

Following in the footsteps of the soon-to-be-retired Hubble, JWST will continue to image the spectacular galaxies and nebulae that have captured the public’s imagination for the past 27 years of Hubble’s career.

But in many ways, the JWST is set to surpass its predecessor. From its solar orbit 1 million miles (1.5 million km) from the Earth, the JWST will be turning its 6 meter (19.7 ft), hexagonal, gold-plated mirror to scientific goals both old and new. Studying exoplanets, star formation, and galactic evolution are all among the JWST’s principle missions, but the one of the most fascinating, yet mystifying, goals of the JWST is to “see the first light in the Universe.”

The evidence of our universe’s past is not in a place, or even a time; it’s all around us.

The notion that we can somehow look back in time to the beginning of the universe, seems to blur the line between science and fiction – until we consider the fact that, to some degree, all telescopes grant us this ‘time-traveling’ ability.

The evidence of our universe’s past is not in a place, or even a time; it’s all around us. Telescopes allow astronomers to image light from distant stars, but light takes time to travel to us. So, when we see the light from a star that is 100 light-years away, we are seeing the light that left the star 100 years ago. For all intents and purposes, we are seeing ‘back in time’ to what the star would have looked like 100 years prior.

Hubble itself can see stars and galaxies more than 13 billion light years away. That means that it is capable of capturing the light that reaches Earth from objects formed just 480 million years after the Big Bang (a relatively small timescale compared to the 13.6-billion-years since our universe’s inception).

In comparison, the James Webb is going to gather data from even further back in time – a mere 100-200 million years after the Big Bang. The way that it is going to do so, however, reveals the JWST website’s eye-catching claim to be a bit more complex than it first may seem.

The JWST will continue making same deep-field observations of super-distant objects that Hubble did and will still be looking directly for extraordinarily old light-sources; however, in addition, the JWST’s cutting-edge instruments will help us learn more about the evolution of the early universe and about the period which the first stars and galaxies were formed: the Reionization Epoch.

A Brief History of the Beginning of Everything

Immediately after the Big Bang, the universe was in a hot dense “soup” of electrons, protons, and neutrons, until it cooled enough to form ionized hydrogen and deuterium (and subsequently helium-4) around 240,000-300,000 years later. It wasn’t long before the newly-produced helium, ionized hydrogen, and the trace amounts of light elements attracted and absorbed the remaining free electrons of the “soup.”

Once this occurred – during the period known as the recombination era – the universe went from opaque to transparent for the first time. Today our universe is mostly transparent. We can see the light from distant stars that has traveled millions of miles to reach us. In the opaque, pre-recombination Universe, however, the dense clouds of materials would have scattered and absorbed most light before it got very far. This meant that, if someone had turned on a lightbulb in the early universe, a person standing only a few meters away wouldn’t have been able to see the light at all.

Following that initial opacity of the universe, the new transparency of the recombination era allowed the light to travel through space mostly unimpeded as it does today, but with one key difference. Due to the dense neutral hydrogen present during this time, the universe would have still been opaque to shorter, bluer wavelengths of light. The person looking for his or her friend with the lightbulb, would be able to see the light, but upon inspecting it with instruments, would realize that only part of the spectrum of the light was getting through.

How can James Webb study something we, by definition, cannot see? By looking at what is not there, of course.

This period of second opacity is known as the universal dark ages: the time after the recombination era, but before the first stars and galaxies formed in the Reionization Epoch.

Reionization refers to the process by which neutral hydrogen, the absorbing culprit behind the opacity of the dark ages, is split back into its component electrons and protons by ultraviolet radiation, thus allowing the high intensity radiation from early stars to travel further. Neutral hydrogen has a tendency to steal high energy, short wavelength light to create ionized hydrogen: a form of hydrogen which doesn’t affect in any way light that passes through it, and is thus fully transparent to light.

The energy required to ionize hydrogen and split it back into particles likely came from the UV radiation of early stars or black holes. Stars forming during this period could have been 30-300 the mass of our sun, a million times as bright, and lived brief lives of a few million years. Astronomers predict that these superhot early stars began to reionize the neutral gases surrounding themselves, thereby creating transparent “ionization bubbles.” These bubbles surrounding each star eventually grew and overlapped with their neighbors – eating up the obscuring neutral hydrogen that surrounded them – to reveal nearly-fully transparent universe we see today.


A graphic representation of the evolution of the universe. Note the depiction of ionization bubbles during the Reionization Epoch.
The upper axis depicts time since the Big Bang and the lower denotes redshift.

Diagram by: National Astronomic Observatory of Japan (NAOJ)

But the light of the first stars was partly obscured by neutral during the dark ages, and only during the late stages of reionization did ionized hydrogen let all wavelengths of light through. So, how can James Webb study something we, by definition, cannot see? By looking at what is not there, of course.

Seeing the Evidence of Reionization

When we analyze light spectra (the broad range of light wavelengths that objects emit), we can learn a lot about the objects that produce the light.

Each chemical element absorbs a different wavelength of light’s broad spectrum of wavelengths. Therefore, by examining at what wavelength pieces of a star’s light spectrum are missing, we can make good estimates about what elements are present in a star. However, not all absorption lines come from the chemical makeup of star itself. When light from the star passes through neutral hydrogen, for example, another absorption line is formed. The wavelength of light that the neutral hydrogen has ‘stolen’ to ionize itself is now missing from the light spectrum.

Current models of the distribution of neutral hydrogen in the modern universe estimate that the further an object is, the more neutral hydrogen clouds light from the object will pass through in its journey to Earth.


Examples of spectra of a distant quasar (at redshift greater than 6) that has a Gunn-Peterson trough (top); a quasar closer than redshift 6 that lacks a trough (bottom).

Figure by: Fan et al. 2001

So, more neutral hydrogen at encountered varying distances from an object means more absorption lines clustered at around the same wavelength in a light spectrum. Therefore, the spectra of super-distant objects should have strong dips in in spectra where many absorption lines have clustered (known as Gunn-Peterson Troughs) that indicate that they formed during reionization when much more neutral hydrogen was present.

To study this phenomenon, scientists chose quasars, or quasi-stellar objects – the objects so immensely bright that we can see them from great distances. By examining the spectra of quasars at different redshifts (read, “distances”), scientists have been able to estimate the point at which reionization ended and our universe was fully transparent: a distance and point in time known as redshift 6. When quasars are further away than redshift 6, and their light is therefore older, their spectra have distinct Gunn-Peterson troughs. Closer than redshift 6, no troughs have yet been found.

However, even a miniscule amount of neutral hydrogen can cause a noticeable dip in the spectrum which means that, just because we see trough-like features, doesn’t necessarily mean that the quasar was formed during reionization.



Graphic by: SG Djorgovski and Digital Media Center, Caltech

Fortunately, there is a way to double check: examining the spectrum for small, isolated transmission spikes – or patchy-absorption – in the Gunn-Peterson trough region of a spectrum would strongly indicate that the quasar’s light encountered not only neutral hydrogen, but the bubbles of ionized hydrogen that early objects created towards the end of reionization, as well.

Scientists have been able to record a handful of quasar spectra that seem to support the current theories about how the light from the first stars led to reionization, but there are still more questions than answers when it comes to the early universe. Luckily, the James Webb Space Telescope was designed to answer those very questions. Equipped with the ability to analyze spectra and see further than any telescope to date, in its quest to see the first light, the JWST aims shed some light of its own on the secrets of the early universe.

About The Author

Madelyn Broome

Madelyn was the 2018 Editor-in-Chief of Innovation, and a former writer and editor for the Space/Physics section. Her piece "Where's the Water?" won the 2019 Gregory T. Pope Prize for Science Writing. She is passionate about science communication and about making science engaging and accessible for people of all ages - though she especially enjoys working to ignite excitement for the sciences in young girls and other underrepresented communities in STEM. When she's not trying to share her enthusiasm for the sciences, she can usually be found exploring, practicing mixed martial arts, archery, lifting, playing soccer, or just generally trying to make up for the dessert she just ate.