What Is Hidden In The Darkness At The Beginning Of Time?

Close your eyes and imagine  yourself within a supervoid. Darkness envelops you, holding  you captive and isolated. With your eyes, even with any amateur  telescope, you see nothing, no stars, no hint of the fuzzy smudges we now  know as our own neighbouring galaxies.

To you, the Universe is empty. The Boötes void is a spherical region 330 million light years across,  approximately 0. 2% the diameter of the entire observable cosmos/universe.

The Milky  Way could fit inside it millions of times over, and yet within a volume of about one million cubic  Mpc, where there should be thousands of galaxies, we have observed only 60 - spiral galaxies,  blue with the light of new star formation. To quote astronomer Greg Aldering: "If the Milky Way had been in the center of the Bootes void—we wouldn't have known  there were other galaxies until the 1960s. " Indeed, were Earth to rest on one edge  of this void, observers on the other side would only now be receiving light from  a time when amphibians dominated the earth, before even the age of the dinosaurs.

Even now, we do not fully understand how such a void came to be. The age of the Universe sets  a limit for the maximum size of any structure, galaxy clusters, and indeed the voids  in between. There should be no single structure in the Universe that is already greater  than tens of millions of light years across.

The Great Nothing, as the void  is known, shatters this limit. Though its absence of structure does give  us an insight into the growth of structure at the earliest of times - the few galaxies  observed within the void following a tube shape, perhaps the crumbled remnants of a  wall between two voids. Individually, these voids would each fit in with the growth  limit, but together they form a supervoid.

And so even here, in the “Great Nothing”,  one of the darkest places in the Universe, there is the light of galaxies, unveiling their  secrets to us through the visible spectrum. To find true darkness, we have to look farther  than the Boötes void, farther back in time. We can only find complete oblivion in  the time before stars and galaxies and planets existed - forebodingly  known as the Cosmic Dark Ages.

Here, the darkness was total, and long-lived.  Although for about 200 million years after the Big Bang the Universe contained a variety of  ingredients that could produce the first stars, none formed. There was no visible light at all.

But why was the cosmos dark for so long? What does this mean for the stars  and galaxies we see around us today? And how can we possibly hope to shed any  light on an era when there was only darkness?

In 1543, Nicolas Copernicus showed the earth isn´t  the centre of the solar system, in the 1920s Jon Oort discovered the sun wasn´t in the centre  of our galaxy, and in that same decade Hubble discovered our galaxy was just one of countless  others. Cosmologically we arent the centre of anything - and that is a sobering realization. This video has been kindly sponsored by Betterhelp, and they are here to help you  work through life´s many realizations.

With BetterHelp, you can tap into a network  of over 30,000 licensed and experienced therapists who can help you with a wide range  of issues. You can then talk to your therapist however you feel comfortable, whether  it’s via text, chat, phone or video call. With BetterHelp, you get the same  professionalism and quality you expect from in-office therapy, but with a therapist who is  custom-picked for you, with more scheduling flexibility, and at a more affordable price.

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In 1995, the then-director of the  Hubble Space Telescope Bob Williams had ring-fenced some personal observing time. He was the envy of the scientific world, holding  precious hours on the world’s greatest telescope, able to look at anything he wanted. At the time Hubble was in the spotlight - but not for the right reasons, having used up billions  of dollars of public funds only to require a crewed space mission to correct a near-fatal  flaw in the system's optics.

The mirror which reflects gathered light into the instruments  needed to be extraordinarily smooth in order for the image to be in focus. Hubble’s mirror was  very smooth indeed, but the first images had been disappointingly blurry, a devastating blow to  the intended science aims of the mission. It was determined that the mirror had an  aberration less than a fiftieth of the width of a human hair that was causing the incoming  rays of light to focus at different points, clouding our view of the cosmos.

For most  space missions, this would have been fatal, but Hubble was in low-Earth orbit, making  a repair mission not unrealistic. Astronauts installed several clever  adaptations and replacements, correcting Hubble’s eyesight to perfection. But now, Williams was under pressure to produce something of worth, something that would  dispel the naysayers in politics, press, and even within the corridors of NASA.

And so he decided to look at nothing. Against the counsel of his colleagues,  Williams and his students steered the eye of the billion-dollar machinery such that it  looked past the planets, sidestepped the stars, and away from the galaxies. Hubble instead  settled its sights on a tiny patch of sky only 1/30th the diameter of the Moon.

And there, it stared, for ten whole days. To our eyes, and any other telescope that  had come before, this patch was barren of structure - but this technology was a  giant leap forward in astronomy - and the fine resolution and sensitivity of the Hubble  Space Telescope revealed shapes in the darkness. Over a long exposure, the new Hubble  could slowly gather photons coming from the farthest and faintest objects in the  Universe - and distant galaxies of every size, shape and colour emerged from the darkness.

We now know this as the Hubble Deep Field, one of the most iconic images in astronomy. As with any photograph, this image is a 2D projection of a 3D landscape - galaxies far  removed in distance appear to be neighbours. It is also a projection of time.

Photons from  the farther galaxies have travelled for longer, and so we see those galaxies as they  were farther back in time. Galaxies can even appear to be colliding, when they  existed as pictured billions of years apart. To understand why, let us consider  the observable universe as constructed of a series of shells around us.

The shell closest us to us is easy to observe because the light travels to our telescopes almost  instantaneously. Light from the Sun takes about eight minutes to arrive to Earth. Move a shell  outwards, perhaps to our nearest neighbouring galaxy, Andromeda, and from here, the light we see  today has taken 2.

5 million years to get to us, released when one of humanity’s earliest  ancestors, Homo Habilis, walked the Earth. This may be a long time for us, but it  is still a short time in terms of the cosmological timeline of 13. 8 billion years.

As  we consider the light from even farther shells, the photons we receive get progressively more  ancient, and so we can see further back in time. And so with Hubble we could finally  observe light from galaxies as they were only hundreds of millions  of years after the Big Bang, so faint and poorly resolved that they are mere  red smudges in the sky - just as our closest galaxies were once smudges on the sky for the  astronomers of the last century and before. We have the means to pick apart the light that  we observe, working out the composition of that galaxy and comparing known element markers  to work out the distance to, and age of, the galaxies we see.

This method uses the same  principle that causes a fire engine’s siren to change pitch as it approaches and passes you  on the street. As the fire engine approaches, the wavefronts of sound pile up, and we  perceive a sound of shorter wavelength, or higher frequency. Conversely, as the fire  engine recedes, the wavefronts are increasingly spaced out, and we hear a lower pitch. 

The inherent siren pitch remains the same, but it is the relative movement between the  source and the listener that causes a shift. Now imagine a galaxy as our siren, except instead  of listening for the sound, we observe the siren light. This light is a specific colour, blue, and  does not change.

As the galaxy recedes from us, however, the light wavefronts stretch out  and we observe a longer wavelength light: the siren gets redder. We call this  shifting of the light spectrum redshift. Certain chemical elements are natural siren lights  in the spectrum, producing a clear emission line at a known frequency.

By measuring how much  that line has shifted in a galaxy’s spectrum, we can calculate the speed at which it is  travelling away from us. Edwin Hubble, the famous astronomer for whom the telescope was named,  observed that the farther the galaxy, the faster it appeared to be travelling away: a keystone  of the Big Bang theory. As a rule of thumb, the oldest light is the most redshifted.

In this  way, we prise apart the thousands of galaxies that appear tangled in the deep field images, and brush  away layers of young galaxies, until we are left with only the oldest, the first of their kind. The latest deep field image from the James Webb Space Telescope covers the area of just  a grain of sand in the sky and yet still, thousands of galaxies emerged from the darkness  - pushing back the cosmic frontier to only a few hundred millions of years after the Big Bang. We are finally reaching a time in the Universe where there was no visible light at  all, a time that even JWST is blind to - the frontier of the First Stars.

But does this mean we can go no further? Are we forever closed off from  the darkness that lay before? In the beginning, light ruled the Universe.

The energy density of radiation far outweighed the  energy density of matter, and interactions between the two were commonplace. So much so that photons  of light quickly broke apart any atoms that formed and, as a result, the photons could not travel  freely in this super-hot dense plasma of matter. This soon changed however.

While both densities reduced as the volume of the Universe increased, radiation  fell faster because of an extra factor: the stretching of light into the longer wavelengths  as the universe expanded, and lower equivalent temperatures - cosmological redshift. After around 380,000 years of expansion, the energy density of radiation had fallen below  that of matter, and the dominion of light in the cosmos was long over. The temperature of  the plasma had in fact cooled to such an extent that atomic nuclei could now reliably  combine with electrons into simple atoms.

Though light was now no longer the dominant  force in the universe, there was an upside - the creation of atoms now meant photons had space  to travel, having their paths diverted only occasionally through interactions with matter. And so light was free - but what would you have actually been able to see? The answer is hard to know for certain - but we can estimate what  the Universe would have looked like.

The radiation at the point of recombination  emitted as a blackbody: an object that emits radiation across the wavelengths with a  predictable shape, as long as you know its temperature. To an onlooker, the Universe  would have seemed to have a warm orange glow. Over the next few hundred million years, as  the Universe expanded further, the dropping temperatures lead that glow to redden with time. 

Finally, it cooled such that there was no longer radiation emitted in the visible part of the  electromagnetic spectrum at all. The embers of the Big Bang faded entirely into the infrared,  microwave and radio wavelengths. Darkness fell.

In the midst of the black,  though, all was not quiet. For another kind of matter was created  in the Big Bang, a kind of matter that is still mysterious to us today: dark matter. In the 1960s, scientists Kent Ford and Vera Rubin measured the orbital velocities of stars in spiral  galaxies and found that something didn’t add up.

Orbital velocity, the speed  that stars orbited the galaxy, is a fine balance. Too slow and gravity causes  the star to spiral inwards, too fast, and the star overcomes gravity and flies out of orbit. The astronomers carefully estimated the stellar mass, and hence gravitational pull, of a variety  of galaxies and found that stars were simply moving far too fast.

Observation after observation  pointed to an uncomfortable truth: there must be a hidden component of mass keeping them in orbit. And that hidden mass was known as “dark matter. ” Calculations show that this ‘dark matter’ makes  up four-fifths of all matter in the Universe, yet it still evades our direct  detection and capture today.

The matter we see around us interacts  with the Universe both gravitationally and electromagnetically. Dark matter, in contrast, only interacts with the gravitational field. It cannot absorb or emit photons and so has no imprint on a spectrum in the same way chemical  elements do.

It does possess a gravitational pull, however, and so high-density  areas of dark matter can grow and collapse through accretion. But only up to a certain point. And this is key - there can be no  stars or planets made of dark matter.

As a cloud of matter collapses, the pressure  of the particles in the cloud fights back. Pressure is linked to temperature: the denser  the particles, the hotter they are, and the more kinetic energy they have to more effectively  create an outward-acting pressure force. Normal matter can convert thermal and kinetic  energy into electromagnetic radiation via the excitation of electrons in colliding atoms.

The atoms move apart at a slower pace and when the electrons naturally return to  their ground states, they emit photons which carry away the parcel of energy from the  cloud. The pressure is reduced permanently. But dark matter does not interact  electromagnetically and so cannot emit photons.

There is no escape valve. This means that once the outwards thermal pressure of the system balances the inwards gravitational  pressure, the collapse is simply halted. And so instead dark matter collapses into  filaments, connecting at their densest points into halos.

Unable to emit light, our Big  Bang observer could not see this vast background web of dark matter, behind the uniform orange or  red glow of the gas. Even in the total darkness that followed as the radiation shifted out of  the visible, the dark matter remained hidden. For close to two-hundred million years,  this remained the state of play - a web of dark matter that was doomed to darkness forever,  and a mist of gas too dilute to ignite fusion.

And so how could the Universe ever  form a star to break the impasse? And hidden as the evidence is  in the total darkness at the beginning of time - how could  we ever know what happened? On 6th May 1937, flames viciously engulfed the  German airship Hindenburg as it attempted to dock with its mooring mast in New Jersey.

Bystanders  could only look on in horror. Pride of the fleet, the ship regularly carried passengers across the  Atlantic Ocean at a stately 80 miles per hour. This journey had been no different until the very  last moment, when the hydrogen cells filling the balloon ignited one by one, causing explosive  damage as the fuel tanks erupted.

Remarkably, just under two-thirds of the  passengers and crew survived, thanks only to its proximity to the ground and the  extended duration of the disaster. Most victims however were severely burnt, and grief ripped  through the families of the 35 lost souls. Airships had been a regular sight across city  skies, carrying army personnel, passengers and freight across and between continents.

Most  airships utilised hydrogen as a way of filling the balloon, as its buoyancy is well above that of  air - but hydrogen also boasts another property: it is extremely flammable. Most evidence points to the cause of the  Hindenburg disaster as a hydrogen leak in the fuel tanks. When this hydrogen mixed with  oxygen in the air, the gas holding up the balloon became ready to blow, and it is thought that  a simple flash of static electricity provided the source of ignition, thus ending the era  of airships with almost immediate effect.

Hydrogen. One proton, one  electron. Simplicity itself.

This most basic atomic structure is odourless  and tasteless, and it will combust when exposed to even the lowest concentrations of air and an  ignition, as seen in the Hindenburg incident. But even that fatal destruction is far from the  full power held within this small orbital system. Nagasaki.

Hiroshima. These cities act as reminders of the  horror that humankind can unleash upon itself just by splitting the atom - fission. But hydrogen fusion-based bombs are far more devastating, and have thankfully not yet been  detonated in an act of war - they are up to 1000 times more powerful and could easily kill the  populations of entire cities in an instant.

And many of the scientists that worked on creating  these nuclear bombs were also physicists and astronomers themselves - looking to the fusion  of hydrogen within stars for inspiration. But at around the same time hydrogen became  the focus of another group of astronomers, for a slightly less bombastic,  but no less remarkable reason. Jan Oort had risen to the top of the  astronomical world by questioning the assumption that the Sun was at the centre  of the Milky Way.

He had shown the Sun was in fact 30,000 light-years out in a galaxy  that was rotating just as a spiral would do. Now however, he was half in hiding, having  protested and resigned over the dismissal of his Jewish colleagues. The war was raging, and  Holland was under German control - research and the exchange of knowledge was fraught with risk.

But the pull of academia was too strong to keep him solely in his country retreat. For Oort was still in pursuit of a perfect model of our galaxy. But to do so, he had to overcome a  challenge.

For our milky way is shrouded in dust. He would need to pierce through the obscuring  interstellar clouds and see further than optical telescopes would currently allow. But how?

The answer came in a smuggled issue of an Astrophysical Journal, which spoke of the  promise of detecting radio waves from the Milky Way. Radio waves have long wavelengths and  are immune to the obscuring dust. In addition, the interstellar gas feeding whole stellar  nurseries might emit radio waves, illuminating the galaxy on much grander scales than individual  stars.

Oort realised that with a suitable radio spectral line, he could map our galaxy far  beyond the boundaries of optical light. And so in 1944, Oort tasked  student Hendrik Christoffel Van de Hulst with searching for  a way to determine the shape of the Milky Way by detecting and mapping the  invisible hydrogen gas that pervaded it. There are many ways in which an atom can  emit light.

Tiny transitions of electrons between two energy levels in an atom cause the  emission of photons with an equivalent energy, or wavelength. Different processes  lead to different wavelengths. Oort and Van de Hulst needed a transition  that was common enough to occur regularly across the sky, and that produced  an intensity such that astronomers could clearly measure its spectral line. 

Having worked meticulously through the atomic transition probabilities,  he found a promising candidate. Within the first energy level of the hydrogen  atom was a hyperfine structure, a splitting of the energy level into sub-levels, populated according  to the electrons’ values of the atomic property, spin. If an electron holds the same spin value  as the proton it orbits, it occupies the higher energy level; if opposite, lower.

Sometimes,  electrons in the higher energy level spontaneously change spin in a rare occurrence called a  spin-flip transition. The electron moves to the lower energy level, emitting a photon of energy  precisely equal to the hyperfine energy gap. This photon has a frequency of 1420 MHz, or  a wavelength of 21cm, placing it within the low-frequency domain of the electromagnetic  spectrum: perfect for a radio telescope.

Van de Hulst presented the 21cm emission line as  the key to mapping hydrogen across the Milky Way, a special second-sight that would reveal  structure far beyond what the stars could tell us. But was the probability of success  too low? The probability of an electron spontaneously flipping in spin was miniscule,  and so to have such a rare event be detectable, the volume of hydrogen would need to be huge. 

We can imagine this like trying to find that elusive, rare trading card in your trading card  collection. If your card makes up 1/100th of those manufactured overall, you will be lucky to find  it in a single pack. Buy several hundred packs, though, and the rare becomes observable, the  Charizard commonplace.

In the mid-20th century, astronomers could not be at all sure of the  amount of hydrogen in space, but there was general scepticism that it could be so much as to  make the 21-cm spin-flip transition observable. Oort salvaged two leftover German antennas  and set to work. However, though he was a well-respected and expert astronomer, the  electrical engineering knowledge required to construct an interstellar listening device was  a unique skill, and one that he entirely lacked.

And unbeknownst to them, at the same time,  a group of physicists in the US had also taken interest in Van de Hulst’s theory  - but with the expertise to follow-up. Edward Purcell´s head had been turned because  of his background in subatomic matter, and his PhD student Harold “Doc” Ewen had a background in  radar physics. They too set to work immediately.

Ewen used plywood, copper plating, and expertly  installed the electronics to construct a pyramidal horn radio antenna. These odd-shaped  devices, like an ice cream cone on its side, were less susceptible to human-made interference  from radios and other electric equipment. The horn was installed hanging out of the fourth-floor  window of the lab on campus, the naïve scientists not foreseeing the risk that students would  enjoy using the horn as a target for snowballs.

The device did not need to be big or steerable,  because they would not be scanning across the skies in their search - whether diffuse or  dense, the hydrogen should be everywhere. And so, on Easter morning in March  1951, they successfully detected the 21-cm Galactic emission line. It  had worked.

Van de Hulst was right. Fortunately a quick two-way  sharing of knowledge began, with the Americans teaching the Dutch  how to construct the experiment, and the Dutch explaining to the Americans the  astronomical context of what they had achieved. This generous sharing of knowledge meant that,  within the space of three months, the global radio astronomy community was primed to map  our Galaxy’s hidden corners and far-off lands.

By searching for such a specific spectral line,  astronomers could now not only identify where the gas lay in the galaxy, but also whether  it was moving towards or away from us, by measuring the redshift of the light. And it is in this way that radio astronomers first showed beyond doubt that the Milky Way was both  a spiral in structure, and that it was rotating. Today, we still search for this line  using ever more advanced telescopes, reaching further and further and still discovering  new arms and branches to our home galaxy.

But what does this mean for the cosmic dark ages? Radio telescopes give us the means to light  up the invisible gas that permeates galaxies. Hydrogen is the most simple of elements, and  the main constituent of normal matter in the Universe.

Before planets, before stars, and  before galaxies, there was just this gas, flowing through the Universe and becoming  trapped in the vast web that dark matter had woven. This gas was still there when darkness  fell, invisible to the optical onlooker. But look again with eyes tuned to a  wavelength far removed from the rainbow.

Look instead with eyes tuned for 21-cm radiation  and the Universe becomes bright with structure, even during the darkest times. The surrounding hydrogen gas follows where the dark matter went before it, collapsing  onto the filaments and halos. Simulations show the hydrogen tracing the dark matter structure  almost perfectly, under the cover of darkness.

The Universe was never truly resting… it was  just hiding its construction work from our eyes. And so, we have found the means to trace the  earliest stages of star and galaxy formation, a single wavelength of light, 21-cm radiation.  The Universe was dark to our eyes for hundreds of millions of years as the ingredients for star  formation gathered and ignited.

But to other eyes, eyes tuned to longer wavelengths, we would have  been able to see these nascent environments, and know where the first stars would form. But do the first stars really  mark the end of these Dark Ages? Would darkness really lift  with the flick of a switch?

Our seas and oceans are the largest habitat  on Earth, and one of the most varied. As we snorkel in crystal clear waters, schools  of brightly coloured fish captivate us. Sinking below the surface, however, and our eyes  steadily become useless as the sunlight dwindles.

By 10 metres down, the reds, oranges and yellows  have been absorbed, and you struggle to see with only 70% of the surface light. By 50 metres down,  the greens and violets have been excised from the rainbow too. And yet, still, the ocean descends,  and still, life is all around you, surviving in a place that cuts you off from your sight.

The fish even become redder as you fall, evolution having noted that in an environment lacking red  wavelengths, they will appear almost invisible. 200 metres marks the twilight zone, a dim,  monochrome world. You begin to see the glow of bioluminescent animals such as the  hatchetfish float past, creating their own light to match the waters above… so they don’t  get seen and snapped up by predators below.

At 300 metres, you are enveloped in darkness,  working with only the light of a moonlit world on the surface. A giant squid glides  serenely past you, all 10 metres of it, its football-sized eyes scanning you for threat. At 850 metres, your eyes can no longer gather enough paltry photons to activate the  optic nerve.

The midnight zone begins, and in some places, may even continue for several  more kilometres. On Earth, then, life has adjusted to total darkness and learned to live despite it,  hidden from the only way our eye knows to look. And so, during the Dark Ages of the  Universe we know too that the black was not empty.

There was structure in  place behind the scenes, a dark matter web that drew in nearby hydrogen and built the  foundations of the Universe brick by brick. And all over the Universe, by about 200  million years after the Big Bang gravity would crush these dense pockets of hydrogen  just enough to ignite the engines of stars. In every direction, the lights blinked  on, slowly illuminating The Dark Ages.

But was the oblivion that preceded this  visible illumination really so empty of light? It may have been dark in the optical, but  was it truly dark in other wavelengths too? Despite emitting across the wavelengths of the  electromagnetic spectrum, the first stars found their nascent cries muffled by the murk  of a deep ocean of hydrogen - the path was not entirely clear for this new light.

While gravity had swept some hydrogen up to make the first stars themselves, there was more  than enough remaining to swaddle the newborns. As the first engines of fusion started up, and  the resulting photons fought their way out from the core to the surface of the star, they  encountered clouds of atoms ready to absorb them before they could make it much further. An observer of these first stars would likely notice little wrong with the light, as hydrogen  absorbs only four visible wavelengths.

The rainbow of its visible spectra therefore would be  complete, except for discrete omissions within the red, cyan, blue and violets. When considering  the entire spectrum, the first stars would have appeared tinted away from the ultraviolet. But before too long, these first stars began to change their surroundings  - and in a surprising way.

Our traditional view of an atom has been  passed down to us from the early 20th century physicist Niels Bohr - a compact  nucleus of protons and neutrons orbited by satellite particles called electrons. But that is not how many of the atoms in our universe pass their time. Almost 99.

9% of the matter in the universe is in the form of a plasma -  a mix of separated electrons and IONS. The highest energy level  in a hydrogen atom is 13. 6 eV.

If the atom absorbs a photon  carrying more than this amount, the electron is energised to such a degree  that it overcomes the bond with its proton, and becomes an autonomous, free particle.  The hydrogen atom is said to be ionized. In the immediate aftermath of the Big Bang, the  high temperatures and frequent collisions rendered all normal matter ionized, before it cooled and  came together as full hydrogen and helium atoms, with their requisite electrons, at  the beginning of the Dark Ages.

But not for long. For quickly following the end of the Dark Ages is a new stage in the history  of the universe - the Era of *Re*ionization. The first photons from a first star and, later,  a first galaxy ionize its immediate surroundings, creating a sphere of ionized hydrogen through  which the next photons can get a little farther before they meet an intact hydrogen atom, hungry  to absorb it.

Over time, these bubbles of ionized hydrogen get larger and larger, connecting  and merging such that the hydrogen gas that we observe around us today is almost all ionized. It is still a mystery exactly how the Universe got to be in the completely ionized state that  we observe it in today. The first stars could not produce enough ionizing photons in their  short lifetime to fully clear the way.

Instead, it seems more likely that, while the first stars  began the process, it was the first galaxies that finished the job. Full of the many and long-lived  second generation stars and a central black hole accretion disk spewing radiation, they  could easily ionize the Universe before the first billion years of time had passed.  Then, one billion years after the Big Bang, in a universe full of ionized gas, light could  travel with the same freedom it can today.

The Epoch of Reionization was complete, and  with it, the Dark Ages was truly over. Together with the Dark Ages, the Epoch of  Reionization is an almost missing era in the timeline of our Universe’s evolution. There  have been a handful of galaxies observed within the latter epoch, but not enough to make  anything but the most tentative scientific conclusions about the state of the Universe over  the whole Era of the First Stars.

Doing so would be like making conclusions about the entire of  human history based on a single bone from each of five random centuries. And so the James Webb  Space Telescope launched on December 25th 2021, expectations were high that we would see many  more galaxies from within that lost time. For Webb is infra-red, allowing it to probe even  further back in space and time than Hubble.

When we peer at the Hubble Deep Field, we  can see a range of galaxies from many epochs, the oldest visibly reddened because of  the expansion of the Universe: redshift. But while galaxies emit across the electromagnetic  spectrum, as we observe the spectrum of light from older and older parts of the universe, there  will be a point at which most of the light has redshifted out of the shorter wavelength domain. Hubble cannot produce images of galaxies whose spectra have redshifted out of the visible  wavelengths.

Webb however, an infra-red instrument, can see light from further into the  past, because it can still detect the photons even though they have redshifted out of the  visible wavelengths, and into the infra-red. Able to look deeper and farther than Hubble,  Webb has already observed some of the earliest galaxies we have ever seen in the universe,  barely 350 million years after the Big Bang. But even this goliath of astronomy has a limit. 

While JWST has provided a fascinating insight into early galaxy formation, to reach further  back - to reach the mysterious Dark Ages, we need a different instrument entirely. Hubble, an optical telescope, and JWST, an infra-red telescope, ultimately provided tiny  samples of the sky, both in image space and across time. To push further, to the earliest moments  of the Epoch of Reionization and the Dark Ages, requires the longest wavelengths we have: radio.

As we look farther back in time, we observe more of the Universe per pixel. Thus, while we may  be able to see the Sun, and galaxies such as Andromeda and the Whirlpool in exquisite  detail, as we move to the earliest times, the resolution achievable is simply so  low that we cannot see individual stars, or even individual galaxies. We need something  on a much larger scale, and radio-loud.

And this is where Van de Hulst´s  hydrogen 21-line comes into its own. As we have seen, in the early universe, clouds  of hydrogen pervaded the Universe - and these can provide a means for us to observe the progression  of the Epoch of Reionization: the 21-cm line. We can observe the Dark Ages as a cacophony  at the redshifted 21-cm wavelength.

Then, as the ionized bubbles grow and merge, we can trace them in the absence of 21-cm  radiation, like a cosmic swiss cheese. The shape and speed they grow can tell us  much about the sources that are producing the radiation: population III stars, baby black  holes and even the earliest forms of galaxy. Radio telescopes hold the key  to lighting up the Dark Ages, but the experiment itself is one of the hardest  in astronomy.

The signal is incredibly small, buried under the photons of our bright galaxy  that cannot help but fall on the same antennas. Is it really possible to find the signal  of reionization and illuminate the Dark Ages? Imagine yourself deep in the Australian outback,  in a desert landscape void of human life, the red dust broken only by  rocks and desiccated shrubs.

The powerful heat of the day shimmers in the  dry air, no hint of recent or forthcoming rain. But as you walk, you see evidence of life:  sheep and cattle grazing in a land that humans largely find too hostile to occupy. These animals don’t just experience this land differently, they see it in  an altogether different light too.

The lenses and the sensors embedded in our  retinas can result in vastly different views and understanding of the same environment. Humans  are usually born with colour-sensitive cones tuned to red, green and blue - but the cows and sheep  you pass cannot see the shades of the red land at all. The snake that slithers past your feet  also lacks the ability to see shades of red, but it has an ability that you do  not: the ability to sense infra-red.

Here, you find another companion, as a scorpion  scuttles in front of you, weaving its way through the metal forest. This scorpion can detect uv  light too, and even has the rare quality of fluorescing when exposed to these wavelengths. The animal kingdom is alive with examples of adaptations to different levels and wavelengths  of light - indeed humans are rare for only seeing in the optical.

Instead, we have emulated the  animal kingdom by creating technology to help us perceive our environment in the uv, infra-red and  x-ray. We have also invented entirely new methods, not shared by any other being on Earth, to  access the longest wavelengths, the radio waves. In the pursuit of long-distance communication, we  built our own interface with the radio Universe, learning through trial and error how to  listen in to a signal from a different room, a different country, a different continent and  then, quite by accident, we picked up the signals of the stars and planets and meteors, and radio  astronomy was born.

We had developed the means to communicate to the distant universe and into  the distant past. Only humans can communicate at these longest wavelengths, having carefully  created for themselves a new kind of eye. As you walk on through the outback, you come  across a strange sight.

Standing on the dirt lie strange metal sculptures. Some are bowtie shaped.  Some look like reflective tables and others look a little like trees, their spikes evoking cacti.

But these bizarre metal structures in the Australian outback are not discarded abstract art,  they are an interface to a Universe that only we have access to. They are a gate to the past, to  a time when the stars were only just forming in the densest nodes of that dark matter web. On Earth, humanity has also woven a web, creating societies and cities that spread out  across the Earth.

This spread is not even. Civilisation built up around rivers and seas,  driven by a need to trade. Over time, the harsher terrains like the outback were abandoned for  the promise of amenities and safety in numbers.

for radio astronomers, aiming to detect the  weakest signals from space, the emptiest voids on Earth are an ideal location to set up their  experiments. Deep in the Australian outback, covering an area of 49,500 square kilometres,  the Murchison shire hosts a population of just 100 people. Here, in one of the quietest places  in the world, away from human technology and interference, radio astronomers have spent over  a decade searching for signs of The Dark Ages.

And in 2018, one of these telescopes received  a signal that was very strange indeed. This telescope is humble, simple even,  and far from the standard image that we associate with astronomy. EDGES is a radio telescope.

Short for “Experiment to Detect the  Global Era of Reionization Signature”, EDGES works like a tv or radio antenna,  receiving radio waves and converting them into a digital signal. It is a sensor, tuned  to that cosmological 21-cm line emitted from the Era of the Reionization and the First  Stars. These photons have been travelling to us from 13.

7 billion years ago, stretching,  redshifting, such that we detect these photons at wavelengths stretched from 21 cm to about 2  metres. By building a telescope to this scale, those wavelengths create a voltage across the  telescope, producing a digital signal that we can examine for its provenance and its intensity. The telescope in effect measures the temperature of the hydrogen across The Dark  Ages, retuning and reobserving at different wavelengths to scan photons  from across hundreds of millions of years.

Though in the beginning the gas is cool,  coalescing around the dark matter web, when the first stars form and begin emitting radiation of  their own, however, they heat their surroundings, imprinting a change in the 21-cm signal. Never moving, never making a sound, its human operators thousands of  miles away, EDGES listened patiently. By observing at different wavelengths, EDGES  could probe further and further back through the Dark Ages, looking for any sign of the first  stars forming, and The Epoch of Reionization beginning.

The data was untidy, the imprint of the  electronics of the telescope and radiation from all around our local Galaxy had to be modelled  and removed carefully, to see if there was a quiet whisper underneath. The team of astronomers found  they could recover a signal below all the noise: a change in temperature at approximately 180 million  years after the Big Bang - potentially the first direct observation of the Era of the First Stars,  a single point of reference: a birth certificate. This discovery alone was momentous, enough  to revolutionise a field starved of data.

But there was more. The temperature of the  gas was all wrong… the data suggested that the gas in the Universe during the Dark Ages  was far colder than any model had predicted. Every model of our Universe, developed over  decades, that fit all other observations were suddenly useless.

Astronomers were shocked,  and there was years of speculation as to the cause of this unexpected cooling. Perhaps there had been errors introduced in the data during the cleaning of the  radiation also picked up from the Galaxy? Perhaps this coolness was an illusion, and  there was a mysterious background of radiation that was leading to the gas to seem cooler?

Or perhaps, and most sensationally, perhaps dark matter could interact with the gas in  early times, cooling it down through collisions. Dark matter is famously nonreactive  and resistant to collisions, otherwise we would have detected it easily in a lab. The claim was  that in a cooler Universe, dark matter behaved differently.

This was an extraordinary idea,  and astronomers met it with healthy skepticism. The need for more extraordinary evidence  spurred the conception and production of a fleet of new telescopes around the world, all  aiming to validate the EDGES result. With time, there was more and more focus on the first, and  most mundane, explanation: the analysis was simply incomplete.

The signal was noise that had not been  correctly removed. The EDGES team had carried out their due diligence, checking all they could, it  was only with wider community help that they could try alternative observation and analysis methods  that could reproduce their own result. SARAS, an antenna floating on an Indian lake, was the first  experiment to confirm with sufficient confidence that the EDGES signal was not to be trusted:  they couldn’t find such an extreme signal.

The old models were retrieved from the  recycling bin, and calm was restored. But what of the birth date of the first  stars? Is it 180 million years after the Big Bang?

Can we trust EDGES enough on this, at  least, if not the exotic nature of dark matter? The search is far from over. The Dark Ages  signal is hundreds to thousands of times smaller than the radiation from our own Galaxy,  and the experiment is one of the toughest in astronomy.

EDGES, and SARAS, have begun that  search, testing methods and new technology, exposing problems in our theories. In that same outback, the Murchison Widefield Array has been listening for the  bubbles of the Epoch of Reionization using bowtie antennas - called “the ear that listens to  the sky” by the local indigenous people who have granted their use of the land. Soon, both are to  be joined by the most ambitious radio astronomy observatory ever built.

The Square Kilometre  Array is to comprise 130,000 antennas spread across the Murchison desert. This telescope is  so sensitive that it could hear an airport radar signal on a planet 10 light-years away, and  it can look back across the first one billion years of our Universe, into the Epoch of  Reionization and the Dark Ages. Even in one of the quietest places on Earth,  the sound of humanity and the interference of the atmosphere is too great for us to look back  into the oldest stages of the Dark Ages.

While we continue gathering data, improving our models  and methods, we also look to the next step. We look for quieter places, such as the Moon.  Testing has already begun to aid the sending of antennas to the far side of the Moon, using the  Moon itself as a shield against the onslaught of mobile phone, radio and satellite signals.

Only by seeking the quietest locations will we be able to truly know the darkest times,  finally shining a light on the Cosmic Dark Ages. And so, the universe was dark for  almost two hundred million years, but really only to our eyes. To the metal antennas in the desert, our interface with the stars, there was never  darkness, just a time of quiet industry.

The gas and dark matter worked together to form stellar  nurseries, and in the longer wavelengths the Universe was bright with 21-cm photons throughout.  But it was the bursting forth of light from the first stars that changed the Universe from  dark to light in the way that we now perceive. Stars would in turn come together to  form galaxies, long-lived and bright.

The Cosmic Dark Ages met their end.