Because light has a finite speed limit, the farther away you look, the deeper into the past you see.

With radio telescopes we can even see the universe as it was just after the big bang, nearly 13.8 billion years ago.

But after that early snapshot, there’s about a billion years we haven’t been able to see clearly.

Cosmologists call this the dark ages,

and probing into this hidden era of the universe can tell us more about how it took shape,

and even the nature of dark matter.

First, let's take it back.

Way, way back to almost the very beginning, about 380,000 years after the big bang

kicked off this whole thing we call existence.

That’s the point when the free protons and electrons were cool enough to start combining

into the simplest and most abundant of atoms, hydrogen.

That’s when the universe goes dark.

Most hydrogen doesn’t emit light in the vast majority of the electromagnetic spectrum.

That’s not to say it’s totally invisible,

it does give off and absorb electromagnetic radiation when it’s electron switches between two states,

but at the relatively long wavelength of slightly over 21 centimeters.

This means that if hydrogen gives off or absorbs radiation,

it should be detectable, what’s known as the hydrogen line.

But plucking out the signal of ancient hydrogen

from the rest of the radiation that we constantly get bombarded with, is tricky.

Since it’s traveled for such a long time to reach us,

the space it’s moved through has itself stretched out quite a bit,

which in turn elongates the wavelength from about 21 centimeters, to one and a half to 20 meters.

The longer the wavelength, the older the signal.

And the deeper the redshift, the weaker the signal becomes,

until its easily masked by noise from the Milky Way,

or human activity like radio broadcasts, or even spark plugs in cars.

Thankfully, the tools cosmologists use to see the history of the universe are getting better all the time.

They’re setting up radio telescopes and antennae around the world,

sometimes in remote locations to avoid human interference,

like an island between South Africa and Antarctica, or on a lake in the Tibetan Plateau.

New technology is making the enormous amounts of data these observatories churn out easier to analyze.

Now scientists think they can finally start sussing out what happened in the dark ages.

There are three periods when hydrogen absorbed or emitted energy that should be discernable.

The first occurred 5 million years after the big bang,

when hydrogen cooled enough to absorb some of the background radiation,

causing a dip in the hydrogen line known as the dark-ages trough.

About 200 million years later, the first stars and galaxies formed,

giving off ultraviolet radiation that made the hydrogen more readily absorb 21-cm photons.

This should appear as a second, more pronounced dip at a shorter wavelength.

Finally, the third major event that affected hydrogen is called the Era of Reionization, or EOR.

Around the universe’s 500 millionth birthday,

the UV radiation from stars and galaxies would have grown so bright

that it would cause hydrogen to fluoresce, emitting 21 cm waves.

But the hydrogen that was too close to the galaxies was bombarded with so much radiation,

that it was stripped of their electrons altogether.

Separated again, the now free protons and electrons would go dark,

so a snapshot of this time period would have ionized bubbles of darkness with brighter radiation in between.

The EOR is the time period most experiments are investigating.

One group of researchers in Australia recently announced that by using a new technique

to process data from a collection of over 4000 antennae called the Murchison Widefield Array,

they were able to generate a 10-fold improvement in their results,

helping them hone in on when the EOR began.

And just last year, an antenna in the Australian outback called EDGES

may have seen the first glimpses of the ionized hydrogen bubbles around the first stars.

The goal is to make a 3D map of these bubbles,

and by examining different wavelengths we’ll be able to see exactly how the universe grew and evolved.

Neat uniform bubbles will tell us that early stars were responsible for the reionization,

while wispy, freeform bubbles would suggest the presence of black holes.

Signals from the EOR could even give us a clue about how to look for dark matter,

indicating whether it’s made up of sluggish and cold particles, or warm ones that are lighter and faster.

Before any of this can happen though,

we need to start finding that 21 cm wavelength in the data we’re collecting.

Once more arrays are online and more data is parsed,

we might be able to shed some light on the universe’s dark ages and the very first days of our galaxy.

Another instrument that will search for the universe’s first days is the James Webb Telescope,

slated to launch in the early 2020s.

Here's a How Close Are We episode on the long awaited telescope.

Is there another astronomical phenomenon that you’d like us to cover?

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