200 most important Astronomy topics - Sykalo Eugen 2023

The Hydrogen Epoch of Reionization Array (HERA)

The Sky Before Stars

Imagine standing alone in the Sahara Desert at midnight. No city lights. No moon. Just the horizon swallowing the stars above. Now take away the stars. Take away the moon. Take away everything but the dark.

That's the early Universe. Not just dark — opaque. What cosmologists call the “Cosmic Dark Ages.” A time after the Big Bang, some 380,000 years in, when the Universe had cooled enough for protons and electrons to form neutral hydrogen atoms — the fog of existence, stretched across the void like a cosmic amniotic fluid. Light couldn’t travel freely yet. There were no stars to shine.

We talk a lot about the birth of the Universe, but rarely about its childhood. What kind of Universe was it before the first stars blinked on? Before galaxies began to coalesce, like dew forming on a spiderweb?

This is the question the Hydrogen Epoch of Reionization Array (HERA) is trying to answer. It doesn’t look for light. It listens for whispers. Radio whispers. Echoes from a time before time knew how to tick.

Reionization: When the Lights Came On

The hydrogen fog didn’t last forever. Gravity, that patient sculptor, began pulling matter into clumps. The first stars — huge, hot, short-lived titans — were born. And with their birth came light, radiation powerful enough to ionize the surrounding hydrogen gas.

This period, called the Epoch of Reionization, was a cosmic revolution. Neutral hydrogen was ripped apart by ultraviolet light. Photons broke the bonds of atoms, returning them to protons and electrons, turning the opaque cosmos into a transparent one. The sky became permeable. Visible.

Think of it as the Universe switching on its own headlights.

But here’s the cosmic irony: we can’t see the Epoch of Reionization. Optical telescopes like Hubble, or even the majestic James Webb, struggle to penetrate this era. The light is too redshifted. Too faint. It's like trying to see a candle through a fog that's a billion years thick.

So instead of looking for light, scientists listen for the hydrogen.

That’s where HERA comes in.

A Telescope Made of Ears

HERA doesn’t look like a telescope. It’s not a dish. It has no lens. Instead, it sprawls across the arid Karoo desert in South Africa, a grid of strange, spidery antennas: 350 radio dishes, each 14 meters wide, each one tuned like a harp string to a specific frequency of ancient hydrogen.

Imagine you’re walking among them. The sun is high, but the instruments point downward, listening for radio signals bouncing off the Earth’s atmosphere. The air buzzes with heat, but the detectors remain still, calm, disciplined.

They are trying to detect a signal so faint, so drowned in cosmic noise, it's been compared to finding a whisper in the middle of a rock concert.

This signal is the 21-centimeter line — the wavelength emitted by neutral hydrogen when the spin of its lone electron flips. It's not luminous. It's not beautiful in the conventional sense. But it’s a Rosetta Stone for the early Universe.

The deeper HERA listens, the further back in time it hears.

The Challenges of Hearing the Past

Now, don’t be fooled. This isn't easy. The signal HERA is searching for is about one part in 100,000 of the radio noise raining down on Earth from everything else — galaxies, quasars, cell towers, satellites, even the Milky Way.

Filtering that out requires absurd precision. Calibration becomes an art form. Scientists spend years modeling the instrument’s own reflections, tweaking, refining, compensating for every curve and every wire. One HERA team member once compared it to "trying to hear a pin drop while your neighbor is vacuuming and playing Led Zeppelin."

Still, they persist. Because if successful, HERA will map the 3D structure of the Universe not through light, but through hydrogen.

That’s not just cool. It’s revolutionary.

Why HERA Matters More Than You Think

"So what?" you might ask. "Why do we care about some ancient hydrogen?"

Because this isn’t just a scientific footnote. Understanding reionization tells us about the formation of the first galaxies, about the distribution of matter, about the nature of dark matter and dark energy.

Think of it like trying to understand a novel by reading only the final chapter. You'd miss the setup, the development, the struggle. The drama of the cosmos lies in its first acts.

HERA gives us those missing pages.

And more than that — it gives us perspective.

The Universe wasn’t always this intricate ballet of galaxies and stars. It began as a blank canvas, and the Epoch of Reionization was the first brushstroke.

A Personal Reflection: Listening to Ghosts

I remember the first time I saw HERA in person. Not on a screen. Not in a journal. But standing there, in the desert.

There was silence, but not stillness. The dishes were alive, straining toward nothingness. I felt as though I were standing at the edge of a cosmic graveyard, listening for the ghosts of stars that died before Earth even existed.

It made me realize: science is not just about answers. It’s about asking the right questions. And one of the most profound questions we can ask is:

What was the Universe like before we were here to see it?

HERA may not give us the full picture. But it gives us a map. And maps are where exploration begins.

We Are the Eavesdroppers

Today, as you read this, HERA is listening. Not to today. Not even to yesterday. But to 13 billion years ago.

That boggles the mind.

We are eavesdropping on the cosmic dawn. On the moment the Universe turned from dark to light. On the first chapters of everything.

Isn't that worth listening for?

Maybe somewhere, out there in that ancient hydrogen haze, the echoes of our own atoms are humming, waiting for us to remember where we came from.