Neutral Hydrogen: The Next Big Game in Cosmology

The majority of the universe remains unmapped, but we have a potential window into it through a peculiar light emitted by nothing other than neutral hydrogen.

Before stars and galaxies lit up the universe, the cosmos was a dark place filled mostly with neutral hydrogen. This was right after the Big Bang and the formation of the CMB—Cosmic Microwave Background. The CMB is like a baby picture of the universe when it was just 380,000 years old. But what came next was a long period called the “Dark Ages.” During this time, the universe didn’t have much going on in terms of visible light because there were no stars or galaxies yet. Frustratingly, most of the volume of the visible universe exists in these Dark Ages, which makes it a very valuable resource to learn about the nature of dark matter and dark energy. But…it was dark, so we can’t just make a bigger telescope and observe it.

Thankfully, the neutral hydrogen that filled the universe during this epoch does emit a feeble kind of light. Due to the quantum mechanical spin flip transition, neutral hydrogen emits radiations with a wavelength of 21 centimeters. However, the Dark Ages were so long ago at this 21cm radiation is redshifted to a wavelength of two meters or more, putting it firmly in the radio band of the electromagnetic spectrum.

In fact, a tiny fraction of the static you hear in your car radio is due to this ancient radiation.

Astronomers can use slightly different wavelengths to map out the extent and evolution of the Dark Ages. Different pockets of neutral gas will emit their radiation at different times, which will correspond to different redshifts.

We expect to see an enormous amount of 21cm radiation at the very longest wavelengths, right at the beginning of the Dark Ages. That’s when the universe was filled with an almost uniform distribution of neutral hydrogen. Then as the first stars and galaxies wake up, they ionize their surrounding gas with powerful blasts of high-energy radiation. So a 21cm map of this era should show holes and pockets in the overall signal. Finally, once most of the neutral hydrogen is wiped away and confined only to cool regions of galaxies, we should see the signal disappear – only to be replaced with the light of galaxies themselves.

However, observing this radiation is a daunting task. That’s because humans are also quite fond of radio emissions, and this signal from the Dark Ages is at least a million times weaker than terrestrial radio broadcasts. Observatories around the world, like the Murchison Wide-field Array in Western Australia and the Hydrogen Epoch of Reionization Array in South Africa have so far failed to find a conclusive signal.

To nail this detection and open up the Dark Ages to exploration, we may have to go off planet. The Lunar Crater Radio Telescope hopes to turn the far side of the Moon into a pristine radio observatory, using the Moon itself to shield the observatory from radio interference. The idea is a long way off, but it might be our only way to to draw a complete map of the cosmos’ past, present, and future.

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