Cosmic Dark Ages (Script) (Patreon)

We live in the stelliferous era. Somewhere between 10 and 1000 billion trillion stars fill the observable universe with light. But there was a time before the first star ignited. A time we call the cosmic dark ages.

In astronomy we study things that are very far away. It’s a powerful challenge because even the brightest objects are almost impossibly faint when you view them from the other side of the universe. But there’s an up side. If the light from some space object took billions of years to get to us then we see that object as it was billions of years ago. In this way we can peer back in time and literally see the past in motion. In fact we’re able to see some of the first stars and galaxies to ever form.  But if we look beyond, both in distance and in time, there is …  nothing. Darkness. For the hundred million years or so between the formation of the first atom and the formation of the first star there were no light sources in the universe. These were the cosmic dark ages. It’s a period of cosmic history rarely discussed because it’s hellishly difficult to observe.  Fortunately scientists are devilishly clever. So what do we know about the time before stars?

The cosmic dark ages began with the event we call recombination. This is something we’ve talked about before, but it never gets dull, right? Prior to recombination, the universe was filled with hydrogen and helium atoms stripped of their electrons  - in other words, ionized - in the searing heat left by the Big Bang. After 400,000 years of expansion things had cooled down enough for nuclei to recapture their electrons and the first atoms formed. The universe became transparent for the first time, and we see the light freed at that moment as the cosmic microwave background. THAT is the oldest light that we see – but it would be a long time before any new light was created.

So came the cosmic dark ages – characterized by two things: the absence of new sources of light and the fog of atomic and molecular hydrogen and helium that filled the universe. Stars that formed from that gas would be the next source of light in the universe, and those stars would also burn away the remnants of that gas, ionizing the universe and beginning the epoch of reionization.  This is how we think this actually played out. I’ll come back to how on Earth we could possibly know this in a bit.

It’s believed that the first stars formed around 150 million years after recombination when tiny fluctuations in density began to collapse under their own gravity. These stars were exceptionally massive because they were unpolluted by the heavier elements of the periodic table. Those elements are actually made in stars – of which there had been none. We already talked about these first stars and why they were so big. For now just take my word for it – they were gigantic and hot – like, ultraviolet-hot. They blasted energetic UV radiation into the surrounding gas and began stripping atoms of their electrons once again. They also died quickly, and their violent supernova explosions contributed to reionization. They also scattered the first heavy elements into the surrounding pristine gas, which would itself collapse into the second generation of stars.

Primitive galaxies – proto-galaxies – formed stars at a prodigious rate, and around these galaxies bubbles of ionized plasma grew as neutral hydrogen was burned away by the growing the ultraviolet aura. By now the dark ages were well over and the universe was in the epoch of reionization. It would take a billion years for those ionized bubbles to grow, overlap, and eventually leave the universe almost fully ionized again – just as we see it today – with only tattered fragments of neutral gas drifting between the growing galaxy clusters.  All this while space was expanding. At the beginning of the dark ages the universe was around one-one-thousandth of its current size. It expanded by a factor of 100 before the epoch of reionization, and it’s expanded by a factor of 10 since then.

That’s quite a story, but how do we know any of this? We do see a handful of primitive galaxies shining out from this time, including one from right near the beginning of reionization when the universe was only 400 million years old. But our best evidence isn’t from the light that reaches us, but rather it’s from the light that never makes it.

The thick neutral hydrogen of the early universe was mostly transparent, but it did block some very particular types of light. Any photon whose energy happened to exactly match an electron energy transition in the hydrogen atom was in danger of being absorbed. Two specific photons were in particular danger of being absorbed: in one case that absorption signaled the end of the dark ages and the second the end of the epoch of reionization.

We’ve actually already talked about the first, so this is just the tl;dr. When the electron in cold hydrogen gas flips its spin direction it either absorbs or emits a radio photon with a wavelength of 21cm. When the first stars ignited they heated the surrounding gas, which caused it to absorb more 21cm radiation than it emitted. This slightly reduced the amount of CMB light at that wavelength. The absorbed wavelength has now been stretched in wavelength – redshifted – by the expanding universe. The amount of that redshift tells us the when the very first stars formed – because it was those stars that enabled this absorption in the first place. They formed around 150 million years after the Big Bang, ending the dark ages.

A different photon tells us about the progress of the epoch of reionization. But before we get to that, I want to take a moment to talk about quasars. It’s relevant, trust me. Not that I should need a reason to talk about quasars.

Remember that first generation of giant stars? They did more than kickstart reionization and produce the first elements. Stars that large and luminous burn out fast and leave behind black holes. These ravenous stellar corpses found themselves in an all-you-can-eat buffet of the gas-rich proto-galaxies. They fed, they merged with each other, they grew. So were born the first supermassive black holes with millions, even billions of times the Sun’s mass – inescapable spheres the size of solar systems. And in the final stage of this feeding frenzy, surrounded by vortices of superheated plasma, these black holes powered the first quasars. They shone with the brightness of hundreds of trillions of Suns, yet they are barely detectable by the best of our telescopes. But we do see them – the faintest red dots in our most sensitive surveys.  As the light of those most distant quasars traveled to us it passed through the last remnants of neutral hydrogen left over from the cosmic dark ages. That gas left its mark.

Back to the second photon of interest. It’s the Lyman-alpha photon – one with a wavelength of exactly 121.57 nanometers. That’s a hard ultraviolet photon that can be absorbed or emitted when an electron jumps between the ground and second electron orbitals of hydrogen. Neutral hydrogen gas is hungry for Lyman-alpha, gobbling up any such photon that it encounters.

So here’s the scenario – a quasar shines out from the epoch of reionization. Lyman-alpha photons can travel a short distance because the quasar has itself ionized a bubble spanning several million light years.  By the time the quasar’s light has reaches the edge of that bubble, the universe has expanded slightly. Photons that were once at the Lyman-alpha wavelength have been redshifted to longer wavelength and were no longer threatened with absorption. Meanwhile, more energetic, shorter wavelength photons get shifted into the danger zone – they got completely absorbed as they quasar’s light entered this region of neutral hydrogen.

The rest of the quasar’s light continues on its way towards us, but the universe also keeps expanding. Wavelength by wavelength, photons get absorbed as they are shifted into the danger zone of Lyman-alpha until much of the quasar’s most energetic ultraviolet photons are absorbed. That happens until the epoch of reionization ends – with electrons detached once more from their atoms, there can be no lyman-alpha transitions. The rest of the quasar’s UV light continues to be shifted into lyman-alpha. But now it is mostly safe from absorption. Now and then that light would pass through a tattered remnant of neutral hydrogen, and whatever photons were unlucky enough to be redshifted into the doomed wavelength at that time would be absorbed.

This is the spectrum of a quasar from the epoch of reionization. All of this light comes from the material falling into the black hole, or being blasted back out again. This is the redshifted Lyman-alpha wavelength – once hard-ultraviolet, but now infrared. Everything to the left of this point was once even more energetic UV, but now it’s gone – it was redshifted through the deadly Lyman-alpha wavelength on its way to us. This is the so-called Gunn-Peterson trough, seen only in quasars that are embedded in the early neutral universe.. This end of the trough is where reionization ended, so that photons to the left of it could potentially reach us. The jagged region is the Lyman-alpha forest, where the quasar light passed through individual clouds of neutral gas, which cut narrow slices out of the spectrum. Oh, and right next to the Lyman-alpha cutoff we see a little light made it through due to the ionized bubble around the quasar itself.

These absorption signatures are probably our most powerful probe of how the epoch of reionization evolved and ended. The width of the Gunn-Peterson trough tells us when the universe finally became fully ionized. And the scant Lyman-alpha light that made it through due to the quasar’s ionization bubble can tell us how much neutral hydrogen was left at the time this quasar was shining.  It lets us track the progress of reionization and even figure out when it must have started.

So, light absorbed from the cosmic background radiation tells us when the dark ages ended, and light absorbed from the first quasars tracks the last phases of the subsequent reionization. The next step will be to peer deeper into the cosmic dark age itself using new generations of extremely sensitive radio telescope to catch more of those elusive 21cm photons. In the meantime, let’s take a moment to be grateful our own light-filled stelliferous era; Probably the only habitable epoch in the past and future history of space time.