The Cosmic Dark Ages

Happy 2021! A new year dawns, as tends to occur every January. Opportunities beckon, fresh challenges present themselves, and, perhaps, hope dares to appear after the nightmare of 2020… it is certainly quite the time to be alive.

Anyway, on to the post! One thing that you’ll find to be conspicuously absent on this site is any discussion of cosmology, or really anything further out than the neighboring star systems. Sure, it’s an aspect of space science like any other, but historically it hasn’t interested me nearly as much—you can’t stand on a galaxy or a star, and many of the universe’s largest denizens, like supermassive black holes, are completely unlike anything humans have ever encountered here on our own planet. The large-scale science of the cosmos is just so huge and so far removed that I have paid little attention to it… until now.

What’s new is that I’m taking an online course on cosmology, through EdX, where noted physicists (including a Nobel Prize winner, Brian Schmidt) lead an exploration of the greatest unsolved mysteries of the universe. The material is surprisingly fascinating and illuminates an area I haven’t studied nearly as much as, say, planetary science. Learning about the early universe is truly a joy, expanding my horizons and perhaps providing fodder for a blog post or two—including this one. Today, I will be discussing the little-known Cosmic Dark Ages.

The Cosmic Dark Ages were, of course, the time when the Cosmic Empire crumbled under its own weight, triggering civil wars, genocides of entire species, and invasions by extra-cosmic outsiders who laid waste to the glittering cities of primordial civilization…

It was pretty much this.

Well, that would be cool, but in fact the Cosmic Dark Ages were just a time when the universe was literally dark. There was, in fact, no light anywhere, unless you count the microwave background left over from earlier epochs. In the visible-light spectrum it would have looked like nothing at all, a field of black populated by wandering neutral hydrogen and helium atoms. This period is believed to have begun about 370,000 years after the Big Bang; exactly when it ended, however, is a matter of ongoing research, and in any case it was a gradual process, with stars and later galaxies forming out of the swirling clouds of dust that filled the universe. Recent research suggests that the first galaxies were organizing themselves as early as 380 million years after the beginning of time, with the process of lighting up the universe continuing until it was basically complete at the billion-year mark.

An artist’s concept of the first generation of stars to ever exist… or, perhaps, footage from a secret time-traveling probe. Image is in the public domain.

The Dark Ages also mark an intermediate era between two periods of widespread ionization. During the first period, ending around 370,000 years in, the universe was very warm and very dense. Hydrogen and a limited amount of other light elements predominated; the non-hydrogen elements, such as helium, had been created between two and twenty minutes after the Big Bang, when the entirety of space was so insanely dense that it functioned as one enormous fusion reactor. Pretty metal, if you ask me. Everywhere you could possibly go you’d encounter conditions like the center of the Sun.

Then, the rapid evolution of the universe got a little less exciting—as space expanded, matter cooled and diffused to the point where fusion could no longer take place. The clouds of hydrogen and helium gas remained ionized for ages afterwards. Electrons, too, floated freely through the primordial soup. Their presence interfered with photons and prevented them from traveling very far in straight lines, effectively making the whole universe an opaque fog beyond which no telescope could ever see. This fog can be seen today as the Cosmic Microwave Background.

The CMB! It was discovered by enormous microwave telescopes back in the 1960s. It’s not quite as heterogenous as the image suggests, with a span of only 0.4 Kelvin between the red portions and the dark blue areas.

The fog dissipated when the universe’s hydrogen and helium cooled enough to regain electrons, forming neutral atoms in place of charged ions—an event known as recombination1. At the same time, the newly formed atoms released photons as they settled into lower energy states. This would be (mostly, see below) the last light for quite some time. There were, after all, no stars to speak of—there was only homogeneous gas, slowly drifting into clumps that would eventually become the objects we know and love today.

Thus commenced the Cosmic Dark Ages. It was a quiet interlude after the intensity of the Big Bang and the tempestuous years afterwards; unfortunately, we cannot observe anything that happened during this time, since in our telescopes it appears only as a transparent void beyond which we see the CMB. There is just nothing to see—with one exception. A quirk of quantum mechanics causes neutral hydrogen to very, very faintly emit photons at a wavelength of 21.106 centimeters, in the microwave portion of the electromagnetic spectrum, and with great difficulty2 researchers can detect this radiation, shedding some light on the Cosmic Dark Ages and their eventual end.

21-centimeter radiation is caused when an electron orbiting a neutral hydrogen atom reverses its spin, causing a slight change in energy levels that releases a photon with a very specific wavelength and frequency. No, I don’t really understand the quantum physics behind it—who does, besides actual quantum physicists? It is an eldritch craft, a counterintuitive, maddening vortex into which few ever peer and return sane…

Who knows? Maybe the Cosmic Dark Ages were actually a time when Cthulhu, Azathoth, Yog Sothoth, and the like roamed freely around the universe, and scientists have been covering it up to prevent the general collapse of human civilization into chaos.

What Hubble really sees when it looks into deep time… Attribution: Dominique Signoret (, CC BY-SA 3.0, via Wikimedia Commons

So, the Cosmic Dark ages saw the cooling of the universe from a whopping 4000 K to 60 K. Towards the end—perhaps as early as 180 million years—the first stars formed, extraordinarily metal-scarce and also hundreds of times larger than our Sun. They were short-lived and there are certainly none of them around today, though telescope observation of distant ones remains in the cards. Galaxies came into being between 300 million to 700 million years after the Big Bang. As the universe lit up again, ultraviolet radiation from these emerging structures ionized the intergalactic medium, creating the diffuse plasma we see today. We should be able to observe this process using 21-centimeter radiation, as expanding “bubbles” in which such radiation was no longer being produced.

About a billion years after the Big Bang, the universe more or less took its current shape, with stars forming galaxies that perpetually hurtled away from each other. The Dark Ages had come to a close, bringing a new and visually much more interesting era. Then, over billions and billions of years, the processes of star and planet formation churned on until they produced a yellow dwarf and a rocky planet orbiting it, soon to be inhabited by a peculiar species of hairless primates…

Thank you for reading! This was certainly a fun romp through deep time and the ultimate origins of the universe, and I hope you had fun reading it, too. I apologize for any scientific errors I may have made. I’m sure there are some—while I tried to stay true to Wikipedia’s take on the current research, all this material is very new and foreign to me, unlike, say, orbital mechanics, which I can grasp more intuitively.

I’ll see you all next week!

  1. Technically just combination, since the electrons and protons had never actually been combined before. But “recombination” is the accepted term.
  2. The main problem is that 21-centimeter radiation is hard to distinguish from the radio signals our civilizations produces in vast quantities. Already, scientists have built a telescope in the middle of the Australian desert to avoid this, and there are proposals to build another such observatory on the far side of the Moon, completely secluded from Earth interference.

Further reading:

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