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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 study. 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, 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 it 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 1/1000 of its current size. It expanded by a factor of 100 before the
epoch of reionization started, 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: 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 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 was emitting. This produced a slight dip in the
the cosmic microwave background spectrum. The absorbed wavelength has now been stretched
– 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 from that quasar 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 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 are no longer threatened with
absorption. Meanwhile, more energetic, shorter wavelength photons get shifted into the danger
zone – they get completely absorbed as the quasar’s light enters this region of neutral
hydrogen.
The rest of the quasar’s light continues on its way towards us, but the universe
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. Then, 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 to 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, each of which cut a narrow slice out of the spectrum.
Oh, and right next to the Lyman-alpha cutoff we see a little light that 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 ages themselves 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 for our own light-filled stelliferous
era; Probably the only habitable epoch in the past and future history of space time.
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Last time we talked about some quantum encryption - ways to secure messages using the weirdness
of quantum mechanics. One thing I learned is that you guys already know a lot about
classical encryption. You dropped a bunch of knowledge - let's see what you had to say.
Sophie Ellen and EebstertheGreat note that both prime numbers in your quantum key need
to be large numbers, not just one of them, as we stated. Doesn't help to multiply some large prime with, like, 7.
That's pretty guessable. On the other hand, who would guess that I'd do something that stupid?
It might just work.
Sophie and several others also point out that internet security isn't doomed in the wake
of quantum computers without us also having quantum cryptography. There are a number of post-quantum
computer security protocols under development that don't rely on prime factoring. Thanks
everyone for emphasizing this. Quantum cryptography may indeed not be essential, however it is
arguable that in certain cases it's the most secure possible system that doesn't include
personally meeting and sharing a key. The reason for that is that assuming the channel
can actually maintain pure quantum states, in order to break quantum crypto you need
to break the laws of physics - rather than just solve really hard math problems.
But, as others mentioned, that condition is a big one - maintaining
a quantum state over long distance is very difficult, requiring relays to reprepare and boost that signal.
Those relays are weak points, especially if the states revert to classical information while
being boosted. We'll talk about quantum relays in an upcoming episode.
In that episode we'll probably conclude that a real quantum internet is a long way off. That said,
the entire point of talking about quantum cryptography was as a way to describe some
of the crazy properties of quantum mechanics. Hopefully we managed to do that.
Tom Areani, regarding the quantum internet quips: "You might or might not have mail!"
Nice. In Schrodinger's Internet, grumpy cat both can and can't haz cheezeburger.