(This post is adapted from a longer, more rambling, and somewhat more technical post I wrote for a group blog, here.)
There have been a few truly transitional moments in the history of the Universe in which something fundamental about the cosmic environment changed. Some of these -- the beginning and end of cosmic inflation, reheating, big bang nucleosynthesis -- altered the very nature of spacetime or the kinds of particles that populated it, and all happened within the first few minutes. The first atoms formed a few hundred thousand years later, marking another milestone. For the 13 billion or so years since then, though, you could argue that it's all been a bit samey. Except for cosmic reionization.
Cosmic reionization can be explained in just a few words: the gas in the Universe went from being mostly neutral to mostly ionized. That might sound trivial, but it turns out that the implications are profound -- reionization is the reason we are able to see other galaxies billions of light-years away, and if we can understand how it happened, we will understand the formation of the very first stars and galaxies in the Universe.
But I should back up for a moment. In order to see why reionization matters, you need to know something about recombination and the cosmic dark ages.
Timeline of the Universe, showing recombination, the dark ages (not even labelled because that epoch just isn't interesting enough, apparently), reionization and the age of galaxies. Source. Credit: Bryan Christie Design)
Great ball of (primordial) fire
Recombination is probably the most inaccurately named event in the history of the Universe, on account of the fact that there was no "combination" before it.* In the beginning, there was the all-encompassing energy-matter-plasma-fireball, the product of the first cosmic explosion, which rapidly expanded in all directions. We sometimes refer to this as the "hot big bang." This fireball was formed mainly of protons and electrons, all of which were hot and unbound and bouncing off photons and generally being really energetic. (Charged particles that aren't bound together are called ions and ionized gas is called plasma, so you could call it a plasmaball instead if you like, but I'll stick with "fireball" for the purpose of dramatic imagery.) In the fireball, the particles and photons were tightly coupled, meaning that they were all mixed up and interacting in a big indistinguishable mess. But as spacetime expanded and the fireball got cooler, the particles lost some of their frenetic energy. Eventually, there came a time when the fireball was cool and diffuse enough that protons and electrons could chill out and become bound atoms. The photons were still there, but now instead of just ricocheting off ions, they could get absorbed by atoms, or sail right by them in the newly abundant spaces between. Some photons still occasionally broke atoms apart, but the Universe was becoming diffuse enough that atoms spent more time bound than not.
Illustration of the transition between the cosmic fireball and the post-recombination Universe. Red spheres are protons, green spheres are neutrons, blue dots are electrons and yellow smudges are photons. The color bar on the bottom represents the average temperature (or energy) of the Universe at that epoch. Source.
(*Terminology note: "Recombination" is also sort of technical term in physics, which in general refers to the joining of an electron and a proton, without regard to whether that particular electron and proton had made up an atom before. In the very early universe, inside the cosmic fireball, hydrogen atoms would sometimes form, but they'd be broken up immediately by energetic photons. The name "recombination," when talking about the epoch, refers to the time when the hydrogen atoms that formed could stay bound for an appreciable amount of time.)
And so, at the epoch of recombination, around 300,000 years after the big bang, the gas went from being ionized to neutral. Recombination set off the decoupling era -- the time when the matter and radiation that were previously tightly coupled (i.e., interacting a lot) became more free to do their own thing. Decoupling is also known as last-scattering, because it was the last moment when photons would immediately be scattered off matter as they flew around. After decoupling, the photons were free to sail around unimpeded and travel for long distances. Which is where the cosmic microwave background (CMB) comes from -- the newly decoupled photons free-streaming through the Universe out of the great primordial fireball.
Map of the cosmic microwave background, the radiation leftover from the primordial cosmic fireball. Tiny fluctuations in the temperature of microwave radiation coming to us from all directions give us clues about how matter was distributed at the earliest times in the Universe. In this rendering, we would be at a tiny dot in the center of the sphere. Source.
And then we wait
The next phase of the Universe was, in many ways, distinctly unexciting. It's called the dark ages. During the dark ages, the Universe was full of cooling neutral gas (mostly hydrogen), and that gas was very very slowly coming together into clumps via gravity. At decoupling, the fluctuations seeding these clumps were more dense than their surroundings by only about one part in 100,000. Those tiny blips, which we see in the CMB, were enough to tip the scales of gravity to draw more matter together into bigger and bigger clumps. But it took a while for anything particularly interesting to happen. Sometime between 100 and 500 million years after the big bang, one of these little clumps became dense enough to form the first star, and that defined the "first light" of the universe. (Of course it wasn't strictly the first light -- the fireball made plenty of light, and we still see it as the CMB -- but it was the first starlight.)
So, if we had a big enough telescope, could we look far enough back into the Universe to see that first star?
Unfortunately, no. It turns out the dark ages were dark for two reasons. One was that there wasn't any (visible) light being produced at the time. The other is that neutral hydrogen is actually pretty opaque to starlight.
Atoms and molecules can only absorb photons at particular frequencies -- those corresponding to transitions between the energy levels of the electrons. During the dark ages, any photon whose energy was in the sweet spot for a hydrogen atom transition would very likely be absorbed. Radio waves or other low-energy photons could get through because there weren't any transitions of the right energies to take them, but visible light was another story. It's easy for a hydrogen atom to absorb visible-light photons and use them to knock its electrons into higher energy levels (the same goes for ultra-violet light). Those atoms release the photons again eventually, but in different directions, so the vast majority of the light produced by the first stars isn't able to make it all the way to our telescopes.
Opacity and transparency. The primordial fireball was opaque like fire is opaque: energetic particles couple with photons and keep them from free-streaming away. The edge of the wall of flame is like the last-scattering surface, where the light is finally free to escape. The dark ages were opaque like fog is opaque: the light was absorbed and scattered and attenuated. Once reionization cleared the "fog" of of the dark ages, light was able to travel unimpeded. Photo sources: here, here and here (but really here).
Here comes the sun(s)...
Once stars were forming in earnest, though, astrophysics really got going, and fun things started to happen. The vast majority of the gas in the Universe (called the intergalactic medium, or IGM) was still neutral at this point -- mostly hydrogen, not doing much -- but each star or galaxy that formed would heat the gas around it and make a bubble of ionized gas. As more and more of these bubbles formed, the intergalactic medium had a sort of swiss-cheese nature, with bubbles of ionized gas growing and coming together, burning away the fog of the dark ages.
Once there were enough stars and galaxies to ionize a significant fraction of the IGM, we finally had reionization: the (aptly named) epoch when the Universe went from being neutral to being fully ionized again. And this time, the universe was much less dense and the starlight could easily pass through the ionized gas, so the IGM became transparent to starlight. And that's why we can see other galaxies -- because there's very little neutral gas left to absorb the light en route.
Artist's conception of bubbles of ionized gas percolating through the IGM during the dark ages. The CMB is at the far left, and the right is the present-day Universe. Source: illustration from a Scientific American article by Avi Loeb, which can be found here.
When did reionization happen? And why does it matter? Second question first: it matters because understanding reionization means understanding how the first sources of light in the Universe formed and how the IGM turned into the galaxies and clusters and all the amazing stuff we see today. Also, it's a major milestone in the Universe's history, and a phase transition of the entire IGM, so it seems important.
Back to the other question: we think reionization happened around a billion years after the big bang, though probably gradually and clumpily and at different times in different places, and we're still trying to pin down the exact epoch. There are a few ways to go about figuring it out. One is to look for the Universe not being transparent. In astronomy, opacity usually manifests as something absorbing light from something behind it. On a foggy day, you know the fog is there because it makes it hard to see things that are far away, not because you really see the water droplets in the air. Reionization is similar -- you know you're getting close to it if some of the light from a distant source (a quasar, generally) is absorbed before it gets to you.
Unfortunately, looking at absorption only tells us roughly when reionization was pretty much over, since it doesn't take much neutral hydrogen (about one part in 100,000) to absorb all the light from a distant quasar.
Another way to pin down reionization is to look at some subtle effects it has on the CMB, but that would take another blog post to even begin to describe, so I'll just say the CMB gives us a pretty good idea of the earliest reionization might have started, but it's hard to get much more than that.
So where does that leave us? We can't use visible light, because that's absorbed as soon as the IGM is slightly neutral. And the CMB tells us a lot about the early universe, and gives us a hint about the beginning of reionization, but doesn't tell us when the bulk of it happened.
Radio astronomy FTW
The big innovation, the thing that institutions all over the world are investing in, is looking for radio signals coming from the neutral hydrogen itself. Neutral hydrogen has a low-energy transition that, when it occurs, emits or absorbs a photon with a wavelength of 21 cm: it's called the 21 cm line. (The frequency is about 1420 MHz.) This wavelength puts it in the radio part of the electromagnetic spectrum, so we see it as radio waves.
The origin of 21 cm radiation. In the higher-energy state, the hydrogen atom's electron and proton are aligned. If one flips its spin, the atom is in a lower-energy state and a 21 cm photon is produced. Source.
The reason 21 cm radiation can let us peer into the dark ages is twofold. One, it's so low-energy that it doesn't take a lot to excite it, so you can get 21 cm radiation being produced even if there's not a heck of a lot going on (just atoms colliding and a few stray photons). The other advantage is that radio waves are really hard for neutral hydrogen to absorb. An atom creates a 21 cm photon in the dark ages, and then the universe expands a little, making that photon just a little longer in wavelength, and then it's too low-energy to be absorbed by anything. So all we have to do is set up a radio telescope and wait for it to arrive here!
Is it here yet? (Photo by Mike Dodds)
Okay, so it's not quite that simple. Because the photons stretch out as the Universe expands, we're really talking about something like 100-200 MHz for "21 cm" photons from the epoch of reionization and the end of the dark ages. There are some major downsides to working at those frequencies. One is that you're now smack in the middle of all sorts of terrestrial radio communication: FM radio, cell phones, satellite transmissions... it's a big mess. Also, at low frequencies, the Earth's ionosphere is highly refractive and can do all sorts of horrible things to your signals as they're coming down from space. Somewhere in the tens of MHz, the ionosphere is completely opaque. So if you want to pick up 21 cm radiation from the epoch of reionization, you have to find a place that's relatively radio-quiet (i.e., unpopulated) to do this sort of study, or you have to find a way to deal with the radio noise. (One example of a relatively radio-quiet place is the Australian outback. Another is the Moon.)
A major challenge that you definitely can't get away from is our own Galaxy. The Galaxy produces a lot of radiation which is extremely bright at the low frequencies we're dealing with here. Galactic radio signals are typically about 10,000 brighter than the signal from reionization. And it doesn't help that the radiation is spatially varying in weird and complex ways. Here's a map of the Galactic radiation at 408 MHz. It's pretty bright, and it gets worse for lower frequencies.
Galactic synchrotron radiation at 408 MHz -- the emission is stronger at lower wavelengths. The color scale here gives the brightness temperature (a measure of the intensity of the signal) in Kelvins. For comparison, the 21 cm reionization signal would be around 10 mK. Source.
In spite of the challenges, there's a lot of effort right now going into building the telescopes to see this signal, because it would allow us to actually probe the IGM in the epoch of reionization. Ideally, we'd get pictures like this:
Simulation of reionization. Source.
Each square represents a bit of the Universe at a different moment in cosmic history, going forward in time as you move left to right and top to bottom. In the upper left-hand panel (0.4 billion years after the big bang), the IGM is largely neutral. In the lower-right hand panel (0.8 billion years), it's ionized. The features in the other panels are ionized bubbles forming and growing. Each of these simulation panels represents just a small patch of sky, but in theory you can imagine doing a full-sky map. Taking into account the expansion of the Universe (and consequent stretching of photons) and tuning the telescope to different frequencies, you ideally get a map of all the neutral hydrogen at each epoch.
I should also point out: the dark ages and epoch of reionization cover a lot of the observable Universe. This sketch shows roughly how much volume is covered by different kinds of observations, where we're in the middle, looking out. The z values are redshift -- a measure of how much the Universe has expanded since that time. (So the edge, the farthest away in space and time, is at a redshift of infinity, since the Universe is infinitely bigger now than at the big bang; the redshift today is zero. Reionization was between redshifts 6 and 10 or so.) In the diagram, the colorful part in the middle contains most of the galaxies we've seen directly. The thick dark circle near the edge is the CMB. Everything inwards of z=50 can be probed with 21cm observations, and almost everything outwards of z=6 can't be seen any other way.
Schematic of how much of the Universe we're seeing with different kinds of observations. Red, yellow and green are optical. The black circle around the edge is the CMB. Everything in blue can be observed with 21 cm radio signals. Source: Tegmark & Zaldarriaga 2009.
If you build it...
There's something of a global competition/collaboration going on right now to try to get at this signal, because it would open a whole new window on the evolution of the Universe. You may have heard of the Square Kilometer Array, which is going to be the world's largest array of radio telescopes when it's completed in a decade or so. It'll be split between South Africa and Western Australia, and one of the key goals of the project is to look deeper into the epoch of reionization than we ever have before, using the 21 cm line. In the meantime, there's the Low-Frequency Array (LOFAR), the Murchison Widefield Array (MWA), and lots of other projects that are just getting going. It's a big industry.
But before we get too excited, I should reiterate that dealing with the foregrounds and instrumental calibration and stuff is hard. There are actually a number of intermediate steps (including getting an all-sky average signal, or doing some kind of statistical detection) that would have to happen before any attempt at mapping (or "tomography"). But mapping remains the ultimate goal. And if we can map out what the neutral hydrogen in the Universe was doing in the first couple of billions of years, we can basically watch the Universe as we know it come into being. And that would be pretty darn cool.
Credit: SPDO/TDP/DRAO/Swinburne Astronomy Productions.