The Cosmic Microwave Background Radiation tells us so much about the Universe. After that era, the Universe went dark. Then, as gas pulled together into the first stars and eventually galaxies, light returned, beginning the Age of Reionization.
241st AAS Meeting (AAS)
What is the Cosmic Microwave Background? (Universe Today)
Epoch of Reionisation (MWA Telescope)
What Is the Big Bang? (NASA Space Place)
What are photons? (Live Science)
How the Cosmic Dark Ages Snuffed Out All Light (Quanta Magazine)
Energy Levels of Electrons (SDSS)
Ionization (Energy Education)
Dwarf Galaxy (ESA/Hubble)
The Pillars of Creation (NASA)
Spitzer Space Telescope views cosmic bubbles in infrared (BBC Sky at Night)
What is ‘red shift’? (ESA)
What is Gravitational Lensing? (CFHTLens)
Transcriptions provided by GMR Transcription Services
Fraser: AstronomyCast, Episode 665, “The Age of Reionization.” Welcome to AstronomyCast, your weekly facts-based journey through the cosmos, where we help you to understand not only what we know, but how we know what we know. I’m Fraser Cain, publisher of Universe Today. With me is Dr. Pamela Gay, a senior scientist for the Planetary Science Institute and the director of Cosmo Quest. Hey, Pamela, how you doing?
Dr. Gay: I’m doing well. Right now, the American Astronomical Society meeting is taking place in Seattle, and I was watching the beginning of a press conference earlier before we went live, and there is so much JWST science that’s going to come out this week. It’s tasty, awesome goodness.
Fraser: Yeah. I got up early, got as much of my news organized and out of the way because I just needed to be prepared for the ongoing tidal wave –
Dr. Gay: Onslaught?
Fraser: The onslaught of news, and normally, when the astronomy panels come out, there’s a few interesting takeaways, but I can go a day without saying, “Okay, that’s really interesting and we should cover that” during the AAS, so I’ll do a couple, but I’m sure this one – each one is gonna be superlative, right? This is gonna be the most of this, and most extreme that, and the first time we’ve ever seen this, and this theory’s been overturned. It’s gonna be a monumental week in astronomy and science, the first time astronomers have got a chance to use JWST, and they are going to run with it.
Dr. Gay: Yeah, and we’re looking at a couple of decades of this, and what I’m also enjoying is there are some hints coming out that it is being considered if they can reboost HST to allow it to keep working for a little bit longer, so we might be looking at this nice era of having good coverage from slightly into the ultraviolet all the way into the far infrared-ish – not too far, because they don’t have a lot of coolant – but it’s a good age for discovery.
Fraser: The cosmic microwave background radiation tells us so much about the universe, but after that era, the universe went dark. Then, as gas pulled together into the first stars and eventually galaxies, light returned, beginning the age of reionization. I’ve always had a really hard time wrapping my mind around those early phases, from the cosmic microwave background, to the dark ages, to the age of reionization. So, can you explain that process for everybody, and I will finally get it into my head?
Dr. Gay: I can try. So, as the story goes, universe formed, there was massive heat and energy, and for the tiniest bit of time, the universe didn’t have any stuff in it, but as it expanded and cooled, expanded and cooled, particles came into existence, and at a point about 370,000 years after the formation of the universe, things cooled enough that all of the previously free-flying electrons were able to find a hydrogen, or helium, or maybe, if they were lucky, a lithium or beryllium atom to glom onto, and this process of the electrons joining in with the atomic nuclei allowed the photons that previously had tried to go somewhere and then gotten absorbed, and tried to go somewhere and gotten absorbed, allowed them to finally just fly free.
And so, this moment of the universe cooling enough that atoms could form and photons could fly free is the moment the cosmic microwave background was formed.
Fraser: So sorry, before we move on, I just wanna sort of put a couple of other notes on that. So, the first thing is if you could stand there when these electrons – sorry, when these photons are first trying to fly around, what would it look like? What color would it be?
Dr. Gay: The best way to think of it that I have is you’re gonna have all the colors, first of all, because this is just continuum radiation, and it’s super hot, so, peaking in the ultraviolet, and at the moment that this occurred, it would be kind of like turning off a neon light or a fluorescent light, where you go from having light coming out, but not getting anywhere, so you’re standing there, and the photons that are lucky enough to have been created right beside your eyeballs go into your eyeballs, but the light itself can’t really get anywhere. It’s trapped in this stuff, so it’s like being inside the neon light.
Fraser: Right. And then, the other thing is – you described it – again, it’s like turning off a neon light, but it wasn’t instantaneous.
Dr. Gay: No.
Fraser: It was a gradual process to go from no light can escape to now, light gets to move for a centimeter, to now, it gets to move for a meter, now, it gets to move for a kilometer, and eventually, it got to move for lightyears.
Dr. Gay: And it’s unclear how fast that process happened. It was something where areas of the universe that had a higher density – well, they also had a higher temperature, and areas that had a lower density had a lower temperature, and those lower-temperature regions were the first to be able to go, “Hi, here’s the cosmic microwave background.”
Now, the thing that always gets me is we talk about how the universe was opaque when all these neutral atoms were formed, but we also say this in the same breath that we say the cosmic microwave background was released, which, clearly, we can see. So, how is it that we’re seeing the cosmic microwave background if we now have an opaque, completely neutral universe? And the answer to that is that for the most part, the cosmic microwave background just passes straight through because of its wavelength, which is a bit confusing.
Fraser: Oh, okay. So, it was a wavelength of light, kind of reddish. I always describe it like the surface of a red giant star, that that wavelength was the one that could actually make it out further into the universe than other wavelengths, and so, we see – if you red-shift the cosmic microwave background all the way – right now, it’s microwave, but if you red-shift it all the way back to the beginning of the universe, it’s roughly the surface of Betelgeuse.
Dr. Gay: And so, this light is able to – for the most part – make it through, but what’s cool is we’re finally starting to be able to detect the early moments in our universe where the universe still had pockets of neutral gas, and these pockets of neutral gas are actually causing defects – irregularities – in the cosmic microwave background, so we can see the epic of reionization and before it using the irregularities in the cosmic microwave background.
Fraser: And so, this pushed into the dark ages?
Dr. Gay: Yes.
Fraser: And so, why were the dark ages dark?
Dr. Gay: Well, there was nothing giving off light. So, you have two different problems going on. First of all, you have no stars, you have no galaxies, you got nothing, and the other thing is you have a cloud of fairly dense neutral hydrogen and helium, and that’s gonna absorb light that’s coming at it once there is light. So, you start off with the dark ages, as there’s just nothing giving off light. So, then, stars start to form, galaxies start to form, and the first ones forming are having some of their light absorbed, and what’s cool is we’re using this absorption of light as seen in distant quasars to start pinpointing the moment in history that reionization occurred.
Fraser: All right, so, give us the physics description of what ionization is sort of as it relates to star stuff.
Dr. Gay: All right. So, you have your hydrogen nuclei. It has a proton, some number of neutrons, depending on what version of hydrogen it is, and a happy little electron if it’s neutral, and that electron has certain allowed energy levels. These allowed energy levels mean that if a photon hits the electron, only if the photon has the right energy level to allow it to jump between the different levels will that electron move. It can be thought of as if you’re on a stairwell, you’re either on step one or step two, you can’t be on step one and a half unless you have antigravity boots, in which case we’ve broken the analogy.
Now, photons come along, and if they have just the right energy level, that electron that absorbs the photon is going to jump to a higher energy level, and if the photon has a high enough energy, that electron’s just gonna go away. No more electron. And, it’s that process of removing an electron that’s called ionization.
Now, with hydrogen, you only have one electron to get rid of. With helium, you have two to try and get rid of that are in their own little energy levels, and so, you need more energy to get rid of both of them, and as you have more and more complex atoms, you can have something that’s singly ionized – that means you got rid of one of its electrons. You can have something that’s doubly ionized – you’ve gotten rid of two of its electrons. And, if you have a fully ionized iron, it’s had a really bad day.
Fraser: Right, with shell after shell of ionized electrons. So, the point being that the electrons are no longer in place around the nucleus, they are free-flowing –
Dr. Gay: Yes.
Fraser: – around, and it’s when – and so, that is ionized, and when the electrons pair up with the nucleus to form more, I don’t know, stable atoms, but anyway –
Dr. Gay: Neutral. They’re electrically neutral.
Fraser: Neutral, yeah. Right. And they are no longer ionized.
Dr. Gay: Yes.
Fraser: So, we’ve got these newly forming – we’ve got all this neutral hydrogen that is left over. So, in other words, the universe was ionized. Then, it became unionized because it had cooled down, and you’ve got all this – just a soup of hydrogen –
Dr. Gay: Yes.
Fraser: – neutral hydrogen, where you’ve got hydrogen with its electron, and then, they collapse together in these star-forming processes to form these stars. They heat up, and then they ionize their surroundings.
Dr. Gay: So, this is where it gets super cool. Pop III stars, which we talked about in the last episode – really, really big. And their high-energy photons have an easy time escaping from the core because there’s no heavy elements in the atmosphere of the star to absorb out those high-energy photons and then rerelease them as lower-energy photons. So, we have Population III stars forming. They are giving off ultraviolet and even higher-energy ionizing radiation.
So, we have Population III stars giving off all of this radiation, and this is where dwarf galaxies finally get to do something awesome. Where you have massive galaxies – massive galaxies are gonna have a whole lot of dense, neutral gas around them because more gas to form more galaxy, essentially. The little tiny dwarf galaxies don’t have as much stuff around them, so the stars inside of them are able to give off this ionizing radiation that can escape in much larger amounts.
Fifty percent of the ionizing radiation in a dwarf galaxy is able to escape the dwarf galaxy, and this light is going to ionize a larger and larger bubble, first around each star, then those bubbles will merge, and then around the galaxy, and now you have all these little dwarf galaxies that are just ionizing a Swiss cheese of growing bubbles of no-longer-neutral gas.
Fraser: So, are there some examples of a very similar situation happening in the universe today? I think about the bubbles in the Pillars of Creation. Is that the same kind of thing, where you’ve got this young, hot star that is blowing up this cavity and ionizing the gas around it?
Dr. Gay: It’s a very similar physical process. The science is the science. So, when you have a star-forming system, you have a giant molecular cloud. First of all, you’re not really going to have molecules, other than H2, in the early universe, so you have a giant molecular cloud of all sorts of different gases, and as it collapses down and fragments, these different fragments are going to form stars that are able to blow bubbles around them, including H2 regions, which is the ionized gas around these young stars.
So, you have the same physics, it’s just much more interesting because you’re throwing in molecules that can now emit their own colors of light, allowing us to peer through the gas and dust using radio and infrared telescopes.
Fraser: And so, once again, if you could be there, standing in space a safe distance or maybe an invulnerable distance from the star – we talked about this last week, these gigantic stars at the very edge of what is possible for a star – what would you see?
Dr. Gay: This would be a case of if you were able to fully protect yourself, it would be very much like being underwater, where your headlight – or in a fog cloud, where your headlight allows you to see a certain distance around you, but beyond that, it’s just opaque. So, as you’re near that one just-formed, starting-to-ionize-the-space-around-it star, you have this place where all of the ionizing radiation has been absorbed and done its job, and beyond it, it’s just opaque gas, and you have a wall.
Fraser: Wow! And it would be – the star would have probably – its stellar winds would probably be off the charts, and so, it would have cleared out any additional materials. You’d have this empty space, and then, whatever distance where the edge of the bubble was forming, you would have this reionized gas in this giant sphere around the star, and if you could somehow navigate, you would see them – these blobs of hydrogen with little gaps inside of them where the stars were forming.
Dr. Gay: And what starts to get super cool to imagine is if you’re hanging out in one of these bubbles – and this would require time travel because there weren’t enough heavy atoms to make anything resembling a civilization, so if you time-traveled into one of these bubbles, it would eventually merge with another bubble, so you’d have this window into another area that you could see into that was ionized, and over time, these bubbles would essentially merge with one another, creating a sphere filled with stars, just like when you look at some star-forming regions, you can see that central region – I’m thinking of the Omega Nebula here – that central region that has been cleared out by the star formation and is still surrounded by gas. No dust back then, no dust.
Fraser: So, how long did this period take, do we think?
Dr. Gay: We’re trying to figure this out, and trying to figure this out is a complicated task on two different fronts. So, on front one, we need to be able to figure out the expansion rate of the universe, so we can translate the red shifts, the amount that the different lines being emitted by atoms have been shifted into years, and we also need to observe.
What I can tell you is the galaxies that have quasars in their hearts that we’ve been able to see in the earliest parts of the universe – they’re giving off extremely bright light that, as it travels from that quasar towards us, it’s going to encounter pockets of gas along the way, and these pockets of gas represent areas that are eventually going to become star formation, the material around galaxies between here and there, and some of them represent not-yet-ionized clouds of material.
And when we look at galaxies at Z=6, we’re seeing pockets of this material, but when we’re looking at Z=5 point something, they’ve looked at a variety of these systems, and it appears that already, at that point, the universe was fully ionized, so it was a very fast process.
Fraser: Can you translate that for the non – what is a 5.6 or a 6? Because I know 14 is – sorry, 20, 17 – these new ones that are coming out of JWST, these are 300 million years after the Big Bang.
Dr. Gay: Yeah. So, we’re thinking that all of this occurred between 150 million and 1 billion years ago, and trying to figure out exactly when in there it was done – we know it started around 150 million. We know it was done by 1 billion. Where in between those two that it was done, we’re still figuring out.
Fraser: Right. And I guess the process is uneven, that you’ve got different amounts of gas, different strengths of stars, different sized collections, which were probably linked up to overdensities and underdensities in the cosmic microwave background, and so, different parts collected together, heated up, cleared out sooner than others.
Dr. Gay: Exactly, and like I said, the really cool thing is the dwarf galaxies literally got to shine, and it was the area around the biggest galaxies that, because they had so much more material that they had to ionize, the areas around the biggest galaxies took a little bit longer to clear the way.
Fraser: So, what are our tools to perceive this time?
Dr. Gay: Well, the best tool that we have is gravitationally lensed galaxies that exist back behind galaxy clusters, and those galaxy clusters help magnify the amount of light that we’re able to get, and then, we point our infrared telescopes, like JWST, at those gravitationally lensed galaxies from the beginning of the universe, and we look for quasars, and once we find those quasars, we use them to figure out where in the quasar’s history do we see what kinds of material – well, essentially grabbing out the light. For the collections of gas that are more close by, it’s a Lyman-alpha forest, and then we’re looking back at the stuff that just hasn’t ionized yet.
Fraser: So, we’re seeing the light of the quasar through the gravitational lens –
Dr. Gay: Yes.
Fraser: – because only by harnessing the gravitational attraction of an entire galaxy cluster can you make a telescope powerful enough to be able to see this period.
Dr. Gay: Yes.
Fraser: You’re looking at these gravitationally lensed quasars, and are you seeing them – you’re seeing the light from the quasar go through pockets of gas and dust –
Dr. Gay: Yes.
Fraser: – that are in various states of ionization. You mentioned the Lyman forest. What is that?
Dr. Gay: So, what’s happening is you have – the background quasar is giving off light, where Lyman-alpha is one of the easier-to-spot lines of light. Now, that Lyman-alpha gas that, in the emission lines can allow us to figure out where quasars are. Now, the quasars, in addition to having this entire suite of emission and absorption lines, also have the continuum radiation that they’re giving off.
That continuum radiation, as it travels towards us, is going to have the hydrogen gas, pockets of hydrogen gas, absorb out different colors, depending on where in history they are. So, the light that was absorbed out at 2 billion years after the Big Bang by hydrogen gas is going to be red-shifted to be one color, and that is a Lyman-alpha line that has now been red-shifted into a color we can detect, from the ultraviolet into the reds.
Dr. Gay: From a different age, it’s going to have a different wavelength, so we have the continuum radiation from the quasar that is going to have this forest of what was hydrogen Lyman-alpha absorption lines from an entire suite of different points in history where there were clouds of gas located to absorb the light.
Fraser: So, the light from this quasar is going through multiple gas clouds –
Dr. Gay: Yes.
Fraser: – and you’re seeing the forest with all of these trees.
Dr. Gay: Now, those forests that you’re seeing aren’t the not-yet-ionized gas left over from the Big Bang. That actually gets its own name, which, as someone with dyslexia, I have to be careful with because it’s called the Gunn Peterson trough, and my brain has decided it should be the Peter Gunn trough, which would be far more amusing.
Fraser: Right, right.
Dr. Gay: So, the Gunn Peterson trough is this gap that we can see, a literal trough in the light from the most distant quasars, that is there from the neutral gas that wasn’t ionized in the beginning, and this is what we’re desperately looking for using JWST, so that by finding enough far-away quasars with these Gunn Peterson troughs – while Peter Gunn plays in the background – we are able to say, okay, so we’re seeing these troughs in quasars at these specific points in history, and then we don’t see them anymore. That tells us everything was ionized by this point in history.
Fraser: Very cool. And, as we mentioned in the beginning of the show, be prepared. There’s probably gonna be a bunch of news coming out about the age of reionization just in this week’s American Astronomical Society. So, hopefully, now you will be better able to understand it, like me – I think I get it now, so I think we’re all right.
Dr. Gay: I’m so glad. It’s not easy.
Fraser: All right. Well, thanks, Pamela.
Dr. Gay: Thank you, Fraser, and I need to thank our wonderful patrons who are out there supporting this show. You allow us to pay all of our humans and, where needed, provide them with health benefits, and that is kind of awesome.
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Fraser: Do it! Join us! Thanks, everyone, and we’ll see you next week.
Dr. Gay: Bye-bye.
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