Podcaster: Paul M. Sutter
Title: AaS! 244: What’s the Most Important Wavelength in All Astronomy?
Organization: INFN Trieste and OSU CCAPP
Link : Twitter @PaulMattSutter, http://www.Facebook.com/PaulMattSutter, and http://www.askaspaceman.com
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Description:
What is 21-cm radiation? What (and who) produces it? What causes it, and what does it tell us about the universe? I discuss these questions and more in today’s Ask a Spaceman!
Bio: Paul Sutter received his Ph.D. in Physics from the University of Illinois at Urbana-Champaign as a Department of Energy Computational Science Graduate Fellow. He then spent three years as a Postdoctoral Fellow in Next-Generation Cosmic Probes at the Paris Institute of Astrophysics, and is currently an INFN Fellow in Theoretical Physics in Trieste, Italy, and a Visiting Scholar at the Ohio State University’s Center for Cosmology and Astro-Particle Physics. He is inexplicably drawn to positions with very long titles.
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Transcript:
What if I told you that there was a secret window? And if you looked through this window, you could see the entire history of the universe unfold before your very eyes.
You could watch as spiral galaxies slowly spin on their axes. You could watch as new stars are born. You could see the emergence of the very first stars and galaxies to ever appear on the cosmic scene.
And through this secret window, if you’re lucky enough and pay enough attention, you might just see an alien. It sounds too good to be true, but this is science, and if we’ve learned anything in our four centuries of scientific exploration of nature, science can produce miracles. Or in this case, science can take advantage of nature’s own miracles.
I’m talking about a curious little feature of the humble hydrogen atom. One proton, one electron. Done.
The simplest atom possible. You can throw a neutron in there if you’re feeling generous. It’s not necessary, but it does add a little bit of fiber.
Now, this proton and this electron are particles, which means that they have a list of properties. Properties like mass and electric charge. And these properties tell us how the particles respond to, in this case, in the case of mass, to the gravitational force, and in the case of electric charge, to the electric force.
And then there’s this other property. A property we call spin. Now, when I say spin, everybody, including myself, thinks of the obvious.
Like, you know, something spinning. Like a Harlem Globetrotter spinning a basketball on their pinky finger. But these are particles, which means they take up no volume in space, so how do they spin?
The answer is they don’t. Well, I mean, they kind of do. Look, it’s really weird and complicated, and it’s one of those many quantum things that we just have to learn to live with because there’s no getting around it, and quantum mechanics doesn’t really care if we understand it or not.
The spin of a particle refers to essentially how it responds to the magnetic force. If I were to take a metal ball and charge it up with electricity, and then set it spinning, and then throw that spinning charged metal ball into a magnetic field, there’s a natural response that the spinning charged metal ball has to that magnetic field. If the spinning ball is spinning in one particular direction, the ball gets deflected in a certain way.
It goes, like, curves up. And if it’s spinning the other way, it curves the other way. And the thing is, particles like electrons and protons do that.
They respond to magnetic fields exactly as if they were charged metal balls, but they’re not charged metal balls, but they still act like they are, so we call it spin because that’s the closest thing we can call it, and we have to move on. And particles like protons and electrons can have one of two choices for their spin. We call these choices up and down because when we shoot these particles through a magnetic field, that’s pointing up and down.
The up-pointing particles go up and the down-spinning particles go down. We could have called these spin states left and right, or A and B, or Alice and Bob, but we went with up and down. In the hydrogen atom, the electron and proton can either have the same direction of the spin, they can either be both be up or both be down, or they can have opposite spins where one is up and the other is down, or one is down and the other is up.
You get the idea. For various quantum mechanical reasons having to do with the overlap of the wave functions and other mumbo-jumbo, when the proton and the electron have the exact same spin, if they’re both up or both down, then in that state, that configuration, that situation, that situation has ever so slightly more energy than the situation when they have opposite-pointing spins. This means that if something causes them to have the exact same spin direction of the proton and the electron point in the same direction with their spins, or they have the exact same spin state, either both up or both down, they would prefer to realign themselves.
They would prefer to have opposite spins. It’s like having opposite spins is a lower energy state. It’s a more relaxed state.
It’s a more natural state than having parallel same-direction spins. It’s like getting a kink in a muscle. Yes, you can function throughout your day and the hydrogen atom can still do all the normal things that hydrogen atoms do, but it’s in a slightly tense state, a slightly more energetic state than it would prefer to be.
And so just like getting a massage and it lowers your energy and it relaxes you, if a hydrogen atom can flip back so that they have opposite spins, the proton and the electron, then everything’s more chill. And that’s what it would much prefer to do, except that this takes a really long time. If you found a hydrogen atom all by its lonesome in the middle of empty space with parallel spins, this little kink in its muscles, and you waited and watched for it to flip back to its normal, more relaxed configuration, the average wait time is around 11 million years.
But here’s the kicker. Last time I checked, there are slightly more than 11 million hydrogen atoms in the universe, which means that if you have a whole bunch of hydrogen atoms all sitting around and a bunch of them get into this aligned state, slightly elevated energy kink in the muscle, walking around your day with a little bit of tension kind of situation, since there are so many of them, a few of them or a lot of them are bound to flip back, get into that relaxed state pretty much all the time. And when they do, this hydrogen atom is going from a high energy state to a low energy state. That’s a difference in energy and that energy goes away.
It emits. The hydrogen atom can emit this energy when it goes from the spin parallel state to the spin opposite state. And if you have a galaxy’s worth of hydrogen atoms, then they’re emitting this energy pretty much all the time.
Now, it’s not a lot of energy. We can calculate it. It’s a 5.8 micro electron volts, which is, I mean, it’s just not a lot of energy, folks. But that energy comes out in a very specific way. It comes out in the form of a single photon of electromagnetic radiation. And we can compute the wavelength of that radiation, the wavelength of that photon at an energy of 5.8 micro electron volts. That comes out to a wavelength of 21 centimeters. 21 centimeters. Hold out your hands in front of you.
Put them yay apart. Have you ever tried to describe how big something is or how far away something is and you’re like, it’s, it’s yay big. And you just kind of hold your hands out vaguely in front of you.
That’s, that’s about 21 centimeters. It’s kind of close, but not very close. That’s wavelength at 21 centimeters.
That’s deep in the microwave band of the electromagnetic spectrum, almost down in the radio, actually. And what’s wild to me is that a hydrogen atom, this tiny little thing, this little solitary atom can just spit out this big wavelength of light. It’s amazing.
Neutral hydrogen. This is hydrogen with one proton and one electron. If you rip the electron away, it becomes ionized hydrogen, which is, which is just a proton by itself.
Neutral hydrogen emits radiation with a wavelength of 21 centimeters. What’s the big deal? Well, big deal is that neutral hydrogen is everywhere.
I mean, hydrogen is by far the most common element in the universe. It’s like three quarters of all the mass of the normal matter in the universe is made of regular hydrogen. So it’s everywhere.
I mean, there’s hydrogen in your, in your body right now, there’s hydrogen in the ground, there’s hydrogen in the air. Yeah. Usually that hydrogen is bound up into other molecules and other chemicals, which is fine, but there’s also plenty of just free floating hydrogen.
And yeah, a lot of the hydrogen is really hot in the universe. It’s ionized. It’s a plasma.
The electrons have been ripped away, but they’re, but space is also really, really cold. And so there are plenty of places where pockets of hydrogen get really cold and they become neutral. They, they don’t have to be in that ionized state.
They can chill out being neutral hydrogen and emitting 21 centimeter radiation, which means our universe is awash in this radiation at exactly a wavelength of 21 centimeters. And 21 centimeter radiation has another special property. It contributes to Patreon.
That’s patreon.com slash PM Sutter. You can join 21 centimeter radiation in your combined contributions and support of this show. And I truly do appreciate it.
That’s patreon.com slash PM Sutter. That’s my name by the way. Although the actual thing, the actual special property is that 21 centimeter radiation is like microwave, almost radio kind of wavelength is really good at cutting through gas clouds, which means that we can see it from far away and use it to see through all sorts of annoying interstellar dust.
And I have to admit, as a cosmological leaning kind of astrophysicist, dust, it just gets in the way. There’s dust everywhere in every galaxy. There’s dust between galaxies.
It obscures our view. It makes it hard to see distant galaxies in the cosmic microwave background. It’s like super annoying.
There are plenty of astronomers and astrophysicists who are really into dust and love this stuff. And they’re like, why do you want to keep getting rid of it when we can study it and learn about the universe? I’m like, okay, dusty friends, dust is useful, but it generally obscures and it’s generally annoying.
It just blocks stuff out. It’s like fog. Here’s 50 light years of fog.
Good luck seeing anything. 21 centimeter radiation, this radiation emitted by neutral hydrogen just sails on through. So if you’ve got a pocket of neutral hydrogen way out there on the other side of the galaxy, it’s surrounded by stars and other nebula and other interesting things.
Maybe an alien civilization that built a big welcome sign. We can’t see any of that. We can’t see the stars.
We can’t see the welcome sign. We can’t see the nebula. It all gets blocked by all this interstellar dust.
But hey, that blob of hydrogen gas emitting that 21 centimeter radiation goes right on through and like, hey, I got that blob. That’s useful. That’s useful.
Let me give you three, just three examples of how powerful and useful 21 centimeter observations can be. That if you could put on 21 centimeter glasses, glasses that let you see light, this special light at a wavelength of 21 centimeters, what would you see? What would you do?
Well, one thing you would do is map the galaxy. Since there are approximately a bajillion hydrogen atoms in the Milky Way galaxy, and they’re all emitting this 21 centimeter radiation pretty much all the time. Yes, it’s super rare, but there’s such an overwhelming number of neutral hydrogen atoms that they’re just glowing all the time.
And because this radiation can reach pretty far distances through interstellar dust, maps of 21 centimeter radiation are our primary vehicle for mapping the structure of the Milky Way. You’ve seen drawings of the Milky Way with some spiral arms, and we’ve got a big bar for a core. It’s pretty cool.
Almost all of that is guesswork. And almost all of that is based on mapping blobs of 21 centimeter radiation, because we can’t see the stars, or we have a really hard time seeing the stars. When we’re looking through the disk of our galaxy, and there’s so much dust in the way, we can’t see the stars.
We can’t directly see the spiral arms, but we can see clumps of neutral hydrogen. And we assume that where there are a lot of clumps of neutral hydrogen, there’s probably a lot of other stuff going on, a lot of star formation, a lot of high density patch. Oh, look, it’s a spiral arm.
And then places where there are not so many clumps of neutral hydrogen, and probably a little bit less density here, not a lot going on. That’s probably a gap. Most of our maps of the Milky Way are based on mapping neutral hydrogen, and we get to use it to measure the rotation of the Milky Way.
Because if that clump of neutral hydrogen over there that we can see from tens of thousands of light years away, or we point our telescopes in that direction, all we see is fog and mist. But then when we tune it to this wavelength of 21 centimeters, boom, we get to see these clumps of neutral hydrogen. If that clump is moving away from us, then its light will be redshifted.
And if it’s moving towards us, its light will be blueshifted. So we can scan all across the galaxy, and we see a whole bunch of clumps moving away from us in this direction, a whole bunch of clumps moving towards us in that direction. Hey, that’s a rotation.
We can use that to measure and nail down the rotation rate of the Milky Way galaxy, which is super cool. How do we estimate the mass of distant galaxies? Neutral hydrogen.
Once again, wow, galaxies really far away, made of lots of hot, glowy stuff and dark matter. And man, they’re really hard to see. It’s really hard to map out stellar light or nebular light.
I don’t know if nebular light is a real astronomy term, but it works for our purposes. But hey, that neutral hydrogen light, man, it lights up like a lighthouse back there. And then we can get a really good handle on that.
And then we can calibrate it with some nearby galaxies where we say, okay, if we see this much neutral hydrogen light at 21 centimeters, that means there’s this much mass in the galaxy. So we calibrate all those relations nearby, where we have good multi-wavelength observations of lots of galaxies. And then we go out in the distant universe, where all we see is a little blob of neutral hydrogen.
That’s all we get, but we can say, okay, okay, I see that much neutral hydrogen, which means this galaxy is probably this big. That’s cool, but there’s more. The second thing is that 21 centimeter radiation is the next big game in cosmology.
So from our vantage point here on the earth, it looks like we’re at the center of the universe. I know we’re not, the universe has no center, but we are at the center of the observable universe. We’re in this tiny little bubble, 90 billion light years across, I guess it’s not so tiny, at the very, very edge of this bubble, right up against the edges of the cosmic microwave background, the most distant thing we can see.
Nearby, we have a whole bunch of galaxies. And then there’s this big blank spot where we don’t have any direct observations. That blank spot is the epoch called the dark ages.
This is the time after the cosmic microwave background did its thing when our universe was 380,000 years old. But before the first stars and galaxies lit up, there’s still stuff there. There’s all sorts of matter, and that matter is slowly building the cosmic web, laying down the seeds of the first stars, the first galaxies, the first black holes, but it’s not glowing.
So that’s why we call it the dark ages, because it’s just kind of there, and we don’t have an easy time seeing it, because it doesn’t light up for us. And in fact, a tremendous volume of the observable universe is in the form of this dark ages. When we look out past the earth, we see our nearby galaxies, and then we see the large-scale structure, and then that kind of fades away.
And then there’s this huge blank volume at the very edge of our bubble that represents an enormous amount of cosmological information, enormous amount of data, tons of structures there evolving that we simply can’t see. But guess what there is? There’s neutral hydrogen.
That’s right, that’s what the cosmic microwave background is. That’s when our universe transitioned from being a plasma to being neutral for the first time. When our universe was 380,000 years old, like in less than 10,000 years, the whole universe neutralized, and you’ve got a literal cosmos filled with neutral hydrogen.
And yeah, it’s glowing. It’s sending out 21 centimeter radiation, but the thing is, it’s super far away, which means there’s a whole lot of redshift going on. The redshift factor here with the dark ages is around a factor of 10, and because of this redshift factor of 10, that means this 21 centimeter radiation is stretched out by at least a factor of 10, which puts it at, let me see here, decimal point, carry the two, ah, 2.1 meters. That’s deeply in the radio. That’s big time radio. That’s straight up radio wavelengths.
What this means is that any radio transmission you might receive, like a TV antenna, a car radio, any of that, a tiny, tiny, tiny portion of it, it’s very, very tiny, but it’s there, is actually this neutral hydrogen emission from the dark ages, which is really fun to think about. So yeah, there are major astronomical observatories gearing up trying to find this neutral hydrogen radiation signal from the dark ages of the universe because it’s literally the only thing lighting up at that time. There ain’t no stars.
There ain’t no galaxies. There’s no UV. There’s no infrared.
There’s not even any radio because you’ve just got neutral hydrogen slowly, slowly, slowly collecting together to form the seeds of the first stars and galaxies. And the only way they’re lighting up is through neutral hydrogen. And what’s even cooler is that we can map this process out because we can take that redshift at different distances, different epochs in the dark ages, we’ll have slightly different redshifts because some will come earlier, some will come later.
So we can start at like 2.1 meters and then start scanning left and right and start to map out this entire mass of neutral hydrogen existing in the very early universe. And we can see what it’s up to. We can potentially map out its entire evolution and learn about things like dark matter and maybe even dark energy and what the universe is made of and how the first structures evolved and what ignited the first stars.
And then as those first stars and galaxies switch on, they flood the universe with high energy light and they actually rip away a lot of the neutral hydrogen. They ionize it again. They rip those electrons away.
And so this neutral hydrogen signal, this global signal, this cosmological signal that fills the sky, actually starts to get little holes in it surrounding the first galaxies. Those holes get bigger and then merge together and then it all washes away. And we can watch this process unfold just by mapping neutral hydrogen 21 centimeter radiation.
There are slight and by slight, tremendous technical challenges to this, mostly because, like I said, 21 centimeter radiation from the dark ages is down in the radio. It has a wavelength of around a couple meters. That’s in the radio and humans, we love our radio.
And so there’s so much human interference. It’s actually really, really hard to dig out and find this signal that is a whole topic of conversation. Feel free to ask.
I think I’ve done some episodes on it, but feel free to ask again, because I’d love to get into the nitty gritty of how we actually try to separate out the human interference and get at these cosmological signals. Anyway, watch that space over the next decades. I’m hoping some really cool things are coming.
Lastly, as if all this wasn’t juicy enough, you get to map the Milky Way galaxy. You get to measure the rotation rate in the Milky Way galaxy. You can map distant galaxies, measure their rotation rates.
We get to map the dark ages of the universe, all thanks to 21 centimeter radiation. If all this wasn’t good enough, we think the best way to talk to aliens is with 21 centimeter radiation, neutral hydrogen. I mean, think about it.
If we’re going to talk to aliens, it’s probably going to be with electromagnetic radiation. I mean, maybe super advanced species prefer neutrino radios or gravitational wave detectors. We’re not there yet.
We can detect neutrinos in gravitational waves, but we can’t exactly use it as a communication system. I don’t know. Maybe they’ll keep an old-timey radio array hanging out just for good time’s sake, and then they blast out a signal every once in a while.
If we had to pick a wavelength on the electromagnetic spectrum, just something to go with, 21 centimeter radiation seems kind of fundamental. I mean, hydrogen is there. It’s the simplest element.
It’s just one proton and one electron, nothing more complicated. The spin-flip transition that goes from aligned to opposite spins seems pretty fundamental. This radiation is already being emitted in the universe all the time.
I mean, if you’re an intelligent civilization, then the going idea is that eventually you’re going to discover hydrogen, you’re going to discover spin, you’re going to discover quantum mechanics, and you are going to discover 21 centimeter radiation. Yeah, you’ll call it something else because you won’t have 21s and you won’t have centimeters, but the same idea is there, that as soon as you start investigating the subatomic nature of the physical universe, you are, this argument goes, guaranteed to find neutral hydrogen radiation, this 21 centimeter radiation. You’ll be like, oh, wow, this property called Gorlak.
When the, instead of spin, I don’t know. That was a bad idea. And 21 centimeter radiation, super common, super fundamental, really good at punching through vast interstellar distances.
It makes for a great social media platform. It makes for a great way for chatting with other people in the galaxy. In fact, the Pioneer plaque, this plaque affixed to the Pioneer probes, which deserves its own episode, please feel free to ask.
I’d love to talk about the Pioneer plaques, has a diagram of the spin flip transition. And all measurements on the plaque are references to multiples of 21 centimeters. So on the plaque, there’s like a picture of humans and the spacecraft itself.
And then there are these like, you know, binary coded measurements. And you’re like, wow, who, how big are these weird creatures that call themselves humans? And you say, oh, wait a minute, wait a minute.
They give us a code that, oh, that’s 21 centimeter radiation. Oh, got it. And they’re, and they’re yay times larger than 21 centimeter radiation.
Oh, super cool. That’s the idea. And yeah, if we see a sudden surge of 21 centimeter radiation coming from a particular direction in the sky, very, very narrow, you know, just looking at a star one day, doing a scan, and then it’s not radio emission, but like, boom, a blast of 21 centimeter radiation, you know, it might just be an alien civilization that thought of the same great idea.
If I had to pick one wavelength of light to view the universe through, you know, if I had to pick the entire electromagnetic spectrum, and I had my choice anywhere in the radio, anywhere in the microwave, anywhere in infrared, visual, ultraviolet, x-ray, gamma ray, the whole deal, and I had to pick one wavelength, I would pick 21 centimeters all the way. Thank you to Jack H. for the question that led to today’s episode.
Thank you to all my Patreon contributors. That’s patreon.com slash pmsutter, where you can support this show. Please keep sending me questions, askaspaceman at gmail.com or the website askaspaceman.com.
Please keep dropping reviews on your favorite podcast platform. It really helps the show visibility. I would like to thank before I go, my top Patreon contributors this month.
There are many more, and I’m so grateful for all of your generosity. But my top ones this month are Justin G, Chris L, Alberto M, Duncan M, Corey D, Robert B, Michael P, Nyla, Sam R, John S, Joshua, Scott M, Rob H, Scott M, Lewis M, John W, Alexis, Gilbert M, Rob W, Jessica M, Jules R, Mike G, Jim L, David S, Scott R, Heather, Mike S, Pete H, Steve S, what what word, Lisa R, Koozie, Kevin B, Michael B, Eileen G, Toho W, Steven W, and Brian O. That’s patreon.com slash pmsutter. And I will see you next time for more Complete Knowledge. You’re listening to the 365 Days of Astronomy podcast.
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365 Days of Astronomy
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