Daniel and Jorge Explain the UniverseWhat mysteries are hiding in the extra-galactic background light?
Daniel and Jorge explore the challenges of detecting light from outside the Milky Way, and the secrets it might reveal.
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Hey Daniel, when you look at the night sky on a good night, how many stars can you see?
Well, if it's a really good dark night without any light pollution, you might see several thousand stars.
Thousands in Los Angeles.
Maybe you forgotten Joshua Tree.
But that's it. Only a thousand stars and the nice guy aren't there, Like trillions of stars out there.
There are oodles of stars out there, but most of them are too dim to see even though they're out.
There, like their photons are not reaching us, or they're too weak.
Photons can travel in infinite distance without ever get tired, but their photons are just so rare that you need like a really big eyeball, maybe Hubble or James Webb, to capture one.
Of them, or just like a long exposure too, right.
Yeah, if you set up your camera for months and months, you'll probably see one of those dim stars.
You need like a slow eyeball.
You need a big or slow eyeball or both.
So I guess what else is out there? Like, what else would you see? Would you see stars and anything else?
We don't know what's out there. Could just be stars and galaxies, or there could be like chocolate bars and frozen bananas waiting for us to discover them.
WHOA, sounds delicious, But will we see those things? Wouldn't they have to glow somehow? Would these be glowing frozen bananas?
Glowing or reflective?
Which case? Do you want to eat them?
I'm glowing with excitement to try them.
I don't know. It sounds a little slippery.
Hi.
I'm Jorge Make, cartoonist and the author of Oliver's Great Big Universe. Hi.
I'm Daniel. I'm a particle physicist and a professor at UC Irvine, and I'm so excited to see what's hiding out there in the universe waiting for us to discover it.
Yeah, there seems to be a lot out there, But do you think it's hiding or we're just not good at seeing.
I don't think it knows or cares about whether we are seeing it, But it has not yet been revealed, so in that sense, it is concealed.
Oh yeah, yeah. But is it really the case that it could anything could be out there? You think then we have a pretty good sense from what we can see around us.
There's still a lot of big questions about what's out there, the stuff nearby, the stuff far away, the stuff in between. And we should never make the assumption that the stuff that's close to us is typical and that it could be used to explain the whole universe.
Well, I guess there could be things hiding within our own galaxy that we can't see it, right, Like we haven't seen all of the Milky Way galaxy.
Oh absolutely, And the center of the galaxy, the most interesting place, is the hardest to see. But the Milky Way itself is so bright and so big that it makes it really hard to see beyond it to other galaxies.
And these other galaxies could be totally different from ours, right.
They could be very different. They could have a very different history. There could be all sorts of stuff going on out there. Deep in the universe that we haven't yet figured out. Most of the photons that come to Earth we don't gather. We mostly ignore them.
You mean, like we're getting all this information from all the cross the universe, but we're not doing anything with it. We're not paying attention.
Yeah, the universe is screaming at us in photons. Everything that's out there in the universe is telling us all about itself. So we have like the whole history of the universe is out there being literally beamed at us. But mostly we're not paying attention. Mostly those photons just like splash on the sidewalk.
I wonder if that could be a good thing. Like, I wonder if at some point it's like TMI Universe. There's some things I don't.
Want to for never TMI. With science, you always want more data so you can know more about at the universe. I want to know the Universe's deepest, darkest, most embarrassing secrets.
I say, you're more of an any person.
Not enough information, absolutely no information is too embarrassing.
Well I hope that's true. But anyways, welcome to our podcast. Daniel and Jorge Explain the Universe, a production of iHeartRadio.
In which no question is too weird, too gross, too icky for us to explore it. We want to know all of the embarrassing details about how the universe was born, how it grew up, and all the messes it made along the way. We want to unravel the deep history of time and understand how the universe got to be the way that it is and why it operates in such an incredible, amazing and beautiful fashion.
Man, you make it sound like crazy fans of the universe we are, or as the kids say these days, stand.
I totally stand our universe absolutely. I will defend it online against haters.
There you go until you turn on it.
If in season seven it does something really weird, then yes, I will turn on it.
But so far, if it jumps the shark, if it jumps the galactic shark, you're like, I like the earlier seasons better.
So far, it's been pretty awesome, and everything we've learned has blown our minds and revealed incredible things about the way the universe works. Not only are the laws that it follows really fascinating and incredible and have weird philosophical consequences for what the nature of reality is, but the stuff that bubbles up from those tiny laws, the huge, the big stuff, the black holes, the galaxies, the quasars, the blazars, all that stuff is just so mind blowingly awesome.
Yeah, it's pretty amazing, as we said before, how much we've been able to figure out about the larger universe, even just about our galaxy and beyond, just from sitting on this little tiny rock in one corner of the galaxy. It's pretty incredible if you think about the scale things and how much we know about what's out there.
But we've only really just begun to observe our universe. We have a few eyeballs capable of picking up really distant object but most of the light that's out there, billions and billions and trillions and trillions of photons that contain super fascinating important information about the history of our universe, we're not capturing them. They're mostly drowned out by bigger, brighter stuff like the lights of Los Angeles.
Yeah, we're sort of washed with information from the universe and we're not really I guess we are capturing it, we're just not recording it, is maybe what you mean. We're like, we're getting a night light, starlight, you know, when I step outside tonight, But I'm not going to be thinking about it or recording it or trying to figure out what it says.
Yeah, most of it just hits the earth, And unless some like gecko is looking up at the night sky, there's no being that's gathering that information. It just like gently warms some rock or some like gum wrapper that's lying on the ground.
But do you think we're missing stuff? Like if I just point a telescope there every once in a while, am I really going to miss anything?
I think there's an incredible amount of deep history out there in the night sky. And if we build zillions of telescopes and point to them in all those directions and just let them accumulate information, we would learn so much about the history of the universe from these really faint sources.
But ironically, if you cover the night sky with telescopes and you wouldn't be able to see the night sky.
We'd see it in a different way.
Well, I guess that's true. We can see it on our phones. It's a little less poetic, and we can see it scientifically. But anyways, it's kind of interesting what is out there and what kind of information we are getting, including even the stuff we might consider being in the background.
Yeah, that's right. The night sky is chock full of stars from our galaxy, but there's a lot of really useful information in the background, information we're mostly missing.
So today on the program, we'll be tackling the question what is extra galactic background light. It's a lot of syllables there for one term.
Yeah, astronomers, you know this one I think they actually need pretty well.
We'll see, we'll see. You've always promised that and most of the time it disappoints.
You have a very unrealistic standard.
If I have to say so, I'm just saying, you know, take a minute to think about.
It, all right, all right, I'll suspend my judgment.
But yeah, it's an interesting question. A lot of words here, extra galactic background light, which should have sounds self apparent, but maybe the extra it throws me off a little bit, like it's extra, like we don't need it, or is it extra like like you get a bonus, Like you know, I pay for a certain amount of galactic background light and I'm getting some extra serving of it.
Or maybe it's just like a bit much universe, like, why are you so extra? Yeah, that's how my teenage daughter would interpret it.
Mmmm, you're being too extra?
Yeah, dad, you're so extra.
Oh my god, that's better than me. Mid. That's like the that's not the worst insult from a right now. That's kind of Mid.
Oh boy, universe, Mid, I've seen.
Better life existence, Mid. But yeah, I guess we'll dig into what all these turns mean and why. It's interesting to think about the extra galactic background light. But as usual for wondering, how many people out there know about this or have any thoughts about what it might be?
Thanks very much to our panel of volunteers who comment on these well named astronomical phenomena and offer their opinions without the chance to google about it. If you would like to play for a future episode, please don't be shy. Write to me two questions at Danielandjorge dot com.
So think about it for a second. What do you think is the extra galactic background light? Here's what people have to say. It must be all the light coming from outside our galaxy.
Either that or one of those LED sets that you can use to make your bedroom lighting extra.
Galactic extragalactic background light is the light produced by Club Andromeda when the Aliens are having a rape.
I don't know something to do with our galaxy, and there's too much light for what there should be by some theory that's calculating, uh, you know, how it fits within the light it should get from other galaxies or our sun. And somehow it's being amplified. So it's extra.
I've heard of it. But I'm thinking that are other galaxies out of our galaxy, and there it's a large background. Most of the stars we see as the background galaxies, and that's the range of galaxies we see in their lights.
Yes, all right, a lot of creative answers here. Some people think extra means party.
I was wondering if anybody was going to say, it's like breaking news, like extra extra, read all about it.
Oh right, that's another use of the word extra. Yeah, if you're from the nineteen twenties, I.
Got a jaunty cap on. I'm standing on a soapbox. I'm telling you, newspaper you.
Get, you get a vest on?
Yeah, yeah, yeah.
You're on this street. I got on my cheeks, hemming outside.
That's how we distribute science these days. We pass it out to OHI g mister, have you read whites In's latest paper. It's a hoot.
But yeah, that's one way to think about it. But a lot of creative answers here. I guess it's sort of self a pair. But like I said, there's some ambiguity in the terms.
Yes, as always.
All right, Well, let's dig into it, Daniel. What is the extragalactic background light?
Basically, the extragalactic background light, or EBL as astronomers like to call it, is all the light emitted by everything else in the universe except for the Milky Way during the entire history of the universe. So it's like all of the photons in the universe not emitted by our galaxy.
Wait what but also not emitted by the background microwave background radiation.
Now, the CMB, the cosmic microwave background, is part of the EBL. The EBL is like the super general version of the CMB. CMB is on only at one wavelength. That's the cosmic microwave background. The EBL is like, well, what's the background in all of the wavelengths?
Oh wait, wait, so the EBL is part of this CMB.
No, the CNB is part of the EBL.
Oh. Okay, but it doesn't come from galaxies, does it.
It doesn't have to come from galaxies. This is the light emitted by everything else in the universe other than our galaxy. So extragalactic means not the Milky Way. So any other star, any other object, any frozen banana floating out there in space, any primordial soup of gas and plasma, everything in the whole history of the universe except for the Milky Way.
Oh, I see, this is not galactic light. It's like extra galactic in the sense of being outside of our galxy.
Yes, take all the photons in the universe and subtract the ones emitted by our galaxy the ones left over, that's the extra galactic background light. So like most of the photons in the universe are the EBL photons I see.
So any photon that we see o in space that doesn't come from a star within or any other source within our Milky Way galaxy exactly.
And the estimate is that there's like ten to the eighty four extra galactic background photons in the universe, So there's a whole lot of them, ten to the eighty four and eighty four to ten with eighty four zeros in front of it. It's a lot.
It sounds like a lot, but I don't know how many come from our galaxy.
I mean, the whole history of the universe. The tiniest fraction come from our galaxy. Almost every single photon in the universe doesn't come from our galaxy. So almost every single photon in the universe is an EBL photon.
Interesting, and this comes from other galaxies or, as we talked about, maybe directly from the Big Bang, right.
It comes from the whole history of the universe. And that's what's super fascinating about it. Some of these photons were made before there were galaxies. Some of them were made before there were stars. Some of them were made during reionization, when these big clouds of neutral gas first started to clump together to form stars. The whole history of the universe is written in these photons. They're out there, they're floating around, and they contain all of this information, but they're very, very tricky to see.
Right. So, like we're getting the light from Andromeda, which is close to us, but we're also getting light from super far away, which also happens to be really a long time ago. That's all mixed in with this in this micro background.
Like, that's right, all that counts as EBL because it's not coming from the Milky Way.
Maybe should be like extra Milky Way background light, right, because it's not just like extra galactic like in the general sense, it's just like our anything outside of our galaxy.
Yeah, extra our galactic background light.
Yeah, there you go. Yeah, because so many in Andromeda, the EBL would be different.
Right, Yeah, that's true. Those astronomers would argue with our astronomers about how to name this thing. That would be fun.
Yeah, there a cartoon is would call my cartoonists. We would duke it out and then I, you know, I would take them out and then I get to name it for the whole universe.
You know, you wouldn't be the first cartoonist to actually name something scientific. You know. Gary Larson actually had an impact on science.
Oh yeah, what did he name?
Ites This hilarious cartoon of a caveman giving a name to those four pokey spikes on the back of a stegosaurus. He calls it the Phagomizer after thag who was killed by a stegosaurus. And it turns out that nobody had actually named that before. So then scientists actually started using phagomizer in their science papers, and now it's the official name for the stegosaurus table.
WHOA see, Now that's a well named thing in science. You should put cartoonists in charge of everything.
Right, Yes, we should name everything after the caveman that was killed by it.
Yeah. No, I'm just saying trust cartoonists, you know, with anything science.
That's the lesson I learned from that. Yeah for sure.
All right, So this seems like a really broad concept. All the light basically, it's all the light in the universe we're getting from outside our galaxy. It seems it's like a lot. Can we make any sense of it? Or is it all just like a wash?
It's a lot, and it contains an incredible amount of information, and it's really varied. It varies across the spectrum from like super high energy gamma rays produced by really distant active galactic nuclei like blazers, all the way down to like really long wavelength radio that might be produced by like dark matter decaying. There's an incredible amount of information across the spectrum. But we want to see across the spectrum. But we want to see it all the way across the spectrum, and we also want to see where it's coming from in the universe. So it's not just like let's just see it. Let's see where it's coming from and what the energy distributions are. Let's use that to learn about the history of the universe.
Because I guess I wonder, like, how do you tell the difference if it's how do you know it's coming if it's coming from our galaxy or from outside of our galaxy.
Yeah, that's really tricky because our galaxy turns out to be really bright. You might think, well, can't you just point your telescope in the night sky and gather some EBL photons. Yeah, but it's sort of like trying to see the Milky Way. If you're in times square. Times square is so bright you can't even see any stars in the sky. So our Milky Way is so bright that we can't see the distant dim things that are hiding behind it. So there's a lot of competition from the Milky Way, and the Milky Way is not something we understand super well, so it's difficult to disentangle which photons come from the extragalactic background light and which photons come from our own Milky Way. There's an interplay there where if we knew really, really well what the extragalactic background light was, it would help us understand the Milky Way. Or if we understood the Milky Way better, we could subtract it from what we see in the night sky and understand better what the extragalactic light is. We would learn so much either way. But right now, the Milky Way outshines everything, and the whole thing is kind of entangled. It's a big mess.
So when you look at the stars of the nice sky, every star he sees is in the Milky Way galaxy. Right, You can't really see stars that are outside of the Milky Way. So any pinpoint you see out there is in the Milky Way galaxy. So if I wanted to see something outside of our galaxy, I would maybe point my to the scope at a spot between the stars. Absolutely, do you get raw light from outside of our galaxy or is there still like dust from our galaxy there that is maybe polluting that light?
So the answer depends on the frequency of light that you're looking for. You're always looking through the Milky Way, and there's always going to be dust that interferes, but it depends on the wavelengths. Some wavelengths of light can penetrate that dust, some wavelengths of light can't, So it's really a different puzzle at different wavelengths, from gamar rays to X rays to ultraviolet light, there's different sources of photons that get confused between the Milky Way and the extragalactic background light. A big factor is the zodiacal light scattering of dust from within our solar system, which makes everything very tricky.
All right, it sounds like we need to part it by frequency, and so let's break down the spectrum of light from outside of our galaxy and see what it tells us about what's out there. But first, let's take a quick break.
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All right, we're talking about the extra milky Way background light. I just renamed it. I called the extra tag tag is a sizer background light.
I'm not even going to comment. It's all up to you at this point.
It's called the extra Larsen or here here we go. We'll call it the extra far side background light because and that is both true and an homage to Gary Larson. Yeah, absolutely right, because anything outside of our galaxy is technically on the far side of the universe.
That's probably what he meant when he was talking about the far side. He meant the other galaxies.
Yes, well, no, I don't think so, but I'm saying in this context it seems like an appropriate But anyways, let's break it down by frequency, you said, because we're getting a wash by all this life from outside of our galaxy. We don't even know if it's coming from outside of our galaxy. But if you break it down by frequency, you can get a better handle on what's going on.
And the main challenges are that most of this stuff is really dim because the sources are really distant. Everything we're talking about is coming from outside our galaxy, which means there's probably millions or billions of light years away. So it's photons have been spread out. They don't get tired, but they do get spread out. So everything out there is really really dim, or there are sources in the milky Way that are brighter than it, or there's like dust and stuff interfering. But the challenge is change as you go through the frequency, just the same way that the night sky changes in frequency. If you look at the night sky and the optical or in the infrared or in the UV, you see a very different picture.
All right, let's break it down, and let's start maybe with the higher frequencies. What do we see at the high frequencies.
So the very highest frequencies, the highest energy photons. We call these things gamma rays for silly historical reasons.
Related to the Hulk. Right, That's where it started.
It started because in the early part of the century we didn't really understand quantum mechanics or radiation, so we just started naming things like alpha rays, beta rays, gamma rays. We had no idea what gamma rays were then later we understood, oh, they're just high energry photons, but they already had this name, so you can't just say, oh, gamma rays and X rays are really the same thing. This artificial distinction between them, we just sort of stuck.
With it, and so they had the Hulk back at the third of the century.
The Hulk was actually the one doing these science experiments. Remember, Bruce Banner is a scientist man. You should read his papers now, they're great stuff.
Is that famous correspondence between Bruce Banner and Albert Einstein right where they're like pal co them.
It was in German though, right, wasn't it in German?
I'm just messing with you, all right, Sorry, I keep going, keep going. What do we see in the gamma rays?
So the gamma ray night sky is mostly surveyed by a space probe called Fermei LAT, which is basically a particle physics detector in space. Photons hid it and they convert into a pair of electrons and positrons, and then we track those. We can use that to measure the energy. So this is our best way to see the night sky in gamma rays. And if you look out in the night sky, you do see a bunch of gamma rays, you see a huge source coming from the center of our galaxy, Like the center of the galaxy emits a very large number.
And that's from the black hole there, right.
That's from all sorts of crazy processes happening in the center of the Milky Way. There's pulsars that emit gamma rays. These are point sources. There's also just a lot of diffuse emission of gamma rays from like really high energy processes, like electrons getting accelerated really really hard and emitting gamma ray photons. The problem is that we don't really understand the center of the galaxy. So if you just look at like the distribution of gamma rays in the center of our galaxy, we don't understand it. We can't explain it. There's lots of mysteries there, and so that's a real problem. If you want to like subtract out the Milky Ways contribution and look at the rest of the universe, you don't really understand what to subtract out.
You just like point your telescope at the center of the galaxy and then point it away from the center of the galaxy to kind of get a sense of what's coming from the center and what is not.
Yeah, you can measure what's happening at the center, but then you also want to understand how that varies across the night sky. To do that, you really need some sort of model so you can interpolate. So then when you're looking at some other place, you can say how much of this is from the center of the galaxy and how much isn't. You can't just like turn off the center of the galaxy in nature and see the rest of it, or turn it back on or vary it. So we need some sort of like understanding of the center of the galaxy in order to extrapolate away from it and like subtract it out from underneath the extragalactic background light.
So we can study the extragalactic background, or so we can study the center of the galaxy or both.
Both, Because mostly the extragalactic background light in the gammorrays we think is coming from the centers of other galaxies. Other galaxies we think are also emitting gamma rays at high rates, and the extragalactic background light in the gamma rays is then mostly from those other galactic nuclei, and so if we could see those, we could understand our own galaxy better, or if we understood our own galaxy better, we could subtract it out and then understand those other galaxies. So it's sort of a chicken and egg problem. We don't really know how to pull this apart, all.
Right, So then what can we see in the gamma rays when we look around us.
Well, there's this long standing mystery, but the center of our galaxy, whether it's sending us signals of dark matter like dark matter, we know this a lot of it in the center of the galaxy, and we wonder if sometimes when two dark matter particles bounce off each other they actually annihilate. Like there might be dark matter and anti dark matter, and it might be possible for it to annihilate and actually produce photons. This is counterintuitive because you think of dark matter as dark, not shining in any electromagnetic spectrum. But there are theories where dark matter will annihilate itself and make very high energy photons. In fact, there's a signal from the center of the galaxy that we don't understand that a bunch of people think is from dark matter. So we don't understand that very well, and we'd love to look for that signal in the centers of other galaxies, basically see if we can reproduce this in the extragalactic background light. But so far we have been able to pull those things apart and understand which photons come from other galaxies.
Wait wait, wait, First of all, are you saying dark matter is not maybe really dark?
Yeah, dark matter might be shining brightly from the center of our galaxy. Oh boy.
And then can we just point our telescope at another galaxy to see what kind of a signal we get from Then.
Yeah, we can do that, and we can see some other galaxies that are very clear, like galaxies with quoasars in them, or blazars, you know, quoasars that are pointed right at us. Those are shooting really high energy photons right at us, and that we can tell like, Okay, it's definitely there. It's definitely there. So that's a part of the extragalactic background light that we can tell. But not every galaxy has an active nucleus, and we're interested in studying those that aren't active because those are the best ones for studying dark matter. But it's not always clear which photons are coming from our galaxy in which photons are coming from other galaxies. Because remember we're inside the Milky Way, and not all of the gamma rays come directly from the center. Some of them are emitted along the galactic plane, and those are still brighter than the emissions from other galaxies.
Emitted by what, well are we This would be emitted by dark matter in the rim of the galaxy, or.
What maybe dark matter in the room the galaxy. But anytime an electron is accelerated, it's going to emit a photon, and so there's some emission of photons from electrons in the galactic plane that cloud our observations.
MM interesting, So looking at these gamma rays might reveal the what's going on with dark matter.
Yeah, it would be super cool if we could make like a map of these gamma rays from other galaxies and then cross correlated with our understanding of like where the density of matter is, Like we have a pretty good understanding of where the galaxies are and the whole cosmic web. If we could cross correlate these gamma rays from like flumps of dark matter, then to be really powerful evidence that maybe these gamma rays really are coming from dark matter, and not just from other sources of gamma rays.
WHOA, would they have like a special signature if they came from dark matter?
Unfortunately not. There's like energy distribution of these things is not that different from the energy distribution we see from other sources, which is what makes it so challenging and why it's so important to understand all the other sources of gamma rays so we can figure out which ones might be coming from dark matter. It's like you're trying to explain the spectrum with a few different blobs, but the blobs aren't that different, so it's hard to tell how much of each blob you need to use to explain the spectrum that you're seeing.
WHOA. So, then if it turns out dark matter is shiny, would you need to change the.
Name of it, Yes, and we'd come to you first.
I'll call it dark light. That's very star wars. All right, Well, that's gamma rays. It might tell us about dark matter. What about the next range of frequencies in the spectrum of this background light?
So taking a step down an energy you get to X rays, and again there's just an arbitrary distinction between gamma rays and x rays, but X rays are lower energy, and here the technology is sort of like a bridge between particle physics detectors that see gamma rays and more traditional telescopes that see lower energy light. Here we have X ray telescopes, and these use like weird X ray optics because X rays are really energetic and really hard to bend using optics, So there's all sorts of weird tricks they use to try to gather and focus X rays. But we have a couple of cool space telescopes and New Star and Chandra up there observing the sky in the X ray What do you.
Mean they are hard to bend like that, You can't focus them. You can focus them with a lens.
Yeah, exactly. Because of the super high energy, they just don't bend very much through a lens, and so you need special techniques to shape these things, basically like wave guides and weird constructions. We had a whole episode about X ray telescopes and Chandra. Check it out for more details on how to build your own X ray telescope.
Right, you can make them out of bones, right, These X rays don't go through bones.
Yes, And we're calling the next one the Fred Flintstone telescope. Exactly.
Yeah, there is.
We're gonna have a Stegosaurus operator. Of course, if Hannah Barbera wanted a fund one, we would name it after them, for sure. There you go.
I'm not sure there are still a.
Row somebody owns that ip.
Oh, well, you should follow the application.
But the night sky in the X ray is really fascinating because this mostly comes from electrons. We were talking earlier about electrons emitting super high energy photons. They also emit X rays and this is a really cool German name for it. It's called bremstra Lung, which means breaking light. Since you have an electron it changes direction because it hits like a magnetic field or something, it has to give off a photon in order to do that, and based on the energy of those electrons the amount of curvature from we give off X rays. So a lot of the night sky in X ray comes from these electrons giving off Bremstra loa.
I mean, these are electrons that are just floating out there in space and if somehow they change direction, they emit an X ray.
Yeah, exactly. Electrons can change direction when they hit a magnetic field or if they like zoom around a black hole or something. Any sort of change of direction or change of velocity an electron will emit a photon.
And so the universe is just full of these electrons or what They're just floating out there like dust or are they like in galaxies or is this the stuff between galaxies?
Electrons are everywhere, man, just low way protons are. You know, most of the universe is hydrogen, but by that we mean protons and electrons, and often it's in plasma form. It's not neutral hydrogen, so there are also clouds of neutral hydrogen. But there's also just a lot of protons and electrons flying out there, both in galaxies and between galaxies. Remember that between galaxies is not as bright because there aren't stars, but a huge fraction of baryonic matter in the universe, meaning protons and electrons, is actually between the galaxies, not in the galaxies. So yeah, electrons are everywhere, and they're emitting X rays whenever they change direction.
Do you consider this noise or is this part of what you want to see or is this getting in the way of the interesting things you want to see.
This is definitely something you want to see because you want to understand all the sources of it, but it's really hard to pin down these sources. Some of it we can associate with the centers of galaxies, so like active galactic nuclei are pumping out these high energy X rays, but a lot of it we can't. I read one study that said that one percent of X rays can be associated with known objects. The rest of it, we're just like, we don't know what made this. So something out there in the universe is like shooting out X rays and we don't know what it is. Maybe it's just a bunch of more active galactic nuclei that have been like red shifted and are faint, but it could also be other weird stuff like early universe black holes, direct collapse black holes that formed during the early universe and emitted X rays.
What do you mean only one percent, like one percent is coming from these electrons floating around, or we're just getting them from things that might have existed sit in the universe a long time ago that we can't sort of see it with their naked eye.
Yeah, we can't associate them with anything we've seen, so we don't know what's making them. They could just be diffuse electrons. It could be a big chunk of it. There could also be a bunch of new astrophysical objects out there emitting X rays that we've just never seen before because we haven't been able to measure the X ray spectrum outside of our galaxy. So there could be these like direct collapse black holes that formed. Like remember we were talking about having the early universe with this famous moment when the universe became neutral. Protons and electrons came together to make hydrogen, and that was the Dark Ages. You had all this neutral hydrogen floating around, but there were no stars yet. At the end of that there's a moment we call reionization when the universe is then pulling those atoms apart again. That's when we think stars started to form. But it's also possible that black holes formed at the same moment. They didn't just collapse into massive stars. Some clouds might have collapsed directly into black holes, and those formations we think left their imprint in the X ray spectrum. So if we could measure really really well, we might see hints of direct collapse black holes from the early universe.
Oh, I see, like this is stuff happened that happened a long time ago, but it happened so far away. We're only just now getting the evidence of these things that happened a long time ago, exactly.
And it's very dim, much dimmer than x rasources in our galaxy, and it's very hard to pull apart. So if we understood the X ray spectrum super duper well, we could ask questions like is there evidence in there for direct collapse black holes or not? But right now it's a big question mark. We don't know which photons come from our galaxy, which photons come from outside the galaxy. So we're like looking for a really tiny signal and we have big question.
Marks, right, big ones. It ninety nine percent. We don't know what it is, question exactly. All right, let's go down to the next frequency range. This is ultraviolet.
Yeah, so ultraviolet photons are super interesting, and our best measurements of the ultraviolet extragalactic background light actually come from the voyager probes. Remember those probes we sent out in the Solar System to take pictures of the planets and then just continue on out into space. They had instruments on board for measuring ultraviolet photons because they were interested in like the atmospheres of those planets and seeing ultraviolet light emitted from those planets to like study atmospheres and all sorts of planetary physics.
Well, so even today we're still getting data from that spacecraft.
I think actually Voyager just shut down. It ran out of power and we're no longer hearing from it. But until very recently we were getting measurements from Voyager. It's a really long lasting probe and it's one of our best ways to understand the UV night sky.
And now what else can we see in this UV light?
So we can't see very much unfortunately, because the galaxy is pretty bright in this UV light. Like planets emit in the UV. Neutral hydrogen in our galaxy absorbs this stuff and emits in the UV. But it's important for understanding the distribution of matter, like we'd love to know more about the baryonic matter, the hydrogen that's between the galaxies. That's where most of the hydrogen in the universe is. If we could separate the UV light that's coming from within our galaxy from the UV light that's coming from outside the galaxy, then we could understand this better. But we don't really have great measurements here, like Voyager was not set up to measure the extragalactic background light in the UV spectrum. It's just like the only thing we have. So's if you look at the whole spectrum of extragalactic background light, there's like a big gap there in the UV because we really have almost no published studies at all. It's just like a big blank.
But I guess what's making these you raise within our galaxy?
So mostly its clouds of neutral hydrogen. Hydrogen, remember, is an atom, and electrons in the atom have certain energy levels, and one of those energy level transitions corresponds with the ultraviolet spectrum, and so neutral hydrogen tends to glow in many different spectrum but one of them is in the UV.
Uh just closed from being hot.
Yeah, exactly. You think of space as cold, but a lot of this interstellar gas and intergalactic gas is actually quite hot in the sense that it has high velocity and each atom can have a significant amount of energy. Even if we know that if you went out there you'd freeze to death, you'd be surrounded by very sparse hot gas. And when we study this stuff, there's a lot that we don't understand like we can try to subtract the Milky Way contribution in the UV, and then there's all sorts of hints that other galaxies are emitting in the UV in ways that we don't understand, Like the Coma cluster is a famous puzzle. We don't understand the UV spectrum from the Coma cluster. What's going on there? Are those galaxies different from ours? Is there something else between us and that galaxy? Are we just not understanding the Milky Way contributions? It's a really open field to study, all.
Right, Pretty cool? I guess I wonder if it was healthy we stopped putting sunblock on all of our telescopes.
Or directly on our eyeballs.
That never helps.
I think that's a bad idea. Don't do that people.
That's right, good health advice here on the Physics podcast. All right, let's get into our maybe more interesting quency spectrum, the optical or visible light spectrum and infrared to see what is out there beyond the bounds of our galaxy. But first, let's take another quick break.
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Ll right, we're talking about light that comes from outside of our galaxy, which that turns out, is like most of the light that we can't see.
Yeah, most of the photons in the universe didn't come from our galaxy. And yet those are the photons that are hardest to see because they're overwhelmed by the light from our galaxy.
Right, because we're so close to our galaxy, we're in it. But also I feel like it's hard because like it's a big universe out there that's been around for a long time. So we're getting stuff at the same time, stuff that's closed, stuff that's far, stuff that's happening now, stuff that's happened, you know, fourteen billion years ago. So it's sort of like this big wash of light that we're getting that you're trying to sift through.
Yeah, it's sort of amazing that you can understand any of it, you know, But the tricks we use are pretty basic. Look at the direction that came from study over time, studied is a function of energy. Using all those ideas, you can try to pull this apart to make a consistent picture of what's out there in the universe generating all these photons, and in the end that's the goal, come up with the whole history of the universe that can explain all the photons that we can see. But first you got to see those photons, right.
Or I guess you can see them. You just don't know what it is that you're looking at.
Yeah, we see a bunch of photons. We'd love to explain them all, and we're hoping that some of those photons give us hints about what's outside our galaxy, not just what's inside our galaxy.
Right, Well, so we've been going through different frequency ranges in the light that we get from outside of our galaxy and how they can reveal different things, and we're down to the visible light spectrum, like what you could see with the naked eye.
Yeah, exactly, And so in the optical of course, we're very curious about what's out there in the universe, and a lot of the light that's in the optical spectrum comes from stars. Right. The optical spectrum exists because we evolve to be able to see light from our Sun, which makes us also able to see with our eyeballs directly light from other stars. And of course there are stars outside the Milky Way, and so a lot of the optical light that's in the extra galactic background light is emitted by stars in other galaxies.
Right, Because our Sun is a pretty typical star in the universe, Like what the kind of light that our Sun puts out is pretty tychopical all stars in the universe.
It's not that unusual. Our star is actually on the larger side compared to your typical star. The most common kind of star in the universe is a red dwarf, which is a little redder and dimmer than our star. But it's not a big deal. Red dwarfs are still mostly in the optical.
Like, if we had been born under a red sun, we would maybe have different eyeball.
Right, yeah, exactly, life could be very different if we evolved around a red dwarf. We have a whole episode about like how unlikely it is for life to have evolved around a weird star like the sun.
All right, So you're saying most of the visible light that we get from outside the galaxy comes from stars that are outside the galaxy, and these are mostly in other galaxies, right, Like, there aren't a lot of stars floating around, not in galaxies.
Yeah, there aren't a lot of stars out there, but there are some. You know, there are stars in like extended halos of galaxies or that were stripped out from the galaxies during a merger or something and are now just like floating out there in space and that's something we'd really like to understand how much light is coming from outside our galaxy, but also outside the galaxies we can identify, you know, from between galaxies.
WHOA wait, you mean there could be a sun out there in between galaxies all by itself with maybe a planet orbiting around it with life. And what would they see in the night sky?
They would only see galaxies, right, so their night sky would be much much darker. They would only see smudges, no pinpoints of light. Oh, they wouldn't see stars. They wouldn't know that their sun is maybe just another star. Necessarily, it's maybe unlikely because a star that experiences those kind of forces would probably also lose its planets and the extreme gravity, being like tossed out of a galaxy. But it's possible that the planets come along for the ride.
Yeah, it makes me a little sad because they wouldn't be able to wish upon a store.
So you might think that this is like the easiest extragalactic background L like to see, because you just like point the hubble or point your eyeball between galaxies and see what's there. But our galaxy is actually really bright in this kind of light, not just from the stars, also from the scattering of dust. We talked about zodiacal light that you can see from Earth. It's like at sunset you can see this like a cone of light, like a hazy pyramid just above the sunrise or sunset. This is the scattering of light off of dust in our solar system, and it's exactly at the frequency we want to use to observe the extra galactic background light. So it's a big problem.
You mean, like the dust that's out there in space reflects visible light exactly, but it must reflect other kinds of lights as well, doesn't it or only does it reflect visible light especially well.
It reflects visible light especially well. To do with like the size of these dust grains that are like usually one to a few hundred microns, and these are a huge cloud of dust all through the center of the Solar system. It extends out we think, like just past Mars, and it might all be Mars's fault. Actually, dust storms on Mars might be kicking up a lot of this stuff. But it reflects light typically in the optical and also in the infrared, and it makes it really really hard to see the extragalactic background light well.
This is it.
I mean, this makes it hard to see even the galactic light, right because all this dust pollution is within our solar system.
Yeah, exactly, it's a big problem for seeing outside of our solar system. You're right, even understanding our galaxy, the zodiacal light is a big issue, all.
Right, Well, what can we tell from the light that is coming from outside of our galaxy in the visible spikrum.
Well, if you look at all the light that's coming from outside of our galaxy, we can't explain it. Like there's a bunch of photons that are out there that just don't match our predictions. Like if you try to say, here, I understand what in the universe, let me predict all the photons we'll see in the optical spectrum, and then you compare that to what we do see, there's a big gap. Like there must be something wrong with our modeling of what's out there in the universe or how it emits. Like the data and our predictions do not agree.
But what do I mean the gap, Like we're missing light.
Yeah, there are more optical photons in the extragalactic background than we can explain. So that means either there's something else out there emitting photons. We haven't accounted for it, or maybe we've underestimated the amount of scattering from this dust the zodiacal background light. But there are photons out there that we can't explain.
Well, meaning like we look out there and we were getting light, but there's no object there to see.
Yeah, a lot of this stuff is diffuse, right, we don't know what's out there emitting it. We can't associate it with any points source, and so we don't know what diffuse sources of this light there are out there in the galaxy. Maybe there are more of these like rogue stars out there in the middle of the galaxy and all their light like adds up to this big diffuse component to explain it, or maybe it's something simpler or like misunderstanding the Milky Way.
Whoa wait, So maybe this mysterious light is just Milky Way light. It's not necessarily extra galactic.
Yeah, exactly. We can't quite pull it apart. But there's a really cool recent technique they're using to try to get a sense for what's outside of our galaxies. So they're can to help pull this thing apart, and that's by using even higher energy photons that come from quasars. So like the super high energy gamma rays we were talking about earlier that emitted from the centers of very distant galaxies. We think we understand the spectrum that should be coming from them, but as they travel through the universe, sometimes they interact with lower energy photons they can get like scattered or reabsorbed or changed to another energy. So if you look at the spectrum that comes from those distant blazars and you see how it's modified from what we expect, you can use that to try to like map how many extragalactic background photons they ran into along the way.
Wait wait, wait, light can run into light. I thought light couldn't collide with other light particles.
No, you're exactly right. In general, light does not interact with light because photons do not have electric charge, so they don't interact directly, but they can interact indirectly like a photon compare produce, turn into an electron and apositron.
Momentarily like randomly.
Yeah, every photon has a probability to pair produce at any moment, and so there's a probability for two photons to interact with this whole episode about light beams crossing and how you can actually study this. It's a rare process, but it does happen. It requires this indirect process through the electron field.
Okay, So then by looking at a source like equaser that we know kind of pretty well and see how the light is modified when it gets to us. By the time it gets to us, you can sort of get a sense of what's out there in between.
Yeah, you can try. It's tricky, like, first of all, you have to be very confident that you understand the unattenuated light, the light that was emitted by the quays are, and then compare it to what we see. Then you also have to convince yourself you understand everything else that could happen to those photons be absorbed or influenced by other things in the universe. That's why they like to use these very high energy sources because they tend to interact less than other photons, so they're more pristine. Oh.
Interesting, And again it's sort of weird that we're getting all this light and we don't know where it's coming from.
Yeah, exactly, But it's cool to be using these like pencil beams of super high energy photons. To get a measure of like the other low energy photons along the way. It's like we're getting information about photons that would never have reached Earth.
Pretty cool, all right. Now. The last frequency range is the low frequency range of light, and that's the infrared.
Yeah, the lowest frequency range is interesting because you know, it's dominated by the microwaves, which we've studied very very well and talked about, and it's sort of like a good example of what you can learn just by looking at the night sky in the microwave. We've learned so much about the early universe because there were microwaves emitted in the very early universe. This moment when protons and electrons came together in the universe became transparent again. Those photons are still around and we've captured them and measured things about the early universe really revolutionized all of cosmology. That's just like a taste of what we could learn from the other spectra. The microwaves, though, are like thirty or forty times brighter than every other wavelength. For the extragalactic background light, it's like the brightest part of the spectrum, which is why it's sort of like the easy thing and the microwaves are like the first by the Apple, But.
We skipped over i infrared though, didn't we In terms of frequency, infrared is higher frequency than microwaves.
Yeah, absolutely, Infrared is higher energy than microwaves. And the cosmic infrared background is also super fascinating and we'd love to study it in more detail because it might have might be rich with information. Infrared light has lots of really interesting point sources, like star forming regions and galaxies at very high redshift that have been red shifted into the infrared. Super interesting to study, and this was actually studied by the same satellite that measured the cosmic microwave background light. There was an instrument on board that was capable of picking up infrared light, but it's much dimmer than the microwave light and so it's much more difficult to study.
Well, I wonder if all these things just kind of get smooshed together, because you know, I know, we've talked about that things that are really far away they might emit light, but by the times that light gets to it, because of the expanding universe, that light gets red shifted. It becomes more redder. So like if you get red light, it could come from basically anything, right.
Yeah, absolutely, you could have super high energy galactic nuclei emitting very high energy photons and by the time they get to us, they're infrared. Like even the cosmic microwave background light. Those photons are super long wavelength, but when they were emitted, they weren't. The plasma that made them was super high energy. They were emitted a very very high frequency. It's only the expansion of the universe that stretched them out all the way down to the microwave. So you're right, everything is piled on top of each other admitted at one wave, just like.
The messiest kind of light we're getting, you know what I mean, Like, because everything piles onto that frequency spectrum as opposed to like the higher frequencies, things don't get bluer.
Yeah, it's a beautiful mess down there at the long wave lengths because everything's red shift to down there into the dustbin of the universe.
Now, this gets us into the microwave range, which we talked a lot about before. The cosmic microwave background, which you said is included in this idea of the extragalactic background.
Exactly because it's generated by plasma that's outside the Milky Way, and so it's definitely extra galactic. It's also the brightest part of the spectrum, and so it's easiest to tackle, and it tells us a lot about the very early universe. So that's why it's sort of the best well known and the best well studied.
So then the stuff we get that we call this CMB the cosmic microwave background, We know for sure what it is like. We always say it's light from the beginning of the universe. How well do we actually know that? Wouldn't it be kind of confounded or nixed together with distance, stars exploding and things like that.
Absolutely, there are other sources in the microwave, including sources from the Milky Way, But everything is easier when you're looking for a bigger signal, right, it's easier to establish, it's easier to subtract a way. Uncertainties in the Milky Way can larger without affecting your measurements because you have a larger signal you're looking for.
What do you mean it's a bigger signal, Like it's more powerful or just a wavelength is bigger?
I mean it's more powerful. There are more photons, like there are forty times as many photons in the microwave than there are in the radio or in the UV or in the optical. The universe is brighter in the microwaves than they are any other spectrum.
Yes, because of the Big Bang, because of the Big Bang? Was a huge source of it? Or why is it.
Because of this early universe plasma? Yeah, not technically the Big Bang, but there's a lot of emission in the very early universe and that then and that got slid all the way down by the expansion of the universe to the microwave, and so it's sitting there as a very bright.
Signal and we sort of know what it is. But then that means it also mean that that one big source the beginning of the universe is drowning out anything else we might want to see in the microwave.
Absolutely, and that's why we've been studying into great detail and trying to understand all the ripples in it and the other sources of it. Again, when you have a brighter signal, just everything is scientifically, you have more data to play with, so you can do more tricks like extrapolating from one region to the other. You have better ways to validate all of your models. It's much much more difficult when you're dealing with faint sources that you're not even sure you're seeing. So seeing that in the microwave and seeing it exactly the temperature we expected was a great confirmation of our understanding of that whole process.
Interesting, and it also heats up barburritos, all right. So then the last frequency range is the radio waves, which is like the lowest frigency, longest wavelength light that's out there exactly.
And these are really cool experiments to see radio emissions from the deep universe. We have these balloon experiments. Well, you have like a radio antenna, but you cool it down using liquid helium, and then you'll launch it up like thirty seven kilometers above Texas or sometimes above the South Pole, so you can gather radio signals from space. You know, you shield it so that it's not just like getting your local NPR station, and you cool it down so it can pick up the most faint signals, and then you try to gather radio signals from deep space. We measure a lot of these things, but we don't understand all the radio waves that we see. Maybe some of them are from dark matter colliding and emitting in the radio Maybe there's some other diffuse emission of radio. Maybe it's just something in the galactic foreground, something else in the galaxy that's emitting in the radio waves we don't yet understand. So we don't actually know if these photons that we're seeing are EBL radio photons or just vanilla Milky way photons.
Mean, like, if they have an actual like source generating them or they're just kind of like noise. Is that what you mean?
Yeah, if there's a point source generating all these radio frequency photons, we probably would have figured that out, could identify it with like the center of the galaxy or some black hole or some pulsar or something. So that would have been easy, and we haven't been able to do that, which means probably it's something diffuse, like maybe dark matter and the whole halo colliding with it or something else. We don't really understand the sources here.
WHOA All Right, So to recap we're getting a lot of light from the universe. Only about one percent of that light comes from our galaxy. Ninety nine percent of the light that we see that when you look at the nice sky comes from outside of the galaxy, and it seems like ninety eight percent of that is all mystery light, Like we don't know what is making that light, where's it coming from? Right, That's kind of what it seems like.
Well, most of the light in the universe is generated outside our galaxy, but most of the light that we see is generated inside our galaxy because we're inside our galaxy. So most of the light that we see in the night sky is coming from the Milky Way. But that's a tiny fraction of all the photons in the universe, and so the rest of the universe is quite dim in comparison to the Milky Way. But that's most of the interesting stuff in it. These photons contain the whole history of the universe, but they're mostly outshined by the brightness of the Milky Way, which just you know, happens to be nearby.
Oh, I see, there's a lot of light out there, but we don't get all of it, is what you're saying. Because we're so close to the Milky Way galaxy, most of the light that we get goes from the Milky Way.
Yeah, it's like you're standing right next to a lighthouse, and so you can't really see anything that's far away. Even if those photons are coming to you, they're mostly outshined by the local sources.
But well, we can't see the cosmic background outside of our galaxy. It's all sort of shrouded in mystery, it seems.
Yeah, it's tricky to disentangle the Milky Way from the other sources. There's a lot of photons we don't understand, a lot of question marks. Over the next few years, we're hoping to turn on more sensitive instruments that can disentangle these things, make better measurements. Maybe this extragalactic background life's going to come into sharper relief, and it might teach us things about the universe. I could reveal all sorts of weird stuff going on in the deep reaches of space and in the deep history of time.
Yeah, and about the makeup of the universe as well. If it tells us about dark matter and maybe dark energy, I think, Daniel, maybe the only solution here is that you're gonna have to leave the galaxy to get a good look at this light. I'll pack a lot of child We have an ex make an extra Daniel, extra galactic Daniel adventure here.
That's going to take a very very long time.
I'll report, but we got time. We got time, right, I.
Got to make it back before next week's episode. Man, I better build a fast ship.
Yeah there you go. Well, first of all, invent the faster than light spaceship, and then then we're talking.
Sounds good?
All right? Well, another reminder of how much of the mystery we still have to observe and discover and figure out. There are still lots of mysteries out there, even in the light that bathes us every night, every day, all around the earth.
That's right, there is so much of the universe we have not yet observed or understood, so much left to discover for you young scientists out there, the next generation of curious explorers.
Or the old ones do right, I'm just going.
To retire and let everybody else figure it out and explain it to me at this point.
Well, hopefully they do it in time, because come on, we're not getting any young Daniel. All right, Well, we hope you enjoyed that. Thanks for joining us, See you next time.
For more science and curiosity. Come find us on social media, where we answer questions and post videos. We're on Twitter, Discord Instant, and now TikTok. Thanks for listening and remember that Daniel and Jorge Explain the Universe is a production of iHeartRadio. For more podcasts from iHeartRadio, visit the iHeartRadio app, Apple Podcasts, or wherever you listen to your favorite shows. When you pop a piece of cheese into your mouth, you're probably not thinking about the environmental impact. But the people in the dairy industry are. That's why they're working hard every day to find new ways to reduce waste, conserve natural resources, and drive down greenhouse gas emissions. House US dairy tackling greenhouse gases, Many farms use anaerobic digestors to turn the methane from manure into renewable and energy that can power farms, towns, and electric cars. Visit you as dairy dot COM's Last Sustainability to learn more.
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