You might be familiar with the cosmic microwave background, but that’s just one of the background radiations astronomers look at. Some are well known and cataloged, while others are just starting to be possible to see. All of them tell us more about our Universe.
Tests of Big Bang: The CMB (NASA)
Big Bang Cosmology (NASA)
The Expanding Universe (SDSS)
How Two Pigeons Helped Scientists Confirm the Big Bang Theory (Smithsonian Magazine)
The Origin of the X-ray Background (IPAC-Caltech)
Evidence for Cosmic Acceleration and Dark Energy (Chandra X-Ray)
The Gamma-ray Background (NASA)
Background Radiation, Gamma Ray (IPAC-Caltech)
Mystery of the Universe’s Gamma-Ray Glow Solved (Space.com)
Extragalactic background light (Wikipedia)
The Universe is not turquoise – it’s beige (New Scientist)
Cosmic neutrino background (Wikipedia)
Background Radiation, Ultraviolet (IPAC-Caltech)
Transcriptions provided by GMR Transcription Services
Fraser: Astronomy Cast Episode 596: The Universe’s Background Noise. Welcome to Astronomy Cast, our weekly facts-based journey through the cosmos, where we help you understand not only what we know, but how we know what we know. I’m Fraser Cain, publisher of Universe Today. With me as always, Dr. Pamela Gay, a Senior Scientist for the Planetary Science Institute, and the director of CosmoQuest. Hey, Pamela! How you doing?
Dr. Pamela Gay: I’m doing well, but it is so cold in the Great Plains part of the United States right now. So –
Dr. Pamela Gay: – cold.
Fraser: You might be shocked to hear that it is also cold here in Canada. Although –
Dr. Pamela Gay: Wow.
Fraser: – it’s been beautiful. Last week, it was wonderful. This week, -7 Celsius.
Dr. Pamela Gay: And you’re on Vancouver Island, where this is not normal.
Fraser: Well, we get a few of those. Yeah, but we are. We are the Florida of Canada… is the best way to describe. People, for the entire rest of Canada, they come out to the west coast, they come to Vancouver Island, and they never go back.
Dr. Pamela Gay: Yeah, it’s –
Fraser: The ferry – the return trips from the island are empty. Nobody ever wants to leave, because our weather is the best for Canada.
Dr. Pamela Gay: Yeah. Someday, we’re retiring to Canada, my Canadian husband and I.
Dr. Pamela Gay: And it’s always the “Where do we go?” And the only thing that you guys have working against you is airports.
Fraser: Well, I mean, there’s a great airport in Vancouver, there’s a great airport in Victoria. Both are international.
Dr. Pamela Gay: But they’re not 20 minutes away.
Fraser: Well, depends if you want to live 20 minutes away from them, but it’s your call. But yeah. Yeah no, absolutely. The point is you’ve got choices. So, –
Dr. Pamela Gay: Yes.
Fraser: – that’s the key. All right. Now, you might be familiar with the cosmic microwave background, but that’s just one of the background radiations that astronomers look at. Some are well known and catalogued, while others are just starting to be possible to see at all. All of them tell us more about our universe. And we’ll talk about that in a second, but first let’s have a break. And we’re back. All right, so what is a background?
Dr. Pamela Gay: It’s that source of signal that is in the back of all of our images. And I literally mean it’s the backmost layer. If you’ve ever used Photoshop, Illustrator, any of these art programs that have different layers, there’s that layer that’s behind everything, that you can set to “Transparent.” Except for our universe doesn’t understand that.
Dr. Pamela Gay: So, no matter what color you’re using to observe our sky, in the spaces between bright objects, there is light, and sometimes even particles and gravitational waves, that are emanating from some background that we’re still in many cases trying to figure out what is.
Fraser: And I think people are most familiar with the cosmic microwave background, only I think because it helped us figure out the entire origin of the universe, the big bang, this mind-bending conclusion that our universe is expanding. But it’s just one of them. There’s tons.
Dr. Pamela Gay: It is. And I have to say also, the fact that the cosmic microwave background was initially blamed on pigeon poop in the detector also really adds to this story. But –
Fraser: You should explain that. If you’re gonna bring up pigeon poop, and not go into more detail, I think you have to.
Dr. Pamela Gay: So, Penzias and Wilson, two research scientists working at Bell Labs, were working to figure out how to improve microwave communications here on the surface of the earth. They built a big old detector. And they were looking for cosmic sources that could interfere with point-to-point signals used in telecommunications. And they found things they expected like Jupiter. Jupiter is loud. But between all the things they expected to find, there is this constant signal. And normally, when you have this kind of a constant signal in a detector that works like a radio detector, you think it’s just noise in the system.
And so, they were trying to get rid of the noise every way possible. And they noticed that pigeons were roosting in their system. So, they gave it a thorough cleaning to remove anything the pigeons may have left within the detector. And it didn’t work. And in the end, after contacting a research group up at Princeton and talking to the folks like Peebles, they realized that what they were seeing matched theoretical predictions of a cold long wavelength background of light that was the stretched-out remains of photons released in the moment when the universe got cool enough for electrons and atomic nuclei to bond together, making our universe transparent for the first time.
Fraser: And this had been theorized that if – I guess, when astronomers were starting to detect that they were seeing galaxies moving away in all directions – that was one line of evidence – that would mean that those things, those galaxies were all close together in the past. Therefore, you should see a time when everything was all in roughly the same region, and it would be opaque, and it would be hot. And then you should see this moment when it all got released into the universe. And that’s what they saw.
Dr. Pamela Gay: And just to be clear, it’s not that there was any center to the universe, any region where things were more compact, it’s the entire universe, the surface of a four-dimensional hypertoroid –
Dr. Pamela Gay: – was a smaller surface in a three-dimensional on top of a four-dimensional object, kind of way. And –
Fraser: That is –
Dr. Pamela Gay: And there’s no center to a surface like that.
Fraser: Right. But, I mean, even if the universe is infinite, –
Dr. Pamela Gay: Yes.
Fraser: – then it was just – I mean, if you could see it – I think we did the math at one point, it’s 3,000 Kelvin. So, back in the day, it was the surface of a red dwarf star. If you could look at it, it would just be this dull red color. Except it went on forever, in all directions.
Dr. Pamela Gay: And all of that light being emanated from every single point in the universe has continued to fly. So, as we look in all directions, we’re seeing photons that were emitted over there, over there, over –
Dr. Pamela Gay: – there. 13.8ish however old we now think the universe is billion years ago. And they’ve been flying towards us ever since then.
Dr. Pamela Gay: And the photons that were emitted where you are, where I am, someone else on the other side of the universe is seeing those.
Fraser: It’s such an amazing concept. If you could – And we’ve talked about this in the past – that you can tune your television to the background temperature of the universe, and some of the static that you’re seeing is these particles. But you’re seeing a new region of space that has emanated these particles. And they have been traveling through space for 13.8 billion years, and they have finally reached your rabbit ears for your television. And then, you’re able to see it. And each second later, you’re seeing another second’s worth of space. And then each year later, you’re seeing a new light years’ worth of space. You’re always seeing the birth pangs of the universe.
Dr. Pamela Gay: And if we were able to watch long enough, we would see the structure change as we’re cutting through different parts of temperature fluctuations. The universe is kind of amazing, and the cosmic microwave background details that amazingness.
Fraser: Right. All right, well we’re gonna go into some of the things that we’ve learned from that in a second, but first, let’s have a break. And we’re back. So, we know what the cosmic microwave background is this time when the entire universe was just a red star. What does it tell us? What do astronomers use it for?
Dr. Pamela Gay: Well, there are slight temperature variations in it that were generated by soundwaves moving through the early universe, when it was still opaque to light, but compression waves. Soundwaves are just compression waves. Those were still able to rattle around. And the size of those soundwaves is a function of the temperature distribution of the universe, of the density profile of the universe over time. And so, we’re able to start to get at how did the universe expand, how dense was it, what is the geometry of the universe? All from those fluctuations, in one color of the cosmic background, that microwave color.
Dr. Pamela Gay: And one of the amazing things is every color of background light, or particles, or gravity waves –
Dr. Pamela Gay: – gives us different information on the early universe.
Fraser: Right. And we’ve done multiple episodes on the cosmic microwave background, and I think we can be safe to say that no part of the universe has given us so much information, asks for so little than the cosmic microwave background. But I love the way you described that. You call it a color that –
Dr. Pamela Gay: Yes.
Fraser: – that there’s red, there’s green, there’s blue, there’s microwave, and there’s gamma radiation, and more colors. So, then, –
Dr. Pamela Gay: Yes.
Fraser: – let’s talk about some of the other colors that – the other background colors of the universe that astronomers look at.
Dr. Pamela Gay: Well, I’m gonna jump straight over to x-ray, because one of the things that always annoyed me growing up with Superman is how is Superman able to use his x-ray vision if his eyes are sensitive to x-rays, and there’s no source of x-rays going through people to allow him to see through them with his x-ray eyes?
Dr. Pamela Gay: Well, –
Fraser: He’s giving cancer to everybody around him.
Dr. Pamela Gay: Well, there’s actually a background –
Dr. Pamela Gay: – of x-ray photons. And so, it’s super faint, it’s super hard to see, it wasn’t really well understood. And so, we started getting the Chandra X-ray Observatory up there. But, there are so many objects out there giving off light in the x-rays, that unless you are entirely precisely focused, it’s going to appear as a smeared-out background of x-ray light. This is coming to us from things like black holes and binary systems that are producing x-rays as that black hole –
Dr. Pamela Gay: – eats some of its companion. It’s coming from hot gas in the cores of galaxy clusters that are giving off light in the x-rays, ‘cause the gas is just that hot. It’s coming from active galaxies. Quasars. All of these high-energy places are giving off x-ray light. And because the universe is so filled with galaxies, and because it was early in the universe that these galaxies were rich in star formation, had the most active quasars, most active galactic nuclei, that background of galaxies is producing a background of x-rays.
Fraser: Right. I had a chance to talk to some x-ray astronomers about this, and the biggest one is this hot gas. That as galaxies are coming together more and more into larger and larger objects, it’s compressing the gas in the galaxy clusters, and causing more and more of this gas to heat up to millions of degrees. You get to a certain point, when it starts to give off these photons of x-ray radiation. And they described it as the way the universe really looks.
Dr. Pamela Gay: Yeah.
Fraser: That what we think is how the universe looks is a sort of poor shadow of the x-ray radiation that is a much truer understanding of the sort of shape and nature and structure and amount of energy that’s out there, which is kind of a fascinating idea, that.
Dr. Pamela Gay: It’s like we’re only seeing the dust moats, and we’re not seeing the air.
Fraser: Yeah. And that it is these huge halos of hot gas that surround the galaxies as they’re coming together, which is kind of amazing. And that’s – so you went all the way to the extreme.
Dr. Pamela Gay: Of course.
Fraser: Right, to the x-ray radiation. What else –
Dr. Pamela Gay: Because Superman.
Fraser: Sure, yeah. And so, that tells us where galaxies are colliding, where stars are forming, where Superman is blasting out x-ray radiation. What are some other colors of light –
Dr. Pamela Gay: So, –
Fraser: – that we see?
Dr. Pamela Gay: To go to the land of Spider-Man, we also have gamma-ray radiation. I don’t know why my brain is on superheroes today. It just is.
Fraser: It’s fine.
Dr. Pamela Gay: But there’s also a gamma-ray background that’s out there. And this one is caused by things a little bit closer to home. These are the moderate distance, still extremely high distant, active galaxies. The star-forming regions. And it’s thought that maybe even dark matter is flickering in the gamma-rays. And so, when we go out, and we’re trying to use telescopes like Fermi to observe the gamma-ray sky, there’s this background that’s always there.
And when we focus in and focus in, taking hour after hour after hour of observations, they start to resolve into galaxies that have an actively-feeding black hole, that have massive jets, that are spewing gamma-ray radiation along with the x-rays in all directions. This is less intracluster medium, less of that gas between the galaxies, and more of the galaxies themselves going, “Hey, look at me!”
Dr. Pamela Gay: In a very dangerous way.
Fraser: All right, we’re gonna talk about some more colors, more background colors of the universe in a second, but first, let’s have another break. And we’re back. All right, we’ve talked about gamma-rays, x-rays, microwaves. What about visible light?
Dr. Pamela Gay: So, visible light is out there, and there’s two different ways of looking at it. First of all, there’s the “If you were able to average across all the stars in all the galaxies, in-between the galaxies, there’s stars between galaxies too. What color would the universe be?” And it turns out, they actually figured out down to the hexadecimal code –
Dr. Pamela Gay: – what color it would be.
Fraser: I remember that.
Dr. Pamela Gay: And this is cosmic latte.
Fraser: Yeah, I remember that. We were at the AAS meeting when they announced the color of the universe. And they actually made a mistake, and then had to do a recalculation to get the right color.
Dr. Pamela Gay: It initially came out as turquoise, or –
Dr. Pamela Gay: – teal. The universe is not bright and dramatic.
Dr. Pamela Gay: It is the color of coffee with too much milk in it.
Fraser: Right. So, if you take every photon that could fall on a sensor of visible light, and you just average it all out, you would get this milky brown color.
Dr. Pamela Gay: It’s – take a glass of milk, add a small shot of coffee, –
Dr. Pamela Gay: – that’s the color of the universe.
Fraser: That’s the color of the universe.
Dr. Pamela Gay: But the crazy thing is exactly how we see that color depends on if we’re looking from Earth, where we have to deal with the stuff like Zodiacal light in our own galaxy. This is the dust that is reflecting sunlight back at us when we look away from the sun.
Dr. Pamela Gay: If we get up past that Zodiacal light, which has been done by the Voyager, Pioneers and New Horizons spacecraft, well New Horizons, which is the youngest and most equipped of these missions, it was able to look out at the universe, and see that it’s actually a slightly different color. And that slightly different color tells us we don’t actually understand how many galaxies are out there. So, when we made predictions, we were wrong.
Fraser: Right. I think we had talked about this, that there are 2 trillion galaxies in the visible universe, and it turns out, there’s probably less. More like the original number. But I love this idea that even the Hubble space telescope experiences light pollution from sunlight flickering off of tiny bits of dust in the solar system. And only by moving a telescope out far out away from the inner solar system, can you get a truly dark sky view of the universe.
Dr. Pamela Gay: And the way to think about this is if you’re in a super dusty room, you’ve just clopped chalkboard erasers or something, and yeah, the window behind you with sunlight streaming in, those illuminated moats of dust – even though the light’s behind you – those illuminated moats of dust will affect your ability to see things on the other side of them. And the dust inside of our solar system –
Dr. Pamela Gay: – is affecting our ability to see beyond.
Fraser: Although chances are, the results of dust inside our galaxy –
Dr. Pamela Gay: Curious.
Fraser: – may affect our ability to see beyond. So, really, truly, if you wanna send a telescope to have a nice dark sky view, you gotta get outside the galaxy. All right, –
Dr. Pamela Gay: Yes.
Fraser: – so we’ve talked about light. Different colors of light. Yes, x-rays are a color. But let’s talk about some other kinds of backgrounds that astronomers are starting to be able to try to pick up. Let’s start with neutrinos.
Dr. Pamela Gay: So, it turns out that before that cosmic microwave background radiation was formed, there was a point in the universe where the particle physics released vast numbers of neutrinos. And just like the photons have gotten cooler and cooler and cooler, and lower energy as the universe has expanded, those neutrinos are also extremely low-energy. So, we haven’t seen them yet except in our models of the universe.
Dr. Pamela Gay: So, are they really there? We know how –
Dr. Pamela Gay: – to test for it, we just don’t know how to build the equipment to test for it yet.
Fraser: And just – a good analogy is you’ve got photons of gamma radiation being generated at the center of the sun, and they’re bouncing around inside the sun on this random walk that could take 50 – 100 thousand years to get out of the sun. Neutrinos are also getting created at the center of the sun, and they’re coming straight out. They pass right through the sun, through the earth, through you. And so, back at the beginning of the universe, the entire universe was that – the interior of a red star. The neutrinos could get out. And so, you’ve got – suddenly, we can’t see what happened for the first 400,000ish years of the universe.
Dr. Pamela Gay: Yeah.
Fraser: But maybe with the neutrinos, we could.
Dr. Pamela Gay: Get all the way down to one second.
Fraser: One second.
Dr. Pamela Gay: Yeah.
Fraser: We’ll be able to see all the way to one second. So, what might we see? What could we learn if we could see that neutrino background?
Dr. Pamela Gay: Theorists have so many different ideas, I’m not even gonna fathom to put something out there. We only really have observational knowledge of what happened at year 400,000ish.
Dr. Pamela Gay: So, it was a muddled, massive heat energy, and particles in constant collision.
Fraser: But we – I mean, we know that we can receive neutrinos when supernovae go off. We know that –
Dr. Pamela Gay: Yes.
Fraser: – we’re receiving neutrinos from the sun. So, –
Dr. Pamela Gay: Yes.
Fraser: – it could very well be if there are densities over densities under densities, that maybe we could map those out to one second after the big bang, which is just a crazy idea. All right, one last really cool idea, and that’s gravitational wave background.
Dr. Pamela Gay: Yes. And this comes from things like primordial black holes evaporating, mergers of things going on in the early universe when we had rapid star formation followed by rapid star death, where we had constant galaxy collisions. All these different things have the ability to generate gravitational waves.
And primordial black holes are again, things that could’ve existed before the cosmic microwave background, in the moments of the cosmic microwave background, and they would’ve evaporated early in the universe. So, they may not still be around. But the gravitational waves that they gave off could still be moving through space, literally collapsing and expanding at the fraction of an atom level, the size of the universe.
Fraser: How far – how close to the beginning of the universe could you get? Could you actually see the jiggly wiggly big bang written into the background of the back of the gravitational waves?
Dr. Pamela Gay: So, we’re still not going to get further back, according to most theories, than that one second that you get with the neutrinos.
Dr. Pamela Gay: Part of the issue is primordial black holes. Don’t totally understand them, in terms of we all don’t totally understand them. And depending on how they live and die, depending on if Hawking was right or wrong, and how black holes evaporate, depending on what model you have for how they formed, you’re going to see gravitational waves’ different points in time. But the cool thing is unlike the neutrinos, we actually have a handle on how we may be able to observe this.
And that handle is pulsars. Spinning neutron stars, if their magnetic field is out of alignment with their rotation axis, that magnetic field every time it comes by can flash us with radio light. And that pulse is very precise, and it’s related to the distance between us and the pulsar. And if that distance changes because of gravitational waves, and we see a number of different pulsars out in a specific direction, changing in a way that makes sense for differences in distance to each of them, we know that that’s probably coming from a gravitational wave passing through the universe, deforming space time on interstellar levels.
Fraser: Wow. I sort of imagine it like buoys sitting on the top of the ocean, floating, bobbing up and down as gravitational waves are passing them.
Dr. Pamela Gay: It’s the tsunami alert system writ much bigger.
Fraser: Yeah. Absolutely fascinating that we have all of these ways to just see the universe.
Dr. Pamela Gay: And I do need to give a shoutout to ultraviolet background. It is the most boring, doesn’t come from anything exciting, but it’s what keeps the universe ionized so that we can actually look across the universe. So shoutout to you, ultraviolet background.
Fraser: And the infrared background?
Dr. Pamela Gay: Well, infrared, microwave, they’re kind of the same thing.
Fraser: That’s true, yeah. Good point. And radio is sort of like microwave too. Do you have some names for us?
Dr. Pamela Gay: I do. Hold on… I, as always, have to thank all of you who are patrons of our show. While we are starting to go out on television, we don’t yet have advertisers. We’re local to the Houston on channel 20.1. I have to admit, I didn’t know that channels now have decimal points, because I don’t watch digital over-the-air TV. But we are over the air on 20 – no, sorry, not 20.1. 20.10 in the Houston metro area.
And the people that allow us to go out to bring all of you our podcast, Twitch, YouTube, television audiences what we do, are this week – the names I’m gonna read are Emily Patterson, Adam Annis-Brown, Just Joe, Nicole Vorisek, Ed of the Universe, Helga Bjørkhaug, Gordon Dewis, Joshua Pierson, Bill Hamilton, Jack Mudge, richard rivera, Frank Tippin, Sydnie Walker, Alexis, Thomas Sepstrup, William Andrews, Ron Thorrsen, Jeff Collins, Harald Bardenhagen, BenFloss, Jordan Turner, Marek Vydareny, Rayvening, Allen M Price, Mark Van Kooy, Jason Thomas, and Arcticfox. Thank you so much to all of you.
Fraser: All right. Well, thanks, Pamela! We’ll see you next week!
Dr. Pamela Gay: Buh-bye!
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