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What the "background noise of the universe" tells us about spacetime's origins REGINA BARBER: You're listening to Short Wave from NPR. Hey, Short Wavers. It's Regina Barber. Can I admit something to you? I learned about the beginning of the universe the way I learn about a lot of things, on TV. [AUDIO PLAYBACK] STEPHEN HAWKING: For thousands of years, people have wondered about the universe. [END PLAYBACK] BARBER: That it all started with a singularity, a brief moment in time called the Big Bang. [AUDIO PLAYBACK] HAWKING: One view of the universe prevailed. [END PLAYBACK] BARBER: But here's the thing. Even though that definition is fairly common, it's not how many scientists talk about the Big Bang now. CHANDA PRESCOD-WEINSTEIN: Today, when you hear people use the phrase, Big Bang, they often mean the time period that we might otherwise call the early universe. So they're often talking about, I don't know, roughly like the first few 100,000 years of space time's existence, the Big Bang era, instead of just the Big Bang. BARBER: That's Chanda Prescod-Weinstein, a theoretical physicist and an expert in cosmology, which is the study of how the universe began. But how do we observe that beginning, that Big Bang era? And why do we talk about it differently now? To understand that, we have to go back 60 years, to a lab with a giant horn-shaped antenna in New Jersey, and to two radio astronomers, Arno Penzias and Robert Wilson. RENEE HLOZEK: You see photos of them, they're standing next to this giant antenna. Arms wide open, looking at the sky. And one of the things that they noticed is it didn't matter where they pointed this detector in the sky, they had a residual noise, kind of a background. BARBER: Renee Hlozek is an observational cosmologist. She's also a spokesperson for the Dark Energy Science Collaboration, which studies the acceleration of the cosmos. HLOZEK: They spent a lot of time trying to figure out what it could be, including the bird poop that had been collected in their detectors. They tried to calibrate all their instruments. BARBER: They thought it was New York maybe? HLOZEK: Yeah. They were just like, where is this coming from. But it wasn't directionally dependent, and that's super important. So if you look anywhere in the sky, you would see this. BARBER: So they put out a paper with their findings, basically saying, we found this background noise and we don't know what it is. HLOZEK: And at the same time, people were saying, oh, if we understand how the nuclei formed in the early universe, we understand that the universe must have been hot, and they must be a background. So in a sense they were looking-- trying to design experiments to look for this. But Penzias and Wilson serendipitously discovered it first. And so as soon as they put out that paper saying, oh, we see this background, the theorists who are working on this were like, oh, shoot, this is exactly what we thought we should get. So it was that discovery of the cosmic microwave background, which we call it now, that, to me, catapulted this era into the modern age. BARBER: So today on the show, the background noise of the universe. We get into what the Big Bang is, why scientists talk about it differently than they used to, and how the cosmic microwave background and its fluctuations help us understand the early universe. I'm Regina Barber, and you're listening to Short Wave, the science podcast from NPR. So we talked about how the cosmic microwave background was found with this like serendipitous accident, basically. And the scientists found out we could detect radiation from the beginning of the universe. And so Renee, based on this background, this CMB, what do we know about the early universe? HLOZEK: OK. So the universe is hot at this early time. And it's so hot that it sort of-- we think of the three phases of matter as solid, liquid, gas. But really, there's a much more important phase of matter to my heart, which is plasma. It's so hot that you don't have atoms. You just have electrons and nuclei, atoms. And because the electrons are free, they can interact with photons. So they basically bounce around as if you're trying to walk through a big crowd or you're at a nightclub. The photons and the electrons are interacting with each other, right? They're bouncing around. It's the same reason why the sun is opaque. And it's opaque because of this bouncing around. We basically say that the photon can't travel very far without interacting with another electron. And so what happens is as everything gets older, everything gets cooler, of course. The same is true of the universe. BARBER: I slowly got that. OK. HLOZEK: So as the universe is cooling. Eventually, it gets cool enough that, in fact, the electrons and protons can now combine, and they can become neutral atoms. And what happens is at that moment, which we call the last scattering surface or the last scattering between photons and electrons, it's kind of like the photons are now free to stream away. And so that is the moment when essentially, we can see the photons because they're now streaming towards us from the distant past. And that radiation is what we call the cosmic microwave background. So it's this light from about 400,000 years after the beginning of the Big Bang, in this Big Bang era. And we can see it on the sky. BARBER: And it's not like only in the sky. It's all around us, right? HLOZEK: Yeah, absolutely. We're actually just swimming in the light of the Big Bang. BARBER: Yeah. HLOZEK: A nice thought that I like to say to people is if you get on the subway in a city, and you sit down, and you feel that the seat is warm, you know that someone was there before you, right? Because you have the heat, residual heat under the chair. And that's the same thing. We look out in everywhere in the sky, and we see this microwave light just everywhere. It's that hot subway seat but all over the sky. BARBER: I have sat in many of those subway seats. I love that analogy. OK, Chanda, but this light in the cosmic microwave background generally has like this very specific temperature of about like 2.726 Kelvin or roughly negative 455 degrees Fahrenheit? I'm just reading these numbers. I don't have them in my brain. Like, shout out to our producer Hannah Chen for that research. Thank you so much. And that's like super cold. Like, how is temperature associated with this light? PRESCOD-WEINSTEIN: Right. So If we think back to the earlier moments in the cosmos, so kind of that hot Big Bang era, we have this time period where the universe is very hot. And as Renee was talking about, there is this plasma. Photons are bouncing around. They're hitting electrons. And then at some point, the universe is just cool enough that those photons start flying freely. They're not running into things. They can kind of travel for a long distances without hitting something. And this is like the Doppler effect, like an ambulance coming towards you and then away from you. First, the sound waves, like, compress, and then the sound waves stretch out. And so the same thing can happen with light, and this is happening as spacetime is expanding. And so this effect is also happening with the light that makes up the cosmic microwave background radiation. And that means that the light, as we see it today, has a different energy and wavelength associated with it than the energy and the wavelength that it started out with, simply because spacetime has had over 13 billion years to expand since that time happened. And so that means, yes, that in the future, like very distant future, like about 4.5 billion years when the sun goes nova, that that temperature is actually going to be different than the one that we measure today. So that temperature that we're measuring today is a snapshot of the CMB, the Cosmic Microwave Background radiation, now. BARBER: OK. So when we're looking at this like cosmic microwave background temperature, what does that tell us about the early universe? HLOZEK: So the key thing is that we're not only interested in that overall temperature, but we're interested in, does the temperature differ slightly from position to position on the sky? Because any temperature differences actually start to give us insights into the theory of how this cosmic microwave background happened and what the physics was like. And what we found is that from place to place on the sky, the cosmic microwave background is different by only one part in 100,000. Now, that's like phenomenally uniform across the sky. It's kind of like saying a drop of water in a gallon, if you put like a tiny drop of food coloring or whatever. But those fluctuations in the temperature from place to place tell us something about what the physics was like at that early time. BARBER: Right. I mean, they tell us about the variations in density during that like early time, which in turn tell us something about the origin of galaxies, right? So what do you use to make these like heat maps of the cosmic microwave backgrounds? Like, how do scientists measure these, like tiny fluctuations? HLOZEK: So the devices are called transition edge sensors. So we don't think of the telescope as a mirror in the same way you do for an optical telescope. We have these huge detectors with all these tiny little individual detectors, thousands and thousands of them into a big array. And then we scan the sky. Typically, we'll scan the sky from different places. So there are a bunch of different experiments. We have a big telescope that has-- that allows us to collect this microwave light. And then at the back of the telescope, there are these detector arrays typically. And those are either put in telescopes in Chile or telescopes at the South Pole or sometimes in space. BARBER: Got it. Got it. What I think I would like to talk about next is, like, how do we use that data for theory? So Chanda, when we have all of this data that's like building up over the decades, and now we're going to probably get even more sophisticated devices in the future, what does that tell theory? What does that tell us about the beginning of our universe? PRESCOD-WEINSTEIN: Yeah. So I will actually just say that this is a hard problem because part of the work is making sure that what we call the data analysis pipeline is well understood, that the instruments are very well calibrated and that we know how those things work. Then assuming all of those things are true, there are a series of assumptions that have to be made about what the correct model of cosmology is in order to actually process the data. And so in fact, this is not something that's widely known. The cosmic microwave background radiation is actually our strongest piece of evidence for the existence of dark matter because you get that match between the theory and the experiment so beautifully if you put dark matter into it. You take dark matter out of it, and the line-- they don't match anymore. BARBER: Dark matter being this, like, invisible, mysterious matter in the universe scientists are trying to understand. PRESCOD-WEINSTEIN: But it is in many ways a little bit like putting a puzzle together, where you're like, if I put the wrong piece here, then none of the other pieces around it are going to fit. BARBER: Which brings me to like my last question. Earlier, we talked about the early cosmic microwave background as opaque. We can see the immediate aftermath of the Big Bang era, but we don't know what caused it, right? Is there any way for us to, like, kind of figure out what caused the Big Bang by looking at all these, like, parameters and trying to fit these models? Does it tell us anything about what happened before the very first milliseconds of time, or is it-- is that impossible? PRESCOD-WEINSTEIN: Yeah. So I think, as with most things in science, we have to get kind of specific about what we're looking for. And on some level, this question is also about what is the boundaries of what science can do and what we think science actually is. I think, like, the bigger point that I would want people to walk away with is that there are times when we come up with ideas that we think we can't test or that we don't know how to test, and that later becomes clear that we actually can test them and that it is a matter of time and human ingenuity. BARBER: Yeah. I mean, that's part of Penzias and Wilson's story, right? Like, they make this discovery by accident, and it turned out to be, like, foundational. That's one of my favorite things about, like, science and being a scientist. And you know this, Chanda and Renee, like it's all about this constant discovery of like new things. PRESCOD-WEINSTEIN: Right. I spend my life on dark matter. It could be that the first real definitive evidence for direct detection of dark matter in the laboratory is going to happen the day after I die. I work every day knowing that that is just how the cookie crumbles. I think my hope as a scientist is not that I will be the one to make the great discovery or idea, and I think that's a very outdated way of thinking about what science is about. My hope is, is that the work that I do now helps to continuously lay the foundation for us to push the boundaries of our understanding forward. Whether it's this generation or a generation like seven generations from now that works out some of the problems that I've committed my life to, science is a multigenerational enterprise. And so I think that that's really the way to think about these questions, is maybe right now, we're not sure that we can test or answer that question. But as scientists, it's not our job to say never. It's our job to figure out, could this be possible later? BARBER: I love that. Thank you so much, Chanda and Renee, for coming to talk with us today. PRESCOD-WEINSTEIN: Thank you for having me. HLOZEK: It's super fun. BARBER: Before we head out, a reminder that we'll be back tomorrow with our regular Short Wave and back Tuesday with our final installment of the Space Camp series. And I have this sneak preview from one of our experts. KATIE MACK: Hey, Shortwavers, it's Katie Mack, cosmologist and connoisseur of cosmic catastrophes. Now that you're experts on the universe as we know it now, join me as we theorize about how it could be in billions or maybe trillions of years. There's still a lot to learn about our ultimate cosmic future. BARBER: This episode was produced and fact checked by Hannah Chen. It was edited by our showrunner Rebecca Ramirez. It was engineered by Maggie Luthar. Julia Carney is our Space Camp project manager. Beth Donovan is our senior director, and Colin Campbell is our senior vice president of podcasting strategy. Special thanks to our friends at the US Space and Rocket Center, home of Space Camp. I'm Regina Barber, and you're listening to Short Wave, the science podcast from NPR. Copyright © 2024 NPR. All rights reserved. Visit our website terms of use and permissions pages at www.npr.org for further information. Accuracy and availability of NPR transcripts may vary. Transcript text may be revised to correct errors or match updates to audio. Audio on npr.org may be edited after its original broadcast or publication. The authoritative record of NPR’s programming is the audio record.