Detecting the acidity of the ocean with sound, the role of lead in human evolution, and how the universe ends

First up on the podcast, increased carbon dioxide emissions sink more acidity into the ocean, but checking pH all over the world, up and down the water column, is incredibly challenging. Staff Writer Paul Voosen joins host Sarah Crespi to discuss a technique that takes advantage of how sound moves through the water to detect ocean acidification.

Next on the show, we visit the lab of University of California San Diego professor Alysson Muotri at the Sanford Consortium, where he grows human brain organoids—multicellular structures that function like underdeveloped brains. Muotri used organoids to compare a protein that appears to be protective in human brains against the effects of lead toxicity with the archaic version of the protein that was present in our extinct cousins, like Denisovans and Neanderthals. His work suggests lead exposure differently affected our ancestors and our archaic cousins, possibly helping us survive to the present day.

Finally, stay tuned for the last in our six-part series on books exploring the science of death. This month, host Angela Saini talks with astrophysicist Katie Mack about how the universe might end and her 2021 book The End of Everything: (Astrophysically Speaking).

Transcript

0:00:02.9 Sarah Crespi: This is The Science Podcast for October 30, 2025. I'm Sarah Crespi. First this week, Staff Writer Paul Voosen joins us to discuss how understanding the physics of how sound moves through water, can help detect ocean acidification. Next on the show, we visit the lab of Alysson Muotri in La Jolla, California, where he grows human brain organoids. These are multicellular structures that function a bit like underdeveloped human brains. He used these organoids to explore differences between us and Neanderthals when it comes to how we handle lead exposure. Finally, stay tuned for the last in our six part series on books exploring the science of death. This month, host Angela Saini appropriately talks with astrophysicist Katie Mack about her 2021 book, The End of Everything, Astrophysically Speaking.

0:00:56.6 SC: Rising carbon emissions are pushing more and more carbon dioxide into the oceans, making them more acidic. How much more and where is not so easy to figure out. This week in Science, Staff Writer Paul Voosen reports on an approach to measuring ocean acidification that focuses on the physics of sound as it travels through water. Hi, Paul, welcome back to the show.

0:01:17.2 Paul Voosen: Hi, good to be here.

0:01:18.4 SC: So what are some of the complications of trying to get a reading of the pH of the ocean?

0:01:24.0 PV: It's something we can do. We've been able to do it, you know, if you have...

0:01:27.7 SC: Little piece of paper?

0:01:27.9 PV: Yeah. The right instrument and you can be there to put the water in your kind of instrument. Problem, of course, is the ocean is large and we do not live in the middle of it. We've often relied on kind of ship collecting campaigns or point measures at single stations in remote areas like off Hawaii. More recently, they are now kind of instruments on robotic floats that can make measure pH in the ocean remotely and beam it back. So there are different ways to get at it.

0:01:58.7 SC: But there are depths to the ocean. There are vast spaces in the ocean. This is something that comes up actually a lot when trying to study our huge bodies of water, is that there's just so much to cover. How to monitor them all, how to make maps of what's happening underneath them is not easy.

0:02:14.3 PV: Our kind of measures for acidification are very much about the surface ocean and a lot of the deeper waters. I mean, they haven't been exposed for centuries to the atmosphere and they will not necessarily reflect acidification as much. But we're also just not measuring them. Some of these Argo floats now go down to 2,000 meters that make these measurements and then there's still a lot of ocean under there as well.

0:02:40.4 SC: Yeah. So let's get into how sound relates to pH in water. Why would you think that it would travel differently if the water had a smaller pH?

0:02:50.5 PV: Yeah. It's weird, huh?

0:02:52.0 SC: Yeah.

0:02:52.2 PV: It's one of the things that really drew me to this work. Apparently science has known since, like, early 1980s that this is a thing. I think since, like, some very studious French scientists did a lot of measurements in a tank that simulated ocean water. So if you have fresh water, this isn't a factor. But in the kind of salinity, all the kind of the mix that makes up ocean water, you have this effect.

0:03:13.6 SC: So freshwater doesn't work. You have to have certain partners in this.

0:03:17.8 PV: So essentially, we'll take one of the main molecules we're talking about, which is borate. So you have this borate in the ocean, and these sound waves, these kind of sound waves travel through it. And when they travel through it, the pressure, little bump they create causes the borate to shift into a different form called boric acid. It's called chemical relaxation. When it does that shift, it steals a tiny bit of energy from that sound wave, and then it doesn't give it back after it crosses through. And so the sound in this specific frequency gets slightly attenuated.

0:03:56.1 SC: So you said boric acid. So I'm assuming there is a pH angle here.

0:04:00.3 PV: As the pH of the ocean goes down with acidification, you get less and less borate able to form, and then you have less opportunity to change into boric acid. I guess you have more boric acid overall. So boric acid is kind of transparent to this. It's just borate that is kind of the factor. And so the fundamental idea is you have this thing that is tied to pH, then you have another measure at a slightly different frequency, another molecule that is not tied to pH. So you can kind of do these comparisons of how sound is traveling through them and get at the pH levels.

0:04:35.3 SC: It sounds very complicated, but I think it makes sense. How do they test whether or not this is at work in the ocean?

0:04:42.3 PV: This is all stems from these efforts by acoustical oceanographers to just figure out new ways of studying the ocean. In the late 2000s, this small group had put together with a Navy contract these hydrophones, essentially sound recorders that you go in the water that could go to great depths. So they put these out, just kind of lowered it down, or it was free fall down, and then would come back up once it reached 6,000 meters. According to the hallway, they were really kind of just doing like, basic physical measurements about complicated things about where sound reflects and all that. So, one thing that had been talked about a lot around this time was, oh, acidification could make the oceans noisier because these low frequency sound waves don't get blocked by these molecules as much anymore, and so they can travel farther. And so this was kind of very much in the zeitgeist at the time. And from this first measurement in 2009, they were pretty sure they saw it. And then it took like 15 years to be sure. Multiple kind of other research cruises that kept trying to kind of test it and they felt confident it was there.

0:05:52.4 SC: Does that mean the ocean is going to get noisier as we get more acidification?

0:05:56.9 PV: Not much. That prompted a big surge of interest and really was found to be a pretty small effect. And actually now people are thinking that it could actually get quieter with global warming, at least kind of the transient phase right now, because warming at the surface can actually prevent ship noise from getting into the waters a bit more.

0:06:18.6 SC: So what we do know is that this could be a potential way to measure the acidification of the ocean and how it kind of relates to, say, what's going on in the atmosphere. But are we going to run into the scale problem again? Like, how do you test the sound of the ocean all over the ocean?

0:06:35.3 PV: So it covers a lot more with one hydrophone. It's integrating the whole column of water kind of above it. So 6,000 meters, if you're able to get down there. You know, one idea is that you could keep this running somewhere at depth on a mooring or something like that for like, areas of particular interest, especially, say you were trying to verify if you're trying to add carbon to the ocean, which has been a big thing talked about in recent years.

0:07:00.2 SC: So if you're trying to make the ocean more alkaline, you could use this to kind of see if you're succeeding in that. What about undersea cables? You mentioned them in your story. I didn't quite see how that would work.

0:07:09.1 PV: This is kind of all the rage. It's called distributed acoustic sensing or fiber sensing. So these cables can pick up earthquakes, like vibrations from earthquakes on the sea floor, but they can also pick up these higher frequencies, theoretically, that they can hear ocean waves or currents. They've talked about that before. And these just sit out there. They have a power source, they stretch for 100km or 200km. And we could potentially be recording those sound waves from the surface and get a measure of pH that way.

0:07:40.6 SC: So not everybody likes this idea. There are some doubts. What are some of the questions out there about this method?

0:07:46.3 PV: It's really the question of how accurate can it feasibly get. The researchers are pretty confident they could get another order of magnitude kind of accuracy, once it's running out there for a while versus kind of just in and out. And if it got to that point, then that'd be on par with what the robotic floats are doing. That's not certain though. They've proven that you can do it. They don't know if it's like a feasible thing, but that's how everything starts.

0:08:14.5 SC: Definitely. You've written before about other projects that are scanning the chemistry of the ocean using probes and there's this large deployment planned. How does this approach that we're talking about here compare with that? And how do they fit together?

0:08:28.7 PV: They've been kind of slowly being deployed in this deployment of 500 of these floats funded by the National Science Foundation will finish next year called Biogeochemical Argo. And it has all these sensors targeting biogeochemistry, including pH. So these have been tested out in the Southern Ocean, had great results. Now they're kind of fanning out. The US is the only place that has really funded these in bulk, and the funding for that ends next year. And they have about like a seven year lifespan. So they'll keep going. But the question is if anyone will pick this back up after, you know, if the US will continue it, if they'll renew the grant. All things like that are questionable right now. These floats are pretty great. And there's the famous Keeling Curve of the atmosphere showing rise in CO2. They'll be able to do the same type of thing for the oceans if we keep them going.

0:09:21.4 SC: It's a lot more of a multi tool, where it's going to look at a spectrum of values as opposed to just pH, pH, pH.

0:09:27.5 PV: There's oxygen, other things. We did a story back when this grant first was selected five years ago or so. The science that will come from it will be very important.

0:09:37.9 SC: Yeah. So I guess we'll just have to wait and see. All right. Paul, thanks so much for talking with me. It's been really interesting.

0:09:42.9 PV: Yeah. My pleasure.

0:09:43.9 SC: Paul Voosen is the Earth Climate and Planetary Science reporter at Science. You can find a link to the story we discussed at science.org/podcast. Next up, we hear about how it's possible modern human brains were better protected from lead poisoning than our cousins like Neanderthals and Denisovans.

0:10:18.5 SC: This fall, I got the chance to travel to San Diego with producer Kevin McLean. We were able to visit a couple different labs that had just published in Science or the Sister Journals. And our first stop was at the Sanford Consortium, where Alysson Muotri grows brain organoids. I was super excited to see an organoid. I've read a lot about what they can and can't do and how they're definitely not many brains in a dish. But I've only ever seen pictures. They can do certain things that brains can do, but they don't have all the cells, or they're kind of developmentally stunted but still, what does a brain in a dish look like? Before our tour and the much anticipated organoid reveal, I sat down with Alysson to talk about the study his group published in Science Advances and the way they use these brain organoids to look deep into human evolutionary history.

0:11:07.7 SC: We know our extinct hominin relatives, like Neanderthals and Denisovans were different from us. Sequencing of ancient DNA shows genomic differences. We also know from that that some of their genes are present in modern humans. If we have enough bones from an extinct relative, they tell us something about appearance and body mechanics, and skulls give us clues to brain size. But none of these tell us much about how their brains function differently from ours. Recently in Science Advances, Alysson Muotri and colleagues wrote about an approach to understanding how extinct hominin brains function differently when it came to lead exposure using brain organoids. Hi, Alysson, welcome to the podcast.

0:11:49.7 Alysson Muotri: Thank you so much. You got it right.

0:11:51.3 SC: Okay. Good. That's very gratifying to hear. Genetic comparisons can tell us, for example, that Neanderthal brains were different because their genes don't match ours exactly. But finding out the function of those changes, those differences in the genes, like when they form proteins or when they control proteins, is a different story. Your lab has this approach. Okay. I need help saying this.

0:12:14.1 AM: Archaeologization.

0:12:15.4 SC: Archaeologization. Which basically means taking an archaic version of a gene and introducing it into a living tissue or cell to see how it functions differently than modern humans today. This idea, you're going to apply it to brain organoids to better understand the effect of these genetic shifts from our ancestors, our relatives, and us. Can you give us the basics of what an organoid is before we go any further?

0:12:40.4 AM: An organoid is a tissue made from stem cells, and usually we use pluripotent stem cells. These are the type of stem cells that are very similar to the embryonic stem cells that can give rise to all the different tissues in the body. A brain organoid is when we drive that stem cells to become a brain tissue. And it's not a mini brain in a dish, but contains all the major cell types and the functionality that's quite similar to a developing human brain.

0:13:07.8 SC: And functionality, you mean like you can see signaling?

0:13:10.5 AM: Signaling, electrophysiology, the formation of complex networks.

0:13:14.5 SC: There's a lot in this paper that we need to talk about, actually. We're going to try to bring lead in now and then we'll get back to organoids. So I always thought that lead exposure, you know, if you think about lead, is bad for your brain, and we know about that from exposures to lead paint or when it was in present in gas in many countries for a very long time, you know, is an industry thing. Like we started smelting metal a long time ago for us, but not for the evolution of humanity. What would make you think that ancient hominins like Denisovans or even Australopithecus like Lucy would be exposed to lead? Where would that come from?

0:13:51.5 AM: Yeah. So that's a great question. And it starts with my collaborators from New York who have these really nice technology where they can analyze the heavy metal presence in the enamel of the tooth. And they were applying that to study autism and other neurodevelopmental conditions to make sure that there's a correlation between these heavy metal and the neurological conditions.

0:14:14.2 SC: So this is in modern people, if you look at their teeth, looking for lead.

0:14:17.9 AM: Yes, exactly. But they also did that on the fossil records, using tooth fossil from ancient hominids, and they originally did from the Neanderthals and realized that the Neanderthals have lead contamination, suggesting that start in early infancy. And then they start exploring how far back is lead contamination. And turns out that it's quite a while. So we are talking about 2 million years ago. We are already exposed to lead. We cannot know precisely how the contamination happened, but we imagine that these ancient hominids, they were looking for caves to escape from winter, for example. And several of these caves have water that is contaminated with lead. So we think that this is most likely what happened.

0:15:05.3 SC: What we're seeing is that people have never been without lead.

0:15:08.1 AM: Exactly, yeah.

0:15:09.0 SC: Or our ancestors, our relatives have never been lead free. It's kind of always been going on. And this was so fascinating to me. The pattern in the teeth from these fossils, and they're from all over the world, they're from China, they're from South Africa, France. They show patterns in teeth of lead exposure.

0:15:24.6 AM: That's correct, yeah.

0:15:25.6 SC: Yeah. And the fact that that lead pattern looks so familiar, and it looks like what happens in modern humans suggests that it's not a result of these teeth being fossilized, but rather this is lead exposure, probably during development.

0:15:40.4 AM: The technology that we use is quite precise and can even tell you the age when the contamination happened. So there is no doubt that the contamination was there. And you can also compare to samples that were not contaminated from modern humans. We are less and less exposed to lead because we know the deleterial effect of lead.

0:16:02.3 AM: Your last few moments before you've seen an organoid, what do you feel?

0:16:06.4 SC: I already got told how big it was. I've only ever seen them glowing, you know, beautifully lit in photographs. Right?

0:16:14.5 AM: The real thing.

0:16:15.5 SC: The real thing is just gonna be clump of cells, my guess. Alysson toured us through some pretty familiar looking lab seeds, benches, shelves up top, lots of equipment boxes piled up. And then took us to a smaller room where people were working in clean hoods, you know, with gear on. And yeah, there were organoids in there.

0:16:36.3 AM: I think Sandra might have even older organoids, like one year old or something.

0:16:40.9 SC: What?

0:16:41.3 AM: Yeah.

0:16:41.7 SC: I didn't realize that you would keep them around so long.

0:16:44.3 AM: In theory, they should not die.

0:16:46.4 SC: Do they continue to differentiate or do they just...

0:16:49.4 SC: We're going to work our way back to the organoids. We're getting there. Starting with genes and proteins that are important for what happens in the brain when we have lead exposure. So we want to look at what happens with modern humans compare it to the ones that are all extinct. So what do we know about the genes that are affected, turned on, turned off by lead?

0:17:07.3 AM: When you are exposed to lead, the cells react in different way. Precisely, your neural progenitor cells. These are the cells that are incorporating during the brain formation. These cells have like a peculiar response to lead and they activate different molecular pathways that are related to neurodevelopment. By looking at that, we can understand why lead is so problematic because it will interfere in the way the brains form, affecting the migration of the cells, the survival of the cells, and finally how the cells connect to each other, forming complex networks. So, lead exposure in early infancy is quite complicated because it definitely affects the brain. So we know that once we study what kind of genes are being mainly affected by lead, it turns out that Nova1, which is a protein, a master regulator of gene expression, is one of the first responsive genes by lead exposure.

0:18:05.7 SC: And that sounds like it makes sense if it is going to influence such an early and complex process that it would be a regulator gene rather than a single protein with one job. So what about Neanderthals and Denisovans? What do their genes look like? Do they have the same version? Is that one of the differences you saw?

0:18:22.4 AM: Yeah. So Nova 1 is actually one of the genes that differentiate us modern humans from our extinct relatives.

0:18:28.9 SC: And there's not that many, so we should probably flag that, right?

0:18:31.3 AM: That is true, yeah. So in the beginning we thought that the world or many, because we're just comparing to the European genetic population. As soon as we start looking into other human populations and contrasting their genomes with the Neanderthals and Denisovans, we can ask what are the genetic variants in protein coding genes that are unique to modern humans? We end up with a list of 61 genes. So it's a really small list.

0:19:00.3 SC: That is a very small list, absolutely.

0:19:01.4 AM: And Nova1 is one of them.

0:19:03.5 SC: And it's related to what happens with lead exposure And so that's a very interesting question to say, well, did they deal differently with lead? I mean, obviously that's not only job in the body, in the brain, but it might actually have a different effect on these archaic humans.

0:19:17.7 AM: And that's exactly the experiment we did. So we have generate brain organoids carrying either the archaic version of Nova1 or the modern version of Nova1. And then we expose these organoids to lead and we ask, I mean, are they responding differently? Basically they all respond in the same molecular pathways. However, on the archaic version, we see a deleterious effect. It's quite toxic for neurons that express FOXP2 gene. It's another gene that is implicated in human language. And that was a big surprise. So that's the major difference between the two species.

0:19:54.3 SC: You did a lot of analysis of your organoids. Like you looked at transcription, you looked at development, you looked at the effect on different cell types across the board, you know, these different conditions with and without lead. The summary of it though seems to be that the modern human was maybe more resilient to lead. Is that kind of your takeaway?

0:20:13.9 AM: That's correct. Yeah. Our FOXP2 expressing neurons do not die, so they're not that toxic to lead. So it suggests that the version, the modern version of the gene, the genetic variant that we all have now is protecting us to lose the FOXP2 neurons.

0:20:31.9 SC: Can you tell us a little bit more about FOXP2? It's kind of a famous gene for being associated with language.

0:20:37.1 AM: It is a gene that when mutated, cause problems with language and communication. So people that carry mutations on those genes, basically the brain, cannot process the language. It is a gene that has been proposed to be different between humans and Neanderthals, but the gene is exactly the same in both.

0:20:55.4 SC: So maybe its regulator is different or its regulator does different things under lead exposure. We can't draw, like a whole story from this. This is just the beginning of the story.

0:21:05.2 AM: Exactly, exactly. Yeah. But we were intrigued by that discovery. I mean, the fact that these FOXP2 gene is expressed in neurons, especially corticothalamic neurons, and they create, like a secretary that's involved in language. And we see that these are the neurons that are more sensitive to lead. And the fact that we carry a resistant variant is quite intriguing. So it favor us to have complex language rather than the Neanderthals or other extinct hominins.

0:21:34.4 SC: That is fascinating. We can't say this is the story of humanity's rise to language, but we can say this is intriguing evidence for that. What else do you think needs to be kind of solidified about this? And why are organoids not the end all be all answer for this kind of question?

0:21:49.5 AM: Right. Although the organoids are a nice tool, there is still like an in vitro system with many limitations that are intrinsic of the model. I mean, a brain organoid is not a fully functional brain. There are lots of missing parts or missing cell types or missing brain regions that we are not taking in account. And it could be that there are compensatory networks. That's why this is like an hypothesis now. So we need better evidence. But I must say that while this manuscript was under review, there was another group who actually humanized the Nova1 in mice.

0:22:24.9 SC: Ooh.

0:22:25.3 AM: Yeah. And that was published a little before our manuscript was accepted. And what they see is that the mouse changed the vocalization.

0:22:35.4 SC: So they need to give a little lead to the mice just. That is fascinating.

0:22:40.4 AM: I think perhaps one important thing is if this is really affecting language. And that's why I think it's probably like a strong selective pressure, because language is our human superpower. Birds can fly, other animals have other superpowers. Language is our superpower. If that's true, if lead is giving us that advantage over the other hominids, it really equipped us with an ability that's an outlier compared to any other species. All the other species will have the archaic version of Nova1. Only us have the modern version that protects our ability to have language. So not even the Neanderthals might have a brain that is able to produce language.

0:23:26.8 SC: It has this irony to it, right? Lead is not good for us. If you have children, they're tested for lead, they say, oh, is there this paint in your house? Like is your kid...

0:23:36.4 AM: Lipstick.

0:23:37.0 SC: Yes.

0:23:37.4 AM: To me, I think it's one of the few observations that we are connecting extinct genes and we're not only looking at how the gene is working on the cells, but also how the environment is affecting tendencies in the cells.

0:23:51.2 SC: Absolutely.

0:23:51.6 AM: We start adding the environmental factors in the equation now.

0:23:55.0 SC: That is so interesting. Absolutely.

0:23:56.9 AM: We never imagined this. Science is way more complex than we can anticipate. So it was a discovery by chance.

0:24:03.6 SC: Someone saw lead and they're like, what's this?

0:24:05.7 AM: We had like a first science paper showing that Nova1 affects neurodevelopment. And one of the remaining questions was, okay, at one point in evolution we got that mutation and it's now fixed. All humans have it. For that to be fixed so quickly it's so beneficial. Right? And what is the benefit of that? That's when I came across Manish, the New York team paper. And within a week or so they were here in San Diego giving a talk. And I talked to him and said, "Do you have any follow up on that? " Said, "Yes. By the way, the whole hominids are contaminated by that."

0:24:43.2 SC: I mean, I was so surprised to see it in so many branches. It is not, not rare. That is just amazing.

0:24:49.9 SC: Super interesting. Is that what's going on right here? What do we see?

0:24:53.1 Speaker 4: This is organoids.

0:24:54.7 SC: Oh, are those... I mean, they look like embryos. Sorry guys. I mean, what am I gonna say?

0:25:01.4 S4: 14 days.

0:25:02.2 AM: 14 days. Ok, very young organoids. In organoids, what you probably are seeing now, it's mostly the neural progenitor cells. More mature cells are not there yet. They are still self organizing. That's why you see some of the structures being formed inside. They're forming what we call rosettes, which is the development of the ventricular zone.

0:25:23.5 SC: It is very different from like an organism starting from one cell going to two. But it's not...

0:25:29.0 AM: Because in the early stages what we do is we block the differentiation of all tissue but the brain.

0:25:35.0 SC: Do you have one that's been around longer?

0:25:38.1 S4: They're in the microscope, you'll see that.

0:25:40.2 SC: Oh, hey, I can actually see them. You know what it's like? It's like when you break open a beanbag chair and it's little styrofoam.

0:25:47.5 S4: This is from February.

0:25:49.5 SC: February? Okay. Yes. We're in the tenth month, so it's.

0:25:54.9 AM: Nine months. So they're just born now.

0:25:56.3 SC: Yes. That is very cool.

0:25:58.0 AM: So each one of those is probably like 5 million cells, 2.5 million neurons in there.

0:26:03.4 SC: Oh, wow. And so when you...

0:26:04.9 SC: Big thank you to Alysson Muotri and his many, many laboratory folks and Kevin McLean for all their help with our visit to the laboratory at Sanford Consortium. And yeah, I got to see an organoid. It looked like a tiny styrofoam ball. Stay tuned for the last in our six part series on books exploring the science of death. This month, host Angela Saini talks with astrophysicist Katie Mack about how the universe might end.

0:26:40.1 Angela Saini: Hi there. I'm Angela Saini, and we've hit the final episode of this year's book series, exploring the topic of death. We've looked at it from every angle, from the science of aging and how to protect our digital remains to how animals experience grief. But for our last book, we're moving away from our life here on this planet and asking how death might look on a universal scale. The author I'm speaking to this month is Katie Mack, a cosmologist at the Perimeter Institute for Theoretical Physics in Canada. Her book The End of Everything, Astrophysically Speaking came out around five years ago, and it surveys the different ways that the entire cosmos might end. Katie, you're a hugely popular public figure in the sciences, especially on social media. What made you choose this topic for your first book, especially given how depressing it could be for some people?

0:27:37.9 Katie Mack: There were a few things. One is that I knew that there were a lot of books about the beginning of the universe and there just weren't very many about the end. And I thought that that was an interesting story that deserved to be told. And I also found that when I give public talks and I talk about the end of the universe and what might happen, people get really emotionally involved in that. And I thought it would be fun to spend some time really digging into those topics.

0:28:03.4 AS: So how do scientists know that the universe will die? I mean, why won't it be here forever? And how much sooner than that end will the earth disappear?

0:28:13.7 KM: It's a question of definition, sort of what it means for the universe to end. In my book, the end that I'm considering is all of the structure in the universe we see today is destroyed. And I think that we have really good reasons to believe that will happen because the universe is evolving and it's changing. And as it's evolving and changing, it is getting, in some sense, less habitable, less suitable for life and existence. And so one way that's happening is that the stars are eventually going to burn out. We're going to work through the fuel for new stars, and there's no sort of renewal for that, as far as we can tell. And also, there are certain principles in physics, like the second law of thermodynamics, that just sort of suggest that things evolve toward disorder. So just taking those facts, the fact that the universe is expanding and that we are kind of working through the material for continued existence and working toward this higher entropy, higher disorder state, suggests that we can't just continue on forever happily, in a beautifully structured cosmos.

0:29:22.3 KM: And so that kind of points toward the idea that the universe will end in some way. For all practical purposes, we won't be around. There won't be anything interesting left in it. But in terms of the timeline, I mean, the earth is kind of doomed by the sun. Eventually, the sun will get brighter, it'll get bigger, we will no longer be in the habitable zone of the solar system, in the sense that the ambient temperature on earth will get too hot for things like liquid water. In something like a billion years, we won't be able to live on this planet at some point. Whether or not we might live somewhere else, I don't know. But the timeline for things like the stars burning out is much, much, much longer. Like trillions and trillions of years for just the stars that we know about now. And then there could be other things that carry on after that.

0:30:09.9 AS: So in all likelihood, humans won't be around to see the end of the universe. With any luck, we might still be around to witness the end of the earth. But your book is divided into five possibilities for how that final, final end might happen. And I'm reluctant to miss out any of them. So I want to kind of take a whirlwind tour through each one in turn. The very first one you write about in your book is a big crunch where the universe, which is now expanding, turns back in on itself. Can you explain how this might happen and what it would look like?

0:30:42.3 KM: Yeah. So we know the universe is expanding right now. We know that distant galaxies are getting farther from us. There's more empty space. And we don't entirely understand all of the reasons for the universe expanding the way it currently is. We know that there's something called dark energy that is making the universe expand faster. But we don't know exactly how that works or what that is. If it changed at some point, that could cause the expansion rate of the universe to change, and it could even change enough to reverse. And so if that happened, we would start to see distant galaxies coming closer. The ambient temperature of the universe would start to go up as all of the radiation in the universe gets kind of compressed into smaller spaces. And eventually things would start to get very hot, very uncomfortable, everywhere in the universe.

0:31:31.1 AS: Right. So kind of like a universal climate change scenario almost.

0:31:35.4 KM: Yeah. Or like the trash compactor scene in Star wars where the moles are kind of caving in. It would start to feel like that in the cosmos. Yeah.

0:31:45.1 AS: And the next one you look at is heat death, which sounds like it might be a bit related. And when you say heat death, what you actually mean is an increase in disorder of particles or energy entropy. How would that scenario play out?

0:31:56.6 KM: So it's just an extension of the idea that the universe is currently expanding. And if it keeps expanding in more or less the way it is now, driven by something we call a cosmological constant, which is a version of dark energy, where the universe just has a kind of expansion property built into it that causes this expansion and causes it to speed up in its expansion, then at some point, the universe just gets really, really empty. Everything gets so spread out, so diffuse, that all of the and energy sort of diffuse through the cosmos, everything kind of decays away.

0:32:31.1 KM: And eventually you get into a situation where the universe is basically pretty empty, only this sort of disordered energy, or this sort of ambient waste heat of everything that ever happened, is kind of very thinly existing in the cosmos. And that leads to what we call the ultimate heat death of the universe, where you get to kind of this maximum entropy state where everything is just disorder and nothing really happens anymore. That's such a long time in the future that we don't really have kind of words to describe how far away that would be. But that's a version of the end of the universe where it's not like space time is falling apart or anything like that. It's just that nothing is happening anymore. Everything's kind of decayed away.

0:33:13.8 AS: So that sounds like the more kind of death by old age. Very gradual. And then you write about a big rip tearing apart the fabric of reality, which sounds so dramatic. How would that happen?

0:33:27.5 KM: So the Big Rip is a version of dark energy that goes wrong. So the kind of dark energy that we think is most likely responsible for the accelerated expansion of the universe is this cosmological constant. It's just kind of built into the cosmos. But if dark energy is driven by something that evolves over time, and specifically something that gets stronger over time, so the acceleration of the universe gets even more powerful, and the expansion speeds up more and more and more in a particular kind of way, then it could get to a point where essentially the space between any two objects becomes infinite in a finite amount of time. That's what the Big Rip refers to, this idea that everything just kind of becomes torn apart. We think that that's something that's very, very unlikely, because the kind of dark energy that would be needed to do that is something that we think doesn't fit very well with our current understanding of how physics works. But we don't really know how dark energy works. And there have been some interesting recent results that have called into question our assumptions about what dark energy is doing. And so it's something that people do occasionally talk about the possibility of what we call phantom dark energy and what that would do to the cosmos.

0:34:37.2 AS: So your book was written five years ago. Does that mean that things that we've learned about dark energy in that space of time has already updated these possibilities?

0:34:47.1 KM: A little bit, potentially. The most relevant thing is there was a recent result from a survey looking at how galaxies are sort of moving through the universe as the universe is expanding. There are hints in that data that maybe dark energy is something more dynamic, more changeable than the cosmological constant. And some hints that the possibility of this phantom dark energy, There could have been some signs of that in the past. It's unclear what we should make of those results because they're, in a sense, sort of preliminary and depends on what kind of data sets you're looking at. And in terms of the interpretation, that's still very much being worked out. But it is an interesting point that when I wrote the book, pretty much all of the data was very much consistent with dark energy is just a cosmological constant. It's not changing. It's not doing anything particularly exciting. Now that's not quite as true.

0:35:41.6 AS: Right. That's interesting. So just going back then to the topics that are covered in your book, the next chapter that you look at is looking at the possibility of vacuum decay, which is this fairly new idea resting on this theory that the vacuum of space isn't, in fact, stable. Why is that, then, a problem for the future of the universe?

0:36:00.3 KM: Yeah. So the idea behind vacuum decay is when we talk about the vacuum in this context, the vacuum really just means the background state of the universe, how physics works in our universe. And we know that there was a time in the very early universe, or we're pretty sure that there was a time in the very early universe when the laws of physics were different, because certain things were different in terms of what kinds of particles existed in the universe, what the different sort of energy fields that exist through the universe, what those were doing. Specifically the Higgs field, which is a sort of energy field that has to do with how particles got mass in the early universe. We know that was different in the early universe. And when it changed, that is what allowed particles to get mass. And it set up kind of the standard model of particle physics, how physics works in our universe. And the big question in that field is, could it change again?

0:36:52.9 KM: And if it did change again, that would change the laws of physics for our universe, and it would change what kinds of particles can exist and how they can interact with each other. That process is known as vacuum decay. We don't know if that'll happen or not. If it does, we don't know when it'll happen. But it would be pretty catastrophic for physics in our universe because it would mean that the fundamental particle interactions we rely on for, like atoms and molecules to exist wouldn't be in play anymore. And maybe the kinds of particles that can exist would not be the same. And so we think it's very unlikely that it could happen. Certainly we think it's extremely unlikely it could happen within the next 10 to the power of 100 years or something like that. But it's one of these things that theorists look into as possibility because we're trying to better understand kind of how the fundamental physics of our universe works.

0:37:44.8 AS: Yeah. It must be incredibly difficult to work on such long timescales. I mean, it's just mind blowing in so many ways. So the final chapter of your book puts forward this theory called bounce based on gravitational waves. And I have to say, this is my personal favorite way for the universe to end. Thank goodness I won't be around to witness it, though. Can you explain this one? What does it mean?

0:38:04.7 KM: When I talk about bouncing cosmologies, there are several different ideas that have come around over the years. Ideas where the universe could expand and then re-collapse and then expand again, or could expand and then get to a point where it sort of sparks the production of new universes after that. One of the possibilities I talk about is an idea where there is a kind of a compression and then that sparks a new big bang. And all of these ideas are a little bit more preliminary, A little bit less explored, but they're fun because they suggest that the conditions of the early universe were set up by a previous cycle, and could also suggest that something might persist as a sort of relic of our universe when it ultimately ends. I think that a lot of people find that really appealing because they like the idea of some kind of continuity.

0:38:55.8 AS: It's comforting in a way that things won't just end completely.

0:39:00.2 KM: But at the moment, I think we're not at a point where we can say definitively if any of those models is going to work out. But things like gravitational waves could give us some ideas of ways to test some of those models. So it would be very exciting to look for clues of those.

0:39:14.9 AS: So which one, out of all these possibilities, do you find the most compelling, then, as a cosmologist working in this space, or have you since then, seen other theories that you think might better fit?

0:39:27.0 KM: The heat death model seems to be most consistent with the data at the moment. It seems to be the way things are trending. But personally, I really like the idea of vacuum decay. I think that it's an exciting thing to work on, where the quantum mechanics of subatomic particles and fields could be so cosmically important that little tweaks to our understanding of particle physics could change how we think the entire universe is going to evolve. And I think that's neat. As somebody who works in cosmology that is sort of tinged with particle physics, I like finding those connections between the very, very biggest and the very, very smallest scales and trying to better understand how everything fits together. And so vacuum decay is particularly fun because it's such a great example of how intertwined the physics of the cosmos are with the physics of the subatomic world.

0:40:24.6 AS: Finally, and this is the final edition of our series, we've been looking at death from lots of different perspectives, but mainly earthly perspectives, not universal ones. But contemplating the death of the universe must put into perspective for you our ultimate insignificance here on earth, that whatever we do, the universe is going to end anyway. What is that like for you? How has writing this book changed the way you think about life?

0:40:50.8 KM: It was really a process of coming to terms with, as you say, how insignificant we are, how little control we have over the universe. When I wrote this book, I got a lot of feedback from people who told me that the concept of vacuum decay was terrifying to them because it's this possibility for the universe ending in a very sudden, unpredictable way. Even if I said it's extremely unlikely, very far in the future, we don't even know if it could happen, people get nervous about that. And it made me think about the fact that there's so much in life that we don't have control over and we kind of have these comforting fictions that we hold about what we do have control over or how even if we die, we have some legacy into the future and that makes it okay. It really made me think about how maybe there's nothing that's going to make it okay in the end. Maybe there's no final wrap up that's like, everything was great. Maybe what we really need to do is to find meaning in the moment, in existence right now and not rely on some future wrap up to make it all have made sense and been worthwhile.

0:41:57.2 AS: I don't know how to feel about that. Katie Mack, thank you so much.

0:42:02.7 KM: Thank you. Thank you. This was lovely.

0:42:04.6 AS: And thanks also to all of you for listening. Like I said, this was our final episode. I hope you've enjoyed all the books. Keep your eyes peeled for our new batch of authors next year.

0:42:18.8 SC: And that concludes this edition of the Science Podcast. If you have any comments or suggestions, write to us at sciencepodcast@aaas.org. To find us on podcast apps, search for Science Magazine or listen on our website science.org/podcast. This show was edited by me, Sarah Crespi and Kevin McLean. We had production help from Podigy. Special thanks to Angela Saini for all her work on the book series this year. Can't wait to see what we do next year. Our music is by Jeffrey Cook and Wen Koi Wen. On behalf of Science and its publisher, AAAS, thanks for joining us.

Cover photo:  NASA/JPL-Caltech/ESA/CXC/Univ. of Ariz./Univ. of Szeged