ARIELLE DUHAIME ROSS: This is “Science Friday” and I’m Arielle Duhaime-Ross in for Ira Flatow. Let’s face it, one day the universe is going to end. Not just the Earth. Not even just our galaxy. All of it. Every star. Every nebula.

And there are a lot of possibilities about how exactly that will happen. In 2020, Ira talked with someone who’s been thinking about the end of the universe. Dr. Katie Mack’s book, “The End of Everything” looks into these possibilities. It’s our SciFri Book Club pick for April and we wanted to go back and listen to that conversation. Here’s Ira.

IRA FLATOW: Welcome, Dr. Mack.

KATIE MACK: Hello. Thanks. Thanks for having me.

IRA FLATOW: So tell me, I’ve got to say this, this is an awfully cheerful book about the end of the universe. Is that how you feel?

KATIE MACK: I think I’m just excited about big things happening in the universe. And I guess I have some professional remove from the idea of everything really ending. Although, there are some points in the book where I do sort of wrestle with that idea that we will have no legacy in the cosmos ultimately. And that is a scary idea.

But it’s fun to think about these big, powerful destructive forces.

IRA FLATOW: Does that bother you that we’re not going to have any legacy? I mean, do you think about that as a scientist?

KATIE MACK: I do think about it sometimes. And sometimes, I think that’s fine, we’re just doing our thing. And someday that’ll be over, and that’s how it should be. And other days, it’s a little bit unsettling. I mean, I’m not fully comfortable with the idea that I’m going to die. And I’m certainly not comfortable with the idea that the whole universe is going to die. But it seems like that’s the way of things.

IRA FLATOW: Give us a short tour of what could possibly go wrong to end our universe?

KATIE MACK: Well, there are several possibilities based on different ways that we look at the data and how we extrapolate what’s happening now in the universe into the future. So in the book, I talk about the Big Crunch, which is this idea that the expansion of the universe that’s currently happening now could reverse and everything could kind of come crashing back together. That’s probably not how it’s going to happen but that’s an idea that’s been kicked around for decades as one of the possibilities.

IRA FLATOW: Let me stop you there, and tell us why that’s probably– you, as a scientist, know or feel it’s probably not going to happen that way.

KATIE MACK: Well, when we look at the expansion of the universe, one of the things that we can observe about it now is that the expansion of the universe is speeding up. That means that the distant galaxies that are moving away from us are moving away from us faster and faster all the time. And that suggests that there’s something in the cosmos that’s accelerating the expansion of the universe. We don’t know what that is. We call it dark energy.

But in the presence of dark energy, it’s hard to imagine that accelerated expansion stopping and turning around and everything coming back. Now, we don’t know for sure because we don’t understand dark energy. But based on what we do know about it, it looks like it’s probably something that’s just going to keep going the way it’s going and the universe will continue to expand forever.

IRA FLATOW: OK, let’s go to option door number two.

KATIE MACK: Well, that’s the heat death. This is what we think is probably the most likely based on how we see the universe evolving now. Again, this is just the universe keeps expanding forever and the end result of that is that everything gets more and more isolated. So galaxies get farther apart from each other, everything gets more and more sort of contained in its own little space, it’s harder for things to interact with each other over these increasing distances.

And so over time, the universe just gets more and more diffuse, darker, colder. There are no new stars forming after a while and particles decay and everything kind of fades and decays away. And ultimately, you’re left with basically just the waste heat of the cosmos.

IRA FLATOW: OK, let’s go to option number three.

KATIE MACK: Well, this is a dramatic one. This is called the Big Rip. And this is based on the idea that maybe dark energy is not what we currently think it is. So our best guess about dark energy right now is that it’s something called a cosmological constant.

Basically, just a property of the cosmos that space time has this sort of expansion built into it. And so if you have some space, it’ll have a tendency to expand. We think that that’s probably what the dark energy is. We’re not entirely certain.

If it’s something else, if it’s something that isn’t just a property of space but some kind of field in the universe that changes over time, it could be something that gets more powerful over time. And if that’s true, then it could actually instead of just moving galaxies away from each other, it could start pulling galaxies apart and pulling stars away from planets, then pulling apart planets and stars themselves, and eventually ripping apart atoms and sort of tearing space itself asunder.

IRA FLATOW: Very interesting. I love talking about dark energy, dark matter. I mean, we talk about we don’t know what 96% of the universe is made out of, right? Dark energy and dark matter. So then how can we make predictions about anything if we don’t know what most of the universe is made out of?

KATIE MACK: Well, we don’t know what it is but we know a lot about how it acts. For dark energy, whatever it is it’s something that’s making the universe expand faster. And we can see the expansion of the universe. We can see how galaxies are moving apart from each other. And because we can see into the past in the universe by looking at very distant things, we’re looking at things as they were billions of years ago. We can see how that expansion has changed over time.

And so we can deduce what dark energy is doing to our universe from those observations. We can survey galaxies all over the universe and see how they’re moving and how they’re relating to each other. With dark matter, dark matter is a very different phenomenon. Dark matter is some kind of invisible matter.

So it’s something that has gravity, it has mass. But we can’t see it and it seems to be the stuff that’s holding galaxies together. It’s sort of this extra gravity where there are clumps of this invisible matter and galaxies and clusters of galaxies and so on are embedded in these clumps of this invisible dark matter.

Now, we don’t know what dark matter is made of. But we can map it out. We can figure out what it’s doing and where it is by how it affects the regular matter, the stars and the galaxies that we do see in the universe. So we might see stars at the edges of galaxies moving around faster than we would expect and we can say, oh, that must mean there’s extra gravity holding them in.

We know a lot about where it is and how it acts. We just don’t know what it’s made of. Now, we have an option number four that it’s sort of a relatively newcomer, is it not?

Yeah. Yeah, so my personal favorite cosmic apocalypse is called vacuum decay. It’s sort of been around since maybe the ’70s or ’80s. But it’s gained a lot of prominence recently because as we’ve learned more about particle physics and as we’ve done things like discover the Higgs boson, which is this particle associated with the Higgs field, a kind of energy field that pervades all of space, it’s allowed us to understand our model of particle physics a little better.

And what we’ve found is that it looks like maybe particle physics or maybe the physics that governs the universe isn’t quite as stable as we thought. What we’re predicting now is that the way that physics works in our universe is not really the only option. And the Higgs field, which is this energy field, it can have different properties and it can change. And so there’s a possibility that somewhere in the universe there will be a quantum event that changes the Higgs field in that point in some single spot in the universe. And that would create a bubble of a different kind of space where physics works differently inside that space and that bubble would expand outward at about the speed of light and just destroy everything in the universe.

And it would be an unpredictable event. We wouldn’t know when or where it would happen. But it would be a totally inescapable bubble of doom. Now, there are reasons that we’re not convinced that that’s definitely going to happen. Because first of all, it depends on the idea that we really understand all of particle physics.

And we’re not quite that arrogant. We know that there are pieces missing to our puzzle. And so it may be that something will come up that’ll prove that vacuum decay can’t happen. But the current equations kind of point that way, which is a very interesting consequence of the calculations people have been doing recently.

IRA FLATOW: If there is a bubble that bubbles up inside our universe, could there be a bubble that bubbles up outside our universe too and have another universe?

KATIE MACK: Well, outside of our observable universe, the region of space that we can actually see with telescopes and so on, we have no idea what’s going on out there. There could be vacuum decay happening there. There could be new universes popping up out of the larger space we’re embedded in. There could have been many universes being created around the same time as ours or popping out of some larger space. There’s a lot that could happen beyond the realm of our observable universe, which is this volume about 46 billion light years across that we can actually get any information from.

IRA FLATOW: So what we visibly can see is not what might be out there. There might be a lot of other stuff out there that is beyond our horizon.

KATIE MACK: Yeah. Yeah, and our horizon is defined by how far away something can be where the light from that thing can have reached us by now in the age of the universe. So if there was a galaxy 46 billion light years away and the light left that galaxy at the moment of the creation of the universe, it would only just be reaching us now. So that defines the distance we can see because anything farther away from that, there just hasn’t been enough time for the light to reach us.

It turns out that because the universe is expanding faster and faster, we’re actually never going to be able to see things that are beyond that point because they’re being carried away from us faster than light can travel by the expansion of the universe. So there’s this real hard edge to what we can see. There’s a lot of interesting mysteries around what might be beyond our horizon.

IRA FLATOW: I know as a theorist, you’re not just inventing these ideas out of thin air. My question is, where does the data come from that’s most useful and important to you?

KATIE MACK: Well, there are a lot of different things we can study. We can look at how galaxies are moving through the cosmos. During the expansion of the universe, we can see galaxies moving apart. We can look at very distant galaxies and see how they’re different from galaxies that are nearby. And that tells us something about how the cosmos has been changing over time.

And we can look at actually the background light from the Big Bang itself. One of the most important pieces of data we have about the early universe, about the evolution of the universe comes from the fact that we can see the afterglow of the Big Bang. So if we look at a galaxy that’s five billion light years away, you know, we’re looking back billions and billions of years. If we look farther away than that, if we look as far out as we can see, we can see the universe as it was at the very beginning.

Because it took 13.8 billion years for the light to get to us from that point, we can see it as it was in the very, very early stages of the cosmos. And what we see when we look as far as we can see is we see this glowing light. This light from a universe that is so young, it’s just finishing up the Big Bang and it’s still in that primordial fire stage. So the early universe was hot and dense and sort of roiling with plasma. And we can see that because we can look so far away that we’re looking so far back in time that we see that hot early young universe.

And that light has in it patterns of little blips of higher density here and lower density over there, and it tells us what the sort of seeds of the structure of the universe looked like and how the universe went from being this kind of fiery, roiling plasma state to this big, cool cosmos with galaxies and clusters of galaxies and so on in it.

ARIELLE DUHAIME ROSS: That’s cosmologist Katie Mack talking to Ira in 2020 about her book “The End of Everything, Astrophysically Speaking”. It’s our SciFri Book Club pick for April. Find out all you need to know, including upcoming events and how to win a free book on our website sciencefriday.com/bookclub. That’s sciencefriday.com/bookclub.

We’d like to finish this hour with a remembrance of Nobel Prize winning psychologist Daniel Kahneman. He died this week at the age of 90. His work turned many traditional ideas about economics upside down, arguing that people often make bad decisions that go against their own self-interest. It’s something he continued to study throughout his career and he wrote about it in “Noise, a Flaw in Human Judgment,” a book he co-authored in 2022. Here he is talking with Ira about that book.

IRA FLATOW: Daniel Kahneman, welcome to “Science Friday”.

DANIEL KAHNEMAN: My pleasure.

IRA FLATOW: Nice to have you. All right, let’s begin talking about this. The title of your book is called “Noise”. What is noise and how is it different from bias?

DANIEL KAHNEMAN: Well, the starting point really is that judgment is a form of measurement. We call it the measurement, where the instrument is the human mind. And so the theory and the concepts of measurement are relevant. Bias, in the theory of measurement, is simply an average error that is not 0. That’s bias.

Noise, in the theory of measurement, is simply variability. So you could measure a line and measure it repeatedly, you’re not going to get, if your ruler is fine enough, you’re not going to get the same measurement twice in a row. There’s going to be variability. That variability is noise.

And you can see that noise is a problem for accuracy because assume that there is no bias. That the average of your measurements is precisely equal to the length of the line. It’s still obviously you’re making mistakes if your judgments or your measurements are scattered around the value. So that’s noise and that’s bias.

IRA FLATOW: So why do people make those mistakes? Why do we have people measuring things and then coming up with different results?

DANIEL KAHNEMAN: Well, there are several reasons. One reason is that really people are inherently noisy. So when you sign your name twice in a row, it doesn’t look exactly the same. We cannot in fact exactly repeat ourselves.

We’re in a series of states. And those states have an effect on the judgments we make. We call that occasional noise. So a judge passing sentences is not the same in the morning and in the afternoon. The judge is not the same in a good mood and in a bad mood.

And then there are two other kinds of noise. To understand the next form of noise, the easiest is, well, let’s stay with the judge. So some judges are more severe than others. Some judges are lenient. We call that level noise because the level of their judgment, there is an individual bias.

But then the most interesting source of noise is that judges do not see the world in the same way. That is if they had to rank defendants for crimes, they would not rank them alike. Some judges are really more severe with young defendants than with old defendants. For other judges, it’s the opposite.

Those differences, which we call pattern noise, they’re really interesting. And they are in quite a few situations, they are the main source of noise.

IRA FLATOW: Is that because that’s where biases may influence the noise because people have different biases that makes it noisy?

DANIEL KAHNEMAN: That’s exactly it. Noise is really produced by the fact that there’s certainly pattern noise that people have different biases.

IRA FLATOW: One last question. I’ve been following your career for a long time and I’ve always wondered what got you and your longtime former psychologist partner the late Amos Tversky so interested in human biases and studying. Where did you fellas decide this was something you wanted to study?

DANIEL KAHNEMAN: Well, it was really ironic research. We found that we were prone to mistakes. It was all about statistical thinking when we started. And we noticed that we had wrong intuitions about many statistical problems. We knew the solutions. And yet, the wrong intuitions remained attractive.

IRA FLATOW: Can you put a finger on why we have so many flaws in our intuitive judgment?

DANIEL KAHNEMAN: So it’s not that we could perform surgery and excise all the sources of biases from human cognition. If you removed all the sources of biases, you would remove a great deal of what makes cognition accurate in most situations. So we are built to reach conclusions not necessarily in a logical way, but in a heuristic way. And heuristic ways of thinking always necessarily lead to some mistake.

Although, on average, they can lead to correct judgments and faster than reason would do.

ARIELLE DUHAIME ROSS: Nobel laureate Daniel Kahneman talked with Ira Flatow in 2022. Daniel Kahneman died earlier this week. He was 90.

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