How can anyone agree on the age of the universe? And how can we be so confident if it’s all just a bunch of models? What if the universe is extra lumpy, does that change anything? I discuss these questions and more in today’s Ask a Spaceman!

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EPISODE TRANSCRIPT (AUTO-GENERATED)

When I say that the universe is 13.77 billion years old, it sounds rather authoritative. And it's not just because of the values after the decimal point. That just makes it sound precise. And we're very proud of achieving that level of precision. Thank you very much. No, it's the extreme amount of confidence that I can just sit here, look you in the eyes, and say in no uncertain terms that we estimate the age of the universe to be 13.77 billion years old. But how? How do we know, like really know, how old the universe is? And what does it even mean for the whole universe to have an age? So let's dig in. In today's episode, I'm going to tell you how we calculate the age of the universe and how we got so dang confident. And then I'm going to present three potential challenges to our method of calculating the universe's age. And I'm going to knock down those challenges one by one with the sheer force of my confidence. And science. Mostly science. Okay, first objection. Can the universe really have a clock? Didn't special relativity, which is kind of important and kind of well-tested for over a century, say that there's no such thing as a universal clock? When we go way back in time to the pre-Einstein era, our conceptions of the structure of the universe were rooted in Newton.

How do objects know when and where to interact with each other? For Newton, what allowed this was a universal clock and a master ruler, some set of standards against which everything else could be measured. No, there wasn't literally a giant clock sitting behind some nebula ticking away cosmic time, and there's no bureau of universal standards and measurements that has, I don't know, a bar of gold that says this is exactly one meter against which all other measurements shall now and forever be made. It's a conceptual idea, a way to frame the mathematics so that everything lines up and makes sense. For Newton, objects and events have no intrinsic sense of time or space. They have to rely on some external entity. We'll go ahead and call that entity spacetime for convenience sake, even though he didn't think of it that way, to know where and when they are at any given place in time. So Einstein destroyed all that, as was his usual style. He said, and so far he's been right, that there are no universal clocks, that there are no master rulers, that every single object in the universe has its own individual special unicorn frame of reference with its own special way of measuring the passage of time and the distances between objects.

That sounds like chaos. And it is. Under special relativity, synchronization is a joke. Something that can happen for brief moments when objects are close together. As soon as they move, it all goes away. But ever the salesman, old Uncle Albert provided the solution to the problem that he created. The mathematics of special relativity. Which tell us how to translate one frame of reference into another. No, we're never going to agree on how long a second is or how wide a meter is, but at least we have the machinery to check notes against each other's. So if there's no master ruler or master clock in the universe, if there's no absolute frame of reference, if everybody and anything has their own version of the passage of time, then how can we say with any confidence at all that the universe is 13.77 billion years old? Doesn't that depend on who's doing the measuring? The answer is... Nah. Special relativity says that the universe doesn't have to have a preferred reference frame, a special absolute scale.

But that doesn't mean there can't be one. Enter general relativity, which is like special relativity but with extra sauce on the side. And that extra sauce is the ability to capture the behavior of spacetime when things get really complex. Complex as in, for example, the evolution of the entire universe. In cosmology, we're faced with a straightforward observational fact. Distinct galaxies appear to be receding away from us. This seems like a complex problem involving gravity and the nature of spacetime, so we turn to general relativity to help us build the model of what's going on. In the simplest and simultaneously most comprehensive model, in physics, by the way, we like to use the word parsimonious, what's the most we can explain with the simplest possible model, aka how cheap can we be and get away with it, is called the FLRW metric. These letters stand for Freeman, Lemaitre, Robertson, and Walker, four folks who in the early 20th century put all of cosmology into a single coherent picture.

And in this picture, this model of how the universe works makes one key distinction that allows for a break from special relativity. Stuff. The universe is full of stuff. Stars, planets, nebulas, your lost socks. There's all sorts of matter just laying around. But it's not just laying around, it's moving around. And it's moving around in a very particular way. It's expanding. As time goes on, the stuff in the universe expands, getting thinner and cooling off. Ah, that gives us a shared history. Epochs in the universe are different. Yesterday, the universe was a little bit smaller, a little bit thicker, and a little bit warmer. Tomorrow, the cosmos will be bigger and thinner and cooler. Because the universe is expanding, the symmetry is broken. The past is different than the future. And any observer in the universe, no matter what galaxy they live in, if they become sophisticated enough to develop cosmology in their version of the FLRW metric... will come to the same conclusion. And they'll all agree that the universe was different in the past.

And they'll be able to calculate how much different the universe was in the past. And from there, calculate how long ago the past was. Which means we can build a universal clock. But wait. Wait, wait, wait, wait, wait, wait. Hold up. This is all based on the assumption that galaxies are receding away from us. And I actually cheated a little. The actual observation isn't that galaxies appear to be flying away. It's that the light from distant galaxies is redshifted. This is a tiny distinction, but in science, details matter. We see galaxies redshifting away from us. That's the original observation. And to be fair, the critical observation today. This result came to us from Edwin Hubble's work in the late 1920s. The common accepted explanation is that the universe is expanding. The light from distant galaxies gets redshifted because as the light travels the long, lonely depths of space, it gets stretched out by cosmic expansion. But for quite a bit of time, it was debatable what exactly was causing this redshift.

It wasn't fully accepted that this was due to the expansion of space. The simplest interpretation, that the galaxies are well and truly flying away from us, doesn't work out so well. That's because it's not just that the galaxies are redshifting. It's that the amount of redshift is proportional to the distance. The greater the distance to a galaxy, the bigger the redshift effect. If the redshift is due to motion, then the galaxies twice as far away from us would have to all know that and conspire to move away from us twice as quickly, which is weird. Another option was proposed by astronomer Fritz Zwicky. Now, old Fritz was a bit of a character. He did a lot of important work. For example, he coined the term supernova, which deserves recognition just by itself, not to mention his early discovery of dark matter. But he also had a habit of going against the status quo. And you know I'm always a fan of cantankerous and crotchety old scientists doing their best to hold back the tide of progress.

One of my favorite quotes comes from him. He said, quote, I have read every paper you ever wrote. I've listened to every presentation you have ever given. And I can tell you quite categorically that I have never found a single original idea that you could honestly call your own. He said this to a colleague, Robert Milliken. Robert Milliken was a Nobel Prize winner, and he was also Zwicky's boss. So yeah, that's the kind of dude we're dealing with. Anyway, Zwicky wasn't super fond of the whole expanding universe idea, so he worked to come up with alternatives, and his most promising alternative came to be known as Tired Light. He hypothesized that maybe there's something funny about life, or something funny about the nature of space between the galaxies, that causes redshift without needing the galaxies to move or the universe to expand. Now, this is a tall order that will require remapping our understanding of physics, but hey, so is the idea of an expanding universe, so at least we're in the same ballpark.

For tired light to work, you need a few things to line up. One, light has to somehow lose energy as it travels, but only over extremely long distances, otherwise we would have seen it by now. Second, the process, whatever it is, can't involve some sort of scattering where light filters through a substance or interacts with something because that would cause the light to spread out on its way here. And at the distances we're talking about, it would cause our images of distant galaxies to get progressively blurrier the farther away they are. And not just blurry because we can't see them well, but like actually blurry even with better telescopes. And this mechanism would have to have the same effect on all wavelengths of light equally, hitting hard x-rays and wobbly radio waves in the exact same way. Oh, and it would have to reproduce the relationship between distance and redshift at extremely large scales, which isn't exactly one-to-one as you go really far out. Oh, and it would have to keep the universe static, which goes against the natural inclinations of general relativity, which say that the universe should be dynamically expanding or collapsing.

Oh, oh, oh, and it would have to be compatible with special relativity and quantum mechanics, neither of which admit a mechanism for just randomly sapping energy out of traveling radiation without any sort of interaction. So tired light is a tired idea. It just doesn't work when you stack up all the hurdles it has to clear. It's not that the whole concept has been proven wrong. That's kind of hard to do in physics. But that every time someone puts a model together of tired light, it just fails and you have to throw it away. And so far, no one's been able to come up with a way to make Tired-like work that agrees with all observations. But something people have been able to do, especially recently, is contribute to Patreon. That's patreon.com slash pmsutter, where you can contribute to this show. I truly appreciate all of your contributions. It keeps this show going. That's patreon.com slash pmsutter. So case closed. Tired Light doesn't work, nor does any other explanation of Galaxy Redshift.

The only left standing is an expanding universe, which we use the FLRW metric to describe, which gives us a universal clock. Well, what if the metric was wrong? The FLRW metric is a model. And you know the saying, all models are wrong, but some are useful. The FLRW metric is parsimonious. It's the simplest conception that captures the most amount of observation. And it is indeed simple. It assumes that at large enough scales, the universe is homogenous. That is roughly the same from place to place. And that this uniform blob of stuff is slowly getting diluted as the universe expands. We make these assumptions because A, it's close enough to reality as we observe it, and B, it makes the math of general relativity, which is notoriously nasty, slightly less nasty. And it's through the language of the FLRW metric that we get our universal clock. In this metric, in these equations, there's a parameter that represents the passage of time. Typically, in these equations, for the passage of time, we use the symbol tau, which is the Greek alphabet precursor to the letter T, so that fits.

And we call this proper time because that also kind of fits. Anyway, the FLRW metric gives you a recipe for figuring out the proper time since the beginning of the universe. You just have to find yourself in a frame of reference that expands with the universe, which means you just have to be chilling, going with the flow, and not moving in any particular direction relative to everything else. Well, how the heck do you do that? It just so happens that the universe gives us a way to figure out how we're moving relative to cosmic expansion, and that's the cosmic microwave background. The CMB was emitted all at the same time, within about a thousand years, but that's close enough, and in all directions all throughout the entire universe all at once. And since it's bombarding us from all directions on the sky, then we can use it to detect our movement. If we make an all sky map of the CMB, and we have multiple times, then if it appears red shifted in one direction and blue shifted in the opposite direction, then we can figure out our movement that goes against the cosmic flow.

Subtract that out, and boom, what do you know? You're now in a frame of reference that's at rest with respect to the entire universe. Pretty zen, if you ask me. And now that you have that, you can calculate your proper time since the beginning of the universe, aka the age of the universe since the Big Bang. We don't need the CMB to do this, but it makes things way easier, so thank you, for once, Cosmos, for making our jobs a little less grueling. But hold on. The FLRW metric, and no, I just can't say that enough this episode, makes an assumption that the universe is super smooth. Apart from things like galaxies and clusters and voids, those don't look super smooth to me. We've all gotten used to the homogenized milk of the universe, and a lumpy cosmos is going to be a little bit hard to swallow. And maybe if we're getting this assumption wrong, then maybe the FLRW metric isn't as useful as we hope. And we're getting the age of the universe wrong, and when we finally meet the aliens, they're going to laugh at us.

This is the argument behind something called the Timescape Theory, which sounds like you'd find it printed in multicolored text on a website designed in 1997. But its creator is an actual cosmologist who was trained in Stephen Hawking's group at the University of Cambridge. His name is David Wiltshire, and he argues that the lumpiness of the universe throws off the FLRW metric. He says that time flows faster in the empty voids than it does in the dense galaxies and clusters. And because of this, our estimates of the age of the universe are off because we're assuming everything is pretty much the same. Now, everything is pretty much the same. We've measured this, but only at big enough scales. A chunk of universe inside a box roughly 100 megaparsecs on a side, that's 100 million parsecs on a side, does indeed look like any other chunk of the universe of the same size. But within that box, the universe is mostly void, somewhere around 70-90% depending on who you're asking or what kind of mood I'm in on a particular day.

The problem with timescape is that it takes a non-standard approach to applying general relativity that doesn't exactly give clear, consistent results. All of this sounds good, and it is true. The universe is mostly empty. Time does flow differently in a deep void than it does compared to a galaxy. But the question is, how big of an effect is it? Is this a tiny, tiny effect that barely matters? And our assumptions behind the FLRW metric are good enough? Or are we completely off base here? And we do need to account for this because the way that clocks tick in the voids versus galaxies is actually way huge. Standard cosmology says, hey, look, the universe is super smooth at super big scales, and then it only gets lumpy once you zoom in a bit. Following this logic, the time dilation inside voids is way small compared to the time dilation in your living room. And while it's real, it's very tiny, like less than a hundredth of a percent. So it's not something we need to worry about. Yes, the clocks are ticking differently, but not enough to mess with our calculations.

Wiltshire says this is bollocks. He says we're starting with an assumption of averageness, then adding in the lumps after. He argues that we should first start with the lumps and then add all of them together to get the big universe. And then average that result. In this approach, the time difference is huge. between the voids and the galaxies. It's big enough to explain away the existence of dark energy altogether and drastically alter the age of the universe depending on where you live. For example, if you live in a galaxy, the universe is 14.2 billion years old. If you live in a void, it's over 18 billion years. But calculating the age of the universe is not the only application of the FLRW metric. We also use it in our simulations of the growth of structure in the universe. If we are doing it wrong, if our approach of assume the universe is smooth at large scales then add some wiggles to it wasn't right, then it would show up in our simulations as arrangements of galaxies in the cosmic web that don't agree with observations.

But the simulations do agree with observations. They just work. And so while this isn't a proof it's at least a strong sign that we're on the right track and that the timescape approach is not. Again, not totally ruled out, but I'm not sweating bullets over here. The FLRW metric appears safe and sound. That the time difference between voids and galaxies isn't big enough of a deal to really affect our calculations. And so we know the age of the universe. But wait, wait, wait, wait, wait, wait, wait, wait, wait, wait, wait, wait, wait, wait. Okay, tired light is out. The universe is expanding. FLRW metric is in. It encapsulates an expanding universe. It's our model of an expanding universe. In its foundational assumption that at large scales, the universe is smooth and homogenous, and then there are these bumps and wiggles on top of it, like galaxies and voids, seems to be the approach that works. It's the one that provides us with all of our results that we can compare against observations.

But isn't the FLRW metric generic? It lays out the basic assumptions about a homogenous universe and then tells us how the universe should behave, but it doesn't say what the universe is made of. And the stuff in the universe dictates exactly how the universe evolves. The FLRW metric is the map. It tells you where all the possible roads are, where all the possible paths of the universe can drive through. But it doesn't tell you exactly which road the universe will take in its evolution. It lays down the rules for how the roads are made, where they're allowed to curve and wind, but not for how long they can stretch. That depends on the ingredients of the universe. Our universe. is following one particular path, one road. And that road is dictated by the ingredients of the universe. How much curvature? How much matter? Both regular and extra spicy dark. How much radiation? How much dark energy? And who knows? Maybe some other components or ingredients that we haven't even dreamed of yet, although for various and sundry technical reasons, matter, radiation, dark energy, and raw curvature pretty much cover all the possibilities that can fit within the FLRW metric.

The FLRW metric alone doesn't give us the age of the universe. It gives us the formula we need to convert a detailed list of observations and measurements about the ingredients and expansion history of the cosmos into a finite age. Proper time, if you want to be precise, since the Big Bang. And that, my dear friends, is up for debate. The current favored model of what the universe is made of, the particular path that the universe is following, is called lambda CDM. For dark energy, we use the Greek letter lambda because Einstein did it once way back when, and who the heck is going to one-up that? And CDM for cold dark matter. Dark matter particles are generally moving slower than the speed of light. According to the Lambda CDM model, according to this path on the FLRW metric map, the universe is 13.77 billion years old. But of course we don't understand dark matter or dark energy. So how in the world can we be confident in the output of a model if we're not confident at all about the inputs? And that's because while we don't know what dark matter and dark energy are to any degree of certainty whatsoever, we do know two very critical things.

We know what they do. And we know how much of them we got. Listen, dark matter is just matter, but dark. A pound of hydrogen and a pound of, I don't know, axions do the exact same thing to the expansion of the universe. They want to slow it down. they plug into the same part of the formula in the FLRW metric. So all we have to do is try to measure the total amount of mass in the universe, which we do often and repeatedly, and we get to fill in the matter column and let later generations sort out what's going on in this particular bucket, what the heck dark matter actually is. We don't care what it is for purposes of deciding the age of the universe. All we need to know is that it's matter, and we need to know how much of it there is. The same is true for dark energy. Who the heck knows what it is? But we can all agree on what it does. It accelerates cosmic expansion. As long as the universe appears to be accelerating, and trust me, it does, then we just have to measure how much of the dark stuff there is, plug it into the FLRW metric, and away we go.

So the Lambda CDM model is our collection of facts about the universe. It's saying, okay, there's this much dark energy, this much dark matter. We've measured these components. Then we plug that into the FLRW metric, and that tells us how old the universe is. Now let's assume that someday we came across some other theory or idea that does throw a monkey wrench into the Lambda CDM model, which is guaranteed to happen at some point because we know we're missing a lot of nuance with that model. Or heck, even the FLRW metric is less likely. So far, that has survived every observational test. But if we were to change either the lambda CDM picture of the universe or the FLRW map that tells us how to translate what's in the universe to an age, wouldn't that change the estimated age of the universe? Yes. But probably not by very much. That's because our models are so successful at explaining all sorts of different cosmological observations that new theories, new ideas have to be somewhat tuned to get them to line up.

If you introduce a new idea, you have to face the same gauntlet of observations that Lambda CDM has already gone through and survived. So if you come up with your new idea, by the time you're tuning it and adjusting it so that it fits all current observations, you are almost guaranteed to land at roughly the same age of the universe because there are aren't a lot of ways to explain all of the data. And even if you kept up, you know, something, a new theory of gravity that didn't even connect to general relativity, a new model of the history of the universe, by the time you got done facing big bang nucleosynthesis, cosmic microwave background, growth of structure, all of it, it's hard to imagine arriving at a different age of the universe. Because ultimately, yeah, we cook up whatever models we want, but nature is the ultimate arbiter. At the end of the day, it's just data. By the time you're done interpreting the data, you're going to get roughly the same result. And there are other independent checks on the age of the universe.

Go out and find the oldest star. The universe can't be younger than that. Go out and find the coldest white dwarf. The universe has to be older than that. Go look at the amount of decaying radioactive elements the universe has to stick around long enough to make all of them. Very generously, those estimates pin the age of the universe to be somewhere between 12 and 15 billion years old. So any model you concoct has to fit within those ranges that are derived from other independent experiments. The upshot of all this is that once you survive the battery of tests and constraints and observations, you generally find that you wind up making the same broad brush predictions as Lambda CDM. You're going to generally find that the universe is expanding. You're going to generally find that most of it is made of something like dark energy that is causing the expansion to accelerate. You're going to find that a big chunk of it is made of some form of dark matter, a matter that doesn't interact with the rest.

you're going to end up with roughly the same results because we have so many observations. There might be a minor tweak here and there, like, ooh, look, void should be a little thicker around the middle or something. But it's hard to change the overall picture. And you might end up changing the age of the universe a little. For example, if we take the recent DESI cosmology results, which I talked about a few episodes back, if we take them at face value, which they hint at evolving dark energy, If we take those at face value, then the age of the universe goes up from 13.77 billion years to around 14.1 billion years. That's a 2.5% difference. 2.5%? I mean, come on, think about it. With all of our models, all of our assumptions, and all of our observations, to think that we're likely within a few percent of the right answer is pretty mind-blowing. Nice job, science. Thanks to Richard G., Michael Z., and James G. for the questions that led to today's episode. Please keep those reviews coming on your favorite podcasting platform that really helps to show visibility.

And more importantly, keep the questions coming. That's askaspaceman at gmail.com or the website askaspaceman.com. And if you are able, I truly appreciate contributions to Patreon. That's patreon.com slash pmsutter. I'd like to thank my top Patreon contributors this month. They are Justin G, Chris L, Alberto M, Duncan M, Corey D, Michael P, Nyla, Sam R, Joshua, Scott M, Rob H, Scott M, Lewis M, John W, Alexis, Gilbert M, Rob W, Jessica M, Jules R, Jim L, David S, Scott R, Heather, Mike S, Pete H, Steve S, Lisa R, Kevin B, Eileen G, Stephen W, Deb A, Michael J, Philip L, and Stephen B. Thank you so much. That's patreon.com slash pmsutter. And I will see you next time for more Complete Knowledge of Time and Space. Thank you.

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