The Distance Between Galaxies Is Worse Than You Think A Calm Journey Across the Cosmos

[00:00:00]Good evening and welcome. Wherever you are and however you found your way here tonight, I'm so glad you've come. Maybe you're lying in the dark with the blankets pulled up close. Maybe you're on a long train home, watching the lights of unfamiliar towns slide past the window. Maybe you've simply reached the quiet end of a long day, and you're ready at last to set it down. However you've arrived, there is nothing you need to do for the next little while but breathe slowly. Let your shoulders soften and allow the world to grow still around you.

[00:00:40]Before we begin, I'd love to know one small thing. Which corner of the world are you settling down in tonight?

[00:00:49]People find their way to these quiet recordings from every kind of place. A flat high above a humming city. A cottage where the wind is the only sound. A narrow bed in a room where the sun set hours ago. And another where it is only just beginning to rise. If you feel like it, you might leave a gentle note somewhere below to say hello and to say where you're listening from.

[00:01:18]There's something wonderfully soothing in the thought of so many people scattered all across the surface of this small spinning world quietly settling in beneath the very same sky. And if these slow, unhurried explorations help you unwind, a soft tap of the like button and a quiet subscribe genuinely help more than you might imagine. A channel like this one is a small and gentle thing, and it grows only when a few kind listeners decide to stay a while. Every like is a quiet signal, whispered to the machinery behind the curtain. That this calm little corner is worth showing to someone else who is lying awake and searching for rest. Every subscription is a soft vote that says, "Please keep making these. It costs you nothing at all. It takes only a moment. And here on this end, it truly means the world. So, thank you from the bottom of my heart.

[00:02:22]Whether this is your very first night with me or one of many.

[00:02:28]Now, let me tell you where we're wandering tonight. Though, I should warn you, it is a very long way. Tonight we are going to try to answer one of the oldest and simplest questions a person can ask. A question you may first have wondered about as a child. Lying in the grass and gazing upward. How big is the universe? Just how far does everything go? We're going to set out from right here from the very room you're resting in. And we are going to travel outward gently and steadily past the moon, past the sun, past the stars, past our whole galaxy and its distant neighbors, all the way to the farthest edge of everything we can see. And then we'll peer ever so softly into the darkness that lies beyond even that. I promise there will be no mathematics to solve, no formulas to memorize, and no test at the end. If anything, my hope is the very opposite of a lesson. I want to take the largest, most staggering numbers that human beings have ever measured and lay them down so gently, one after another, that they begin to feel less like facts to be learned and more like a lullaby of distances.

[00:03:52]And if you drift off to sleep somewhere along the way, long before we reach the edge, please don't think of it as missing the ending. That is exactly what this is for. It would be the kindest compliment you could possibly give me.

[00:04:06]Here is the thing I find so beautiful and so strangely comforting about the sheer size of the universe. It is so vast that our everyday worries when we hold them up against it seem to soften and shrink until they are almost nothing at all. And yet that very same vastness holds us and cradles us. It is the great quiet home in which everything we have ever loved has taken place. To feel small beneath the stars is not to feel worthless.

[00:04:42]It is, I think, to feel a kind of peace.

[00:04:45]The simple peace of being one small, warm, living thing, resting for a single night inside something almost unimaginably immense. So, settle in and make yourself as comfortable as you can and close your eyes whenever you like.

[00:05:03]Let's leave the ground behind us ever so slowly and begin the long gentle journey outward to discover together just how big the universe truly is.

[00:05:17]Before we take our first real step outward, I have to be honest with you about something. And there's a certain relief in simply admitting it. The human mind is not built to understand the size of the universe. It isn't a failing on your part or on mine. Our minds were shaped over countless generations to handle the distances that mattered for a life lived on the ground. The length of a day's walk, the throw of a stone, the distance across a valley to the next hill. Numbers beyond the few thousand begin to blur together into a soft, shapeless fog that we simply label a lot. And that's perfectly fine for daily life. But it means that when someone tells you a star is 100 trillion km away, the number lands with a dull thud and rolls straight off. 100 trillion.

[00:06:14]It's just noise. It could be 10 times more or 100 times less and it would feel exactly the same. So before we go anywhere, we need a better ruler. We need a gentler way to measure the heavens. One that our tired minds can actually hold on to. And here is the lovely trick that astronomers use. The one we'll lean on all night long.

[00:06:39]Instead of measuring distance in kilome or miles, we'll measure it in time.

[00:06:46]In how long it takes light to make the journey.

[00:06:50]Light is the fastest thing there is.

[00:06:52]Nothing in all the universe travels more quickly. In a single second, a beam of light races about 300,000 km.

[00:07:01]To give that some shape, in the time it takes you to say the word one, a flash of light could circle the entire Earth, not once, but 7 and 12 times. It is, by any human measure, impossibly absurdly fast. If you could travel at the speed of light, you could cross from one side of your country to the other in the blink of an eye and never even feel yourself move. So, here is our ruler.

[00:07:31]We'll say that the distance light travels in 1 second is one light second.

[00:07:37]The distance it travels in a minute is a light minute. and the distance it covers in a whole year, a full year of that unimaginable earth circling speed, never stopping, never slowing, we call a light year. A light year is not a measure of time, even though it sounds like one. It is a measure of distance, the enormous stretch of space that light, the fastest thing in existence, can cross in one year.

[00:08:08]Hold on to that idea gently.

[00:08:11]because it is the key to the whole night. From here on, whenever a distance sounds too vast to picture, we won't reach for a number with too many zeros to count. We'll simply ask a quieter question. How long would it take a beam of light to travel there? And in the answer, we'll begin to feel the true shape of the cosmos.

[00:08:36]Because here is the humbling part. The part that tells you just how large a place we live in. Light is so fast that here on Earth, it seems to arrive everywhere instantly. Flip a switch and the whole room is lit before you can even register it. And yet, as we travel outward tonight, we're going to watch that same lightning fast light begin to struggle.

[00:09:03]We'll watch it take seconds, then minutes, then years, then thousands and millions and billions of years just to cross from one place to another. The universe is so large that it makes the fastest thing there is look slow. And that more than any number is how you begin to feel how big the universe really is.

[00:09:28]So, let's begin close to home with the nearest thing in the whole sky.

[00:09:34]Let's start our journey with the very first stop out from the Earth. The pale familiar face of the moon. The moon feels close, doesn't it? On a clear night, it hangs there, low and bright, seeming almost near enough to touch. It looks like a friendly lantern just above the rooftops.

[00:09:58]But that nearness is a gentle illusion.

[00:10:01]And the moon is our first quiet lesson in how the sky hides its true distances.

[00:10:08]The moon sits on average about 384,000 km away from us. In our new language of light, that means the moon is a little over one light second away.

[00:10:23]When you gaze up at it, the light landing softly in your eyes left the moon's surface just over a second ago.

[00:10:31]You are, in a very real sense, seeing the moon, not quite as it is, but as it was a heartbeat ago. It's the closest thing in the sky. And already we are looking ever so slightly into the past.

[00:10:49]Now, one light second may not sound like much after all our talk of grand distances, but let me try to make that gap feel real because it is far far wider than it looks.

[00:11:04]If you took every other planet in our solar system, Mars and Venus and mighty Jupiter and ring Saturn, all of them lined up side by side, you could fit them all comfortably into the empty space between the Earth and the Moon and still have room to spare. All that gaping distance tucked into the little gap between us and that lantern in the sky. Here's another way to feel it. When astronauts first traveled to the moon, riding a top some of the most powerful machines humanity has ever built, roaring away from the Earth faster than a rifle bullet, the journey still took them about 3 days. 3 days of steady travel through the empty dark just to reach the nearest object in the heavens.

[00:11:54]The moon is not a lantern above the rooftops. It is a whole world a quarter the width of the earth floating in a vast and silent gulf.

[00:12:06]Let me give you a small model to hold in your mind. One will keep building on as we go. Imagine the earth shrunk down to the size of a basketball resting on the floor in front of you. On that scale, how big would the moon be and how far away? The moon would be about the size of a tennis ball and it would not be in your hand or across the room. It would be about seven and a half meters away, most of the way across a large room or clear to the other side of a modest garden. A basketball and a distant tennis ball with a wide empty stretch of nothing in between. That is the earth and the moon to scale. Already the sky is beginning to open up beneath us. And yet in the grand scheme of the night ahead, the moon is practically sitting in our lap. It is the one other world close enough that human beings have actually walked upon it. And even that took the boldest journey our species has ever made. So let's hold the moon gently in mind as our first small step. And then let's push onward much much farther out to the great fire at the center of everything. The star we call the sun. If the moon taught us that the sky hides its distances, the sun is about to teach us that it hides them on a truly staggering scale.

[00:13:38]The sun sits about 150 million km away from the earth. In our language of light, that is roughly eight light minutes.

[00:13:51]Let that settle for a moment.

[00:13:54]When you step outside on a bright morning and feel the warmth of sunlight on your face, that light did not arrive instantly. It left the surface of the sun about 8 minutes ago and has been racing across empty space ever since. at that unthinkable earth circling speed just to reach your skin. If the sun were to somehow vanish this very instant, you would carry on in full daylight for 8 more minutes, entirely unaware before the sky finally went dark. The sunlight you feel is always 8 minutes old. And the sun itself is enormous, far beyond anything our earthbound minds are ready for. It is a vast ball of glowing gas so large that you could fit well over a million earths inside it. If the Earth were a small marble, the sun would be a great sphere taller than a two-story house. It holds more than 99% of all the material in our entire solar system.

[00:14:58]Everything else, all the planets, all the moons, all the comets and asteroids and dust, everything else is just the leftover crumbs drifting around that one colossal fire.

[00:15:12]Now, let's return to our scale model and let it grow.

[00:15:17]Remember, on our last model, the Earth was a basketball.

[00:15:22]But to fit the whole solar system into imagination, we need to shrink things much further.

[00:15:28]So let's make the sun the size of a large beach ball and set it down at one end of a long field. On that scale, how big is the Earth and where is it? The Earth would be a tiny peppercorn, a little speck you could barely see, and it would sit about 26 m away from that beach ball. A good long walk across the grass. Tiny Earth, a whole peppercorn that far from the sun. And this is where the true emptiness begins to reveal itself.

[00:16:03]Because the planets are not crowded together like fruit in a bowl. They are scattered across almost incomprehensible gulfs of empty space. On our beach ball model, the giant planet Jupiter, a marble now, would be more than 100 meters from the sun. And distant Neptune, the farthest of the major planets, would be the better part of a kilometer away, 3/4 of a kilometer of near total emptiness with only a handful of tiny specks scattered across it. The solar system is not really a place full of planets. It is a place full of emptiness with a few small worlds drifting lonely and far apart through the dark. And how far does it all reach?

[00:16:52]We have a wonderful way to feel this thanks to a little machine we sent out into that darkness.

[00:16:58]In the 1970s, humanity launched a small spacecraft called Voyager and set its sailing outward past all the planets on a one-way journey into the deep. It flew past Jupiter, past Saturn, sending back the first close portraits of those distant worlds, and it kept going and going for decade after decade. It was only after more than 35 years of constant travel in the year 2012 that this little craft finally crossed the outer boundary where the sun's influence fades and true interstellar space begins.

[00:17:40]35 years at a speed far faster than any rifle bullet just to reach the edges of our own son's little kingdom. And even that is not truly the end of the solar system. Far far beyond the planets, out in the deep, cold dark, our sun is thought to be surrounded by an immense faint shell of icy fragments. The frozen leftovers of the sun's birth, drifting in a great sphere that may reach outward for something like a full lightyear or even more, a lightyear. The distance light itself takes a whole year to cross. And our son's family may extend nearly that far. Our little solar system, it turns out, is not so little after all. But it is about to look impossibly small because we are ready now to leave it entirely and to face the true emptiness that lies between the stars.

[00:18:43]Here is where our journey takes its first truly great leap.

[00:18:47]the one that separates the cozy neighborhood of our sun from the enormous ocean of the galaxy. We are about to travel to the nearest star beyond our own. And the gulf we must cross to get there is so wide that it changes everything about how we think of the sky.

[00:19:07]Every star you have ever seen, every point of light scattered across a clear night is a sun. A great blazing ball of fire, much like our own, and many of them far larger.

[00:19:21]They look like tiny cold silver pin pricks, only because they are so unfathomably far away. And the nearest of them all, the closest star to our sun, our very next door neighbor in the cosmos, is a small faint red star called Proxima Centauri.

[00:19:42]How far away is it? A little over four light years. 4 years for light itself.

[00:19:49]The fastest thing there is. The thing that crosses from the sun to the Earth in a mere 8 minutes to make the journey.

[00:19:57]That same light which took 8 minutes to reach your morning would need to travel for more than four long years without pausing, without slowing to reach the very next star.

[00:20:11]When you look at that faint red star tonight, you are seeing light that left it over 4 years ago. It is quite literally a glimpse into the past.

[00:20:22]Let's return one more time to our scale model because it makes this gulf almost heartbreakingly clear. Remember, we had shrunk the sun down to the size of a beach ball with the Earth a peppercorn 26 m away and distant Neptune 3/4 of a kilometer out. On that very same scale, where would the nearest star be? Would it be down the road in the next town? On that scale, where our whole solar system fits within about a kilometer and a half, the nearest star would be another beachballsiz sun sitting roughly 6 and a half thousand km away. That is like standing in one country and having your nearest neighbor be on another continent entirely with nothing, nothing at all in all the vast space between you but empty, silent, dark. That is the true nature of the space between the stars.

[00:21:22]It is not crowded. It is the loneliest emptiness imaginable.

[00:21:28]A void so wide that even our fastest spacecraft could barely begin to cross it. Remember little Voyager?

[00:21:37]The craft that took 35 years just to leave our son's neighborhood. If we pointed it towards that nearest star and let it fly, it would take something like 70,000 years to arrive.

[00:21:51]70,000.

[00:21:54]Thousand years.

[00:21:56]Longer than all of recorded human history, many times over, just to reach the single closest star to our own.

[00:22:06]Let me try one last way to make this goal feel real in the plain terms of everyday travel. Forget spacecraft for a moment and imagine you could simply drive there in an ordinary car, humming along at a steady motorway speed, never once stopping to rest or sleep or refuel.

[00:22:26]How long would each journey take to reach the moon? Driving without pause would take you about half a year. to reach the sun. That same steady drive would stretch to roughly 170 years. Two or three long human lifetimes spent entirely behind the wheel.

[00:22:47]And to reach the nearest star, driving in your little car day and night without ever stopping, it would take you something on the order of 50 million years to arrive. 50 million years of driving just to reach the very next star along from our own. There is simply no ordinary human way to hold a distance like that in the mind. It lies beyond every scale we live our lives by. And that gently is exactly the point. And I want you to sit with that for a moment drowsily without any alarm because there is a strange piece in it. The stars are not close and cluttered. They are separated by these tremendous hushed gulfs of nothing. Each one a distant fire tending its own small family of worlds unimaginably far from all the others. When you look up at a sky full of stars, you are not looking at a crowd. You are looking at a scattering of lonely lighouses.

[00:23:52]Each one impossibly far from the next.

[00:23:56]their light crossing years of darkness just to reach your eyes. And here is the truly humbling part. The thought that finally begins to open up the real size of things. That nearest star four light years away, so distant that it staggers the mind. It is one of our very closest companions. Because our sun and that neighbor and hundreds of billions of other stars are all bound together, drifting as one in a single vast island of light. And it's time now to pull back and see the whole of it. The great spiral of stars we call home. On a truly dark night, far from the lights of any town, if you let your eyes adjust and look up, you may see a faint, pale band of hazy light arching all the way across the sky, like a river of mist or a trail of spilled sugar. People have gazed at it and told stories about it for as long as there have been people.

[00:25:05]We call it the Milky Way. And what you are seeing when you look at that soft glowing band is our own galaxy seen from the inside.

[00:25:17]A galaxy is a vast island of stars held together by gravity all slowly turning through space as one. And our galaxy, the Milky Way, is a great flattened spiral, a colossal, slowly rotating pin wheel of stars, gas, and dust.

[00:25:37]That faint band across the sky is what the main disc of our galaxy looks like when you are living inside it. Looking out through the crowded plane of stars.

[00:25:48]We are not looking at the Milky Way from outside. We are inside it. One small point of light within the swirl gazing out at the rest. So how big is it? Let's use our ruler. Our whole solar system, remember the sun and all its planets and its faraway shell of ice is at most a couple of light years across. Our galaxy, the Milky Way, is about 100,000 light years across, 100,000.

[00:26:20]A beam of light which crosses our entire solar system in a matter of hours would need to travel for a 100,000 years without stopping just to cross from one edge of our galaxy to the other. If our whole solar system were shrunk to the size of a small coin, our galaxy on that same scale would still be wider than a continent. And it is not a lonely place.

[00:26:47]Our galaxy holds a truly staggering number of stars.

[00:26:52]Somewhere between 100 billion and 400 billion of them. Let's simply say hundreds of billions.

[00:27:00]Hundreds of billions of blazing suns.

[00:27:03]Each one on average separated from its neighbors by those same enormous lonely gulfs of light years that we just crossed to reach Proxima Centuri.

[00:27:14]Hundreds of billions of stars and very likely far more planets than stars all turning together in one grand glittering wheel. And it isn't only stars. In just the last few decades, we have learned something that would have amazed the astronomers of the past. Planets are not rare at all. Nearly every star you can see is very likely circled by worlds of its own in just the way our sun is.

[00:27:44]Which means our single galaxy holds not merely hundreds of billions of stars, but in all likelihood, even more planets than stars, hundreds of billions of worlds, and quite possibly a great many more scattered all through this one glittering wheel. rocky worlds and giant ones, frozen worlds and scorched ones, worlds with rings and worlds with a dozen moons. The overwhelming majority of which no one will ever name or map or see.

[00:28:18]And remember, this is only our galaxy, only our own single island among the countless islands still ahead of us tonight.

[00:28:28]And where are we in all of this? We are not at the center. We are not anywhere special at all. And there is a quiet comfort in that. Our sun sits about 2/3 of the way out from the middle in a calm and unremarkable spot along one of the galaxy's great spiral arms. A quiet suburb of the Milky Way, some 26,000 lighty years from the crowded brilliant core.

[00:28:58]From our peaceful corner, we drift along with all our neighboring stars, circling the distant center in a slow, majestic orbit. And how slow is that orbit, almost beyond belief? Our sun, carrying the Earth and all of us along with it, is sweeping around the center of the galaxy at an enormous speed. And yet the galaxy is so vast that it still takes our sun something like 230 million years to complete a single lap.

[00:29:30]230 million years for one trip around when our sun last passed through this exact spot in its orbit. The earliest dinosaurs were only just beginning to walk the earth. We have made only a small handful of these great galactic laps in the entire history of our planet. The wheel turns but it turns so slowly and so grandly that whole ages of the earth pass in a single quiet revolution.

[00:30:00]So here we are then a small warm world circling one ordinary star which is itself one of hundreds of billions of stars all drifting together in a single glittering island a 100,000 lightyear wide. You might think surely that must be close to the whole of it. Surely that is nearly everything. But we have barely begun because for a long time human beings believed our galaxy was the entire universe. And then we discovered to our astonishment that it is only one island among a great and uncountable multitude.

[00:30:38]But before we sail out to those other islands, let's pause on a gentler question. one you may already be wondering how could anyone possibly know all this. How do we measure a distance of 100,000 lighty years? It's worth stopping here in the quiet to ask a very reasonable question. We've been throwing around these enormous distances. Light minutes, light years, a galaxy 100,000 light years wide. But no one has ever traveled these distances. No tape measure reaches to the stars. So, how on earth could anyone actually know how far away they are? The answer is one of the most beautiful and clever detective stories in all of science. And it helps the whole vast picture feel trustworthy rather than like a fairy tale.

[00:31:33]Astronomers build up their knowledge of cosmic distances step by step, like climbing a ladder, where each rung reaches a little farther than the one below it. The first rung of the ladder is something you can feel with your own two eyes. Right now, hold one thumb up at arms length and close one eye, then the other. Back and forth.

[00:31:59]Watch how your thumb seems to jump back and forth against the far wall behind it. It isn't moving, of course. You're simply looking at it from two slightly different positions, your two eyes. And the closer your thumb is to your face, the bigger that jump appears.

[00:32:19]This is the whole secret. If you know how far apart your two eyes are, and you measure how much your thumb seems to shift, you can work out exactly how far away your thumb is without ever touching it. Astronomers do exactly this with the nearer stars. As the Earth travels around the sun through the year, we ourselves are carried from one side of our orbit to the other, a shift far, far wider than the gap between two eyes. And when they look at a nearby star from these two different vantage points, 6 months apart, they can see it shift ever so slightly against the distant background of farther stars. It is a tiny, tiny shift, far too small to notice by eye. But by measuring it with great care, they can work out the distance to that star using nothing more than patience, geometry, and the width of our own orbit.

[00:33:22]This is the first solid rung of the ladder, and it reaches out to the nearest stars with beautiful precision.

[00:33:30]But that trick only works for stars close enough to show a measurable shift.

[00:33:35]For anything farther, we need to climb higher. And the next rung was handed to us by a quiet, painstaking astronomer named Henrietta Levit, who in the early 1900s made one of the most important discoveries in the history of measuring the sky. She was studying a particular kind of star, a type that slowly pulses, growing brighter and then dimmer and then brighter again, over and over, like a lighthouse breathing. and she noticed something remarkable and precise.

[00:34:12]The slower a star of this kind pulsed, the more brilliant it truly was. The rhythm of its pulsing told you reliably exactly how much light it was really giving off. Now, why does that matter?

[00:34:28]Because it gives us a way to measure distance across enormous gulfs. Think of a candle. A candle up close is bright.

[00:34:37]The very same candle far away looks dim.

[00:34:41]If you know how bright a candle truly is up close, then by seeing how faint it appears from a distance, you can work out precisely how far away it must be.

[00:34:52]Levit's pulsing stars are like candles whose true brightness we can read simply by timing their rhythm. Find one of these stars in a distant cluster, time its pulse, and you know how bright it really is.

[00:35:07]Measure how faint it looks from here and the difference tells you the distance.

[00:35:12]Suddenly we could measure our way out across thousands and then millions of light years. There is something I find deeply reassuring about this way of building knowledge and it changes how the whole night's journey feels.

[00:35:29]Each rung of this ladder is checked against the one below it. The pulsing lighthouse stars are calibrated against nearby stars whose distances we already know from that simple shift of perspective.

[00:35:44]So we are never merely guessing, never plucking numbers out of the air. We are extending carefully and patiently a chain of measurement that begins with something as humble and certain as the width of the Earth's own orbit and reaches link by tested link out toward the edge of the visible cosmos.

[00:36:07]So when you hear these vast distances tonight, you can rest in the knowledge that behind each one lies not a wild guess, but generations of quiet, painstaking, self-checking work.

[00:36:21]And the ladder climbs higher still. For the most distant reaches, where even those pulsing stars grow too faint to see, astronomers turn to a certain kind of exploding star, a stellar death so brilliant and so reliably, precisely bright that it can be seen clear across the universe and can serve as a candle of known brightness at almost unimaginable distances, rung by rung, from the shift of nearby stars to the pulsing lighouses to the brilliant dying stars.

[00:36:57]Each method calibrated carefully against the one below it. We have built a trustworthy ladder of distances that reaches from our own backyard out to the farthest galaxies.

[00:37:10]So when I tell you a number tonight, know that it is not a guess. It is the hard one result of generations of patient, clever people measuring the heavens with light and geometry and care. And now with our ladder in hand, we're ready to leave our galaxy behind and cross the truly cosmic gulf to the other islands of stars.

[00:37:36]For most of human history, people assumed that our galaxy, that great glowing band across the sky, was the entire universe.

[00:37:47]Everything that existed, all the stars there were gathered into one grand island of light surrounded by infinite empty darkness. It was a reasonable thing to believe, and it turned out to be one of the greatest underestimates in all of history.

[00:38:06]For centuries, astronomers had noticed scattered among the sharp points of the stars a number of faint fuzzy little patches of light. Soft, cloudy smudges in the dark. Nobody was quite sure what they were. Some thought they were clouds of glowing gas within our own galaxy, but others wondered something far more daring. What if those faint smudges were not clouds at all, but entire other galaxies?

[00:38:37]Separate islands of stars so unfathomably far away that their hundreds of billions of suns blurred together into a single dim glow. It became one of the great questions in astronomy. Were those little spirals part of our galaxy, or were they whole universes of their own, impossibly far beyond it?

[00:39:01]The question was finally settled in the 1920s and the answer changed our sense of everything.

[00:39:09]Using the very distance ladder we just spoke of, those patient pulsing lighthouse stars, astronomers managed to pick out individual pulsing stars inside one of those faint spiral smudges, a patch in the sky we now call the Andromeda galaxy. They timed the pulsing. They read off the true brightness. And when they worked out the distance, the answer was so enormous that it settled the debate forever.

[00:39:39]That faint smudge was not inside our galaxy at all. It was another galaxy entirely. A separate island of hundreds of billions of stars drifting in the dark, unimaginably far away.

[00:39:54]How far? The Andromeda galaxy lies about 2 and a half million light years from us. Let that number wash over you slowly. The light landing softly in your eyes when you look at Andromeda and you can just see it faintly with your naked eye on a dark night. That light left it two and a half million years ago. It began its journey when our own distant ancestors were just learning to walk upright on the plains of Africa.

[00:40:25]It has been crossing the empty dark at the speed of light for 2 and a half million years. And only tonight does it finally arrive to land in your eye as the faintest of glimmers. When you look at Andromeda, you're looking farther back in time and across a greater gulf than at almost anything else a human being can see without a telescope. It is the most distant thing most of us will ever glimpse with our own eyes. An Andromeda is not alone and neither are we. Our Milky Way and Andromeda and a few dozen smaller galaxies all drift together in a loose gathering that astronomers call our local group. A small cluster of galaxies bound gently by gravity spread across several million lighty years of space. A group of islands, you might say, in the same distant corner of the sea. And here is a soft and lovely fact to rest on.

[00:41:31]Andromeda and our own Milky Way are slowly drifting toward one another. And billions of years from now, long after tonight, they will gently merge into a single greater galaxy.

[00:41:45]But that is a slow and peaceful dance, unfolding over time scales so vast that there is nothing at all to fear in it.

[00:41:55]It is simply the quiet way of the cosmos.

[00:42:00]So now our sense of scale has shifted again and dramatically. Our galaxy, that island of hundreds of billions of stars, a 100,000 lighty years wide. It is not the universe.

[00:42:16]It is one island among many, one member of a small local gathering which is itself just a quiet neighborhood in something far far larger. Because these galaxies too gather together into greater and greater structures. And when we pull back far enough to see how all the galaxies in the universe are arranged, we find one of the most breathtaking and unexpected patterns in all of nature.

[00:42:46]Let's rise up now higher than we have ever gone. And imagine looking down on a truly enormous stretch of the universe.

[00:42:55]A piece so large that entire galaxies, each with their hundreds of billions of stars, become mere dots of light. What would we see? You might expect the galaxies to be scattered evenly through space like grains of sand spread smoothly across a beach. But that is not what we find at all. And the true pattern is so beautiful that it took astronomers by surprise. The galaxies are not spread out evenly. Instead, they gather together. First into small groups like our local group. Then those groups gather into much larger collections called clusters where hundreds or even thousands of galaxies are drawn together by gravity. And those clusters in turn gather into truly colossal structures called superclusters.

[00:43:50]Our own local group, it turns out, is just a tiny outlying thread of a vast supercluster of galaxies.

[00:44:00]A structure so immense it has been given a beautiful name, Lania.

[00:44:06]A word from the Hawaiian language meaning immense heaven or immeasurable sky.

[00:44:13]Lania contains something like 100,000 galaxies and stretches across more than 500 million lighty years.

[00:44:22]500 million lightyear. And our entire galaxy with all its hundreds of billions of stars is just one tiny speck drifting near its outer edge.

[00:44:36]But here is the truly astonishing part.

[00:44:39]The pattern that reveals itself only at the very largest scales. If you map out where all these clusters and superclusters lie across billions of light years, you find that they are not scattered at random. They are strung together into immense glowing threads and filaments.

[00:45:00]great bridges of galaxies stretching across unfathomable distances.

[00:45:06]And these threads connect and cross, weaving together into a vast, delicate, three-dimensional latice that spans the entire visible universe. Astronomers call it the cosmic web. It is one of the most beautiful images in all of science.

[00:45:25]And I want you to picture it gently as you rest. Imagine the finest, most delicate spid's web, or the soft branching threads of foam in a bubble bath. Or perhaps most hauntingly of all, the fine tangled network of cells inside a living brain. The universe on its very grandest scale looks like all of these long luminous filaments of galaxies woven together into a great cosmic web.

[00:45:59]And strung along those threads like dew drops on a spider's silk are the galaxies and clusters and superclusters.

[00:46:08]All the light in the universe gathered along these delicate strands. And between the threads, emptiness, vast, almost perfectly empty regions called voids, enormous, hushed bubbles of near nothingness, hundreds of millions of light years across where scarcely a single galaxy is to be found.

[00:46:31]The cosmic web is mostly these great empty voids with the galaxies gathered only along the thin walls and filaments that wind between them like soap film stretched around the empty spaces in a mound of bubbles.

[00:46:49]If you could somehow see the whole universe at once, it would not look like a scattering of stars. It would look like a vast glowing foam-like tapestry.

[00:47:01]Bright threads and dark hollows woven together on a scale so large that our galaxy, our local group, even mighty Lanni, are little more than glinting specks within it. It's worth resting for a moment on those great empty voids, because they may be the most quietly humbling part of the whole picture. We tend to imagine space as mostly full, brimming with stars and galaxies.

[00:47:31]But on the very largest scale, the truth is nearly the opposite. Most of the universe by far is these enormous silent voids. vast rounded hollows of nearperfect emptiness, so large that a lone galaxy a drift in the middle of one would find its nearest neighbors impossibly far away with almost nothing but darkness in between. All the light in the cosmos, all the galaxies, all the clusters, all the superclusters is gathered only along the thin glowing threads and delicate walls that wind between these hollows. Like the shining film of soap that outlines the empty spaces in a great mound of bubbles, the universe on its grandest scale is mostly gentle emptiness, faintly traced in light. This great cosmic web is the largest pattern that exists. The very architecture of the universe itself, the way all matter has arranged itself over billions of years, drawn slowly together by gravity into these vast luminous threads.

[00:48:43]And now we come to the final dizzying question of scale. We have traveled outward and outward from the moon to the sun. From the sun to the stars, from the stars to our galaxy, to the other galaxies, to the clusters and superclusters and the great cosmic web itself. So just how far does it all go? How large is the whole thing? To answer that, we have to understand one final extraordinary fact.

[00:49:16]The strange and wonderful truth that the universe is not sitting still, it is growing.

[00:49:23]Here is one of the most surprising discoveries human beings have ever made.

[00:49:27]And it is the key that finally unlocks the true size of the universe.

[00:49:32]For most of history, people assumed that the universe, whatever its size, was fixed and still, a great unchanging stage, the same yesterday, today, and forever.

[00:49:46]But a little under a hundred years ago, astronomers made a discovery that overturned that idea completely.

[00:49:53]The universe is not still at all. It is expanding, growing. Every distant galaxy on average is rushing away from us and away from every other galaxy in every direction.

[00:50:09]Astronomers discovered this by studying the light from distant galaxies.

[00:50:14]It turns out that when something that gives off light is moving away from you, its light gets stretched ever so slightly and shifts toward the redder end of the rainbow. And when they looked carefully at the galaxies scattered across the sky, they found that nearly all of them showed this telltale stretching.

[00:50:34]Almost every galaxy in the universe is moving away from us. And there was a pattern discovered by a thoughtful astronomer studying these faint smudges of light. The farther away a galaxy is, the faster it is receding.

[00:50:51]The most distant galaxies are fleeing from us at truly enormous speeds.

[00:50:56]Now, at first this might sound a little unsettling or even lonely, as though every galaxy in the cosmos were fleeing from us in particular, as though we had done something to offend the whole universe. But the truth is far gentler and far stranger than that. And once you see it, it becomes one of the most peaceful ideas in all of science.

[00:51:21]The galaxies are not really flying away through space like fragments from an explosion with us at the center.

[00:51:30]Instead, it is space itself. The very fabric between the galaxies that is quietly stretching and growing. And as that space grows, it carries all the galaxies along with it, gently drawing them apart from one another everywhere, all at once. Let me give you a soft picture to hold. Imagine a loaf of raisin bread sitting in a warm oven slowly rising. The raisins are the galaxies and the dough is space itself.

[00:52:03]As the dough rises and swells, every raisin drifts away from every other raisin. Pick any raisin you like and imagine sitting on it and you would see all the other raisins moving away from you. And the ones farthest away would appear to move away the fastest because there is more swelling dough between you and them. No raisin is at the center. No raisin is being singled out. The whole loaf is simply growing and every point within it drifts gently apart from every other. That is our universe. We are not at the center of the fleeing galaxies.

[00:52:44]There is no center. Every point in the cosmos sees the same gentle drifting apart because space itself everywhere is quietly growing.

[00:52:56]There is one more gentle point worth making about this stretching of space because it often trips people up. When we say the galaxies are rushing apart, we do not mean that they are hurtling through space like debris scattered by a blast. Each galaxy in its own little neighborhood is sitting more or less still. It is the space between the galaxies that is growing. New distance quietly appearing everywhere all the time in the great gaps between the islands of stars within a single galaxy where gravity holds everything in a firm and loving embrace. The stars are not drifting apart and our own galaxy is not swelling. It is only across the immense empty gulf between galaxies where gravity's grip grows faint that this stretching of space wins out gently carrying the galaxies away from one another. So it is not really that everything is flying apart. It is that the emptiness itself, the great vacant spaces between the islands of light is slowly and silently growing wider. And this discovery carried within it a profound and beautiful implication.

[00:54:14]If the universe is expanding, if all the galaxies are drifting apart and the space between them is growing, then we can imagine running the whole thing backward like a film in reverse.

[00:54:28]Rewind and the galaxies draw closer together. Rewind further and the space between them shrinks and everything crowds nearer and nearer.

[00:54:40]Keep rewinding far enough and you reach a moment when everything in the entire universe, all the galaxies, all the stars, all the matter that would ever become you and me and this small warm world was gathered together unimaginably hot and dense into a single tiny brilliant point.

[00:55:02]And from that beginning, everything has been expanding and cooling and unfolding ever since.

[00:55:10]This was the birth of the universe.

[00:55:13]A beginning we have come to call rather grandly the big bang. And by carefully measuring just how fast the universe is expanding today and running the film backward, astronomers can work out how long ago that beginning must have been.

[00:55:30]The answer is one of the most important numbers we have ever measured. The universe, as best we can tell, is about 13.8 billion years old. 13.8 billion years since that first bright moment.

[00:55:47]And that number, as we are about to see, holds the final key to the true size of everything.

[00:55:55]Now we arrive at the great question we set out to answer. And we finally have the tools to face it. How big is the universe? And here we meet one of the most subtle and wonderful ideas in all of cosmology. So let's take it very slowly and gently, one soft step at a time. We just learned two crucial facts.

[00:56:21]first that light for all its incredible speed still takes time to travel years, millions of years, billions of years to cross the great cosmic gulfs. And second, that the universe has a finite age about 13.8 billion years since the beginning. Put those two facts together and something remarkable falls out. It means there is a limit to how far we can possibly see. Think about it this way.

[00:56:52]If a galaxy is so far away that its light would need more than 13.8 billion years to reach us, then that light simply hasn't arrived yet. It has been traveling since the very beginning of the universe and it is still on its way.

[00:57:09]So no matter how powerful our telescopes become, there is a certain distance beyond which we cannot see empty anything at all. Not because there is nothing there, but simply because the light from those places has not had enough time in the whole history of the universe to cross the gulf and reach us.

[00:57:32]It is as though we sit at the center of a great sphere and the surface of that sphere is the farthest distance from which light has had time to arrive.

[00:57:44]Astronomers call this sphere the observable universe. It is not the whole universe. It is simply the part we are able to see. The part close enough that its light has reached us by now. So here is the natural question. How big is that sphere? If the universe is 13.8 billion years old and light travels one lightyear each year, you might reasonably guess that we could see about 13.8 billion lighty years in every direction. And that would be a perfectly sensible guess. But it is not quite right. And the reason why is one of the most delightful twists in the whole story. Because remember, while that ancient light has been traveling toward us all these billions of years, the space it was traveling through has been stretching and growing the entire time.

[00:58:41]The universe has been expanding beneath the light's feet, carrying its source ever farther away.

[00:58:49]So imagine a beam of light that set out toward us near the very beginning of the universe from some distant patch of matter. It has traveled for 13 billion years to reach us. But during all that time, the place it started from has been carried farther and farther away by the expansion of space.

[00:59:11]By the time its light finally arrives in our telescopes tonight, that original patch of matter is no longer 13 billion light years away. It has been swept along by the growing cosmos to a distance far greater than that.

[00:59:28]When astronomers do the careful accounting, they find that the edge of the observable universe, the farthest we can possibly see, now lies about 46 billion light years away from us in every direction, not 13, but a 46 billion lightyear.

[00:59:49]The observable universe is a sphere some 93 billion lightyear across.

[00:59:55]Take a slow breath and let that settle over you like a blanket.

[01:00:01]93 billion light years from one side of the visible universe to the other. And remember what a single light year is.

[01:00:10]The distance that light, the fastest thing there is, crosses in a whole year.

[01:00:16]A distance so vast it swallows our entire solar system many times over. Now stack up 93 billion of them side by side. That is the width of the part of the universe we can see. And scattered all throughout that unthinkable volume are the galaxies. Not a few, not thousands, but by careful estimates something like 2 trillion galaxies.

[01:00:43]Two trillion islands of stars, each holding its own hundreds of billions of suns. Each of those sons very likely surrounded by its own worlds. The numbers have grown so large that they have quietly slipped past anything the mind can truly hold. And still, this is only the part we can see.

[01:01:06]Let me try to give that number, two trillion galaxies, just a little more shape. Though I should warn you, it can't truly be done. You may have heard it said that there are more stars in the universe than there are grains of sand on all the beaches of the earth. It has the ring of the kind of lovely exaggeration one isn't quite meant to take literally. But when you actually sit down and work it out, it turns out not to be an exaggeration at all. It is simply true and a gentle understatement at that. gather up every grain of sand on every beach and in every desert across the whole face of our planet.

[01:01:51]Already a number far too large to picture. And the stars scattered through the observable universe outnumber them.

[01:01:59]And not by a little, but by a wide and comfortable margin.

[01:02:05]And remember what each of those stars can be. Around a great many of them drift planets of their own whole worlds with their own long mornings and the quiet evenings.

[01:02:18]Their own silent oceans and unclimbed mountains turning slowly in the dark that no eye will ever see and no traveler will ever reach.

[01:02:32]The universe is not merely large. It is furnished endlessly and extravagantly with more suns and more worlds than there are grains of sand to count them by. And every last one of them, every star, every world, every glowing thread of the cosmic web has been quietly sending its light out across the darkness toward us. There is a wonderful gift hidden inside the fact that light takes time to travel. And I want to share it with you now because it is one of the most beautiful ideas in all of astronomy.

[01:03:10]Because light is slowly is slow to cross these great distances.

[01:03:16]It means that whenever we look out into space, we are also always looking back in time. When you look at the moon, you see it as it was 1 second ago. When you look at the sun, you see it as it was 8 minutes ago. When you gaze at the nearest star, you see light that left it 4 years ago. Look at the Andromeda galaxy and you are peering 2 and a half million years into the past. And the farther out we look, the deeper into the past we see. The most distant galaxies our great telescopes can find are so far away that we are seeing them as they were billions of years ago when the universe itself was young. To look out into the depths of space is to look down through the long history of the cosmos layer upon layer like reading the rings of an ancient tree.

[01:04:15]So here is a natural and wondrous question. If looking farther out means looking further back in time, then what is the very farthest thing we could possibly see? What lies at the deepest edge of the past? And the answer is one of the most extraordinary things human beings have ever detected. If we look out as far as we possibly can in every direction past all the galaxies, we come at last to a faint uniform glow that fills the entire sky. It is the oldest light in all the universe and it is the afterglow of the beginning itself.

[01:04:56]Here is what it is. In the earliest moments of the universe, everything was so unimaginably hot and dense that it was not yet made of stars or galaxies at all. It was a thick glowing fog of raw material, so hot and so crowded that light could not travel freely through it. It was trapped, scattered endlessly like light lost inside a dense bank of cloud. But as the universe expanded and cooled, there came a moment about 380,000 years after the beginning when the fog finally thinned and cleared and light was set free to travel across the cosmos for the first time. That light, the light released at the very moment the universe became clear, has been traveling ever since, for nearly the whole age of the universe. And it is still arriving right now tonight from every direction in the sky. We call it the cosmic microwave background. And it is quite literally the baby photograph of the entire universe. A picture of the cosmos as it was when it was only a few hundred,000 years old, long before there were any stars.

[01:06:19]Over the billions of years it has been traveling, this ancient light has been stretched by the expanding universe until it is no longer a visible glow, but a soft whisper of microwaves, extraordinarily faint and cold, just a few degrees above the coldest temperature possible. the absolute zero of deepest space. It is so gentle that it went unnoticed until in the 1960s, two scientists testing a large antenna kept picking up a faint, steady hiss they could not get rid of, no matter what they did. They cleaned their equipment. They even chased away some nesting birds, thinking the birds might be the cause. But the hiss remained coming from every direction in the sky, day and night. And at last they realized with astonishment what they had found.

[01:07:15]That gentle uniform hiss was the afterglow of the birth of the universe.

[01:07:20]The oldest light there is falling softly on the earth from all around.

[01:07:26]This is the true edge of what we can ever see. Not a wall, not a boundary in space, but a wall in time.

[01:07:35]When we look out to the cosmic microwave background, we are looking as far back as light itself allows to the moment the universe first became clear. We cannot see past it because before that moment, there was no free light to see. It is the farthest, oldest, gentlest light in existence, filling the whole sky all the time. A soft and constant reminder of where everything came from. And it brings us at last to the deepest question of the night. The one that lies just beyond that ancient wall of light.

[01:08:15]We have traveled an enormous way tonight all the way out to the edge of the observable universe. That sphere 93 billion lightyears across filled with two trillion galaxies bounded by the oldest light there is. And now we come to the question that everything has been building toward the one you have perhaps been quietly wondering this whole time.

[01:08:41]Is that the whole universe? Is that sphere everything there is? Or does it go on beyond what we can see? Put most simply, is the universe finite, or is it infinite? And here I have to share with you the honest and rather beautiful answer, which is this. We do not know.

[01:09:03]But we know enough to say something wonderful about it. So let me draw the crucial distinction as gently as I can because it is the heart of the whole matter. The observable universe, the part we can see is finite. It has a definite size 93 billion lightyear across because there has only been so much time for light to reach us. But the observable universe is almost certainly not the whole universe.

[01:09:34]It is simply the part close enough for its light to have arrived. Beyond that edge, there is every reason to believe the universe continues. More galaxies, more cosmic web, more of everything, stretching on past the horizon of what we can ever see. The edge of the observable universe is not the edge of the universe. It is only the edge of our view like the horizon at sea. When you stand on the shore and look out at the horizon, you are not seeing the edge of the ocean. You are only seeing as far as the curve of the world allows.

[01:10:14]The sea goes on well beyond the line you can see. And so almost certainly does the universe.

[01:10:23]And here is a thought that gently turns the whole picture inside out. one worth holding softly as you rest because the edge of the observable universe is simply the farthest that light has had the time to travel to us.

[01:10:40]It means our sphere of visible universe is centered on us. Not because we are special in any way, but only because we happen to be the ones doing the looking.

[01:10:51]A creature on some world in a distant galaxy billions of light years from here would have its own observable universe.

[01:11:01]A sphere of the very same size centered on itself. Its horizon would reach out to places forever hidden from us and ours would reach to places forever hidden from it. There is no single edge that everyone shares and no true center anywhere to be found. Every point in the cosmos sits as far as it can ever tell at the very middle of its own vast sphere of visible sky. Each of us at the center of our own horizon. A drift in a universe that very likely has no center at all. So how far does it go beyond our horizon? Is it merely much larger than the part we see? Or is it truly infinite going on forever without any edge at all?

[01:11:55]This is where we reach the genuine limit of human knowledge. And there is a kind of peace in arriving there. We have one important clue and it comes from measuring the overall shape of space. It turns out you can ask whether the universe on the largest scales is curved. Whether space bends back on itself like the surface of an enormous ball or whether it lies flat and straight stretching on without curving.

[01:12:25]And when astronomers measure this with exquisite care using that ancient light from the beginning, they find that the universe appears to be flat.

[01:12:36]astonishingly precisely flat as far as we can possibly measure with no detectable curve at all. And a flat universe as far as we understand it is one that may well be infinite, truly endless, going on forever in every direction with no edge, no boundary, and no center anywhere at all. Now, our measurements can never be perfect. So we cannot say for certain that it is infinite. It might instead be finite, but simply so much vaster than the part we can see that it appears flat to us.

[01:13:15]The way a small patch of an enormous sphere looks flat when you stand upon it. But either way, the conclusion is humbling almost beyond words. The universe is at the very least immensely larger than the two trillion galaxies we can see. And it may quite possibly go on without end forever.

[01:13:39]Let that thought settle over you softly without any fear as you rest. The two trillion galaxies of the observable universe. That 93 billion lightyear sphere so vast it defeats all imagining.

[01:13:55]that may be only a tiny tiny fraction of a universe so large that the word big loses all meaning. We may be one small warm world in one ordinary galaxy in one thread of one supercluster, a drift in something that has no edge at all. And rather than making us feel lost, I find there is something deeply peaceful in it. The sense of resting within something so boundless and so gentle that it will hold us and everything we love without ever running out of room.

[01:14:32]There is a question that almost everyone asks the moment they hear that the universe is expanding and it is such a good and natural question that we must give it the gentle answer it deserves.

[01:14:46]If the universe is growing, getting bigger all the time, then what is it growing into? What lies outside it? In the space it is expanding to fill.

[01:14:58]Surely if something is getting larger, there must be some room around it for it to grow into. Where is the edge and what is beyond it? It is one of the most reasonable questions in the world. And yet the answer is one of the strangest and most beautiful ideas we have. The universe is not expanding into anything.

[01:15:21]There is no outside for it to grow into.

[01:15:24]No surrounding empty room, no edge that it is pushing against. When we say the universe is expanding, we do not mean it is a ball of stuff getting bigger within some larger empty space. We mean that space itself, the very fabric of distance is stretching everywhere all at once. It is not that the galaxies are moving out into some pre-existing void.

[01:15:51]It is that new space is quietly appearing between them everywhere, growing the distances from within. There is no need for an outside because it is not expanding into anything. It is simply becoming more of itself.

[01:16:09]I know that is a difficult thing to picture because everything in our daily experience grows into some surrounding space. But this is one of those places where the universe gently refuses to behave like the small familiar things we know. Go back to that image of the raisin bread rising. From inside the loaf, every raisin sees the others drifting away as new dough appears between them. The raisins don't need somewhere to go. The dough simply makes more of itself in every direction. Our universe is like an endless loaf rising everywhere at once with no crust, no pan, and no oven around it. Just the dough quietly growing forever from within. And here is a final remarkable twist. One of the most important discoveries of recent times. You might think that the expansion of the universe would be gradually slowing down, gently coasting to a stop, held back by the gravity of all the matter within it. The way a ball thrown upward slows as it rises. For a long time, that is what astronomers expected to find. But at the very end of the 20th century, they made a startling discovery.

[01:17:29]The expansion of the universe is not slowing down at all. It is speeding up.

[01:17:35]The galaxies are drifting apart faster and faster as time goes on, driven by some mysterious presence woven into the fabric of empty space itself.

[01:17:46]something we understand so poorly that we have given it only a placeholder name, dark energy. We do not know what it truly is. We only know that it fills all of space and that it is gently, relentlessly pushing the universe to expand ever faster. And this speeding up expansion carries a quiet and rather poignant consequence for the far far future. As the universe expands faster and faster, the most distant galaxies are being carried away from us more and more quickly until in the unimaginably distant future, they will be receding so fast that their light can never reach us again.

[01:18:30]One by one, over billions upon billions of years, the far galaxies will slip silently beyond our horizon, out of view forever, until the sky grows quieter and lonelier than it is tonight. But that is a slow fading spread across time scales so vast that they make even the age of the universe seem like a single breath.

[01:18:54]For us tonight, the sky is still full, brimming with two trillion galaxies of light. And that, I think, is a reason to feel not sadness, but a soft and simple gratitude that we are here to see it while it shines.

[01:19:12]We have come now to the very frontier, the outermost edge of what human beings know, and the beginning of what we can only wonder about.

[01:19:24]We have traveled all the way from the room you're resting in, out past the moon and the sun, past the nearest star and the hundreds of billions of stars in our galaxy, past the two trillion galaxies of the observable universe, out to that ancient wall of light and the flat, perhaps endless space beyond it. And here at the true limit of knowledge, the honest scientist can only fall quiet and wonder along with the rest of us.

[01:20:01]So, let's spend our last few moments together, drifting gently through the biggest questions of all, the ones no one can yet answer. What lies beyond the edge of what we can see? The most likely answer, and the simplest, is more of the same. More galaxies, more clusters, more of the great cosmic web stretching on far past our horizon. Looking on the whole, much like the part we can see, there is nothing special about our particular spot in the cosmos. We have no reason to think we sit at any center or near any true edge.

[01:20:43]So in all likelihood, the universe simply carries on beyond our view, vast and gentle and much the same, farther than any light will ever let us look.

[01:20:57]But some thinkers wonder about possibilities stranger and grander still. And it is lovely to let the mind wander among them as you drift toward sleep, holding them lightly the way you'd hold a dream.

[01:21:13]If the universe truly is infinite, truly endless, going on forever, then some have pointed out a curious and dizzying consequence in an infinite universe with matter arranged in only so many possible ways.

[01:21:31]Patterns would eventually have to repeat. Somewhere out there, unfathomably far beyond our horizon, far past any place we could ever reach or see, there might be another world much like this one, another quiet room, another warm and living creature, listening to a soft voice in the dark, and drifting off to sleep. It is only a possibility, a strange consequence of endlessness, and no one knows if it is truly so. But it is a gentle thought to hold.

[01:22:08]Others wonder whether our entire universe, all two trillion galaxies of it, and everything beyond our horizon too, might itself be just one among many. Perhaps some suggest our universe is a single bubble in a far greater sea.

[01:22:28]And there are other universes, other bubbles, each with its own vast reach of galaxies forever separate from ours.

[01:22:37]This is a bold and unproven idea. A whisper at the very edge of what physics can even guess at. And it may be that we can never know whether it is true. It lives for now in the realm of wondering.

[01:22:52]But there is something wonderful in the fact that we can even ask the question that a small soft creature on a small warm world can lie awake at night and wonder whether there are other universes beyond its own. And perhaps that is the most beautiful place of all to leave our journey, not at an answer, but at the edge of a great and gentle mystery.

[01:23:19]Because the truth, the honest and marvelous truth, is that the universe is so much larger than we can see and so much stranger than we can fully grasp.

[01:23:30]That some of its deepest questions may remain forever open, waiting quietly for minds not yet born to wonder at them, just as we wonder tonight. So, here we are at the end of our long journey outward. Having traveled in our imaginations from a quiet room all the way to the edge of everything and past it into mystery, let's gather it all up now softly and set it gently down beside us as the night deepens.

[01:24:04]We began close to home and we learned that even the moon, that friendly lantern in the sky, is a whole world across a vast gulf of dark. We felt the sun's light arrive 8 minutes old and wandered out across the enormous emptiness of our own solar system.

[01:24:24]We crossed the tremendous lonely gulf to the nearest star and then pulled back to see our galaxy. An island of hundreds of billions of suns 100,000 light years wide. And still we kept rising past our galaxy to its neighbors, past the neighbors to the clusters and the superclusters out to the great glowing cosmic web. And finally to the very edge of the observable universe, a sphere 93 billion lightyear across, holding 2 trillion galaxies wrapped in the oldest light there is. And beyond even that, we found a universe that may stretch on flat and gentle, perhaps without any end at all. It is almost too much to hold, isn't it? And yet, here is the thought I'd like to leave with you as you drift towards sleep. In all that unthinkable vastness, all those trillions of galaxies, all those countless suns, all that endless growing space, there is, as far as we know, only one small blue world where someone is lying quietly in the dark tonight, wrapped in warmth, breathing slowly, alive and aware, and able to wonder at it all. That is you.

[01:25:45]In a universe of almost limitless size, you are a rare and precious thing, a small, warm place where the cosmos has woken up just enough to marvel at its own immensity.

[01:25:59]To feel small beneath such a sky is not a sad thing. It is, I think, one of the most peaceful feelings there is. All the worries of the day held up against 93 billion light years grow quiet and soft and very far away. And what remains is simply this. You resting safely, held within something vast and ancient and beautiful beyond all measure. The universe is unimaginably large and you are unimaginably small. And both of those things tonight are perfectly gently all right.

[01:26:44]There's an old feeling that comes over many of us when we stand beneath a truly dark sky far from any town and let our eyes drink in the stars. A feeling that is somehow both dizzying and calming at once.

[01:26:59]I think what we are feeling in that moment is exactly this. the gentle collision of our own smallalness with the vastness overhead.

[01:27:10]And I have come to believe it is one of the healthiest feelings a person can have. It quiets the noise. It softens the sharp edges of the day. It sets our small troubles down ever so gently against a backdrop so enormous that they cannot help but grow lighter. The universe is very big and you are very small and there is nothing at all to fear in that. Only a deep and ancient kind of peace.

[01:27:41]Thank you truly for wandering all this way with me out to the edge of everything and back again.

[01:27:49]Wherever you are on this small spinning world, whatever tomorrow may hold, I hope you feel just now wonderfully small and wonderfully safe beneath the same vast and patient sky that shelters us all. Rest easy. The stars will keep their long, slow watch above you all through the night.

[01:28:13]Sleep well.