[0:00]14 billion years ago, give or take there was a bang, a big one. Everything that exists came out of it. The whole universe, all the galaxies, all the stars, every atom in your body, every atom in the chair you are sitting on. All of it was produced in that one colossal event.
[0:17]And then it was over, done, finished. We are living in the aftermath, like walking through the ashes the morning after the bonfire. The sky is quiet, the excitement happened a long time ago, and we missed it by about 14 billion years.
[0:31]Too bad. That is what most people believe. That is the picture carried around in the heads of nearly everyone who has heard the phrase big bang and never looked much further into it, and it is completely wrong.
[0:43]See, the trouble with calling it the Big Bang is that the name itself does all the damage before the physics even gets a chance to explain itself.
[0:51]A bang is something that happens and then stops. You hear a bang, it is loud, it is dramatic, maybe there is a flash and then silence.
[0:59]So people quite naturally picture this cosmic explosion, this mother of all firecrackers, and they figure the interesting part is ancient history. It went off, it is finished, we are just the cold debris drifting.
[1:11]Somebody should have come up with a better name, but they did not, and now we are stuck with it.
[1:16]But that is not what the equations say, that is not what the instruments detect. That is not what the measurements tell us, and I want to take this apart piece by piece very carefully because once you see what is actually going on, you cannot unsee it.
[1:31]You will walk outside after this talk and look up at the sky and know in a way you cannot un-know that the thing people call the Big Bang is not something that happened. It is something that is happening right now, while we sit here together.
[1:44]Let me begin by being fair to the wrong picture. I want to make the wrong picture as compelling as I possibly can, because the people who hold this picture are not being foolish, they have good reasons, very good reasons.
[1:56]Go back to 1929. Edwin Hubble is working at the Mount Wilson observatory, perched in the San Gabriel Mountains above Pasadena, California.
[2:06]He has access to what was then the most powerful telescope on the planet, a 100-inch reflecting telescope, an enormous piece of glass that gathered more light than anything humans had ever built.
[2:17]And night after night, Hubble is patiently studying the light from distant galaxies, these fuzzy patches in the sky that people were not even sure were separate galaxies at the time.
[2:27]They called them nebulae, just foggy smudges. What Hubble does is take the light from each distant galaxy and spread it out into a spectrum, the way a prism breaks white light into a rainbow.
[2:38]And when you do that with starlight, you see dark lines at very specific wavelengths. These are absorption lines. Each chemical element absorbs light at its own particular set of wavelengths, like a fingerprint.
[2:50]Hydrogen has its pattern, calcium has its pattern, iron has its pattern. You can read the chemistry of a star from across the universe just by looking at where these dark lines fall, and Hubble notices something peculiar.
[3:03]In the distant galaxies, all the lines are shifted, not randomly, they are all displaced in the same direction toward the red end of the spectrum, toward longer wavelengths.
[3:13]Now, you know what this means if you have ever stood by a road and listened to an ambulance pass by. The siren sounds higher in pitch as the ambulance approaches, and it drops lower as it drives away. Sound waves get compressed when the source is moving toward you, and they get stretched when the source is moving away.
[3:29]We call it the Doppler effect. Light does the same trick. If a galaxy is moving away from you, the light waves arriving from it are stretched to longer wavelengths. Longer wavelengths of visible light look redder, so we call it a red shift.
[3:44]Hubble sees this redshift in every distant galaxy he examines, every single one in every direction, all receding.
[3:52]The universe is not sitting still, it is flying apart. And then he discovers something even more remarkable.
[3:59]The redshift is not random. There is a pattern to it. Though the farther away a galaxy is, the faster it is receding. A galaxy twice as far away is receding twice as fast.
[4:13]Three times as far, three times as fast, a perfectly clean linear relationship. Distance multiplied by a constant equals recession velocity.
[4:22]We call it Hubble's Law, and we call that constant the Hubble constant. It tells you the rate at which the universe is spreading out.
[4:29]So you take this beautiful observation, and you do the obvious thing, you run the film backward in your mind. If everything is flying apart today, then yesterday everything was a little closer together. Last year, closer still. A million years ago, much closer. A billion years ago, dramatically closer. Keep rewinding.
[4:50]All the matter in the universe, packed tighter and tighter, denser and denser, hotter and hotter until you reach a point roughly 14 billion years in the past where everything was crushed into an almost unimaginable temperature and density.
[5:03]Temperatures in the trillions of degrees, densities that make the core of the sun look like a gentle vacuum. And from that state the universe expanded rapidly, violently, the Big Bang.
[5:15]And this picture has the deeply satisfying quality of a story with a clear beginning, a middle, and an end. The beginning was the bang, the middle was the long expansion and cooling.
[5:25]Atoms formed, stars ignited, galaxies assembled, planets coalesced, life emerged, and the end is now us. Us, sitting in a vast cold, gently drifting universe, watching the remnants of an ancient explosion coast outward in slow motion, the energy spent, the fire long extinguished.
[5:45]The numbers supported beautifully, and I mean beautifully in the way physicists use that word, which is to say the math works out so cleanly that it almost feels like cheating.
[5:56]In the first second after the expansion began, the temperature was about 10 billion degrees. At that temperature, matter exists in a state you have never encountered and never will, not on this planet.
[6:08]There are no atoms, no nuclei, just a furious soup of protons, neutrons, electrons, photons, neutrinos, all smashing into each other at tremendous energies, converting back and forth between forms.
[6:21]A proton gets hit and turns into a neutron, a neutron decays back into a proton. Particles and antiparticles are being created and annihilated in pairs. It is chaos, absolute roaring, white-hot chaos.
[6:34]And then the universe cools and things start to freeze out. The neutrinos stop interacting and fly off on their own, a ghostly background that is still out there today, by the way, though we have never managed to detect it directly.
[6:47]The ratio of neutrons to protons gets locked in. By about three minutes after the expansion, the temperature has dropped to roughly a billion degrees.
[6:56]Still unimaginably hot, but cool enough for something new to happen. Protons and neutrons can now stick together. Nuclear fusion begins, not the sustained fusion of a star, which takes millions of years of gravitational pressure.
[7:11]This is fast, explosive cosmological fusion, happening everywhere in the universe simultaneously. Protons and neutrons slam together and form deuterium, a heavy form of hydrogen.
[7:22]Deuterium nuclei fuse into helium 3, then helium 4, a tiny trace of lithium 7 is produced. And then within about 20 minutes, the temperature drops too low for fusion to continue, and the process stops, 20 minutes.
[7:37]The entire nuclear history of the early universe, the forging of the lightest elements, happened in a window of about 17 minutes. That is all the time there was between the temperature being high enough for fusion to start, and too low for it to continue.
[7:51]And the proportions it produces are specific. The theory predicts about 75% hydrogen by mass, about 25% helium with trace amounts of deuterium and lithium.
[8:02]When we look at the oldest, most pristine gas clouds in the universe, the ones that have not been contaminated by later stellar processing, we find exactly those proportions, 75 and 25.
[8:16]The helium fraction matches, the theory and the measurement agree with startling precision, and this is one of the great triumphs of the standard cosmological model.
[8:25]Then, after about 380,000 years, the universe cooled to roughly 3,000 degrees. Still hot by everyday standards, but cool enough for a pivotal thing to happen.
[8:37]Electrons, which had been zipping around freely, too energetic to be captured, finally slowed down enough to be grabbed by atomic nuclei. Hydrogen atoms formed, and for the first time light could travel freely through the universe without slamming into a charged particle every fraction of a second.
[8:52]Before this moment, the universe was opaque, a dense fog of plasma. After it, the universe became transparent, the lights came on, so to speak.
[9:00]Then gravity did its patient work. Gas clumped under its own weight. Stars formed and burned. The heavier elements, carbon, oxygen, iron, were cooked inside massive stars and scattered into space when those stars exploded.
[9:17]New generations of stars formed from the enriched debris. Planets condensed, chemistry grew more complex. Eventually biology appeared, and here we are 14 billion years after the beginning, in a universe with a background temperature of 2.7 degrees above absolute zero, cold, quiet, settled, story over.
[9:38]Accepted, it is not. And before I start dismantling this picture, I want to do something. I want you to feel what the wrong picture feels like, really viscerally, so that when I tear it apart, you feel the loss.
[9:50]Because the wrong picture is comforting. It has a neatness to it. Everything happened, and then everything settled down, and now we are just here. If you believe this picture and you could somehow rewind time, you would see an incredible sequence.
[10:05]Right now, everything is cold and spread out. Go back a billion years, the galaxies are a little closer together. Go back 5 billion years and our sun has not formed yet.
[10:16]The gas cloud that will become our solar system is still scattered through the spiral arm of the Milky Way. Go back 10 billion years, and many of the galaxies we see today are still assembling from smaller pieces, merging, colliding, building up.
[10:29]Go back 13 billion years, and the first stars are just flickering on. The universe is mostly dark gas, and the cosmic microwave background is just a few hundred million years old, still blazing hot. Rewind further, past the point where the first stars formed,
[10:45]past the dark ages, when the universe was an expanding cloud of neutral hydrogen gas with no light sources at all.
[10:51]Back to 380,000 years, when the universe goes opaque. The fog closes in, you cannot see through it. Everything is a glowing plasma, about 3,000 degrees, filled with light that bounces between charged particles like a pinball in an infinite machine.
[11:08]Go further back, the plasma gets hotter. Atoms cannot hold together, nuclei cannot hold together. Further back, the distinct forces of nature begin to merge.
[11:20]Our physics starts to break down. And at the very beginning, what we can calculate runs into a wall. The equations give infinities, densities and temperatures become formally infinite, which in physics is a polite way of saying we do not have a theory that works there.
[11:32]What happened at the very beginning, at time zero, or whether there even was a time zero, we do not know.
[11:38]Some people ask what came before the Big Bang, and the honest answer is that we do not know, and it may be that the question does not even have a meaning.
[11:46]If time itself began with the expansion, then asking what came before is like asking what is north of the North Pole. The question uses a framework that does not apply.
[11:56]But I want to be clear, we do not know this for certain. It is possible that time extends before what we call the Big Bang.
[12:04]It is possible that our expansion was triggered by some larger process in some grander structure. We genuinely do not have the physics to answer this question yet.
[12:13]And I would rather tell you plainly that we do not know than invent a pretty story. What we do know is what happens starting from about a trillionth of a second after the expansion began.
[12:23]From that point forward, the physics is tested, confirmed and reliable. And from that point forward, the expansion has been continuous. And here is where the comfortable picture starts to crack.
[12:34]Because if you thought the expansion was a one-time event, a push at the beginning that set everything in motion and then stopped, the evidence is going to disagree with you in three very specific ways.
[12:44]Thread number one. A hiss that would not go away. In 1964, two radio engineers at Bell Telephone Laboratories in Homdel, New Jersey, named Arno Penzias and Robert Wilson, were trying to calibrate a large horn-shaped microwave antenna.
[13:02]This antenna had been built for satellite communication experiments, and they wanted to use it for radio astronomy. They were careful, methodical, thorough, and they had a maddening problem.
[13:13]No matter where they pointed the antenna, no matter what direction in the sky, no matter what time of day, no matter what season of the year, they detected a faint, persistent hiss of microwave radiation. It was everywhere, uniform, constant, and it was not supposed to be there.
[13:29]They checked every piece of equipment, they recalibrated the receiver, they tightened every connection. They discovered that pigeons had been roosting inside the horn of the antenna, and leaving behind what they delicately described in their paper as a white dielectric material, which is a polite way of saying pigeon droppings.
[13:47]They swept it out, they cleaned the antenna thoroughly. They even caught and relocated the pigeons. The pigeons came back, by the way. Pigeons always come back, but the hiss was not from the pigeons.
[13:58]The hiss remained exactly as it had been before, coming from every direction equally, with an intensity corresponding to a temperature of a few degrees above absolute zero.
[14:10]What Penzias and Wilson had found, almost entirely by accident, was the cosmic microwave background radiation, the afterglow of that moment, 380,000 years after the expansion began, when the universe first became transparent.
[14:24]That first freely traveling light had been journeying across the universe ever since, for nearly 14 billion years, stretched by the expansion of space, from visible orange glowing light into microwaves, and it was arriving at their antenna right then, right at that moment, just as it is arriving at instruments all around the world right now, this second, from every direction, filling the entire observable universe with a faint uniform glow.
[14:53]If the Big Bang were truly over, if it was simply a historical event, locked away in the deep past, why is its light still raining down on us? Not from some particular corner of the sky, from everywhere simultaneously, continuously.
[15:09]If you built a sensitive enough microwave detector and pointed it at any patch of sky, you would find it. About 1% of the static on an old television set was actually the cosmic microwave background.
[15:20]You were watching the afterglow of the beginning of the universe without knowing it. But that does not sound like something that finished a long time ago.
[15:31]That sounds like something you were sitting inside of. But maybe you push back. Fine, you say. Light takes a long time to travel. The universe is big, so of course ancient light is still arriving. That is just an echo, like hearing thunder minutes after the lightning.
[15:44]The event itself is long over, we are just catching the tail end of the sound. All right, that is a reasonable objection. Let me pull a second thread.
[15:52]Go back to Hubble's observation. The galaxies are receding from us. I said that in the present tense, and I meant it deliberately. Not they receded, not they once moved apart. They are moving apart right now, today.
[16:07]We measure the redshifts of galaxies continuously with instruments far more sensitive than anything Hubble had access to. The recession is happening as we speak. The expansion has not wound down.
[16:20]It has not settled into some static aftermath. It is active and measurable and ongoing. And uh, in 1998, the expansion story took a turn that genuinely shocked the physics community.
[16:32]Two independent research teams were studying a special category of exploding star, called a Type 1A supernova. Now, these particular supernovae are extraordinary cosmic tools.
[16:42]They all explode with roughly the same peak luminosity, the same intrinsic brightness, so if you measure how bright a Type 1A supernova appears from Earth, you can figure out how far away it is, because dimmer means farther.
[16:54]And if you measure its redshift, you know how fast the space between us and it is expanding. Do this for dozens of supernovae at different distances, and you can map out how the expansion rate has changed over time.
[17:05]The team led by Saul Perlmutter at Lawrence Berkeley National Laboratory and the team led by Brian Schmidt and Adam Riess, starting from the Australian National University and Johns Hopkins, respectively.
[17:18]Both expected the same thing. They expected to find that the expansion was decelerating, slowing down. This was the obvious prediction. Every bit of matter in the universe attracts every other bit of matter through gravity.
[17:30]All that gravitational pull should act like a brake pedal on the expansion. The only real question was how strong the braking was. Would the expansion slow to a halt and reverse, collapsing everything back into a hot, dense state?
[17:43]Or would it slow down but never quite stop, coasting forever? Both teams found something else entirely. The expansion is not decelerating, it is accelerating. The distant supernovae were dimmer than they should have been.
[17:58]Farther away than the braking model predicted, which meant the expansion had been speeding up, not slowing down. Something was pushing the universe apart with increasing force.
[18:08]We gave this mysterious push a name, dark energy, which sounds like we know what it is, but we emphatically do not. And I want to be very honest about this because in physics giving something a name can create the illusion of understanding.
[18:21]We understand the effect of dark energy extremely well. We can measure it, we can write equations for how it influences the expansion, but what it actually is at a fundamental level, we do not know, not yet, maybe not for a long time.
[18:35]One possibility is that dark energy is a property of space itself. That empty space is not truly empty but has an inherent energy, a kind of baseline hum, and this energy pushes space apart.
[18:47]Einstein actually proposed something like this in 1917, a term in his equations he called the cosmological constant, which he later reportedly called his greatest blunder.
[18:57]Because he introduced it for the wrong reasons, but the mathematics he wrote down may turn out to describe something real, something intrinsic to the geometry of spacetime.
[19:07]Another possibility is that dark energy is a new kind of field, something that fills all of space but changes slowly over time, sometimes called quintessence.
[19:15]In this picture, the acceleration we see today might not last forever. It might strengthen or weaken or change character over billions of years. We do not have enough data to distinguish between these possibilities with certainty.
[19:28]What we do know is that dark energy makes up roughly 68% of the total energy content of the universe.
[19:35]Ordinary matter, the protons and neutrons and electrons that make up everything you have ever seen or touched, accounts for only about 5%.
[19:43]Dark matter, another mysterious component that interacts gravitationally but not with light, accounts for about 27%.
[19:50]The universe is overwhelmingly made of things we do not understand. We live in the 5%. And the dark energy has dominated the expansion for roughly the last 5 billion years, since approximately the time our sun and solar system were forming.
[20:03]Before that, the expansion was actually decelerating as the old models predicted, because dark matter and ordinary matter were dense enough that their gravitational pull was stronger than the dark energy push.
[20:15]But as space expanded, the matter thinned out, and the dark energy, which does not thin out because it is a property of space itself, eventually won. The universe shifted gears from decelerating to accelerating about 5 billion years ago, and it has been picking up speed ever since.
[20:32]But notice what this tells us about our question. A regular explosion decelerates after it goes off. The shrapnel spreads, gravity pulls, air resistance slows things down, and eventually everything settles.
[20:45]An explosion that is speeding up is not an explosion that is over. Something is actively feeding it energy. Something is actively driving the expansion right now, today, this very second.
[20:55]The universe is not coasting on old momentum. It has an engine, and the engine is running. Thread no. 3, and this one demolishes the explosion picture completely.
[21:05]When I described galaxies moving apart, I was using sloppy language, dangerously sloppy, and the sloppiness is exactly where the deepest misconception lives.
[21:16]The galaxies are not moving through space away from each other, not in the way shrapnel moves outward through the air after a bomb detonates. Something far stranger is happening. Space itself is expanding.
[21:29]The galaxies are for the most part, sitting nearly still in their local patches of space. It is the space between them, the actual geometry of the universe, that is stretching, growing, producing new distance where there was less distance before.
[21:44]This is the single most important idea in all of cosmology, and it is the one that people get wrong most often. And they get it wrong because nothing in everyday experience prepares you for the idea that the emptiness between objects can change.
[21:58]We are used to objects moving through a fixed, unchanging stage. The stage itself does not stretch. The floor under your feet stays put, but the universe does not work that way.
[22:09]I should tell you, by the way, where the name Big Bang actually came from because the origin is almost comically ironic. The name was coined by Fred Hoyle, a brilliant British astronomer, who did not believe in the theory at all.
[22:19]He preferred a competing idea called the Steady State Model, in which the universe had no beginning and has always existed in roughly its present form.
[22:28]In a radio broadcast on the BBC in 1949, Hoyle used the phrase Big Bang dismissively, almost mockingly, to describe the idea he thought was wrong, and the name stuck.
[22:38]The most successful theory in the history of cosmology is saddled with a name that was invented as an insult by one of its opponents. And the name itself encodes the very misconception we are trying to dismantle.
[22:50]A bang, a single event, an explosion, all wrong. Let me try to build some intuition for what is actually happening.
[22:59]Take a balloon, uninflated. Draw dots all over the surface with a marker. Lots of little dots spread evenly. Each dot is a galaxy. The rubber surface of the balloon is space, not the air inside, not the room around it, just the two-dimensional surface itself.
[23:17]That surface is your entire universe. The ants on the surface cannot see off the surface, they cannot perceive inside the balloon or outside the balloon. The surface is all there is.
[23:28]Now, inflate the balloon slowly. What happens? Every dot moves away from every other dot. An ant standing on any particular dot, looks around and sees all the other dots receding.
[23:40]The more distant dots recede faster. The ant might think, I must be at the center of an explosion. Everything is fleeing from me. But it is not at the center. There is no center on the surface of a sphere. Pick any other dot.
[23:53]The ant sitting there sees exactly the same pattern. Every dot looks like the center. No dot actually is. Um, there is no special point, no edge, no boundary. The expansion is happening in the rubber itself, uniformly everywhere.
[24:10]And the critical thing to notice is that the dots are not sliding across the rubber. They are sitting exactly where they were drawn. The rubber between them is what is stretching.
[24:19]New rubber is appearing, if you like, between every pair of dots. The distances grow, not because of motion through space, but because of the creation of more space. That is the universe. Space is the rubber.
[24:32]Galaxies are the dots. The expansion is not objects flying outward from a center. It is the fabric of space itself growing everywhere, all at once. No center, no edge, nothing outside it, for it to expand into. The universe is not expanding into anything. Space is just getting bigger.
[24:52]And this is still happening. Let me try another angle. Imagine you are baking raisin bread. You knead the dough, scatter raisins through it evenly. Each raisin is a galaxy. The dough is space.
[25:08]You slide the loaf pan into the oven. The dough rises. It expands uniformly. Every raisin drifts away from every other raisin. Not because the raisins are pushing through the dough, the dough between them is stretching, puffing up, creating more dough.
[25:22]A bug sitting on any one raisin sees all the other raisins retreating. And the farther away a raisin is, the more expanding dough sits between them, so the faster it appears to recede. That is Hubble's Law, falling right out of the rising bread.
[25:37]And the bread is still rising, the oven is still on. We are inside the loaf right now. A very good question comes up at this point.
[25:45]If space is expanding everywhere, why are you not expanding? Why is the Earth not getting bigger? Why is the distance between you and the person next to you not growing?
[25:55]Why does your kitchen table still fit in your kitchen? Because the expansion is breathtakingly gentle on small scales, and it is completely overpowered by all the other forces that bind things together.
[26:05]Gravity holds the Earth in one piece. Electromagnetic forces hold the atoms of your table in their crystal lattice. The strong nuclear force holds atomic nuclei together. These forces are monstrously stronger than the expansion on the scales of everyday life.
[26:21]The numbers make this vivid. The expansion rate, the Hubble constant, is about 70 kilometers per second per megaparsec.
[26:29]A megaparsec is about 3.26 million light-years, so over a distance of 3 million light-years of truly empty space, the expansion produces a recession velocity of about 70 kilometers per second.
[26:42]That is significant over cosmological distances, over billions of light-years, recession speeds become enormous, even exceeding the speed of light in some cases, which is allowed because nothing is traveling through space faster than light.
[26:58]Space itself is expanding, but on the scale of a galaxy, the expansion amounts to a tiny, tiny fraction of a kilometer per second, completely negligible, compared to the gravitational forces holding the galaxy together.
[27:12]On the scale of a solar system, effectively zero. On the scale of your body, the atoms in your left hand are being pushed away from the atoms in your right hand at a rate so infinitesimal that you would need to wait many, many times the current age of the universe for the expansion to amount to even a single atom's width of extra distance.
[27:30]Gravity wins locally, the strong force wins locally. Electromagnetism wins locally. The expansion of space only wins where there is nothing holding things together. Out in the vast empty voids between galaxy clusters, where there is essentially no matter, no gravitational binding, out there the expansion reigns.
[27:50]Space stretches unopposed, distances grow, and thanks to dark energy, they grow faster every year. Now, I want to take you to the edge of what we can see because something extraordinary lives at that boundary.
[28:02]Light moves at a finite speed, roughly 186,000 miles per second, or about 300 million meters per second. That means when you look at a distant galaxy, you are not seeing it as it is right now.
[28:16]You are seeing it as it was when the light left it. A galaxy 1 billion light-years away is being seen as it looked 1 billion years ago. We are looking backward in time.
[28:25]The deeper into space we peer, the further back in the past we see. And there is a limit. The observable universe has a horizon. Not a wall, not a barrier you could touch or see.
[28:38]A horizon, like the horizon on the ocean, where your line of sight runs out. Beyond the cosmic horizon, light has not had time to reach us since the universe first became transparent, about 380,000 years after the expansion started.
[28:51]The cosmic microwave background is the image of that horizon, the oldest light, the most distant surface we will ever observe. The radius of this observable universe is about 46 billion light-years.
[29:04]And that number baffles people. How can the observable universe be 46 billion light-years in radius if the universe is only about 14 billion years old? The light cannot have traveled 46 billion light-years in 14 billion years.
[29:18]And this is where the expansion does something sneaky. The light from the cosmic microwave background has been traveling for almost 14 billion years, true.
[29:27]But while that light was in transit, the space it was traveling through was stretching, continually. The source that emitted the light was much, much closer to our location when the light left, perhaps about 42 million light-years away.
[29:41]But in the intervening 14 billion years, the expansion of space carried that source farther and farther away, so the point of origin is now 46 billion light-years from us, even though the light itself traveled through only 14 billion years of expanding space.
[29:56]Think of it this way. Imagine you are walking across a very long rubber sheet. You step off one end, heading for the other. But while you walk, someone is stretching the sheet, adding length to it.
[30:08]By the time you reach the far side, the sheet is much longer than it was when you started. The distance you actually walked is less than the current length of the sheet because the sheet grew while you were on it.
[30:20]That is what happens to every photon crossing the universe. The photon departs from a distant galaxy, it flies toward us at the speed of light, but space stretches while it travels.
[30:31]By the time it arrives, the galaxy it came from is much farther away than it was when the light departed. The expansion made the journey longer even while the photon was making it.
[30:40]And here is a detail that really bends the mind. Some galaxies are so far away that the space between us and them is expanding faster than the speed of light, which means photons leaving those galaxies right now, at this very moment, heading straight toward us, will never arrive.
[30:54]They will never bridge the gap because the gap is growing faster than light can cross it. Those galaxies are in a very real sense, gone from our universe.
[31:04]We can still see their ancient light, photons that left long ago when the galaxy was closer, but their present-day light will never reach us. They are beyond our cosmic event horizon, invisible, unreachable, already lost, and we do not even notice.
[31:20]And this brings me to something very elegant. The cosmic microwave background, when it was emitted, was not microwave radiation. It was visible light, glowing orange-red, roughly 3,000 degrees, like the surface of a dim red star.
[31:35]Imagine the entire sky in every direction, blazing with this warm orange glow. That was the universe at 380,000 years old.
[31:44]But over the nearly 14 billion years since, the space those photons traveled through expanded enormously, by a factor of about 1,100, and the light waves expanded with the space.
[31:55]Their wavelength stretched, what was once visible orange light has been stretched into microwaves. Invisible to the eye but detectable by radio antennas and satellite instruments.
[32:06]That stretching is direct, measurable physical evidence of how much space has expanded since those photons were released. Every photon in the cosmic microwave background carries a record of the expansion, stamped into its wavelength.
[32:17]Like a ruler that has been stretched and still shows the marks. And new photons from that ancient surface are arriving every instant. The expansion is not a fossil, it is alive, writing its signature on the light of the universe in real time, continuously.
[32:31]Let me turn to the future now, because the future of an accelerating universe is strange, and I will be honest with you, a little frightening. If the acceleration continues, and every measurement we have today suggests it will,
[32:44]then something remarkable unfolds over enormous time scales. Distant galaxies already receding from us, will recede faster and faster. The light from those galaxies will be stretched to longer and longer wavelengths.
[32:57]Their images will grow dimmer and redder until they fade from detectability entirely. And eventually their recession speed will exceed the speed of light. Space between us and them will be expanding so quickly that a photon leaving that galaxy will never be able to cross the gap.
[33:12]It will be running on a treadmill that is speeding up faster than it can run. When that happens, the galaxy vanishes from our observable universe. Not because it has been destroyed, it is still out there, burning and spinning and full of worlds.
[33:25]But its light can no longer reach us. It has been carried beyond our cosmic horizon by the relentless expansion of space. This will happen galaxy by galaxy, cluster by cluster, over trillions of years.
[33:38]Our observable universe will shrink, not in the sense that space contracts, but in the sense that less and less of the universe will be visible to us. The other galaxies will wink out like street lights in a fog, one by one, as the expansion carries them beyond reach.
[33:53]In the very, very far future, trillions upon trillions of years from now, an astronomer living in whatever our Milky Way has merged and evolved into, will look out at the sky with the finest instruments imaginable and see absolutely nothing. No distant galaxies, no cosmic web, no great walls and voids, no large scale structure of any kind.
[34:15]Just their own local group of gravitationally bound stars, drifting in what appears to be a vast, limitless, featureless, empty and perfectly static void.
[34:24]The cosmic microwave background itself will have been stretched to wavelengths so preposterously long that no conceivable instrument could detect it. The photons will still exist technically, each one stretched to a wavelength larger than the observable universe itself, functionally invisible, gone as far as any observation is concerned.
[34:44]And it gets worse. Even the chemical evidence will be misleading. By that time, the primordial hydrogen and helium will have been processed through many generations of stars.
[34:55]The original ratios, the 75/25 split that we use today to confirm Big Bang nucleosynthesis, will have been scrambled beyond recognition. The fossil record of the early universe, written in the abundances of light elements, will have been overwritten.
[35:10]Every line of evidence we currently rely on to deduce the history of the universe will be either gone or unreadable. That future astronomer will examine the universe and draw a completely rational conclusion.
[35:21]The universe is small, static, unchanging and eternal. It has always been this way. There are no other galaxy clusters. There is no expansion. There was no Big Bang. They will arrive at exactly the picture of the cosmos that astronomers held before Hubble made his measurements in the 1920s.
[35:41]The expansion will have erased every trace of its own existence, a process that destroys its own evidence. I find that unsettling in a way I cannot quite shake.
[35:51]But not just the loneliness of it, not just the idea of intelligent beings surrounded by emptiness they cannot explain, but the epistemological horror of it, the philosophical vertigo. Those future scientists will be brilliant, possibly more brilliant than us.
[36:06]They will have technology we cannot imagine. They will have every reason to trust their instruments and their observations, and their instruments will tell them something that is absolutely, categorically, completely wrong. Not because the instruments are broken, not because the scientists are careless,
[36:23]but because the evidence has been physically removed from the universe by the very phenomenon they cannot detect. The truth will be real but unobservable. The data will be clean but misleading.
[36:32]Nature itself will be lying to them, not through deception but through expansion. It makes you wonder, is something like that happening to us right now? Are there truths about the universe that are real but that the expansion has already hidden from our view?
[36:47]Things that happened before our observable horizon formed that we will never, ever have access to? Almost certainly, yes. We see a bubble, a large bubble, 46 billion light-years across, but a bubble nonetheless.
[37:02]What lies beyond it? What the universe looks like on scales far larger than our horizon, we do not and cannot know from observation.
[37:11]We can speculate, we can extrapolate, but we cannot see, we cannot. The expansion has already drawn that curtain. But we are lucky. We happen to live during the window, the cosmological window, when the evidence is visible.
[37:27]The microwave background is detectable. Galaxy redshifts are measurable. The acceleration was only discovered in 1998.
[37:36]We exist during the brief era, the cosmic springtime, when the universe is willing to show its hand to anyone curious enough to look carefully. That window will not stay open forever, and we did not earn it.
[37:48]We are simply here at the right time. Let me draw all of this together because I want the full picture to land with its full weight.
[37:55]The Big Bang is not a moment in the distant past. It is a process. It is the ongoing expansion and cooling of the universe from a hot, dense initial state.
[38:07]That moment of extreme temperature and density that most people imagine when they hear the words Big Bang, that is just the opening note of a symphony. The symphony is still playing, the cooling is still happening.
[38:19]The expansion is still running, faster now than ever before, driven by dark energy. You are inside the Big Bang. Every photon from a distant galaxy that arrives at your telescope, a little redder than it would be if space was sitting still, that is the Big Bang happening right now.
[38:36]Every faint microwave whisper picked up by radio antennas, humming at a temperature of 2.7 degrees above absolute zero, that is the Big Bang still speaking.
[38:45]Every new sliver of space appearing between superclusters of galaxies, space that did not exist a moment ago, that is the Big Bang continuing its work. It never stopped. There is no post-Big Bang era. But there is no aftermath.
[39:01]We live in the Big Bang, the way a fish lives in the ocean. We are so thoroughly immersed in it that we mistake it for stillness. We mistake it for background. We think of it as history, when it is in fact the present.
[39:13]And this is maybe the most surprising thing of all. The Big Bang is not dramatic in the way people expect. It does not feel like an explosion. There is no roar, no heat, no violence.
[39:25]At this stage in the process, 14 billion years in, the Big Bang feels like a quiet Tuesday afternoon, like a cup of coffee cooling on a desk, like birds outside the window.
[39:37]That calm, ordinary feeling is what the Big Bang feels like at this temperature, at this density, at this moment in its history.
[39:44]14 billion years ago, it felt like a furnace. In 14 billion more years, it will feel even quieter than now, even colder, even emptier. We happen to be here during the lukewarm phase, the phase where interesting chemistry is possible.
[39:58]The brief window where matter can organize itself into creatures that wonder about their own origins. I think the reason this matters beyond the pure intellectual thrill, is that it transforms what you mean when you say now.
[40:09]When you say now, you picture something ordinary, something calm. Just another quiet afternoon. Nothing cosmic is happening. But cosmologically, now is a single frame in a 14-billion-year-old process of expansion and cooling that is in full swing.
