What Happens If You Zoom All The Way Out?

In 1973, the yet to be famous physicist Stephen Hawking found himself behind the Iron Curtain. He was attending a conference on the formation of nature's most extreme objects, black holes. It was the height of the Cold War, a tense time between East and West, and both sides were bristling with nuclear weapons that could obliterate each other in minutes. And yet here, in Warsaw, Poland, there appeared to be a thaw. Scientists from both blocs aimed to meet and exchange ideas, talking cutting edge science in a fragile oasis of intellectual freedom. Those from the West desperately wanted to know what had been found by those working on the other side. What had they discovered that they had not been able to share? Indeed, Hawking himself had heard whispers of remarkable Soviet advances, and there were also rumors of new insights into cosmology and the Big Bang itself. And one name surfaced again and again in these discussions, like an echo across disciplines. Surely, Hawking thought, this name couldn't belong to a single person. Perhaps it was a collective, a cabal of geniuses working under a shared pseudonym. Indeed, how else, wondered Hawking, could one name be tied to such a dizzying array of breakthroughs? Eventually, Hawking would put a face to this extraordinary man, a balding face in his 50s with dark circular glasses perched on his nose. Hawking quickly realized that there was no school of physicists, no hidden brotherhood of secret geniuses. It was just one extraordinary man with one extraordinary mind, the extraordinary mind of Yakov Borisovich Zeldovich. For decades, hidden behind the Iron Curtain, Zeldovich had been instrumental in shaping Soviet science. And with his exceptional intellect, was both a prized asset and a captive of that system. Unfortunately, he did not live to see the collapse of the Soviet Union, dying in 1987. And yet, while his name remains relatively obscure in the West, his contributions will be legion. But for our story, it is Zeldovich's work on the largest things in the universe that is most important. His first ingenious insights into supermassive, super ancient cosmic objects that span hundreds of millions of light years across. Mega structures so big that they dwarf the Milky Way 10,000 times over. And so old that they may hold the secret to how it all began. Behind every History of the Universe documentary, are many, many hours of research. From our expert writers, Paul, Colin, and Geraint, to scouring countless papers, to our talented editors and animators like Siji and Manuel keeping their visuals as accurate as they are stunning, History of the Universe is the result of real people with a passion for bringing real astrophysics and cosmology to life. And in this research, Opera is an invaluable tool. It can be easy to get lost in a research rabbit hole with lots of tabs open at once. But with the Opera browser, you can group your dozens of tabs into neat, organized islands. You can quickly give these islands names and colors, allowing you to jump around easily through everything you have open without getting lost. This is of course great on deep dives for work, but also very handy in our personal lives when buying gifts and booking trips as you can expand and collapse them as needed. The names and colors make everything so easy. Indeed, your most recently used tabs even have a subtle tab trace underscore, making it even easier to find that tab you're looking for at just a glance. The darker the underscore, the more recently you've visited the tab. We also love the video pop out feature. It's easy to detach a video while you're watching and move it inside or even outside of the browser. This is also perfect for multitasking. So, to experience this yourself, download Opera via my link in the description. In the late 1970s and early 1980s, a quiet revolution was unfolding in the way we saw the universe. Astronomers were no longer content with counting galaxies as distant smudges of light on photographic plates. They wanted to map their positions in three dimensions to reveal the true architecture of the cosmos. They wanted an atlas of the heavens. And among those leading this charge to become cosmic cartographers, were two American astronomers, Margaret Geller and John Huchra. Their work was audacious in its simplicity. They set out to measure the speeds of many thousands of galaxies across the sky and use the rule that underscores the expanding universe. The faster a galaxy receded, the further away it must be. With the galaxy speeds measured by some of the world's largest telescopes, Geller and Huchra began to assemble a 3D atlas of the universe. Collecting the sparse photons from distant sources took many hours at the telescopes, but as time wore on, an amazing picture began to emerge. For what they found was not some random scattering of points, as if galaxies had simply been thrown across the heavens. Instead, they saw a distinct pattern. A vast cosmic web of filaments and voids. And as the data built up, there came another shock. In 1989, Geller and Huchra unveiled a feature so enormous it defied comprehension. A wall of thousands of galaxies stretching hundreds of millions of light years across the cosmos. They called it the Great Wall. And that would be just the beginning. Following the discovery of the Great Wall, astronomers would continue to find almost incomprehensibly large structures across the sky. The filaments within the cosmic web. And indeed, many that followed would be even bigger. Dwarfing the Great Wall is the Sloan Great Wall, found in 2003 within the Sloan Digital Sky Survey and spanning approximately a 60th of the entire observable universe. Roughly equally long is the South Pole Wall, which is twice as close to us at only 500 million light years away. And between these great filaments, the gaps in the web, are the voids, massive regions of underdensity. The Sloan Wall frills the edges of the famous Boötes void, which is more than 300 million light years wide. And the South Pole Wall winds around the much larger Eridanus Supervoid, which some studies put at more than twice the size of Boötes. However, these discoveries have been more than just a cartographer's triumph. For they raised clear questions about the fundamental nature of our universe. Just what forces could have sculpted this grand design across the depths of time and space? Long before the Great Wall was revealed, Zeldovich had been wrestling with the question of how structure could form and grow in the universe. And he'd realized that the cosmos of matter that emerged from the Big Bang must have been exceptionally smooth. But it could not have been completely smooth. If the universe had been perfectly smooth at its moment of creation, it would remain that featureless sea of matter today with no stars, no galaxies, and no structure. So, instead, Zeldovich imagined the universe as a vast sea of matter gently rippled by primordial fluctuations. He also realized that the only force capable of shaping the universe must be gravity. With its long reach, it would amplify these tiny irregularities over time, pulling matter into sheets and filaments. This was not chaos, but a slow choreography dictated by the precise laws of physics. He saw that the collapse of matter was unlikely to be neat, symmetrical, or even spherical. The collapse was more likely to be irregular and uneven. And it was from these ideas that came the notion of the Zeldovich pancake. This is a striking metaphor for the collapse of matter. Zeldovich realized that in an uneven collapse, one direction should collapse the fastest. In his picture, as this one dominant axis rapidly shrinks, gravity squeezes matter into thin planes. And so, this means that the universe's first structures were not neat spheres, but immense flattened slabs. Continued collapse would then produce filaments and dense nodes. It was a vision that foreshadowed the cosmic web well before astronomers mapped the cosmos in any detail. The elegance of Zeldovich's picture lay in its simplicity, yet its implications were profound. It suggested that minute fluctuations seeded the universe's grand design. Well, the pancakes were the scaffolding upon which the cosmic architecture would rise. And it would be the advent of computers that would help prove these ideas. What Zeldovich had sketched in elegant theory could now be tested in virtual synthetic universes. Their work revealed that Zeldovich's pancakes were just the beginning. The universe was not a simple grid, but a sprawling network of interconnected strands. And it showed that an unseen cosmic matter was central to it. In the decades before the 1980s, observations had revealed that the stars we see are not the dominant stuff of the universe, and that another mass known as dark matter dominated the cosmos. Indeed, the sheer amount of dark matter astonished the scientific community with almost 100 times more unseen mass than in the trillions of stars we can see. And it was this dark matter, with its dominant pull of gravity, that collapsed into clusters, sheets, and filaments. The raw material of stars, mainly hydrogen and helium, just came along for the gravitational ride. But unlike the boring and simple gravitational physics of dark matter, which can only slowly gather and not fully collapse, gas can interact electromagnetically, and thus collide, cool, and eventually collapse, bursting into light as new stars roar into being as high density knots in the collapsing material. Although this picture did have an unsettling conclusion. When we look at any galaxy, including our own Milky Way, what we are seeing is the proverbial icing on the cake. All the stars nestled together are bound by the gravity of dark matter, and it is this distribution of unseen matter that stretches far beyond the visible edge of the galaxy and links it into the cosmic web that fills the universe. By the late 20th century, cosmologists felt that they had a good idea about the formation and evolution of galaxies. But while it seemed clear that galaxies had grown from seeds, drawing in more and more matter, the question of the origin of those galactic seeds remained. Theoretical ideas had bounced around for a decade or so, but in 1989, a space telescope was launched. The Cosmic Microwave Background Explorer, or COBE for short, whose mission was to directly image the earliest epochs of the universe. First observed in the 1960s, the cosmic microwave background radiation is the afterglow of creation itself. The intense radiation of the initial moments now cooled to a feeble glow at a few degrees above absolute zero that entirely fills the cosmos today. COBE found that the microwave background was an almost perfect blackbody spectrum of light. A spectrum so pure that it could only have been forged in the furnace of the Big Bang. This was not just a clue, this was confirmation of the universe's fiery birth and the idea of a searingly hot initial few moments. But then came the subtler truth hidden in the tiniest of ripples. COBE revealed fluctuations at the level of one part in 10,000. Faint wrinkles over the entire fabric of the sky, and it was these that were the seeds from which galaxies would grow. The fingerprints of structure etched into the infant cosmos. In those patterns, we glimpse the universe as it was 380,000 years after its birth. And so, what we are seeing in the CMB is the first sketch of the cosmic web drawn in the language of gravity and time. These delicate variations would swell into filaments and clusters, shaping the grand design we inhabit today. But, that wasn't the end. For it was not only random fluctuations that were found in this window into the early universe. There was something else. Something bigger. Something more organized. And so, to find out what caused these patterns, we must look deeper still. Back before the cosmic microwave background was formed, to an era when the universe was a seething plasma. Hot, dense, and waiting for the first light to break free. In the early 1980s, a small group of astronomers known as the Seven Samurai set out to tackle one of the biggest questions in cosmology. Just how fast is the universe expanding? The Seven Samurai were David Burstein, Roger Davies, Alan Dressler, Sandra Faber, Donald Lynden-Bell, Roberto Terlevich, and Gary Wegner. Each bringing their own expertise, but together forming a team that would reshape our understanding of cosmic motion. Their observational program was ambitious. They focused their work on measuring the distances and velocities of hundreds of elliptical galaxies, but the payoff was immense. For with that catalog in hand, the Seven Samurai could begin to map the motions of galaxies across the local universe. Where they were going and how fast. And what they uncovered was extraordinary. Their measurements revealed that galaxies were not simply drifting apart with the smooth expansion of the universe. Instead, they saw that there must be a vast region of space containing an immense quantity of mass that was exerting an immense gravitational pull, drawing galaxies towards it. This mysterious concentration of mass was called the Great Attractor, not as it turned out a single object, but a sprawling region of space dominated by clusters of galaxies and dark matter. For cosmology, this was a revelation. The cosmological principle assumes that on large scales, the universe is homogeneous and isotropic, same everywhere and in every direction. But the Great Attractor showed that even on scales of hundreds of millions of light years, the distribution of matter is far from uniform. Gravity creates flows, and these flows can distort our view of the universe. And now, 40 years later, that logic has led to remarkable discoveries, discoveries even larger and more consequential than the Great Attractor itself. The beautiful campus of the University of Queensland sits on a big, lazy bend of the Brisbane River. And it is here that Cullen Howlett, a young, up and coming cosmologist, is making his mark in understanding the nature of the universe. But of course, modern cosmology is not confined by geography, and Cullen works with leading researchers across the globe. And this includes Brent Tully at the University of Hawaii, a veteran when it comes to mapping the universe. Together, they were exploring the latest and most detailed map of the cosmos we have, known as Cosmic Flows 4. This modern data set, the distant descendant of the remarkable work of the Seven Samurai, is a compilation of decades of careful measurements of more than 50,000 galaxies, charting their positions and motions across vast stretches of space, revealing the hidden architecture of the universe. And it was in this cutting edge three dimensional map of the cosmos that they stumbled upon something unexpected. For beyond the simple clusters and superclusters of galaxies, they found a structure unlike anything they'd seen before. A vast spherical shell of galaxies, so large it appeared to defy intuition. They named it Ho'oleilana, a Hawaiian phrase meaning from deep darkness comes murmurs of awakening. And it is an immense bubble, a billion light years across, written into the very fabric of the cosmos. But, what could have possibly created such a vast megastructure? To unravel this mystery, we must go back to behind the Iron Curtain, back inside the incredible mind of Yakov Borisovich Zeldovich. Zeldovich was a dominant figure and rightly held in the highest regard. It was not for nothing that he boasted of having three gold stars to his orders of Lenin, while Brezhnev only had two. There was a simple anecdote that sums up the forceful character of Zeldovich, as well as his pride for the sheer quantity of things he discovered during his lifetime. In 1986, the year before his death, Zeldovich took part in a visit to Rome as a delegate of the space program of the Soviet Union. Finally free of the restrictions put upon him in previous visits to the West, he thoroughly enjoyed attending conferences and engaging in open discussion with as many people as he could. And it was during the event in Rome that he was introduced to Pope John Paul II, and upon their meeting, gifted his Holiness two large red volumes of his own papers. The story goes that the Pope thanked him warmly, to which Zeldovich replied, "Not just thanks, these are 50 years of my work." Zeldovich had long understood that the early universe that emerged from the Big Bang was a seething cauldron of light and matter. Indeed, for hundreds of thousands of years, it was so hot that matter was plasma, with all electrons torn from the nuclei of atoms. In this state, the light and matter were locked together, sloshing around as a single cosmic fluid. However, the tiny ripples that seeded the matter in the universe were there, and so gravity immediately began to draw dark matter together. The plasma of matter and light followed along, but unlike dark matter, this cosmic fluid possessed pressure that pushed back, resisting gravity's pull. This meant that the matter light plasma would collapse, but then the pressure from the electromagnetic radiation would become too great, pushing it back out again. This cycle would then repeat, causing the plasma to bounce back and forth. And so this struggle would set up oscillations, sound waves rippling through the primordial fluid, the oldest sound in the universe. While Zeldovich blazed the trail with these ideas, it would be Jim Peebles and J.T. Yu in the West who formalized the complex physics of these oscillations. They showed how matter in overdense regions compressed under gravity, but then rebounded as plasma pressure fought back. The continued compressions and reflections initiating a complex series of peaks and troughs in the plasma density. The universe resonated like a vast instrument with these waves moving at half the speed of light through the photon baryon sea. This was not a gentle hum, but a full symphony playing out over hundreds of thousands of years. You can think of these waves like ripples on a pond expanding outwards after you throw in a stone. But the universe was not struck by a single stone. It was more like tossing in a handful of pebbles, each sending out its own expanding rings. The result was a complex pattern of interference, peaks and troughs overlapping across space. However, as the cosmos expanded and cooled, the music began to fade. And indeed, at about 380,000 years after the Big Bang, the universe became too cool to sustain the plasma, and electrons and protons combined into neutral hydrogen. The first neutral atoms were created in the universe, and the bond between matter and light was severed. The fog lifted, and photons of light streamed freely across space. This is, rather unhelpfully, called cosmic recombination. Named that way because it was proposed before the Big Bang theory was finished. Physicists at the time believed that the nuclei were reuniting with their electrons after a period of separation. Not that these were the first neutral atoms ever, and the re in recombination stuck. This was the moment the universe finally became transparent, the creation of the cosmic microwave background. The sound waves were frozen in their tracks, and so the freed light carried with it a frozen record of the oscillations, a snapshot of the cosmic symphony etched into the microwave sky we see today. Indeed, faint variations observed by the COBE satellite in the cosmic microwave background are actually tiny temperature fluctuations, signatures of both these oscillations and the gravitational imprints at the time of recombination. Galaxies would form preferentially in the peaks, where gravity had a head start, and less so in the troughs as matter was drawn away. The frozen waves were thus written into the distribution of galaxies, a record of this early symphony preserved across billions of years. But again, it is not so simple as the ripples being a single note. Multiple waves radiated outwards, leaving a more complex pattern, like the overtones of a musical instrument. Indeed, it is these harmonics that are etched into the cosmic web, a silent score that cosmologists can read today in the clustering of galaxies. But how can astronomers find this complex pattern written into the distribution of galaxies? The answer lies in something called a correlation function. It sounds intimidating, but at its heart, it is a very simple idea. We take our catalog of galaxies and start counting. How many pairs of galaxies are there that are separated by 1 million light years? How many by 2 million? And so on, building up a picture of the typical separation between galaxies across the cosmos. If the universe were completely random, the correlation function would be smooth with no special distances standing out. But, we know the early universe was not random. The waves rippled through the primordial plasma spreading out from tiny peaks in density. When the universe became transparent, these waves froze in place, leaving a preferred scale. The distance between the splash where the pebble was thrown and the crest of the ripple when the wave expansion halted. And this means that the correlation function should reveal something remarkable. At certain separations, there should be a slight excess of galaxy pairs, a bump in the curve. There should be a preferred distance called the sound horizon at recombination, which is the maximum distance those waves could travel before the universe cooled and photons streamed free. It is the fossil of a sound wave written into the cosmic web. But, of course, if astronomers are to measure these peaks in the correlation function, they need immense catalogs of galaxies, not just a few hundred or a few thousand, but many millions spread across vast volumes of space. And as well as this, for each galaxy, we need accurate distances, not just positions on the sky. Without this, the subtle signal of the baryon acoustic oscillations will be lost in the noise. And so, in the United States, plans were laid for a survey unlike any attempted. The Sloan Digital Sky Survey, known colloquially as SDSS, would use a dedicated 2 and 1/2 meter telescope at Apache Point Observatory in New Mexico to map millions of galaxies across a quarter of the sky. It was a bold vision, for this survey would measure not just positions, but also obtain spectra, giving precise redshifts, and therefore, distances. The Sloan Digital Sky Survey would become the gold standard for mapping the cosmos, a resource that transformed cosmology. And yet, on the other side of the world, a complementary effort was also taking shape. The 2-degree Field Galaxy Redshift Survey was initiated on the Anglo-Australian Telescope, a 3.9 meter telescope perched on Siding Spring Mountain in New South Wales. A special instrument was constructed to ease the mapping, an electronic Hydra with many eyes, with a revolutionary fiber optic system able to collect the light from up to 400 individual galaxies at a time. This leap in efficiency made mapping across the southern sky possible, and so the friendly race between the north and south was on, night after night, collecting data that was distributed to the collaborative teams around the world. Positions were confirmed, velocities measured, and patch by patch by patch across the sky, the most detailed three dimensional maps ever made of the cosmos began to take shape. By the early 2000s, results began to flow in. The immense catalogs from SDSS and 2dF were steadily growing larger, and so astronomers began to measure the correlation functions of galaxies. This was the moment of truth. Would the frozen echoes of the early universe reveal themselves in the cosmic web? Teams on both sides of the globe worked feverishly, crunching through millions of galaxy positions and redshifts, and then, almost together, the announcements came. It was SDSS that reported the first detection, but 2dF was hot on its heels. The result was almost a dead heat, a triumph for both hemispheres. In the northern sky and the southern sky alike, the same feature appeared, an unambiguous peak in the correlation function. That peak was not random. It sat precisely at the scale predicted from the physics of the early universe, the distance sound waves should have been able to travel in the primordial plasma before the cosmos became transparent. They had found the fossil of a wave frozen into place for billions of years and written into the distribution of galaxies. This was a profound moment. The baryon acoustic oscillations, once a theoretical curiosity, were now an observational reality. The music of the early universe had been heard again, not in sound, but in the silent arrangement of galaxies across the sky. And 20 years later, our first individual baryon acoustic oscillation was found and named Ho'oleilana, a vast, silent witness to the music of creation. To quote discoverer Brent Tully, "We were not looking for BAO. However, visual examination of maps from the Cosmic Flows 4 compilation of galaxy distances revealed a structure that invited further inspection." The sheer size of Ho'oleilana is hard to grasp. It rests roughly 800 million light years away from the Milky Way and is so large that it contains not only Huchra and Geller's Great Wall, but the Sloan Great Wall as well, amongst various other superclusters. The Boötes void rests near its center, encased within its namesake supercluster, the Boötes supercluster. From a width of roughly half a million light years when it was frozen into the cosmic microwave background 13.8 billion years ago, Ho'oleilana now stands stretched to over a billion light years in diameter. And yet, even with all this understanding of the early structure of the universe, one mystery remained. We could describe the oscillations, the sloshing of plasma and light, but it still did not answer why the universe had the initial imperfections. The origin of these first ripples was hidden in an even earlier epoch, far beyond the reach of the cosmic microwave background, waiting for a deeper theory to explain. Science can be a strange beast. When you sit down with your paper and pen or your keyboard, or when you're adjusting the setting of your experiment, you don't know if the results you'll find will have any impact, or indeed whether anyone will really care. Black holes were first proposed by English clergyman John Michell in the 1780s, only to be forgotten about for nearly 200 years. George Gamow came up with quantum tunneling in the late 1920s, long before later work would highlight its importance in areas as wide ranging as superconductors and nuclear fusion in the hearts of stars. Even the idea of dark matter took many years to have an impact. Its first observations in 1933 going largely ignored until the thread was picked back up again in the 1970s. Indeed, this is why guessing who will get the next Nobel Prize can be a tricky prospect, because it can be years or even decades before the importance of a piece of work and its reach are clear. And this was the situation in the 1970s when two physicists, Timothy Bunch and Paul Davies, were exploring the quantum nature of fields in curved spacetime. Their work was guided by simple theoretical curiosity. They weren't thinking about galaxies or the cosmic microwave background, yet what they found would fundamentally shape our understanding of both. And so to truly understand what they discovered, we must first understand what a quantum fluctuation is. To begin with, think of an electron. Perhaps the picture that pops into your head is something like a little ball bearing, a solid particle that carries an electric charge and whizzes around the outside of the nucleus of an atom. However, they're not quite so simple. Instead, an electron is quantum, a quantum wave of probability that tells you the chance of finding the electron here or there or moving at this speed or that. But just what is this electron wave wiggling in? This is where the notion of the quantum field comes in. You can think of a quantum field like a body of water, and just like water can carry ripples and waves, so can the quantum field. But unlike a pool of water, however, the electron quantum field pervades all of space and carries the uncountable ripples that represent all of the electrons in all parts of the cosmos. And indeed, it's not just electrons, but all pieces of reality. Quarks, photons, neutrinos, all the particles are ultimately ripples in their own quantum fields. At a fundamental level, the universe is nothing but multiple seas of quantum waves, rippling with quantum particles, and interacting through quantum forces. And even that is not the end of it. Imagine a region of space where electrons are completely absent. The electron quantum field is still there, but with no electrons, surely there should be no electron ripples. And with no electron ripples, the electron quantum field should sit still like an unbroken pond. And yet, we need to remember that quantum waves are waves of probability, and it is these probabilities that introduce uncertainty into the quantum picture. This means that the quantum field can never ever be totally calm. Instead, it bubbles and boils as ripples come and go with no seeming origin. This quantum field frothing is shaped by the inherent uncertainty of quantum mechanics. And the ripples also aren't totally ephemeral. They of course represent electrons, or quarks, or whatever the nature of the field is. And so, the bubbling and boiling are particles popping in and out of existence. To physicists, this seething is known as quantum fluctuations, a fundamental and statistical property of quantum fields. And this means that there is a quantum energy that exists in what we might think of as the emptiest of spaces. Though being at the truly minuscule quantum scale means that these fluctuations are tiny, barely detectable. Unless of course that is something blows them up. And this brings us back to Timothy Bunch and Paul Davies. Their focus was on the mathematics of quantum theory in an expanding universe, and how the world of the quantum that rules the small scale universe would evolve in the dynamic spacetime background of the cosmos. And out of this work came the Bunch-Davies vacuum, a particular quantum state that behaves in a well understood way in a universe that is stretching. It was elegant and rigorous, a natural choice for describing how quantum fields evolve in a rapidly evolving universe. And yet, when they published their work, they barely caused a ripple. Cosmology at the time was dominated by classical ideas, and quantum subtleties seemed far removed from anything observable. The Bunch-Davies state was filed away as a theoretical curiosity, interesting to specialists but irrelevant to the big questions of the day. Indeed, few could imagine that this quiet piece of theory would one day underpin our entire understanding of the origin of structure in the universe. But that all changed with the arrival of the theory of inflation. By the close of the 1970s, cosmologists were wrestling with numerous deep puzzles. Why is the universe so smooth on the largest scales, yet sprinkled with just the right amount of structure? Why does space itself appear so flat when gravity should have curved it long ago? And why do we see no relics of exotic particles predicted by high energy physics? These were cracks in the standard Big Bang model, and they demanded an explanation. And so Alan Guth, a young particle physicist, was drawn to these problems. And his calculations led him to a radical idea. What if, in its earliest moments, the universe underwent a brief but dramatic burst of expansion? A period where space itself inflated exponentially, flattening it out and diluting the relics far across the cosmic horizon. And indeed, that wasn't all that inflation would do. It would also provide a mechanism for seeding structure. The epoch of inflation was terrifyingly short, lasting only 10 to the power of minus 32 seconds, a duration far, far smaller than we could ever measure in a laboratory. And yet, in this briefest of epochs, the universe expanded by almost 10 to the power of 30, smoothing out irregularities in spacetime and stretching the quantum ripples locked into the Bunch-Davies states. These microscopic quantum fields were stretched to cosmic scales. Tiny quantum fluctuations described by states like the Bunch-Davies vacuum were blown up by the dramatic expansion. And so, when inflation ended, much less than a second after the universe began, these patterns were imprinted into the matter field, frozen as real variations in density. Their uncertainty gone. And it would be these blown up fossils of the quantum world that would become the initial scaffolding for galaxies, clusters, and the entire cosmic web. Every structure we see today, from the smallest dwarf galaxy to the Milky Way itself, can trace its origin to these quantum whispers. Inflation turned the microscopic into the macroscopic, transforming quantum uncertainty into architecture. In the ripples of the cosmic microwave background, we see the fingerprints of quantum physics written across the sky. And as well as this, physicists could chart the mathematical properties of these matter fluctuations, not the precise pattern across the universe, but their statistical properties. When they compared their mathematical predictions to the patterns observed in the cosmic microwave background as observed by COBE, they found an almost exact agreement. An agreement that only improved as more sensitive telescopes were dispatched into orbit to stare at the early universe. One example of these mathematical predictions is what is called the scalar spectral index, a spectrum showing how many large, medium, and small fluctuations there are in the CMB. Using later measurements from the Planck telescope, a descendant of COBE, physicists have been able to match the CMB spectrum to the spectrum predicted by inflation to remarkable accuracy. Inflation models generally predicted a scalar index slightly less than one, and the Planck missions measured this number to be 0.96 to an accuracy of five sigma. And so, from these seeds, stars and galaxies formed. Clusters gathered along filaments, leaving great voids between them. Even our own galaxy traces its origin to one of these quantum ripples, a faint whisper from the earliest moments of time. Inflation therefore didn't just solve existing cosmological puzzles, it connected the smallest scales of physics to the largest structures in the universe. But, our story is not over. For all of these discoveries are not just intellectual curiosities, satisfying answers to ancient questions. For we can use them as tools. For these patterns in the sky allow us to chart the very fundamental properties of our expanding universe. His name is among the most famed astronomers, but before he embarked on his stellar career, Edwin Hubble had already lived a remarkable life. He'd studied law at the University of Oxford, had served in the First World War, and even dabbled in boxing. But his true passion lay in the stars. And in the 1920s, he turned to astronomy with a bold ambition. He would use the world's largest telescope, the 100 inch Hooker telescope on Mount Wilson, to answer one of the biggest questions imaginable. Just how big is the universe? At the time, many believed the Milky Way was everything. A potentially endless sea of stars that had existed forever. Faint smudges of light had been spotted in the night sky, but many argued that they were little more than clouds of gas. Some, however, began to suspect that these were island universes, vast congregations of suns, each a distant reflection of our own Milky Way, adrift in the depths of space. And so it was Hubble's observations that settled the debate, resolving the smudges and identifying them as individual galaxies in their own right. But it was his observations of a particular kind of star, the Cepheid variable, that truly shattered the notion of distances over the cosmos. This was possible because the speed at which Cepheid variables pulsed was related to their brightness, and their intrinsic brightness was always the same. And so, by measuring their timing and thus their brightness, Hubble could calculate how far away they were, thus revealing that those clouds of gas were in fact separate galaxies far beyond our own. And through the rest of the 20th century and into the 21st, astronomers have further refined their observations in an effort to refine Hubble's initial observations to great detail and to reach out more distantly into the cosmos. But to see even further, astronomers have had to use other galactic features to chart the distant cosmos, as even the brightest Cepheids are invisible at cosmic distances. Luckily, to begin with, exploding stars, supernovae, have proved to be valuable replacements. Now known as standard candles, illuminating cosmic distances over billions of light years. And yet, despite this, measuring distances in astronomy is still not easy. Every method carries uncertainties, and as careful as astronomers are, there's always worries that errors have snuck in as they piece together the true scale of our universe. Indeed, as in all good science, repetition in measurements amongst different groups and using different techniques is hugely important. And this is where baryonic acoustic oscillations have found their greatest use. In reading the frozen ripples in the cosmic microwave background, we have found a new cosmic ruler one that dwarfs the rest. To begin with, we have to remember that these waves had only 380,000 years to expand before they were frozen when the universe became transparent. And so, from knowing the physics and thus the speed of these plasma waves, cosmologists can calculate precisely their physical scale. These measurements give cosmologists a new way to chart the life and times of the universe. By comparing the physical scale of the sound horizon, the extent to which the waves in the plasma could spread, to its apparent size in the cosmic microwave background, they've been able to calculate the expansion rate of the universe with exquisite precision. And yet, despite this seemingly clear success, this is also where the story takes a twist. For the number they have found does not match the value measured from nearby galaxies and supernovae. Two methods, two answers, and a growing tension that has become one of the biggest puzzles in modern cosmology. It is called the Hubble tension, and it's a problem that has covered countless pages of academic journals. Local measurements using Cepheids and supernovae suggest the universe is expanding faster than the rate inferred from the early cosmos, as determined from the cosmic microwave background. And indeed, the discrepancy is not trivial, with the significance of the gap between the two sets of measurements only getting larger as observations have become more refined. It is now so large as to make astronomers wonder if something fundamental is missing from our picture of the universe. And so, what could explain this discrepancy? Some suggest that the tension doesn't really exist, as the local measurements are possibly biased, perhaps by unrecognized systematics in the calibration of distances. Indeed, it's important to remember measuring distances in astronomy is always hard. Others argue that the early universe calculations might be incomplete, hinting at new physics beyond the standard model of cosmology. Could dark energy be evolving? Is there an extra species of particle? Or a subtle twist in gravity itself? These possibilities are tantalizing, and they've resulted in a kind of wild west of theoretical speculation, but none has yet provided a definitive answer. For now, it seems we have to accept that the Hubble tension remains a tantalizing mystery. And that is not the end of the uses for baryon acoustic oscillations, as astronomers now use these frozen ripples in new ways to reveal the fundamental properties of the universe. The secret is to combine two sets of data, the cosmic microwave background, which reveals the scale of the physical waves at recombination, with the vast catalogs obtained from modern surveys. These galaxy surveys reveal the same imprint of waves written into the cosmic web, but at a time billions of years after recombination. So, by comparing these two epochs, astronomers can trace how the universe has expanded over deep time. This is, at heart, a very elegant method. The cosmic microwave background provides the starting point, the ruler calibrated by the physics of the early universe. And the galaxy surveys can measure how that ruler appears at different redshifts. Our standard cosmological model, with a universe containing matter and an unchanging form of dark energy, predicts a smooth expansion. And so, the apparent size of the baryon acoustic oscillations seen in galaxy surveys should follow a precise, predictable pattern. Any deviation from that pattern, possibly signaling new physics. And this brings us to DESI, the Dark Energy Spectroscopic Instrument. Conceived in the early 2010s and coming online in 2021, DESI represents the culmination of decades of effort to understand the accelerating universe, spurred on as it was by the unexpected discovery in the late '90s of the acceleration of expansion and the presence of dark energy. However, DESI is not a single device, but a sophisticated assembly of cutting edge technologies. At its heart lies a robotic fiber positioning system capable of placing 5,000 optical fibers with exquisite precision across the telescope's focal plane. Each fiber feeds light from a distant galaxy or quasar into one of 10 spectrographs, allowing DESI to capture the fingerprints of cosmic expansion on an industrial scale. This engineering marvel enables DESI to undertake the most ambitious galaxy survey ever attempted. Over 5 years, it will chart tens of millions of galaxies and quasars, constructing a three dimensional atlas not just of space, but of time as well, spanning 11 billion years of history. With this map, DESI can measure the baryon acoustic oscillation scale at multiple epochs, turning those frozen ripples in the primordial plasma into a detailed chronicle of how the universe has grown and stretched under the influence of dark energy. Multi-year programs such as DESI often release their data in chunks, data releases that build up to a final catalog of everything they observe. And so, in 2025, DESI released its second chunk with more than 14 million galaxies and more than a million and a half quasars mapped across the cosmos. In the time since its release, astronomers have dug into the data, teasing out the signal of the baryon acoustic oscillations, and the results have been surprising, to say the least. Over the multiple epochs, the signal of the oscillations rises clearly above the noise, with no doubt that this relic of the early universe is still imprinted on the cosmos today. But in reconstructing the expansion history from these signals, astronomers have found that it doesn't match the predictions of our simplest picture of dark energy. Instead, the data hints at quite strange behavior. As if dark energy were not constant, but has been changing and evolving over billions of years. Of course, it's not quite yet definitive, but as more data flows in, this signal of strange dark energy could become more significant, or like so many other interesting signals in physics, it could melt back into statistical noise. But it is already enough to make cosmologists pause. Could this be the first sign that our picture of dark energy is incomplete? Or is there something deeper, a new ingredient in the cosmic recipe? If dark energy is not constant, but instead evolves with time, the implications for cosmic history are profound. In our standard picture, a steady dark energy acts like a gentle, but relentless push, causing the universe's expansion to accelerate ever faster. But if its strength has changed over billions of years, the story becomes far more intricate. Perhaps in the distant past, dark energy was weaker, allowing gravity to dominate for longer, thus impacting the growth of structure. Or perhaps it was stronger, driving an earlier surge of expansion that left its mark on the distribution of galaxies we see today. And ultimately, looking ahead, an evolving dark energy could rewrite the ultimate fate of the entire universe. For if its influence continues to grow, the universe may race towards a big rip, where galaxies, stars, and even atoms are torn apart by runaway expansion. And if it fades, the acceleration could stall, and gravity might one day halt the expansion, leading to a slow collapse. Of course, between these extremes lies a spectrum of possibilities, each demanding new physics to explain why the vacuum of space is not the simple, immutable backdrop we once imagined. And finally, it's tempting to wonder whether these two puzzles, the hints of evolving dark energy from DESI and the stubborn Hubble tension, are not separate mysteries at all, but threads of the same tapestry. If the strength of dark energy has shifted over cosmic time, then our extrapolations from the early universe to the present day may be subtly flawed, introducing the discord we see between local and primordial measurements of expansion. A single, coherent picture could emerge, one that reshapes our understanding of cosmic acceleration and the physics driving it. As DESI's map grows and new observations sharpen the view, we may find that these anomalies are not accidents, but signposts pointing towards a deeper truth. And so, in that Warsaw conference on cosmology and black holes of 1973, Stephen Hawking could scarcely have imagined the universe we now see. Back then, cosmology was still a fledgling science, its foundations resting on a handful of observations and a great deal of speculation. Yet among those whispers of Soviet breakthroughs, Zeldovich was already sketching a vision of how structure might emerge. Fast forward half a century, and the cosmos has yielded its secrets in ways that would have astonished even Zeldovich. We now map the universe not in hundreds or thousands of galaxies, but in tens of millions, tracing its architecture across billions of years. Those delicate ripples in the early plasma, the baryonic acoustic oscillations, have become tools, rulers stretching across the cosmic web. We see hints that dark energy may not be constant. We puzzle over the Hubble tension. And in these mysteries, the spirit of Zeldovich lingers. The audacity to imagine beyond the obvious, to seek unity in complexity. From the fragile oasis of Warsaw to the robotic precision of DESI, the journey has been long, but the quest remains the same. To understand where it all came from and where it is going. You've been watching the entire history of the universe. Don't forget to like and subscribe and leave us a comment to tell us what you think. Thanks for watching and we'll see you next time. </content> </invoke>