Skip to content
Published on

The Big Bang — How the Universe Began

🇰🇷KO🇺🇸EN🇯🇵JA
Split View필사 모드
Authors

A Beginning — We Are All Made of Stardust

Have you ever looked up at the night sky? When you leave the glow of the city behind and stand somewhere truly dark, thousands of stars come pouring down over your head as if spilling from above. In front of that spectacle, almost everyone eventually arrives at the same question. Where did all of this come from? When, and how, did the universe begin?

This is one of the oldest questions humankind has ever held. Ancient peoples answered it with myth, philosophers with contemplation. Yet the moment when that answer crossed over from myth and speculation into the realm of telescopes, observation, and testable evidence was only a little more than a hundred years ago. Measured against the whole of human history, it is a remarkably recent development. We are fortunate enough to live among the very first few generations who can speak of the origin of the universe in the language of science.

Let me share one astonishing fact first. The atoms that make up your body — the calcium that hardens your bones, the iron that paints your blood red, the oxygen you draw in with every breath — were all forged long ago in the heart of some distant star. We are, quite literally, beings made of stardust. And the raw material that built those stars, the lightest elements of all, hydrogen and helium, was shaped within mere minutes after the universe was born.

In this essay I want to follow the journey of how humankind came to know the beginning of the universe. The astronomers who sat through the night before their telescopes, the two engineers who discovered the echo of the cosmos in a noise they had stumbled upon by accident, and the mysteries of a universe filled with things we cannot see. I will try to unfold 13.8 billion years of time as if it were a single story.

Let me make one promise in advance. There will be almost no difficult equations in this essay. Instead, through everyday scenes we all recognize — a loaf of raisin bread, a balloon swelling up, the siren of an ambulance fading into the distance — I will trace the vast story of the cosmos. Cosmology may be the most ambitious story humankind has ever composed, yet its central ideas are astonishingly simple and beautiful. Let us walk through it together, slowly.

The Old Belief in a Motionless Universe

For a very long time, people believed the universe never changed. The stars were always in their places, and the cosmos remained exactly as it was, without beginning or end, forever. Even the great Einstein thought so at first. When he completed his general theory of relativity in 1915, he was unsettled to find that his own equations insisted the universe must be either contracting or expanding. To preserve a static universe, he forced an extra term into his equations, a term he called the cosmological constant.

But nature does not bend to human preconceptions. In the 1920s, the insights of two men began to crack this solid belief.

One of them was Georges Lemaitre, a Belgian priest and physicist. In 1927 he solved Einstein's equations and arrived at the conclusion that the universe might be expanding. He went further still, daring to imagine more boldly. If the universe is expanding, then rewinding time should gather everything into a single point. He called this the primeval atom, or the cosmic egg. It was the seed of the idea we call the big bang today.

Around the same time, the Russian mathematician Alexander Friedmann likewise drew solutions for an expanding universe out of Einstein's equations. Sadly, he died young and never confirmed for himself that his insight had been correct, but the fundamental equations that describe the expansion of the universe today carry his name. Science is like a chorus in this way, a great picture completed as the insights of many people gather across time.

Curiously, the name big bang itself is said to have come from the astronomer Fred Hoyle, who disliked the theory and coined the phrase somewhat mockingly during a radio broadcast. A name meant as criticism became the most famous term in all of science.

Here I want to pause and say a little more about Lemaitre. A Catholic priest and at the same time a physicist of the highest caliber, he was careful all his life not to mingle faith and science. He was wary, in fact, of having his theory that the universe began at a single point too easily equated with religious accounts of creation. His conviction that science must be handled in the language of science and faith in the language of faith is an attitude well worth chewing over even today, whenever we think about the relationship between science and worldview. The simple fact that the coldest equations and the deepest reverence could coexist without conflict inside one person's mind is, in itself, a fascinating story.

At first, Einstein did not much care for Lemaitre's idea. According to one account he told Lemaitre that his calculations were correct but his physical intuition was abominable. Yet as time passed and observational evidence accumulated, Einstein is said to have regretted the cosmological constant he had forced in, calling it the greatest blunder of his life. Even the greatest of scientists had to set down his preconceptions before nature. Intriguingly, the cosmological constant would later return to the stage in the form of dark energy, a story we will meet again further on.

Hubble Sees the Galaxies Recede

The man who added decisive evidence to the theory was the American astronomer Edwin Hubble. The fact that one of the most famous telescopes in the universe today bears his name, the Hubble Space Telescope, shows just how greatly he changed humanity's picture of the cosmos.

In the 1920s, the Mount Wilson Observatory in California housed the largest telescope in the world at the time, a 100-inch instrument. On cold nights Hubble sat before that enormous lens, gathering the light of distant objects. In those days people still thought that our own Milky Way was the entire universe. Faint objects like Andromeda were taken to be nothing more than nebulae within the Milky Way.

Inside these objects Hubble found a special kind of star called a Cepheid variable. The brightness of such a star changes with a regular period, and from that period one can tell how bright the star truly is. By comparing its true brightness with its apparent brightness, one can measure its distance. It is much like judging distance from the fact that a streetlamp of standard brightness looks dimmer the farther away it stands. The result was a shock. Andromeda was not inside the Milky Way at all but another vast galaxy lying at a distance almost impossible to imagine. The universe was far, immeasurably larger than anyone had supposed.

Then, in 1929, Hubble discovered an even more astonishing pattern. When he analyzed the light of the galaxies, almost every one of them was receding from us. And the more distant a galaxy was, the faster it was moving away.

It is worth pausing to weigh the gravity of this discovery. It was not merely an observation that a few galaxies happened to be moving. It was a declaration that the entire universe was changing. The cosmos, which we had believed would remain forever the same, was in fact larger today than yesterday and would be larger still tomorrow. This was a discovery that made humankind humbly reconsider its place once again. Just as Copernicus had taught that the Earth is not the center of the universe, Hubble's observations revealed that even the cosmos is not a fixed stage but a living thing in ceaseless change.

The Doppler Effect of Light and Redshift

How can we tell that a galaxy is receding? The secret hides in the color of its light.

You have surely experienced how the siren of an ambulance rises in pitch as it approaches and falls as it recedes. This is the Doppler effect, which arises because the waves of sound are compressed or stretched. Light is also a wave, so the same thing happens. When a star or galaxy moves away from us, the wavelength of its light is stretched and shifts toward the red end of the spectrum. We call this redshift.

The relationship Hubble discovered can be tidied into a simple proportion.

recession speed = Hubble constant times distance

In symbols:  v = H0 times d

where
  v  : the speed at which a galaxy recedes
  d  : the distance to the galaxy
  H0 : the Hubble constant (currently about 70 kilometers per second per megaparsec)

This simple law — that the greater the distance, the faster the recession — meant that the universe as a whole was expanding.

The Analogy of the Raisin Bread

Here we must clear up a common misunderstanding. If all galaxies are receding from us, does that mean we are at the center of the universe? It does not.

Imagine a loaf of raisin bread rising in the oven. As the dough swells, every raisin moves away from every other raisin. From the viewpoint of any one raisin, all the other raisins appear to be retreating. And the more distant a raisin is, the faster it recedes, because more dough lies stretching between them.

The universe is the same. The galaxies are not flying through empty space; rather, the space between the galaxies is itself stretching. That is why the universe has no special center. No matter which galaxy you stand on, all the other galaxies appear to be moving away from you.

In other words, the redshift of a distant galaxy is not produced by that galaxy fleeing swiftly across space. It arises because, while the light was traveling toward us, the space in between stretched, and the wavelength of the light stretched along with it. This phenomenon, often called cosmological redshift, is direct evidence not of the motion of galaxies but of the expansion of space itself. Once you grasp this subtle yet crucial distinction, it becomes far clearer why the expansion of the universe has no special center.

A deeper truth hides within this analogy. When we picture a loaf of raisin bread, we naturally imagine its outer crust, the edge of the dough. But the universe has no such edge. Whether the universe stretches on without end, or whether it curves back upon itself like a surface, in neither case is there a boundary like the crust of a loaf that we tend to imagine. The expansion of the universe is not a spreading outward from some center; it is the stretching of space itself, in which every point recedes from every other point at the same time. It is only natural that this is hard to picture in the mind. Our intuition grew up inside some space; it has never experienced space itself stretching.

One thing to add: galaxies very near to us do not obey this rule. Our Milky Way and the Andromeda galaxy, for instance, are actually approaching each other and are expected to collide and merge into one in the distant future. At close range the gravity between the two galaxies is stronger than the expansion of space. The expansion of the universe becomes clearly visible only on a sufficiently large scale, between objects sufficiently far apart, such as clusters of galaxies.

Rewinding Time — A Hot Beginning

If the universe is expanding, then let us rewind time like a film reel running backward. The galaxies draw closer and closer, the universe grows smaller and smaller, and as it does it grows hotter and denser. Trace it back far enough and all matter and energy gather into a single state, unimaginably small and hot. That is the moment of the big bang.

Here we must underline an important point. The big bang was not an explosion that went off somewhere in empty space. There was no outer space lying around for an explosion to occur in. Space and time themselves began together in that very moment. The big bang was not an event that happened within space; it was the beginning of space itself. This point runs against intuition and is hard to grasp, but it lies at the heart of modern cosmology.

How, then, can we believe such a theory? After all, no one was there to witness 13.8 billion years ago. The reason science is so great is that even about an unseen past it can put forward testable predictions and then confirm them. The big bang theory rests on two decisive pieces of evidence.

The Drama of the First Moments

Before we turn to the evidence, let us briefly look at how breathtaking the very first moments after the big bang were. The early history of the universe is divided not into minutes or seconds but into instants far shorter still.

In the earliest moment, the universe was hot and dense beyond imagining. At this time the four fundamental forces we know today — gravity, the electromagnetic force, and the two nuclear forces — are thought to have still been fused into one, or to have been splitting off from one another in sequence. As the universe cooled, these forces revealed themselves one by one. Just as water, cooling, changes from vapor to liquid and from liquid to ice, the universe too changed its nature in stages as it cooled.

The universe then became something like a hot soup seething with the tiniest particles that make up matter. In this era, particles called quarks drifted about freely, and as the universe cooled further they bound together to form protons and neutrons, the raw material of atomic nuclei. And, as we have already seen, within a few minutes after the big bang these materials combined to forge the nuclei of the lightest elements. When we read 13.8 billion years as a single story, its most dramatic scenes are in fact compressed into the first few minutes — no, into a span no longer than the first few seconds.

Of course, about the very earliest moment, the very first instant of the big bang, our physics still cannot give a complete answer. In that extreme environment we would need at once a theory of gravity, which governs the very large, and a theory of quanta, which governs the very small, and humankind does not yet possess a finished theory to join the two seamlessly. The first moment of the universe remains one of the hardest unsolved problems modern physics must face.

The First Piece of Evidence — An Echo Filling the Universe

If the big bang truly happened, then the traces left behind by that hot early universe should still remain throughout the entire cosmos. In the late 1940s, several physicists made the following prediction. If the universe was once so hot, then the light that filled it in that era would have cooled as the universe expanded, and today it should be spread everywhere across the cosmos in the form of very cold radio waves. This is the cosmic microwave background, often shortened to the CMB.

Pigeon Droppings and a Nobel Prize

This prediction was confirmed in the most unexpected way imaginable.

In 1964, at Bell Laboratories in New Jersey, two engineers named Arno Penzias and Robert Wilson were working with an enormous horn-shaped antenna for a radio communication experiment. No matter how hard they tried, a noise refused to go away. Whichever direction of the sky they pointed the antenna, by night or by day, even as the seasons changed, the same faint hiss persisted.

The two of them did everything they could to find the cause. They even cleaned out the droppings left by pigeons that had nested inside the antenna. Still the noise remained. As it turned out, it was not a flaw in the equipment but a signal coming from the universe itself. The leftover heat of the big bang, the echo of a universe that had cooled over 13.8 billion years, was what they had caught by chance.

At nearby Princeton University, a research team had just been preparing to deliberately search for that very same signal. When the two groups met and pieced the puzzle together, humankind held in its hands the most powerful evidence for the big bang. Penzias and Wilson received the Nobel Prize in Physics in 1978 for this discovery. It would be no exaggeration to call it a secret of the universe found while cleaning.

There is a deep lesson about the nature of science in this story. Penzias and Wilson did not build their antenna in order to uncover the origin of the universe. They were simply doing the practical work of radio communication. Yet their diligence in not ignoring a persistent noise, in digging to the very bottom of its cause, led to a discovery that will live in the history of humankind. The great moments of science often arrive this way, to people who refuse to look away from a small strangeness that will not disappear. Had they dismissed the noise as a fault of their equipment and let it lie, the echo of the universe might have stayed buried in silence a while longer.

If you remember old televisions, recall the flickering black-and-white static that appeared on a channel with no broadcast. Astonishingly, a tiny fraction of that static came from this very cosmic microwave background. A part of the light left behind by the universe 13.8 billion years ago was flickering inside the television in our living room. The beginning of the universe is not only in some far-off place; it seeps quietly into the very middle of our everyday lives.

The Oldest Light, the Most Uniform Light

This cosmic microwave background is the oldest light we can see anywhere in the universe. It carries the appearance of a moment about 380,000 years after the big bang. Before that, the universe was so hot and dense that light could not travel freely and was trapped as if inside a fog. At the moment the universe had cooled enough for atoms to form stably, light was at last set free, and that light has crossed 13.8 billion years to reach us now.

Later, space telescopes such as COBE, WMAP, and Planck measured this light with great precision. They revealed that the cosmic microwave background is astonishingly uniform in every direction. In terms of temperature it is very cold light, about 2.7 degrees above absolute zero, roughly minus 270 degrees Celsius. And yet there exist tiny temperature differences at the level of one part in a hundred thousand, and these very small blemishes became the seeds from which galaxies and stars and we ourselves were later born.

This map of the cosmic microwave background is often called a baby photograph of the universe. Since it captures the universe at just 380,000 years old, it is indeed a portrait of the newborn era compared with the 13.8-billion-year-old universe of today. What is remarkable is that this single map holds, packed within it, clues to the age of the universe, the proportions of its composition, the history of its expansion, and even its future fate. By precisely analyzing the pattern of these faint blemishes, scientists drew out almost every number we have discussed in this essay. A single cold map of light turned out to be the resume of the entire universe.

The Second Piece of Evidence — The Elements the Universe Forged

The second powerful piece of evidence for the big bang theory lies in the proportions of the elements that exist in the universe.

For the first few minutes after the big bang, the universe was hot enough for nuclear fusion to occur. During this brief era the lightest elements were made. This is called primordial nucleosynthesis. What was forged in this period was mainly hydrogen and helium, along with a very small amount of lithium. The window for this fusion opened only briefly and then closed. As the universe kept cooling, the temperature dropped low enough to halt fusion within just a few minutes. The elements made before that short window closed were nearly all the matter that filled the universe for the hundreds of millions of years that followed.

The big bang theory predicts the proportions of these elements precisely. About three quarters of the ordinary matter in the universe should be hydrogen and about one quarter helium. And when we actually observe the stars and gas throughout the universe, we find exactly those proportions. The moment the numbers calculated on the chalkboard match the numbers measured by the telescope is the moment we come to trust a theory.

Where, then, did the heavier elements like carbon, oxygen, and iron come from? These were made not in the big bang but over long ages inside the stars that were born afterward. When a star reaches the end of its life and explodes as a supernova, these elements were scattered into the universe, and their remnants gathered to form the next generation of stars and planets, and in the end, us. This is precisely what we meant at the beginning when we said we are made of stardust.

To savor this fact once more is truly to feel wonder. Each oxygen atom you have just breathed in, each grain of calcium beneath your fingernails, each atom of iron in your blood was scattered into the universe by some enormous star meeting its death billions of years ago. Our bodies are walking records of the cosmos, holding within them the history of at least one generation of stars that lived and died. When the astronomer Carl Sagan said that we are made of star stuff, it was not a poetic metaphor but a literal scientific fact. Perhaps we who look up at the universe are, in truth, one of the ways the universe looks in upon itself.

Seen this way, the story of the big bang is not a tale about some distant cosmos belonging to others; it is the oldest birth record of our very selves. The first few minutes in which hydrogen and helium were forged, the cosmic dawn when the first star was lit, the supernova explosions that scattered the heavy elements — all those scenes joined together one after another and at last reached you, reading these words now. We live in the most recent line of a long story spanning 13.8 billion years.

The tale of primordial nucleosynthesis has another fascinating side. The proportions of light elements that the theory predicts depend very sensitively on exactly how much ordinary matter the universe held just after the big bang. Therefore, by tracing backward from the observed proportions of hydrogen, helium, and lithium, we can independently estimate how much ordinary matter exists in the universe. Astonishingly, the value obtained this way agrees beautifully with the value drawn from the analysis of the cosmic microwave background we saw earlier. When two entirely different lines of evidence point to the same conclusion, scientists become confident at last that they are on the right path.

Cosmic Inflation — An Explosive Growth in an Instant

The big bang theory was powerful, but it also left unanswered questions. One of the greatest puzzles was why the cosmic microwave background is so uniform in every direction. How could opposite ends of the universe, so far apart that not even light could ever have passed between them, possibly have the same temperature? It is as strange as two people who have never once met somehow having exactly the same body temperature.

Let me unfold this a little more. For two objects to reach the same temperature, they normally have to exchange heat and come into balance. Just as hot coffee turns lukewarm when you pour in cold milk. Yet the opposite ends of the universe lie so far apart that not even light could travel between them once over the age of the universe. How could they have reached the same temperature without ever once making contact? This is called the horizon problem. The big bang theory alone could not explain this uniformity well, and it was precisely to fill this gap that the idea of cosmic inflation arose.

In the 1980s, physicists put forward the bold hypothesis of cosmic inflation to solve this problem. In an unimaginably brief instant just after the big bang, the universe expanded at a colossal rate, faster even than light. So fast and so vast was it that a region smaller than an atom is said to have swelled in a flash to the size of a grapefruit.

Inflation explains why the universe is uniform. A region that, before inflation, was very small and well mixed and shared the same temperature swelled rapidly to become the enormous universe we see today. It is rather like dots drawn on a small balloon: blow the balloon up large and the dots move far apart, yet the original pattern is preserved. Inflation also held that it magnified faint quantum fluctuations from the earliest era up to cosmic scales, sowing the seeds from which galaxies would later cluster.

Inflation solves another long-standing puzzle as well. On the largest scales the universe is almost perfectly flat. For the universe to be so precisely flat is a very special condition, but if inflation occurs, then whatever shape the universe began with, its rapid swelling smooths it out so that it appears flat to our eyes. It is like the way the surface of a small balloon looks curved, yet if you blow that balloon up to the size of the Earth, the surface where you stand feels flat. When a single hypothesis explains several seemingly unrelated puzzles all at once, scientists begin to take that hypothesis seriously.

Inflationary theory fits a number of observations well, yet not all of its details are settled. It remains an area of active research in modern cosmology.

The most beautiful legacy inflation left us is the idea that the grand structure of the universe in fact arose from extremely tiny quantum fluctuations. In the quantum world, even utterly empty space trembles ceaselessly at the faintest level. Before inflation, this trembling was so small it must have seemed to mean nothing. Yet inflation stretched these tiny tremblings in a flash to astronomical scales, inscribing on the universe a pattern in which some places were ever so slightly denser and others ever so slightly sparser. As time passed, gravity drew the dense places together more densely still, and from this grew stars, galaxies, and clusters of galaxies. The root of every structure that adorns the night sky today is a fleeting quantum trembling just after the big bang. This story, that the smallest thing shaped the largest, is among the most poetic truths the universe tells us.

And this very pattern remains before our eyes as the faint temperature differences of one part in a hundred thousand in the cosmic microwave background we saw earlier. In other words, the map of blemishes in the cosmic microwave background is the fingerprint inflation left behind, and at the same time a blueprint drawn in advance of where galaxies would be born. Herein lies the reason that the exquisite temperature map produced by the Planck satellite is so precious to scientists.

The Number 13.8 Billion Years

The age of the universe is estimated at about 13.8 billion years. How did such a precise number come about?

Several independent methods point to similar answers. The method of measuring the rate at which the universe expands and calculating backward, the method of precisely analyzing the faint pattern of the cosmic microwave background, and even the method of measuring the ages of the oldest stars in our galaxy. The fact that these methods, setting out by different roads, all arrive in the neighborhood of 13.8 billion years tells us that the number is no mere guess.

It is not easy to grasp the scale of this time. There is a famous analogy called the cosmic calendar, which compresses the 13.8-billion-year history of the universe into a single year. If the big bang occurred at midnight on January 1, then the Sun and the Earth do not come into being until September. The dinosaurs appear and vanish in late December, and the entire recorded history of human civilization is compressed into the last few seconds of December 31. A single human lifetime does not even reach the blink of that final second.

Standing before this calendar stirs a strange feeling. On one hand, we are endlessly small and brief beings within the vast time of the universe. To think that all of human civilization amounts to only the last few seconds of a year cannot help but make one humble. On the other hand, the fact that we, living within those very last few seconds, have figured out the history of the entire year feels boundlessly great. That smallness and greatness coexist in the same place may, perhaps, be the true nature of a human being standing before the universe.

The Cosmic Distance Ladder

To work out an age of 13.8 billion years, we must first be able to measure the distances to galaxies accurately. Yet measuring distances in the universe is trickier than it sounds. We cannot carry a ruler out to them, after all. Astronomers use a clever method called the cosmic distance ladder. They measure nearby distances with one method, use those results to calibrate another method that reaches a little farther, and use that in turn to reach farther still, widening their range of distances rung by rung as if climbing a ladder.

On the lower rungs of the ladder stand the Cepheid variables we saw earlier. On the rungs above them lies a particular kind of supernova, whose peak brightness at the moment of explosion is nearly constant, so that it can be used like a standard candle. When such a supernova goes off in some galaxy, its apparent brightness lets us gauge the distance to that galaxy. It was thanks to these distance measurements, stacked rung upon rung, that Hubble's law, the accelerating expansion of the universe, and the age of the universe could all be measured.

Interestingly, a small puzzle has lately arisen over the rate of cosmic expansion, the Hubble constant. The value measured through the distance ladder and the value obtained by analyzing the cosmic microwave background disagree ever so slightly. This discrepancy is called the Hubble tension. It might be a simple measurement error, or it might be a clue to new physics we do not yet know. What this small disagreement means is one of the most hotly debated topics in cosmology today. In science, when two numbers that ought to match diverge just a little, the door to the next great discovery often swings open.

A Timeline of Cosmic History

just after the big bang ~ an instant less than a second
   Inflation. The universe swells explosively.

up to about 3 minutes after the big bang
   Primordial nucleosynthesis. Hydrogen and helium are made.

about 380,000 years after the big bang
   The universe cools and atoms form. Light becomes free.
   The light set free at this moment is today's cosmic microwave background.

about 100 to 200 million years after the big bang
   The first stars and galaxies are born. The cosmic dawn.

about 9.1 billion years after the big bang (about 4.6 billion years ago)
   The Sun and the Earth form.

about 13.8 billion years after the big bang (the present)
   Now, as we look up at the night sky and ask this question.

The Cosmic Dawn

Looking at the timeline, you can see that even after light became free about 380,000 years after the big bang, the universe held no shining stars for a long while. Scientists call this era the cosmic dark ages. Atoms had only just formed, but no star had yet been lit; it was a dark, quiet, fog-like time.

Then gravity slowly began its work. Following the faint pattern of density that inflation had inscribed, matter gathered and gathered again in the slightly denser places. Clouds of gas contracted under their own weight, their centers growing hotter, until at last they reached the critical point where nuclear fusion ignited. In that moment the very first star in the universe shone. This is thought to have happened roughly between 100 and 200 million years after the big bang. This first light, which ended the long, long dark ages, is called the cosmic dawn.

These first stars are thought to have been quite different from the stars of today. Since there were as yet no heavy elements, these stars, made only of hydrogen and helium, burned very large and bright, lived short lives, and exploded as supernovae. And those explosions scattered heavy elements into the universe for the first time. The raw material for the next generation of stars and planets, and one day for life itself, sprang from the deaths of these first stars. The stars of the cosmic dawn burned themselves up and left behind the seeds for everything that would come.

Between one star and the next there flows a kind of inheritance. The ashes scattered by the death of one generation of stars shape the next generation, and those stars in turn die, leaving behind richer elements. Our Sun is a relatively late-born star, formed only after this inheritance had passed down several times over. Thanks to that, the Earth beside the Sun could be supplied with enough of the elements life requires, like carbon, oxygen, and iron. That we can exist here at all is owed to the long story of countless stars that lived and died before us.

This is exactly why the latest observing instruments, such as the James Webb Space Telescope, strive so hard to peer into that distant past. To witness the cosmic dawn directly is the same as journeying back to the oldest homeland of us all.

The Unseen Part of the Universe — Dark Matter and Dark Energy

In the universe we imagine to be full of stars and galaxies, the matter we can actually see makes up only 5 percent of the whole. The remaining 95 percent is something invisible.

Dark Matter — The Unseen Hand

In the mid-twentieth century, astronomers noticed something strange. Galaxies were rotating far too fast. The gravity of the stars and gas we can see should not have been able to hold against such rapid rotation, and the galaxies ought to have flown apart. And yet the galaxies were holding their shape just fine.

It meant that some invisible matter was adding extra gravity. This something, which neither emits nor absorbs light and so cannot be seen by a telescope, yet clearly reveals its presence through gravity, is called dark matter. It is estimated to make up about 27 percent of the entire universe, but no one yet knows what it actually is. It is one of the greatest mysteries of modern physics.

Let us try a thought experiment here. On a windy day, how do we know that the invisible wind is there? The wind itself cannot be seen, but from the sway of the branches and the flutter of a flag we know the wind exists. Dark matter is the same. We cannot see it directly, but we read its presence from the way galaxies rotate rapidly without flying apart, and from the way distant light bends around enormous bodies. The universe teaches us again and again that something unseen is not the same as something that is not there.

The evidence for dark matter is not the rotation of galaxies alone. The degree to which giant clusters of galaxies bend light, the details of the pattern left in the cosmic microwave background, even the way the grand structure of the universe is woven — many entirely different observations all point to the same conclusion. That invisible matter abounds in the universe. When independent lines of evidence speak with one voice, the conclusion is not easily toppled.

Dark Energy — An Accelerating Expansion

A still more astonishing story arrived in 1998. Astronomers observed distant supernovae to trace how the expansion of the universe had changed over time. The result everyone expected was this: since gravity draws all things together, the expansion of the universe should be slowing down as time goes on.

But the observations showed exactly the opposite. Far from slowing, the expansion of the universe was growing faster and faster. Something was pushing the universe apart ever more quickly, against gravity. This unidentified force is called dark energy. It appears to make up about 68 percent of the entire universe, but its true nature is, again, a deep mystery.

One striking detail tied to this discovery is that the research teams did not at first believe their own observations. They had set out to measure how much the expansion of the universe was slowing, yet the data kept pointing to the opposite answer, that the expansion was speeding up. At first they suspected a mistake somewhere and checked again and again. But however many times they looked, the result was the same, and at last they accepted this astonishing fact. To bow honestly before the result nature shows you, even when it is not the answer you had hoped for, is the hardest and yet the most important virtue of a scientist. This discovery led to the Nobel Prize in Physics in 2011.

What is intriguing is that this dark energy is deeply connected to the very cosmological constant that Einstein, as we saw, came to regret. The term he had inserted to make a static universe and then discarded came back to life, in altered form, as a leading candidate to explain the accelerating expansion. Einstein never realized that the idea he had called the blunder of his life was in fact pointing toward one of the greatest secrets of the universe. In the history of science, it is not rare for an idea once cast aside to return in an unexpected guise.

A Comparison of the Universe's Composition

ordinary matter (stars, planets, gas, and us)    : about 5 percent
dark matter (unseen, acting through gravity)     : about 27 percent
dark energy (the force accelerating expansion)   : about 68 percent

What this breakdown tells us is a humbling truth. Everything we see and touch and know is a mere 5 percent of the universe. Of the remaining 95 percent, we know only that it exists; we do not yet know what it is. The universe remains a vast unknown to humankind.

Here I want to stress one thing. The dark in the names dark matter and dark energy does not mean anything frightening or ominous. It is simply an honest confession that we have not yet seen it and do not know its nature. Science begins with the honesty of calling what we do not know unknown. To mark our ignorance clearly rather than hide it is the way of science, the way that lets the next generation fill that place in. Perhaps someone reading these very words will one day be the one to uncover the nature of this 95 percent.

Scientists around the world are still pouring great effort into catching the identity of dark matter. Detectors waiting deep underground for the faint trace a dark matter particle might leave behind, vast particle accelerators, and giant telescopes that precisely survey the entire sky are all aimed at this mystery. No decisive answer has come yet, but humankind keeps tenaciously hurling its questions into this darkness.

The Telescope That Travels Back Through Time

There is one special gift in the work of studying the universe that no other science enjoys. It is the ability to see the past directly. A geologist digs into the earth to read the traces of the past, and a paleontologist revives vanished life from fossils. But an astronomer, simply by gazing at the sky, sees the past directly with the eyes, in the most literal sense.

However fast light may be, it is not infinitely fast. Because it takes time for light to reach us, to see a distant object is to see that object's past. To look at the Sun is to see the Sun as it was about 8 minutes ago, and to look at a galaxy millions of light-years away is to see that galaxy as it was millions of years ago. The farther we look, the deeper into the past we peer.

So astronomers have striven, through telescopes that see farther, to observe directly the appearance of the universe when it was young. The Hubble Space Telescope captured galaxies from billions of years ago, and the James Webb Space Telescope, active in recent times, looks in infrared into an even more distant past, close to the era when the very first stars and galaxies in the universe were just being born. The more distant the light, the more its wavelength is stretched by the expansion of the universe and shifted toward the infrared, and the Webb telescope was designed precisely to catch this stretched light.

Such observations are not mere collections of photographs. The big bang theory predicts what kinds of galaxies should exist, and in what forms, in the early universe, and when a telescope actually peers into that era, we can test the theory's predictions. Some observations agree well with theory, and some show the unexpected, galaxies that seem to have grown too quickly at an earlier time than predicted, setting scientists to puzzling happily. This process, in which theory and observation push and pull and refine one another, is the face of living science. Cosmology is not a finished body of knowledge stuffed and mounted in a museum; it is an ongoing story being updated before the telescope at this very moment.

Myths and Truths Surrounding the Big Bang

Let me clear up some common misunderstandings about the big bang.

First, the big bang was not an explosion that occurred at some single point. As I have said, it was an event in which space itself began to expand everywhere at the same time. There was no empty space for an explosion to spread into.

Also, the question of what existed before the big bang is hard to answer, at least within current physics. If time itself began with the big bang, then the expression before that struggles to carry any meaning. It may be a situation rather like asking what lies farther north than the North Pole. Once you stand at the North Pole, every direction is simply south; there is no such direction as farther north. The phrase before the big bang may, in a similar way, be a question that does not even hold together. Of course, this is a topic of active debate, and several hypotheses have been proposed.

That does not mean the question is meaningless, however. Some theories hold that our universe was born from the contraction of a previous universe, while others hold that time began smoothly at the big bang so that the very concept of before disappears. These are all serious scientific inquiries, but no way has yet been found to tell them apart by observation. Whether humankind will one day be able to answer this question, or whether it will remain a horizon we can never reach, no one knows. To honestly admit that we do not know the answer, and yet not to give up the question, is the attitude science takes before this enormous query.

Finally, the big bang theory is a highly successful scientific theory that explains how the universe has evolved; it does not completely explain the ultimate origin of how something arose from nothing. Science carries us as far as observable evidence can reach, and about what lies beyond it speaks honestly of not knowing. That honesty is precisely the strength of science.

When Two Theories Clashed

The big bang theory was not the accepted view from the start. In the mid-twentieth century, two rival theories stood in tense opposition: the big bang theory and the steady-state theory. The steady-state theory argued that even as the universe expands, new matter continually comes into being to fill the empty places, so that the universe always keeps a similar appearance. The picture of a universe without beginning or end seemed attractive to many. Which side was correct was settled, in the end, by observation. The key differences between the two theories can be summarized as follows.

ItemBig Bang TheorySteady-State Theory
Beginning of the universeSet out from a single hot state about 13.8 billion years agoEternal, with no beginning
Change of the universeCools and evolves over timeAlways keeps a similar appearance on large scales
Cosmic microwave backgroundMust existHard to explain naturally
Light element proportionsPredicts them preciselyDifficult to explain
Current standingEstablished as the standard theory by observationPushed aside before decisive evidence

The discovery of the cosmic microwave background and the agreement of the light element proportions clearly raised the hand of the big bang theory in this contest. Intriguingly, Fred Hoyle, the very man who coined the name big bang, was among those who supported the steady-state theory to the end. This history shows well that in science the winner is decided not by whose voice is louder but by which side fits the observations better.

Questions We Cannot Yet Answer

The big bang theory is the most successful cosmic story humankind possesses, yet around it there still hang great questions left unresolved. To lay these questions out honestly is not something to be ashamed of; rather, it shows just how honest and how alive science is.

First, what happened in the very first instant of the big bang? As I said earlier, to explain that extreme moment we need a new physics that binds gravity and quantum theory into one. Several candidates have been proposed, such as string theory and loop quantum gravity, but there is as yet no final answer confirmed by observation.

Second, what is the true nature of dark matter and dark energy? That we do not know the essence of these two presences, which make up 95 percent of the universe, means that we have read only the cover and a few pages of the book that is the cosmos, while most of its text remains unopened.

Third, is our universe everything there is? Some theories raise the possibility of a multiverse, in which inflation happens not just once but ceaselessly in many places, so that our universe may be only one among countless universes. This is a most fascinating idea, but because directly observing other universes is exceedingly difficult, it remains for now in the realm of hypothesis awaiting verification. Science welcomes such bold imagination, yet holds to the principle of drawing no conclusions without evidence.

These unsolved questions tell us that cosmology is not a finished discipline but one that has only just stepped into its most interesting chapter. The fact that there is far more we do not know than what we do know is a reason not for despair but for excitement. Where the discoveries yet to unfold will carry us, no one yet knows.

The Future of the Universe — Several Possible Endings

Having spoken of the beginning, one grows curious about the end. What will become of the universe? For now we cannot say with certainty, but scientists offer a few scenarios. The fate of the universe divides according to how dark energy behaves from here on.

When we speak of the future of the universe, we need to be honest. This is not a fact confirmed by observation, like the evidence for the big bang, but a careful prediction based on present knowledge. So long as we do not know the nature of dark energy, which makes up 95 percent of the universe, no ending can be settled upon. Please take the scenarios below not as a decided fate but as a map of the possibilities we can sketch right now.

The Big Freeze (Heat Death)

This is the scenario that best fits current observations. If dark energy keeps accelerating the expansion of the universe as it does now, the galaxies will draw farther and farther apart until at last they vanish from one another's view. Stars will burn through their fuel and go out one by one, and the material for new stars to be born will run dry. The universe of the very distant future becomes ever darker, colder, and emptier space. A state in which all energy has spread out evenly so that nothing more can happen is called heat death, or the Big Freeze. Measured in time, it is a matter of an unimaginably remote future.

There is something strangely forlorn about this scenario. If some intelligent being looks up at the universe in the distant future, it may be after the accelerating expansion has already carried the other galaxies beyond view. They might conclude that there is nothing beyond their own galaxy, that the universe has always been just this much from the very start. Neither the fact that the universe is expanding nor the cosmic microwave background, the evidence of the big bang, would reach them then. That we can read the history of the universe now may be because we live, perhaps, in a fortunate stretch of cosmic time when the evidence still lingers at our side. This fact makes the night sky we are looking at now feel like a most precious gift.

The Big Crunch

If, at some future moment, gravity were once again to prevail, the expansion of the universe might halt and begin to contract in reverse. Everything would draw close and grow hot once more, finally gathering into a single point as if the big bang were running backward. This is called the Big Crunch. That said, given the current observations of accelerating expansion, the likelihood of this scenario is regarded as rather low.

The Big Rip

Another possibility is the case in which dark energy grows ever stronger as time goes on. Should that happen, the force of expansion would grow fiercer and fiercer, tearing galaxies and stars and planets, and finally even atoms, to shreds. This extreme ending, in which every structure of the universe is ripped to pieces, is called the Big Rip. Yet this too is only one hypothesis among several.

Setting these three scenarios side by side, we can see that the fate of the universe rests, in the end, on a single unknown: the nature of dark energy. If dark energy remains unchangingly constant, we head toward the Big Freeze; if it weakens and yields its place to gravity, toward the Big Crunch; if it grows ever stronger, toward the Big Rip. Humankind, having uncovered the beginning of the universe, is now searching for the key that decides its end. That key is hidden within the vast darkness that is 95 percent of the universe.

Which scenario is the truth can only be known once we understand the nature of dark energy more deeply. To ask after the end of the universe is also, in the end, to ask after the unknown presence that makes up 95 percent of it.

None of these scenarios is something that will happen while we are alive, or even while our species exists. The end of the universe lies tens of billions of years away, or far beyond even that remote distance in time. So there is no need to fear it at all. What is fascinating, rather, is the very fact that we have come to be able to calculate and imagine so distant a future in earnest. To reckon the beginning by reaching back across 13.8 billion years, and then to sketch the ending toward a future hard even to imagine, to hold both ends of that immense span of time within one person's mind, is a remarkable power of the human intellect.

A Quiz to Think About for a Moment

To help gather together what you have read, here are a few questions. Try first to recall the answers on your own, then check the explanations below. Far more valuable than getting the answer right is the process of explaining for yourself why it is so.

Question 1. If all galaxies are receding from us, does that mean our galaxy is the center of the universe?

Question 2. What was the noise that Penzias and Wilson discovered by accident in 1964?

Question 3. About what percentage of the entire universe does the ordinary matter visible to our eyes make up?

Question 4. About how many years is the age of the universe estimated to be?

Question 5. Was the big bang an explosion that occurred somewhere in empty space?

Question 6. Where were heavy elements such as carbon, oxygen, and iron made?

Now for the explanations.

Explanation 1. No. As in the analogy of the raisin bread, because space itself stretches everywhere, from any galaxy's viewpoint the other galaxies appear to recede. The universe has no special center.

Explanation 2. It was the cosmic microwave background. It is the leftover heat of the oldest light in the universe, set free about 380,000 years after the big bang. This discovery became the decisive evidence for the big bang theory.

Explanation 3. About 5 percent. Of the rest, about 27 percent is dark matter and about 68 percent is dark energy, and the nature of neither has yet been uncovered.

Explanation 4. About 13.8 billion years. Several independent methods — the rate of expansion, analysis of the cosmic microwave background, the ages of the oldest stars, and more — all point to similar values.

Explanation 5. No. The big bang was not an explosion that occurred within some empty space but the beginning of space and time themselves. There was no outer space for an explosion to spread into.

Explanation 6. Not in the big bang, but inside the stars that were born afterward and in the process of supernova explosions. Just after the big bang, mainly hydrogen and helium were made, and the heavy elements were forged through the lives of stars and scattered into the universe. This is where the saying that our bodies are made of stardust comes from.

How did it feel to read the explanations? It is all right even if you missed a question or two. These are, after all, things that all of humankind could not answer until barely a hundred years ago. That we now know the answers is itself a gift built up by the efforts of countless scientists.

What the Story of the Big Bang Leaves Us

To tell the story of the origin of the universe is, in the end, also to tell the story of ourselves. The greatest gift the big bang theory has given humankind is not merely the number that the universe is 13.8 billion years old. It is a true, testable story of where we came from and through what journey the matter that makes us up has gathered here, now.

This story teaches us humility. We know only 5 percent of the universe, and we are small beings living an instant within a vast span of time. At the same time, this story gives us pride. For that small being read the history of the entire universe with nothing but telescopes, equations, and stubborn curiosity. To hold both humility and pride together may, perhaps, be the balance of mind that science cultivates in us.

The story of the big bang is also a fine example that shows at a glance how science works. A bold hypothesis is proposed, testable predictions follow, those predictions are confirmed by observation, and the questions that remain lead the next research. This cycle. The prediction and discovery of the cosmic microwave background, the agreement of the light element proportions, the unexpected observation of the accelerating expansion of the universe — all of this is a living example of that very cycle. To understand the big bang is not merely to know one thing about the universe but also to learn the very way in which humankind draws near to truth.

And this story is not yet finished. The identity of dark matter, the nature of dark energy, the first instant of the big bang, the ultimate fate of the universe — great questions still await us. The task of finding their answers is left to the next generation. Some child looking up at the night sky tonight may one day be the one who fills in these blanks.

In Closing — The Mind That Seeks to Understand the Universe

Let us return to the night sky of the beginning. We now know that that starlight is not a simple point but a story that has crossed 13.8 billion years of time. Some starlight reaches our eyes across the universe even after the star itself has already vanished. For us to look at the night sky is, in fact, to look at the past of the universe.

In following this essay we have walked quite a long road. The universe we believed was motionless was in fact expanding, and tracing time back revealed a hot beginning, which left its traces in a cold echo filling the universe and in the proportions of the light elements. The universe is 13.8 billion years old, what we see and touch is a mere 5 percent of it, and the rest remains in darkness. And the end of the universe lies open along several branching paths. All of this humankind has figured out in barely more than a hundred years.

What is more astonishing still is that we, beings born in one corner of the universe, have come to be able to reckon the history of the entire cosmos. That within the 13.8 billion years of cosmic time a mind arose seeking to understand the universe itself is, in itself, a thing of wonder. For we who are made of stardust are asking back about the very stars and universe that made us.

When you think about it, this whole story began with one person who sat all night before a telescope, two engineers who refused in the end to look away from a noise that would not disappear, and physicists who accepted the uncomfortable truth their equations spoke. The grand story of the origin of the universe was written, in the end, by the perseverance of ordinary people who followed a small curiosity all the way through. They had one thing in common. They admitted what they did not know, and even so they never stopped asking.

The big bang theory is not a finished, final answer but the most honest and powerful story humankind has reached so far. We know only 5 percent of the universe, and at its beginning and its end great question marks still hang. But to know that we do not know, and to keep lifting our telescopes toward that unknown without ceasing to question, is the road humankind has walked toward the stars.

Perhaps the greatest mystery of the universe is not the universe itself but the fact that a being capable of understanding that universe was born within it. That stardust gathered to become a thinking being, and that being in turn asks after the origin of the stars and the cosmos — this cycle is a true story more wondrous than any myth. So long as we ask after the beginning of the universe, within that very question the universe is coming to know itself.

Something to Ponder

I encourage you to look up at the sky tonight. And let these questions come to mind. Some of the starlight we see may be the last farewell of a star that has already vanished. How does that fact change our sense of time and distance? What attitude toward human knowledge does the fact that 95 percent of the universe remains unknown lead us to take? And if the universe has an end, what meaning does knowing that end give to the way we live today?

To stand before questions that have no fixed answer may, perhaps, be the greatest gift the universe gives us.

Finally, I would like to recommend one more thing. The next time you have a chance to look at the night sky with someone, share one of the stories you read today. That the starlight has crossed 13.8 billion years, that our bodies are made of stardust, and that 95 percent of the universe remains unknown. The greatest joy of science lies not in knowing alone but in sharing that wonder. The story of the universe shines brightest when we look up at it together.

References

Discuss on Twitter • View on GitHub