 Reality in the Shadows, or what the heck's the Higgs? Chapter 7 A Sky of Shadows Do you live in the country? Do you live in the city? Either way, if it's a clear night, step outside and look up. Even with a great deal of distracting light from a busy city, you should still be treated to a few gems in the night sky. If you're in the Northern Hemisphere, you may see the bright objects that make up the constellation of Orion. If you're in the Southern Hemisphere, you may see the bright objects that make up the constellation called the Southern Cross. So what are those objects and the universe made from? You might be tempted to think those bright points, adrift in a sea of darkness, are all that there is to the cosmos. Our sun is a star. Most of those points you see in the night sky are also stars, like our sun. Some are bigger than our sun, some are smaller, but most of them are bright spheres, made mostly of hydrogen, ignited in nuclear fire under the pressure of their own gravity. Perhaps the universe is only made from the stuff of stars. Astronomers, astrophysicists and physicists thought this for a very long time. They were wrong. Let's look more closely at the night sky. In doing so, in trying to shine more light onto the shadows of reality, we will find that there are more shadows lurking in the sky than we could have dreamed. Earlier, we had a peek at the structure of the cosmos. We learned that Galileo Galilei and those scientists inspired by his approach used the telescope to collect visible light from the cosmos, allowing them to see finer detail of things that are very far from the Earth. In doing so, they learned that the Earth is only one of many planets and other astronomical bodies that orbit the sun. Later generations of astronomers learned that the sun is but one of many stars. Still later, they learned that the stars are gravitationally collected together into whirling galaxies. Those galaxies bound together into clusters, superclusters, and so on. This is the visible universe. Well, it's the universe that's visible to us, and those few wavelengths of light to which our eyes are sensitive. It's not our fault that we're so limited in our ability to see. The most prominent wavelengths of light to penetrate the Earth's atmosphere, originating from the sun, are those we call visible light. If a world is bathed in such wavelengths, then it is natural that being able to see these wavelengths would confer a strong survival advantage to any species able to collect and interpret them. The hand of natural selection is strong here, and it's no accident that organisms with an enhanced ability to interpret those wavelengths would survive to pass along their genes to following generations. We are the product of our environment. Our eyes are one end of a long term of natural selection. Not perfect, but pretty good at what they do. We humans are clever. We are not bound only by the gifts nature has given us. We possess creativity. We have invention. And over the centuries during which we have embraced more and more technology as a means to enhance our lives, we have learned to see what our eyes cannot. There are many more wavelengths out there than our eyes can see. For instance, if we were to consider electromagnetic waves with wavelengths shorter than what our eyes can see, we would be in the realm of ultraviolet light, discovered in 1801 by physicist Johann Ritter. And then, going further, we would be in the realm of X-rays, which were systematically studied and understood by physicist Wilhelm Röntgen in 1895. And going even shorter, finally, at the very shortest wavelengths, we would be down to gamma rays, discovered by chemist and physicist Paul Villard in 1900. Short wavelengths do not easily penetrate the Earth's atmosphere. To pursue astronomy with them, one must put telescopes or other such instruments high in the atmosphere, or even above the atmosphere in an orbit around the Earth. If you could see these light waves with your eyes, you would see a sky wherein stars glow not only invisible light, but in the ultraviolet light that is caused by their immense temperature. You would see the afterglow of dead stars and the radiation of their death explosion. If you were lucky, you will see the bright x-ray glow emitted by gas that is whipped into a frenzy by the forces of neutron stars or black holes. All of these things are invisible to the naked eye, but properly equipped, you would see the most cataclysmic explosions known in the universe, gamma ray bursts, whose origins are only now coming into our knowledge. They flare and fade, visible across the inconceivable distances between our galaxy and the other galaxies that litter the night sky. Now let us consider wavelengths longer than what our eyes can see. We enter first the realm of the infrared wave, discovered by astronomer William Herschel in 1800, and then we find microwaves. Although micro, they are still longer in wavelength than the shorter wavelengths we just described. And then we enter the realm of the radio wave, longer still, predicted by James Clerk Maxwell in 1867 when he unified the theories of electricity and magnetism and later actually found by Heinrich Hertz in 1887. If your eyes could be retuned to see these long infrared wavelengths, you would see a sky in which the coolest stars, so cool that their red light is very faint, glow bright in infrared. You would see the quick pulsing of spinning neutron stars as they chirp out regular signals in the radio range of wavelengths. You would see the glow left over from the very beginnings of the universe itself, the heat of the Big Bang coming at you from all directions as a nearly featureless omnipresent bath of microwaves. Humans have developed a wide variety of technologies to capture these unsealable longer and shorter wavelengths and transform them into electrical signals. These signals can be fed into electronics and computers as software to be reconfigured into numbers, graphs and images that our eyes can see. We are very good at revealing the unseen and converting it into forms we can see so that we can understand that which otherwise would elude our senses. This is technological evolution, extending the five familiar senses of the human body into new realms where the paths of natural selection themselves did not take us. As an example, there is the Fermi Gamma Ray Space Telescope, launched on June 11th, 2008. Fermi has been operating since then in orbit around the Earth. It possesses many instruments, one of which is a sandwich of detector technologies that might feel right at home in a particle collider detector like ATLAS or CMS at the LHC. This sandwich is designed to capture gamma rays as they pass through the telescope, allowing the energy in the gamma rays to convert to matter as a pair of particles, an electron and its anti-matter counterpart, the positron. By precisely measuring the energy deposits left by the electrically charged pair, the instrument can determine the original trajectory of the parent gamma ray. This allows astronomers and astrophysicists who use the Fermi data to point very precisely back in the sky to where the gamma ray originated. Fermi, like all other instruments designed to transform the invisible into the visible, is an eye that sees what our eyes cannot. Fermi has captured a huge number of gamma rays this way, and has given us a picture of the Milky Way and beyond in color we were not aware of even a few years ago. It is very tempting to assume that all we can see is all that there is. Sometimes though, when we apply that assumption to the universe, we earn a surprise. The universe answers with something unexpected. Such are the beginnings of discovery. Let us look at a particular discovery. This is the story of gravity and light, and all the shadows these twin messengers have revealed. The story begins with the assumption that atoms, the atoms of everyday life, whose fingerprints are seen in the light emitted by all catalogued bodies in the visible universe, make up everything. The story ends with the revelation that for every one gram of atoms that can be found by using light, there appears to be five other grams of matter that neither emits nor absorbs any light. This story begins with the astronomer Fritz Zwicky. Born in Bulgaria in 1898, Zwicky was sent to live in Switzerland at the age of six for the purpose of having him study commerce. Zwicky was to be made into a businessman, but found himself more interested in mathematics and physics. At the prestigious Swiss institution known today as ETH Zurich, roughly translated as the Federal Institute of Technology in Zurich, he earned a degree in experimental physics and emigrated to the United States in 1925 to work with the eminent American physicist Robert Millican, famous for performing a painstaking experiment with small oil droplets and electric fields in order to determine the smallest unit of electric charge, a fascinating story in and of itself. This put Zwicky at the California Institute of Technology, Caltech, surrounded by some of the most famous names in science. He became known for the discovery of neutron stars, and his work earned him full professorship at Caltech in 1942. It is for another piece of work, however, that Zwicky matters to this part of the story. He wanted to determine the mass present in galaxy clusters. Galaxy clusters are vast collections of galaxies, often hundreds or thousands of such bodies collectively interacting via their mutual gravitational attractions. We cannot travel to each galaxy in a cluster. That is not possible. Instead, we must use the known laws of physics and observational methods to infer the quantities we wish to know. Zwicky wanted the mass. Being clever, he used two independent approaches that should have confirmed each other to make the measurement. Knowing that atoms respond to and emit light, Zwicky measured the amount of light emitted by galaxies in a cluster, converting that number into a number to represent the mass of the cluster. All the stars and hot gas that make up galaxies can be measured using the light emitted by them. The relationship between light and mass is then used to convert one number into the other. The second approach used motion. As the galaxies in galaxy clusters whirl about each other in a collective gravitational dance, one can measure the motion of the galaxies in the cluster and, using Newton's law of gravity, infer the gravitational mass that is present. Two independent techniques in search of one number, an experimentalist's dream. But Zwicky hit a wall. He applied his ideas to the coma cluster of galaxies. When he made the two measurements and transformed them into masses, he found that the gravitational mass represented by the motion of the coma cluster was about 400 times larger than the luminous mass represented by the atoms emitting light. Since then, other astronomers have repeated these measurements using increasingly more accurate techniques. They do not find a number quite as dramatic as 400 times, but with our present greater understanding of galaxies and clusters, the mismatch between the gravitational and the luminous mass is still not explained by experimental error or by any other error that we can determine that can be described by our present knowledge of light, atoms, and gravity. Zwicky concluded that the coma cluster must be home to an unseen form of matter, one that does not emit nor absorb light readily. He merely called it Dunkle Mattery, dark matter, a literal description in German of an unknown class or classes of matter that are invisible to light. The invisibility of this dark matter is not limited to the visible wavelengths of light that were more commonly used by astronomers in the 1930s when Zwicky did his work on the coma cluster. Indeed, peering at galaxies using gamma rays or ultraviolet or infrared or radio, these haven't filled in the gap between gravitational and luminous mass. From the perspective of galaxy clusters, we are left with a mystery. What is this dark matter? The problem isn't just with galaxy clusters. No problem applies also to individual galaxies, which are much smaller and less massive than a cluster of hundreds or thousands of galaxies. Nonetheless, in the behavior of galaxies there is too an unseen hand that plays a role in their structure and behavior. To understand this, we need to meet the next important scientist in our story. Vera Rubin was born in 1928 as Vera Cooper. She was born in Philadelphia and later moved to Washington, D.C. There she developed an interest in astronomy, pursuing it at Vassar College, earning her Bachelor of Arts in Astronomy. Intending to continue her work in graduate school, she applied to places like Princeton University. However, at the time she applied, Princeton did not admit women to their graduate astronomy program. They would not do so until 1975. She enrolled for a master's at Cornell University, which she earned in 1951. She went on to earn her Ph.D. at Georgetown University, working with George Gamow, famous for his work in understanding radioactive decay and for advancing the Big Bang hypothesis at a time when it was not considered a serious theory of the formation of the cosmos. She earned her Ph.D. in 1954 and continued to work at Georgetown for 11 years until she joined the faculty at the Department of Terrestrial Magnetism at the Carnegie Institution for Science in Washington. There she met and forged a research partnership with Kent Ford, which eventually led to the study of the rotation of galaxies. Ford developed an advanced spectrometer that, married with a telescope, allowed for very precise measurements of the atomic spectra of stars at different points in nearby galaxies. Galaxies are huge. The Andromeda galaxy, one of the nearest to our own Milky Way, contains about 1.2 trillion stars and takes up an apparent visual area in the sky, similar to that taken up by the full moon. Why don't we see this as easily as we see the moon in the night sky? It's simple. Although Andromeda contains about 1.2 trillion stars, it's so far away from the Milky Way that the light from those stars is so very, very, very faint compared to the light from the stars in our own interstellar neighborhood or those in our own galaxy. If you were to aim 100 super bright flashlights at a friend 10 feet away, that friend would be blinded. Walk those flashlights 60 miles away and it would be incredibly difficult to see them compared to the bright street lights, window lights, or car headlights that would now dominate the local landscape. Ruben and Ford used their partnership, she with expertise in the astronomy of galaxies and he with expertise in instrument design to study the motion of stars in the Andromeda galaxy. With the right telescope hooked up to the right spectrometer, Ruben could easily see the starlight from Andromeda and fingerprint its atoms using the advanced spectrometer. But it gets better than that. Using the same principle that causes an ambulance siren rushing toward you to be higher in pitch but moving away from you to be lower in pitch, the so-called Doppler shift, Ruben noted shifts in pitch or frequency of the light in the spectra of stars at different distances from the center of Andromeda. This allowed her to create a rotation curve, a chart of the speed of rotation as a function of distance from the center of rotation. Since gravity is what binds stars together in a galaxy, those stars and their motion must obey the law of gravity and the laws of conservation of energy and momentum. Together those laws command that thou shalt not orbit too fast as you move farther and farther from the center of rotation. Consider our own solar system as a model. Mercury, the planet closest to the sun, makes one orbit in about 90 days at a speed of 107,000 miles per hour. Venus does the same thing in about 225 days at a speed of 78,000 miles per hour. Earth makes one orbit every 365 days, what we call one year, at a speed of 66,000 miles per hour. Mars does it in about 690 days at a speed of 53,600 miles per hour. Jupiter takes about 4,333 days at a speed of 29,200 miles per hour. See a pattern? The farther out from the sun, the slower a planet orbits the sun. Johann Kepler codified this in his laws of planetary motion, which are really a re-expression of Newton's laws of motion that we discussed earlier. Kepler's work preceded Newton's, so we learned only later the exact why of planetary motion. It's gravity. And so it should be with galaxies. Stars that are closer to the galactic center should generally orbit more quickly than those farther away. This is the commandment of gravity and energy and momentum. So what did Rubin observe? She observed that stars at the outskirts of galaxies like Andromeda are moving nearly as fast as stars that are closer to the center of the galaxy. This made no sense. According to an accounting of the mass of a galaxy such as Andromeda, using the light emitted by that galaxy, there is too little mass within the orbit of such far-flung stars to permit them to orbit so quickly. They should fly off into intergalactic space. How can it be then that galaxies can become so large, yet remain stable if their outermost stars are moving so fast? One explanation is what Zwicky found in his earlier studies of the coma cluster of galaxies. There could be an unseen form of matter whose influence is felt through gravity, but goes unseen using light. Non-luminous matter would solve this problem of galactic rotation. Dark matter could be a real thing, and play a more important role even on the scale of galaxies than anyone would have guessed. Other explanations have been offered, of course, for the apparent deviation from what is expected of the gravitational influence of luminous matter. For instance, what if the laws of gravity, general relativity, are not correct when applied to the largest scales, the size of things like galaxies or galaxy clusters? What if gravity needs to be modified in some way to explain what Zwicky, Ruben, and others have observed? The idea of modified gravity survived side by side with the idea of dark matter for a while, until observations made in the early 2000s. To close out this discussion of dark matter, let's look at two final players in this story, the WMAP collaboration and the bullet cluster. We begin by considering the early universe. If dark matter exists, influencing galaxies and clusters of galaxies, shouldn't it also have existed in the early universe? This is an excellent question. How might we answer it? Perhaps, if we look at a relic from the very early universe, we might detect the fingerprints of dark matter there. If we do, it will be strong evidence that dark matter is a valid explanation for filling the gap we observe between the motion of stars and galaxies and the motion expected from the gravity of luminous matter. The cosmic microwave background, or CMB, the light left over from the Big Bang, is precisely such a relic. In the 1990s, it was established that this light matched exactly a Big Bang prediction that such light, emitted after the universe cooled enough for hydrogen to form, has been streaming through the universe, unaffected except by the expansion of space-time for billions and billions of years. But what can we learn by studying this light? Let's consider some of its history. It was first detected by Arno Penzias and Robert Wilson. They are both American radio astronomers, using radio telescopes to understand the universe. Penzias and Wilson, working at Bell Laboratories in 1965, were using a very sensitive, six-meter radio antenna with the goal of detecting weak signals that had been bounced off of metal balloon satellites. As they pursued their goal, they continually encountered a curious problem. There was a constant noise in the signal, one that could not be removed by any means they attempted, including, famously, scraping the pigeon droppings off their antenna. At first, Penzias and Wilson were unable to interpret their observation. Fortunately, the physicist Robert Dickey was nearby at Princeton. Dickey, working with the equations of general relativity, had already rediscovered an earlier prediction that if the Big Bang was the correct description of the birth of the universe, there should be light left over from it that would, today, have been shifted in frequency by the expansion of space-time into the radio band. As a result of this work, he was one of only a handful of people who could have solved the puzzle and be able to supply the theoretical interpretation that could tie Penzias and Wilson's observation back to the Big Bang. The Big Bang theory made very specific predictions about the way in which energy is apportioned to each range of frequencies present in the light left over from the beginning of time. This is a famous shape, known as the Black Body Spectrum, the kind of radiation that is emitted by a body that absorbs all frequencies of light impacting it, re-radiating that energy. Is the noise that Penzias and Wilson had detected coming from all directions in the sky truly relic radiation from the Big Bang? To be more confident, one must always test a claim further and further. To do this, many experiments were devised. One of the most famous and successful was Kobe, the cosmic background explorer, a satellite host to a suite of instruments that would reveal the character of the radio waves Penzias and Wilson had detected. Launched in 1989, the NASA-Kobe mission published its first most definitive results on this radiation in 1992, revealing that the spectrum of this radiation was exactly that of a Black Body Spectrum, as predicted by the Big Bang theory. Not only did this help to cement the Big Bang theory as a correct theory of the early universe, it initiated a new era of cosmic microwave background science that continues to yield discoveries to this day. Is the spectrum of this radiation a perfect, smooth curve? That is, is it featureless, continuous, unbroken, or is there some fine detail lurking in the light from the Big Bang? If there is structure, what caused it? What could have perturbed the light from the Big Bang before it streamed freely through the cosmos, reaching us here on Earth billions of years later? Light and matter interact. Nowhere is this more beautifully summarized than in the standard model, a theory of the interaction of matter and forces. When matter interacts with light, it leaves an imprint on the light. Maybe some frequencies of light go missing as a result. Maybe the intensity of the light is affected. When light encounters a region of clumped matter, it may miss the clump, but is bent by passing through its gravitational field. This too leaves lasting effects on the light. In fact, if anything warps space-time and light passes through that warping, this will have lasting effects on the light that can be detected later. Can we see such effects in the light left over from the Big Bang? To answer this question, the astrophysics and cosmology communities pushed hard and constructed more precise and ambitious instruments. One of the most famous of those is the WMAP experiment. WMAP stands for the Wilkinson Microwave and Isotropy Probe, a collaboration of scientists that operated and analyzed data from the WMAP instrument observed a fine pattern of temperature distortions in the CMB. These distortions are the very fingerprints we sought by studying the pattern of these distortions and comparing them to the behavior of space-time, matter, and energy described in the general theory of relativity. One can build a profile of the universe when it was very young. It was just 380,000 years after its birth, when that cosmic microwave background light was freed to stream across the universe. What we learned from the WMAP experiment and its successor experiment, the Planck satellite, still ripples through the physics community. Normal matter, the atoms that are so well described by the standard model, count for just 4.86% plus or minus 0.10% of the energy density of the universe. From all we know, from all that we have studied for thousands of years, all the stars we see, all the galaxies that contain them, all the hydrogen and helium and all the other elements and stuff we know about in the universe, these total to just about 5% of the energy density of the cosmos. So what is the rest? Well, the good news for the discoveries of scientists like Swicky and Rubin is that a great deal of it is in a form of matter unlike that which is described by the standard model. It is slow moving. It is cold. There's a lot of it, a lot more of it than there is of normal matter. It matches well with the observation of a non-luminous matter component of galaxies and galaxy clusters. It's dark matter. How much of it is there? The data from WMAP and Planck indicates that dark matter presently makes up 25.89% plus or minus 0.57% of the energy density of the universe. That means that for every 1 gram of normal matter, there are 5 other grams in the universe of a form of matter whose properties lie beyond the standard model. Fully 85% of the matter in our universe is a shadow whose immense contribution to the universe through its gravitational effects have shaped the very cosmos in which we live daily. But is it really matter? The evidence from the cosmic microwave background is one piece of a much larger pattern of evidence that says it is. Let's look at one last spectacular piece of evidence. This piece combines all the greatest hits of the universe that we've learned about so far, light, atoms, gravity, and galaxies. This evidence comes from one of a now large group of collisions called the bullet cluster. The bullet cluster is an astronomical object that is made from two large clusters of galaxies that long ago collided with each other and have since continued on their way. The collision concluded about 150 million years ago when a smaller cluster of galaxies, the actual bullet cluster, passed through a much larger one. While the galaxies in the cluster missed each other, the hot intergalactic gas that follows along with the galaxies collided. This gas also makes up most of the luminous matter present in the cluster. X-ray imaging reveals that the hot colliding gas lags the actual clusters which have moved on beyond the gas by a huge distance. Light reveals where the galaxies presently are, and light reveals where the hot gas is presently located. So we can now ask, where is the bulk of the mass from this pair of once colliding clusters? Is it following the galaxies or following the gas? To answer this, we turn to gravity. Regions of clusters and gas that contain a lot of mass will also warp space-time commensurate with its mass. Light passing through such regions will be bent as if by a glass lens. By looking at the lensing of light that came from behind clusters and has since passed through those clusters and gas, we can figure out where most of the mass is actually concentrated. This has been done, and it reveals that while most of the luminous matter trapped in the gas is stuck between the clusters, the most dense concentrations of mass are focused around the galaxies and not the gas. What does this mean? If a modified gravity theory were the correct explanation, the lensing would mostly be expected to follow the gas because, if there's only normal matter, that's where all the mass and thus all of the gravitational effects should be found. But that is not what is observed. Instead, the bulk of the gravitational effects follow along with the galaxies, even though the galaxies account for less of the total cluster mass than does the gas. Curious! This implies that, as with galaxy rotation curves, the motion of galaxies and galaxy clusters and the fingerprints in the cosmic microwave background, an unseen and new form of matter is present in galaxies. It interacts primarily via gravity, whereas normal matter experiences all of the forces of the standard model more strongly than it does gravity. Because gravity is the force that seems to be most expressed by this non-luminous matter, it, like the galaxies, just doesn't suffer meaningful collisions as the clusters pass through each other. The bullet cluster is one of at least 70 such systems that have been catalogued. A broad analysis of these colliding galaxy clusters has revealed that the gravitational lensing and thus the bulk of the mass follows along with the galaxies in the collision, even when the intergalactic and interstellar gas heated during the collisions lags behind the galaxies after the collision. The evidence for a new, non-luminous form of matter, dark matter, has grown stronger as more evidence has been accumulated. The composition of dark matter is an active area of investigation. To date, there is no definitive direct evidence of having detected the constituents of dark matter by any means other than their bulk gravitational behavior. The astute reader will have noted that if one adds together the observed normal and dark matter energy densities determined from the cosmic microwave background, one does not arrive at the total energy density of the universe. 25.89% plus 4.86% is not 100%. It's just 30.75%. So what is the rest? If all the matter in the universe accounts for just under a third of the energy density of the entire cosmos, what is missing? What makes up most of the universe? To begin to understand this question, and physicists and astronomers are only just beginning to understand this, we have to consider the deaths of stars and the vast distances between things in the universe. How did we know how far away the moon was before humans began visiting it in the 1960s? The answer is a wonderful trick of light and geometry called parallax. Consider that you are sitting in the passenger seat of a car. Look out the window. Have you ever noticed how objects close to you, like a mailbox on the side of the road, appear to whip by you much faster and rapidly cover a much more dramatic distance than do objects that are distant from you, such as buildings far off that are close to the horizon or the moon? They appear to stand still. The large motion of close things compared to the much slower motion of distant things when you change your position is called parallax. It has an exact mathematical description. Using parallax, one can measure the distance to things that are too far away for a ruler or tape measure to be used. Parallax is how astronomers measure the distance to nearby things. By nearby, we mean things like the moon, the planets, and even neighboring stars. But when you want to measure things that are millions or hundreds of millions or billions of years away at the speed of light, you can no longer use parallax. The motion of the thing whose distance you're trying to measure is just not appreciable against the background of even more distant objects. Astronomers have developed a toolkit for making such difficult measurements called the cosmic distance ladder that is applied in various ways as the distances we want to measure become larger and larger and larger still. For very distant objects like galaxies that are hundreds of millions or billions of light years away, almost nothing works as well as a dying star. Well, a certain type of dying star. Stars die when they exhaust the hydrogen nuclear fuel in their core and under the collapse caused by gravity as their outward radiation pressure lessens begin to burn the heavier elements that are the byproducts of their long hydrogen burning phase. At first, the star burns helium, flaring in brightness and size to become much larger than its original size as it feeds on this heavier nuclear fuel. Once the helium is exhausted, it collapses again and ignites a new phase of burning of even heavier elements going up the chain of nuclei from helium to carbon to oxygen and beyond. A star like our sun will reach the phase of burning through its helium producing lots of carbon and oxygen, but will stop there. Why would our sun stop there? Our sun is a middling main sequence star having a mass that puts it in the middle of the typical stars you find in the universe. At that mass, a star can burn helium, but when it reaches carbon or oxygen, its collapse under its own mass is not sufficient to ignite the next phase of nuclear burning. A star in this stage gently blows off its atmosphere forming around it a structure called a planetary nebula and leaves behind a beautiful white-colored hot core of carbon and sometimes carbon and oxygen. This beautiful stellar remnant is a white dwarf. In fact, this is how our sun is expected to die. It will bloat when it burns helium, then collapse again and then hit a wall as to its ability to continue burning nuclear fuel. Its atmosphere will blow off from the heat of its core and the exposed core will burn bright for trillions of years slowly cooling from white dwarf to black dwarf over the remaining history of the universe. It's a beautiful picture. Our sun will end its days like a great nuclear diamond shining in the night surrounded by those planets that are left in the solar system after it bloats to become a red giant as it begins to die. This is a quiet retirement for a mid-sized star like ours. Not all stars are this lucky. Not all stars have a quiet retirement. And as can be said of some human lives, the trouble begins with an unreliable companion. It is estimated that about one-third of the stars in the Milky Way have at least one companion star, another star that dances around the first under the influence of gravity. There's no reason to believe that the proportion of such companion stars is any different in other galaxies. Sometimes one star will die gently and become a white dwarf. If the companion star is a red giant, either a red giant star from birth or a red super giant resulting from the start of the death of the companion star, retirement gets interesting for the white dwarf. Red giants are so large that their outer atmospheres are far from the core and are more loosely bound by gravity. The neighboring white dwarf, exerting its own gravitational attraction, may begin to slurp the atmosphere off the red giant companion. Over time, the slow march of atmospheric hydrogen from the companion onto the white dwarf causes not only a disk of gas to form around the white dwarf, but for that stolen gas to infall down onto the core, accumulating new mass onto the white dwarf. There is a limit to how long this can continue. If the mass of the white dwarf reaches a total mass that is 1.3 times the mass of our own sun, it comes out of retirement in a spectacular way. The slow burning of the white dwarf can no longer resist the growing gravitational inward pull of its accumulated mass and the star begins to collapse in a runaway and catastrophic process. This causes a huge explosion, blowing the white dwarf to pieces in the process and likely destroying the poor companion star as well. This is known as a type 1A supernova. The light from such an event can be seen with a telescope across the universe. The light that results from this process has a behavior in time and an intensity that is predictable because the process happens the same way every time. A white dwarf forms with a mass less than 1.3 times that of our sun. It feeds on the atmosphere of a companion star until it reaches the threshold mass. A runaway collapse begins that in just seconds destroys the star and creates the light we see from our vantage point in another galaxy. Because this process is so regular, you can look at it at any distance and knowing the measured faintness of the light correct that faintness back to the original brightness. In doing so, you take into account the vast distance that would make the light so faint and bingo, you have the distance from us to the type 1A supernova. Suddenly, the end of a peaceful retirement of a star becomes a meter stick for determining the distances to things in the cosmos. This is an incredibly useful tool. In 1998, two teams of astronomers used type 1A supernova surveys to map distances in the universe as a function of time. They wanted to know how the expansion of the universe which began during the Big Bang has been changing over the course of its history. By looking at type 1A supernovas closer to Earth and those very far away from Earth, it is possible to see how distance scales have changed in the universe over time. The farther away a supernova, the older it is and the earlier in the universe it detonated. It took all this time for its light to reach us on Earth. But here again, general relativity helps us to predict what we should see. General relativity takes into account the geometry of space time and the matter and energy content of the universe, making definitive predictions about how bright a given supernova should be if its light has traveled over a certain distance in a certain amount of time. If the universe is dominated by matter, for instance, then distance supernovas would be just a little bit brighter to us here on Earth, then if the universe were made of both matter and, for instance, a cosmological constant, a universal energy density of empty space that can act to push space time apart faster and faster. So what did these two teams see? The teams, one led by astrophysicist Sol Perlmutter called the Supernova Cosmology Project, or SCP, and the other, co-led by Adam Rice and Brian Schmidt and called the High-Z Supernova Search Team, both catalogued dozens of Type 1A supernovas, some going back in time as far as about 8 billion years ago. What they found was that the most distant of these supernova were fainter than would have been expected if the universe was dominated by matter, as was the common belief in the 1990s. Instead, a better explanation for what they saw was that the universe was not only expanding, but more recently, that expansion had accelerated, making objects that are far away appear fainter than they should be at this particular time in the life of the universe. This was a stunning revelation. Yes, the universe was expanding, but the expansion was not slowing, or even remaining constant. Rather, the expansion was speeding up. A few years later, when the results from the WMAP analysis of the cosmic microwave background was completed, and later still, when the Planck analysis of the same light was completed with more precision, the picture became a lot clearer. There was another player in the life of the cosmos during the beginning of time, and more prominent now. That player behaved not like matter, but like some kind of energy density that is present in space itself. Einstein would have called this a cosmological constant, although his purpose in inventing it was to try to hold the universe constant. What he described as his, quote, greatest blunder, unquote, now finds a new home in our understanding of the cosmos, not for the purpose of holding its size constant, but instead to act like a negative pressure, pushing space time apart faster and faster as the universe ages, accelerating its expansion. So the missing 69.25% of the energy density that wasn't explained by matter is explained by something far stranger, the seeming energy of empty space itself. Rather than being a sub-dominant player in the universe, it now appears to be the most prominent player in the cosmos, gradually making the universe bigger and bigger at a faster and faster rate. We have two mysteries. In trying to shine light on the universe, we have revealed shadows that account for more of its mass than can be explained by the familiar shapes of matter and forces described by the standard model. We have also revealed that matter itself is really just a bit player in the universe, relegated to less than a third of the energy density of the universe. The rest is a kind of dark energy that acts to accelerate the expansion of the universe. Some of these new shadows increase the mass of the universe, guiding its clumping by exerting more gravity on normal matter than normal matter alone could achieve. And some of the new shadows press on the walls of the cave, stretching those walls to make the cave larger and larger, doing so faster and faster. If dark energy is but the energy of empty space, perhaps quantum mechanics can come to the rescue. After all, in the quantum realm, particles and antiparticles are free to pop into existence and disappear again. This makes the vacuum alive with virtual particles, whose fleeting existence has real consequences for the universe, including making empty space not empty, but filling it with a kind of quantum energy. Perhaps this is dark energy. But alas, the calculations have been done. The results are surprising. You might have expected that we would tell you that the standard model doesn't offer enough virtual particles to come into and go out of existence. Rather, the opposite is true. The standard model would predict too much dark energy, more than is observed to exist in the cosmos. This calculation doesn't overshoot by a little. It over-predicts spectacularly badly. The calculations overshoot the amount of dark energy in the universe, not by a factor of 2 or even 10, but by more than 100 factors of 10 multiplied times each other. Such a spectacular miss cries for an explanation. As we leave the puzzles of dark matter and dark energy for other topics, we must keep these twin modern mysteries in mind. The standard model may hold no particles that can explain dark matter, but a more all-encompassing theory of nature must. This is a stiff demand on an expanded theory of nature. In addition, that theory must explain why dark energy exists and why it must be so much smaller than would be predicted by the vacuum energy of the standard model alone. Echoing the beginning words of this chapter, do you live in the country? Do you live in the city? It doesn't matter. If it's a clear night, go outside and look up. Don't look at the stars. The temptation of their light is a distraction. The universe is ruled not by those bright gems, but by the voids and the shadows that lie between them. Reality in the shadows, or what the heck's the higgs, was written by S. James Gates Jr., Frank Blitzer, and Stephen Jacob Sikula, published by YBK Publishers in 2017. This chapter was read by Stephen Jacob Sikula. The music is a public domain recording made by Norman McGill, available from the MetaMath website. The recording is a musical representation of the proof of Proposition 5.18 from Isaac Newton's Principia Mathematica.