How Did the Universe Get Going?
Over the past several decades, physicists and astronomers have constructed a remarkable paradigm, known as the big bang model, to describe the expanding universe. By combining observations of distant galaxies, the cosmic microwave background (CMB) radiation, and the abundances of the lightest elements with the fundamental theories of gravity and atomic and nuclear physics, researchers have been able to account, in large measure, for the evolution of the universe from the first few seconds until the present (see Box 4.1).
In the wake of this progress, there are new and deeper questions to be answered and mysteries to be explained: Is the universe flat in its spatial directions, and if so, why? Where did the structure around us—from individual galaxies to the great walls of galaxies—originate? What went bang and started the expansion? Surprisingly, the answers to these questions about phenomena on the largest imaginable scales may well be found in the physics of the smallest scales. This chapter describes the questions and challenges raised by the puzzle of the earliest evolution of the universe and the opportunities for providing answers.
BIG BANG COSMOLOGY: THE BASIC MODEL
Observations of the recession of distant galaxies confirm that the universe is expanding. As the universe expands, the density of both matter and radiation decreases with time. Thus, in the past, the universe was much denser and much hotter than it is today. Today, the universe is filled with the cosmic background radiation, the residual heat from the big bang, which has been cooled by the expansion of the universe. This radiation fills space; there are roughly 400 microwave background photons in each cubic centimeter of space.
BOX 4.1 WHAT WE KNOW ABOUT THE BIG BANG AND HOW WE KNOW IT
The big bang model embodies our accumulated knowledge about how the universe began and evolved to its present state. Like most scientific theories it is not static, but rather is constantly being tested and extended. Nor does it exist in a vacuum—its foundation being Einstein’s theory of general relativity.
Testing the big bang model (or any theory) requires a theoretical framework—in this case, general relativity. If the predictions of the big bang model agree with the data, then both the big bang model and general relativity are being tested (a failure of either one would lead to discrepancies). The fundamental set of observations that support the big bang model are the expansion of the universe; the existence of the cosmic microwave background (CMB) radiation; the 0.001 percent variations in the intensity of the CMB that reflect the primeval lumpiness in the distribution of matter, which seeded all the structure seen today; and the abundance pattern of the lightest elements (hydrogen, deuterium, helium, and lithium) seen in the most primitive samples of the cosmos. A large number of other observations are also consistent with the big bang model.
Within the context of the big bang model, parameters that describe the key features of our universe are measurable. There has been great progress in recent years in improving the precision of these measurements: the temperature of the CMB has been measured to four digits, T = 2.725 K; the expansion rate of the universe (or Hubble constant) has been determined to a precision of 10 percent, H0 = 63 to 77 km/sec/Mpc; the time back to the big bang has been determined to a precision of about 15 percent, t = 11.5 billion to 14.5 billion years; the average density of matter and energy has been measured and found to be between 95 and 125 percent of the critical density; independently, space has been shown to be uncurved (“flat”) to a precision of about 4 percent; and the rate of change in the expansion rate has been measured, indicating that the expansion seems to be speeding up, not slowing down.
Some of these measurements require further assumptions or information beyond the assumption of general relativity; for example, to determine the time back to the big bang requires both the Hubble constant and knowledge about the matter and energy makeup of the universe. The curvature of space was determined from measurements of the size of hot and cold spots on the microwave sky and involves a minor, but nonetheless additional, assumption about the nature of the lumpiness in the distribution of matter. Some of the cosmological parameters test the basic consistency of the framework; for example, the time back to the big bang can be compared to the age of the oldest stars, between 10 billion and 14 billion years. Within the margin of error the universe is older than the oldest objects within it.
Our present understanding of the big bang takes us back to a time when the universe was a soup of elementary particles, a few microseconds after the big bang. Current attempts to extend the big bang model, such as inflation, aspire to describe even earlier moments in the universe and to answer deeper questions, such as, How did the lumpiness arise? What made the universe flat? What
was the dynamite behind the big bang? The key idea of inflation is a tremendous growth spurt that occurred during the earliest moments and was caused by physics that is not yet well understood, but whose basic consequences are quite clear. They include the prediction that the universe is flat, and that the lumpiness arose from quantum fluctuations and the existence of a background of gravity waves. Testing these basic predictions tests the inflationary framework. If inflation passes these tests, and the early signs (e.g., the flatness of the universe) are that it will, then more detailed aspects of its predictions can be addressed and can lead to an understanding of the underlying cause of inflation. The array of tests awaiting inflation is quite elaborate. For example, its prediction about the lumpiness in the distribution of matter leads to a detailed statistical description of how structure forms, once the nature of the dark matter is specified. The evidence points to the dark matter consisting of slowly moving particles (cold dark matter), and the cold dark matter scenario opens inflation to a whole array of new tests.
There is no doubt that inflation, even if correct in broad outline, will not be the last word on our understanding of the origin of the universe.
Looking outward in space is equivalent to looking backward in time. When they observe the nearby Andromeda galaxy, astronomers are detecting photons that left that galaxy 2 million years ago. Five hundred thousand years after the beginning, the temperature of the CMB radiation was 3000 K. At that temperature, hydrogen, the dominant atomic component of the universe, was ionized and existed as free protons and electrons. While cosmic background photons move freely through neutral hydrogen, they scatter easily off electrons. Thus, observing the microwave background radiation involves detecting photons that last interacted with matter during this early epoch, when the matter was mostly ionized, and provides a snapshot of the infant universe.
NASA’s Cosmic Background Explorer (COBE) satellite accurately measured the energy spectrum of this background and found that it agreed with the big bang model’s prediction of a thermal spectrum to better than 1 part in 10,000. Although CMB observations are measuring the physical conditions 400,000 years after the big bang, it is possible to use the big bang model to extrapolate back to early times.
Closer to the beginning of the big bang, the universe gets hotter and denser. Three minutes after the big bang, the temperature of the background radiation was roughly 1 billion K. At these high temperatures, neutrons and protons collided and fused to form most of the deuterium and helium in the universe. The big bang theory accurately accounts for the abundance of
these light elements. This success is one of the primary tests of the big bang model (see Box 4.2 for more on precision cosmology).
When the universe was younger than about 10 microseconds, neutrons and protons did not exist as such. Rather, there was a soup of their constituents, quarks and gluons. One of the scientific goals of the relativistic heavy ion collider at Brookhaven National Laboratory is to confirm that at sufficiently high temperatures matter exists as a quark-gluon plasma.
At 10 micro-microseconds after the big bang, the temperature of the universe was roughly comparable to the highest energies that are now achievable at the largest particle accelerators. At these high temperatures, electrons and positrons (electrons’ antiparticles) collided to produce a vast cornucopia of particles. Most of these particles did not survive to the present: They were destroyed through either annihilation or decay. However, the electron, the neutrino, and the proton may not be the only survivors. Some theories of particle physics predict that there may be other stable particles, such as axions and neutralinos (see Chapter 4, section “Refining the Big Bang: The Inflationary Paradigm”). If these exist, then they would have been created in this powerful cosmological accelerator and would have survived today as fossil relics of the earliest moments.
Moreover, as Chapter 3 points out, astronomical evidence actually suggests the existence of some new kind of particle. Since the 1970s, astronomers have known that the mass in galaxies, inferred through its gravitational influence on motions within the galaxies, vastly exceeds the mass in visible stars. Over the past two decades, they have eliminated all of the usual suspects: low-mass stars, clouds of molecular or ionized gas, massive planets, and even supermassive black holes. These particles, products of the first microsecond of the big bang, may account for the bulk of the matter in the universe and may even be detectable in laboratory and astronomical searches (see Chapters 3 and 5).
REFINING THE BIG BANG: THE INFLATIONARY PARADIGM
Despite its successes in explaining the expansion of the universe, the abundance of light elements, and the properties of the CMB, the big bang model is incomplete. It does not explain why the universe is so large and so uniform. It requires that physically disconnected regions of space all simultaneously start expanding at the same moment and at the same rate. At the beginning of this expansion, the kinetic energy of the expansion must have nearly perfectly balanced the gravitational energy counteracting the expansion. Without this nearly perfect balance, the universe would either have
BOX 4.2 PRECISION COSMOLOGY
We stand at the brink of a new era of exploration in cosmology and particle physics. The coming years will witness multiple probes of the deep relationship between physics at the highest energies and the details of the early universe and dark matter and dark energy. What are the dark matter and dark energy, and how are they related to the physics of the hot early universe? Thanks to new tools, scientists are now entering the age of precision cosmology, a phrase that only a decade ago would have been considered an oxymoron.
The new tools include measurements of the cosmic microwave background to microkelvin accuracy; redshift surveys that include samples as large as 1 million galaxies; x-ray instruments with the spatial resolution of the Hubble Space Telescope (Chandra X-ray Observatory) and spectral resolution matching that of the best optical instruments (XMM-Newton); new specialized detectors to search for dark matter particles; and in the coming years even more powerful probes.
Taken together, observations of the effects of dark matter and dark energy and of the fluctuations in the remnant radiation from the big bang will soon allow percent-level determinations of several cosmological parameters— the expansion rate (Hubble constant), the density of ordinary matter, and the curvature of space—testing the foundations of current understanding of the universe as well as the framework of general relativity itself.
The highest energies characterizing the frontier of particle physics, unattainable in any conceivable accelerator on Earth, are reached in the big bang. Perhaps some day the remnant cosmic gravitational-wave noise from the turbulent first moments will be detected. It is already possible to map the small (30 microkelvin) temperature variations in the remnant microwave radiation from the fireball that was our observable universe when it was 1,000 times smaller than it is today. These variations reveal the underlying gravitational effects of dark mass-energy fluctuations left over from an even earlier time: a filtered glimpse of the primordial universe.
New satellite experiments measuring the polarization of this microwave radiation could reveal more details of the primordial universe. The systematic mapping of the gravitational distortions of images of the distant universe by intervening matter (gravitational lensing) enables charting the development of structure in the universe and probing both dark matter and dark energy. Large samples of distant supernovae and of galaxies and clusters will be used to study the expansion history of the universe and thereby get at the nature of the dark energy. Will the current models survive these tests? The answer to this question is not certain, but it is certain that discoveries arising from this new exploration will illuminate the origins of our world.
collapsed long ago in a big crunch or have expanded so rapidly that it would be nearly devoid of matter today. The big bang model also does not explain the origin of the lumpiness in the distribution of the matter that grew to form stars and galaxies.
One of the great successes of cosmology over the past two decades has been the development and initial testing of the “inflationary paradigm,” which extends the big bang model and explains the large size and uniformity of the universe as well as the origin of the lumpiness. In the inflationary scenario, vacuum energy, not ordinary matter, dominated the energy density of the universe during the first moments of the big bang. This vacuum energy drove a rapid expansion of the universe, which homogenized it by stretching a microscopic patch to a size much larger than our visible universe. The effect of the vacuum energy of inflation is similar in many respects to that of the dark energy that physicists think may be driving the recently observed acceleration of the expansion of the universe, although the underlying physical mechanism may be different (see Chapter 5). This expansion made the geometry of the universe nearly flat; this is one of the basic predictions of inflation.
Astronomical observations, however, suggest that the density of ordinary matter is not sufficient to satisfy the equations of Einstein gravity for a flat universe, in apparent contradiction with the inflationary prediction. The total amount of matter inferred from astronomical observations falls short of that needed for a flat universe by a factor of about 3. However, recent observations of distant supernovae seem to indicate that the additional matter/energy to make the universe flat exists as a new state of mass-energy, dubbed dark energy. Moreover, measurements of the tiny temperature variations in the CMB across the sky also point to a flat universe (see Chapter 5).
The nature of this dark energy and of dark matter remains a mystery and is the focus of Chapter 5. However, the absence of a precise identification of dark matter particles and the lack of a fundamental understanding of the nature of dark energy do not prevent calculations within the inflationary paradigm that connect the physics of the early universe to observations of the CMB and of galaxies and clusters today. These calculations are key to testing inflation.
HOW DID THE UNIVERSE GET ITS LUMPS?
The 30 microkelvin variations in the temperature of the CMB from point to point on the sky indicate that the initial big bang explosion was not
perfectly uniform. These variations in the CMB temperature, discovered by the COBE satellite and studied in more than 20 other experiments since (see Figure 4.1), indicate that the initial distribution of matter in the universe was lumpy, varying by about 0.001 percent from place to place. Such a small deviation from perfect smoothness may seem unimportant, but it is absolutely crucial: The attractive effect of gravity acting over the past 13 billion years has turned this tiny lumpiness into the structure that exists today. (In fact, it continues to exert an inexorable pull on matter, as the collision in Figure 4.2 suggests.) Moreover, the level of lumpiness revealed by COBE and other experiments is just what is needed to account for the structure observed today. This is one of the great successes of the hot big bang cosmology.
The existence of the lumpiness that was revealed by COBE raises a very fundamental question: How did the universe get its lumps?
The Heisenberg uncertainty principle prohibits precise knowledge of the energy density of the universe at the atomic scale, which leads to a fundamental source of lumpiness. Unfortunately, the tremendous mismatch between the subatomic length scales on which quantum fluctuations are important and the astrophysical scales associated with the structure in the universe renders this uncertainty principle possibility completely irrelevant in the standard big bang model.
Inflation changes all that. The tremendous spurt of growth that is the hallmark of inflation bridges the gap, stretching subatomic scales to astrophysical size. Further, inflationary models make detailed predictions for the statistical properties of the lumpiness that arises from quantum fuzziness. There are three key tests of the inflationary prediction that the largest structures in the universe owe their origin to the quantum fuzziness of the subatomic world.
The first test involves a comparison of the structure that exists in the universe today with that expected from the inflationary picture. If, as is currently believed, the dark matter consists of particles that are moving slowly (so-called cold dark matter), then calculations and computer simulations show that the gravitational clumping of this dark matter around the fluctuations left from inflation leads directly to the formation of galaxies and clusters of galaxies. The large-scale distribution of the galaxies and clusters that emerge from these calculations agrees well with the observed distribution. More precise tests will come when larger, more precise surveys of the universe—such as the Sloan Digital Sky Survey and the Two-Degree Field mapping project—are complete (see Figure 4.3).
The second test can directly probe the primeval lumpiness itself, before gravity has enhanced it. The lumpiness produced by inflation also manifests itself in local variations in the temperature of the CMB, which are directly related to the lumpiness in the distribution of matter. The inflationary model makes detailed predictions for the statistical properties of the fluctuations on the cosmic background microwave sky. The predictions are consistent with the CMB fluctuations measured by the COBE satellite on large angular scales (see Figure 4.4) and by a host of other CMB experiments on smaller angular scales. More definitive measurements will be made by a combination of Earth-based, balloon-borne, and spaceborne instruments over the first decade of the new century.
In particular, NASA’s Microwave Anisotropy Probe (MAP), launched in June 2001, is mapping the entire microwave sky using measurements from
over a million independent patches and will provide precision tests of the temperature fluctuation predictions of the inflationary model (see MAP in Figure 4.5). Later in the decade, the European Space Agency’s Planck Surveyor will make even higher-resolution maps of the whole sky. If the data are consistent with inflation, then the statistical properties of the CMB fluctuations can be used to learn much about the universe. Included in this list are basic cosmological parameters, such as the size of the universe and the rate at which it is expanding, the average density of ordinary matter, the average density of dark matter particles, and the amount of dark energy, as well as basic parameters of inflation.
The third test involves a detailed comparison of the two maps of the universe just discussed with other observations, including the distribution of dark matter revealed by gravitational lensing surveys (see Chapter 5). These
comparisons not only will test inflation but also will determine cosmological parameters more precisely and test the underlying cosmological framework.
Although measurements of microwave background temperature fluctuations alone have the ability to discount the inflationary model or to establish it as a basic tenet of cosmology, they will not allow researchers to distinguish among different versions of inflation. Most of the current versions of inflation are simple models that show that the inflationary physics is plausible, but they are not yet closely linked to theories of the elementary particles physics such as string theory, the current best hope for unifying gravity with the other fundamental forces.
THE ORIGIN OF MATTER: WHY ARE WE HERE?
Physicists speculate that during the first microsecond of the big bang, the universe underwent a series of phase transitions. Prior to the “electroweak” phase transition, electromagnetic and weak forces were unified in a single electroweak force. Afterward, this unification or symmetry was broken and the two forces had rather different properties. Similarly, prior to the “grand unification” phase, the electroweak force and the strong force were unified as a single force. Physicists speculate that the universe began in a state of symmetry among all the forces and with equal amounts of matter and antimatter (if inflation took place, the balance between matter and antimatter is automatic). Then, around the time of either the electroweak phase transition or the grand unification phase transition, a sequence of events called baryogenesis is believed to have occurred that was responsible for the origin of the slight imbalance between matter and antimatter. As a result of baryogenesis, in the early universe there were 299,999,999 antiprotons for every 300,000,000 protons. During the first minutes of the big bang, the antiprotons annihilated and destroyed all but the one excess proton. Without baryogenesis, all of the protons in the universe would have been annihilated through collisions with an equal number of antiprotons, leaving no ordinary matter left to form stars and planets. The details of baryogenesis are not yet understood in any detail. When they are, baryogenesis will be as fundamental a part of our understanding of the evolution of the universe as big bang nucleosynthesis, which explains the origin of the lightest nuclei, is today.
Three decades ago, the Soviet physicist and dissident Andrei Sakharov identified the key ingredients for generating the matter-antimatter imbalance. These include reactions that do not create or destroy baryons and antibaryons in pairs and reactions that are not the same for matter and antimatter. Experi-
ments can probe these reactions. If baryons can be created and destroyed, then protons could be unstable. Grand unified theories predict that the proton is unstable but very long-lived. Terrestrial experiments have searched for a variety of modes into which the proton could decay and have set remarkable limits on the order of 1032 years for its lifetime. Further progress will require even larger detectors. Ongoing studies are attempting to determine the feasibility of this next effort (see Chapter 3). Reactions that occur at different rates for matter and antimatter (so-called CP violation; see Chapter 2) are under intense study at particle accelerators. It is also possible the needed baryon asymmetry arose from an inequality between neutrinos and antineutrinos and involves CP violation in the neutrino sector.
GRAVITATIONAL WAVES: WHISPERS FROM THE EARLY UNIVERSE
Gravitational waves produced in the first moments of the universe can propagate directly to detectors without being altered by the intervening matter. Observations of primordial gravitational waves, although very challenging, are potentially the most powerful probe of the early universe. What are the possible sources of cosmic gravitational waves?
Today, all quarks are bound together into either protons or neutrons. However, during the first 10 microseconds of the big bang, the temperature of the universe was so high that unbound quarks moved freely in a state of matter called quark-gluon plasma. The transition from free quarks to bound quarks as the universe cooled is called the quantum chromodynamics phase transition.
Other phase transitions probably occurred. At least two symmetry-breaking phase transitions are expected: the electroweak and grand unification symmetry-breaking phase transitions, described above.
If they were violent enough, any of these phase transitions could have produced a cosmic background of gravitational waves—the gravity-wave static that new instruments can detect. The characteristic wavelength of these emitted gravitational waves corresponds to the size of the visible universe when this phase transition occurred. Compactification of putative extra dimensions may have produced a spectrum of gravitational waves. Various versions of string cosmology also predict a background of gravitational waves. Since current understanding of the physics of strings is incomplete, these sources of gravitational waves are even more speculative than the sources associated with phase transitions. Gravitational-wave observations are a unique window onto to the earliest moments of the universe and to physics that is not accessible in an Earth-based laboratory.
Currently, the best-motivated source of primordial gravitational waves is associated with inflation. These gravitational waves arise directly as quantum fluctuations in space-time itself. Their amplitude is directly related to the energy scale of inflation. In some inflationary models, there is a predicted relationship between the amplitude and wavelength of the gravitational waves and the amplitude and wavelength of the density fluctuations that grew to form galaxies. These density fluctuations are the dominant source of CMB fluctuations, but gravitational waves can also make a contribution, depending on the specific relationship between amplitude and wavelength. The upcoming MAP and Planck Surveyor satellites will be able to detect the gravitational wave contribution if it is present to a sufficient degree.
An even more promising avenue is to study the polarization of the CMB radiation. The electrons and protons in the early universe can respond to gravitational waves. They can also scatter and polarize light, producing both variations in the microwave temperature and a particular pattern in the polarization of the microwave radiation on large angular scales that carries the imprint of the gravitational wave perturbations. Since both density fluctuations and gravitational waves produce variations in the microwave background temperature, it is difficult to detect gravitational waves with temperature measurements alone. However, gravitational waves produce a unique pattern of polarization fluctuations that can be easily distinguished from that produced by density fluctuations.
So far, no gravitational wave signature in the polarization of the CMB has been detected. (Recent results from the DASI experiment have proven the ability to measure the polarization of the CMB, as seen in Figure 4.6). A variety of techniques and technologies are being tried in small-scale ground-based and balloon experiments. NASA’s MAP satellite has the sensitivity to detect the polarization predicted to arise from the density fluctuations that seeded structure. Whether or not MAP is able to make this detection depends upon the unknown competing polarization signal associated with point sources and other galactic and extragalactic foregrounds. Unlike the experiments studying temperature fluctuations, the key sources of systematic errors have not been identified and controlled for. However, over the next few years, the combination of MAP and several ground-based and balloon-based experiments will measure the galactic polarization foregrounds over a wide range of frequencies and should enable detection of the polarization signal produced by density fluctuations. If the gravitational-wave signal is particularly strong, then these experiments may be able to detect it. However, detection of the gravitational-wave signal will likely
require a new satellite beyond MAP and the Planck Surveyor. Measurements of polarization fluctuations with high sensitivity on a large angular scale will require the benign environment of space. Once the upcoming generation of experiments improves understanding of the foregrounds and of the potential sources of systematic error, it should be possible to begin designing the next-generation polarization satellite.
A second method is the direct search for gravitational-wave static not associated with any particular astrophysical event (such as a catastrophic collision of two black holes) but rather constituting a random background radiation. The first major step is being taken with the ground-based Laser Interferometer Gravitational-wave Observatory (LIGO) and with parallel projects in Europe and Japan, which should be collecting data in 2 years and for which extensive upgrades are planned over the coming decade. However, these detectors will not operate at long wavelengths. A spaceborne laser interferometric detector could be sensitive to very-long-wavelength gravitational waves. Although gravity-wave detectors have been designed to detect point sources of gravitational radiation, they might be able to observe the diffuse radiation from the inflationary era of the early universe predicted by some models, and they would certainly be sensitive to waves from phase transitions or other exotic epochs that have been hypothesized. As the history of the CMB shows, the detection of a diffuse-noise background is far more difficult than detection of point sources. Some researchers have begun to envision multiple space-based detectors operating simultaneously and optimized to detect the cosmic background of gravitational waves.
EVEN BEFORE INFLATION: THE INITIAL CONDITIONS
While the big bang model and the inflationary universe paradigm answer many of the questions about the physical conditions in the early universe, they do not answer the most fundamental cosmology question: How did the universe begin? One approach to understanding the initial conditions for the universe is quantum cosmology, an attempt to apply the laws of quantum mechanics to the universe itself. Familiar laws of physics, such as Newton’s laws or Einstein’s theory of relativity, describe how physical systems evolve given an externally specified set of initial conditions. In cosmology, there is no “rest of the universe” from which to set those conditions, so a complete cosmological theory of initial conditions is needed in addition to dynamical physical laws. Whether quantum cosmology will lead to such an understanding of initial conditions, and, if so, whether it will
make predictions that can be tested are deeply interesting but very open questions.
The inflationary paradigm is a bold attempt to extend the big bang model backward to the first moments of the universe. It uses some of the most fundamental ideas in particle physics (e.g., symmetry breaking and vacuum energy) to answer many of the basic questions of cosmology. Because of these deep connections, advances in elementary particle physics and cosmology will come hand in hand.
A number of opportunities are now ripe for answering some of the central questions of the inflationary picture. Is the inflationary picture correct? If so, what is the physics of inflation? Were there phase transitions in the early universe associated with changes in the symmetries of the underlying physics? Are the gravitational whispers detectable? It may well be possible to answer he boldest question we can ask—How did the universe begin?
How Did the Universe Begin?
In the coming years, observations will provide more stringent tests of the inflationary model. Experiments that map the fluctuations of the microwave background on finer angular scales, together with weak gravitational lensing surveys, can measure the size and the rate of expansion of the universe, the density of ordinary and dark matter, and the basic parameters of inflation. Measurements of microwave background polarization fluctuations will be sensitive to primordial gravitational waves, possibly yielding clues to the physics that underlies inflation.
What Are the New States of Matter at Exceedingly High Density and Temperature?
The direct detection of long-wavelength gravitational waves will enable scientists to “listen to” phase transitions in the early universe. This ability will require new experiments, probably space-based, designed to look for the background of gravitational waves produced in the early universe. If successful, these observations could reveal exotic states of matter in the hot, early universe, including quark-gluon plasma.