V. PHYSICS OF THE EARLY UNIVERSE

In contrast to the observational studies of the CMBR, galaxies, and large-scale structure, the field of the physics of the early universe involves concepts that are less familiar, more theoretical, and more daunting. A little background, supplementary to the sidebars (The Early Universe, The Cosmic Picture) is helpful.

As described in the sidebars (The Early Universe, The Cosmic Picture), the universe is cooling and decreasing in density as it expands. Since temperature is simply a measure of mean energy per particle, the energy available for particle interactions is also declining. If we imagine running the universe's clock backward toward zero, before the first 100 seconds, the CMBR would be blindingly hot and energetic, and more and more energetic events would become possible, including the creation of multitudes of elementary particles that are not stable at the present energy density of the universe.

An important concept of modern physics is the phase transition, the idea that the nature of the interactions of particles can change with available energy. The freezing and boiling of water are familiar examples of phase transitions. In the early universe, the nature of physical law itself is thought to have undergone a series of phase transitions, with enormous consequences for the physics of that era. The history of the early universe and the laws of physics are intimately intertwined--each one can be illuminated by studying the other. Moreover, energies in the Big Bang reached up to 10¹⁴ times higher than any conceivable terrestrial accelerator, and these energies probe realms that are inaccessible to our laboratory experiments. For this reason the Big Bang is sometimes called the "poor man's particle accelerator."

The aim of early-universe cosmology is to trace the successive transitions of forces and particles from the earliest, fiercely hot moments of the Big Bang to the epoch of atom formation at 4,000 K (see section II on the cosmic microwave background radiation). Along the way, at certain critical temperatures, precipitous changes in the state of the universe occurred. Some of these changes have observable consequences, called relics, that persist to this day and provide key tests of models. Some of these relics-the ratio of photons to atoms and the nature of the large-scale fluctuations in the matter distribution as detected by the COBE satellite and studies of large-scale structure--are discussed above. Other fundamental questions, such as why the universe is homogeneous on the largest scales, and what is the nature of dark matter, also appear to have their answers in the physics of the early universe.

The sidebar "The Early Universe" treats the first 100 seconds after the Big Bang. During this period, the universe was opaque to all forms of electromagnetic radiation, and so astronomers cannot make direct observations of the events at these early times. Nevertheless, important experimental and observational information can be obtained that leads to reasonable physical inferences about events and conditions during this period. Progress in the study of the early universe has been spectacular in recent years.

The earliest state of the universe that can be addressed by physical theories is called the quantum gravity era (see sidebar, "The Cosmic Picture"), because during that era the temperature and density of the universe were so high that gravity must be described by a quantum field theory of some kind. The four forces of nature (gravity, the weak force of radioactive decay, electromagnetism, and the strong nuclear force) were probably completely unified during this era, and space and time could not be differentiated. After this almost unimaginably remote epoch, the universe cooled sufficiently for gravity to be described by Einstein's general relativity theory, but the temperature was still sufficiently high that the other three forces of nature remained unified (the grand unification era). Many theorists believe the phase transition that marked the end of the grand unification era was followed by a period of inflation. (Inflation is discussed below.)

As time progressed and the universe cooled further, additional phase transitions occurred, such as the end of the symmetry between weak interactions and electromagnetic interactions and the transition from free quarks to quarks bound into hadrons. (Protons and neutrons are the most familiar example of hadrons.)

At very high temperatures quantum interactions create and destroy particles, leading to an equilibrium in the number density of each particle type. However, particle interaction rates depend on temperature, and as the universe cooled, interactions occurred less frequently. When an interaction rate became small relative to the expansion rate of the universe, the equilibrating process effectively ended. The abundances of the reacting particles were thereafter fixed. Although the expansion rate of the universe slowed as it became cooler and less dense, particle interaction rates decreased even more rapidly.

Although the abundance of atoms relative to photons is likely to be a relic from an earlier, hotter epoch whose physics is not yet well understood, the conditions that existed when the universe was 1 second old can now be explored in the high-energy physics laboratory. Cosmologists are therefore more confident in their modeling of the constituents and the physical processes at work. For example, by the time the universe was 1 second old, it had cooled and expanded to the point where neutrinos, whose weak interaction with other matter decreases as a function of temperature, effectively ceased interacting with matter. This change acted to stabilize the ratio of neutrons to protons. With further cooling, positrons (antielectrons) annihilated most of the electrons, and neutrons quickly attached to the protons, forming all the deuterium (an atomic nucleus consisting of one proton and one neutron) and most of the helium now present in the universe.

Thus, many interactions that were close to thermal equilibrium in the early universe later froze out at a predictable epoch, and left relics, some of which survive to the present day (see Table 1). One example is the abundance of primordial helium and deuterium, discussed in more detail below.

Primordial Nucleosynthesis and Dark Matter

Along with the Hubble expansion and the cosmic microwave background radiation, one of the pillars of the Big Bang theory is its successful prediction of the abundances of the light elements deuterium, helium, and lithium. The Big Bang theory says that when the universe was about 1 second old and had a temperature of 10¹⁰ K, nuclear processes should have started that eventually yielded certain well-specified abundances for these light elements (see Table 1). The abundances of these light elements have all been found to be in agreement with the predictions of Big Bang theory within the accuracy of the measurements. Even the abundance of lithium relative to hydrogen, predicted to be 1 part in 10 billion, matches the observations. Furthermore, the Big Bang theory predicts that the abundances will fit well only if there are no more than three families of neutrinos-a condition that was confirmed recently at the Large Electron-Positron (LEP) Collider in Geneva, Switzerland. Thus, the Big Bang theory's detailed predictions, even though they are based on the nature of the universe when it was only 1 second old, have been confirmed by observations and experiments.

The light elements shown in Table 1, with abundances ranging from about 23 percent for helium to 1 part in 10¹⁰ for lithium, all fit with the Big Bang theoretical predictions on one condition-that the one adjustable parameter of the theory, W_B (the ratio of the mean density of ordinary matter (baryons) to the critical density in the universe), has a value between 0.01 and 0.1. The nucleosynthesis calculations thus imply either that (1) the total density of the universe is much less than the critical density and it will never stop expanding, or (2) the dominant component of the universe is not ordinary (baryonic) matter.

Table 1. Observed Relic Values

The value of W_B predicted by primordial nucleosynthesis can be compared to that observed in the universe. The mass density of luminous material in stars and galaxies is small, W_visible < 0.01, while the hot gas in galaxy clusters, which astronomers can detect by its x-ray emission, contributes perhaps W_gas ~ 0.03. The sum of these values lies in the range consistent with primordial nucleosynthesis. At the same time, dynamical models based on the relative motions of galaxies, and the way spiral galaxies rotate, argue that galaxies have more mass than is seen in their detected stars and gas. These dynamical arguments imply that each galaxy has an invisible halo of dark matter that is about 10 times the visible mass. Moreover, consideration of large-scale flows seems to indicate a still larger amount of dark matter on scales much larger than single galaxies. Perhaps there is even enough to be consistent with a total matter density of W ~ 1, or at least W > W_B. This implies that there must exist some unknown form of matter that dominates the mass density of the universean awkward situation for cosmologists.

There are other theoretical reasons to expect that W = 1, or in other words that the total mass density is exactly equal to the critical value that just closes the universe. W is an unstable quantity in an expanding universe. If W is below 1 it will rapidly become much less than 1 as expansion proceeds. Conversely, if W is greater than 1, it will grow to values much greater than 1. Only if W = 1 does it stay at 1; all other values diverge to either zero or infinity. A finite, non-zero value of W today, other than W = 1, implies that it must have been extremely close to 1 at the beginning of the universe. Cosmologists have puzzled over this fine-tuning problem for decades, but just in the past decade or so, considerations of the early universe have motivated a sensible resolution to this question--inflation.

Epoch of Inflation and Grand Unified Theories of Matter

After the Big Bang, the temperature of the early universe was so high that the four fundamental forces of nature are believed to have been merged. In the grand unification era that followed, the grand unified theory (GUT) predicts that all the forces except gravity were of equal strength. Modern theories of these forces involve a concept known as symmetry breaking, in which the lowest-energy state (the vacuum) is not symmetric at the low temperatures of the present universe. As time progressed, the temperature decreased, and the vacuum underwent a phase transition from a symmetric state of higher energy. The higher energy of the "false vacuum" can in principle act like a non-zero cosmological constant, L ' 0, which, according to Einstein's general relativity theory, can drive an extremely rapid, accelerating expansion of the universe. This expansion is called inflation (see "Measuring the Cosmological Parameters" in section IV) and is supposed to have occurred in the first instants after the creation of the universe (see "The Cosmic Picture" and "The Early Universe"). In a very short time (10^-32 s), the early universe may have expanded by a greater factor than it has in the billions of years since. Thus, inflation is intimately connected with our understanding of elementary-particle physics.

Inflation beautifully explains three long-standing problems of cosmology. In the normal theory, regions of space separated by a distance greater than the distance light has traveled in the time since the Big Bang are effectively disconnected from each other. In traditional, non-inflationary models, there is no reason for such regions to be similar. For example, since disconnected regions can never have exchanged energy, why should they be at exactly the same temperature? Energy exchange between nearby regions could result in small patches with uniform temperature, but CMBR measurements tell us that large regions are nearly equal in temperature. Because inflation can quickly expand an extremely tiny volume into a vastly larger region of space, it would allow a small, uniform patch to expand to cover our entire observable universe, leading to a nearly uniform temperature for the CMBR.

At the same time, there must remain some minimum level of bumpiness even in the uniform patches, because quantum mechanics and the uncertainty principle require it. Inflation magnifies these tiny fluctuations into the CMBR anisotropy that astronomers see today and the large-scale structure of matter in the universe. Indeed, the variation in amplitude of fluctuations of different angular size is consistent with the expectations of the inflationary model. Inflation takes microscopic quantum noise and blows it up to create the seeds of galaxies and large-scale structure.

A third advantage of inflation is that it forces the spatial curvature of the universe to be negligibly small on a cosmological scale so that space is flat (i.e., euclidean geometry applies). This flatness is a direct consequence of the tremendous expansion expected during inflation. A small closed surface such as a balloon has an obvious curvature, but if expanded to the size of Earth, its curvature is much less apparent. The absence of curvature in an inflationary universe would imply that today, the density parameter W should be close to unity. Thus, an inflationary phase in the early universe naturally solves the fine-tuning problem mentioned above.

The panel emphasizes that inflation is an idea, not a complete or well-tested physical theory. In addition to the original version described here, many different variants have been presented, some with inflation occurring during the quantum gravity era, others with inflation occurring much later, each driven by different mechanisms. Although our understanding of particle physics is incomplete at these energies, and we have no understanding of the details of the inflationary epoch, inflation is an attractive concept because of its ability to resolve several long-standing cosmological conundrums. Many cosmologists are convinced that such an episode must have occurred.

Particle Theory and Dark Matter Candidates

Astronomers have found strong evidence for a major dark matter component of the universe; the visible matter does not add up to the total amount of matter measured by dynamical means. Could the dark matter be ordinary baryonic matter in a form that doesn't shineperhaps brown dwarf stars, black holes, or hot intergalactic gas? Apparently not, according to the calculations of primordial nucleosynthesis, which work only if the density of baryons is less than 0.1 of the critical value (W_B < 0.1). Thus the bulk of the dark matter must be composed of an unknown form of matter. What could it be? Particle physics has some candidates that are discussed below, and the astronomical behavior of dark matter offers some clues. Observations show that the dark matter is much less clumped than the visible matter. Therefore, the two kinds must interact only weakly, mainly via the gravitational force. Computer simulations of the formation of large-scale structure also provide valuable information about the behavior of the dark matter, which plays an important role in shaping the structure. These studies show that the non-baryonic dark matter candidates can be divided into two categories depending on the velocity with which the particles were moving when the universe became dominated by matter ("The Cosmic Picture" and "The Early Universe"). During this epoch, a rapidly moving particle (e.g., because its mass is small) is considered hot dark matter; a slowly moving particle is considered cold dark matter. Currently, the cold dark matter candidates, or a mixture of hot and cold, give the best agreement between computer simulation results and the observed large-scale structure.

Most cosmologists believe that the unknown matter needed to explain the "missing mass" exists in the form of some yet-undetected elementary particle-a particle that is fundamentally different from ordinary matter. Such a particle would be a relic of some process in the high-energy-physics era, but whether from the grand unification era or some later era is not known. Clearly, it is important to identify this non-baryonic dark matter, by direct searches and by accelerator experiments, with particle theory providing guidance to focus the experiments. Chief among the theoretical elementary-particle candidates for non-baryonic dark matter are weakly interacting massive particles (WIMPs), axions, and neutrinos with finite mass. Of these, only neutrinos are known to exist, but they are usually assumed to have zero mass. The experimental upper limit for the electron neutrino mass is about 7 electron-volts (eV; the mass of the electron is about 511,000 eV). A sea of primordial neutrinos with this mass would provide sufficient dark mass to make W = 1. However, neutrino dark matter would be hot and so does not work well by itself in computer simulations of the observed large-scale structure. Another class of phenomena from particle physics, called cosmic strings or textures, can be added to act as seeds for cosmic structure, or some cold dark matter (e.g., axions) can be added to make the results look more like the observations.

The most likely dark matter candidates from particle theory are added to a mix of ordinary matter and thermal radiation in a gigantic computer model that simulates the complex physics of an expanding universe that contains collapsing clumps of matter. These simulations are complex and difficult to do. The goal is to find a set of parameters and values that produces a simulation with clumps that have a large-scale structure much like that seen by astronomers. Another approach to identifying the dark matter particles is to search for them directly with techniques drawn from experimental particle physics.

WIMPs

There are good theoretical and experimental reasons to suspect that a new symmetry exists in nature, known as supersymmetry, which might enable gravity to be unified with the weak, electromagnetic, and strong forces. If supersymmetry exists, then every fundamental particle of ordinary matter and radiation has a supersymmetric partner particle, as yet undetected. The lightest supersymmetric particle cannot decay (because there are no lighter particles to decay into) and would therefore have survived from the time of the early universe until now. Such a particle's interactions with ordinary matter would be very weak, and current accelerator experiments tell us that the mass of the lightest supersymmetric particle is greater than 20 GeV (billion electron-volts; the mass of the proton is about 1 GeV)--massive for an elementary particle. Thus WIMPs make an ideal candidate for non-baryonic dark matter in the universe. They are imagined to be only weakly associated with luminous matter, for example, forming a loosely bound halo around our galaxy and others.

Laboratory detectors are now under construction in several countries to look for a flux of WIMPs with mass in the range from 5 to 100 GeV. Early results from conventional detectors have already set useful limits on the flux of WIMPs, and new efforts are starting based on entirely new cryogenic detectors. If WIMPs form a halo around our galaxy, then they constantly bombard Earth, but only rarely would a WIMP interact with an atom. Searches for WIMPs are conducted by measuring the recoil energy expected from the occasional collision between a WIMP and the nucleus of an atom in the detector. The experiments are extraordinarily difficult because the expected event rate for WIMP interactions is very low, somewhere between 0.001 to 1 event per kilogram of detector per day. Furthermore, the energy deposited in the detector for each event is small. However, the biggest problem in these experiments is the confusion generated by similar signals coming from natural radioactivity and cosmic rays. The experiments are therefore conducted deep underground to greatly reduce the cosmic ray flux and use extremely pure materials to minimize radioactive contamination. Like all direct searches for dark matter, these are high-risk experiments because of their technological challenges and because of the absence of precise predictions for the mass and the behavior of the WIMP candidates. But they are also experiments with potentially huge payoffs--understanding the missing mass and opening a new chapter in particle physics.

Axions

The axion is an unusual particle whose existence has been postulated for reasons related to charge-parity (CP) conservation, a symmetry of the strong interaction in elementary-particle physics. If axions actually exist, they do not behave like most particles, which move independently and randomly with different directions and energies. Instead, axions are expected to move coherently, behaving more like a slowly moving sea of particles. Theory allows only a narrow range of possible masses for the axion, near 10^-5 eV--the opposite extreme from the possible mass for WIMPs. Nevertheless, if the axion exists with this mass, its total cosmological mass density could still dominate the universe.

Dark matter axions could be detected based on the prediction that axions can change into photons in a strong magnetic field. If tuned to the proper frequency, a microwave cavity embedded in a strong magnetic field appears to spontaneously produce electromagnetic energy, or photons. Axion-induced oscillations would occur only in a narrow frequency range. By tuning the cavity to different frequencies a range of possible axion masses could be scanned. Two prototype detectors have been built, and results from these experiments are expected in the next few years. If the axion is detected, it will be a triumph of experimental ingenuity and the verification of a remarkable theoretical concept.

Neutrinos

In the early universe, neutrinos were as abundant as any other particle species. Although neutrinos ceased to interact significantly with other matter when the universe was only 1 second old, they have not disappeared, and today neutrinos are believed to make up a background sea of radiation similar to the CMBR. Because neutrinos are so abundant (100 per cubic centimeter, on average), they would dominate the mass density of the universe if they had even a little mass. Neutrino masses can be probed by accelerator experiments looking for one species of neutrino spontaneously changing into another species. The rate of transformation depends on the difference in mass between the species. Sensitive experiments of this sort are under way at the Fermi, Brookhaven, and Los Alamos national laboratories, and at CERN.

There is also an astrophysical method for measuring the masses of neutrinos. When a massive star explodes as a supernova, 99 percent of its energy is carried away by neutrinos. The neutrinos are predicted to be emitted in a brief, intense pulse. Measuring the amount that the pulse has spread out when it arrives at Earth allows estimation of limits on the masses of the neutrinos. This method was pioneered by two underground experiments that detected the neutrino pulse from the 1987 supernova in the Large Magellanic Cloud. A supernova in our own galaxy could provide enough data to make a much better measurement of neutrino mass if detectors were operating when it occurred. A future supernova closer to Earth might yield a sufficient flux of neutrinos of all species, so that estimates or improved limits on the masses of different species of neutrinos could be inferred. It is important that the detectors be ready when the next supernova in the Milky Way occurs, given that the previous one was recorded 400 years ago, and opportunities for observing such events occur only rarely.

Summary of the Study of the Early Universe

The success of the Big Bang theory of nucleosynthesis gives reason to hope that particle physicists and cosmologists can reach even farther back into the early universe with theories and experiments. The key tasks are to extend our knowledge of physics at the highest energies and to find self-consistent explanations of all of the phenomena astronomers see in the universe today. The abundances of the light elements, CMBR fluctuations, the composition and structure of matter, and the homogeneity and geometry of today's universe are examples of observable phenomena that have roots in the early universe. To study these and other relics of the Big Bang, astronomers and physicists use traditional optical and radio telescopes and particle accelerators. A wide variety of special-purpose instruments are also in use, such as underground dark matter detectors and small microwave radiometers on balloons and at the South Pole. The essential strategy comes mostly from theorists working at the boundary of particle physics, nuclear physics, and astrophysics-in the emerging field of particle astrophysics.

In the past decade much common ground has been found between the physics of the very small (elementary particles) and the physics of the very large (cosmology). The early universe offers the particle theorist the ultimate laboratory for testing exotic theories of unification and high-energy phenomena. The concepts of particle theory offer the cosmologist physical explanations for the origins of otherwise mysterious phenomena such as the fine-tuning problem (W » 1), the source of the fluctuations that gave rise to large-scale structure in the universe, and the nature of the dark matter. Identification of the missing dark matter and the testing of the concept of inflation are the major challenges ahead.