accelerating expansion of the universe. This expansion is calledinflation (see “Measuring the Cosmological Parameters” in section IV) and is supposed to have occurred in the first instantsafter the creation of the universe (see pp. 5-8). In a very shorttime (10−32 s), the early universe may have expanded by a greater factor thanit has in the billions of years since. Thus, inflation is intimatelyconnected 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 greaterthan the distance light has traveled in the time since the Big Bangare effectively disconnected from each other. In traditional, non-inflationarymodels, there is no reason for such regions to be similar. For example,since disconnected regions can never have exchanged energy, why shouldthey be at exactly the same temperature? Energy exchange betweennearby regions could result in small patches with uniform temperature,but CMBR measurements tell us that large regions are nearly equalin temperature. Because inflation can quickly expand an extremelytiny volume into a vastly larger region of space, it would allowa 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 bumpinesseven in the uniform patches, because quantum mechanics and the uncertaintyprinciple require it. Inflation magnifies these tiny fluctuationsinto the CMBR anisotropy that astronomers see today and the large-scalestructure of matter in the universe. Indeed, the variation in amplitudeof fluctuations of different angular size is consistent with theexpectations of the inflationary model. Inflation takes microscopicquantum noise and blows it up to create the seeds of galaxies andlarge-scale structure.
A third advantage of inflation is that it forces the spatial curvatureof the universe to be negligibly small on a cosmological scale sothat space is flat (i.e., euclidean geometry applies). This flatnessis a direct consequence of the tremendous expansion expected duringinflation. A small closed surface such as a balloon has an obviouscurvature, but if expanded to the size of Earth, its curvature ismuch less apparent. The absence of curvature in an inflationary universewould imply that today, the density parameter Ω should be close tounity. Thus, an inflationary phase in the early universe naturallysolves the fine-tuning problem mentioned above.
The panel emphasizes that inflation is an idea, not a complete orwell-tested physical theory. In addition to the original versiondescribed here, many different variants have been presented, somewith inflation occurring during the quantum gravity era, others withinflation occurring much later, each driven by different mechanisms.Although our understanding of particle physics is incomplete at theseenergies, and we have no understanding of the details of the inflationaryepoch, inflation is an attractive concept because of its abilityto resolve several long-standing cosmological conundrums. Many cosmologistsare convinced that such an episode must have occurred.
Particle Theory and Dark Matter Candidates
Astronomers have found strong evidence for a major dark matter componentof the universe; the visible matter does not add up to the totalamount of matter measured by dynamical means. Could the dark matterbe ordinary baryonic matter in a form that doesn't shine—perhaps brown dwarfstars, black holes, or hot intergalactic gas? Apparently not, accordingto the calculations of primordial nucleosynthesis, which work onlyif the density of baryons is less than 0.1 of the critical value(ΩB < 0.1). Thus the bulk of the dark matter must be composed of an unknownform of matter. What could it be? Particle physics has some candidatesthat are discussed below, and the astronomical behavior of dark matteroffers some clues. Observations show that the dark matter is muchless clumped than the visible matter. Therefore, the two kinds mustinteract
only weakly, mainly via the gravitational force. Computer simulationsof the formation of large-scale structure also provide valuable informationabout the behavior of the dark matter, which plays an important rolein shaping the structure. These studies show that the non-baryonicdark matter candidates can be divided into two categories dependingon the velocity with which the particles were moving when the universebecame dominated by matter (see pp. 5-8). During this epoch, a rapidlymoving particle (e.g., because its mass is small) is considered hotdark matter; a slowly moving particle is considered cold dark matter.Currently, the cold dark matter candidates, or a mixture of hot andcold, give the best agreement between computer simulation resultsand the observed large-scale structure.
Most cosmologists believe that the unknown matter needed to explainthe “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-physicsera, but whether from the grand unification era or some later erais not known. Clearly, it is important to identify this non-baryonicdark matter, by direct searches and by accelerator experiments, withparticle theory providing guidance to focus the experiments. Chiefamong the theoretical elementary-particle candidates for non-baryonicdark matter are weakly interacting massive particles (WIMPs), axions,and neutrinos with finite mass. Of these, only neutrinos are knownto exist, but they are usually assumed to have zero mass. The experimentalupper 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 primordialneutrinos with this mass would provide sufficient dark mass to makeΩ = 1. However, neutrino dark matter would be hot and so does notwork well by itself in computer simulations of the observed large-scalestructure. Another class of phenomena from particle physics, calledcosmic strings or textures, can be added to act as seeds for cosmicstructure, or some cold dark matter (e.g., axions) can be added tomake the results look more like the observations.
The most likely dark matter candidates from particle theory are addedto a mix of ordinary matter and thermal radiation in a gigantic computermodel that simulates the complex physics of an expanding universethat contains collapsing clumps of matter. These simulations arecomplex and difficult to do. The goal is to find a set of parametersand values that produces a simulation with clumps that have a large-scalestructure much like that seen by astronomers. Another approach toidentifying the dark matter particles is to search for them directlywith techniques drawn from experimental particle physics.
There are good theoretical and experimental reasons to suspect thata new symmetry exists in nature, known as supersymmetry, which mightenable gravity to be unified with the weak, electromagnetic, andstrong forces. If supersymmetry exists, then every fundamental particleof 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 wouldtherefore have survived from the time of the early universe untilnow. Such a particle's interactions with ordinary matter would bevery weak, and current accelerator experiments tell us that the massof the lightest supersymmetric particle is greater than 20 GeV (billionelectron-volts; the mass of the proton is about 1 GeV)—massive foran elementary particle. Thus WIMPs make an ideal candidate for non-baryonicdark matter in the universe. They are imagined to be only weaklyassociated with luminous matter, for example, forming a loosely boundhalo around our galaxy and others.
Laboratory detectors are now under construction in several countriesto look for a flux of WIMPs with mass in the range from 5 to 100GeV. Early results from conventional detectors have already set usefullimits on the flux of WIMPs, and new efforts are starting
based on entirely new cryogenic detectors. If WIMPs form a halo aroundour galaxy, then they constantly bombard Earth, but only rarely woulda WIMP interact with an atom. Searches for WIMPs are conducted bymeasuring the recoil energy expected from the occasional collisionbetween a WIMP and the nucleus of an atom in the detector. The experimentsare extraordinarily difficult because the expected event rate forWIMP interactions is very low, somewhere between 0.001 to 1 eventper kilogram of detector per day. Furthermore, the energy depositedin the detector for each event is small. However, the biggest problemin these experiments is the confusion generated by similar signalscoming from natural radioactivity and cosmic rays. The experimentsare therefore conducted deep underground to greatly reduce the cosmicray flux and use extremely pure materials to minimize radioactivecontamination. Like all direct searches for dark matter, these arehigh-risk experiments because of their technological challenges andbecause of the absence of precise predictions for the mass and thebehavior of the WIMP candidates. But they are also experiments withpotentially huge payoffs—understanding the missing mass and openinga new chapter in particle physics.
The axion is an unusual particle whose existence has been postulatedfor reasons related to charge-parity (CP) conservation, a symmetryof the strong interaction in elementary-particle physics. If axionsactually exist, they do not behave like most particles, which moveindependently and randomly with different directions and energies.Instead, axions are expected to move coherently, behaving more likea slowly moving sea of particles. Theory allows only a narrow rangeof 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 densitycould still dominate the universe.
Dark matter axions could be detected based on the prediction thataxions can change into photons in a strong magnetic field. If tunedto the proper frequency, a microwave cavity embedded in a strongmagnetic field appears to spontaneously produce electromagnetic energy,or photons. Axion-induced oscillations would occur only in a narrowfrequency range. By tuning the cavity to different frequencies arange of possible axion masses could be scanned. Two prototype detectorshave been built, and results from these experiments are expectedin the next few years. If the axion is detected, it will be a triumphof experimental ingenuity and the verification of a remarkable theoreticalconcept.
In the early universe, neutrinos were as abundant as any other particlespecies. Although neutrinos ceased to interact significantly withother matter when the universe was only 1 second old, they have notdisappeared, and today neutrinos are believed to make up a backgroundsea of radiation similar to the CMBR. Because neutrinos are so abundant(100 per cubic centimeter, on average), they would dominate the massdensity of the universe if they had even a little mass. Neutrinomasses can be probed by accelerator experiments looking for one speciesof neutrino spontaneously changing into another species. The rateof 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 ofneutrinos. When a massive star explodes as a supernova, 99 percentof its energy is carried away by neutrinos. The neutrinos are predictedto be emitted in a brief, intense pulse. Measuring the amount thatthe pulse has spread out when it arrives at Earth allows estimationof limits on the masses of the neutrinos. This method was pioneeredby two underground experiments that detected the neutrino pulse from the 1987
supernova in the Large Magellanic Cloud. A supernova in our own galaxycould provide enough data to make a much better measurement of neutrinomass if detectors were operating when it occurred. A future supernovacloser to Earth might yield a sufficient flux of neutrinos of allspecies, so that estimates or improved limits on the masses of differentspecies of neutrinos could be inferred. It is important that thedetectors be ready when the next supernova in the Milky Way occurs,given that the previous one was recorded 400 years ago, and opportunitiesfor observing such events occur only rarely.
Summary of the Study of the Early Universe
The success of the Big Bang theory of nucleosynthesis gives reasonto hope that particle physicists and cosmologists can reach evenfarther back into the early universe with theories and experiments.The key tasks are to extend our knowledge of physics at the highestenergies and to find self-consistent explanations of all of the phenomenaastronomers see in the universe today. The abundances of the lightelements, CMBR fluctuations, the composition and structure of matter,and the homogeneity and geometry of today's universe are examplesof observable phenomena that have roots in the early universe. Tostudy these and other relics of the Big Bang, astronomers and physicistsuse traditional optical and radio telescopes and particle accelerators.A wide variety of special-purpose instruments are also in use, suchas underground dark matter detectors and small microwave radiometerson balloons and at the South Pole. The essential strategy comes mostlyfrom theorists working at the boundary of particle physics, nuclearphysics, and astrophysics—in the emerging field of particle astrophysics.
In the past decade much common ground has been found between thephysics of the very small (elementary particles) and the physicsof the very large (cosmology). The early universe offers the particletheorist the ultimate laboratory for testing exotic theories of unificationand high-energy phenomena. The concepts of particle theory offerthe cosmologist physical explanations for the origins of otherwisemysterious phenomena such as the fine-tuning problem (Ω ≈ 1), the sourceof the fluctuations that gave rise to large-scale structure in theuniverse, and the nature of the dark matter. Identification of themissing dark matter and the testing of the concept of inflation arethe major challenges ahead.
Cosmologists work to better understand the contents, structure, andevolution of the universe over vast stretches of space and time andover enormous ranges of density, temperature, and energy. Theorists,observers, and experimenters use a diverse assortment of techniquesand instruments to answer questions of the most fundamental kind.Progress over the past three decades, but especially since the maturingof space science, has been astonishing. Recent observational resultsfrom the COBE satellite have revolutionized how cosmologists thinkabout structure in the universe, and current efforts to map the fluctuationsin the CMBR on smaller angular scales promise to show us detailsof the thermal history of the universe and to measure its fundamentalparameters. Observations of large-scale structure in the universeas measured by the galaxy distribution have made dramatic progressin recent years, challenging all available theoretical models. Ourknowledge of large-scale flows is still incomplete, but, when thework is finished, cosmologists will be much closer to understandingthe role of dark matter in the universe. The new generation of gianttelescopes such as the 10-m Keck are making remarkable observations,and the Keck, along with its newly constructed twin and the two 8-mGemini telescopes, will enable study of the distant universe in waysnever before conceivable. The HST has already made seminal contributions,including a measurement of the Hubble constant that gives the universea surprisingly young age. We expect dramatic advances in our understandingof galaxy and large-scale structure evolution, as well as new testsof global curvature, to emerge from these studies in the coming decade.We still have no clear idea of the nature of the ubiquitous darkmatter, but it is almost certainly a remnant of the early universe.Several experiments are under way to detect, or eliminate, candidatesfor this dark matter, and these will make substantial progress inthe coming decade.
The problems of cosmology are particularly well posed, but many ofthe solutions have remained elusive for decades. At last, with theaccelerating influx of new data from numerous advanced experimentaland theoretical techniques, fundamental questions about the natureof our physical world are beginning to be answered. Cosmologistseagerly await the exciting new insights that will surely come inthe next decade. If another briefing on cosmology is written 10 yearsfrom now, it will undoubtedly bear little resemblance to this one.
The cosmological constant, which measures the energy density of avacuum. Ω: The ratio of the average total density of matter to thecritical density required to close the universe and eventually stopits expansion. Ω can be broken down into its components accordingto the type of matter involved.
The ratio of the mean density of baryons to the critical densityrequired to close the universe.
Variation with direction.
A hypothetical elementary particle whose existence might explaincertain particle physics experiments. A candidate for cold dark matter.
A massive, strongly interacting elementary particle, such as a protonor a neutron.
Ordinary matter as we know it consists largely of baryons, as opposedto hypothetical matter that might theoretically exist. Both kindshave mass, and either kind can be dark matter.
Big Bang theory:
The theory that the universe began with all matter and energy concentratedto very high density and temperature some 15 billion years ago. Thepresent universe expanded from that epoch and is still expanding.In the hot Big Bang theory, the ratio of photons to atoms is large,say ~109, as astronomers now observe.
An object that has become so dense that, through the effects of generalrelativity, its contents are no longer accessible to the outsideuniverse. No light from the surface can escape to the outside, hencethe term “black.”
A hypothetical star not sufficiently massive to ignite hydrogen burning,with mass less than 10 percent of the mass of the Sun.
Charge-coupled device. An electronic image detector used in modernvideo cameras and astronomical instruments.
The European Laboratory for Particle Physics.
A universe expanding slowly enough to be braked by gravity. In thiscase, Ω ≥ 1. If Ω > 1 (above critical density), the universe will eventually recollapse.If Ω = 1 (at critical density), the universe will expand forever,but ever more slowly.
An assemblage of many galaxies.
Cosmic microwave background radiation.
The Cosmic Background Explorer satellite.
A product of a primordial physical process. The cosmic microwavebackground radiation is a cosmic remnant.
Cosmic velocity flow:
Alterations in the regular movement of celestial objects away fromeach other caused by the gravitational attraction of other nearbyobjects. These flows give an indication of total mass, both luminousand nonluminous.
The study of the contents, structure, and evolution of the universefrom the beginning of time to the infinite future.
Charge-parity conservation. The symmetry of properties under a reflectionin space and reversal of charge.
The density of matter that would just halt the expansion of the universe.The dividing line between a collapsing and an ever-expanding universe.
Matter that does not emit enough light or other radiation to be observeddirectly. Most of the matter in the universe is believed to be ofthis type. Cold dark matter had a low velocity compared to the speedof light during the epoch of recombination. An example would be elementaryparticles with mass about equal to that of a proton or higher. Hotdark matter had a high velocity (near the speed of light) duringthe epoch of recombination. An example would be light elementaryparticles.
A period characterized by the dominance of a particular physicalprocess, such as the formation of the light elements from protonsand neutrons.
Epoch of photon decoupling:
See epoch of recombination.
Epoch of recombination:
The time when electrons and nuclei were combining to form atoms andthe universe was 1,600 times smaller than its present size. Alsocalled the epoch of photon decoupling and the epoch of atom formation.
An electron-volt, a measure of energy equal to that gained by anelectron passing through a potential difference of 1 volt. Also aunit of particle mass. Electrons have a mass of about 0.511 MeV (millionelectron-volts); protons have a mass of about 938 GeV (billion electron-volts).
A universe where space is euclidean (zero curvature). If Λ = 0, a flat universe has Ω = 1. If Λ is non-zero, then Λ + Ω = 1.
Forces of nature:
The four basic forces of physics: gravity, electromagnetism, andthe weak and strong interactions.
The disequilibrium by which relics are formed in the universe.
A large assemblage of stars. Our own galaxy, the Milky Way, contains1011 stars.
Grand unification era:
The era when the universe cooled sufficiently for gravity to be describedby Einstein's general relativity theory, but where the temperaturewas still sufficiently high that the other remaining three forcesof nature remained unified.
Grand unified theories:
Theories that combine the strong, electromagnetic, and weak interactionsinto one unified theory.
The process whereby a small lump in an expanding universe can growunder gravity, pulling in surrounding matter and ultimately collapsingto form an object like a galaxy or cluster of galaxies.
A celestial object that distorts the image of another object behindit by virtue of the fact that its gravity affects the propagationof the light from the background object.
The Hubble constant. A measure of the expansion rate and age of theuniverse.
A strongly interacting particle such as a proton or neutron.
The matter surrounding a galaxy.
Edge of the portion of the universe visible to us. Light signalsfrom beyond this point have not had time to reach Earth yet.
Hubble Space Telescope.
The principle that any two distant celestial objects (e.g., galaxies)move away from each other at a speed that is proportional to thedistance between them, due to the homogenous expansion of space.
A rapid expansion appearing as an early phase in some cosmologicalmodels, which solves several problems of cosmology.
Light of wavelength longer than the reddest part of the visible spectrum.
The material between galaxies.
Under terrestrial conditions, most matter has an equal amount ofpositive and negative charge, so that its net charge is zero. Athigh temperatures, the charges separate in a process called ionization.
The NASA, British, and Dutch Infrared Astronomy Satellite, whichwas flown in 1983.
The two new, state-of-the-art ground-based 10-meter optical telescopeslocated on Mauna Kea, Hawaii.
The Large Electron-Positron Collider. A particle accelerator at CERN.
A class of elementary particles including electrons, muons, and tauons.
If a small, dark body is directly in the line of sight to a brightbackground star, the brightness of the background star may appearto increase because of bending of the light rays by the dark body.
Very light (possibly massless) particles that are emitted in theprocess of radioactive decay. There are three species, associatedwith electrons, muons, and tau-leptons. They interact with ordinarymatter through the weak force.
The process by which the elements are built up from protons and neutrons.
A universe expanding faster than the retarding pull of gravity. Ithas less than critical density, Ω < 1, and expands forever.
The period of time immediately after the creation of the universe(10−43 s) during which quantum gravity is the principal phenomenon governingthe evolution of the universe.
The deceleration rate for the expansion of the universe, which isrelated to the curvature of space.
The elementary constituents of hadrons or baryons (such as protonsand neutrons).
An object with a large redshift and an inferred luminosity oftenhundreds of times that of a normal galaxy.
The shifting of light toward the red end of the spectrum that occurswhen the observed light source is receding from the observer.
Space Infrared Telescope Facility. A proposed orbiting infrared telescope.
A celestial object whose intrinsic brightness is known or can beestimated by some physical principle and whose observed brightnessis therefore useful as a tool to measure distance.
A space-time symmetry that would imply the existence of partnersto all elementary particles, with quantum spins of one-half a unithigher or lower. Often used in constructing theories that unify gravitywith the three other forces.
The characteristic distribution of radiation as a function of frequencythat is emitted by a body at a well-defined temperature, also calleda black-body spectrum.
A method to determine galactic distances. Big, luminous galaxiesrotate faster than small, faint ones. The connection between thetwo is given by the Tully-Fisher relation.
The concept that two or more forces that seem distinct in today's universe could, at higher energies (or temperature), merge to becomeone force.
All of space and time taken together.
The Very Large Array. An array of 27 radio telescopes in New Mexico,capable of adjustable spacing along a Y-shaped track, up to a radiusof 27 km.
Very Long Baseline Array. A newly completed radio interferometeroperated by the National Radio Astronomy Observatory. Capable ofproducing images with angular resolution of one thousandth of a secondof arc.
The interactions of elementary particles that are responsible forradioactive decay.
Weakly Interacting Massive Particle. A candidate for dark matter.