In addition to providing an account of events from a fraction of a second (the time of big-bang nucleosynthesis) to the present, the hot big-bang cosmology, supplemented by the standard model of particle physics and other advances in our understanding of the fundamental particles and their interactions, provides a firm foundation for speculations about much earlier times, back to 10-43 sec after the beginning (earlier, the quantum nature of gravity and possibly space-time itself must be considered).
These speculations involve a crucial connection between the inner space of elementary particles and the outer space of cosmology. That connection is simple: when the Universe was young, it was a hot soup of the fundamental particles of nature, quarks, antiquarks, electrons, positrons, neutrinos, antineutrinos, photons, gluons, and other particles. To understand the earliest moments of creation, one has to understand the fundamental particles and how they interact with one another. The highly successful, standard model of particle physics provides the information needed to take us back to about 10-11 sec; ideas about how the forces and particles are unified (e.g., supersymmetry, grand unification and superstring theory) are needed to discuss the Universe at even earlier times.
The duality of inner space/outer space connection is also worth noting: The quark soup of the early Universe can be recreated at particle accelerators by colliding high-energy particles together; the early Universe, with its sea of extremely energetic particles that are constantly colliding, can be used to study the forces and particles at energies beyond the reach of terrestrial accelerators (see e.g., Kolb & Turner 1990). Motivated by interesting and sometimes compelling speculations about fundamental physics and the unification of the forces of nature, the past fifteen years have seen much discussion of the earliest history of the Universe. These cosmological speculations have allowed cosmologists to address the most fundamental questions they face; conversely, cosmology has given particle physicists access to a new laboratory with virtually unlimited energy.
The most compelling idea to arise from the synthesis of elementary particle physics with cosmology is Inflation + Cold Dark Matter (Guth 1982; Blumenthal et al. 1984). It is an expansive paradigm, deeply rooted in fundamental physics, and it has the potential to extend our understanding of the Universe back to 10-32 sec and to address most of the fundamental questions poised by the hot big-bang model. Inflation + Cold Dark Matter holds that most of the dark matter consists of slowly moving elementary particles; that the Universe is flat; and that the density perturbations that seeded all the structure seen today arose from quantum mechanical fluctuations on scales of 10-23 cm or smaller. It took a while for cosmological observers and experimentalists to take this paradigm seriously enough to try to disprove it; now, in the 1990s, it is being tested in a serious way - and is passing the tests with flying colors. (David Schramm played a very crucial role in this regard - he urged the more conservative observers to take these new ideas seriously, and with equal fervor, he urged particle cosmologists to make testable predictions.) My thesis for this debate is that the first evidence supporting the fundamental tenets of Inflation + Cold Dark Matter have been presented this year.
The key feature of inflation - and the one responsible for its name - is the tremendous burst of expansion: In 10-32 sec the Universe grew in size by a factor greater than it has since! I will not discuss the details of what caused this burst of expansion; it suffices to say that it is related to vacuum energy. This tremendous growth in the size of the Universe means that all we can see today originated from an extraordinarily tiny bit of the whole Universe. A tiny bit of any space appears smooth and flat - take the earth for example - and this leads to the first key prediction of inflation: the Universe should appear flat and thus the total energy density should equal the critical density. Further, it explains the large-scale regularity seen today. (The subsequent expansion of the Universe does not change the curvature or regularity; to be very precise, inflation does not predict an exactly flat Universe, and only predicts that a region much, much larger than the observable Universe is smooth.)
The quantum world of subatomic particles is in constant turmoil, fluctuating and changing. Because we do not live in the subatomic world, we are unaware of these quantum fluctuations. However, the extraordinary burst of expansion stretches quantum fluctuations to astrophysical scales, making them relevant for the Universe. And, in a well defined way, this quantum turmoil leads to the primeval lumpiness in the distribution of matter in the Universe. The quantum-born, inflation-produced fluctuations are of a form known as Gaussian scale-invariant curvature fluctuations (Guth & Pi 1982; Hawking 1983; Starobinskii 1982; Bardeen et al. 1983). Inflation-produced, quantum fluctuations in space time itself lead to a relic background of gravitational waves that are an additional ``smoking gun'' signature of inflation. On to the cold dark matter part. Inflation predicts a flat Universe; that is, total energy density equal to the critical energy density. Inflation does not predict what form or forms the critical density takes; we must rely upon astrophysical clues and measurements. Since ordinary matter (baryons) only contributes about 5% of the critical density, and there is good evidence that the total amount of matter is 40% of the critical density, there must be another form of matter in addition to baryons. The leading possibility is elementary particles left over from the earliest, fiery moments. Because of the high temperatures that existed early on, the full zoo of elementary particles was represented. Of interest for cosmology, are particles that are long lived or stable, and interact sufficiently weakly so that they would not have annihilated by the present. Generically, they fall into two classes - fast moving, or hot dark matter; and slowly moving, or cold dark matter. Neutrinos are the prime example of hot dark matter - they move quickly because they are very light. Axions and neutralinos are examples of cold dark matter. Neutralinos move slowly because they are very heavy (fifty to five hundred times the mass of a proton) Axions are extremely light (one millionth of a millionth the mass of an electron), but were produced in a very, very cold state (Bose - Einstein condensate). Both the axion and neutralino are as of yet hypothetical particles: they are predicted by theories that unify the forces and particles of Nature, but they are not yet ruled in or ruled out by experiment.
Motivated by since-refuted experimental evidence that neutrinos have enough mass to account for the critical density, hot dark matter was carefully studied in the 1980s and found wanting (White et al. 1983). With hot dark matter structure in the Universe forms from the top down: large things, like superclusters form first, and then fragment into smaller objects such as galaxies. This is because fast moving neutrinos erase lumpiness on small scales by moving from regions of greater density into regions of lower density. Observations now very clearly indicate that galaxies formed at redshifts z ~ 2-4, clusters formed at redshifts z ~ 0-1, and superclusters are just forming today. So neutrinos are out, at least as the major component of the dark matter. This leaves cold dark matter. (While apparently not a major ingredient in the cosmic mix, neutrinos may play the role of a needed cosmic spice; as discussed earlier, there is now experimental evidence that at least two of the neutrino species are massive.)
Cold dark matter particles cannot move far enough to smooth out lumpiness on small scales, and so structure forms from the bottom up: galaxies, followed by clusters of galaxies, and so on. The bulk of galaxies should form around redshifts of 2-4, followed by clusters at redshifts 0-1 and superclusters today. This is just what the observations of the young Universe made by the Keck 10-meter telescopes and the Hubble Space Telescope indicate.
Figure 4. Constraints used to determine the best-fit CDM model: PS = large-scale structure + CBR anisotropy; AGE = age of the Universe; CBF = cluster-baryon fraction; and H0 = Hubble constant measurements. The best-fit model, indicated by the darkest region, has H0 60-65 km s-1 Mpc-1 and 0.55-0.65. Evidence for its smoking gun signature - accelerated expansion - was presented in 1998 by Perlmutter et al. and Riess et al.
The cold dark matter model with a cosmological constant, referred to as CDM by the experts, is consistent with an enormous body of cosmological and astrophysical data, from the determinations of the age of the Universe to the pattern of hot and cold spots in the CMB (see Figs. 4, 5, and Turner 1997; Krauss & Turner 1995; Ostriker & Steinhardt 1995). And now, its dramatic prediction, that the Universe should be speeding up rather than slowing down, has been verified (Riess et al. 1998; Perlmutter et al. 1998). In CDM the dark energy exists in the form of spatially constant vacuum energy (Einstein's cosmological constant). It accounts for 60% of the critical energy density, but plays no direct role in the formation of cosmic structure because it cannot clump.
Figure 5. The same data in Fig. 1, but averaged and binned to reduce error bars and visual confusion. The theoretical curve is for the CDM model with H0 = 65 km s-1 Mpc-1 and M = 0.4 (Figure courtesy of L. Knox).