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11 Intro cluct ion The Stanciarc! Moclel The recent renaissance in the development of cosmology has oc- curred mainly within the context of the hot big-bang models, whose governing gravitational equations were derived more than 50 years ago. These are the models currently employed by most cosmologists be- cause they are the simplest and most natural ones in accord with the observations. For example, big-bang models are compatible with (1) the isotropy of radiation backgrounds and galaxy counts, (2) the galaxy redshift-distance relation, (3) the observed ages of the oldest stars and meteorites, (4) the cosmic microwave background radiation tempera- ture of the universe (~3 K), (5) the present mean density of matter, (6) the rate of expansion and deceleration of the universe, and (7) the abundance of primordial elements. The renaissance in cosmology was sparked by the realization that the microwave background radiation and the light-element abundances are remnants of a hot big bang, but it has been driven by the successful application of a broad range of observations and theory to difficult cosmological problems. Currently, revolutionary ideas concerning the relationship between microscopic physics and the large-scale structure and evolution of the universe are being actively studied and tested. Figure 11.1 shows the past history of the universe, according to a standard big-bang model and including recent ideas from particle physics. The contribution of the various particles to the mass-energy density is plotted versus time since the extrapolated epoch of infinite 87

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88 COSMOLOGY lo - as 40~35s AG E BIG BANG COSMOLOGY ~ o-42s ~ o2s ~ 05yr 104y r 11 ' ~ , NOW ,~ NAG RAVITY ,----STRONG E E ~ o80- - z ~o4Q c) I1J He LLJ 1 ~ 40-4 Rodiat~on EA K Dominated cat. l x ? ~ ~ ELECTROMAGNETIC ~o45- ~ odo- ~ ~o5- ~ _~_ mace _m~~c2 me c2 - ~ s .t W.! In In I`J ~ Z o J at ~1 I Matter Dominated E In 1; . 0,: .. m do-25 l ~0 20 ~0-~5 to SO- 5 SCA L E FACTOR ( ~ + ~ )~4 - ~o29 _ Y - lo24 -4O49 -4044 IL -409 404 Pc FIGURE 11.1 The history of the universe is depicted here in terms of the mass-energy density of the different types of particles that were present at various epochs. At early times, many types of elementary particles (X, W. Z. quarks, gluons, it, a, e, fly, v, . . . ) existed. Most disappeared because of particle-antiparticle annihilation when the particles' kinetic energies became less than their rest-mass energy. Neutrons (n) and protons (p) were produced from quarks at about 10-5 second, and light nuclei were produced from nucleons at about 10~ seconds. The three barriers indicate the epoch beyond which we cannot '~see" via each of the three types of particles. In addition, at energies higher than that available at accelerators we have little direct knowledge of the laws of physics (hence, the dashed curves). density. It is this mass-energy density that controls the expansion rate in the standard model, so the distance scale factor (z + 11* can also be shown. Most of what we know about the universe comes from astronomical observations of optical and radio sources at the extreme right-hand edge of Figure 11.1, between the present and a scale factor of z + 1 = 2, corresponding to a time when the universe was about one half its present age. * z = cosmological redshift = [A(source) - A(lab)]/A(lab). So, the distance scale, which is proportional to wavelength, changes as z + 1.

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INTROD UCTIONTHE STANDARD MODEL 89 In the hot big-bang model the 3-K microwave background is a remnant of primordial blackbody radiation. However, the radiation is strongly scattered before the photon barrier at z ~ 103 because at earlier times the radiation temperature was high enough to ionize hydrogen and permit Thomson scattering. This strong coupling of radiation and matter before the photon barrier also tended to keep the matter from clumping into stars, galaxies, or larger systems bound by gravity. Decoupling of the matter and radiation allowed the process of galaxy formation to begin, leading eventually to the complex large- scale structure seen today. Since the 3-K photons were last scattered at the photon barrier (unless matter is reionized), their current prop- erties carry the imprint of this epoch (z ~ 103, T ~ 104 K, age = t ~ 105 years). Note the tremendous range of physical conditions that the model encompasses, with densities reaching 1094 g/cm3 at the Planck era where the unknown laws of quantum gravity prevail. The bold exten- sion of our present knowledge of physics into the early universe represents the greatest extrapolation in all of science. However, this extrapolation provides a unique opportunity to derive observable consequences from the laws of physics that we imagine to operate under such conditions. As we shall indicate below, relic particles (produced in the early universe and still present today) may provide the key, or perhaps the spectrum of residual density fluctuations will be our deepest probe. Even more likely, the observational breakthrough to the particle-physics era will come in some entirely unanticipated way; new ideas are currently appearing at a rapid rate.