1. 1998, A MEMORABLE YEAR FOR COSMOLOGY

The birth of the hot big-bang model dates back to the work of Gamow and his collaborators in the 1940s. The emergence of the hot big-bang as the standard cosmology began in the late 1960s, with the discovery of the microwave background radiation, the establishment of its black-body spectrum, and the success of big-bang nucleosynthesis. By the 1970s, the hot big-bang was being referred to as the standard cosmology. Today, it is well established and provides an accounting of the Universe from a fraction of a second after the beginning when the Universe was a hot, smooth soup of quarks and leptons to the present, some 13 Gyr later. The standard cosmology rests upon three strong observational pillars: the expansion of the Universe; the cosmic microwave background radiation (CBR); and the abundance pattern of the light elements, D, ³He, ⁴He, and ⁷Li, produced seconds after the bang (see e.g., Peebles et al, 1991).

The standard cosmology leaves fundamental questions unexplained: the matter/antimatter asymmetry, the origin of the smoothness and flatness of the Universe, the nature and origin of the primeval density inhomogeneities that seeded all the structure in the Universe, the quantity and composition of the dark matter that holds the Universe together, and the nature of the big-bang event itself. This has motivated the search for a more expansive cosmological theory.

In the 1980s, a new paradigm emerged, deeply rooted in fundamental physics with the potential to extend our understanding of the Universe back to 10^-32 sec and to address the fundamental questions poised by the hot big-bang model. That paradigm, known as inflation + cold dark matter, holds that most of the dark matter consists of slowly moving elementary particles (cold dark matter), 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 awhile for the observers and experimentalists to take this theory seriously enough to try to disprove it, and in the 1990s it began to be tested in a serious way.

1998 could prove to be a watershed year in cosmology, as important as 1964, when the CBR was discovered. The crucial new data include a precision measurement of the density of ordinary matter and of the total amount of matter, both derived from a measurement of the primeval deuterium abundance and the theory of BBN; and the first fine-scale (down to 0.3°) measurements of the anisotropy of the CBR; and a measurement of the deceleration of the Universe based upon distance measurements of type Ia supernovae (SNe1a) out to redshift of close to unity. Together, these measurements, which are harbingers for the precision era of cosmology that is coming, provide the first plausible, complete accounting of the matter/energy density in the Universe and evidence that the primeval density perturbations arose from quantum fluctuations during inflation. In addition, there exists a body of evidence in support of the cold dark matter theory of structure formation.

The accounting of matter and energy goes like this (in units of the critical density): light neutrinos, at least 0.3%; bright stars and related material, 0.5%; baryons, 5%; cold dark matter, 35%; and vacuum energy (or something similar), 60%; for a total equalling the critical density (see Fig. 1). The recently measured primeval deuterium abundance (Burles & Tytler, 1998) and the theory of big-bang nucleosynthesis accurately determine the baryon density (Schramm & Turner, 1998), _B = (0.02 ± 0.002)h^-2 0.05 (for h = 0.65). Using the cluster baryon fraction, determined from x-ray measurements, f_B = M_baryon / M_TOT = 0.07 ± 0.007 (Evrard, 1996), and assuming that clusters provide a fair sample of matter in the Universe, _B / _M = f_B, it follows that _M = (0.3 ± 0.05)h^-1/2 0.4 ± 0.1. That _M >> _B is strong evidence for nonbaryonic dark matter; the leading candidates are axions, neutralinos and neutrinos.

Figure 1. Summary of matter/energy in the Universe. The right side refers to an overall accounting of matter and energy; the left refers to the composition of the matter component. The upper limit to mass density contributed by neutrinos is based upon the failure of the hot dark matter model and the lower limit follows from the evidence for neutrino oscillations (Fukuda et al, 1998).

The position of the first acoustic peak in the angular power spectrum of temperature fluctuations of the CBR is a sensitive indicator of the curvature of the Universe: l_peak 200 / ₀, where R_curv² = H₀^-2 / |₀ - 1|. Measurements now span multipole number l = 2 to around l = 1000 (see Fig. 2); while the data do not yet speak definitively, it is clear that ₀ ~ 1 is preferred. Several experiments have new results around l = 30 - 300, and should be reporting them soon. Ultimately, the MAP (launch in 2000) and Planck (launch in 2007) satellites will cover l = 2 to l = 3000 with precision limited essentially by sampling variance, and should determine ₀ to a precision of 1% or better.

Figure 2. Summary of current CBR anisotropy measurements, where the temperature variation across the sky has been expanded in spherical harmonics, T(, ) = i alm Ylm and Cl <|alm|2>. The curves illustrate CDM models with 0 = 1 and 0 = 0.3. Note the preference of the data for a flat Universe (Figure courtesy of M. Tegmark).

The same angular power spectrum that indicates ₀ ~ 1 also provides evidence that the primeval density perturbations are of the kind predicted by inflation. The inflation-produced Gaussian curvature fluctuations lead to an angular power spectrum with a series of well defined acoustic peaks. While the data at best define the first peak, they are good enough to exclude models where the density perturbations are isocurvature (e.g., cosmic strings and textures): in these models the predicted spectrum is devoid of acoustic peaks (Allen et al, 1997; Pen et al, 1997).

The oldest approach to determining ₀ is by measuring the deceleration of the expansion. Sandage's deceleration parameter, q₀ - ( / R) / H₀² = ₀ / 2[1 + 3p / ], depends upon both ₀ and the equation of state. Accurate measurements of the (luminosity) distance as a function of redshift allow the deceleration to be determined. Accurate distant measurements to some fifty or so SNe1a, with redshifts as large as one, carried out by two groups (Riess et al, 1998; Perlmutter et al, 1998) indicate that the Universe is speeding up, not slowing down (i.e., q₀ < 0). The simplest explanation is a cosmological constant, with ~ 0.6. This result fits neatly with the CBR determination that ₀ = 1 and dynamical measures that indicate _M ~ 0.4: the "missing energy" exists in a smooth component that cannot clump and thus is not found in clusters of galaxies.

While the evidence for inflation + cold dark matter is not definitive and we should be cautious, 1998 could well mark a turning point in cosmology as important as 1964. Recall, after the discovery of the CBR it took a decade or more to firmly establish the cosmological origin of the CBR and the hot big-bang cosmology as the standard cosmology.