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,
fB = Mbaryon /
MTOT = 0.07 ± 0.007
(Evrard, 1996),
and assuming that clusters provide
a fair sample of matter in the Universe,
B /
M =
fB, 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:
lpeak
200 /
0, where
Rcurv2 = 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.
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.