Introduction

The title not withstanding, this is not a history of dark matter. Until we know what the dark matter is, we cannot know its history. Instead, this is a brief history of how astronomers converged to the view that most of the matter in the universe is dark. This paper deals principally with the early studies which helped to answer the questions “Are rotation curves flat? If so, why?” It also includes some early history in deciphering the signature of clusters of galaxies as gravitational lenses, which seems to have been little investigated. This account covers the years up to 1980; achievements since 1980 are science, not history. Several excellent, informative brief histories exist, and interested readers should see Trimble (1987, 1995) and van den Bergh (1999). We can all thank Sidney van den Bergh for correctly translating Zwicky's “dunkle (kalte) materie” as “dark (cold) matter” and finally putting to rest the myth that Zwicky called it “missing matter.”

The notion that there are stars that are dark was a common one in the 18th and 19th Century. Walt Whitman's (1855) lines in Leaves of Grass, “The bright suns I see and the dark suns I cannot see are in their place” and Bessel's “Foundation of an Astronomy of the Invisible” (Clerke 1885 and reference therein) are early manifestations of this belief.

The primordial abundances of deuterium, helium, and lithium probe the baryon density of the universe only a few minutes after the Big Bang. Of these relics from the early universe, deuterium is the baryometer of choice. After reviewing the current observational status (a moving target!), the BBN baryon density is derived and compared to independent estimates of the baryon density several hundred thousand years after the Big Bang (as inferred from CMB observations) and at present, more than 10 billion years later. The excellent agreement among these values represents an impressive confirmation of the standard model of cosmology, justifying—indeed, demanding—more detailed quantitative scrutiny. To this end, the corresponding BBN-predicted abundances of helium and lithium are compared with observations to further test and constrain the standard, hot, big bang cosmological model.

Introduction

As progress is made towards a new, precision era of cosmology, redundancy will play an increasingly important role. As cosmology is an observational science, it will be crucial to avail ourselves of multiple, independent tests of, and constraints on, competing cosmological models and their parameters. Furthermore, such redundancy may provide the only window on systematic errors which can impede our progress or send us off in unprofitable directions.

We present photometric observations of an apparent Type Ia supernova (SN Ia) at a redshift of ∼1.7, the farthest SN observed to date. The supernova, SN1997ff, was discovered in a repeat observation by the Hubble Space Telescope (HST) of the Hubble Deep Field-North (HDF-N), and serendipitously monitored with NICMOS on HST throughout the Thompson et al. GTO campaign. The SN type can be determined from the host galaxy type: an evolved, red elliptical lacking enough recent star formation to provide a significant population of core-collapse supernovae. The classification is further supported by diagnostics available from the observed colors and temporal behavior of the SN, both of which match a typical SN Ia. The photometric record of the SN includes a dozen flux measurements in the I, J, and H bands spanning 35 days in the observed frame. The redshift derived from the SN photometry, z = 1.7±0.1, is in excellent agreement with the redshift estimate of z = 1.65 ± 0.15 derived from the U300B450V606I814J110J125H160H165Ks photometry of the galaxy. Optical and near-infrared spectra of the host provide a very tentative spectroscopic redshift of 1.755. Fits to observations of the SN provide constraints for the redshift-distance relation of SNe Ia and a powerful test of the current accelerating Universe hypothesis. The apparent SN brightness is consistent with that expected in the decelerating phase of the preferred cosmological model, ΩM ≈ 1/3, ΩΛ ≈ 2/3.

The planet Uranus was discovered in 1781 by the British astronomer William Herschel. Not long after its discovery, astronomers charting the orbit of Uranus found small discrepancies between the predicted and observed positions of the planet. In September 1845, British astronomer John Adams proved mathematically that the deviations in Uranus' orbit could not result merely from the gravitational pull of the other known planets and he predicted the existence of another, previously undetected planet in the solar system. The eventual discovery of the planet Neptune in September 1846 by the German astronomer Johann Galle thus marked the first detection of astronomical “dark matter” whose presence was first deduced by its gravitational effects. However, in the history of physics, we also find a case in which the assumption about the existence of an unseen medium was later proven to be totally wrong. Until 1887, physicists assumed that aether—a substance that pervades all space—was a necessary medium for the propagation of light. A famous experiment by American researchers Albert Michelson and Howard Morley not only showed unambiguously that this medium does not exist, but the experimental results also set Einstein on the road to a new theory of space and time—special relativity.

Astrophysicists today are faced with a similar “Neptune vs. aether” dilemma.

A straightforward interpretation of the MACHO microlensing results in the direction of the Magellanic Clouds suggests that an important fraction of the baryonic dark matter component of our Galaxy is in the form of old white dwarfs. If correct, this has serious implications for the early generations of stars that formed in the Universe and also on the manner in which galaxies formed and enriched themselves in heavy elements. I examine this scenario in some detail and in particular explore whether the searches currently being carried out to locate local examples of these MACHOs can shed any light at all on this scenario.

Introduction

A conservative estimate of the mass of the Galaxy out to a distance of about 2/3 of that of the Large Magellanic Cloud is MG = 4 × 1011 M⊙ (Fich & Tremaine 1991). With a total luminosity in the V-band of 1.4 × 1010 L⊙ (Binney & Tremaine 1987) the Galactic mass to light ratio in V (M/Lv) out to 35 kpc is ∼ 30. Since normal stellar populations do not generally produce M/Lv ratios higher than about 3, this is usually taken as evidence for an important component of dark matter within an extended halo surrounding the Galaxy.

FRIIb radio galaxies provide a tool to determine the coordinate distance to sources at redshifts from zero to two. The coordinate distance depends on the present values of global cosmological parameters, quintessence, and the equation of state of quintessence. The coordinate distance provides one of the cleanest determinations of global cosmological parameters because it does not depend on the clustering properties of any of the mass-energy components present in the universe.

Two complementary methods that provide direct determinations of the coordinate distance to sources with redshifts out to one or two are the modified standard yardstick method utilizing FRIIb radio galaxies, and the modified standard candle method utilizing type Ia supernovae. These two methods are compared here, and are found to be complementary in many ways. The two methods do differ in some regards; perhaps the most significant difference is that the radio galaxy method is completely independent of the local distance scale and independent of the properties of local sources, while the supernovae method is very closely tied to the local distance scale and the properties of local sources.

FRIIb radio galaxies provide one of the very few reliable probes of the coordinate distance to sources with redshifts out to two. This method indicates that the current value of the density parameter in non-relativistic matter, Ωm, must be low, irrespective of whether the universe is spatially flat, and of whether a significant cosmological constant or quintessence pervades the universe at the present epoch.

X-ray clusters provide excellent constraints on cosmological parameters such as ΩM. I will describe measurements of cluster masses and of cluster evolution. The cluster baryon fraction and the evolution of the cluster temperature function strongly constrain the mean density of matter in the universe (ΩM). The constraints are consistent with ΩM = 0.2–0.5, with best fit values of ΩM = 0.3–0.4. The systematic uncertainties are of the same size as the statistical uncertainties, even with the small number of clusters in our current temperature surveys (ΔΩM ∼ 0.1.) Thus, reduction of the uncertainties in these methods requires not only an increased number of hot massive clusters in a given sample but much better quantification of the systematics, a goal which demands not only more clusters but clusters with a range of properties and redshifts. The current constraints are not particularly sensitive to the particular form or value of the acceleration parameter Λ and therefore these constraints provide an limit on cosmological parameters complementary to the limits imposed by the cosmic microwave background studies and by the Type Ia supernovae at cosmological distances.

Introduction

I seek to make the following three points in this review:

1. (a) Clusters of galaxies are excellent targets for cosmological studies.

2. (b) Existing studies have already placed very strong constraints on the mean density of matter in the universe.

3. (c) These constraints are nearly orthogonal to constraints from the cosmic microwave background and type Ia supernovae.

4. […]