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. […]

One of the most fundamental questions in cosmology is: How much matter is there in the Universe and how is it distributed? Here I review several independent measures—including those utilizing clusters of galaxies—that show that the mass-density of the Universe is only ∼ 20% of the critical density. Recent measurements of the mass-to-light function—from galaxies, to groups, clusters, and superclusters—provide a powerful new measure of the universal density. The results reveal a low density of 0.16 ± 0.05 the critical density. The observations suggest that, on average, the mass distribution follows the light distribution on large scales. The results, combined with recent observations of high redshift supernovae and the spectrum of the CMB anisotropy, suggest a Universe that has low density (Ωm ≃ 0.2), is flat, and is dominated by dark energy.

Introduction

Theoretical arguments based on standard models of inflation, as well as on the demand of no “fine tuning” of cosmological parameters, predict a flat universe with the critical density needed to just halt its expansion (1.9 × 10–29 h2 g cm–3). Observations, however, reveal only a small fraction of the critical density, even when all the unseen dark matter in galaxy halos and clusters of galaxies is included. There is no reliable indication that the matter needed to close the universe does in fact exist. Here I review several independent observations of clusters of galaxies which indicate, independently, that the mass density of the universe is sub-critical.

Milgrom has proposed that the appearance of discrepancies between the Newtonian dynamical mass and the directly observable mass in astronomical systems could be due to a breakdown of Newtonian dynamics in the limit of low accelerations rather than the presence of unseen matter. Milgrom's hypothesis, modified Newtonian dynamics or MOND, has been remarkably successful in explaining systematic properties of spiral and elliptical galaxies and predicting in detail the observed rotation curves of spiral galaxies with only one additional parameter—a critical acceleration which is on the order of the cosmologically interesting value of CH〪. Here I review the empirical successes of this idea and discuss its possible extention to cosmology and structure formation.

Introduction

Modified Newtonian dynamics (MOND) is an ad hoc modification of Newton's law of gravity or inertia proposed by Milgrom (1983) as an alternative to cosmic dark matter. The motivation for this and other such proposals is obvious: So long as the only evidence for dark matter is its global gravitational effect, then its presumed exitance is not independent of the assumed form of the law of gravity or inertia on astronomical scales. In other words, either the universe contains large quantities of unseen matter, or gravity (or the response of particles to gravity) is not generally the same as it appears to be in the solar system.

A close scrutiny of the microlensing results towards the Magellanic clouds reveals that the stars within the Magellanic clouds are major contributors as lenses, and the contribution of MACHOs to dark matter is 0 to 5%. The principal results which lead to this conclusion are the following:

1. (i) Out of the ∼17 events detected so far towards the Magellanic Clouds, the lens location has been securely determined for one binary-lens event through its caustic-crossing timescale. In this case, the lens was found to be within the Magellanic Clouds. Although less certain, lens locations have been determined for three other events and in each of these three events, the lens is most likely within the Magellanic clouds.

2. (ii) If most of the lenses are MACHOs in the Galactic halo, the timescales would imply that the MACHOs in the direction of the LMC have masses of the order of 0.5 M⊙, and the MACHOs in the direction of the SMC have masses of the order of 2 to 3 M⊙. This is inconsistent with even the most flattened model of the Galaxy. If, on the other hand, they are caused by stars within the Magellanic Clouds, the masses of the stars are of the order of 0.2 M⊙ for both the LMC as well as the SMC.

3. (iii) If 50% of the lenses are in binary systems similar to the stars in the solar neighborhood, ∼10% of the events are expected to show binary characteristics.

4. […]

There are now two cosmological constant problems: (i) why the vacuum energy is so small and (ii) why it comes to dominate at about the epoch of galaxy formation. Anthropic selection appears to be the only approach that can naturally resolve both problems. This approach presents some challenges to particle physics models.

The problems

Until recently, there was only one cosmological constant problem and hardly any solutions. Now, within the scope of a few years, we have made progress on both accounts. We now have two cosmological constant problems (CCPs) and a number of proposed solutions. In this talk I am going to review the situation, focusing mainly on the anthropic approach and on its implications for particle physics models. I realize that the anthropic approach has a low approval rating among physicists. But I think its bad reputation is largely undeserved. When properly used, this approach is quantitative and has no mystical overtones that are often attributed to it. Moreover, at present this appears to be the only approach that can solve both CCPs. I will also comment on other approaches to the problems.

The cosmological constant is (up to a factor) the vacuum energy density, ρv.

For physicists, recent developments in astrophysics and cosmology present exciting challenges. We are conducting “experiments” in energy regimes some of which will be probed by accelerators in the near future, and others which are inevitably the subject of more speculative theoretical investigations. Dark matter is an area where we have hope of making discoveries both with accelerator experiments and dedicated searches. Inflation and dark energy lie in regimes where presently our only hope for a fundamental understanding lies in string theory.

Introduction

It is a truism that the development of astronomy, astrophysics, cosmology relies on our understanding of the relevant laws of physics. It is thus no surprise that my astronomy colleagues tend to know more classical mechanics, electricity and magnetism, atomic and nuclear physics than my colleagues in particle theory.

As we consider many of the questions which we now face in cosmology, we must confront the fact that we simply do not know the relevant laws of nature. The public often asks us “What came before the Big Bang?” We usually think of this as requiring understanding of physics at the Planck scale. But at present we can't even come close. Ignorance sets in slightly above nucleosynthesis, and becomes severe by the time we reach the weak scale. Some of the questions which trouble us will be settled by experiment over the next decades; some require new theoretical developments. Needless to say, it is possible that much will remain obscure for a long time.

Recent measurements of temperature fluctuations in the cosmic microwave background (CMB) indicate that the Universe is flat and that large-scale structure grew via gravitational infall from primordial adiabatic perturbations. Both of these observations seem to indicate that we are on the right track with inflation. But what is the new physics responsible for inflation? This question can be answered with observations of the polarization of the CMB. Inflation predicts robustly the existence of a stochastic background of cosmological gravitational waves with an amplitude proportional to the square of the energy scale of inflation. This gravitational-wave background induces a unique signature in the polarization of the CMB. If inflation took place at an energy scale much smaller than that of grand unification, then the signal will be too small to be detectable. However, if inflation had something to do with grand unification or Planckscale physics, then the signal is conceivably detectable in the optimistic case by the Planck satellite, or if not, then by a dedicated post-Planck CMB polarization experiment. Realistic developments in detector technology as well as a proper scan strategy could produce such a post-Planck experiment that would improve on Planck's sensitivity to the gravitational-wave background by several orders of magnitude in a decade timescale.

The simplest models for the formation of large-scale structure are reviewed. On the assumption that the dark matter is cold and collisionless, LSS data are able to measure the total amount of matter, together with the baryon fraction and the spectral index of primordial fluctuations. There are degeneracies between these parameters, but these are broken by the addition of extra information such as CMB fluctuation data. The CDM models are confronted with recent data, especially the 2dF Galaxy Redshift Survey, which was the first to measure more than 100,000 redshifts. The 2dFGRS power spectrum is measured to ≲ 10% accuracy for k > 0.02 h Mpc–1, and is well fitted by a CDM model with Ωmh = 0.20 ± 0.03 and a baryon fraction of 0.15 ± 0.07. In combination with CMB data, a flat universe with Ωm ⋍ 0.3 is strongly favored. In order to use LSS data in this way, an understanding of galaxy bias is required. A recent approach to bias, known as the ‘halo model’ allows important insights into this phenomenon, and gives a calculation of the extent to which bias can depend on scale.

Structure formation in the CDM model

The origin and formation of large-scale structure in cosmology is a key problem that has generated much work over the years. Out of all the models that have been proposed, this talk concentrates on the simplest: gravitational instability of small initial density fluctuations.

History's time line swept through 1963 with a breathtaking pace. The community of nations was about to welcome the birth of its newest member, Kenya, which that year attained independence from Great Britain. The Vietnamese military, meanwhile, was in the process of overthrowing the regime of Ngo Dinh Diem, deepening the US involvement in Southeast Asia and setting the stage for a decade of discordant relations among the superpowers. Ironically, this was also the year in which the first test ban agreement between the USA and the Soviet Union was ratified, concluding a nervous endeavor to ease growing nuclear tensions. For the individuals in society, the issue of women's rights resurfaced, promoted by Betty Friedan's just-released book Feminine Mystique. And while readers were being exposed to the idea of a modern woman discarding her traditional role, humanity as a whole was gaining some leverage over nature with the discovery of a vaccine against the measles. Many remember 1963 for the tragic assassination of President John F. Kennedy.

This tessellation of historical markers stirring the world in 1963 formed quite a backdrop for two minor events that would lead, over time, to the eventual uncloaking of the most powerful objects in the universe. At Mount Palomar Observatory, Maarten Schmidt was pondering over the nature of a starlike object with truly anomalous characteristics, while Roy Kerr, at the University of Texas, was making a breakthrough discovery of a solution to Albert Einstein's (1879–1955) general relativistic field equations.

Other than the spectacle of an obscured event horizon quivering before a bright sheet of background light, the most spectacular blackhole phenomenon astronomers can witness from the remoteness of Earth is a relativistic jet of plasma piercing the darkness of intergalactic space. Among the most dizzying cosmic displays in nature, these funnels of energetic particles probe the medium surrounding roughly one in 20 known supermassive black holes. A prominent jet was evident on the very first quasar photograph (of 3C 273), and glows even more brilliantly as a high-energy ray of light in modern Chandra images (see Fig. 1.2). For the most part, however, black-hole jets manifest themselves in a “parallel” universe – indeed, their ghostly apparitions pre-empted the discovery of supermassive black holes by several decades, though without any portent of what they would later reveal. And once again, astronomers can thank the telephone company for facilitating one of the most amazing advances in the history of science, on a par with the discovery – six decades later – of the cosmic microwave background radiation through the commercialization of space.

Not long after a demonstration that the substance of light behaves like a series of waves undulating through time and space, Guglielmo Marconi (1874–1937) successfully initiated transatlantic communications in 1901 using wireless radio.

Though some Hubble images of distant galaxies feature destructive collisions that could trigger quasar activity, others show that many normal, undisturbed aggregates of stars are oblivious to the cosmic thunder within their midst. This is an indication that a variety of mechanisms – some quite subtle – may be responsible for igniting a quasar. Whatever the formative process is, however, these supermassive objects seem to have spared their hosts from any obvious damage, so their prodigious outpouring of matter and radiation may be a shortlived phenomenon. Still, this observation is not sufficient to guide astronomers toward the identification of a coherent, single pattern of quasar birth and growth.

For years, astrophysicists concerned with the nature of supermassive black holes have been asking themselves a cosmological “chicken and the egg” question: “Which came first, the gargantuan pit of closed spacetime, or the lively panorama of gilded stars and glowing gas that we call a galaxy?”

Prior to a remarkable recent discovery that now seems to have answered this question for the majority of cases, the evidence in favor of black holes appearing first was anchored by the telling observation that the number of quasars peaked 10 billion years ago, early in the universe's existence. The light from galaxies, on the other hand, originated much later – after the cosmos had aged another 2 to 4 billion years.