The origin of the biosphere

The biosphere is the ensemble of the life-supporting regions of the terrestrial globe. It is made up of the oceans and fresh waters, plus the atmosphere and the layer of soil (spread over the continents) that contains organic matter. Water is the predominant component of the biosphere, in which the atmosphere plays an important role, and where the many compounds of carbon are essential because they are needed by life. In the outer crust of the Earth, however, inorganic carbon is by far the most abundant component, in the form of carbonates (limestone, dolomite, etc.). Heat easily decomposes carbonates and frees carbon in the form of carbon dioxide (CO2); thus this process is the principal source of volcanic CO2.

For a long time, the origin of the biosphere remained a mystery, because no fossil trace exists from the first billion years of the Earth's evolution.

The nervous system

Living organisms learned very early on that the detection of changes happening in their environment was essential if they were to survive. Some photosynthetic algae had already learned how to move by wagging their flagellum, in order to look for places where lighting was optimal. The first multicellular organisms increased their chances of survival by detecting changes in their own body, and thus coordinate the response from their different cells.

Contact between one cell and its neighbors developed in the plants by means of enzyme exchange; that is, in a purely chemical way. It is a very slow process which leads to tropisms (such as the flower that turns toward the Sun, or the foliage that seeks light). But tropisms are too slow for animals; quick reactions for attack or defense are essential. A faster communication system appeared: some cells specialized in sending swift electrical impulses from one cell to another, as if along an electric wire. The nervous system began to emerge.

The life of the stars

At the time when the first quasars showed their dazzling brilliance, there were still no atoms of carbon, nitrogen, or oxygen, no metals in the Universe, nor any solid stuff (earth, rocks, etc.). It is tempting to think that the extreme temperatures reached in the accretion disks of quasars would have already produced new elements. But we must remember that the rings that reached a sufficiently high temperature were eventually absorbed by the black hole and disappeared from our Universe before having had a chance to make complex molecules.

The first stars formed out of a gaseous mixture of about 76% hydrogen and 24% helium, plus a few traces of rare light elements, such as lithium, or rare isotopes like deuterium or 3He. Table 3.1. Major thermonuclear reactions within stars, classified by increasing ignition temperature, from 10 million degrees for hydrogen, up to 6 billion degrees for iron

The minimum stellar mass able to reach this ignition temperature is indicated in solar mass units M⊙.

The grand beginning

Let us summarize the initial events in their chronological order. First, we can imagine a quantum fluctuation in the void, which began everything. The perfect symmetry of the little bubble of pure energy is unstable and breaks up spontaneously. We follow it at the instant when it is still smaller than a proton. It inflates exponentially while creating its space–time dimensions and, after 10-32 second, it is already larger than the present Solar System. This exponential ‘inflation’ (see Appendix F) creates all the matter and all the radiation still present in the Universe.

After that, the phase transition ends. The change of state has forever broken the initial symmetry. From now on, only the nuclear forces will remain confined between the quarks, whereas the forces of gravity and electromagnetism now act at a distance. The Universe will continue its expansion in an almost linear manner, restrained merely by gravitation, right up to the present.

But where did all this energy come from, with its ability to create the enormous bulk of matter and radiation that we see in the stars and the galaxies?

From the Big Bang to the human brain, it has taken the universe some fifteen billion years of cosmic, physical, chemical, and biological evolution to reach a stage where, on our own little speck of dust, it is beginning to look into itself and ponder its origin, nature, and significance.

How did it all happen? What is known, suspected, or assumed of each of the steps whereby time and matter first arose out of nothing, elementary particles condensed out of the original plasma, and, out of them, in turn, the atoms of the various elements came to be? Of the steps whereby galaxies were born, spawning billions of stars, many probably surrounded by planetary systems? Of the steps whereby, on one particular planet, which happened to combine a special set of physical conditions, life emerged and evolved, finally leading to conscious, thinking beings?

How much of this extraordinary history is due to deterministic forces, how much to chance? Did it happen only once? Or does the cosmos contain many planets that have given rise to life, perhaps even to intelligent life? What is it about the cosmological constants that endows our universe with its unique properties? Is only one such universe possible? Or are there many universes, of which ours happens to bear life and mind, and thus to be knowable, because of a special combination of cosmological constants? What triggered the Big Bang? A creative act of God? Or just randomly fluctuating nothingness?

The evolutionary thread

This book has tried, chronologically, to tell a history of the Universe that began with the Big Bang and continues up to our existence. In spite of many uncertain details and incomplete interpretations, the remaining gaps have not obscured a clear thread of ascent toward a greater and greater complexity, going from atoms to molecules to life, from bacteria to animals to humans, from early cultures to societies to civilizations.

It now remains for us to ponder on the vistas that we have opened up, in order to try to see what they reveal, and to understand the nature of what could still be concealed. Still following the thread of chronology, as long as it remains useful, let us first consider what could have happened before the Big Bang.

The ‘Augustinian era’

In 1952, George Gamow wittily proposed calling the period that might have occurred before the Big Bang the ‘Augustinian era’, because Saint Augustine was the first to raise the question of knowing what God did before He created Heaven and Earth.

Matter is made up of two kinds of elementary particle, quarks and leptons, each having six varieties divided into two groups of three:

Quarks

Leptons

Usual matter

Mesons

Forces of nature

Four types of force of different symmetries occur among matter particles; these forces are carried by particles of a new kind:

1. (1) electromagnetism, carried by the photon (light, X-rays, γ rays, etc.)

2. (2) the weak force is carried by three particles: W+, W- and Z0

3. (3) the strong force is carried by eight ‘colors’ of gluons

4. (4) gravitation is carried by the graviton

The standard model unifies electromagnetism and the weak force, as if the three particles W+, W- and Z0 were actually heavy photons.

The existence of the top quark had been predicted for a score of years; it was finally observed in 1994, completing the list of elementary particles.

Humankind has just awoken to the cosmic origins of the human adventure and I have striven to reconstruct this story as science now sees it. In order to keep it accessible to the enquiring reader, I have not used mathematics but followed the thread of chronology.

Recently, it has become possible to tell this story without leaving too many gaps. At the beginning of this century, the different natural sciences, originating in the same discipline of natural philosophy, had become specialized; they were cut off from one another. Now, physics, cosmology, astronomy, chemistry, geology, paleontology, biochemistry and biology are coming together again.

So my thread follows the Universe from the moment of its beginning; I tell how its expansion led to the formation of the galaxies and stars. The pursuit of the thread shows us how life could finally emerge, through the most probable cosmic processes, while using the most abundant elements made in the crucible of the stars' cores.

I have tried to keep the story simple; I cannot say that I have always succeeded. Occasionally, I had to cut short explanations that might have become tedious. The pages that seem too difficult can be skipped; in order to help the reader to follow the thread to the final goal, I have tried to give short summaries throughout the book.

The purpose of the first chapter is to familiarize the reader with the extremely large and the extremely small.

The standard model does not try to explain the cause of the Big Bang. It starts from the present conditions of average density and temperature in the Universe. If we go backwards in time, the Universe was smaller; its density and temperature can be computed for some typical epochs in the past.

1. (1) Average density. The present Universe is very unhomogeneous, so that its average density can be estimated only by using a very large volume, for instance, a cube of 500 million light-years on each side, for which the total mass of millions of galaxies can be assessed. The average density found by this method is a little less than 10-30 g/cm3.

2. (2) Average temperature. There are now still 3 billion photons at 2.7 K in the fossil radiation coming directly from the Big Bang, for each hotter photon arriving from the stars. These stellar photons are therefore the insignificant and negligible traces left by the fireworks from the primordial explosion. The average temperature of the Universe is 2.7 K.

3. (3) Expansion velocity. Its rate is given by the Hubble constant, H, which can be taken, for instance, as H = 25 km/second per million light-years.

In Table E.1, results have been rounded to the nearest factor of 10. The Universe cooled, as for example in an explosion gases cool as they fill a larger and larger volume, so that it is easy to compute the temperature and density at different times in the past.

The study of globular cluster systems (GCSs) has long been motivated, at least in part, by the idea that these systems can be used as fossil records of the formation history of their host galaxies (e.g. Harris and Racine 1979; Harris 1991). As described in the previous chapter, empirical information concerning GCSs has grown tremendously in both quantity and quality in recent years. This growth has led to more discriminating tests of models of the formation and evolution of galaxies through the properties of their globular cluster systems. Understanding galaxy formation and evolution is one of the primary challenges in extragalactic astronomy and cosmology. In this chapter, we describe models of galaxy formation and the constraints placed on these models by observations of globular cluster systems.

Models of galaxy formation

Galaxy properties

In the search for a physical model of galaxy formation and evolution, one of the primary questions is why galaxies have such a wide variety of morphologies, star formation histories, and stellar kinematics. One specific issue of great interest is why some galaxies are ellipticals which have old stellar populations, are dynamically hot, and follow de Vaucouleurs' surface brightness profiles (see Section 5.2), and others spirals which have been forming stars at roughly a constant rate over a Hubble time in a rotationally supported, exponential disk.

The dynamical differences between ellipticals and spirals are interesting, particularly since these galaxies have roughly similar mass densities.

Like most astronomical objects, globular clusters exhibit a range of properties and characteristics, but certain features are common to the majority of them. Due to their relative proximity, globular clusters within the Milky Way are the best–studied, and most of the ‘typical’ properties described in this chapter are based on observations of these objects. Unless otherwise stated, the data used in this and the following chapter are taken from the McMaster catalog described by Harris (1996; see also Harris and Harris 1997). Differences between the globular clusters of the Milky Way and other galaxies are summarized at the end of this chapter.

Color–magnitude diagrams

The luminosity and temperature of a star are dependent on its mass, age, and chemical composition. Color–magnitude diagrams of globular clusters have long been the subject of intensive study because they reflect these fundamental properties of the constituent stars. Figure 2.1 shows the color–magnitude diagram of M5 and illustrates a number of the basic features of globular cluster color–magnitude diagrams. These include the main sequence, the giant branch, and the horizontal branch, each of which is discussed in the following subsections.

Main sequence

One of the key features of globular clusters is the well-defined main sequence extending from the turn–off to fainter magnitudes and redder colors (see Figure 2.1). Globular cluster stars on the main sequence derive their energy from the conversion of hydrogen to helium in the stellar core. The low–luminosity end of the main sequence shown in Figure 2.1 is determined by the magnitude limit of the observations.

At present, there is no widely accepted theory of globular cluster formation. In this chapter, we describe some of the general ideas that have been proposed in this area, and compare these ideas with the constraints placed on globular cluster formation models by the observations described in earlier chapters.

One piece of evidence that has played an important role in the development of this field is the lack of current globular cluster formation in the Milky Way today. Open clusters and stellar associations form quite happily at the present epoch in the Galactic disk, but globular clusters do not. Until relatively recently, a significant fraction of astronomers would have probably argued that globular cluster formation was something that only occurred in the early universe. Others have claimed for some time that the most massive young star clusters found in the Large Magellanic Cloud and other similar environments are genuine analogs of the old globular clusters of the Milky Way and other galaxies. As discussed in Chapter 5, there is now evidence that globular clusters are currently forming in merging and interacting galaxies. While it is premature to regard this evidence as conclusive, there seems to be a growing acceptance of the idea that globular clusters can, under certain circumstances, form at the present epoch. As we show below, this possibility has significant consequences for models of globular cluster formation.

The globular cluster system of the Milky Way consists of over 150 known members. It is likely that not all the clusters have been detected, primarily because of obscuration by the Galactic bulge. The best estimate for the total population is around 180 objects. The system is centrally concentrated, with roughly half of the globular clusters residing within about 5 kpc of the Galactic center. However, the most remote clusters extend to beyond 100 kpc from the center of the Galaxy. The system is more usefully considered as two or more distinct subsystems. The majority of globular clusters form a roughly spherical, metal-poor, halo distribution. Recent evidence suggests that this halo population may itself be a composite system. A smaller number of globular clusters are relatively metal-rich and have the spatial and kinematic characteristics of a thick disk population. It has also been suggested that these metal-rich clusters can more properly be regarded as belonging to the bulge population of the Milky Way.

In this chapter, we examine the range of globular cluster properties, correlations between these properties, and how the characteristics of globular clusters vary with position within the Milky Way. We describe the evidence that has led to the separation of Milky Way globular clusters into distinct subsystems. The dynamical evolution of the Galactic globular cluster system and constraints on the properties of the initial Milky Way globular cluster system are also discussed. Finally, we consider the important question of what the Milky Way globular cluster system reveals about the formation and evolution of the Galaxy.