Extrapolating the present motion of expansion of the universe backward in time, we conclude that the early universe must have been very dense. And extrapolating the (adiabatic) expansion of the cosmic background radiation backward in time, we conclude that the early universe must have been very hot. Thus, at an early time, the universe must have been very different from what it is now. There were no stars and no galaxies, but only a uniform hot plasma, consisting of free electrons and free nuclei. The chemical composition of the early universe must also have been different. The heavy elements (that is, elements other than hydrogen, deuterium, helium, and lithium) in our immediate environment were formed by nuclear reactions in the cores of stars, so these elements did not exist in the early universe. At very early times, the violent thermal collisions would have prevented the existence of any kind of nuclei, and the matter in the universe must have been in the form of free electrons, protons, and neutrons. At the earliest times, even the protons and neutrons would have been disrupted, and the universe must have contained a mix of quarks, gluons, and other elementary particles.

The observed expansion of the universe and the observed cosmic background radiation provide the empirical basis for a Friedmann-Lemaître model of the universe with a Big Bang, sometimes called the Standard Model. Further evidence supporting this model is provided by calculations of the synthesis of helium in the universe. Although stars make helium by the thermonuclear burning of hydrogen, most of the helium in the universe must be primordial, since it is found even in stars that have not yet burned long enough to accumulate a significant amount of helium. This primordial helium was formed by nuclear reactions in the early universe at about 100 s, and the abundance of this helium (relative to hydrogen) can be calculated by examining the thermal equilibrium attained by protons and neutrons in reactions in the early, hot universe. The numbers obtained by such calculations of the helium abundance are in excellent agreement with the observational data. The abundances of other light elements formed in the early universe can be calculated similarly.

I find the slow emergence of infrared astronomy a moving story. It began with the revolutionary discovery by the self-taught William Herschel in 1800 of invisible radiation from the Sun, the significance of which took so long to be fully appreciated. Reading his methodical and imaginative papers makes his genius clear. Piazzi Smyth made the next step, with detection of infrared radiation from the Moon in 1856. I found his book about his expedition to Tenerife captivating, one of the great works of scientific popularization from the Romantic era. There followed another 50 years of painstaking work trying to detect the brightest stars and planets in infrared light and the slow progress of stellar infrared astronomy in the first half of the twentieth century, culminating in the work of Harold Johnson and his group. In 1930, Robert Trumpler discovered the key ingredient for understanding the infrared sky: interstellar dust.

We then come to the titans of the modern era, Frank Low and Gerry Neugebauer, repeatedly being told they were wasting their time as they tried to push astronomy into the infrared. They and their colleagues from many different groups deserve immense credit for their pioneering work in the 1960s and 1970s. This led to the explosive development of infrared astronomy following the launch of the IRAS satellite in 1983. It was such an exciting time to see the clouds of interstellar dust directly shining at us in infrared light and to find some of the most distant galaxies known at that time, infrared monsters convulsed in huge bursts of star formation. Considering where infrared astronomy had been only a decade or so earlier, it was wonderful to be using IRAS as a cosmological probe, linking the infrared galaxy distribution to the cosmic microwave background radiation left over from the Big Bang itself. IRAS was followed by ever more sophisticated space missions: ISO, Spitzer, Akari and today Herschel. And then there have been the giant ground-based telescopes, working either in the optical and near infrared or at submillimetre wavelengths. To be searching in the 1990s for submillimetre sources one thousand times fainter than those we had been trying to observe only 20 years earlier seemed like magic.

It has been a huge challenge to try to write the whole story of infrared astronomy, from the discovery of infrared radiation from the Sun by William Herschel (1738–1822) in 1800 through to the present day, to the discoveries being made by the space mission named after Herschel. I wanted to make this story accessible to the general reader with some interest in science but no scientific background. At the same time I wanted to make it a full and accurate account. Having lived and worked through the great period of infrared astronomy, from the 1960s to the present, I know many of the major figures whose work is described here, and I wanted to do them justice.

To reach the general reader, I have had to continually simplify the text, moving more complex and detailed material to the notes. Astronomy is a branch of physics, and physics is not an easy subject for someone who has perhaps not even studied it at school. I’ve provided a glossary of technical terms and tried to keep them to a minimum. The notes and very full bibliography allow the interested reader to explore the full details of a major area of science.

I have written about what I know, the science of infrared astronomy, and haven’t attempted to give the full story of the technological developments required to make this science possible. To give some idea of the huge army of people who work to provide the tools for science, an infrared space mission generally involves more than one thousand people in its design and construction. I was drawn into infrared astronomy in the early 1970s by my friend, and colleague at Queen Mary College London, Peter Clegg.

Introduction

The preceding chapter focused on singular events in the later history of the Earth - the flowering of multicellular complex organisms at the start of the Phanerozoic eon and the widespread extinction of species some 65 million years ago at the close of the Cretaceous period. Although these events stand out in their drama and the mystery of their causes, any understanding of the interactive history of life and Earth's environment cannot rest on their study alone. Throughout the Phanerozoic, and before, the relatively steady rhythms of plate tectonics brought continental masses together and then moved them apart, creating new seafloor and destroying old. The process of great landmasses moving around the planet must have had profound effects on the environment, and indeed this is seen to be the case in the geologic record.

This chapter begins by reconsidering plate tectonics with an eye to understanding the apparently cyclical creation and break up of multicontinent landmasses, or supercontinents. We consider the effects of such supercontinent cycles on the amount of volcanic activity, and hence atmospheric chemistry, on the ocean circulation patterns, on mountain building, and hence on the available area for storage of continental snow and ice deposits. Such considerations touch on a major theme of the latter portion of Earth history, the comings and goings of great ice ages. Finally, we draw our attention in detail to a particularly warm time in recent Earth history, the Cretaceous period.

Introduction

In Chapter 1, we became acquainted with the scale of the solar system - the stage upon which planetary evolution is set. However, the formation of elements out of which planets and life came into being involved the universe of stars and galaxies - a scale much larger than the solar system - and the microscopic world of atoms, which involves size scales much smaller than that of our ordinary experiences. In this chapter we explore how cosmic distances are gauged, and then begin to acquaint ourselves with the basic building blocks of matter.

Scientific notation

Although the book is written with the nonmathematically inclined reader in mind, the discussion of numbers, both large and small, cannot be avoided if we are to gain a true understanding of Earth and its place in the cosmos. Numbers of interest in science range over enormous magnitudes (Figure 2.1). The number of protons contained in a single star, our Sun, is of order 1,000,000,000,000,000,000,000, 000,000,000,000,000,000,000,000,000,000,000,000; the size of an individual proton (itself made up of smaller elementary units) is of order 0.0000000000001 cm. (The term of order refers to how many powers of 10 a number contains, rather than the specific numerical value it has; hence 200 is of order 100, 40 is of order 10, etc.) These numbers are inconvenient to write down and manipulate in even the simplest mathematical expressions.

The IRAS surveys had demonstrated the origin of the dipole anisotropy in the cosmic microwave background (CMB) and seemed to have shown that the mean density of the universe could be close to the critical value. They had also shown that there was more structure on large scales than expected in the standard ‘cold dark matter’ model for the universe. In this chapter, I describe how the Cosmic Background Explorer (COBE) mission, launched in November 1989, measured the spectrum of the background radiation with unbelievable accuracy and then went on to detect the expected small fluctuations in the background that would be the precursors of galaxies and clusters of galaxies today. However, these fluctuations turned out to be stronger than expected, confirming the IRAS conclusion that an additional ingredient was needed to explain this structure. The choice seemed to be between massive neutrinos or a cosmological constant. This and many other conundrums about the universe were settled by the WMAP mission, launched in 2001. The beautiful detail of WMAP’s measurements of CMB fluctuations on both small and large angular scales heralded a new era of precision cosmology.

The Cosmic Background Explorer and the Spectrum of the Cosmic Microwave Background

During the IRAS mission of 1983, I recall that Mike Hauser, who took a lead role in developing the IRAS extended emission maps, was already working on the Cosmic Background Explorer mission. As far back as 1974, NASA had received three proposals for cosmological background radiation missions in response to a call for small- or medium-sized astronomical missions, which were known as the ‘Explorer’ series of missions. Although the mission selected on that occasion was IRAS, NASA chose members from each of the three cosmic background proposal teams to get together and propose a joint satellite concept. In 1977, this team converged on the idea of a polar orbiting satellite, COBE, that could be launched by either a Delta rocket or the space shuttle.

Introduction

The Phanerozoic eon is a major division in the fossil record that dates radioisotopically at a bit younger than 600 million years before present. Its geologic marker is the appearance of numerous complex multicellular organisms in the fossil record. This eon has no counterpart on any other planet, even if Mars harbored simple life-forms within the first billion years of its history. On Phanerozoic Earth, life began to occupy just about every conceivable niche on land, sea, and air. Geologically, Earth was more or less modern in form as the eon opened: the total continental mass was comparable to that today, modernstyle plate tectonics were operating, and oxygen levels in the atmosphere were approaching present-day values.

The Phanerozoic eon is divided into eras, eras into periods, and periods into epochs. The boundaries between most of the periods are defined by extinction episodes in which a number (sometimes very large) of species disappear and are replaced in the sedimentary fossil record above that point by new species. Although the resulting story of complex multicellular organisms is too large to tell in detail in this book, some of the highlights are shown in Figure 18.1.

The presence of multicellular organisms per se was not new. Multicellular bacterial colonies had existed since the Archean; multicellular algae (for example, green seaweed) made their appearance shortly after the first unicellular eukaryotes in the fossil record. In each of these, and many other cases, there is little or no specialization among cells, and only limited communication.

Gravitational effects cannot propagate with infinite speed. This is obvious both from the lack of Lorentz invariance of infinite speed and from the causality violations that are associated with signal speeds in excess of the speed of light. Since the speed of light is the only Lorentz-invariant speed, we expect that gravitational effects propagate in the form of waves at the speed of light.

As a concrete example, consider an apple that hangs on a tree. At some time, the stem of the apple breaks and the apple falls to the ground, which means there is a sudden change in the terrestrial mass distribution. The gravitational field surrounding the Earth must then adapt itself to this new mass distribution. The change in the field will not occur simultaneously throughout the universe – at any given point of space the change will be delayed by a time equal to the time needed for a light signal to travel from the Earth to that point. Hence the disturbance in the gravitational field propagates outward at the speed of light. Such a propagating disturbance is a gravitational wave.

Introduction

The close of the Hadean and opening of the so-called Archean eon is defined and characterized by the oldest whole rock samples found on Earth, 4.0 billion years old. At the opening of the Archean, Earth had an atmosphere rich in carbon dioxide, with perhaps some nitrogen and methane but little molecular oxygen, and liquid water was stable on its surface. Mantle convection had begun producing oceanic basalts and continental-type granitic rocks. The rate of impacts of asteroidal and cometary fragments had decreased significantly. The Moon, formed from Earth at the end of accretion some half billion years before, could be seen in the terrestrial sky.

By 3.5 billion years ago, rocks were present that record definitive evidence for life; more controversial evidence exists back to almost 3.9 billion years. Large sedimentary or layered formations in ancient limestones contain concentric spherical shapes, stacked hemispheres and flat sheets of calcium carbonates (calcite), and trapped silts. These stromatolites are best understood as the work of bacteria from 3.5 billion years ago, precipitating calcium carbonate in layers as one of the byproducts of primitive photosynthesis. (Present-day active stromatolite-forming colonies can be found in Shark Bay, Australia.) If the interpretation is correct, life on Earth was present then and somewhat earlier as well, because such bacteria constitute already reasonably well-developed organisms.

Introduction

The beginning of the Proterozoic eon is set formally by geologists at 2.5 billion years before present. However, the transition between the Archean and the Proterozoic is not a sharp one. From about 3.2 billion to 2.5 billion years ago, rocks with a modern granitic composition made a widespread appearance in the geologic record. Prior to this time, rocks making up the Archean continents had a composition different from modern granites in several important respects. Beginning around 3.2 billion years ago in what is now Africa, and extending to 2.6 billion years ago on the Canadian shield, large quantities of modern-type granites were produced. We can collect these rocks today and date them by use of radioisotopes. How did the original Archean continents form? Why was there a transition in chemical composition of the rocks roughly halfway through the Archean? What might Earth have been like today if this eruption of new rock types had not occurred? As we see, the transformation wrought on Earth's primitive continents may have been an inevitable consequence of their increasing coverage of Earth's surface.

What might have been inevitable on Earth was apparently difficult or impossible on the other terrestrial planets. No evidence for large granitic masses exists on any other planet. Venus bears two crustal masses that resemble continents, but the details of their geology suggest that they are more similar to primitive Archean continents than to our modern ones and, even then, the connection is a weak one.

Introduction

One of the most fiercely debated social issues grounded in science today is whether humans are affecting the climate of the planet on which we live. While the basic question is a scientific one, the implications potentially touch every aspect of our lives. We are facing, for the first time in human history, a planet-wide transformation of our environment wrought by human activities. In this chapter the evidence and mechanisms are discussed along with the potential impacts of humankind's effect on climate.

The records of CO2 abundance and global temperatures in modern times

Ice cores contain trapped bubbles of air, which, provided they can be properly dated, represent a record of the composition of air over time. Because of the weight of overlying layers of ice, compressing the pores in the ice, it is very difficult to extend the record back as far as that for temperature derived from the isotopic composition of the water itself. In fact, the manner in which the air bubbles were originally trapped in ice results in their movement upward or downward relative to the ice itself, making age determination a challenge.

Figure 22.1 displays CO2 values from an ice core collected in Greenland. The dating of the air was achieved by taking advantage of a byproduct of nuclear weapons testing: the isotope 14C reached a peak in Earth's atmosphere, from the detonation of nuclear bombs, in 1963. Using this peak in heavy carbon, geochemists M. Wahlen of Scripps Institution of Oceanography and colleagues determined that the trapped air was displaced by the equivalent of 200 years relative to the ice surrounding it.

All astronomy starts from the night sky, the picture of the universe we see with our own eyes: the stars and constellations, the Milky Way. We have become used to the idea that there is a universe beyond the constellations, revealed by the giant telescopes of the astronomers. We can still imagine the Hubble Space Telescope to be a giant extension of our own eyes, and its images are still images made in visible light. Much harder to grasp is the universe that is revealed in invisible light, such as the infrared radiation that is the subject of this book. I’ve called the book Night Vision for two reasons. Firstly, infrared sensors and binoculars are already widely used to aid seeing in the dark, or night vision. And secondly, my goal is to try to make the infrared universe as familiar to you as the night sky, so that when you look out at the night sky you can also imagine what it would look like with infrared eyes, and therefore see the new vision of the night sky that the infrared gives us.

Above all, infrared astronomy is about the cool, dusty universe. Spread between the stars are tiny grains of dust, similar to sand and soot, and these absorb the light from stars and reradiate the energy as infrared radiation. There are dense clouds of gas and dust within which new stars are forming, and only with infrared light can we peer into them. Dying stars and massive black holes are often shrouded in dust, which shines in the infrared. And the cool bodies of the Solar System, planets, comets and asteroids are mainly radiating infrared light. In the infrared and submillimetre parts of the spectrum, we see galaxies in formation, undergoing violent bouts of star formation often caused by collisions between smaller fragments. And we see the cool glow left over by the hot fireball phase of the Big Bang itself.