Cosmology_A Very Short Introduction Page 2
The Universe in myth
Most early cosmologies are based on some form of anthropomorphism (the interpretation of something which is not human, in terms of human characteristics). Some involve the idea that the physical world is animated by wilful beings that can help or hinder mankind, others that the physical world itself is inanimate but can be manipulated by a god or gods. Either way, creation myths tend to explain the origin of the Universe in terms of entities whose motives can be understood, at least partly, by human beings.
There are many differences in creation myths around the world, but there are also some striking similarities. For one thing, their imagery often incorporates the idea of a supreme craftsman. The beauty of the natural world is thus represented as the handiwork of a skilled artisan, examples of which are found in all cultures. Another recurring image is the growth of order from chaos, mirroring the progressive organization of human society. Yet another parallel is the Universe as a biological process. The most striking examples of this occur in myths that depict the cosmos as forming from an egg or seed.
The Babylonian version of Genesis, the Enuma Elish, contains these elements. This myth dates from around 1450 BCE, but is probably based on much older Sumerian versions. In its account of the creation, the primordial state of disorder is identified with the sea. From the sea emerges a series of gods representing fundamental properties of the world, such as the sky, the horizon, and so on. Two of these deities, Marduk and Tiamat, fight and Tiamat the sea-goddess is killed. Marduk makes the Earth from her body.
China also furnishes interesting illustrations. One involves the giant Pan Gu. In this story, the cosmos began as a giant egg. The giant slept inside the egg for thousands of years before he awoke and broke free, shattering the egg in the process. Some parts of the egg (the lighter and purer bits) rose up to form the heavens while the heavier, impure parts formed the Earth. Pan Gu held up the heavens with his hands while his feet rested on the Earth. As the heavens drifted higher, the giant grew taller to keep them in contact with the Earth. Eventually Pan Gu died, but his body parts were put to good use. His left eye became the Sun, his right eye the Moon. His sweat became the rain, his hair the plants of the Earth, and his bones the rocks.
1. The Babylonian God Marduk. Marduk is credited with the imposition of cosmic order after the destruction of Tiamat, the embodiment of primordial chaos, shown here at his feet in the form of a horned dragon. Many mythologies around the world incorporate the idea that order arose from chaos, and the theme survives in some aspects of modern scientific cosmology.
There are as many creation legends as there have been cultures, and I have no space to give more examples here. Whether African, Asian, European, or American, it is striking how many formal similarities these myths display.
The Greeks
Western science has its roots in Greece. The Greeks, of course, had their own gods and myths, many of them borrowed from neighbouring cultures. But alongside these more traditional elements they began to establish a system of principles for scientific enquiry. The identification of cause and effect, still an essential component of scientific theories, was down to the Greeks. They also realized that descriptions and explanations of observed phenomena could be phrased in mathematical or geometrical rather than anthropomorphic terms.
Cosmology began to emerge as a recognizable scientific discipline within the overall framework of rational thought constructed by the Greeks, notably through Thales (625–547 BCE) and Anaximander (610–540 BCE). The word cosmology itself is derived from the Greek ‘cosmos’, meaning the world as an ordered system or whole. The emphasis is just as much on order as on wholeness, for in Greek the opposite of ‘cosmos’ is ‘chaos’. The Pythagoreans of the sixth century BCE regarded numbers and geometry as the basis of all natural things. The advent of mathematical reasoning, and the idea that one can learn about the physical world using logic and reason marked the beginning of the scientific era. Plato (427–348 BCE) expounded a complete account of the creation of the Universe, in which a divine Demiurge creates, in the physical world, imperfect representations of the structures of pure being that exist only in the world of ideas. The physical world is subject to change, whereas the world of ideas is eternal and immutable.
Aristotle (384–322 BCE), a pupil of Plato, built on these ideas to present a picture of the world in which the distant stars and planets execute perfect circular motions, circles being a manifestation of ‘divine’ geometry. Aristotle’s Universe is a sphere centred on the Earth. The part of this sphere that extends as far as the Moon is the domain of change, the imperfect reality of Plato, but beyond this the heavenly bodies execute their idealized circular motions. This view of the Universe was to dominate Western European thought throughout the Middle Ages, but its perfect circular motions did not match the growing quantities of astronomical data being gathered by the Greeks from the astronomical archives made by the Babylonians and Egyptians. Although Aristotle had emphasized the possibility of learning about the Universe by observation as well as pure thought, it was not until Ptolemy’s Almagest, compiled in the second century CE, that a complete mathematical model for the Universe was assembled that agreed with all the data available.
The Renaissance
Much of the knowledge acquired by the Greeks was lost to Christian culture during the dark ages, but it survived in the Islamic world. As a result, cosmological thinking during the Middle Ages of Europe was rather restricted. Thomas Aquinas (1225–74) seized on Aristotle’s ideas, which were available in Latin translation at the time while the Almagest was not, to forge a synthesis of pagan cosmology with Christian theology which was to dominate Western thought until the sixteenth and seventeenth centuries.
The dismantling of the Aristotelian world-view is usually credited to Nicolaus Copernicus (1473–1543). Ptolemy’s Almagest was a complete theory, but it involved applying a different mathematical formula for the motion of each planet and therefore did not really represent an overall unifying system. In a sense, it described the phenomena of heavenly motion but did not explain them. Copernicus wanted to derive a single universal theory that treated everything on the same footing. He achieved this only partially, but did succeed in displacing the Earth from the centre of the scheme of things. It was not until Johannes Kepler (1571–1630) came along that a completely successful demolition of the Aristotelian system was achieved. Driven by the need to explain the highly accurate observations of planetary motion made by Tycho Brahe (1546–1601), Kepler replaced Aristotle’s divine circular orbits with ellipses.
The next great development on the road to modern cosmological thinking was the arrival on the scene of Isaac Newton (1642–1727). Newton was able to show, in his monumental Principia (1687), that the elliptical motions devised by Kepler were the natural outcome of a universal law of gravitation. Newton therefore re-established a kind of Platonic level of reality, the idealized world of universal laws of motion. The Universe, in Newton’s picture, behaves as a giant machine, enacting the regular motions demanded by the divine Creator and both time and space are absolute manifestations of an internal and omnipresent God.
Newton’s ideas dominated scientific thinking until the beginning of the twentieth century, but by the nineteenth century the cosmic machine had developed imperfections. The mechanistic world-view had emerged alongside the first stirrings of technology. During the subsequent Industrial Revolution, scientists had become preoccupied with the theory of engines and heat. These laws of thermodynamics had shown that no engine could work perfectly for ever without running down. In this time there arose a widespread belief in the ‘Heat Death of the Universe’, the idea that the cosmos as a whole would eventually fizzle out just as a bouncing ball gradually dissipates its energy and comes to rest.
Towards the modern era
Another spanner was thrown into the works of Newton’s cosmic engine by Olbers (1758–1840), who formulated in 1826 a paradox that still bears his name although it was discussed by many b
efore him, including Kepler. Olbers’ Paradox emerges from considering why the night sky is dark. In an infinite and unchanging Universe, every line of sight from an observer should hit a star, in much the same way as a line of sight through an infinite forest will eventually hit a tree. The consequence of this is that the night sky should be as bright as a typical star. The observed darkness at night is sufficient to prove the Universe cannot be both infinite and eternal.
Whether the Universe is infinite or not, the part of it accessible to rational explanation has steadily increased. For Aristotle, the Moon’s orbit (a mere 400,000 km) marked a fundamental barrier, beyond which the human mind could not reach. To Copernicus and Kepler the limit was the edge of the Solar System (billions of kilometres away). By the eighteenth and nineteenth centuries, it was being suggested that the Milky Way, a structure now known to be at least a billion times larger than the Solar System, was the entire Universe. This suggestion had a rival, the idea that many strange spiral ‘nebulae’ discovered scattered across the sky were objects very similar to our Milky Way but seen at immense distances. These objects would come to be called galaxies. A ‘Great Debate’ took place in the early years of the twentieth century between these two opposing ideas, which I will discuss in Chapter 4. Thanks largely to Edwin Hubble (1889–1953), it is now known that the Milky Way is indeed only one of hundreds of billions of similar galaxies.
The modern era of cosmology began in the early years of the twentieth century, with a complete rewrite of the laws of nature. Albert Einstein (1879–1955) introduced the principle of relativity in 1905 and thereby demolished Newton’s conception of space and time. Later, his general theory of relativity also supplanted Newton’s law of universal gravitation. The first great works on relativistic cosmology by Friedmann (1888–1925), Lemaître (1894–1966), and de Sitter (1872–1934) formulated a new and complex language for the mathematical description of the Universe. Einstein’s theory plays such a fundamental conceptual role in modern cosmology that I will devote much of the next chapter to it.
But while these conceptual developments paved the way, the final steps towards the modern era were taken not by theoretical physicists, but by observational astronomers. In 1929, Edwin Hubble, who had only recently shown that the Universe contained many galaxies like the Milky Way, published the observations that led to the realization that our Universe is expanding. Finally, in 1965, Penzias and Wilson discovered the cosmic microwave background, proof (or as near to proof as you’re likely to see) that our Universe began in a primordial fireball – the Big Bang.
Cosmology today
The modern era of scientific cosmology began with Einstein’s general theory of relativity, published in 1915, which made possible a consistent mathematical description of the entire Universe. According to Einstein’s theory, the properties of matter and motion are related to deformations of space and time. The importance of this for cosmology is that space and time are no longer thought of as absolute and independent of material bodies, but as participants in the evolution of the Universe. General relativity allows us to understand not the origin of the cosmos in space and time, but the origin of space and time themselves.
Einstein’s theory forms the basis of the modern Big Bang model, which has emerged as the best available description of the expanding Universe. According to this model, space, time, matter, and energy all came into existence as a primordial fireball of matter and radiation at extremes of temperature and density about 15 billion years ago. A few seconds after the beginning, the temperature had decreased to a mere 10 billion degrees and nuclear reactions began to make the atoms from which we are all made. After about 300,000 years the temperature had fallen to a few thousand degrees, releasing the radiation we now observe as the cosmic microwave background. As this explosion expanded, carrying space and time with it, the Universe cooled and rarefied. Stars and galaxies formed by condensing out of the expanding cloud of gas and radiation. Our present-day Universe contains the ashes and smoke left over from the Big Bang.
Chapter 5 describes the Big Bang theory in more detail. Most cosmologists accept it as being essentially correct, as far as it goes. It explains most of the things we know about the bulk properties of the Universe, and can account for most relevant cosmological observations. But it is important to realize that the Big Bang is not complete. Much of modern cosmological research is driven by the desire to fill the gaps in this otherwise compelling framework.
For one thing, Einstein’s theory itself breaks down at the very beginning of the Universe. The Big Bang is an example of what relativity theorists call a singularity, a point where the mathematics falls to pieces and measurable quantities become infinite. While we know how the Universe is expected to evolve from a given stage, the singularity makes it impossible to know from first principles what the Universe should look like in the beginning. We therefore have to piece this together using observations rather than pure thought, like archaeologists trying to reconstruct a city from ruins. Modern-day cosmologists are therefore collecting huge quantities of detailed data so that they can try to piece it all together to make a picture of how the Universe began.
Technological developments over the last twenty years have accelerated progress in observational cosmology to a remarkable extent, and we are truly in a ‘Golden Age’ of cosmic discovery. Observational cosmology now includes the construction of huge maps of the distribution of galaxies in space, showing the remarkable large-scale structure of filaments and sheets. These surveys are complemented by deep observations being made with, for example, the Hubble Space Telescope. The Hubble Deep Field is such a long exposure that it can see galaxies at distances so huge that it has taken light much of the age of the Universe to reach us from them. Using observations like this we can see cosmic history unfolding. For example, microwave astronomers are now able to make pictures of the structure of the early Universe by observing ripples in the cosmic microwave background produced in the primordial fireball. Planned satellite experiments, such as MAP and the Planck Surveyor, will probe these ripples in more detail over the next few years and the results they produce should plug many of the gaps in our understanding of how the Universe is put together.
Astronomical observations can be used to measure the rate of cosmic expansion, how this is changing with time, and also to probe the geometry of space by applying the principles of triangulation on an enormous scale. In Einstein’s theory, light rays do not necessarily travel in straight lines because space distorts in response to the gravity produced by massive bodies. Over cosmological distances, this effect can close the whole of space-time back on itself (like the surface of a sphere) causing the paths of parallel light rays to converge. It could also produce an ‘open’ Universe in which light rays diverge. Poised in between these two alternatives is the ‘normal’ idea of flat space in which Euclid’s laws of geometry apply. Which of these alternatives is correct depends upon the total cosmic density of matter and energy, which the Big Bang theory cannot itself predict.
The Big Bang theory underwent a major theoretical overhaul in the early 1980s, when particle physicists took up cosmology as a way of trying to understand the properties of matter at the extremely high energies their particle accelerators couldn’t reach. These theorists realized that the early Universe would be expected to undergo a series of dramatic transformations known as phase transitions, during which its expansion would accelerate by an enormous factor in a tiny fraction of a second. Such a period of ‘inflation’ is expected to flatten out the curvature of space leading to a definite prediction that the Universe should be flat. This seems to be consistent with the cosmic surveys mentioned above. Recent suggestions that the expansion of the Universe may be accelerating even now, suggest the existence of a mysterious dark energy that is perhaps some relic of the earlier inflationary phase.
Cosmologists have also applied modern supercomputers to the business of trying to understand the condensation of clumps of cosmic material into stars and galaxies
as the Universe expands and cools. These calculations have suggested that this process requires the existence of huge concentrations of exotic material, dense enough to assist the growth of structure, yet producing no starlight. This invisible stuff is called dark matter. Computer predictions of the structure formed are in close agreement with the huge maps being made by the observers lending further support to the Big Bang theory.
The interplay between these new theoretical ideas and new high-quality observational data has catapulted cosmology from the purely theoretical domain and into the field of rigorous experimental science. This process began at the beginning of the twentieth century, with the work of Albert Einstein.
Chapter 2
Einstein and all that
We are all aware of the effects of gravity. Objects fall to Earth when we drop them. It’s harder to run uphill than down. To a physicist, however, there is much more to gravity than its effects on our everyday lives. For one thing, the larger the scale of things being considered, the more important gravity becomes. Gravity pulls the Earth around the Sun, and the Moon around the Earth, and it causes tides. On the scale of things relevant to astronomy, gravity is the prime mover. So if you want to understand the Universe as a whole, you have to understand gravity.