Astronomy and Cosmology: Setting the Cosmic Calendar: Arguing the Age of the Cosmos and Earth
Astronomy and Cosmology: Setting the Cosmic Calendar: Arguing the Age of the Cosmos and Earth
Astronomy and Cosmology: Setting the Cosmic Calendar: Arguing the Age of the Cosmos and Earth
Cosmology and geochronology, the studies of the age of the universe and Earth, respectively, have generally been separate disciplines that relied on separate sources of evidence. Cosmologists tend to study the ages of star clusters and the speed of the universe's expansion to determine its age; geochronologists study many factors, including erosion, ocean salinity, Earth's cooling, fossils, and—most importantly—the decay of radioactive elements in rocks and meteorites. Various contradictions between these categories of evidence long seemed to exist, but all these puzzles have since been solved by the collection of better information.
In the mid-twentieth century scientists determined that Earth formed from a primordial disk of gas and dust orbiting the young sun about 4.5 billion years ago; both Earth and the sun contain heavy elements forged in the hearts of older generations of stars. By the late twentieth century scientists had calculated the age of the universe itself at 13.7 billion years.
Historical Background and Scientific Foundations
Scientific and Cultural Preconceptions
Creation myths were passed from generation to generation by most ancient societies. For example, the Egyptians believed that in the beginning, Nun, the father of the gods, caused an island of dry land to emerge from the primordial waters. In the Hindu belief system, the universe passes through endless repetitive cycles of destruction and rebirth, each lasting about 34 billion years. Few creation myths, however, if any, specified a particular age for Earth or considered that the universe as a whole might be far larger than the known heavens, or of a different age.
Ancient Greek philosophers argued among themselves about the history of Earth. Plato (428–348 BC) held that everything had been created by a divine craftsman or Demiurge at some point in the past; his most famous student, Aristotle (384–322 BC), maintained that the universe had always existed and always would, without beginning or end.
Beginning with Hebrew scripture and in Jewish, Christian, and Muslim societies as well, assumptions about the world's beginnings were based mostly on the Hebrew text known today as the biblical book of Genesis. According to this narrative, the universe had a definite beginning. Its history is unique and noncyclic, more like the telling of a story than like the turning of a wheel.
Fossils, which became important to our understanding of Earth's age beginning in the nineteenth century, were known to ancient cultures but were not usually seen as the remains of living things. Because of their distinctive and beautiful patterns, they were sometimes used in Neolithic times (starting about 8000 BC in the Near East, about 4000 BC in Europe) as tomb offerings. Some Native Americans made personal ornaments out of them
The Greeks, however, correctly identified fossils as the remains of living creatures. Pythagoras (580–500 BC), for example, deduced that by finding seashell fossils at high altitudes those mountains must have been under water at some earlier time. The Greek view did not persist, however, and by the Middle Ages the prevalent European view was that fossils had formed in the earth by some natural process (much as we now understand crystals to be formed). When philosophers did consider fossils as imprints of long-dead plants and animals, they usually assumed that they had been created by the great Flood described in Genesis.
An unbroken genealogy is given in the Bible from the first man, Adam, to early historical times. Until as recently as the nineteenth century, many people assumed that it is possible to calculate Earth's age from the ages of the men named in this genealogy. The Christian philosopher and bishop Theophilus of Antioch (d. AD 180) made one of the first such estimates in the second century AD, calculating that Earth was created in 5529 BC. Throughout the Middle Ages there was general agreement, based on calculations such as Theophilus's, that the world had been created about 6,000 years before. The matter did not seem to be of particularly great importance and was not widely debated for many centuries. Both culturally and scientifically the question was taken to be both settled and of little importance until the Scientific Revolution began to unsettle established thoughts about the physical world beginning in the late 1500s.
The Science: How Old Is Earth?
In seventeenth-century England, scholarly interest in the age of the world became widespread. Scientists made many calculations of the date of creation based on the chain of births and deaths recorded in the Hebrew Bible. However, because of ambiguities in the original texts and differences between various ancient copies, there was no single definite way to calculate the date of creation from the Bible. The most famous estimate was made by Archbishop of Ireland James Ussher (1581–1686), who announced that creation took place on the evening of Saturday, October 22, 4004 BC.
IN CONTEXT: LORD KELVIN STRIKES OUT
In the late nineteenth century, English physicist Sir William Thomson, Baron Kelvin (1824–1907), committed one of the most famous blunders in the history of science.
Charles Darwin (1809–1882), proponent of evolution by natural selection, calculated in the 1850s that the erosion of certain English landscapes must have taken over 300 million years to occur, considered a shocking figure at that time. Darwin even opined that this was “a mere trifle” compared to the time intervals implied by some other geological features. This was an important concept because Darwin believed that natural selection needed vast stretches of time to produce the many fossil records seen in the modern world.
Thomson, who disliked the theory of evolution, decided to set Darwin straight. He began his calculations by assuming that Earth and the sun formed at about the same time by the falling-together of particles. The friction of the collisions between these particles would release energy, accounting for the interior heat of Earth and the heat of the sun. After forming, Earth and the sun began to cool. Lord Kelvin concluded in 1868 that Earth was at most 100 million years old. Other physicists soon decided that this figure was too high, that Earth could be no more than 20 to 40 million years old.
But they were wrong. Planets do, as Thomson assumed, start out hot because of the conversion of gravitational potential energy to heat: Jupiter, Saturn, and Neptune still radiate more heat than they receive from the sun. Earth, however, being comparatively small, radiated away most of its heat of formation. Despite the loss of this original heat, its interior has been kept hot (12,992° F/7,200°C at the core) as energy is released by the radioactive breakdown of certain elements, especially potassium, thorium, and uranium. Since radioactivity wasn't discovered until 1896, however, Thomson and his fellow physicists could not consider it in their calculations. Darwin and the geologists were right: Earth is not a few dozen million years old, but about 4.5 thousand million years old.
In the late 1600s, as the Scientific Revolution spread to more and more areas of thought, speculation began about how natural processes had formed various landscapes over time. These led directly to the possibility that Earth might be much older than 4,000 to
10,000 years. For example, English skeptic and fossil collector Edward Lhwyd (1660–1709) observed that in a certain valley in Wales littered with thousands of boulders, a fresh boulder rolled down the slopes two or three times in a human lifetime. Assuming a constant rate of boulder-fall, Lhwyd calculated from the number of boulders in the valley that Earth must be about 200,000 to 300,000 years old. Lhwyd's boulders were actually deposited by glaciers much more recently than that, and Earth itself is actually about 15,000 times older than he guessed, but he was on the right track.
Speculation about Earth's great age flourished in the 1700s along with the infant science of geology. Scottish geologist James Hutton (1726–1797) argued in the 1780s that Earth's surface is recycled endlessly by the slow erosion of exposed rocks, deposit of eroded rock grains (sediments) at the bottoms of oceans, and uplift of those sediments, now hardened to rock, to be exposed again to erosion. “Time,” Hutton told the Royal Society of Edinburgh in 1781, “which measures everything in our idea, and is often deficient to our schemes, is to nature endless and as nothing.” He asserted that Earth's history is cyclic and devoid of significant change, having “no vestige of a beginning,—no prospect of an end.” He did not speculate on Earth's exact age.
In the nineteenth century, geology split into two contending schools of thought: gradualism and catastrophism. Gradualists, Hutton's intellectual heirs, taught that Earth's surface had been shaped over vast stretches of time by gradual, observable, ongoing processes such as erosion. Earth was shaped grain by grain, by erosion and deposition, over very great stretches of time—millions of years. Catastrophists, of whom French scientist Georges, Baron Cuvier (1769–1832) was the most famous, insisted on a much younger Earth and explained geology in terms of catastrophes—sudden, violent events such as volcanic eruptions and a worldwide biblical Flood.
Gradualism became the majority view in geology from about 1850 onward. A major figure in this shift was English geologist Sir Charles Lyell (1797–1875), whose arguments for gradualism persuaded Charles Darwin (1809–1882), proponent of evolution by natural selection, to look for gradualistic causes operating over deep time on the structure of both landscapes and living things.
Support for gradualism came partly from the new science of paleontology (the study of fossils) and stratigraphy, the study of Earth's thousands of rock layers, or strata (singular “stratu”). For centuries most European thinkers had seen fossils either as natural formations akin to crystals or relics of the biblical Flood. If fossils were relics of the Flood, they must have all been laid down at the same time, in a matter of months. In the early nineteenth century, however, this view was challenged. Not only was the concept of deep time—an age for the world far greater than 5,000 or 10,000 years—gaining ground, so were theories of evolution, several of which had already been put forward in the 1700s by thinkers such as Erasmus Darwin (1731–1802, Charles's grandfather) and Jean-Baptiste de Monet, chevalier de Lamarck (1744–1829).
In France, Cuvier published a study of Parisian geology, including fossils. In 1816, Englishman William Smith (1769–1839) began to publish his meticulous Strata Identified by Organized Fossils and Stratigraphical System of Organized Fossils. These cataloged fossils in British rocks and announced two basic principles of stratigraphy. First, the fossils found in each layer of rock are mostly distinct from those found in other layers of rock. Second, deeper strata are older than the layers above them, except in cases where it can be shown that geological movements have turned the rocks upside down. Over the next 30 years, various scientists greatly advanced the new science of biostratigraphy—the study of sequences of rock layers based on the fossils found within them. The entire sequence of strata found in Earth, from oldest at the bottom to most recent at the top, is termed the geologic column.
Stratigraphy alone cannot precisely date rocks or fossils, however. Layers of particles can be deposited at greatly different rates, so merely measuring their thickness or counting their number does not reveal their age. However, by becoming acquainted with the basic structure of the geologic column, paleontologists of the early nineteenth century did the essential work that eventually allowed others to measure the depth of deep time. The sheer height of the column, its great number of layers, indicated great amounts of time, though nobody could yet say exactly how much. Darwin estimated the vertical thickness of some sedimentary rock formations at 13.7 miles (22 kilometers), which clearly could not have been laid down by the normal processes of erosion in less than many millions of years, and certainly not by a single great Flood.
Darwin and others also noticed the fossils of the geologic column contained a suggestive series of fossils: the oldest layers tended to contain simpler creatures, the most recent ones more complex creatures. This suggested a history of development, rather than the sudden and nearly simultaneous creation of all creatures, as suggested by literal readings of Genesis. Also, nearby layers tended to contain creatures with similar forms. There were jumps between the layers, just as there are jumps between the individual frames of a motion picture, but viewed as a whole the layers told a story of continuous change. That story must have taken place over a very long time, and Darwin argued that it was a story of evolutionary descent from one species to another.
In his groundbreaking book On the Origin of Species by Means of Natural Selection (1859), Darwin displayed the stratigraphic evidence for an ancient Earth: “the lofty pile of sedimentary rocks in Britain gives but an inadequate idea of the time which has elapsed during their accumulation. The consideration of these various facts impresses the mind almost in the same manner as does the vain endeavour to grapple with the idea of eternity.” To Darwin, the geologic column hinted both that Earth was ancient and that life on Earth had changed over deep time in an evolutionary way.
In the mid- to late 1800s, disagreement over Earth's age grew. Physicists, of whom Sir William Thomson, Baron Kelvin (1824–1907) was typical, argued that Earth could not be as old as the geologists and biologists said it was, because its interior was still hot. If it had formed hundreds of millions of years ago, they reasoned, it would be cooler by now. These scientists were wrong—Earth is, in fact, 4.5 thousand million years old, not the mere 100 million or less proposed by Lord Kelvin and others—but they had some excuse, for the true source of Earth's present-day interior heat, radioactivity, had not yet been discovered.
The discovery of radioactivity by French physicist Antoine-Henri Becquerel (1852–1908) in 1896 not only reconciled the physicists to the idea of a very old Earth, but gave geologists a new tool with which to study its age. In 1902 English physicist Ernest Rutherford (1871–1937) and his colleagues proposed that radioactivity could be explained by the breakdown of atoms into other elements, and that energy is released in this process. Some of these new elements would also be radioactive, and would break down into still others in a sequence of transformations (termed a radioactive decay series), that ended when a stable element, such as lead, was produced. Since this process of radioactive breakdown proceeds at a predictable rate and is very slow for some elements, including the common metal uranium 238, the age of a rock sample can be judged by the amounts of various elements it contains. In concept, the process is not very different from judging how long a piece of cheese has been in the refrigerator by seeing how much of it has turned into mold.
In 1905 American scientist Bertram Boltwood (1870–1927) estimated from uranium and lead ratios that certain rocks were 400 million to 2.2 billion years old. At first, some scientists disputed these figures. For example, Irish scientist John Joly (1857–1933) argued that uranium's radioactive decay rate might not be constant over long time periods, as Boltwood had assumed. These objections were answered by further research, vindicating the new technique.
In 1911 a breakthrough paper was published by British geologist Arthur Holmes (1890–1965). Using radioactive dating methods, Holmes specified dates for certain parts of the geologic column. The geologic column was now more than a list of rock layers: it was also a calendar of Earth's history. Holmes dated the Silurian/Ordovician period at 430 million years of age and the Devonian at 370 million years. By 2004, almost a century later, these estimates had been adjusted to 443.7 and 416 million years respectively. The original estimates had proved remarkably accurate.
Over the next few decades, Holmes and others systematically dated the geologic column, piece by piece. By the 1930s they estimated Earth to be about 2 to 3 billion years old; by the late 1940s they had produced reasonably accurate dates for the entire geologic column. In 1947 Holmes calculated Earth's age as at least 3.35 billion years. In 1954 scientists adjusted this estimate by comparing terrestrial mixtures of lead isotopes (atoms of the same element with different numbers of neutrons in their nuclei) to those found in meteorites. Their new estimate of Earth's age was 4.5 billion years. This figure has withstood a half century of further testing based on several independent sources of evidence.
In the early twentieth century, even as estimates of Earth's age were being firmly grounded on radioactive dating, ideas about the age of the universe as a whole were also beginning to take shape. German physicist Albert Einstein (1879–1955) perfected his theory of general relativity in 1915 to explain gravity and describe the overall structure of the universe. It allowed mathematically for the universe to be expanding, stable, or collapsing, so Einstein, assuming that the universe had always existed, adjusted his equations to stabilize it. In 1918, however, red-shifted galaxies were first observed. Red shift occurs when a light source, such as a distant galaxy, moves away from Earth. The color of the observed light is made redder (shifted toward the red end of the spectrum) by the motion; the faster the recession, the redder the light. Red shift revealed an expanding universe, and an expanding universe could not have been expanding forever.
In 1922 the big bang theory was proposed by Russian astronomer Alexander Friedmann (1888–1925) and Belgian astronomer and priest Georges-Henri Lemaître (1894–1966). According to this theory, now accepted by scientists, the universe originated in a great explosion at some definite time in the past. In 1929 American astronomer Edwin Hubble (1889–1953) announced his discovery of a mathematical law linking red shift firmly to big bang theory. According to Hubble's law, the amount of red shift from a distant galaxy is directly proportional to its distance: a galaxy twice as far away has twice the red shift.
Hubble's law allowed the age of the universe to be calculated. By running the universe's expansion backward (mathematically), one could specify how long ago all the galaxies were together at a single point—the beginning. However, the age derived from Hubble's law was embarrassingly small—a mere 1.5 billion years. Geochronology had already established that Earth itself was much older. The problem turned out to be with the value estimated for Hubble's constant of proportionality between red shift and distance.
In the following years, the Hubble constant was refined, and by the 1950s, the age of the universe, as estimated from expansion, had grown to 10 billion years. This gave plenty of time for Earth to have formed a few billion years ago. Yet another apparent contradiction persisted, however: the oldest objects in the universe, globular star clusters (relatively small, spherical groupings of thousands of stars that orbit the Milky Way and other galaxies) seemed to be older than the universe itself.
Astronomers estimate the age of a star cluster by comparing how many young blue stars it contains to how many old red stars it contains. In the 1980s the oldest globular star clusters were estimated to be 16 to 20 billion years old, but the age of the universe as derived from big bang theory and red shift observations was still only about 10 billion years. As recently as 1997 the oldest star clusters were estimated at 12 to 18 billion years of age, older than the universe. At that time, debate still swirled around the question of the universe's age, with competing teams of researchers arguing the point, sometimes angrily.
But improved measurements gradually ate away at the apparent contradiction from both ends. By 1999 the oldest globular star clusters were estimated at only 12.2 billion years, just a little older than the age of the universe estimated from cosmic expansion. In 2003 these disputes were finally put to rest: A NASA satellite launched in 2001, the Wilkinson Microwave Anisotropy Probe (WMAP), had accumulated enough data about the universe-permeating microwave glow left over from the big bang to make an accurate estimate of the age
of the universe. The answer turned out to be 13.7 billion years, give or take about 1% (200 million years). The globular clusters were, then, younger than the universe, and all was well. In 2006 further data from WMAP narrowed the uncertainty to a mere 60 million years—less than one half of one percent. The universe is 13.7 billion years old.
Impact on Science
In the nineteenth century, discovery of the great age of Earth and of the universe in which it resided changed the framework in which geology, biology, and astronomy operated. New meanings could now be perceived in old rocks. Darwin could not only conceive of the process of evolution by natural selection, but could find evidence in the rocks that there had been ample time for that process to produce what he called, in the famous last sentence of On the Origin of Species, “endless forms most beautiful and most wonderful.”
In astronomy, the idea of an old Earth implied that the entire universe was old, too. This opened up the possibility that the stars, like man, had evolved over many millions of years. Today, the process by which stars form, change, and expire is known as stellar evolution. Without the knowledge that Earth and the universe have existed for billions of years, science could not begin to accurately unravel the story of the world, nor solve many particular puzzles about its nature.
Impact on Society
The nineteenth-century discovery of the great age of Earth disturbed many people. They had based their views of the meaning of life on certain traditional religious teachings, including literal interpretations of the Bible, and these views often included the belief that the entire universe, including both stars above and Earth below, had been created by God for human beings, who had themselves been created from dust a mere 4,000 years or so ago. Now, however, scientists claimed that human beings had descended from other animals and that they had existed for only a tiny fraction of Earth's history. As American novelist Mark Twain (1835–1910) put it, “If the Eiffel Tower were now representing the world's age, the skin of paint on the pinnacle-knob at its summit would represent man's share of that age.” The new knowledge was both awesome and irksome. For those whose values had depended on the idea that our species loomed large on the stage of universal history, it was disastrous.
It is therefore not surprising that many religious authorities attacked Darwin and the geologists for their claims about evolution and deep time. In a famous 1860 debate, English philosopher Thomas Huxley (1825–1895) successfully defended Darwin against Church of England Bishop Samuel Wilberforce (1805–1873) at Oxford University, an encounter usually cited as an important defeat for traditional religious views. Other scholars, however, such as David Livingstone, noted that some nineteenth-century religious thinkers adapted their views to the new science rapidly and even defended them against conservative religious critics. The Catholic Church, which had blundered in the early seventeenth century by officially condemning the Copernican view that Earth goes around the sun, refrained from condemning evolution and ancient-Earth ideas.
Modern Cultural Connections
In recent decades, the age of the universe has been a matter of intense scientific debate. These were resolved in 2003 with the proof, from WMAP data, that the universe is almost exactly 13.7 billion years old. Estimates of the ages of the oldest stars and of Earth itself now are consistent with this number.
Yet fundamental issues remain. When scientists say that the universe is 13.7 billion years old, they are referring to how much time has elapsed since its explosion from a primordial fireball, the big bang. Whether the universe existed in some form before that point is uncertain. Some physicists suggest that the universe may be far older, perhaps even infinite. This would be the case if the history of the universe since the big bang were simply the latest repetition of an endless cycle of rebirth and destruction (as in ancient Hindu belief).
According to these cyclic models, the cosmic expansion we are presently observing will someday reverse, ending in a Big Crunch as all matter and energy reunite at a single point many billions of years hence. This will be followed by a new big bang and a new cycle of expansion and contraction. Such cyclic or oscillatory models were first introduced in the 1930s and are still taken seriously by cosmologists.
Outside the scientific arena, the age of Earth remains an important issue for millions of people worldwide, especially creationists—people who believe that the human race (and usually other species as well) were created directly by God without evolution. Creationists who additionally claim that Earth was created only about 10,000 years ago are termed “young-Earth creationists.” Although it is difficult to estimate how many young-Earth creationists there are today, news accounts have cited a figure of about 5% for the population of Australia, a stronghold of creationism. If this is correct and a similar figure holds for the United States, another industrialized, English-speaking country where creationism is popular, there are about 15 million people in the United States alone who believe that Earth is only 10,000 or so years old. The actual figure may be higher: In a 1997 Gallup poll 44% of Americans agreed with the statement that “God created man pretty much in his present form at one time within the last 10,000 years.”
The political and cultural struggle between young-Earth creationists, scientists, and mainstream religious thinkers who accept the scientific world-picture is prominent in the United States and some other countries. For example, young-Earth creationists have sought to alter science textbooks used in public schools to counter evolution, the big bang, and the great age of Earth. It should be noted that there is no debate within the scientific community about the factuality of evolution or the great age of Earth and universe. In Europe, creationism is comparatively rare.
See Also Astronomy and Cosmology: A Mechanistic Universe; Astronomy and Cosmology: Big Bang Theory and Modern Cosmology; Astronomy and Cosmology: Cosmology; Astronomy and Cosmology: Western and Non-Western Cultural Practices in Ancient Astronomy; Astronomy and Space Science: Astronomy Emerges from Astrology.
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