Cosmology

• 1. Introduction

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  Cosmology is the study of the nature of the universe itself. Not one galaxy, not one solar system, not one planet, but everything that exists, has ever existed, or ever will. When you look up at the night sky, you are not just seeing the present. Theoretical astrophysicist David N. Spergel has described cosmology as a "historical science" because "when we look out in space, we look back in time" due to the finite nature of the speed of light. The further away a star appears, the older the light reaching your eyes. You are, in a sense, an archaeologist of the cosmos every time you look up. This raises questions that have driven human inquiry across civilizations: How did the universe begin? How old is it? What is it made of? And what will become of it? The word cosmology first appeared in English in 1656, in Thomas Blount's Glossographia, where it carried the meaning of "a speaking of the world". But the questions it names are far older than any dictionary.

• 2. Ancient Maps of Heaven

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  Greek philosophers Aristarchus of Samos, Aristotle, and Ptolemy each proposed a different theory of the cosmos, and for centuries their ideas shaped everything that came after. The geocentric model associated with Ptolemy placed a stationary Earth at the center of everything, with planets moving in circular epicycles around it. Ptolemy's model was, by one measure, the most successful universe model in history: it held sway for well over a thousand years. Charles Kahn, a historian of philosophy, traced the origins of ancient Greek cosmology back even further, to Anaximander, who proposed a purely mechanical model around 560 BCE. In Anaximander's vision, the Earth floated still at the center of the infinite, unsupported by anything. A ball of flame had once surrounded it like bark on a tree, then broke apart to form the rest of the universe. What Anaximander and his successors shared was the ambition to explain the cosmos through reason rather than through stories of gods and monsters. The philosopher Marcus Aurelius captured the stakes of this inquiry plainly: "He who does not know what the world is does not know where he is, and he who does not know for what purpose the world exists, does not know who he is, nor what the world is."

• 3. The Copernican Turn

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  Nicolaus Copernicus, and subsequently Johannes Kepler and Galileo Galilei, dismantled the Ptolemaic arrangement in the 16th century by placing the Sun at the center. This shift is widely regarded as one of the most famous examples of epistemological rupture in physical cosmology. Isaac Newton's Principia Mathematica, published in 1687, then gave the heliocentric model its physical backbone. Newton described the law of universal gravitation for the first time in that work. His theory accounted for the anomalies in earlier systems caused by gravitational interaction between the planets, and it resolved them. What made Newton's contribution philosophically decisive was a principle that carried Copernicus's logic further: the bodies on Earth obey exactly the same physical laws as every celestial body in the sky. This idea, called the Copernican principle, meant the universe was not divided into earthly and heavenly realms with separate rules. It was one continuous system, operating under a single set of laws, open to investigation from the ground beneath your feet.

• 4. The Expanding Universe

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  Modern scientific cosmology is widely considered to have begun in 1917, when Albert Einstein published his final modification of general relativity in a paper titled "Cosmological Considerations of the General Theory of Relativity". That paper, not widely available outside Germany until the end of the First World War, opened a new era of theorizing. It prompted cosmogonists including Willem de Sitter, Karl Schwarzschild, and Arthur Eddington to explore the astronomical consequences of the new mathematics. In 1922, Alexander Friedmann introduced the idea of an expanding universe containing moving matter, directly challenging the long-held assumption that the cosmos was static and unchanging. A parallel debate was reaching its own climax during this same period. Early cosmologists including Heber Curtis and Ernst Opik were determining that some of the nebulae visible in telescopes were separate galaxies lying far beyond our own. Curtis argued that spiral nebulae were independent star systems; Harlow Shapley, an astronomer at Mount Wilson, insisted the Milky Way was the whole cosmos. The disagreement became formalized as the Great Debate, which reached its peak on the 26th of April 1920 at a meeting of the U.S. National Academy of Sciences in Washington, D.C. Edwin Hubble settled the question in 1923 and 1924 by detecting Cepheid variable stars in the Andromeda Galaxy and establishing their distance as well beyond the edge of the Milky Way. Hubble's subsequent discovery of the redshift in 1929 then provided direct observational support for the expanding universe.

• 5. Building the Big Bang

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  Belgian priest Georges Lemaitre proposed the Big Bang model in 1927. His vision of an initial high-density state, which he called the "primeval atom", described a universe that began in extreme compression and expanded outward. Lemaitre is considered the father of the Big Bang model. Further confirmation arrived in 1964, when Arno Penzias and Robert Woodrow Wilson discovered the cosmic microwave background radiation, the faint thermal glow left over from the universe's earliest moments. The Lambda-CDM model, a specific parameterization of the Big Bang that incorporates dark matter and dark energy, became the dominant framework of modern physical cosmology. Its predictions were tested against observations from the COBE, WMAP, and Planck satellites, from large galaxy redshift surveys including 2dfGRS and SDSS, and from observations of distant supernovae and gravitational lensing. When these observations matched the predictions of the cosmic inflation theory, many researchers began calling the era since around 1990 the "golden age of cosmology". At the Planck 2014 meeting in Ferrara, Italy, on the 1st of December 2014, astronomers reported a precise breakdown of the universe's composition: 4.9% atomic matter, 26.6% dark matter, and 68.5% dark energy, with an age of 13.8 billion years.

• 6. Cosmology Beyond Physics

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  Not all cosmologies have been scientific in the modern sense. Religious and mythological cosmology draws on creation myths, eschatological traditions, and sacred texts, and these frameworks have described the universe in radically different terms across human history. The Hindu cosmology of the Rigveda, dated roughly between 1700 and 1100 BCE, described a cyclical universe in which primal matter remains manifest for 311.04 trillion years before returning to an unmanifest state for an equal length of time. The Babylonian literature from roughly 2300 to 500 BCE described a flat, circular Earth covered by a solid dome, all floating within infinite "waters of chaos". Fakhr al-Din al-Razi, working between 1149 and 1209 CE, proposed a multiverse in which God has the power to fill an infinite outer space with an infinite number of worlds. In Diderot's Encyclopedie, cosmology was divided into four branches: uranology, the science of the heavens; aerology, the science of the air; geology, the science of the continents; and hydrology, the science of waters. That classification captures something important. For much of human history, asking "what is the cosmos" and asking "what is the world around me" were the same question. Modern cosmology has narrowed its scope to the testable. But the broader tradition it grew from stretched across physics, philosophy, religion, and myth simultaneously.

• 7. Open Questions

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  Physical cosmology has become a precise, predictive science, but it has not closed every question it inherited. Some questions about the universe remain beyond the scope of scientific inquiry and may only be addressed through philosophical approaches. What is the first cause of the universe, if any? Does the cosmos have a purpose? Does consciousness play a role in the existence of reality? These questions have been posed under the categories of monism, pantheism, atomism, and teleology, among others. In 2014, the BICEP2 collaboration announced what appeared to be a detection of gravitational wave imprints in the cosmic microwave background, a finding that would have been a major confirmation of inflationary theory. The result was later found to be spurious: the supposed signal was in fact due to interstellar dust. That episode is a reminder that even in cosmology's golden age, the distance between a claimed discovery and a confirmed one remains large. The universe that began, according to the best current measurement, 13.799 plus or minus 0.021 billion years ago is still telling its story, and the instruments used to hear it are still being refined.