Cosmology

Except for the few stars in the foreground (which are bright and easily recognizable because only they have diffraction spikes), every speck of light in the composite photo is an individual galaxy, some of them as old as 13.2 billion years; the observable universe is estimated to contain more than 2 trillion galaxies. From the Hubble eXtreme Deep Field.

Cosmology (from Ancient Greek κόσμος (cosmos) 'the universe, the world' and λογία (logia) 'study of') is the study of the nature of the universe, the cosmos. The term cosmology was first used in English in 1656 in Thomas Blount's Glossographia, with the meaning of "a speaking of the world". In 1731, German philosopher Christian Wolff used the term cosmology in Latin (cosmologia) to denote a branch of metaphysics that deals with the general nature of the physical world. Cosmology is investigated by scientists, including astronomers and physicists, as well as philosophers, such as metaphysicians, philosophers of physics, and philosophers of space and time. Because of this shared scope with philosophy, theories in physical cosmology may include both scientific and non-scientific propositions and may depend upon assumptions that cannot be tested. Religious or mythological cosmology is a body of beliefs based on mythological, religious, and esoteric literature and traditions of creation myths and eschatology.

In the science of astronomy, physical cosmology is concerned with the study of the chronology of the universe, the observable universe's origin, its large-scale structures and dynamics, and the ultimate fate of the universe, including the physical laws that govern these areas. Physical cosmology is a sub-branch of astronomy that is concerned with the universe as a whole. Modern physical cosmology is dominated by the Big Bang Theory which attempts to bring together observational astronomy and particle physics; more specifically, a standard parameterization of the Big Bang with dark matter and dark energy, known as the Lambda-CDM model.

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.

Physics and astrophysics have played central roles in shaping our understanding of the universe through scientific observation and experiment. Physical cosmology was shaped through both mathematics and observation in an analysis of the whole universe. The universe is generally understood to have begun with the Big Bang, followed almost instantaneously by cosmic inflation, an expansion of space from which the universe is thought to have emerged 13.799 ± 0.021 billion years ago. Cosmogony studies the origin of the universe, and cosmography maps the features of the universe.

In Diderot's Encyclopédie, cosmology is broken down into uranology (the science of the heavens), aerology (the science of the air), geology (the science of the continents), and hydrology (the science of waters).

Metaphysical cosmology has also been described as the placing of humans in the universe in relationship to all other entities. This is exemplified by Marcus Aurelius's observation that a man's place in that relationship: "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."

Physical cosmology is the branch of physics and astrophysics that deals with the study of the physical origins and evolution of the universe. It also includes the study of the nature of the universe on a large scale. In its earliest form, it was what is now known as "celestial mechanics," the study of the heavens. Greek philosophers Aristarchus of Samos, Aristotle, and Ptolemy proposed different cosmological theories. The geocentric Ptolemaic system was the prevailing theory until the 16th century when Nicolaus Copernicus, and subsequently Johannes Kepler and Galileo Galilei, proposed a heliocentric system. This is one of the most famous examples of epistemological rupture in physical cosmology.

Isaac Newton's Principia Mathematica, published in 1687, was the first description of the law of universal gravitation. It provided a physical mechanism for Kepler's laws and also allowed the anomalies in previous systems, caused by gravitational interaction between the planets, to be resolved. A fundamental difference between Newton's cosmology and those preceding it was the Copernican principle—that the bodies on Earth obey the same physical laws as all celestial bodies. This was a crucial philosophical advance in physical cosmology.

Modern scientific cosmology is widely considered to have begun in 1917 with Albert Einstein's publication of his final modification of general relativity in the paper "Cosmological Considerations of the General Theory of Relativity" (although this paper was not widely available outside of Germany until the end of World War I). General relativity prompted cosmogonists such as Willem de Sitter, Karl Schwarzschild, and Arthur Eddington to explore its astronomical ramifications, which enhanced the ability of astronomers to study very distant objects. Physicists began changing the assumption that the universe was static and unchanging. In 1922, Alexander Friedmann introduced the idea of an expanding universe that contained moving matter.

In parallel to this dynamic approach to cosmology, one long-standing debate about the structure of the cosmos was coming to a climax – the Great Debate (1917 to 1922) – with early cosmologists such as Heber Curtis and Ernst Öpik determining that some nebulae seen in telescopes were separate galaxies far distant from our own. While Heber Curtis argued for the idea that spiral nebulae were star systems in their own right as island universes, Mount Wilson astronomer Harlow Shapley championed the model of a cosmos made up of the Milky Way star system only. This difference of ideas came to a climax with the organization of the Great Debate on 26 April 1920 at the meeting of the U.S. National Academy of Sciences in Washington, D.C. The debate was resolved when Edwin Hubble detected Cepheid Variables in the Andromeda Galaxy in 1923 and 1924. Their distance established spiral nebulae well beyond the edge of the Milky Way.

Subsequent modelling of the universe explored the possibility that the cosmological constant, introduced by Einstein in his 1917 paper, may result in an expanding universe, depending on its value. Thus the Big Bang model was proposed by the Belgian priest Georges Lemaître in 1927 which was subsequently corroborated by Edwin Hubble's discovery of the redshift in 1929 and later by the discovery of the cosmic microwave background radiation by Arno Penzias and Robert Woodrow Wilson in 1964. These findings were a first step to rule out some of many alternative cosmologies.

Since around 1990, several dramatic advances in observational cosmology have transformed cosmology from a largely speculative science into a predictive science with precise agreement between theory and observation. These advances include observations of the microwave background from the COBE, WMAP and Planck satellites, large new galaxy redshift surveys including 2dfGRS and SDSS, and observations of distant supernovae and gravitational lensing. These observations matched the predictions of the cosmic inflation theory, a modified Big Bang theory, and the specific version known as the Lambda-CDM model. This has led many to refer to modern times as the "golden age of cosmology".

In 2014, the BICEP2 collaboration claimed that they had detected the imprint of gravitational waves in the cosmic microwave background. However, this result was later found to be spurious: the supposed evidence of gravitational waves was in fact due to interstellar dust.

On 1 December 2014, at the Planck 2014 meeting in Ferrara, Italy, astronomers reported that the universe is 13.8 billion years old and composed of 4.9% atomic matter, 26.6% dark matter and 68.5% dark energy.

Religious or mythological cosmology is a body of beliefs based on mythological, religious, and esoteric literature and traditions of creation and eschatology. Historically, religious cosmology and myths have intertwined with scientific and philosophical understandings, but recent scientific cosmology has challenged many aspects of religious cosmology. Generally, scientific cosmology is grounded in empirical evidence and observations, while religious and mythological cosmology relies on the persuasive arguments by prophets, philosophers, or mathematicians.

Representation of the observable universe on a logarithmic scale. Distance from the Sun increases from center to edge. Planets and other celestial bodies were enlarged to appreciate their shapes.

Cosmology deals with the world as the totality of space, time and all phenomena. Historically, it has had quite a broad scope, and in many cases was found in religion. Some questions about the Universe are beyond the scope of scientific inquiry but may still be interrogated through appeals to other philosophical approaches like dialectics. Some questions that are included in extra-scientific endeavors may include:

• What is the origin of the universe? What is its first cause (if any)? Is its existence necessary? (see monism, pantheism, emanationism and creationism)
• What are the ultimate material components of the universe? (see mechanism, dynamism, hylomorphism, atomism)
• What is the ultimate reason (if any) for the existence of the universe? Does the cosmos have a purpose? (see teleology)
• Does the existence of consciousness have a role in the existence of reality? How do we know what we know about the totality of the cosmos? Does cosmological reasoning reveal metaphysical truths? (see epistemology)

Charles Kahn, a historian of philosophy, attributed the origins of ancient Greek cosmology to Anaximander.

Table notes: the term "static" simply means not expanding and not contracting. Symbol G represents Newton's gravitational constant; Λ (Lambda) is the cosmological constant.

• Bragg, Melvyn (2023). "The Universe's Shape". bbc.co.uk. BBC. Melvyn Bragg discusses shape, size and topology of the universe and examines theories about its expansion. If it is already infinite, how can it be getting any bigger? And is there really only one?

• "Cosmic Journey: A History of Scientific Cosmology". history.aip.org. American Institute of Physics. 2023. The history of cosmology is a grand story of discovery, from ancient Greek astronomy to -space telescopes.

• Dodelson, Scott; Schmidt, Fabian (2020). Modern Cosmology 2nd Edition. Academic Press. ISBN 978-0128159484.

• Charles Kahn. 1994. Anaximander and the Origins of Greek Cosmology. Indianapolis: Hackett.

• "Genesis, Search for Origins. End of mission wrap up". genesismission.jpl.nasa.gov. NASA, Jet Propulsion Laboratory, California Institute of Technology. December 2017. About 4.6 billion years ago, the solar nebula transformed into the present solar system. In order to chemically model the processes which drove that transformation, we would, ideally, like to have a sample of that original nebula to use as a baseline from which we can track changes.

• Lyth, David (12 December 1993). "Introduction to Cosmology". arXiv:astro-ph/9312022. These notes form an introduction to cosmology with special emphasis on large scale structure, the cmb anisotropy and inflation. Lectures given at the Summer School in High Energy Physics and Cosmology, ICTP (Trieste) 1993.) 60 pages, plus 5 Figures.

• "NASA/IPAC Extragalactic Database (NED)". ned.ipac.caltech.edu. NASA. 2023. April 2023 Release Highlights Database Updates

  • "NASA/IPAC Extragalactic Database (NED)". ned.ipac.caltech.edu. NASA. 2020. NED-D: A Master List of Redshift-Independent Extragalactic Distances

• Sophia Centre. The Sophia Centre for the Study of Cosmology in Culture, University of Wales Trinity Saint David.

• Choquet-Bruhat, Yvonne (2014). Introduction to General Relativity, Black Holes, and Cosmology. Oxford University Press. ISBN 9780191936500.

• Lectures on Cosmology by David Tong

• Wright, Edward L. (24 May 2013). "Frequently Asked Questions in Cosmology". Los Angeles: Division of Astronomy & Astrophysics, University of California, Los Angeles.