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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.[1] From the Hubble eXtreme Deep Field.

Cosmology (from Ancient Greek κόσμος (cosmos) 'the universe, the world' and λογία (logia) 'study of') is a branch of physics and metaphysics dealing with 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".[2] 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.[3] 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, cosmology is concerned with the study of the chronology of the universe.

Physical cosmology is the study of the observable universe's origin, its large-scale structures and dynamics, and the ultimate fate of the universe, including the laws of science that govern these areas.[4] It 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. 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;[5][6] 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.[7]

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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.[8] 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).[9]

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."[10]

Discoveries

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Physical cosmology

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Main article: Physical cosmology
See also: Observational cosmology

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"[11] (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.

Part of a series on
Physical cosmology
Full-sky image derived from nine years' WMAP data
Early universe
Backgrounds
Expansion · Future
Components · Structure
Components
Structure
Experiments
Scientists
Subject history

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.[12] 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.[13][14] 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[15] which was subsequently corroborated by Edwin Hubble's discovery of the redshift in 1929[16] and later by the discovery of the cosmic microwave background radiation by Arno Penzias and Robert Woodrow Wilson in 1964.[17] 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,[18] WMAP[19] and Planck satellites,[20] large new galaxy redshift surveys including 2dfGRS[21] and SDSS,[22] 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".[23]

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.[24][25]

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.[26]

Religious or mythological cosmology

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See also: Religious cosmology

Religious or mythological cosmology is a body of beliefs based on mythological, religious, and esoteric literature and traditions of creation and eschatology. Creation myths are found in most religions, and are typically split into five different classifications, based on a system created by Mircea Eliade and his colleague Charles Long.

  • Types of Creation Myths based on similar motifs:
    • Creation ex nihilo in which the creation is through the thought, word, dream or bodily secretions of a divine being.
    • Earth diver creation in which a diver, usually a bird or amphibian sent by a creator, plunges to the seabed through a primordial ocean to bring up sand or mud which develops into a terrestrial world.
    • Emergence myths in which progenitors pass through a series of worlds and metamorphoses until reaching the present world.
    • Creation by the dismemberment of a primordial being.
    • Creation by the splitting or ordering of a primordial unity such as the cracking of a cosmic egg or a bringing order from chaos.[27]

Philosophy

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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.[28] 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:[29][30]

  • 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.[31]

Historical cosmologies

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Further information: Timeline of cosmological theories
Name Author and date Classification Remarks
Hindu cosmology Rigveda (c. 1700–1100 BCE) Cyclical or oscillating, Infinite in time Primal matter remains manifest for 311.04 trillion years and unmanifest for an equal length of time. The universe remains manifest for 4.32 billion years and unmanifest for an equal length of time. Innumerable universes exist simultaneously. These cycles have and will last forever, driven by desires.
Zoroastrian Cosmology Avesta (c. 1500–600 BCE) Dualistic Cosmology According to Zoroastrian Cosmology, the universe is the manifestation of perpetual conflict between Existence and non-existence, Good and evil and light and darkness. the universe will remain in this state for 12000 years; at the time of resurrection, the two elements will be separated again.
Jain cosmology Jain Agamas (written around 500 CE as per the teachings of Mahavira 599–527 BCE) Cyclical or oscillating, eternal and finite Jain cosmology considers the loka, or universe, as an uncreated entity, existing since infinity, the shape of the universe as similar to a man standing with legs apart and arm resting on his waist. This Universe, according to Jainism, is broad at the top, narrow at the middle and once again becomes broad at the bottom.
Babylonian cosmology Babylonian literature (c. 2300–500 BCE) Flat Earth floating in infinite "waters of chaos" The Earth and the Heavens form a unit within infinite "waters of chaos"; the Earth is flat and circular, and a solid dome (the "firmament") keeps out the outer "chaos"-ocean.
Eleatic cosmology Parmenides (c. 515 BCE) Finite and spherical in extent The Universe is unchanging, uniform, perfect, necessary, timeless, and neither generated nor perishable. Void is impossible. Plurality and change are products of epistemic ignorance derived from sense experience. Temporal and spatial limits are arbitrary and relative to the Parmenidean whole.
Samkhya Cosmic Evolution Kapila (6th century BCE), pupil Asuri Prakriti (Matter) and Purusha (Consiouness) Relation Prakriti (Matter) is the source of the world of becoming. It is pure potentiality that evolves itself successively into twenty four tattvas or principles. The evolution itself is possible because Prakriti is always in a state of tension among its constituent strands known as gunas (Sattva (lightness or purity), Rajas (passion or activity), and Tamas (inertia or heaviness)). The cause and effect theory of Sankhya is called Satkaarya-vaada (theory of existent causes), and holds that nothing can really be created from or destroyed into nothingness—all evolution is simply the transformation of primal Nature from one form to another.[citation needed]
Biblical cosmology Genesis creation narrative Earth floating in infinite "waters of chaos" The Earth and the Heavens form a unit within infinite "waters of chaos"; the "firmament" keeps out the outer "chaos"-ocean.
Anaximander's model Anaximander (c. 560 BCE) Geocentric, cylindrical Earth, infinite in extent, finite time; first purely mechanical model The Earth floats very still in the centre of the infinite, not supported by anything.[32] At the origin, after the separation of hot and cold, a ball of flame appeared that surrounded Earth like bark on a tree. This ball broke apart to form the rest of the Universe. It resembled a system of hollow concentric wheels, filled with fire, with the rims pierced by holes like those of a flute; no heavenly bodies as such, only light through the holes. Three wheels, in order outwards from Earth: stars (including planets), moon, and a large Sun.[33]
Atomist universe Anaxagoras (500–428 BCE) and later Epicurus Infinite in extent The universe contains only two things: an infinite number of tiny seeds (atoms) and the void of infinite extent. All atoms are made of the same substance, but differ in size and shape. Objects are formed from atom aggregations and decay back into atoms. Incorporates Leucippus' principle of causality: "nothing happens at random; everything happens out of reason and necessity". The universe was not ruled by gods.[citation needed]
Pythagorean universe Philolaus (d. 390 BCE) Existence of a "Central Fire" at the center of the Universe. At the center of the Universe is a central fire, around which the Earth, Sun, Moon and planets revolve uniformly. The Sun revolves around the central fire once a year, the stars are immobile. The Earth in its motion maintains the same hidden face towards the central fire, hence it is never seen. First known non-geocentric model of the Universe.[34]
De Mundo Pseudo-Aristotle (d. 250 BCE or between 350 and 200 BCE) The Universe is a system made up of heaven and Earth and the elements which are contained in them. There are "five elements, situated in spheres in five regions, the less being in each case surrounded by the greater – namely, earth surrounded by water, water by air, air by fire, and fire by ether – make up the whole Universe."[35]
Stoic universe Stoics (300 BCE – 200 CE) Island universe The cosmos is finite and surrounded by an infinite void. It is in a state of flux, and pulsates in size and undergoes periodic upheavals and conflagrations.
Platonic universe Plato (c. 360 BCE) Geocentric, complex cosmogony, finite extent, implied finite time, cyclical Static Earth at center, surrounded by heavenly bodies which move in perfect circles, arranged by the will of the Demiurge[36] in order: Moon, Sun, planets and fixed stars.[37][38] Complex motions repeat every 'perfect' year.[39]
Eudoxus' model Eudoxus of Cnidus (c. 340 BCE) and later Callippus Geocentric, first geometric-mathematical model The heavenly bodies move as if attached to a number of Earth-centered concentrical, invisible spheres, each of them rotating around its own and different axis and at different paces.[40] There are twenty-seven homocentric spheres with each sphere explaining a type of observable motion for each celestial object. Eudoxus emphasised that this is a purely mathematical construct of the model in the sense that the spheres of each celestial body do not exist, it just shows the possible positions of the bodies.[41]
Aristotelian universe Aristotle (384–322 BCE) Geocentric (based on Eudoxus' model), static, steady state, finite extent, infinite time Static and spherical Earth is surrounded by 43 to 55 concentric celestial spheres, which are material and crystalline.[42] Universe exists unchanged throughout eternity. Contains a fifth element, called aether, that was added to the four classical elements.[43]
Aristarchean universe Aristarchus (c. 280 BCE) Heliocentric Earth rotates daily on its axis and revolves annually about the Sun in a circular orbit. Sphere of fixed stars is centered about the Sun.[44]
Ptolemaic model Ptolemy (2nd century CE) Geocentric (based on Aristotelian universe) Universe orbits around a stationary Earth. Planets move in circular epicycles, each having a center that moved in a larger circular orbit (called an eccentric or a deferent) around a center-point near Earth. The use of equants added another level of complexity and allowed astronomers to predict the positions of the planets. The most successful universe model of all time, using the criterion of longevity. The Almagest (the Great System).
Capella's model Martianus Capella (c. 420) Geocentric and Heliocentric The Earth is at rest in the center of the universe and circled by the Moon, the Sun, three planets and the stars, while Mercury and Venus circle the Sun.[45]
Aryabhatan model Aryabhata (499) Geocentric or Heliocentric The Earth rotates and the planets move in elliptical orbits around either the Earth or Sun; uncertain whether the model is geocentric or heliocentric due to planetary orbits given with respect to both the Earth and Sun.
Quranic cosmology Quran (610–632 CE) Flat-earth The universe consists of stacked flat layers, including seven levels of heaven and in some interpretations seven levels of earth (including hell)
Medieval universe Medieval philosophers (500–1200) Finite in time A universe that is finite in time and has a beginning is proposed by the Christian philosopher John Philoponus, who argues against the ancient Greek notion of an infinite past. Logical arguments supporting a finite universe are developed by the early Muslim philosopher Al-Kindi, the Jewish philosopher Saadia Gaon, and the Muslim theologian Al-Ghazali.
Non-Parallel Multiverse Bhagvata Puran (800–1000) Multiverse, Non Parallel Innumerable universes is comparable to the multiverse theory, except nonparallel where each universe is different and individual jiva-atmas (embodied souls) exist in exactly one universe at a time. All universes manifest from the same matter, and so they all follow parallel time cycles, manifesting and unmanifesting at the same time.[46]
Multiversal cosmology Fakhr al-Din al-Razi (1149–1209) Multiverse, multiple worlds and universes There exists an infinite outer space beyond the known world, and God has the power to fill the vacuum with an infinite number of universes.
Maragha models Maragha school (1259–1528) Geocentric Various modifications to Ptolemaic model and Aristotelian universe, including rejection of equant and eccentrics at Maragheh observatory, and introduction of Tusi-couple by Al-Tusi. Alternative models later proposed, including the first accurate lunar model by Ibn al-Shatir, a model rejecting stationary Earth in favour of Earth's rotation by Ali Kuşçu, and planetary model incorporating "circular inertia" by Al-Birjandi.
Nilakanthan model Nilakantha Somayaji (1444–1544) Geocentric and heliocentric A universe in which the planets orbit the Sun, which orbits the Earth; similar to the later Tychonic system.
Copernican universe Nicolaus Copernicus (1473–1543) Heliocentric with circular planetary orbits, finite extent First described in De revolutionibus orbium coelestium. The Sun is in the center of the universe, planets including Earth orbit the Sun, but the Moon orbits the Earth. The universe is limited by the sphere of the fixed stars.
Tychonic system Tycho Brahe (1546–1601) Geocentric and Heliocentric A universe in which the planets orbit the Sun and the Sun orbits the Earth, similar to the earlier Nilakanthan model.
Bruno's cosmology Giordano Bruno (1548–1600) Infinite extent, infinite time, homogeneous, isotropic, non-hierarchical Rejects the idea of a hierarchical universe. Earth and Sun have no special properties in comparison with the other heavenly bodies. The void between the stars is filled with aether, and matter is composed of the same four elements (water, earth, fire, and air), and is atomistic, animistic and intelligent.
De Magnete William Gilbert (1544–1603) Heliocentric, indefinitely extended Copernican heliocentrism, but he rejects the idea of a limiting sphere of the fixed stars for which no proof has been offered.[47]
Keplerian Johannes Kepler (1571–1630) Heliocentric with elliptical planetary orbits Kepler's discoveries, marrying mathematics and physics, provided the foundation for the present conception of the Solar System, but distant stars were still seen as objects in a thin, fixed celestial sphere.
Static Newtonian Isaac Newton (1642–1727) Static (evolving), steady state, infinite Every particle in the universe attracts every other particle. Matter on the large scale is uniformly distributed. Gravitationally balanced but unstable.
Cartesian Vortex universe René Descartes 17th century Static (evolving), steady state, infinite System of huge swirling whirlpools of aethereal or fine matter produces gravitational effects. But his vacuum was not empty; all space was filled with matter.
Hierarchical universe Immanuel Kant, Johann Lambert 18th century Static (evolving), steady state, infinite Matter is clustered on ever larger scales of hierarchy. Matter is endlessly recycled.
Einstein Universe with a cosmological constant Albert Einstein 1917 Static (nominally). Bounded (finite) "Matter without motion". Contains uniformly distributed matter. Uniformly curved spherical space; based on Riemann's hypersphere. Curvature is set equal to Λ. In effect Λ is equivalent to a repulsive force which counteracts gravity. Unstable.
De Sitter universe Willem de Sitter 1917 Expanding flat space.

Steady state. Λ > 0

"Motion without matter." Only apparently static. Based on Einstein's general relativity. Space expands with constant acceleration. Scale factor increases exponentially (constant inflation).
MacMillan universe William Duncan MacMillan 1920s Static and steady state New matter is created from radiation; starlight perpetually recycled into new matter particles.
Friedmann universe, spherical space Alexander Friedmann 1922 Spherical expanding space. k = +1 ; no Λ Positive curvature. Curvature constant k = +1

Expands then recollapses. Spatially closed (finite).

Friedmann universe, hyperbolic space Alexander Friedmann 1924 Hyperbolic expanding space. k = −1 ; no Λ Negative curvature. Said to be infinite (but ambiguous). Unbounded. Expands forever.
Dirac large numbers hypothesis Paul Dirac 1930s Expanding Demands a large variation in G, which decreases with time. Gravity weakens as universe evolves.
Friedmann zero-curvature Einstein and De Sitter 1932 Expanding flat space

k = 0 ; Λ = 0 Critical density

Curvature constant k = 0. Said to be infinite (but ambiguous). "Unbounded cosmos of limited extent". Expands forever. "Simplest" of all known universes. Named after but not considered by Friedmann. Has a deceleration term q = 1/2, which means that its expansion rate slows down.
The original Big Bang (Friedmann-Lemaître) Georges Lemaître 1927–1929 Expansion

Λ > 0 ; Λ > |Gravity|

Λ is positive and has a magnitude greater than gravity. Universe has initial high-density state ("primeval atom"). Followed by a two-stage expansion. Λ is used to destabilize the universe. (Lemaître is considered the father of the Big Bang model.)
Oscillating universe (Friedmann-Einstein) Favored by Friedmann 1920s Expanding and contracting in cycles Time is endless and beginningless; thus avoids the beginning-of-time paradox. Perpetual cycles of Big Bang followed by Big Crunch. (Einstein's first choice after he rejected his 1917 model.)
Eddington universe Arthur Eddington 1930 First static then expands Static Einstein 1917 universe with its instability disturbed into expansion mode; with relentless matter dilution becomes a De Sitter universe. Λ dominates gravity.
Milne universe of kinematic relativity Edward Milne 1933, 1935;

William H. McCrea 1930s

Kinematic expansion without space expansion Rejects general relativity and the expanding space paradigm. Gravity not included as initial assumption. Obeys cosmological principle and special relativity; consists of a finite spherical cloud of particles (or galaxies) that expands within an infinite and otherwise empty flat space. It has a center and a cosmic edge (surface of the particle cloud) that expands at light speed. Explanation of gravity was elaborate and unconvincing.
Friedmann–Lemaître–Robertson–Walker class of models Howard Robertson, Arthur Walker 1935 Uniformly expanding Class of universes that are homogeneous and isotropic. Spacetime separates into uniformly curved space and cosmic time common to all co-moving observers. The formulation system is now known as the FLRW or Robertson–Walker metrics of cosmic time and curved space.
Steady-state Hermann Bondi, Thomas Gold 1948 Expanding, steady state, infinite Matter creation rate maintains constant density. Continuous creation out of nothing from nowhere. Exponential expansion. Deceleration term q = −1.
Steady-state Fred Hoyle 1948 Expanding, steady state; but unstable Matter creation rate maintains constant density. But since matter creation rate must be exactly balanced with the space expansion rate the system is unstable.
Ambiplasma Hannes Alfvén 1965 Oskar Klein Cellular universe, expanding by means of matter–antimatter annihilation Based on the concept of plasma cosmology. The universe is viewed as "meta-galaxies" divided by double layers and thus a bubble-like nature. Other universes are formed from other bubbles. Ongoing cosmic matter-antimatter annihilations keep the bubbles separated and moving apart preventing them from interacting.
Brans–Dicke theory Carl H. Brans, Robert H. Dicke Expanding Based on Mach's principle. G varies with time as universe expands. "But nobody is quite sure what Mach's principle actually means."[citation needed]
Cosmic inflation Alan Guth 1980 Big Bang modified to solve horizon and flatness problems Based on the concept of hot inflation. The universe is viewed as a multiple quantum flux – hence its bubble-like nature. Other universes are formed from other bubbles. Ongoing cosmic expansion kept the bubbles separated and moving apart.
Eternal inflation (a multiple universe model) Andreï Linde 1983 Big Bang with cosmic inflation Multiverse based on the concept of cold inflation, in which inflationary events occur at random each with independent initial conditions; some expand into bubble universes supposedly like the entire cosmos. Bubbles nucleate in a spacetime foam.
Cyclic model Paul Steinhardt; Neil Turok 2002 Expanding and contracting in cycles; M-theory Two parallel orbifold planes or M-branes collide periodically in a higher-dimensional space. With quintessence or dark energy.
Cyclic model Lauris Baum; Paul Frampton 2007 Solution of Tolman's entropy problem Phantom dark energy fragments universe into large number of disconnected patches. The observable patch contracts containing only dark energy with zero entropy.

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.

See also

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References

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Sources

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  • Bragg, Melvyn (2023). "The Universe's Shape". bbc.co.uk. BBC. Retrieved 23 May 2023. 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. Retrieved 23 May 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. Download full text: Dodelson, Scott; Schmidt, Fabian (2020). "Scott Dodelson - Fabian Schmidt - Modern Cosmology (2021) PDF" (PDF). scribd.com. Academic Press. Retrieved 23 May 2023.
  • 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. Retrieved 23 May 2023. 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.
  • Leonard, Scott A; McClure, Michael (2004). Myth and Knowing. McGraw-Hill. ISBN 978-0-7674-1957-4.
  • 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. Retrieved 23 May 2023. April 2023 Release Highlights Database Updates
  • Sophia Centre. The Sophia Centre for the Study of Cosmology in Culture, University of Wales Trinity Saint David.
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Cosmology

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Cosmology is the scientific study of the as a whole, encompassing its large-scale properties, origin, evolution, structure, and ultimate fate. It employs the to address fundamental questions about the , including its composition and dynamics, drawing on observations from telescopes and missions to test theoretical models. The prevailing model of the universe's origin is the , which posits that the cosmos began approximately 13.8 billion years ago as an extremely hot and dense state, expanding rapidly from a singularity-like condition. This event created all matter, energy, and radiation, followed by cosmic inflation—a brief period of exponential expansion—and subsequent cooling that allowed the formation of subatomic particles, nuclei, and eventually atoms. Key evidence includes the (CMB) radiation, the remnant heat from the early universe, discovered in the 1960s and mapped in detail by missions like the (WMAP). The theory has been refined through 20th-century discoveries, transforming cosmology from philosophical speculation into a rigorous empirical . Modern cosmology reveals a universe composed of approximately 5% ordinary matter (such as stars, planets, and gas), 27% (invisible mass inferred from gravitational effects on galaxies and clusters), and 68% (a mysterious force driving the cosmos's expansion). Observations of distant supernovae in 1998 indicated accelerating expansion according to the standard ΛCDM model, with the universe's expansion—first noted by in 1929—not slowing but speeding up, likely leading to an ever-expanding fate. However, recent results from the (DESI) as of 2025 suggest that may evolve over time, potentially indicating a transition to decelerated expansion in the current epoch. provides the gravitational scaffolding for cosmic structures, while dominates the universe's energy budget, posing unresolved challenges for . Ongoing research in cosmology leverages advanced observatories, such as the , , and upcoming facilities like the , to probe early formation, map large-scale structures, and test models of and . Projects like the (DESI) analyze distributions to quantify 's influence, while deep-field imaging reveals the 's evolution from the "Dark Ages" after recombination—about 380,000 years post-Big Bang—to the era of around one billion years later, when the first stars ignited. These efforts continue to refine our understanding, bridging cosmology with and to explore the 's deepest mysteries.

Branches of Cosmology

Physical Cosmology

is the branch of astronomy and that examines the origin, , large-scale , and ultimate fate of the universe through the application of physical laws and observational data. It focuses on the measurable aspects of the , integrating principles from to describe geometry and dynamics on cosmic scales. Unlike broader cosmological inquiries, emphasizes testable models derived from , such as the distribution of galaxies and cosmic expansion. A foundational assumption in physical cosmology is the cosmological principle, which posits that the universe is homogeneous—meaning matter is evenly distributed on the largest scales—and isotropic, exhibiting no preferred direction when observed from any point. This principle simplifies the mathematical description of the universe, enabling the use of the Friedmann-Lemaître-Robertson-Walker metric to model its expansion. Homogeneity ensures that observers in different locations see similar large-scale structures, while isotropy implies uniformity in all directions, supported by observations of the cosmic microwave background. Physical cosmology draws heavily on interdisciplinary connections, particularly with to understand the early universe's high-energy conditions, for gravitational effects on cosmic scales, and for phenomena like quantum fluctuations during . These ties allow cosmologists to probe fundamental questions, such as the nature of and , by linking microscopic particle interactions to macroscopic universe evolution. Modern physical cosmology relies on advanced computational tools, including large-scale N-body simulations run on supercomputers to model the formation of cosmic structures from initial density perturbations. These simulations, such as those using the Hardware/Hybrid Accelerated Cosmology Code (HACC), replicate the gravitational clustering of and baryons over billions of years, providing predictions testable against observations. A key framework in this field is the , the current standard cosmological paradigm, which incorporates (CDM), a (Lambda) representing , and ordinary matter to explain the universe's composition and expansion history. This model successfully accounts for the observed flat geometry and accelerated .

Philosophical Cosmology

Philosophical cosmology explores the through rational inquiry, addressing fundamental existential questions that transcend empirical observation, such as why the exists and what its ultimate structure or purpose, or , might be. These inquiries often invoke a priori reasoning to probe the nature of reality, the origins of existence, and humanity's place within the , contrasting with scientific approaches by prioritizing logical and metaphysical analysis over testable hypotheses. Central to this field is the , which posits that the existence of the contingent implies a necessary first cause or ultimate ground of being, challenging thinkers to reconcile contingency with an explanatory foundation. Ancient philosophers like laid foundational ideas in this tradition by conceiving the universe as eternal and ungenerated, a spherical, finite whole governed by natural where celestial bodies move in perfect circles due to their inherent nature. In his , Aristotle argues that the is everlasting, exempt from generation and decay, with as an eternal, unchanging cause sustaining its order, thus viewing the universe's structure as inherently purposeful and directed toward perfection. This eternalist framework influenced subsequent thought, emphasizing a harmonious, self-sustaining without beginning or end. Immanuel Kant further advanced philosophical cosmology by examining the antinomies of pure reason, conflicts arising when reason attempts to grasp the universe's totality through categories like and . In the , Kant delineates four antinomies, including debates over whether the world has a beginning in time or is infinite, and whether it is composed of simple parts or infinitely divisible, demonstrating that such cosmological ideas lead to irresolvable contradictions when treated as objects of theoretical knowledge. These antinomies highlight reason's limits in comprehending the universe's ultimate structure, suggesting that existential questions about origins and wholeness remain speculative rather than resolvable through logic alone. Key concepts in philosophical cosmology include debates over eternalism and presentism regarding time's nature and the universe's temporal structure. Eternalism holds that all moments in time—past, present, and future—are equally real, implying a block universe where temporal existence is fixed and unchanging, aligning with views of an atemporal cosmic whole. In contrast, presentism asserts that only the present moment exists, rendering the past and future unreal, which raises questions about the universe's persistence and the reality of cosmic evolution. Another enduring concept is the problem of the one and the many, which interrogates how the universe can be a unified whole (the "one") while comprising diverse, plural entities (the "many"), a tension explored in metaphysical terms from ' monism to ' emanation from The One. This issue probes the coherence of cosmic unity amid multiplicity, influencing reflections on the universe's telos as either a singular harmonious order or a dynamic interplay of parts. In modern philosophical cosmology, debates extend to speculative scenarios like the , which posits that our perceived reality might be a computationally generated simulation run by advanced posthumans, raising profound questions about the nature of and observation. Philosopher argues in his seminal paper that if advanced civilizations can simulate ancestor realities, the vast number of such simulations implies a high probability that we inhabit one, thereby challenging traditional notions of a "real" universe. Similarly, the offers a brief conceptual framework for understanding why the universe permits observers like humans, stating that we must observe a compatible with our , without invoking empirical fine-tuning. Introduced by , it underscores the observer's role in cosmological reasoning, prompting reflections on purpose and contingency. Philosophical cosmology distinguishes itself from by emphasizing a priori logic and metaphysical speculation over observational data and empirical models, treating the as a singular entity that defies repeatable experimentation. While relies on testable predictions within and , philosophical approaches grapple with and the uniqueness of the , using reason to explore unobservable aspects like ultimate origins or teleological purpose. This focus on conceptual limits ensures that existential inquiries remain insulated from scientific falsification, preserving a for rational deliberation on humanity's cosmic significance.

Religious and Mythological Cosmology

Religious and mythological cosmologies offer symbolic frameworks for understanding the universe's origin, structure, and purpose, rooted in sacred narratives that emphasize divine agency and cosmic harmony rather than empirical observation. These traditions classify creation processes into distinct types, such as ex nihilo (creation from nothing), where a supreme deity summons existence through will or word; from chaos, involving the differentiation of primordial disorder into ordered realms; and contrasts between linear time, progressing toward an endpoint like or redemption, and cyclical time, featuring eternal repetitions of creation, decay, and renewal. In , particularly , , the Genesis account exemplifies ex nihilo creation, portraying a singular who forms the heavens, , and all in six days through divine commands, establishing a linear progression from chaos to ordered culminating in human . This doctrine, articulated in early Jewish and Christian texts like 7:28 and the writings of around 180 CE, underscores God's transcendence and absolute power, distinguishing it from surrounding ancient Near Eastern myths that assumed pre-existent matter. Hindu cosmology, drawn from Vedic and Puranic texts, embodies cyclical time through the system, where cosmic history unfolds in repeating eras of declining righteousness: the virtuous (1,728,000 human years), followed by Treta (1,296,000 years), Dvapara (864,000 years), and the current (432,000 years), together forming a mahayuga of 4.32 million years that recurs in vast kalpa cycles lasting billions of years. These cycles, first detailed in the (circa 3rd century BCE–4th century CE) and expanded in the , reflect divine intervention by the creator, the preserver, and the destroyer, promoting an ethical view of (cosmic order) that wanes and revives eternally. Indigenous cosmologies worldwide often feature earth-diver myths, where divine or animal figures plunge into primordial waters to retrieve , forming the from placed on a foundational element like a or , symbolizing emergence from aquatic chaos. This motif predominates in North American traditions, such as those of the and , with the widest distribution among Native American narratives, emphasizing communal cooperation among creator beings to establish land amid a watery void. Central to these cosmologies are concepts like divine intervention, where gods actively shape reality, and sacred geography, mapping the through interconnected realms such as world trees (e.g., the Norse or Mayan ) that link heavens, earth, and underworlds, the latter often depicted as subterranean domains of ancestors or the dead. In , for instance, the giant emerges from the void of and is slain by and his brothers, whose body parts form the world—flesh as earth, blood as oceans, bones as mountains, and skull as sky—illustrating emergence from chaos via sacrifice, as preserved in Snorri Sturluson's (13th century). The Mayan Popol Vuh, a K'iche' sacred text compiled in the from pre-Columbian oral traditions, outlines creation through trial and error by the creator deities Heart of Sky and Plumed Serpent, who discard mud and wooden humans before succeeding with fashioned from divine essence, integrating sacred geography with maize mountains and trials to affirm human-divine reciprocity. These narratives profoundly shape cultural practices: they inspire rituals reenacting creation, such as Hindu yuga-aligned festivals or Indigenous earth-renewal ceremonies, influence art through depictions of cosmic axes like world trees in Mayan codices and Norse carvings, and underpin ethics by promoting harmony with divine order, as in Abrahamic calls for stewardship or Hindu adherence to .

Historical Development

Ancient and Classical Cosmologies

Ancient Mesopotamian cosmology envisioned the as a flat, disk-shaped floating on a primordial , enclosed by a solid celestial dome that held back the upper waters. This model, derived from texts, portrayed the cosmos as a multi-layered structure with the at the center, surrounded by mountains supporting the dome, through which and deities moved in predictable paths. Similarly, ancient Egyptian cosmology depicted the as a flat plane beneath a vaulted , personified by the goddess Nut arching over the world, with her body forming the dome separating the terrestrial realm from the watery chaos above. The sun god traversed this dome daily by boat, rising in the east and setting in the west, reinforcing a geocentric framework tied to flood cycles and agricultural life. In , early philosophical cosmologies emerged from the Milesian school, where proposed water as the fundamental principle (arche) underlying all matter and change, observing its role in nourishment and transformation across natural phenomena. His successor, , advanced this by introducing the —an infinite, boundless, and indeterminate substance—as the origin of the , from which opposites like hot and cold separated to form the ordered world, including a cylindrical suspended freely in space. The Pythagoreans, emphasizing numerical harmony, conceived the universe as a series of concentric spheres carrying celestial bodies, producing an inaudible "music of the spheres" through their proportional motions, reflecting cosmic order and mathematical beauty. Later, synthesized these ideas into a of four eternal elements—, air, fire, and water—combined and separated by the forces of (attraction) and Strife (repulsion), explaining cosmic cycles without a single originating substance. Ancient Indian Vedic cosmology, as articulated in the Rigveda, described the universe as emerging from a cosmic sacrifice or primordial unity, with cyclical time (yugas) governing creation, preservation, and dissolution in vast, repeating epochs. The cosmos was structured in three realms—earth, atmosphere, and heaven—interconnected by a world axis (axis mundi), where the sun, moon, and stars followed divinely ordained paths, blending observation with ritualistic explanations of natural order. In parallel, ancient Chinese Taoist cosmology centered on the Tao as the undifferentiated source of all, manifesting through the dynamic balance of yin (passive, feminine, dark) and yang (active, masculine, light) forces, which interplayed to generate the five elements (wood, fire, earth, metal, water) and sustain cosmic harmony without a fixed center. These ancient and classical cosmologies shared geocentric assumptions, placing the at the universe's core with heavens revolving around it, often supported by mythological or qualitative reasoning rather than systematic empirical testing. Lacking quantitative measurements or falsifiable predictions, they prioritized intuitive explanations of observed celestial motions and natural cycles, limiting predictive power and integration of contradictory evidence.

Medieval to Early Modern Cosmologies

During the Middle Ages, cosmology largely adhered to the geocentric model established by Claudius Ptolemy in his Almagest around 150 CE, which posited Earth as the fixed center of the universe surrounded by concentric celestial spheres carrying the Moon, Sun, planets, and stars in uniform circular motion. To account for observed irregularities such as retrograde planetary motion, Ptolemy introduced epicycles—small circular orbits upon which planets moved while their centers revolved around larger deferents centered near Earth—along with eccentrics and equants to refine predictions and maintain the philosophical ideal of perfect circular paths in the heavens. This system, rooted in Aristotelian principles of a divided cosmos with changeable sublunary realms below immutable heavenly spheres made of aether, dominated European and Islamic scholarship for over a millennium, providing a mathematical framework that aligned with theological views of a divinely ordered universe. Islamic scholars during the (8th–13th centuries) advanced Ptolemaic astronomy through precise observations and innovations, preserving and critiquing ancient texts while integrating them with empirical methods. Abu Rayhan (973–1048 CE), a Persian , contributed significantly by measuring Earth's using trigonometric techniques from a mountain vantage, estimating it at approximately 6,340 km (3,939 miles)—remarkably accurate to within 1% of the modern mean of 6,371 km (3,959 miles)—and confirming the planet's through observations of horizon dip and stellar positions, consistent with the scholarly consensus of his time. In works like Al-Qanun al-Mas'udi (The Mas'udic Canon), refined for astronomical calculations, cataloged coordinates for over 600 locations, and speculated on Earth's possible , though he ultimately favored a geocentric model with gravitational tendencies drawing celestial bodies toward the center. These efforts, building on earlier translations and observatories like those in , enhanced the Ptolemaic system's predictive power and emphasized empirical verification over pure philosophy. In medieval Christian , cosmology intertwined Ptolemaic with theological doctrine, portraying the as a hierarchical reflection of divine order where 's centrality symbolized humanity's spiritual significance. This synthesis culminated in literary depictions like Dante Alighieri's (completed around 1320), which envisioned a geocentric cosmos of nine concentric transparent spheres encircling : the , Mercury, , the Sun, Mars (fortitude), , Saturn, the , and the Primum Mobile, powered by angels and ascending toward , God's unchanging realm of pure light and love. Dante's structure integrated Aristotelian virtues with Christian salvation—Hell's nine circles burrowed into , as an intermediary mountain, and Paradise's spheres representing progressive beatitude—thus mapping physical astronomy onto moral and eschatological journeys, reinforcing the Church's view of a finite, theocentric balanced between material imperfection and celestial perfection. The transition to early modern cosmology began with the , challenging geocentric orthodoxy by proposing a heliocentric model where the Sun occupied the center, rotated daily on its axis, and orbited annually alongside other . outlined this in (On the Revolutions of the Heavenly Spheres), published in 1543 just before his death, after decades of refining observations to simplify calculations and eliminate Ptolemy's equant, though it initially circulated privately in a 1514 manuscript. This work sparked debate by demoting from cosmic centrality, aligning with and mathematical elegance, yet faced resistance for contradicting scriptural interpretations and . Building on Copernicus, (1571–1630) analyzed Tycho Brahe's precise naked-eye observations of Mars to derive empirical laws of planetary motion in (1609): follow elliptical orbits with the Sun at one focus, and a line from the Sun to a sweeps equal areas in equal times, establishing a dynamical foundation for without circular assumptions. These developments marked the shift from medieval synthesis toward observation-driven models, setting the stage for further astronomical inquiry.

19th and 20th Century Advances

In the , astronomers grappled with Olbers' paradox, which questioned why the is dark in an infinite, static filled with stars, as formulated by Heinrich Wilhelm Olbers in 1823. This paradox highlighted inconsistencies in classical models, suggesting limitations such as a finite universe or absorption, and spurred debates on cosmic scale. Concurrently, advances in stellar revolutionized the field; Joseph von Fraunhofer's 1814 observations of solar absorption lines laid groundwork, while Angelo Secchi's 1860s classifications of stellar spectra into types based on line features established the foundations of stellar . These techniques enabled chemical analysis of distant stars, revealing compositions like and dominance, and shifted astronomy toward quantitative physics. Estimates of the Sun's age also emerged as a key 19th-century challenge, with calculating in the 1860s that gravitational contraction could sustain for 20 to 40 million years, assuming no sources beyond that mechanism. This estimate, derived from thermodynamic principles, conflicted with geological evidence for an older , underscoring tensions between astrophysics and other sciences. By the early , Albert Einstein's publication of on November 25, 1915, provided a new gravitational framework, incorporating curvature to describe cosmic dynamics. This theory enabled cosmological applications, moving beyond Newtonian limits. In 1922, derived non-static solutions to Einstein's field equations, proposing an expanding or contracting universe with variable density, thus challenging static models. These solutions offered mathematical descriptions of dynamic cosmologies, influencing subsequent theoretical work. built on this in 1927 by proposing that the universe expanded from a highly dense initial state, integrating Friedmann's equations with early data, and later developing the primeval atom hypothesis in 1931 to describe its origin as a single quantum entity that disintegrated. Hubble's 1929 observations at confirmed galactic recession, measuring velocities proportional to via Cepheid variables, providing empirical support for expansion. The 1930s saw refinements in recession models, with Richard Tolman developing tests like the relation to distinguish expanding from static universes, as outlined in his 1934 text on relativity and cosmology. These models predicted dimming of in expanding space, aiding validation of dynamic theories. By 1948, , , and proposed the steady-state theory, positing continuous matter creation to maintain constant density amid expansion, adhering to the perfect . This alternative to evolving models sparked debate, emphasizing uniformity over time. The post-World War II era marked a transition in cosmology, driven by radio astronomy's emergence and larger telescopes, which enabled deeper observations and theoretical synthesis, setting the stage for integrated big bang frameworks.

Foundations of Physical Cosmology

General Relativity and Cosmological Models

, formulated by in , provides the theoretical foundation for modern cosmological models by describing gravity as the curvature of caused by mass and energy. The theory's core is encapsulated in the , which relate the geometry of to the distribution of matter and energy within it. These equations enable the construction of models that describe the large-scale structure and evolution of the universe, assuming homogeneity and isotropy on cosmic scales. The Einstein field equations are given by Gμν=8πGc4Tμν,G_{\mu\nu} = \frac{8\pi G}{c^4} T_{\mu\nu}, where GμνG_{\mu\nu} is the Einstein tensor, derived from the Ricci curvature tensor RμνR_{\mu\nu} and the Ricci scalar RR as Gμν=Rμν−12RgμνG_{\mu\nu} = R_{\mu\nu} - \frac{1}{2} R g_{\mu\nu}, with gμνg_{\mu\nu} the metric tensor; TμνT_{\mu\nu} is the stress-energy tensor representing the density and flux of energy and momentum; GG is Newton's gravitational constant; and cc is the speed of light. Physically, the left side encodes the curvature of spacetime, while the right side sources it through matter and energy content. Einstein derived these equations through an iterative process between November 4 and 25, 1915, building on the and the requirement of . Starting from the vacuum equations Gμν=0G_{\mu\nu} = 0, which describe empty , he incorporated matter by analogy to in Newtonian gravity, ∇2Φ=4πGρ\nabla^2 \Phi = 4\pi G \rho, generalizing it to curved . The derivation involved computing for motion, forming the Riemann tensor for , contracting to the Ricci tensor, and ensuring conservation laws via the Bianchi identities, which imply ∇μTμν=0\nabla^\mu T_{\mu\nu} = 0. This form was finalized on November 25, 1915, after testing against the perihelion precession of Mercury. In 1917, Einstein extended the field equations to include a cosmological constant Λ\Lambda, motivated by the desire for a static, finite universe in line with contemporary astronomical views: Gμν+Λgμν=8πGc4Tμν.G_{\mu\nu} + \Lambda g_{\mu\nu} = \frac{8\pi G}{c^4} T_{\mu\nu}. The term Λgμν\Lambda g_{\mu\nu} acts as a uniform energy density with negative pressure, representing a repulsive force to balance gravitational attraction in a closed universe. Einstein introduced Λ\Lambda to satisfy the condition for a static solution, solving the modified equations for a hyperspherical geometry with constant radius, where matter density ρ\rho and Λ\Lambda are tuned such that Λ=4πGρ/c2\Lambda = 4\pi G \rho / c^2. This Einstein static universe was, however, later shown to be unstable to perturbations. The static model faced challenges when demonstrated in 1922 that the field equations without Λ\Lambda admit dynamic, expanding solutions. Friedmann assumed a homogeneous, isotropic universe with a stress-energy tensor Tμν=(ρ+p/c2)uμuν+pgμνT_{\mu\nu} = (\rho + p/c^2) u_\mu u_\nu + p g_{\mu\nu}, where ρ\rho is , pp is , and uμu^\mu is the . By solving the equations, he derived solutions where the factor a(t)a(t) evolves with time, yielding parabolic (k=0k=0), hyperbolic (k<0k<0), and elliptic (k>0k>0) geometries, all non-static. Friedmann's work revealed that the universe could expand from a dense state or contract, overturning the static paradigm. Independently, in 1927 generalized Friedmann's solutions, incorporating Λ\Lambda and linking them to early astronomical data on redshifts. Lemaître's analysis confirmed expanding models, proposing a originating from a "primeval atom" that decays into , though he emphasized the mathematical framework over the explosive origin. His solutions aligned with Friedmann's but included observational estimates, predicting a linear velocity-distance relation. To model such universes, the Robertson-Walker metric is employed, which describes a homogeneous and isotropic : ds2=−c2dt2+a(t)2[dr21−kr2+r2dθ2+r2sin⁡2θdϕ2],ds^2 = -c^2 dt^2 + a(t)^2 \left[ \frac{dr^2}{1 - k r^2} + r^2 d\theta^2 + r^2 \sin^2\theta d\phi^2 \right], where a(t)a(t) is the time-dependent scale factor, r,θ,ϕr, \theta, \phi are comoving coordinates, and k=−1,0,+1k = -1, 0, +1 determines the spatial curvature (open, flat, closed). This form assumes the , with spatial slices of constant curvature. Howard Robertson and Arthur Walker derived this metric in 1934-1935 by requiring that the geometry satisfy the conditions for uniform expansion and , using group-theoretic arguments on the symmetry of the and conformal transformations. Robertson's kinematic approach emphasized observable distances, while Walker's focused on Milne's kinematical relativity into . Applying the Einstein field equations (with Λ\Lambda) to the Robertson-Walker metric yields the Friedmann equations, governing the universe's dynamics: (a˙a)2=8πG3ρ−kc2a2+Λc23,\left( \frac{\dot{a}}{a} \right)^2 = \frac{8\pi G}{3} \rho - \frac{k c^2}{a^2} + \frac{\Lambda c^2}{3}, a¨a=−4πG3(ρ+3pc2)+Λc23.\frac{\ddot{a}}{a} = -\frac{4\pi G}{3} \left( \rho + \frac{3p}{c^2} \right) + \frac{\Lambda c^2}{3}. The first equation relates the Hubble parameter H=a˙/aH = \dot{a}/a to density, curvature, and Λ\Lambda, analogous to an energy conservation law for the expanding universe. The second describes acceleration, showing deceleration for matter/radiation (p>0p > 0) unless balanced by Λ\Lambda. These were first obtained by Friedmann in 1922 for Λ=0\Lambda = 0 and extended by Lemaître. They form the basis for all standard cosmological models.

The Big Bang Theory

The Big Bang theory describes the universe's origin as a hot, dense state that expanded and cooled over time, leading to the formation of fundamental particles, nuclei, atoms, and large-scale structures observed today. This model, rooted in , posits that the emerged from an approximately 13.8 billion years ago, with its expansion driven by the governing a homogeneous, isotropic . The theory successfully predicts key observables, such as the (CMB) temperature and light element abundances, providing a framework for understanding the universe's thermal history from the earliest moments. The timeline begins at t=0t = 0, where the singularity of infinite and , beyond which classical breaks down and is required. Immediately following, the Planck epoch (t<10−43t < 10^{-43} s) encompasses scales where gravitational and quantum effects are unified, with temperatures exceeding 103210^{32} K; during this phase, the universe's fundamental forces may have been indistinguishable. As the universe expanded and cooled to around 1 MeV (at t≈1t \approx 1 s), quarks and gluons formed hadrons, transitioning into the hadron epoch. Big Bang nucleosynthesis (BBN) occurred between 1 and 20 minutes after the singularity, when temperatures dropped to about 0.1 MeV, allowing light nuclei to form from protons and neutrons. A key feature is the deuterium bottleneck, where the low binding energy of deuterium (2.224 MeV) causes it to be photodissociated by the ambient radiation until the universe cools sufficiently; this delays heavier element synthesis until the reverse reaction dominates. The primary reaction is p+n⇌2H+γ,p + n \rightleftharpoons {}^2\mathrm{H} + \gamma, with the neutron-to-proton ratio freezing at about 1/6 prior to BBN due to weak interaction decoupling, ultimately yielding primordial abundances of ~75% hydrogen, ~25% helium-4 by mass, and trace deuterium, helium-3, and lithium-7. The early universe was radiation-dominated, with energy density scaling as ρr∝a−4\rho_r \propto a^{-4} (where aa is the scale factor), leading to expansion governed by a∝t1/2a \propto t^{1/2}. This era persisted until matter density ρm∝a−3\rho_m \propto a^{-3} became comparable, marking the transition to matter domination at redshift z≈3400z \approx 3400 or about 50,000 years post-Big Bang, after which a∝t2/3a \propto t^{2/3}. In the matter-dominated phase, gravitational clustering began to shape the large-scale structure, continuing until the recent onset of dark energy influence. The observed baryon asymmetry, with a baryon-to-photon ratio η≈6×10−10\eta \approx 6 \times 10^{-10}, reflects an imbalance between matter and antimatter that survived annihilation, leaving the universe predominantly matter-filled. This requires processes violating baryon number conservation, charge conjugation (C) and combined CP symmetry, and departing from thermal equilibrium, as outlined in the Sakharov conditions. Two fine-tuning issues in the standard Big Bang model motivate extensions: the horizon problem, where distant CMB regions exhibit uniform temperature (~2.725 K) despite lacking causal contact in a radiation-dominated expansion, implying initial hypersurface homogeneity finer than 1 part in 103010^{30}; and the flatness problem, where the density parameter Ω\Omega must have been tuned to within 1 part in 106010^{60} at the Planck time to yield the observed near-critical density today (Ω≈1\Omega \approx 1). These challenges highlight the need for mechanisms to set the initial conditions without extreme precision. The current age of the universe, derived from CMB data and the standard Λ\LambdaCDM model, is 13.787±0.02013.787 \pm 0.020 billion years.

Inflationary Universe

The inflationary universe model posits a phase of rapid, exponential expansion in the very early universe, occurring approximately 10^{-36} seconds after the , which addresses key shortcomings of the standard Big Bang theory. Proposed by in 1980 and detailed in his 1981 paper, this scenario is driven by a hypothetical scalar field called the inflaton, denoted as ϕ\phi, with an associated potential V(ϕ)V(\phi). During inflation, the energy density of the inflaton field dominates, causing the scale factor of the universe to grow by a factor of at least e60e^{60} or more, far exceeding the subsequent expansion in the radiation-dominated era. The dynamics of inflation rely on the slow-roll approximation, where the inflaton field evolves gradually down its potential, allowing the expansion to proceed quasi-exponentially. Introduced by Andreas Albrecht and Paul J. Steinhardt in 1982, this regime is characterized by two key slow-roll parameters: the first, ϵ=12(V′V)2\epsilon = \frac{1}{2} \left( \frac{V'}{V} \right)^2, measures the relative change in the Hubble rate, and the second, η=V′′V\eta = \frac{V''}{V}, quantifies the field's acceleration. Inflation occurs when ϵ≪1\epsilon \ll 1 and ∣η∣≪1|\eta| \ll 1, ensuring that the potential energy V(ϕ)V(\phi) remains nearly constant, mimicking a de Sitter spacetime with nearly constant Hubble parameter HH. Common potentials, such as the quadratic V(ϕ)=12m2ϕ2V(\phi) = \frac{1}{2} m^2 \phi^2 or exponential forms, satisfy these conditions over sufficient e-folds of expansion, typically 50–60, to match observations. This exponential growth resolves several fine-tuning problems in the standard Big Bang model. The horizon problem, where distant regions of the cosmic microwave background appear uniform despite never having been in causal contact, is solved because these regions were within a single causal patch before inflation, allowing thermal equilibrium to be established prior to the expansion. Similarly, the flatness problem, requiring the density parameter Ω\Omega to be finely tuned close to 1 today, is addressed as inflation drives Ω\Omega exponentially toward unity by stretching any initial curvature to negligible levels. Additionally, the monopole problem—predicting excessive magnetic monopoles from grand unified theory phase transitions—is mitigated because inflation dilutes their density by many orders of magnitude, pushing them beyond the observable universe. Quantum fluctuations of the inflaton field during slow-roll inflation provide the primordial seeds for large-scale structure formation. These vacuum fluctuations, stretched to superhorizon scales, generate scalar perturbations in the gravitational potential, leading to a nearly scale-invariant power spectrum P(k)∝kns−1P(k) \propto k^{n_s - 1}, where kk is the wavenumber and the spectral index ns≈1n_s \approx 1 for simple models. Detailed calculations by James M. Bardeen, Paul J. Steinhardt, and Michael S. Turner in 1983 show that ns=1−6ϵ+2ηn_s = 1 - 6\epsilon + 2\eta, yielding a slight red tilt (ns<1n_s < 1) consistent with cosmic microwave background data, while tensor perturbations from gravitational waves produce a comparable but suppressed spectrum. Variants of the inflationary model include eternal inflation, proposed by Andrei Linde in 1986, where quantum fluctuations prevent the inflaton from uniformly reaching the slow-roll minimum. In this picture, inflation ends in some regions, forming "bubble universes" that undergo reheating and evolve into hot Big Bang cosmologies, but continues indefinitely in others due to stochastic field excursions, resulting in an eternally self-reproducing multiverse structure. This framework extends chaotic inflation scenarios, where initial field values vary across space, ensuring perpetual expansion on global scales.

Observational Evidence

Expansion of the Universe

The expansion of the universe is primarily evidenced by the observed redshift of light from distant galaxies, indicating that space itself is stretching over time. In 1929, Edwin Hubble published observations showing that the radial velocities of extragalactic nebulae are proportional to their distances, establishing the foundational relation v=H0dv = H_0 d, where vv is the recession velocity, dd is the distance, and H0H_0 is the Hubble constant representing the current expansion rate. This law implies a homogeneous, isotropic expansion on large scales, with nearby galaxies receding faster the farther they are from the Milky Way. Redshift zz is quantified as z=Δλ/λz = \Delta \lambda / \lambda, the fractional increase in wavelength of light emitted at wavelength λ\lambda. For low redshifts (z≪1z \ll 1), this approximates the classical : z≈v/cz \approx v/c, where cc is the speed of light, linking observed redshifts directly to recession velocities in . At higher redshifts, the cosmological redshift arises from the expansion of space, governed by the scale factor a(t)a(t) in the Friedmann-Lemaître-Robertson-Walker metric, such that 1+z=1/a(tem)1 + z = 1 / a(t_\text{em}), where temt_\text{em} is the emission time and a(t0)=1a(t_0) = 1 today. This stretching of photon wavelengths occurs as light travels through expanding space, distinct from local Doppler shifts due to peculiar motions. Measurements of H0H_0 have evolved significantly since Hubble's initial estimate of approximately 500 km/s/Mpc, which suffered from distance calibration uncertainties. Subsequent refinements using Cepheid variable stars and other distance indicators yielded values around 50–100 km/s/Mpc through the mid-20th century, but systematic errors in the cosmic distance ladder persisted. Modern determinations converge near H0≈70H_0 \approx 70 km/s/Mpc, yet a notable tension exists: local measurements using Type Ia supernovae and Cepheids, refined with James Webb Space Telescope data, give H0=73.3±0.9H_0 = 73.3 \pm 0.9 km/s/Mpc (as of 2024), while early-universe constraints from cosmic microwave background data yield H0=67.4±0.5H_0 = 67.4 \pm 0.5 km/s/Mpc, differing at approximately 5σ significance and prompting investigations into new physics or systematics. Type Ia supernovae serve as effective standard candles due to their consistent peak absolute magnitude, approximately -19.3 in the B-band, arising from the thermonuclear explosion of white dwarfs reaching the . By calibrating their apparent magnitudes against distances from Cepheid variables in host galaxies, astronomers measure luminosity distances to high-redshift events, enabling precise tests of expansion. In 1998, observations of such supernovae at redshifts 0.16≤z≤0.620.16 \leq z \leq 0.62 revealed that distant explosions appear fainter than expected in a decelerating universe, indicating an accelerating expansion driven by a positive cosmological constant or dark energy component. This discovery, independently confirmed by complementary datasets, reshaped cosmology and earned the 2011 . To interpret these observations in an expanding framework, comoving coordinates are employed, which fix the relative positions of galaxies amid expansion, with the proper distance scaling as d(t)=a(t)χd(t) = a(t) \chi, where χ\chi is the comoving distance. The luminosity distance dLd_L, which relates observed flux to intrinsic luminosity via f=L/(4πdL2)f = L / (4\pi d_L^2), is given by dL=(1+z)∫temt0c dt/a(t)d_L = (1 + z) \int_{t_\text{em}}^{t_0} c \, dt / a(t) in a flat universe, incorporating redshift dimming from time dilation and photon energy loss. This metric allows mapping redshift to distance, confirming across cosmic scales and revealing the transition from deceleration to acceleration around z≈0.7z \approx 0.7.

Cosmic Microwave Background

The cosmic microwave background (CMB) is the thermal radiation left over from the , originating from the epoch of recombination when the universe cooled sufficiently for electrons and protons to form neutral hydrogen, decoupling photons from matter approximately 380,000 years after the initial expansion. This radiation provides a snapshot of the early universe, serving as a key probe of cosmological parameters and the initial conditions set during . Its near-perfect blackbody spectrum, with a temperature of 2.725 K, confirms the hot model and indicates the universe was once in thermal equilibrium. The discovery of the CMB occurred in 1965 when Arno Penzias and Robert Wilson, using a horn antenna at Bell Laboratories, detected an excess noise temperature of about 3.5 K isotropic across the sky, initially attributed to equipment issues but later identified as cosmic radiation. Their measurement, published in the Astrophysical Journal, established the existence of this uniform background radiation, earning them the 1978 Nobel Prize in Physics. Subsequent observations refined the blackbody nature of the spectrum, with the Cosmic Background Explorer (COBE) satellite's Far Infrared Absolute Spectrophotometer (FIRAS) instrument confirming deviations from a perfect blackbody at less than 50 parts per million, solidifying its relic status. Small temperature anisotropies in the CMB, at the level of ΔT/T≈10−5\Delta T / T \approx 10^{-5}, encode information about density fluctuations and gravitational potentials in the early universe. These arise primarily from the Sachs-Wolfe effect, where photons climbing out of potential wells formed by primordial density perturbations experience a gravitational redshift, imprinting temperature variations on large angular scales. On smaller scales, the anisotropies result from acoustic oscillations in the photon-baryon plasma before recombination, manifesting as peaks in the angular power spectrum CℓC_\ell. The first peak, located at multipole moment ℓ≈220\ell \approx 220 corresponding to an angular scale of about 1 degree, arises from the fundamental mode of these oscillations and its position indicates a spatially flat universe with curvature parameter Ωk≈0\Omega_k \approx 0. Higher peaks reflect baryon loading and damping effects, providing constraints on baryon density and other parameters. The CMB also exhibits polarization patterns, divided into E-modes (curl-free, sourced by scalar density perturbations) and B-modes (curl patterns, primarily from tensor perturbations like primordial gravitational waves produced during inflation). E-mode polarization, detected at levels comparable to temperature anisotropies, correlates with the temperature power spectrum and helps break degeneracies in parameter estimation. B-modes remain undetected at primordial levels, with current upper limits on the tensor-to-scalar ratio r<0.036r < 0.036 (95% CL) from BICEP/Keck ground-based experiments, though they offer the most direct evidence for inflationary gravitational waves if observed. Key missions have mapped the CMB with increasing precision. COBE's Differential Microwave Radiometer (DMR) in 1992 first detected anisotropies at 7σ\sigma significance, confirming predictions of the Big Bang model. The Wilkinson Microwave Anisotropy Probe (WMAP), operating from 2001 to 2010, provided full-sky maps with resolution down to arcminutes, measuring the power spectrum peaks and yielding parameters like Hubble constant H0=70.4±1.4H_0 = 70.4 \pm 1.4 km/s/Mpc and matter density Ωm=0.272±0.020\Omega_m = 0.272 \pm 0.020. The Planck satellite, from 2009 to 2013, delivered the highest-resolution maps, with its 2018 legacy results refining parameters to percent-level precision: H0=67.4±0.5H_0 = 67.4 \pm 0.5 km/s/Mpc, Ωm=0.315±0.007\Omega_m = 0.315 \pm 0.007, and spectral index ns=0.965±0.004n_s = 0.965 \pm 0.004, while confirming the flatness and acoustic peak structure. These measurements underscore the CMB's role in validating the Λ\LambdaCDM model.

Large-Scale Structure and Dark Components

The large-scale structure of the universe manifests as a vast cosmic web, comprising dense filaments of galaxies, sheet-like walls, and expansive voids that occupy much of the volume. Observations from the (SDSS) have mapped this structure in three dimensions, revealing a network where galaxies cluster along filaments and walls, separated by voids spanning tens to hundreds of megaparsecs. These mappings, based on spectroscopic redshifts of millions of galaxies, demonstrate that approximately 80% of the universe's volume consists of underdense voids, with the remaining matter concentrated in the interconnected filaments and walls. Dark matter, an invisible component inferred from gravitational effects, plays a crucial role in shaping this structure by providing the gravitational scaffolding for matter to clump. Evidence for dark matter first emerged from galactic rotation curves, where stars and gas in spiral galaxies orbit at unexpectedly high velocities far from the center, implying a massive, unseen halo extending beyond visible matter. Gravitational lensing further confirms this, as massive galaxy clusters distort background light more than expected from visible mass alone, revealing dark matter concentrations through weak lensing shear measurements. Cosmological parameters from the Planck satellite indicate that dark matter constitutes about 27% of the universe's energy density, with the total matter density parameter Ω_m ≈ 0.315, including both dark and baryonic components. Baryonic acoustic oscillations (BAO) imprint a characteristic scale on this structure, serving as a standard ruler for measuring cosmic expansion. Originating from sound waves in the early universe's plasma, these oscillations froze at recombination, leaving a preferred separation of ~150 Mpc between galaxy overdensities today, as detected in SDSS galaxy clustering data. This scale, calibrated by cosmic microwave background observations, traces the distribution of baryonic matter within the dark matter-dominated web. Galaxy formation within this framework follows a hierarchical process in the cold dark matter (CDM) paradigm, where small dark matter halos merge over cosmic time to build larger structures. Numerical simulations in the ΛCDM model reproduce observed clustering by simulating gravitational collapse and merging, starting from tiny density fluctuations that grow into galaxies and clusters. A compelling demonstration of dark matter's distinct nature comes from the (1E 0657-558), a merging galaxy cluster observed in 2006. Weak lensing maps show the gravitational mass—dominated by dark matter—offset from the hot intracluster gas detected in X-rays, indicating that dark matter passed through the collision without significant interaction, unlike baryonic gas which slowed due to electromagnetic forces. This separation provides direct empirical evidence for collisionless dark matter, ruling out modifications to gravity as the sole explanation for observed dynamics.

Contemporary Issues and Frontiers

Dark Energy and the Fate of the Universe

Dark energy constitutes the dominant component of the universe's energy budget, driving its accelerated expansion and comprising approximately 70% of the total energy density parameter Ω_tot. Observations of type Ia supernovae provide the initial evidence for this component, indicating a positive cosmological constant with Ω_Λ > 0 at over 5σ confidence when combined with other data. The (CMB) anisotropies measured by the Planck satellite yield Ω_Λ = 0.6847 ± 0.0073 in the flat ΛCDM model. (BAO), as traced by galaxy clustering, further constrain the matter density to Ω_m = 0.295 ± 0.015 from recent (DESI) measurements, implying Ω_Λ ≈ 0.705 assuming spatial flatness. In the standard ΛCDM paradigm, dark energy is characterized by a cosmological constant Λ, for which the equation of state parameter is fixed at w=pρ=−1w = \frac{p}{\rho} = -1, where pp is the pressure and ρ\rho is the energy density; this value ensures constant energy density over cosmic time, unlike matter or radiation. Alternative dynamical models propose scalar fields to explain dark energy. Quintessence models feature a slowly rolling scalar field with equation of state −1<w<−1/3-1 < w < -1/3, allowing the energy density to evolve gradually and potentially alleviating fine-tuning issues associated with Λ. Phantom energy models, in contrast, predict w<−1w < -1, resulting in an increasing energy density that accelerates expansion more aggressively. The value of ww profoundly influences the universe's long-term evolution. In the ΛCDM scenario with w=−1w = -1, the universe undergoes eternal expansion, culminating in the heat death or Big Freeze, where galaxies recede beyond interaction, star formation ceases, and the cosmos approaches absolute zero temperature over trillions of years. Phantom energy with w<−1w < -1 leads to a Big Rip singularity, where the accelerating expansion overcomes all bound structures—first galaxies, then star systems, planets, and ultimately atoms—in a finite time, potentially as soon as 20-30 billion years from now. A Big Crunch, involving recollapse to high density, remains possible only in models where dark energy decays sufficiently or if the universe is closed without dominant Λ, though current data disfavor this. Recent observational campaigns have begun probing potential deviations from w=−1w = -1. The DESI Year 1 BAO results from 2024, covering over 6 million galaxies and quasars up to z ≈ 2.3, are consistent with ΛCDM at the 1-2σ level but show mild preferences for dynamical in combinations with and CMB data, hinting at evolving ww with tensions around 2σ. The space telescope, launched in 2023 and commencing its wide survey in 2024, has released early imaging data revealing millions of galaxies, with full BAO and weak lensing analyses expected to tighten constraints on models by 2026; pre-2025 data already underscore ongoing H_0 tensions that could signal deviations from constant Λ. By 2025, 's Data Release 2 extended BAO measurements to 14 million objects, strengthening evidence for possible time-varying with w(z) deviations at the 2-3σ level, though full confirmation awaits later releases. As of November 2025, further analyses indicate evidence for time-varying at up to 4σ significance when combined with data, suggesting it may evolve or decay, challenging the constant Λ model.

Recent Discoveries

Since its operational debut in 2022, the (JWST) has revolutionized our understanding of the early universe by detecting an unexpectedly high number of bright and massive galaxies at s greater than 10, corresponding to less than 500 million years after the . These observations, including confirmed examples like at z=13.2 and subsequent discoveries such as JADES-GS-z14-0 at z=14.32, indicate that galaxy formation proceeded more rapidly than predicted by standard hierarchical models, potentially requiring revisions to efficiencies or growth rates. Initial photometric estimates for candidates like CEERS-93316 suggested even higher redshifts around z≈16, but spectroscopic follow-up refined it to z=4.9, underscoring the challenges in early confirmations while highlighting the overall abundance of luminous systems. The Hubble tension, a longstanding discrepancy in measurements of the Hubble constant H₀, has persisted and intensified in recent years, with local determinations from the SH0ES team using Cepheid-calibrated Type Ia supernovae yielding H₀ ≈ 73 km/s/Mpc, in contrast to the early-universe value of H₀ ≈ 67.4 km/s/Mpc inferred from Planck data. Recent measurements through 2025, including JWST data, show local H0 values ranging from 70.4 ± 2.1 km/s/Mpc (CCHP team using tip of the ) to 73.0 ± 1.0 km/s/Mpc (SH0ES), with the tension debated at 3-5σ and possible signs of resolution due to or new physics. Independent constraints from gravitational-wave standard sirens, analyzed using 47 binary mergers from the LIGO-Virgo-KAGRA GWTC-3 catalog released in 2023, provide a model-independent estimate of H₀ ≈ 64^{+17}_{-13} km/s/Mpc (68% confidence), aligning more closely with the CMB but with large uncertainties due to limited electromagnetic counterparts. The , launched in July 2023, began delivering science results in 2024, with its first major data release in March 2025 unveiling catalogs of over 26 million galaxies and detailed mappings of cosmic structures to probe and energy distributions. Early observations, including wide-field images sharper than ground-based telescopes by a factor of four, preview Euclid's capability to measure (BAO) across cosmic time, offering constraints on the universe's expansion history complementary to ongoing surveys. Similarly, the (DESI) has advanced BAO mapping with its 2024 Year 1 results, analyzing over 6 million galaxies and quasars to measure the sound horizon scale with 0.7% precision, providing robust distance ladders from z=0.3 to z=2.3. These data refine the equation of state parameter w to -0.99^{+0.15}_{-0.13} in a constant-w model, consistent with a but showing mild hints of dynamical evolution when combined with and CMB datasets.

Open Questions and Alternative Theories

One of the most profound open questions in cosmology is the , which addresses why the observed value of the Λ\Lambda is so extraordinarily small compared to theoretical expectations from . In the standard Λ\LambdaCDM model, Λ\Lambda drives the current accelerated , with its measured density contributing about 70% of the total energy budget, yet quantum vacuum fluctuations predict a value up to 120 orders of magnitude larger. This vast discrepancy, often termed the in this context, arises because the of quantum fields should gravitate like Λ\Lambda, but no mechanism in known physics explains the fine-tuning required to suppress it to the observed level of approximately 10−4710^{-47} GeV4^4. Seminal analyses, such as Weinberg's review, highlight this as a core tension between and , potentially signaling the need for new physics like or dynamical cancellation mechanisms. Alternative theories seek to resolve such puzzles without invoking dark components central to Λ\LambdaCDM. Modified Newtonian Dynamics (MOND), proposed by Milgrom, modifies gravity at low accelerations to explain galactic curves without , predicting flat velocities for accelerations below a0≈10−10a_0 \approx 10^{-10} m/s2^2, which aligns with observations in many spiral galaxies. While MOND successfully reproduces phenomena like the Tully-Fisher relation without unseen mass, it faces challenges in cluster scales and requires relativistic extensions like TeVeS to incorporate cosmology, where it predicts a varying effective Λ\Lambda tied to Hubble expansion. Cyclic models, such as the Steinhardt-Turok ekpyrotic scenario, propose an infinite sequence of universe cycles driven by collisions in higher-dimensional space, avoiding the of the and naturally suppressing Λ\Lambda through entropy dilution across cycles. In this framework, each cycle expands and contracts slowly, with accelerated expansion arising from a rather than a constant Λ\Lambda, offering a test against by predicting distinct signatures. Quantum gravity approaches further probe these open issues by quantizing spacetime itself. Loop quantum cosmology (LQC), developed by Ashtekar and Bojowald, applies loop quantum gravity to cosmological spacetimes, replacing the Big Bang singularity with a quantum bounce where the universe contracts to a minimal volume before expanding, resolving the hierarchy problem by dynamically adjusting effective Λ\Lambda through holonomy corrections that cap curvature at Planck scales. This predicts deviations in cosmic microwave background power spectra at high multipoles, potentially observable with future experiments. In string theory, the landscape paradigm, articulated by Susskind, posits a vast ensemble of 1050010^{500} or more vacua arising from compactifications of extra dimensions, where our universe's small Λ\Lambda is anthropically selected from the distribution, as only low-Λ\Lambda environments allow structure formation and observers. Bousso and others have formalized this via the string theory landscape, linking it to eternal inflation where bubble nucleation populates diverse vacua, though it raises multiverse implications without direct falsifiability. The of the universe, quantified by η≈6×10−10\eta \approx 6 \times 10^{-10}, remains unexplained beyond leptogenesis in standard models, prompting alternatives that generate the matter-antimatter imbalance via effects. , occurring during the around 100 GeV, relies on bubble nucleation in the early universe, amplified by strong first-order transitions in extensions like the two-Higgs-doublet model, producing η\eta through transport of left-handed fermions across bubble walls. GUT-scale , as in grand unified theories, generates asymmetry via heavy particle decays violating B-L symmetry, with Affleck-Dine mechanisms in supersymmetric models using scalar fields to produce baryons non-thermally during reheating. These mechanisms satisfy Sakharov's conditions—baryon number violation, C and , and departure from equilibrium—and are constrained by limits, offering testable predictions like enhanced neutron-antineutron oscillations. In a cosmological context, the —questioning the absence of detected despite the universe's age and scale—intensifies with expanding horizons that limit . The contains about 101110^{11} galaxies, yet cosmic expansion recedes distant civilizations beyond our after roughly 10 billion years, implying that only a of the universe's remains causally connected over cosmic . Tipler's argument extends this, positing that advanced civilizations would colonize the galaxy rapidly via self-replicating probes, but inflation and acceleration isolate regions, resolving the paradox by suggesting intelligent life is rare or confined to local bubbles. Recent analyses incorporate this into Λ\LambdaCDM, estimating the stellar formation rate implies 102010^{20} potential sites, yet no signals due to temporal and spatial isolation.

Philosophical Implications

Cosmological Arguments

Cosmological arguments in posit that the existence and structure of the imply the presence of a necessary being or first cause, often identified as . These arguments draw on cosmological insights, such as the universe's apparent beginning, to infer a transcendent cause beyond the physical realm. Rooted in medieval and early modern thought, they emphasize causation, contingency, and necessity as pathways to explaining why the exists at all. Thomas Aquinas, in his Summa Theologica, presents the third of his Five Ways as an argument from possibility and necessity. He observes that many things in nature are contingent, meaning they can exist or not exist, as evidenced by their generation and corruption. If everything were contingent, there would have been a time when nothing existed, and from nothing, nothing could arise, leading to the absurdity that nothing exists now. Therefore, there must be a necessary being whose existence is not derived from another, serving as the ultimate ground for all contingent beings; Aquinas identifies this as . Gottfried Wilhelm Leibniz advances a contingency-based cosmological argument, grounded in the principle of sufficient reason, which holds that every fact or truth must have an explanation. The existence of contingent things—the world as a whole—forms a contingent fact that cannot be sufficiently explained by other contingent things alone, as this would lead to an without ultimate reason. Thus, there must be a necessary being whose essence includes existence, providing the sufficient reason for the contingent universe; Leibniz terms this necessary being . The , revived in modern form by , asserts that whatever begins to exist has a cause, the began to exist, and therefore the has a cause. This cause must be timeless, spaceless, immaterial, and immensely powerful, pointing to a personal creator. The argument's second premise relies on philosophical arguments against an actual of events and scientific evidence from the , indicating a finite past of approximately 13.8 billion years. Contemporary formulations strengthen the by incorporating the Borde-Guth-Vilenkin theorem, which demonstrates that any universe undergoing average expansion cannot be past-eternal but must have a finite , with geodesics terminating in the past. This theorem applies to inflationary models, including attempts at , implying a boundary to and supporting the universe's beginning, thus bolstering the need for an external cause. Critiques of these arguments often invoke to challenge the necessity of a first cause. For instance, models like quantum fluctuations in a vacuum state suggest the could arise uncaused from "nothing," where quantum laws permit spontaneous creation without violating , as seen in proposals like the Hartle-Hawking no-boundary condition. Philosophers such as Wes argue that the overlooks uncertainties in extrapolating the to an absolute beginning, given quantum effects near the singularity, and question whether empirical causation applies to the universe's origin. contends that the argument presupposes a creator without addressing whether the universe's origin requires one, especially if quantum indeterminacy allows uncaused events.

Multiverse and Fine-Tuning

The fine-tuning of the describes the remarkable precision of certain fundamental parameters that appear necessary for the formation of galaxies, , atoms, and ultimately . A striking example is the Λ\Lambda, which governs the accelerated and is observed to be extraordinarily small, tuned to approximately 1 part in 1012010^{120} compared to the Planck scale. This precision is essential because a value even slightly larger would cause the to expand too rapidly for bound structures like galaxies to form, while a negative value could lead to rapid collapse. This observation was highlighted in Steven Weinberg's analysis, where he derived an upper bound on Λ\Lambda based on the requirement for galaxy formation. Another key instance of fine-tuning involves the Higgs (vev), which sets the scale for particle masses in the . The Higgs vev is fine-tuned to a value around 246 GeV, far below the Planck scale, enabling the stability of atoms and the production of elements heavier than helium through . Without this tuning, protons might decay too quickly or atomic binding energies could fail to support complex chemistry. considerations suggest that the allowed range for the Higgs vev is narrow, constrained by the need for sufficient carbon production in stars and long-lived hadrons, as explored in early applications of the to parameters. To address this fine-tuning without adjustments, the hypothesis proposes an ensemble of universes with varying physical constants, where our universe is one that permits observers due to selection effects. In the inflationary bubble , arising from , quantum fluctuations during create disconnected bubble universes, each potentially with different energies and expansion rates. This framework, developed from models of slow-roll , predicts a perpetual branching of space-time regions, leading to diverse cosmological outcomes. Complementing this, the posits a of possible states in , estimated at around 1050010^{500} distinct flux vacua in type IIB compactifications on Calabi-Yau manifolds, each corresponding to different low-energy effective theories and constants. This landscape provides a theoretical basis for varying fundamental parameters across universes. The formalizes how observers select fine-tuned universes within a . The weak (WAP) asserts that the observed values of constants must be consistent with the existence of conscious observers, as we could not exist in universes incompatible with life. In contrast, the strong (SAP) posits that the universe is compelled to produce observers by its inherent structure. Extending this, John Archibald Wheeler's participatory suggests that observers retroactively influence the universe's through measurement, effectively participating in its realization. These principles shift the explanation from coincidence to a in a ensemble. Despite its explanatory power, the faces significant critiques. A primary concern is : since other universes lie beyond our cosmic horizon, predictions about them cannot be empirically verified, potentially rendering the hypothesis unfalsifiable and more philosophical than scientific. However, recent proposals as of 2024 suggest indirect tests of the in a context, such as detecting primordial gravitational waves from cosmic via satellites like LiteBIRD (planned launch 2032) and searching for fuzzy axions—light particles that could explain certain cosmological features but not —using future experiments; if fuzzy axions are ruled out as , it would support the rarity of life-permitting conditions. Additionally, some philosophers argue that inferring a large from our single commits the inverse , akin to assuming many coin tosses from one streak of heads without independent evidence for multiple trials. These issues highlight tensions between multiverse theories and standard scientific methodology. The and fine-tuning debate underscores a contrast between intentional , where parameters are deliberately set for , and naturalistic selection, where biases in a diverse ensemble explain our observations without purpose. This framework resolves fine-tuning puzzles by invoking statistical inevitability across immense possibilities, though it remains contested due to evidential challenges. Eternal variants of , briefly, contribute to this by generating ongoing bubble nucleation, while the Λ\Lambda problem exemplifies broader open questions in .

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