Cosmology is the scientific and philosophical study of the origin, structure, evolution, and ultimate fate of the universe as a whole. It draws on physics, mathematics, astronomy, and philosophy to address questions about the universe's large-scale properties: its age, composition, geometry, and dynamics. The field divides broadly into physical cosmology, which applies observational and theoretical physics to the observable universe, and philosophical or metaphysical cosmology, which addresses questions about existence, causation, and the nature of space and time that extend beyond empirical reach.

The dominant framework in physical cosmology is the Lambda-CDM (ΛCDM) model, which describes a universe approximately 13.8 billion years old that began in an extremely hot, dense state - referred to as the Big Bang - and has been expanding ever since. This model incorporates ordinary (baryonic) matter, cold dark matter (CDM), and a cosmological constant (Λ) representing dark energy, which is invoked to explain the observed accelerating expansion of the universe first reported in the late 1990s. According to current measurements, ordinary matter constitutes roughly 5% of the universe's total energy content, with dark matter accounting for approximately 27% and dark energy approximately 68%.

The Cosmic Microwave Background (CMB) - a nearly uniform thermal radiation field permeating the universe - is treated as a key observational pillar of the standard model, as its properties align with predictions of a hot early universe. Large-scale structure surveys, supernova distance measurements, and gravitational lensing observations provide additional evidentiary support.

Several areas within the standard framework remain unresolved. The nature of dark matter and dark energy has not been directly confirmed by any detection experiment; both are inferred from gravitational effects and cosmological fits. The “Hubble tension” - a statistically significant discrepancy between measurements of the universe's current expansion rate derived from the CMB and those derived from local distance-ladder measurements - has not been resolved and is an active area of debate. The earliest moments of the universe, including the hypothesized inflationary epoch, remain outside direct observational reach. Questions about initial conditions, fine-tuning of physical constants, and the universe's ultimate fate (heat death, Big Rip, Big Crunch, and others) remain open.

Philosophical cosmology addresses questions that physical models leave open by design: why there is something rather than nothing, whether the universe requires a first cause, whether it is infinite or finite in extent or duration, and how physical laws themselves originated. These questions intersect with theology, metaphysics, and the philosophy of science.

The Big Bang framework and the ΛCDM model represent the prevailing consensus among professional cosmologists and astrophysicists. See Cosmology - Physics Consensus for a description of the scope and basis of that consensus. Consensus on the standard model does not resolve the philosophical questions the field raises, nor does it settle internal disputes over dark energy models, inflationary theory, or the Hubble tension.

Standard Big Bang / ΛCDM - The mainstream scientific position holds that the universe originated in a hot, dense initial state approximately 13.8 billion years ago and has evolved under known and postulated physical laws since. See Standard Big Bang Viewpoint.

Steady State and non-standard cosmological models - Prior to the late 20th century, steady-state models proposed a universe without a temporal beginning, continuously creating matter to maintain constant density. Modern variants and alternatives to ΛCDM, including plasma cosmology and modified-gravity models, dispute aspects of the standard account. See Alternative Cosmological Models Viewpoint.

Fine-tuning and the anthropic principle - The observed values of physical constants appear narrowly constrained to permit the existence of complex structures and life. Some cosmologists and philosophers argue this requires explanation, either through multiverse hypotheses, design inference, or a revised understanding of physical law. Others regard the inference as a selection effect requiring no further explanation. See Fine-Tuning Viewpoint.

Theistic and creationist cosmologies - Various religious traditions maintain cosmological accounts that differ from or conflict with the standard scientific model, including young-universe creationism, old-universe creationism, and theistic frameworks that accept the Big Bang while interpreting it as consistent with or as evidence for divine creation. See Theistic Cosmology Viewpoint.

Cyclic and eternal universe models - Some theoretical frameworks, including conformal cyclic cosmology (Penrose) and ekpyrotic models, propose that the universe undergoes repeated cycles or that the Big Bang was not an absolute beginning. See Cyclic Universe Viewpoint.

Islamic astronomical cosmology - Classical Islamic scholarship developed sophisticated astronomical and cosmological frameworks that both preserved and extended Greek cosmological thought, and in some cases anticipated elements of later European science. See Islamic Astronomy Viewpoint.

• Cosmology - History
• Cosmology - Debate
• Cosmology - Physics Consensus
• Big Bang
• Dark Matter
• Dark Energy
• Multiverse
• Philosophy of Cosmology
• Fine-Tuning
• Islamic Astronomy Viewpoint

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