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CosmologyEdit

Cosmology is the science that studies the universe as a whole—the origin, composition, evolution, and large-scale structure of all that exists. It brings together physics, astronomy, and mathematics to explain how a vast, increasingly complex cosmos emerges from a few fundamental laws and initial conditions. The discipline rests on a disciplined method: formulate hypotheses that yield testable predictions, compare those predictions with observations, and refine the models accordingly.

A central scaffold in modern cosmology is the cosmological principle: on sufficiently large scales, the universe looks the same in every direction and from every location. When combined with the theory of gravity, this principle leads to a mathematical description of expansion and geometry through the Friedmann equations, which relate the growth of the scale factor to the energy content of the cosmos. The general relativistic framework underpins these equations, with solutions that describe a universe that can be open, flat, or closed in shape and that expands or contracts over time. For the past century, this synthesis has explained the broad outlines of cosmic history, from the hot, dense beginnings to the cooler, structured universe we observe today. See General relativity and Friedmann–Lemaître–Robertson–Walker metric for foundational methods and models.

Observational cosmology gathers data from the oldest light in the universe, the cosmic microwave background, and from the distribution and behavior of matter at large scales. Precise measurements of the cosmic microwave background by missions such as Planck (spacecraft) reveal a remarkably uniform background with tiny anisotropies that encode information about the early universe, its contents, and its geometry. The same data, together with observations of distant supernovae, baryon acoustic oscillations, gravitational lensing, and the rotation curves of galaxies, map the composition and expansion history of the cosmos. These observations point to a universe that is currently dominated by two enigmatic components—dark matter, which clumps and drives structure, and dark energy, which accelerates cosmic expansion—along with ordinary matter that forms stars, planets, and life. See Cosmic microwave background, Dark matter, Dark energy, and Baryon acoustic oscillations for linked discussions.

The leading framework is the standard model of cosmology, commonly called ΛCDM. It posits a hot early universe that expanded and cooled, allowing photons and baryons to decouple and form the first atoms, followed by the growth of structure under gravity. In this model, ordinary matter constitutes a small fraction of the total energy density; the rest is dark matter and dark energy. The framework relies on an evolving universe described by the equations of General relativity and a near-uniform early state that developed into the complex cosmic web seen in modern surveys of galaxies and clusters. The names of key elements—Big Bang, inflation, cosmological constant, dark matter, dark energy—are linked concepts in the broader mosaic of cosmology, each supported by multiple lines of evidence and subject to ongoing refinement. See Big Bang, Cosmological inflation, Cosmological constant, Dark matter, and Dark energy.

Foundational concepts that guide current practice include the cosmological constant as the simplest explanation for late-time acceleration, the nature of dark matter as a non-baryonic form of matter that interacts primarily through gravity, and the inflationary epoch that smooths and flattens the early universe while generating the seeds of structure. The standard model also rests on well-tested physics, such as the expansion history described by the Friedmann equations and the metric structure dictated by General relativity; it accommodates the observed abundances from Big Bang nucleosynthesis and the imprint of acoustic oscillations in the distribution of matter. See Cosmological constant, Big Bang nucleosynthesis, and Cosmological inflation for focused topics.

Contemporary cosmology is marked by both strong consensus and significant questions. The ΛCDM model provides a compelling, quantitative account of a wide range of data with a relatively small set of parameters. Yet several tensions have emerged, most notably in measurements of the Hubble constant, which describe the current rate of cosmic expansion. Local measurements and early-universe inferences yield values that do not fully align within the same framework, prompting discussions about possible new physics, systematic effects, or extensions to the standard model. See Hubble constant for more on this topic.

There is also debate about the interpretation of dark energy and whether a simple cosmological constant suffices or a dynamic form of energy is required. Related discussions include the possibility of modifications to gravity on cosmological scales, alternative histories of the early universe, and the role of fine-tuning in explaining the observed acceleration. Proposals range from modest extensions of the standard model to more speculative ideas, such as a broader multiverse or anthropic reasoning to account for observed conditions. Supporters emphasize the tractability and testability of testable theories, while critics argue that untestable or highly speculative explanations should remain outside the boundaries of a robust scientific program. See Dark energy, Modified gravity, Anthropic principle, and Multiverse.

Alongside the mainstream view, cosmology also contends with historical alternatives and ongoing refinements. The steady-state theory, once a competitor to the Big Bang scenario, is now largely out of favor due to its inability to explain the cosmic microwave background and observed evolution; nevertheless, its historical role illustrates how empirical data can shift consensus. Other ideas, such as different early-universe scenarios or modifications to gravity, continue to be explored as potential complements or replacements for parts of the standard picture. See Steady state theory and Friedmann–Lemaître–Robertson–Walker metric for context.

Cosmology remains deeply empirical: predictions are checked against observations, new data refine or revise prevailing models, and the dialogue between theory and measurement continues to shape our understanding of the cosmos. The field intertwines with related disciplines—such as the physics of the early universe, the study of galaxy formation, and the investigation of the fundamental forces—yet it is the coherence between a few universal principles and a suite of high-precision measurements that keeps the standard model as the most persuasive description we have today. See Cosmology for a general overview and Galaxy formation for the connection to structure on smaller scales.

See also