The Big Bang theory is the prevailing cosmological model describing the origin and evolution of the observable universe. It holds that the universe began approximately 13.8 billion years ago in an extremely hot, dense state and has been expanding ever since. The theory addresses the earliest observable conditions of the universe and its subsequent development, not the ultimate origin of matter or energy. It is supported by multiple independent lines of evidence, including the observed redshift of distant galaxies, the cosmic microwave background radiation (CMB), and the measured abundances of light elements consistent with predictions of Big Bang nucleosynthesis.

The Big Bang theory is built on two foundational pillars: general relativity, which describes the large-scale structure and dynamics of spacetime, and the cosmological principle, which holds that the universe is homogeneous and isotropic at large scales. Edwin Hubble's 1929 observation that galaxies are receding from one another at velocities proportional to their distance provided the first major empirical basis for an expanding universe. The discovery of the CMB by Arno Penzias and Robert Wilson in 1965 - a near-uniform thermal radiation field permeating all directions of space - was widely regarded as strong confirmation of the model's prediction that the early universe was hot and opaque.

The Lambda-CDM model (Lambda Cold Dark Matter) is the current standard model of cosmology, extending the basic Big Bang framework to include dark energy (represented by the cosmological constant, Lambda) and cold dark matter. Together, these two components are thought to constitute roughly 95% of the total energy content of the universe, though neither has been directly detected. The nature of dark matter and dark energy remains an active area of research and debate.

Certain questions remain outside the explanatory scope of the standard Big Bang model. The theory does not address what, if anything, preceded the initial singularity, or whether the concept of “before” applies at all given that time itself may have originated with the universe. The flatness problem, the horizon problem, and the magnetic monopole problem led to the development of inflationary cosmology in the 1980s - a proposed rapid exponential expansion in the universe's earliest moments - which is now incorporated into most mainstream accounts but remains a distinct hypothesis with its own contested elements. Alternatives and extensions to inflation, as well as challenges to the standard model, are subjects of ongoing theoretical work.

The Big Bang theory holds broad consensus across the physical sciences, including cosmology, astrophysics, and particle physics. Major scientific institutions and peer-reviewed literature treat it as the best-supported account of the universe's early history. Consensus exists on the core framework; consensus is narrower or absent on extensions such as inflationary models, the nature of dark matter and dark energy, and interpretations of the earliest moments near the singularity.

Standard Big Bang / Lambda-CDM - The mainstream scientific position holds that the universe originated in a hot, dense state approximately 13.8 billion years ago and has expanded and cooled since. Standard Cosmology Viewpoint

Inflationary cosmology - A widely adopted extension positing a period of extremely rapid exponential expansion in the universe's first fraction of a second, proposed to resolve several anomalies in the basic Big Bang model. Not universally accepted even among proponents of the Big Bang framework. Inflationary Cosmology Viewpoint

Quantum gravity and no-boundary proposals - A family of approaches including the Hartle-Hawking no-boundary proposal and string-theoretic pre-Big Bang models (Veneziano et al.) that attempt to describe the origin of the universe within a quantum gravitational framework, in some cases eliminating the initial singularity entirely. Distinct from cyclic models in that they do not posit a prior classical phase. Quantum Gravity Cosmology Viewpoint

Steady State theory - A now-minority position, historically prominent, holding that the universe has no beginning and that matter is continuously created to maintain a constant density as the universe expands. First proposed by Hoyle, Gold, and Bondi in 1948; most proponents abandoned it following the discovery of the CMB, though a quasi-Steady State variant was later developed by Hoyle, Burbidge, and Narlikar. Steady State Viewpoint

Cyclic and bouncing cosmologies - A family of models proposing that the Big Bang was not an absolute beginning but a transition from a prior contracting phase or a preceding cycle, including the Ekpyrotic model and loop quantum cosmology proposals. Cyclic Cosmology Viewpoint

Plasma cosmology / Electric Universe - A heterodox position arguing that electromagnetic forces, rather than gravity and the Big Bang framework, better explain large-scale cosmic structures. Rejected by mainstream cosmology but maintained by a small number of researchers. Plasma Cosmology Viewpoint

Creationist and religious cosmologies - Various theological traditions hold that the universe was created by a divine being, with accounts ranging from young-earth creationism (which rejects the Big Bang timeline entirely) to theistic interpretations that treat the Big Bang as consistent with or even supportive of a doctrine of creation. Creationist Viewpoint | Theistic Cosmology Viewpoint

• Big Bang Theory - History
• Big Bang Theory - Debate
• Big Bang Theory - Cosmological Consensus
• Universe
• Cosmology
• Cosmic Microwave Background
• Big Bang Nucleosynthesis
• Dark Matter
• Dark Energy
• Inflationary Cosmology
• Steady State Theory
• Fine-Tuning Argument

1. Planck Collaboration, “Planck 2018 results. VI. Cosmological parameters,” Astronomy & Astrophysics 641 (2020): A6. doi:10.1051/0004-6361/201833910
2. Edwin Hubble, “A Relation between Distance and Radial Velocity among Extra-Galactic Nebulae,” Proceedings of the National Academy of Sciences 15, no. 3 (1929): 168-173.
3. Arno A. Penzias and Robert W. Wilson, “A Measurement of Excess Antenna Temperature at 4080 Mc/s,” Astrophysical Journal 142 (1965): 419-421.
4. Alan H. Guth, “Inflationary universe: A possible solution to the horizon and flatness problems,” Physical Review D 23, no. 2 (1981): 347-356.
5. Fred Hoyle, Thomas Gold, and Hermann Bondi, “The Steady-State Theory of the Expanding Universe,” Monthly Notices of the Royal Astronomical Society 108 (1948): 252-287.
6. Fred Hoyle, Geoffrey Burbidge, and Jayant V. Narlikar, A Different Approach to Cosmology (Cambridge: Cambridge University Press, 2000).
7. Paul J. Steinhardt and Neil Turok, Endless Universe: Beyond the Big Bang (New York: Doubleday, 2007).
8. James B. Hartle and Stephen W. Hawking, “Wave function of the Universe,” Physical Review D 28, no. 12 (1983): 2960-2975.
9. Gabriele Veneziano, “Pre-big-bang origin of our universe from string cosmology,” Classical and Quantum Gravity 9 (1992): S335-S347.