The Big Bang is a physical cosmology in which the universe evolved from an early hot, dense phase and has expanded and cooled over 13.8 billion years to form present‑day structures such as galaxies and clusters, consistent with general relativity and multiple, independent observations. Big-bang model | Encyclopaedia Britannica;
Planck mission overview | ESA.
Historical development
- –In 1927 the Belgian physicist and priest Georges Lemaître derived an expanding‑universe solution of Einstein’s equations and connected it to the observed galaxy redshifts; an English translation appeared in 1931.
MNRAS (1931) translation of Lemaître 1927;
IAU press release on “Hubble–Lemaître law”.
- –In 1929 Edwin Hubble reported a correlation between galaxy distances and recessional velocities, now termed the Hubble–Lemaître law.
PNAS/ADS abstract of Hubble 1929;
Britannica: Cosmological expansion.
- –In the 1940s George Gamow, Ralph Alpher, and Hans Bethe developed hot Big Bang nucleosynthesis, explaining light‑element abundances.
Physical Review 73 (1948): “The Origin of Chemical Elements”;
APS News background.
- –The term “Big Bang” was popularized by Fred Hoyle in a 1949 BBC broadcast while advocating a Steady-state theory alternative.
Observational pillars
- –Cosmic expansion: galaxy redshift–distance proportionality indicates metric expansion of space.
- –Cosmic microwave background (CMB): relic radiation from recombination (~380,000 years after the Big Bang) with near‑perfect blackbody spectrum and small anisotropies, mapped by COBE/WMAP/Planck.
ESA: Planck and the CMB;
NASA WMAP overview;
OSTI: COBE DMR anisotropy papers (1992).
- –Primordial nucleosynthesis: predicted abundances of H, He, D, and Li formed within minutes are broadly consistent with observations, constraining the baryon density and relativistic species.
- –Large‑scale structure and baryon acoustic oscillations (BAO): galaxy clustering and Lyα forest measurements provide a standard‑ruler consistent with ΛCDM and CMB results.
SDSS eBOSS cosmology results;
A&A BAO at z≈2.35.
The hot early universe and thermal history
- –Inflationary epoch: a brief period of accelerated expansion can resolve the horizon and flatness problems and seed adiabatic, nearly scale‑invariant perturbations.
- –Nucleosynthesis (t ≈ 3–20 minutes): light nuclei formed as the plasma cooled below ~1 GK; deuterium and helium abundances serve as sensitive probes of early‑universe physics.
Nuclear Science and Techniques (2024) on primordial D/H.
- –Recombination and photon decoupling (t ≈ 380,000 years): neutral hydrogen formed as the temperature fell to ~3000 K; photons began free‑streaming, creating the CMB observed today at T ≈ 2.725 K.
- –Structure formation: tiny CMB anisotropies (ΔT/T ~ 10⁻⁵) trace initial density fluctuations that grew via gravitational instability into galaxies and clusters.
Cosmological parameters and composition
- –Precision CMB measurements yield an age of about 13.8 billion years and a matter–energy budget of roughly ~4.9% baryonic matter, ~26.8% dark matter, and ~68.3% dark energy in the concordance ΛCDM model.
- –Representative parameters inferred from Planck include a Hubble constant H₀ ≈ 67 km s⁻¹ Mpc⁻¹ (CMB‑inferred), which, when combined with BAO and other probes, tightly constrains spatial curvature and matter density.
Late‑time acceleration
- –Type Ia supernova observations in the late 1990s showed that the expansion is accelerating, commonly attributed to dark energy consistent with a cosmological constant Λ.
Nobel Prize in Physics 2011.
Key missions and instruments
- –COBE established the blackbody CMB spectrum and detected large‑scale anisotropies; WMAP refined full‑sky anisotropy maps; [Planck (spacecraft)] delivered high‑precision temperature and polarization data.
Alternative and earlier competing models
- –The Steady-state theory (Bondi, Gold, Hoyle, 1948) posited continuous matter creation to maintain constant density in an eternally expanding universe; evidence such as the CMB and radio source counts led to its abandonment in favor of the hot Big Bang.
Recognition and consolidation of physical cosmology
- –The modern framework of physical cosmology, including interpretation of CMB anisotropies and structure formation within the Big Bang paradigm, was recognized with the 2019 Nobel Prize in Physics to James Peebles.