Physical cosmology

Physical cosmology is the scientific study of the universe as a whole, examining its large-scale structure, origin, evolution, and ultimate fate. It relies on cosmological models—mathematical descriptions that apply universal physical laws to the cosmos—to interpret astronomical observations and address fundamental questions concerning cosmogony, chronology, and structure. Modern cosmology integrates work from theoretical physics, observational astronomy, particle physics, relativity, and plasma physics, drawing upon both empirical data and theoretical frameworks.
The discipline emerged from the application of the Copernican principle, which asserts that the same physical laws operate universally, and from the development of Newtonian mechanics, which offered the first systematic description of gravitational dynamics. Twentieth-century advances, particularly Einstein’s theory of general relativity and the discovery of the expanding universe, transformed cosmology into a precise, observation-driven science.

Emergence of modern cosmology

A major turning point occurred in 1915 with general relativity, which recast gravity as the curvature of spacetime. Einstein initially favoured a static cosmos and introduced the cosmological constant to counteract gravitational collapse. His 1917 model depicted a finite, unbounded static universe; however, it proved unstable, as small perturbations would inevitably lead to contraction or expansion.
Independent solutions by Alexander Friedmann in the early 1920s showed that general relativity permitted dynamic universes—expanding or contracting, and with open, flat, or closed geometry. Later, Georges Lemaître derived the same expanding-universe solutions and proposed that the cosmos began from a “primeval atom”, anticipating what would become known as the Big Bang theory.
Observational evidence accumulated rapidly. Work by Vesto Slipher and others detected redshifts in spiral nebulae, and in 1929 Edwin Hubble demonstrated that these nebulae were distant galaxies whose recessional velocity increased with distance. This linear relation, now known as Hubble’s law, implied that the universe is expanding uniformly, consistent with Friedmann–Lemaître cosmology.

Competing models and the rise of the Big Bang

The expanding universe spurred competing theories. Lemaître’s Big Bang model proposed a hot, dense origin, while Fred Hoyle, Thomas Gold, and Hermann Bondi advanced the steady-state model, in which matter is continuously created to preserve a constant cosmic density. For several decades both models attracted support.
Decisive evidence favouring the Big Bang emerged in 1965 with the discovery of the cosmic microwave background (CMB), predicted as residual radiation from an early hot universe. Precise CMB measurements by later missions reaffirmed an early epoch of extreme temperature and density. By the 1990s, the Big Bang theory—as refined into the ΛCDM model—had become the consensus framework.
ΛCDM, the standard cosmological model, describes a universe dominated by cold dark matter and dark energy. Although the physical nature of these components remains uncertain, the model accurately matches diverse observations, including large-scale galaxy surveys, CMB anisotropies, and supernova distance measurements.

Observational advances and recent developments

Dramatic improvements in instrumentation, from high-resolution space telescopes to deep multiwavelength surveys, have revolutionised cosmology since the late twentieth century. Observations of distant Type Ia supernovae revealed the accelerating expansion of the universe, suggesting a dominant form of dark energy. Studies of baryon acoustic oscillations and gravitational lensing further constrained cosmological parameters.
Inflationary cosmology predicts that gravitational waves were produced in a rapid expansion shortly after the Big Bang. While their direct detection remains challenging, ongoing missions continue to search for characteristic signatures in the CMB.
Theoretical work by Roger Penrose and Stephen Hawking in the 1960s demonstrated that classical general relativity implies an initial gravitational singularity, although more recent quantum-gravity hypotheses have attempted to avoid or reinterpret the singularity. Some newer theoretical proposals consider cosmological models without a true beginning, positing cyclic, bouncing, or emergent universes.
In 2023, observations from the James Webb Space Telescope prompted renewed debate about aspects of the ΛCDM model, as early galaxies appeared more massive or evolved than predicted, motivating ongoing refinement of theoretical frameworks.

Cosmic matter, energy, and nucleosynthesis

According to standard cosmology, the lightest chemical elements—principally hydrogen and helium—were synthesised during Big Bang nucleosynthesis within the first few minutes of cosmic time. Subsequent element formation occurs within stars through stellar nucleosynthesis, leading to heavier nuclei up to the iron group. The extremely high binding energies of these elements make their formation energetically favourable in stellar cores.
Cataclysmic events such as supernovae and neutron-star mergers fuel rapid nucleosynthesis of elements heavier than iron, enriching the interstellar medium. Gravitational collapse into black holes powers some of the most energetic astrophysical phenomena, including quasars and active galactic nuclei, driven by accretion processes in galactic centres.
Despite progress, some cosmic phenomena—most notably the accelerating expansion—cannot be explained by ordinary matter or radiation alone. Dark energy, often associated with vacuum energy or the energy of quantum fields in empty space, is introduced to account for this behaviour. Its physical origin remains uncertain.

Cosmological energy considerations

Defining total energy in an expanding universe is problematic in general relativity. Unlike in Newtonian mechanics, no global conservation law applies straightforwardly to dynamic spacetime. Photons lose energy through cosmological redshift, and this “lost” energy does not transfer to other systems, raising questions about the meaning of conservation on cosmic scales. Some theorists maintain that energy conservation remains valid in a generalised form, while others argue that conservation is not defined for the universe as a whole.
As the universe expands, the energy density of radiation decreases more rapidly than that of non-relativistic matter, owing to the additional redshift effect on photon energies. Consequently, the dominant cosmic energy component has shifted over time—from radiation in the earliest epochs, to matter for most of cosmic history, and now to dark energy.

Cosmology as a multidisciplinary science

Cosmology today stands at the intersection of theory and observation, drawing on disciplines such as quantum mechanics, high-energy physics, astrophysics, and mathematical physics. Its central task is to construct models that explain observed cosmic structures—from galaxies and large-scale filaments to the earliest fluctuations in the CMB—while addressing profound questions about the origins and long-term evolution of the universe.