Physical cosmology

Physical cosmology emerged in the early 20th century from the combination of Einstein's general relativity with astronomical observations. Albert Einstein's field equations, published in 1915, described gravity as the curvature of spacetime and allowed for a dynamic universe. In 1917, Willem de Sitter and others found solutions that suggested expansion, but it was Edwin Hubble's 1929 observation of a linear redshift–distance relation for galaxies that provided the first direct evidence for cosmic expansion.¹ This discovery, combined with theoretical work by Alexander Friedmann and Georges Lemaître, led to the Big Bang model, which posits that the universe began from a hot, dense state and has been expanding ever since.

The Big Bang model is the prevailing cosmological description of the universe's origin. It is supported by three key observational pillars: the cosmic microwave background (CMB) radiation, the abundance of light elements (primordial nucleosynthesis), and the large-scale distribution of galaxies. The CMB, discovered in 1965 by Arno Penzias and Robert Wilson, is a nearly uniform bath of microwave radiation that is a relic of the universe when it became transparent, about 380,000 years after the Big Bang.² The precise measurements of the CMB by the COBE, WMAP, and Planck satellites have confirmed the predictions of the Big Bang model and provided detailed information about the universe's composition and geometry.

Observations of galaxy rotation curves, gravitational lensing, and the CMB indicate that the universe is dominated by two mysterious components: dark matter and dark energy. Dark matter, which does not emit or absorb light, makes up about 27% of the universe's energy density and is inferred from its gravitational effects. Dark energy, which makes up about 68%, is thought to be responsible for the accelerated expansion of the universe, discovered in 1998 through observations of distant Type Ia supernovae.³ The nature of both components remains one of the biggest unsolved problems in physics.

The standard model of cosmology is the Lambda-CDM model, which combines a cosmological constant (Lambda) with cold dark matter (CDM). It successfully explains a wide range of observations, including the CMB power spectrum, the distribution of galaxies, and the abundance of galaxy clusters. The model is parameterized by a small set of cosmological parameters, such as the Hubble constant, the matter density, and the spectral index of primordial fluctuations. However, the model faces challenges, including the 'Hubble tension'—a discrepancy between measurements of the expansion rate from the early universe (CMB) and the local universe (supernovae).

Cosmologists use a variety of observational probes to test models and constrain parameters. These include the CMB, galaxy surveys (such as the Sloan Digital Sky Survey), baryon acoustic oscillations, weak gravitational lensing, and the Lyman-alpha forest. Upcoming projects like the James Webb Space Telescope and the Euclid mission aim to improve measurements of dark energy and the epoch of reionization. Additionally, the detection of gravitational waves from merging black holes and neutron stars opens a new window into the early universe, potentially probing physics at energies far beyond the reach of particle accelerators.⁴

Beyond the mainstream narrative, physical cosmology includes several niche but significant topics. For instance, the 'axis of evil' is a puzzling alignment in the CMB that may hint at anomalies in the standard model. The 'Great Attractor' is a gravitational anomaly in the local universe that influences the motion of galaxies. The concept of 'cosmic variance' limits the precision of CMB measurements because we can only observe one universe. Also, the 'Bullet Cluster' provides direct evidence for dark matter, as the separation of dark matter from ordinary matter during a collision is observed. Furthermore, the 'cosmological constant problem'—the huge discrepancy between theoretical predictions and observed value—remains an open puzzle.⁵

Big Bang

The prevailing cosmological model for the origin of the universe, describing its expansion from a hot, dense state.

Cosmic microwave background (CMB)

The thermal radiation left over from the Big Bang, filling the universe and providing a snapshot of the early universe.

Dark matter

A form of matter that does not emit or absorb light, inferred from its gravitational effects.

Dark energy

A mysterious energy component causing the accelerated expansion of the universe.

Lambda-CDM model

The standard model of cosmology, incorporating a cosmological constant and cold dark matter.