Cosmology Glossary

Periodic fluctuations in the density of visible baryonic matter caused by acoustic density waves in the primordial plasma, leaving a characteristic scale imprinted in the galaxy distribution.

The cosmological model describing the universe's origin from an extremely hot, dense state approximately 13.8 billion years ago, followed by continuous expansion and cooling.

A hypothesized phase of exponentially rapid expansion in the very early universe, lasting a tiny fraction of a second, that explains the large-scale uniformity and flatness of the observable universe.

Thermal radiation filling the observable universe nearly uniformly, released about 380,000 years after the Big Bang when the universe became transparent to photons.

The large-scale distribution pattern of matter in the universe, consisting of interconnected filaments of galaxies and galaxy clusters surrounding vast, nearly empty voids.

A constant term (Lambda) in Einstein's field equations representing a uniform energy density of the vacuum, which produces a repulsive gravitational effect and is the simplest model for dark energy.

The precise average density of matter and energy needed for the universe to be spatially flat. It equals approximately 9.47 x 10^-27 kg/m^3, or about 5.7 hydrogen atoms per cubic meter.

A form of energy with negative pressure that permeates all of space and drives the accelerating expansion of the universe, constituting roughly 68% of the total energy density.

A form of matter that does not interact electromagnetically and is detectable only through its gravitational effects on visible matter, radiation, and the large-scale structure of the universe.

The fine-tuning puzzle of why the universe's spatial geometry is measured to be very close to flat (Euclidean), which requires the density to be extremely close to the critical density in the early universe.

A set of equations derived from general relativity for a homogeneous, isotropic universe that govern the expansion dynamics, relating the scale factor to the energy density and pressure of the universe's contents.

Einstein's theory of gravitation that describes gravity as the curvature of spacetime caused by mass and energy, providing the foundational equations for cosmological models.

The bending of light from distant sources by the gravitational field of intervening massive objects, as predicted by general relativity. It is used to detect dark matter and measure mass distributions.

The puzzle of why the cosmic microwave background has a nearly uniform temperature across the entire sky, even though widely separated regions were never in causal contact in the standard Big Bang model.

The rate of expansion of the universe at the present time, expressed in kilometers per second per megaparsec, with current measurements yielding values between approximately 67 and 73 km/s/Mpc.

The statistically significant discrepancy between measurements of the Hubble constant from early-universe probes (CMB, ~67.4 km/s/Mpc) and late-universe probes (supernovae, ~73 km/s/Mpc).

The hypothetical set of all possible universes, including our own, which may arise from mechanisms such as eternal inflation. Each universe could have different physical constants, laws, or initial conditions.

The production of light atomic nuclei (hydrogen, helium, lithium) during the first few minutes after the Big Bang, when temperatures and densities were sufficient for nuclear fusion reactions.

The spherical region of the universe from which light has had time to reach Earth since the Big Bang, with a comoving radius of about 46.5 billion light-years.

The earliest period of the universe's history, from time zero to about 10^-43 seconds after the Big Bang, when quantum gravitational effects dominated and current physical theories break down.

The epoch approximately 380,000 years after the Big Bang when the universe cooled enough for free electrons and protons to combine into neutral hydrogen atoms, allowing photons to travel freely for the first time.

The stretching of electromagnetic radiation to longer wavelengths, caused in cosmology by the expansion of space. Higher redshifts correspond to greater distances and earlier cosmic times.

A dimensionless quantity in cosmological models that describes how the distances between objects in the universe change over time due to expansion. It is conventionally set to 1 at the present epoch.

A point in spacetime where density and curvature become infinite according to general relativity, such as the initial state of the Big Bang or the center of a black hole. It signals a breakdown of the theory.

An astronomical object with a known intrinsic luminosity, such as Type Ia supernovae or Cepheid variables, used to measure distances by comparing apparent and absolute brightness.