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The rate of expansion of the Universe, representing the proportionality constant in Hubble's Law, typically measured in kilometers per second per megaparsec (km/s/Mpc).

A period in the early Universe during which it is thought to have expanded exponentially, smoothing and flattening out any irregularities and leading to a universe that appears largely the same in all directions (isotropic).

The epoch in the Universe's history when electrons combined with protons to form neutral hydrogen atoms, allowing photons to travel freely, which we now observe as the Cosmic Microwave Background.

The assumption that the distribution of matter and energy in the Universe is homogeneous and isotropic when viewed on a sufficiently large scale.

A cosmological theory that describes the early development of the Universe, which started from a singular point and has been expanding ever since.

The process by which small fluctuations in the density of the Universe grow over time due to gravitational attraction, leading to the formation of stars, galaxies, and clusters.

The redshift caused by the expansion of space itself, rather than through the Doppler effect of moving celestial objects.

A type of matter that does not emit, absorb, or reflect light, making it invisible to current electromagnetic observations, but whose presence can be inferred from gravitational effects on visible matter.

The thermal radiation left over from the time of recombination, about 380,000 years after the Big Bang, which provides a snapshot of the infant Universe.

A theory that proposes a period of extremely rapid (exponential) expansion of the universe during its first few moments (fractions of a second), driven by a hypothetical inflaton field.

The process of creating new atomic nuclei from pre-existing nucleons (primarily protons and neutrons), which occurred during the first few minutes of the Universe's existence.

An unknown form of energy that is hypothesized to permeate all of space and is driving the accelerated expansion of the Universe.

The 'normal' matter which makes up the stars, planets, and all other visible structures in the Universe, consisting of protons, neutrons, and electrons.

The philosophical consideration that observations of the Universe must be compatible with the conscious and sapient life that observes it, influencing certain cosmological theories.

The question of why the night sky is dark if the Universe is infinite and uniformly filled with stars; resolved by the understanding that the Universe is dynamic and expanding.

The process by which a galaxy is formed and develops over time due to gravitational interactions, gas accretion, star formation, and mergers with other galaxies.

The observation that the recessional velocity of galaxies (measured by their redshift) is directly proportional to their distance from us, suggesting the Universe is expanding.