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A luminous and extremely distant celestial object, powered by a supermassive black hole at its center, which emits immense amounts of energy.

A period in the early universe during which it is thought to have expanded exponentially rapidly in a very short time, solving the horizon and flatness problems.

The observation in physical cosmology that galaxies are moving away from each other at a rate proportional to their distance, described by the equation $v = H_0 \times d$, where $v$ is the galaxy's recessional velocity, $H_0$ is the Hubble constant, and $d$ is the distance to the galaxy.

The increase in the wavelength of light from an object which is moving away from the observer, indicative of the expansion of the universe.

Radiation left over from the early stages of the universe in Big Bang cosmology, now observed as a uniform background radiation at a temperature of approximately 2.7 Kelvin.

A form of matter that does not emit light or energy and, therefore, cannot be directly observed. Its existence is inferred from gravitational effects on visible matter and cosmic radiation.

The shape of space-time as described by General Relativity, which can be flat, open, or closed, depending on the density of the universe.

The apparent paradox that the night sky is dark although the infinite universe and the countless stars should fill it with light, explained by the finite age of the universe and the expansion of space.

The maximum distance from which light has had enough time to reach us since the beginning of the Universe, dictating the observable universe's limits.

The standard model of Big Bang cosmology that includes the cosmological constant (Λ) representing dark energy, and cold dark matter (CDM) as key components of its structure.

A series of methods by which astronomers determine the distances to celestial objects, built upon one another to extend from our solar system to the edges of the observable universe.

A point in space-time at which density and gravitational forces are predicted to become infinite, often associated with the centers of black holes and the state of the universe at the beginning of the Big Bang.

A mysterious form of energy that is hypothesized to permeate all of space and is responsible for the accelerated expansion of the universe.

A term introduced by Einstein in his field equations of General Relativity, representing an unknown form of energy that fills space homogeneously and can cause the expansion of the universe to accelerate.

A set of equations derived from Albert Einstein's General Theory of Relativity that govern the expansion of space in homogeneous and isotropic models of the universe.

The redshift of light that is caused by the expansion of the universe, stretching the light's wavelength as it travels through space.

Fluctuations in the density of the visible baryonic matter of the universe, caused by acoustic density waves in the primordial hot plasma of the early universe.

A hypothetical particle that is a candidate for dark matter; it interacts with normal matter via gravity and possibly through the weak nuclear force but not electromagnetically or strongly, making it difficult to detect.

The scientific theory that describes the early development and shape of the universe, which started from a small singularity around 13.8 billion years ago and has been expanding ever since.