Glossary items by topic: Cosmology

Accelerating Universe

A model for the universe in which a repulsive force counteracts the attractive force of gravity, driving all the matter in the universe apart at speeds that increase with time. Recent observations of distant supernova explosions suggest that we may live in an accelerating universe.

Big Bang

A broadly accepted theory for the origin and evolution of our universe. The theory says that the observable universe started roughly 13.7 billion years ago from an extremely dense and incredibly hot initial state.

Closed Universe

A geometric model of the universe in which the overall structure of the universe closes upon itself like the surface of a sphere. The rules of geometry in a closed universe are like those that would apply on the surface of a sphere.

Cosmic Microwave Background

Radiative energy filling the universe that is believed to be the radiation remaining from the Big Bang. It is sometimes called the “primal glow.” This radiation is strongest in the microwave part of the spectrum but has also been detected at radio and infrared wavelengths. The intensity of the cosmic microwave background from every part of the sky is almost exactly the same.

Cosmological Principle

This principle states that the distribution of matter across very large distances is the same everywhere in the universe and that the universe looks the same in all directions. According to this principle, our view of the universe is like the view from a boat on an ocean, which is essentially the same for any other person on any other boat on any other ocean. Measurements of matter and energy in the universe on the largest observable scales support the cosmological principle.


The investigation of the origin, structure, and development of the universe, including how energy, forces, and matter interact on a cosmic scale.

Critical Density

The minimum average density that matter in the universe would need in order for its gravitational pull to slow the universe’s expansion to a halt.

Dark Matter

Matter that is too dim to be detected by telescopes. Astronomers infer its existence by measuring its gravitational influence. Dark matter makes up most of the total mass of the universe.

Flat Universe

A geometric model of the universe in which the laws of geometry are like those that would apply on a flat surface such as a table top.

Grand Unified Theory (GUT)

A theory stating that that strong and weak nuclear forces and electromagnetic forces are varying aspects of the same fundamental force.

Hubble’s Law

Mathematically expresses the idea that the recessional velocities of faraway galaxies are directly proportional to their distance from us. Hubble’s Law describes the relationship of velocity and distance by the equation V=Ho * d, where V is the object’s recessional velocity, d is the distance to the object, and Ho is the Hubble constant. Essentially, the more distant two galaxies are from each other, the faster they are traveling away from each other. American astronomer Edwin Hubble discovered this relationship in 1929 when he observed that galaxies and clusters of galaxies were generally moving away from each other.

Hubble Constant (Ho)

A number that expresses the rate at which the universe expands with time. Ho appears to be between 60 and 75 kilometers per second per megaparsec.

Open Universe

A geometrical model of the universe in which the overall structure of the universe extends infinitely in all directions. The rules of geometry in an open universe are like those that would apply on a saddle-shaped surface.

Primordial Nucleosynthesis

Element building that occurred in the early universe when the nuclei of primordial matter collided and fused with one another. Most of the helium in the universe was created by this process.

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