COSMOS Glossary

Cosmological Glossary

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Editor's note: so far this glossary consists of a few entries copied from other sites. Hopefully I'll have tome to add a bit more material MAK110803

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Anisotropy This term is frequently used to describe the temperature fluctuations in the Cosmic Microwave Background. The word means not uniform, or not the same. (Cosmology Glossary - Western Washington University)


Baryon any of a family of heavy particles that includes the proton and neutron. Baryons are fermions composed of three quarks in the standard model, while mesons are bosons composed of a quark and an antiquark. (from Glossary of Cosmology - KeyWen)

Big Bang Theory Theory of the creation of the Universe first introduced by George Gamow in the 1940's. Describes the Universe as starting very small, dense, and hot about 15 billion years ago and then expanding into our modern day Universe, and continuing to expand today. (Cosmology Glossary - Western Washington University)


Cosmic Microwave Background radiation The electromagnetic radiation "leftover" after the decoupling of photons from matter about 300,000 years after the Big Bang. We study the perturbations or disturbances in the radiation in hopes of one day discovering the ultimate fate of our Universe. (Cosmology Glossary - Western Washington University)

Cosmology The study of the evolution of the Universe and everything in it. (Cosmology Glossary - Western Washington University); the study of the largest scale structures and dynamics of our universe (Glossary of Cosmology Principles).

Critical Density The minimum density needed for gravity to overcome the expansion of the Universe and cause it to eventually collapse. (Cosmology Glossary - Western Washington University)


Dark Matter the exotic ingredient required by the Big Bang model to account for most of the mass in the universe ; it is invisible to current instruments, does not not emit, absorb, or scatter any light, or does not interact with light the way typical matter does. (from Glossary of Cosmology - KeyWen)

Decoupling Term that describes the event 300,000 years after the Big Bang where the Universe had finally cooled enough so that ordinary matter could form and photons could travel in a free path. During decoupling the Universe goes from opaque to transparent. (Cosmology Glossary - Western Washington University); the conjectured time and place when all cosmic microwave background radiation was produced, closely associated with Recombination. also called Decoupling Era, or Surface of Last Scattering (Glossary of Cosmology Principles).


Early Universe the period immediately following the initial singularity or "big bang", when the entire cosmos was as hot and dense as a thermonuclear reaction. Under these conditions, it was filled homogeneously and isotropically with an incredibly high energy density and concomitantly huge temperatures and pressures. Physicists use particle accelerators to try to deduce what the early universe may have been like The early Universe was radiation or energy dominated, unlike the present Universe which is matter dominated. (from Glossary of Cosmology - KeyWen)

Event Horizon Term relating either to the outer perimeter of a black hole, or to how far we can see into the Universe. Because the Universe has a finite age, we cannot observe any part of the Universe that is farther away than its age in light years. As long as the Universe continues to expand at a rate less than the speed of light, our event horizon will continue to get bigger as a function of time. (Cosmology Glossary - Western Washington University)


Galaxy an accumulation of stars and dust and gas that is held together by gravity. The gravitational pull within a galaxy is stronger than the force of the Hubble Expansion, so the elements of a galaxy do not expand away from each other. Galaxies can be spiral-shaped, elliptical, or irregular. (Cosmology Glossary - Western Washington University)

Galaxy Cluster A collection of galaxies that is drawn together by gravity. Galaxy clusters are the largest objects in the Universe that have a gravitational pull strong enough to overcome the Hubble Expansion, and thus galaxies in a cluster to not expand away from each other. (Cosmology Glossary - Western Washington University)


Helium the second most abundant element in the known Universe after hydrogen, constituting 23% of the elemental mass of the universe. It is a colorless, odorless, and non-toxic gas, Most of the helium in the universe was formed during the nucleosynthesis epoch. (from Glossary of Cosmology - KeyWen)

Homogeneous When we talk about a homogeneous Universe, we are making the assumption that the Universe is uniform, or has the same large-scale makeup throughout. This means that the matter density of a large-scale region, the amount of galaxies, stars, gas and dust per a certain volume, is pretty much the same anywhere in the Universe. (Cosmology Glossary - Western Washington University)

Hubble Expansion Used to describe the expansion of the Universe based primarily on the redshifted light of distant galaxies. The relationship between recession velocity and distance is described by Hubble's Law. (Cosmology Glossary - Western Washington University)

Hydrogen the most abundant, and also the lightest and simplest, element in the known Universe. (from Glossary of Cosmology - KeyWen)


Isotropic An isotropic Universe has no preferred direction. It behaves the same in every direction. For example, the redshift of distant galaxy clusters looks the same in the direction of Orion as it does in Scorpius. (Cosmology Glossary - Western Washoe University)


Redshift When a radiating object moves away from us, we observe a redshift in its light, the light waves it emits are being stretched, that is, shifting to the red part of the spectrum). (from Cosmology Glossary - Western Washington University)


Steady State Theory Theory of the creation of the Universe that says the Universe has been and always will be like it is today. It assumes that the Universe is uniform, infinite, and not expanding. As a competing theory to the big bang, the steady state theory has been generally discredited because of its inability to explain such things as the cosmic microwave background radiation, the evolution of galaxies, the cosmic redshift, and other observations.(Cosmology Glossary - Western Washington University)

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content by MAK110729, quotes from Wikipedia. Edited RFVS111026

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