Glossary of Terms
This term is frequently used to describe the temperature fluctuations in the Cosmic Microwave Background. The word means not uniform, or not the same.

Big Bang Theory
Theory of the creation of the universe first introduced by George Gamow in the 1940s where the universe started as very small, dense, and hot about 15 billion years ago and then expanded into our modern day universe and continues to expand today.

Cosmic Microwave Background
This is the light 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.
Curious about the CMB? Check out Wayne Hu's CMB Tour:

The study of the evolution of the universe and everything in it.
To find more on cosmology go to

Critical Density
The minimum density needed for gravity to overcome the expansion of the universe and cause it to eventually re-collapse.

Term that describes the event 300,000 years after the Big Bang where the universe had finally cooled off enough so that ordinary matter could form and photons could travel in a free path. During decoupling the universe goes from opaque to transparent.

Electromagnetic Spectrum
This contains all the different types of light that we can physically detect ranging from gamma rays to radio waves. The Electromagnetic Spectrum distinguishes between the different types of waves by their wavelengths.
To find out more about the Electromagnetic Spectrum, go to:

Event Horizon
Term relating to how far we can see into the universe, because the universe is only about 15 billion years old, we cannot observe any part of the universe that is farther away than 15 billion 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.

A galaxy is a accumulation of stars 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 to not expand away from each other. Galaxies can be spiral-shaped, elliptical, or irregular.

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.

Geocentric Universe
A model of the universe that puts Earth at its center.

Heliocentric Universe
A model of the universe that puts the sun at its center.

Hubble Expansion
Used to describe the expansion of the universe based on the redshifted light of distant galaxies. The relationship between recession velocity and distance is described by Hubble's Law, where [recessional velocity of an object]=[Hubble constant]*[distance of object from Earth]

When we talk about the universe being infinite in relation to steady state theory, we can talk about several things. First, steady state theory predicts the universe to be infinitely big, without limits. Secondly, steady state theory says the universe is infinitely old, as old as time itself.

An isotropic universe has no preferred direction. It acts the same in every direction. For example, the redshift of distant galaxy clusters looks the same from our location as it does from another distant cluster in the universe.

Not Expanding
A universe that is not expanding is what we call static. According to steady state theory, the universe just is. It cannot be expanding because steady state theorists think this is the way the universe has always looked. A universe that is expanding would have looked very different at a time billions of years ago.

Primordial Soup
The name for the time in the evolution of the universe where the universe was to hot to form ordinary matter. The universe was opaque and glowed due to photons that were being continuously scattered by electrons.

When a radiating object moves away from us, we observe a redshift in its light, or the light waves it emits are getting longer (shifting to the red part of the spectrum).

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.
For more about steady state theory go to:

When we talk about a homogeneous universe, we are making the assumption that the universe is uniform, or has the same makeup throughout. So, we figure that the matter density of our local region, or lets say the amount of galaxies, stars, gas and dust per a certain volume is pretty much the same anywhere in the universe.

Visible Spectrum
This is the part of the Electromagnetic Spectrum that contains the light we can see. The colors in the visible Spectrum from longest wavelength to shortest are: red, orange, yellow, green, blue, and violet.

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