# Terminology

## Terminology

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A list of frequently used terms.

Term | Brief description |
---|---|

Airy disk | Airy disk - In optics, the Airy disk is described as the best-focused spot of light that a perfect lens with a circular aperture can make. |

Anisotropy | Anisotropy - Structural property of non-uniformity in different directions, as opposed to Isotropy. |

Axis of Evil | Axis of Evil - The "axis of evil" is a name given to the apparent correlation between the plane of the Solar System and aspects of the Cosmic Microwave Background (CMB) in which the location of Earth is shown to have a greater "significance" than might be expected by "chance". |

Coulomb's Law | Coulomb's Law - force between two electrically charged particles at rest |

Cosmic Microwave Background | Cosmic microwave background (CMB, CMBR), or relic radiation, is microwave radiation that fills all space in the observable universe. |

Covariance | Covariance is a measure of the relationship between two known variables and to what extent, they change under transformation. Example: Lorentz Transformations facilitate a change in the x-axis (distance) and t-axis (time) as a function of the Gamma factor:
Length contraction: Time dilation: By using using covariance (a change in the x and t coordinate) the speed of light will remain constant in all frames that use this transformation. Special Relativity makes use of linear orthogonal transformations for frames moving at a constant velocity. The General Theory expands covariance to accelerated frames, but maintains the same changes to the x and t coordinates. |

Classical mechanics | Classical mechanics is a physical theory describing the motion of objects. |

Dynamics | Dynamics - Predictions of future motion based on real forces with respect to a fix lab frame. Real forces are defined relative to the analysis, often including variables like: friction, resistance, mass, momentum, and energy. |

Experiment | Experiment - A test under controlled conditions that is made to demonstrate a known truth, examine the validity of a hypothesis, or determine the efficacy of something previously untried. |

Flatness problem | Flatness problem - The flatness problem (also known as the oldness problem) is a cosmological fine-tuning problem within the Big Bang model of the universe, problems arise from the observation that some of the initial conditions of the universe appear to be fine-tuned to very 'special' values. |

Horizon problem | Horizon problem (also known as the homogeneity problem) - is a cosmological fine-tuning problem within the Big Bang model due to the difficulty in explaining the observed homogeneity of causally disconnected regions of space |

Isotropy | Isotropy - In physics and geometry, isotropy is uniformity in all orientations, as apposed to Anisotropy. |

Kinematics | Kinematics - Future predictions of motion based on observation - no regard for causal mechanism or forces. |

Invariance | Invariance - Any physical quantity is invariant when its value remains unchanged under specific coordinate transformations. |

Inverse-square law | The Inverse-square law applies to strength of Light and radio waves over distance. |

Rayleigh Criterion | Rayleigh Criterion - determines the minimum angle between two point objects that can be resolved by a circular aperture, lens, or mirror. |

Statics | Statics - Statics is the branch of classical mechanics that is concerned with the analysis of force and torque acting on a physical system that does not experience an acceleration, but rather is in equilibrium with its environment. |

Tired light | Tired light - A class of hypothetical redshift mechanisms that was proposed as an alternative explanation for the redshift-distance relationship. |

Twilight | Twilight - light produced by scattering sunlight in the upper atmos, when the Sun is below the apparent horizon. |