An Etymological Dictionary of Astronomy and Astrophysics
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An operation of tensor algebra that is obtained by setting unlike indices equal and summing according to a summation convention.

→ contraction; → tensor.

A generalization of the tensor concept that like a tensor transforms, except for the appearance of an extra factor, which is the → Jacobian matrix of the transformation of the coordinates, raised to some power, in transformation law. The exponent, which is a positive or negative integer, is called the weight of the tensor density. → weight of a tensor density. Ordinary tensors are tensor densities of weight 0. Tensor density is also called → relative tensor.

→ tensor; → density.

A field of space and time each point of which has multiple directionality, and is describable by a tensor function.

→ tensor; → field.

The perturbation in the → primordial Universe plasma caused by → gravitational waves. These waves stretch and squeeze space in orthogonal directions and bring about → quadrupole anisotropy in incoming radiation temperature.

→ tensor; → perturbation.

The total number of → contravariant and → covariant indices (→ index) of a → tensor.

→ tensor; → rank.

A theory put forward to provide a basis for a relativistic generalization of the → MOdified Newtonian Dynamics (MOND) paradigm. TeVeS is based on three dynamical fields: a tensor field, a vector field, and a scalar field. In contrast to general relativity, it has two metrics, an Einstein metric and a physical metric. TeVeS has attracted considerable attention, since it can explain many galactic and cosmological observations without the need for → dark matter. Proposed by J. D. Bekenstein, 2004, "Relativistic gravitation theory for the modified Newtonian dynamics paradigm", Phys. Rev. D, 70, 083509, arXiv:astro-ph/0403694.

→ tensor; → vector; → scalar; → theory.

A measure of the energy level of a → thermodynamic system. It represents the amount of → work obtainable when the system undergoes a → change. The main types of thermodynamic potential are: → internal energy, → enthalpy, the → Helmholtz free energy, and the → Gibbs free energy.

→ thermodynamic; → potential.

1) To utter a threat against; menace.
2) To be a menace or source of danger to (Dictionary.com).

From M.E. thretnen, from O.E. thrêatnian, → threat

Infinitive from harš, → threat.

A very hard, silver-white to steel-gray metal with a body-centered cubic crystalline structure; symbol W. Atomic number 74; atomic weight 183.85; melting point about 3,410°C; boiling point 5,660°C; specific gravity 19.3 at 20°C. The chemical element was discovered by the Swedish chemist Carl-Wilhelm Scheele in 1781. Tungsten metal was first isolated by the Spanish chemists Don Fausto d'Elhuyar and his brother Don Juan Jose d'Elhuyar in 1783.

The name derives from the Swedish ng sten "heavy stone". The chemical symbol, W, is derived from the Ger. wolfram, which was found with tin and interfered with the smelting of tin.

A constant the value of which is characteristic for any given → tensor density.

→ weight; → tensor; → density.

An → X-ray source that does not display emission → outbursts, in contrast to → X-ray transients.

→ X-ray; → persistent.

A very familiar antenna array, which is the commonest kind of terrestrial TV aerial to be found on the rooftops of houses. It consists of a single "feed" or "driven element," usually a dipole antenna. The rest of the elements help transmit the energy in a particular direction. These antennas typically operate in the HF to UHF bands (about 3 MHz to 3 GHz), although their bandwidth is typically small. In astronomy Yagi antennas are used as elements in some → radio interferometers. Same as Yagi-Uda antenna.

Named after the Japanese electrical engineer Hidetsuga Yagi (1886-1976); → antenna.

The potential function that is associated with the strong, short-ranged force resulting from the exchange of massive particles between two → nucleons in the same atomic nucleus. The potential has the form of V(r) = C. (1/r) exp (-K.r), where r is the distance between the nucleons and C and K are constants, giving measures of the strength and range of the force respectively.

In honor of the Japanese physicist Hideki Yukawa (1907-1981), winner of the 1949 Nobel Prize in Physics; → potential.