An Etymological Dictionary of Astronomy and Astrophysics
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1) The energy in the form of heat emitted by an object by virtue of its temperature.
2) The total potential and kinetic energies associated with the random motions of the particles of a material. The quantity of thermal energy possessed by a body determines its temperature. The thermal energy which is absorbed, given up, or transferred from one material to another is heat.
3) The characteristic energy of → thermal neutrons at room temperature, about 0.025 eV.

→ thermal; → energy.

The minimum energy necessary for the occurrence of some chemical/physical effect.

→ threshold; → energy.

The sum of all forms of energy involved in a system.

→ total; → energy.

A particle belonging to the most energetic population of → cosmic rays with an energy above ~ 10²⁰ → electron-volts. The UHECRs constitute a real challenge for theoretical models, because their acceleration requires extreme conditions hardly fulfilled by known astrophysical objects. See also → UHECR puzzle, → Greisen-Zatsepin-Kuzmin cutoff.

→ ultra- + → high-energy cosmic ray.

A neutrino particle accelerated to energies above 10¹⁸ → electron-volts. They are produced by the interaction of → ultra-high-energy cosmic ray (UHECR)s with the → cosmic microwave background radiation. Also called → cosmogenic neutrinos. See also → Greisen-Zatsepin-Kuzmin limit.

→ ultra-; → high; → energy; → neutrino.

In particle physics the lowest energy allowed by field quantization when all fields are in their → ground states. Vacuum energy is predicted to arise from → virtual particles that fluctuate in and out of existence, as manifested by the → Casimir effect. The cosmological → dark energy is postulated to be related to vacuum fluctuations. There is however an enormous discrepancy with the predictions of quantum field theory. In this theory the value of vacuum energy density is expected to be roughly of the order ρ_v≅ E_max⁴, where E_max is the maximum energy at which the field theory is valid. At energies of the order of the → Planck energy, E_Pl≅ 10¹⁹ GeV, vacuum energy might be roughly: ρ_v≅ E_Pl⁴≅ 10⁷⁶ GeV⁴. On the other hand, the vacuum energy density in standard cosmological model is given by: ρ_Λ = Ω_Λ.ρ_crit, where Ω_Λ is the → density parameter for the → cosmological constant and ρ_crit is the → critical density. More explicitly, ρ_Λ = Ω_Λ . 3 H²/(8πG). Using present-day values of Ω_Λ (0.7) and H (70) leads to ρ_Λ = 10^-46 GeV⁴. Therefore, the discrepancy between the prediction and the observed value is 122 orders of magnitude.

→ vacuum; → energy.

The energy due to the vibration of the molecules making up atoms (→ molecular vibration). A molecule in space can have energies in various forms: → rotational energy, vibrational energy, or electronic energy. These energies of molecules are → quantized and a particular molecule can exist in different rotational and vibrational → energy levels. The molecules can move from one level to another level only by a jump involving a finite amount of energy. → Quantum mechanics predicts that any molecule can never have zero vibrational energy, that is atoms can never be completely at rest relative to each other. The harmonically oscillating molecules can undergo vibrational changes determined by simple selection rules obtained from → Schrödinger equation.

→ vibrational; → energy.

The → work of the resultant force exerted on a particle equals the change in kinetic energy of the particle.

→ work; → energy; → principle.

The lowest possible energy that a → quantum mechanical system may possess. It is the energy of the → ground state of the system. The term "zero point" refers to the observed fact that → vacuum fluctuations persist at → absolute zero temperature. Same as → vacuum energy.

→ zero; → point; → energy.