An Etymological Dictionary of Astronomy and Astrophysics
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Empty space undergoing → vacuum birefringence.

→ birefringent; → vacuum.

A peculiar, hypothetical state of matter which is predicted to exist by current theories of → elementary particles, including the → grand unified theories. Unlike the ordinary vacuum, a false vacuum has a large → energy density and a large → negative pressure. A false vacuum is the driving force behind the rapid expansion in the → inflationary model of the → Universe.

→ false; → vacuum.

Empty space influenced by very strong → magnetic field. Same as → birefringent vacuum.

→ magnetized; → vacuum.

1) A space completely empty of matter but not achievable in practice on Earth.
2) A state of lowest energy in a → quantum field theory.

L. vacuum "an empty space, void," noun use of neuter of vacuus "empty," related to vacare "to be empty."

Xala' loan from Ar.

A highly → magnetized vacuum behaving as a prism for the propagation of light, as predicted by → quantum electrodynamics (QED). Attempts to detect this phenomenon in the laboratory have not yet succeeded in the 80 years since it was predicted (Heisenberg & Euler, 1936, Z. Physik, 98, 714). This effect can be detected only in the presence of enormously strong → magnetic fields, such as those around → neutron stars. Owing to the large inferred magnetic fields (B ~ 10¹³ G, → gauss), radiation from these sources is expected to be substantially polarized, independently of the mechanism actually responsible for the → thermal emission. The strongest magnetic field so far created in a laboratory is less than 10⁶ G lasting only for several tens of milliseconds. A large observed → polarization degree is, however, expected only if QED polarization effects are present in the magnetized vacuum around the star. The detection of a strongly → linearly polarized signal would therefore provide the observational evidence of QED effects in the strong-field regime. Recently a team of astrophysicists (Mignani et al. 2016, arXiv/1610.08323) have detected → linear polarization toward the neutron star RXJ1856.5-3754 (at a significant degree of around 16%). This finding is likely due to the boosting effect of vacuum birefringence occurring in the area of empty space surrounding the neutron star.

→ vacuum; → birefringence.

An enclosure from which air is removed.

→ vacuum; → chamber.

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.

A quantum field theory a process in which an electromagnetic field gives rise to virtual electron-positron pairs that in turn exert electromagnetic fields of their own, in a manner similar to classical dielectric polarization.

→ vacuum; → polarization.