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TOPIC: Physical constants

Blobrana

L

Posts: 131433

Date: Feb 17 23:32 2014

RE: Physical constants	Permalink

Title: Cosmology: the search for twenty-four (or more) functions Author: John D. Barrow We enumerate the 4(1+F)+2S independent arbitrary functions of space require to describe a general relativistic cosmology containing an arbitrary number of non-interacting fluid (F) and scalar fields (S). Results are also given for arbitrary space dimension and for higher-order gravity theories, where the number increases to 16+4F+2S. Both counts are subject to assumptions about whether the dark energy is a cosmological constant. A more detailed analysis is provided when global homogeneity is assumed and the functions become constants. This situation is also studied in the case where the flat and open universes have compact spatial topologies. This changes the relative generalities significantly and places new constraints on the types of expansion anisotropy that are permitted. The most general compact homogeneous universes containing Friedmann models are spatially flat and described by 8+4F+2S constants. Comparisons are made with the simple 6-parameter lambda-CDM model and physical interpretations provided for the parameters needed to describe the most general cosmological models. Read more (15kb, PDF) __________________

Blobrana

L

Posts: 131433

Date: Aug 26 01:17 2011

Fundamental constants	Permalink

Title: Measuring space-time variation of the fundamental constants with redshifted submillimetre transitions of neutral carbon Authors: S. J. Curran, A. Tanna, F. E. Koch, J. C. Berengut, J. K. Webb, A. A. Stark, V. V. Flambaum (Version v2) We compare the redshifts of neutral carbon and carbon monoxide in the redshifted sources in which the fine structure transition of neutral carbon, [CI], has been detected, in order to measure space-time variation of the fundamental constants. Comparison with the CO rotational lines measures gives the same combination of constants obtained from the comparison fine structure line of singly ionised carbon, [CII]. However, neutral carbon has the distinct advantage that it may be spatially coincident with the carbon monoxide, whereas [CII] could be located in the diffuse medium between molecular clouds, and so any comparison with CO could be dominated by intrinsic velocity differences. Using [CI], we obtain a mean variation of dF/F = (-3.6 ±8.5) x 10^-5, over z = 2.3 - 4.1, for the eight [CI] systems, which degrades to (-1.5±11) x 10^-5, over z = 2.3 - 6.4 when the two [CII] systems are included. That is, zero variation over look-back times of 10.8-12.8 Gyr. However, the latest optical results indicate a spatial variation in alpha, which describes a dipole and we see the same direction in dF/F. This trend is, however, due to a single source for which the [CI] spectrum is of poor quality. This also applies to one of the two [CII] spectra previously used to find a zero variation in alpha²/mu. Quantifying this, we find an anti-correlation between \|dF/F\| and the quality of the carbon detection, as measured by the spectral resolution, indicating that the typical values of >50 km/s, used to obtain a detection, are too coarse to reliably measure changes in the constants. From the fluxes of the known z > 1 CO systems, we predict that current instruments are incapable of the sensitivities required to measure changes in the constants through the comparison of CO and carbon lines. We therefore discuss in detail the use of ALMA for such an undertaking Read more (87kb, PDF) __________________

Blobrana

L

Posts: 131433

Date: Jul 26 17:09 2011

Physical constants	Permalink

The latest revision of fundamental quantities bodes well for the proposed overhaul of the international system of units. Every four years, a task force of the Committee on Data for Science and Technology (CODATA), headquartered in Paris, releases a short-hand description of the physical world: the latest, painstaking determination of the values and uncertainties of more than 300 physical constants, from the familiar speed of light, c, to the more obscure mass of the tau lepton. The most recent figures, from 2010, were first placed online in June and announced on 19 July by the US National Institute for Standards and Technology (NIST) in Gaithersburg, Maryland. Read more __________________

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