摘要:

Recent key observational results on the variation of the fine‐structure constant, the proton‐to‐electron mass ratio and the gravitational constant are reviewed. The necessity to substantiate the dark sector of cosmology and to test gravity on astrophysical scales is also emphasized. © 2004 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1835171

出版商:AIP    

年代:1904

数据来源: AIP

摘要:

Recent ideas from particle physics introduce the possibility that the universe could be a higher‐dimensional spacetime, with our observable part of the universe being a 4‐dimensional “brane” surface. In this brane world scenario, Standard Model particles and fields are confined to the brane, while gravity propagates freely in all dimensions. At low energies, gravity is localized at the brane, behaving approximately as a 4‐dimensional field, and general relativity is recovered with small corrections. But at high energies in the early universe, gravity behaves as a truly higher‐dimensional field, “leaking” off the brane, and this leads to significant modifications to general relativistic dynamics and perturbations. These modifications can bring new features into cosmology, and at the same time, they are subject to strong constraints from high‐precision observations. This review gives an introduction to the dynamics and perturbations of simple 5‐dimensional brane world models for cosmology. © 2004 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1835172

出版商:AIP    

年代:1904

数据来源: AIP

摘要:

The best‐motivated alternatives to general relativity are scalar‐tensor theories, in which the gravitational interaction is mediated by one or several scalar fields together with the usual graviton. The analysis of their various experimental constraints allows us to understand better which features of the models have actually been tested, and to suggest new observations able to discriminate between them. This talk reviews three classes of constraints on such theories, which are qualitatively different from each other: (i) solar‐system experiments; (ii) binary‐pulsar tests and future detections of gravitational waves from inspiralling binaries; and (iii) cosmological observations. While classes (i) and (ii) impose precise bounds respectively on the first and second derivatives of the matter‐scalar coupling function, (iii)a prioriallows us to reconstruct the full shapes of the functions of the scalar field defining the theory, but obviously with more uncertainties and/or more theoretical hypotheses needed. Simple arguments such as the absence of ghosts (to guarantee the stability of the field theory) nevertheless suffice to rule out a wide class of scalar‐tensor models. Some of them can be probed only if one takes simultaneously into account solar‐system and cosmological observations. © 2004 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1835173

出版商:AIP    

年代:1904

数据来源: AIP

摘要:

Conformal embedding of closed‐universe models in a de Sitter background suggests a quantisation condition on the available conformal time. This condition implies that the universe is closed at no greater than the 10&percent; level. When a massive scalar field is introduced to drive an inflationary phase this figure is reduced to closure at nearer the 1&percent; level. In order to enforce the constraint on the available conformal time we need to consider conditions in the universe before the onset of inflation. A formal series around the initial singularity is constructed, which rests on a pair of dimensionless, scale‐invariant parameters. For physically‐acceptable models we find that both parameters are of order unity, so no fine tuning is required, except in the mass of the scalar field. For typical values of the input parameters we predict the observed values of the cosmological parameters, including the magnitude of the cosmological constant. The model produces a very good fit to the most recent CMBR data, predicting a low‐lfall‐off in the CMB power spectrum consistent with that observed by WMAP. © 2004 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1835174

出版商:AIP    

年代:1904

数据来源: AIP

摘要:

Primordial perturbations in the density of radiation and matter can be related to vacuum fluctuations of scalar fields during inflation in the very early universe. In the standard inflaton scenario this inevitably leads to a Gaussian distribution of adiabatic density perturbations, but alternative models such as the curvaton scenario or inhomogeneous reheating can yield non‐Gaussian or non‐adiabatic signatures. © 2004 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1835175

出版商:AIP    

年代:1904

数据来源: AIP

摘要:

We provide a general overview of the evolution of vortex string networks and of their cosmological consequences. We describe the velocity‐dependent one‐scale model for string evolution and use it to discuss their basic large‐scale features. We discuss their basic cosmological consequences, in particular for the cosmic microwave background. We also briefly review other cosmological implications such as primordial backgrounds of gravitational waves and ultra‐high energy cosmic rays. © 2004 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1835176

出版商:AIP    

年代:1904

数据来源: AIP

摘要:

Chameleons are scalar fields whose mass depends on the environment, specifically on the ambient matter density. While nearly massless in the cosmos, where the matter density is tiny, their mass is of order of an inverse millimeter on Earth, where the density is high. In this note, we review how chameleons can satisfy current experimental constraints on deviations from General Relativity (GR). Moreover, we study the cosmological evolution with a chameleon field and show the existence of an attractor solution, akin to the tracker solution in quintessence models. We discuss how chameleons can naturally drive the observed acceleration of the universe. © 2004 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1835177

出版商:AIP    

年代:1904

数据来源: AIP

摘要:

Considering an anisotropic universe filled with a perfect fluid and a number of minimally coupled and massive scalar fields, we look for the necessary conditions on these fields such that the universe reaches an isotropic equilibrium state. Supernova data puts additional constraints on this final state. © 2004 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1835178

出版商:AIP    

年代:1904

数据来源: AIP

摘要:

We present the first results from a dedicated survey with ISAAC on the Very Large Telescope (VLT) to constrain variations in the fine‐structure constant &agr; in the range 2 <z< 3 using the forbidden [O III] transitions at 4959Å and 5007Å, as pioneered at lower redshift by Bahcall et al. (2004). We describe our data analysis in detail and show that we can use the atmospheric OH lines to wavelength calibrate the spectra to very high accuracy. The resulting method gives useful constraints but further work remains to remove sources of systematic uncertainty. © 2004 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1835179

出版商:AIP    

年代:1904

数据来源: AIP

摘要:

We present a summary of recent constraints on the value of the fine‐structure constant at the epoch of decoupling from recent observations made by the Wilkinson Microwave Anisotropy Probe (WMAP) satellite. Within the set of models considered, a variation of the value of &agr; at decoupling with respect to the present‐day value is now bounded to be smaller than 2&percent; (6&percent;) at the 95&percent; confidence level. We point out that the existence of an early reionization epoch as suggested by the above measurements will, when more accurate cosmic microwave background polarization data is available, lead to considerably tighter constraints. We find that the tightest possible constraint on &agr; is about 0.1&percent; using CMB data alone—tighter constraints will require further (non‐CMB) priors. © 2004 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1835180

出版商:AIP    

年代:1904

数据来源: AIP