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1.
We consider toy cosmological models in which a classical, homogeneous, spinor field provides a dominant or sub-dominant contribution to the energy-momentum tensor of a flat Friedmann-Robertson-Walker universe. We find that, if such a field were to exist, appropriate choices of the spinor self-interaction would generate a rich variety of behaviors, quite different from their widely studied scalar field counterparts. We first discuss solutions that incorporate a stage of cosmic inflation and estimate the primordial spectrum of density perturbations seeded during such a stage. Inflation driven by a spinor field turns out to be unappealing as it leads to a blue spectrum of perturbations and requires considerable fine-tuning of parameters. We next find that, for simple, quartic spinor self-interactions, non-singular cyclic cosmologies exist with reasonable parameter choices. These solutions might eventually be incorporated into a successful past- and future-eternal cosmological model free of singularities. In an Appendix, we discuss the classical treatment of spinors and argue that certain quantum systems might be approximated in terms of such fields. 相似文献
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Excitation of trans-stilbene with 306 nm pulses generates a narrower transient spectrum than with 265 nm. These spectra do not evolve in the same manner, suggesting that the spectral changes with 265 nm result from vibrational cooling. Spectral differences at longer times suggest there is a slow cross-relaxation component in the inhomogeneous distribution. 相似文献
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A variety of simple alkyl and aryl isocyanides have been polymerized using 0.5% NiCl2 in ethanol as a catalyst. The resulting poly(iminomethylenes) have been characterized by carbon-13 NMR spectroscopy and their polystyrene-equivalent molecular weights have been determined by gel permeation chromatography. Straight chain aliphatic isocyanides having from three to ten carbon atoms in the chain form readily solyble polymers having molecular weights (Mw) in the general range 10,000 to 30,000. Neopentyl isocyanide unlike tert-butyl isocyanide forms an insoluble polymer. A number of new soluble aryl isocyanide polymers have been obtained. However, aryl isocyanides having a single alkyl substituent (CH3, C2H5, CF3) in the ortho position give only insoluble polymers, whereas aryl isocyanides having alkyl substituents in both ortho positions (e.g., 2,6-(CH3)2C6H3NC and 2,4,6-(CH3)3C6H2NC) fail to polymerize under these conditions. The highest molecular weight soluble aryl isocyanide homopolymer is obtained from 3-CH3OC6H4NC(Mw = 26,000). The trimethylsilyl substituted isocyanide (CH3)3SiCH2CH2NC has been obtained from LiCH2NC and (CH3)SiCH2Cl and gives a brown soluble homopolymer with a molecular weight (Mw) of 19,000. 相似文献