1,721,860 research outputs found
Constraints on pre-big bang models for seeding large-scale anisotropy by massive Kalb-Ramond axions
No full textWe discuss the conditions under which pre-big bang models can fit the observed large-scale anisotropy with a primordial spectrum of massive (Kalb--Ramond) axion fluctuations. The primordial spectrum must be sufficiently flat at low frequency and sufficiently steeper at high frequency. For a steep and/or long enough high-frequency branch of the spectrum the bounds imposed by COBE's normalization allow axion masses of the typical order for a Peccei--Quinn--Weinberg--Wilczek axion. We provide a particular example in which an appropriate axion spectrum is obtained from a class of backgrounds satisfying the low-energy string cosmology equations.We discuss the conditions under which pre-big bang models can fit the observed large-scale anisotropy with a primordial spectrum of massive (Kalb--Ramond) axion fluctuations. The primordial spectrum must be sufficiently flat at low frequency and sufficiently steeper at high frequency. For a steep and/or long enough high-frequency branch of the spectrum the bounds imposed by COBE's normalization allow axion masses of the typical order for a Peccei--Quinn--Weinberg--Wilczek axion. We provide a particular example in which an appropriate axion spectrum is obtained from a class of backgrounds satisfying the low-energy string cosmology equations
Pre-big-bang in string cosmology
No full textThe duality-type symmetries of string cosmology naturally lead to a pre-big-bang phase of accelerated evolution as dual counterpart of the decelerated expansion of standard cosmology. We discuss several properties of this scenario, including the possibility that tracks of the pre-big-bang may be found either in the spectrum of relic gravitons or in the distortion they induce on the cosmic microwave background.The duality-type symmetries of string cosmology naturally lead to a pre-big-bang phase of accelerated evolution as dual counterpart of the decelerated expansion of standard cosmology. We discuss several properties of this scenario, including the possibility that tracks of the pre-big-bang may be found either in the spectrum of relic gravitons or in the distortion they induce on the cosmic microwave background
The pre-big bang scenario in string cosmology
No full textWe review physical motivations, phenomenological consequences, and open problems of the so-called pre-big bang scenario in superstring cosmology. (C) 2002 Elsevier Science B.V. All rights reserved
Isospin Mixing of Narrow Pentaquark States
Full text linkInterpreting the recently discovered narrow exotic baryons as pentaquark states, we discuss, along an old argument of ours, the isospin mixing occurring within the two doublets of and Q=0 states lying inside the (-cascade) sector. We argue that, at least within the Jaffe-Wilczek assignment, presently available data already indicate that mixing should occur at an observable level in both charge sectors, with mixing angles that can be predicted in terms of ratios of observable mass splittings.Interpreting the recently discovered narrow exotic baryons as pentaquark states, we discuss, along an old argument of ours, the isospin mixing occurring within the two doublets of and Q=0 states lying inside the (-cascade) sector. We argue that, at least within the Jaffe-Wilczek assignment, presently available data already indicate that mixing should occur at an observable level in both charge sectors, with mixing angles that can be predicted in terms of ratios of observable mass splittings.Interpreting the recently discovered narrow exotic baryons as pentaquark states, we discuss, along an old argument of ours, the isospin mixing occurring within the two doublets of Q=−1 and Q=0 states lying inside the S=−2 ( Ξ -cascade) sector. We argue that, at least within the Jaffe–Wilczek assignment, presently available data already indicate that mixing should occur at an observable level in both charge sectors, with mixing angles that can be predicted in terms of ratios of observable mass splittings
Singularity and exit problems in two-dimensional string cosmology
No full textA broad class of two-dimensional loop-corrected dilaton gravity models exhibit cosmological solutions that interpolate between the string perturbative vacuum and a background with asymptotically flat metric and linearly growing dilaton. The curvature singularities of the corresponding tree-level solutions are smoothed out, but no branch-change occurs. Thus, even in the presence of a non-perturbative potential, the system is not attracted by physically interesting fixed points with constant dilaton, and the exit problem of string cosmology persists.A broad class of two-dimensional loop-corrected dilaton gravity models exhibit cosmological solutions that interpolate between the string perturbative vacuum and a background with asymptotically flat metric and linearly growing dilaton. The curvature singularities of the corresponding tree-level solutions are smoothed out, but no branch-change occurs. Thus, even in the presence of a non-perturbative potential, the system is not attracted by physically interesting fixed points with constant dilaton, and the exit problem of string cosmology persists
Electromagnetic origin of the CMB anisotropy in string cosmology
No full textIn the inflationary scenarios suggested by string theory, the vacuum fluctuations of the electromagnetic field can be amplified by the time-evolution of the dilaton background, and can grow large enough to explain both the origin of the cosmic magnetic fields and of the observed CMB anisotropy. The normalization of the perturbation spectrum is fixed, and implies a relation between the perturbation amplitude at the COBE scale and the spectral index n. Working within a generic two-parameter family of backgrounds, a large scale anisotropy \Delta T/T\simeq 10^{-5} is found to correspond to a spectral index in the range n\simeq 1.11 - 1.17.In the inflationary scenarios suggested by string theory, the vacuum fluctuations of the electromagnetic field can be amplified by the time-evolution of the dilaton background, and can grow large enough to explain both the origin of the cosmic magnetic fields and of the observed CMB anisotropy. The normalization of the perturbation spectrum is fixed, and implies a relation between the perturbation amplitude at the COBE scale and the spectral index . Working within a generic two-parameter family of backgrounds, a large scale anisotropy is found to correspond to a spectral index in the range
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