Transition from decelerated to accelerated cosmic expansion in braneworld universes

Braneworld theory provides a natural setting to treat, at a classical level, the cosmological effects of vacuum energy. Non-static extra dimensions can generally lead to a variable vacuum energy, which in turn may explain the present accelerated cosmic expansion. We concentrate our attention in models where the vacuum energy decreases as an inverse power law of the scale factor. These models agree with the observed accelerating universe, while fitting simultaneously the observational data for the density and deceleration parameter. The redshift at which the vacuum energy can start to dominate depends on the mass density of ordinary matter. For Omega = 0.3, the transition from decelerated to accelerated cosmic expansion occurs at z approx 0.48 +/- 0.20, which is compatible with SNe data. We set a lower bound on the deceleration parameter today, namely q > - 1 + 3 Omega/2, i.e., q > - 0.55 for Omega = 0.3. The future evolution of the universe crucially depends on the time when vacuum starts to dominate over ordinary matter. If it dominates only recently, at an epoch z < 0.64, then the universe is accelerating today and will continue that way forever. If vacuum dominates earlier, at z > 0.64, then the deceleration comes back and the universe recollapses at some point in the distant future. In the first case, quintessence and Cardassian expansion can be formally interpreted as the low energy limit of our model, although they are entirely different in philosophy. In the second case there is no correspondence between these models and ours.Comment: In V2 typos are corrected and one reference is added for section 1. To appear in General Relativity and Gravitatio

Unified dark energy models : a phenomenological approach

A phenomenological approach is proposed to the problem of universe accelerated expansion and of the dark energy nature. A general class of models is introduced whose energy density depends on the redshift $z$ in such a way that a smooth transition among the three main phases of the universe evolution (radiation era, matter domination, asymptotical de Sitter state) is naturally achieved. We use the estimated age of the universe, the Hubble diagram of Type Ia Supernovae and the angular size - redshift relation for compact and ultracompact radio structures to test whether the model is in agreement with astrophysical observation and to constrain its main parameters. Although phenomenologically motivated, the model may be straightforwardly interpreted as a two fluids scenario in which the quintessence is generated by a suitably chosen scalar field potential. On the other hand, the same model may also be read in the context of unified dark energy models or in the framework of modified Friedmann equation theories.Comment: 12 pages, 10 figures, accepted for publication on Physical Review

Equation of state for Universe from similarity symmetries

In this paper we proposed to use the group of analysis of symmetries of the dynamical system to describe the evolution of the Universe. This methods is used in searching for the unknown equation of state. It is shown that group of symmetries enforce the form of the equation of state for noninteracting scaling multifluids. We showed that symmetries give rise the equation of state in the form $p=-\Lambda+w_{1}\rho(a)+w_{2}a^{\beta}+0$ and energy density $\rho=\Lambda+\rho_{01}a^{-3(1+w)}+\rho_{02}a^{\beta}+\rho_{03}a^{-3}$, which is commonly used in cosmology. The FRW model filled with scaling fluid (called homological) is confronted with the observations of distant type Ia supernovae. We found the class of model parameters admissible by the statistical analysis of SNIa data. We showed that the model with scaling fluid fits well to supernovae data. We found that $\Omega_{\text{m},0} \simeq 0.4$ and $n \simeq -1$ ($\beta = -3n$), which can correspond to (hyper) phantom fluid, and to a high density universe. However if we assume prior that $\Omega_{\text{m},0}=0.3$ then the favoured model is close to concordance $\Lambda$CDM model. Our results predict that in the considered model with scaling fluids distant type Ia supernovae should be brighter than in $\Lambda$CDM model, while intermediate distant SNIa should be fainter than in $\Lambda$CDM model. We also investigate whether the model with scaling fluid is actually preferred by data over $\Lambda$CDM model. As a result we find from the Akaike model selection criterion prefers the model with noninteracting scaling fluid.Comment: accepted for publication versio

The growth factor of matter perturbations in an f(R) gravity

The growth of matter perturbations in the $f(R)$ model proposed by Starobinsky is studied in this paper. Three different parametric forms of the growth index are considered respectively and constraints on the model are obtained at both the $1\sigma$ and $2\sigma$ confidence levels, by using the current observational data for the growth factor. It is found, for all the three parametric forms of the growth index examined, that the Starobinsky model is consistent with the observations only at the $2\sigma$ confidence level.Comment: 15 pages, 5 figure

Extended Theories of Gravity and their Cosmological and Astrophysical Applications

We review Extended Theories of Gravity in metric and Palatini formalism pointing out their cosmological and astrophysical application. The aim is to propose an alternative approach to solve the puzzles connected to dark components.Comment: 44 pages, 11 figure

Experimental study on rectangular CFT columns with high-strength concrete

In the 1999 AISC-LRFD, the in-filled concrete strength (f(c)') of concrete-filled tube (CFT) columns is limited to a maximum value of 55 MPa (N/mm(2)). That limiting value is raised to 70 Wa in the 2005 AISC-LRFD. This study aims to assess if the LRFD CFT column formulas are applicable to intermediate to long rectangular columns with higher concrete strengths. Twenty four specimens with f(c)' varying between 29 and 84 MPa were tested. Various formulas and relevant provisions for CFT columns as specified in the major design codes including AISC-LRFD, EC 4, AS-5100, and CSA S16-01 were examined and compared. The design CFT strength (P-u) predicted by the AISC-LRFD formulas and the test results (P-test) were found to be in good agreement. The higher f(c)' limiting value of 70 MPa proposed in the 2005 AISC-LRFD appears acceptable. The test results reveal that the 1999 AISC-LRFD design strengths are conservative and tend to penalize these CFT columns with higher concrete strength. (c) 2006 Elsevier Ltd. All rights reserved

Experimental investigation on built-up columns

This study presents a direct experimental verification of the AISC-LRFD slenderness ratio formulas for built-up compression members. Comparisons of slenderness ratios as specified in various design codes including the AISC-ASD, the AISC-LRFD, the AS-4100 and the CSA S 1601, are also provided. Test results concluded from this study show that the 1999 LRFD formulas generally result in conservative design strengths. However, the 2005 LRFD yields slightly higher strength due to the use of a higher resistance factor of 0.9 instead of 0.85. The design strengths determined based on the slenderness ratio formulas of the Australian Code (AS-4100) are even more conservative. For back-to-back snug-tight bolted sections, the Canadian Code (CSA S 16-01) gives virtually the same design outcome as the LRFD. While for back-to-back welded sections, the CSA S 16-01 tends to yield a less conservative design. (c) 2006 Elsevier Ltd. All rights reserved

Investigation on slenderness ratios of built-up compression members

This study provides a direct experimental verification of the AISC slenderness ratio formulas for built-up compressive members. The comparison oil various code-specified slenderness ratios or provisions, which used in the AISC-ASD, AISC-LRFD, AS-4100, and CSA S16-01, are presented. The 0.75 rule, which states that the slenderness ratio of component element of built-up member should not exceed three-fourths times the governing slenderness ratio of built-up member, seems justified according to the tests. The governing slenderness ratio of built-up member could be either the modified or the unmodified one-as specified in the AISC Specifications. The test results indicate that the built-up members with component slenderness ratio of 0.75 to 1.0 times the governing slenderness ratio (modified or unmodified) could also furnish a quite encouraging design outcome. The use of separation ratio (alpha) in built-up Compression members results in the decrease of design strength when compared to one with no use of separation ratio. (C) 2008 Elsevier Ltd. All rights reserved

Numerical evaluation on warping constants of general cold-formed steel open sections

The calculation of warping constant (C-w) for an open thin-walled open section is a tedious and difficult task and thus presenting an obstacle to routine design. Although C-w formulas and values for selective cold-formed steel sections are available in the AISI design manuals, most practicing engineers have limited idea of evaluating the warping constants for sections not listed in the AISI design manuals. This paper proposes a detailed step-by-step numerical procedure for evaluating the warping constant of a general open thin-walled section. Comparison shows virtually no difference between the calculated values and those listed in the AISI design manuals. The proposed procedure is a reliable and useful tool for computing the warping constant for an arbitrary cold-formed steel open section, which does not need a sophisticated computer software

The Compressive Strength of Slender C-shaped Cold-formed Steel Members with Web Openings

This study investigated the compressive strength of slender C-shaped cold-formed steel members with web openings. The effective sectional area concept was adopted to conduct the analysis of cold-formed compressive members. The load-carrying capacity of compression member was calculated based on the sectional effective width as described in the AISI Standard. The study tested a total of 21 channel specimens including C-shaped cross-sections with and without web openings. Comparisons were made between the test results and the predictions based on both the AISI Standard and the ANSYS analysis. The results indicated that all the observed failure modes were of the flexural-torsional buckling classification. It was found that the reduction in the compression strength of the specimens with web openings seems to be negligible. In addition, the predictions based on the AISI Standard were conservative as compared with the test results of the specimens with web openings. Moreover, it appeared that the ANSYS finite element analysis was able to predict the ultimate loads and failure modes of the specimens. In the absence of test data and for the purpose of preliminary design, the ANSYS appears to provide quite all encouraging prediction capability