Reconstructing a model of quintessential inflation

We present an explicit cosmological model where inflation and dark energy both could arise from the dynamics of the same scalar field. We present our discussion in the framework where the inflaton field $\phi$ attains a nearly constant velocity $m_P^{-1} |d\phi/dN|\equiv \alpha+\beta \exp(\beta N)$ (where $N\equiv \ln a$ is the e-folding time) during inflation. We show that the model with $|\alpha|<0.25$ and $\beta<0$ can easily satisfy inflationary constraints, including the spectral index of scalar fluctuations ($n_s=0.96\pm 0.013$), tensor-to-scalar ratio ($r<0.28$) and also the bound imposed on $\Omega_\phi$ during the nucleosynthesis epoch ($\Omega_\phi (1 {\rm MeV})<0.1$). In our construction, the scalar field potential always scales proportionally to the square of the Hubble expansion rate. One may thereby account for the two vastly different energy scales associated with the Hubble parameters at early and late epochs. The inflaton energy could also produce an observationally significant effective dark energy at a late epoch without violating local gravity tests.Comment: 18 pages, 7 figures; added refs, published versio

Accelerating universe from warped extra dimensions

Accelerating universe or the existence of a small and positive cosmological constant is probably the most pressing obstacle as well as opportunity to significantly improving the models of four-dimensional cosmology from fundamental theories of gravity, including string theory. In seeking to resolve this problem, one naturally wonders if the real world can somehow be interpreted as an inflating de Sitter brane embedded in a higher-dimensional spacetime described by warped geometry. In this scenario, the four-dimensional cosmological constant may be uniquely determined in terms of two length scales: one is a scale associated with the size of extra dimensions and the other is a scale associated with the expansion rate of our universe. In some specific cases, these two scales are complementary to each other. This result is demonstrated here by presenting some explicit and completely non-singular de Sitter space dS$_4$ solutions of vacuum Einstein equations in five and ten dimensions.Comment: 7 pages; extended (from journal) version, minor typos fixed, refs adde

Cosmic Acceleration from M Theory on Twisted Spaces

In a recent paper [I.P. Neupane and D.L. Wiltshire, Phys. Lett. B 619, 201 (2005).] we have found a new class of accelerating cosmologies arising from a time--dependent compactification of classical supergravity on product spaces that include one or more geometric twists along with non-trivial curved internal spaces. With such effects, a scalar potential can have a local minimum with positive vacuum energy. The existence of such a minimum generically predicts a period of accelerated expansion in the four-dimensional Einstein-conformal frame. Here we extend our knowledge of these cosmological solutions by presenting new examples and discuss the properties of the solutions in a more general setting. We also relate the known (asymptotic) solutions for multi-scalar fields with exponential potentials to the accelerating solutions arising from simple (or twisted) product spaces for internal manifolds.Comment: 23 pages, 3 figures; added a summary Table, PRD versio

Warped compactification on curved manifolds

The characterization of a six- (or seven)-dimensional internal manifold with metric as having positive, zero or negative curvature is expected to be an important aspect of warped compactifications in supergravity. In this context, Douglas and Kallosh recently pointed out that a compact internal space with negative curvature could help to construct four-dimensional de Sitter solutions only if the extra dimensions are strongly warped or there are large stringy corrections. That is, the problem of finding 4-dimensional de Sitter solutions is well posed, if all extra dimensions are physically compact, which is called a no-go theorem. Here, we show that the above conclusion does not extend to a general class of warped compactifications in classical supergravity that allow a non-compact direction or cosmological solutions for which the internal space is asymptotic to a cone over a product of compact Einstein spaces or spheres. For clarity, we present classical solutions that compactify higher-dimensional spacetime to produce a Robertson--Walker universe with de Sitter-type expansion plus one extra non-compact direction. Such models are found to admit both an effective four-dimensional Newton constant that remains finite and a normalizable zero-mode graviton wavefunction. We also exhibit the possibility of obtaining 4D de Sitter solutions by including the effect of fluxes (p-form field strengths).Comment: 24 pages, 1 figure; v5 significant changes in the presentation, published (journal) versio

On compatibility of string effective action with an accelerating universe

In this paper, we fully investigate the cosmological effects of the moduli dependent one-loop corrections to the gravitational couplings of the string effective action to explain the cosmic acceleration problem in early (and/or late) universe. These corrections comprise a Gauss-Bonnet (GB) invariant multiplied by universal non-trivial functions of the common modulus $\sigma$ and the dilaton $\phi$. The model exhibits several features of cosmological interest, including the transition between deceleration and acceleration phases. By considering some phenomenologically motivated ansatzs for one of the scalars and/or the scale factor (of the universe), we also construct a number of interesting inflationary potentials. In all examples under consideration, we find that the model leads only to a standard inflation ($w \geq -1$) when the numerical coefficient $\delta$ associated with modulus-GB coupling is positive, while the model can lead also to a non-standard inflation ($w<-1$), if $\delta$ is negative. In the absence of (or trivial) coupling between the GB term and the scalars, there is no crossing between the $w -1$ phases, while this is possible with non-trivial GB couplings, even for constant dilaton phase of the standard picture. Within our model, after a sufficient amount of e-folds of expansion, the rolling of both fields $\phi$ and $\sigma$ can be small. In turn, any possible violation of equivalence principle or deviations from the standard general relativity may be small enough to easily satisfy all astrophysical and cosmological constraints.Comment: 30 pages, 8 figures; v2 significant changes in notations, appendix and refs added; v3 significant revisions, refs added; v4 appendix extended, new refs, published versio

Late-time Cosmic Dynamics from M-theory

We consider the behaviour of the cosmological acceleration for time-dependent hyperbolic and flux compactifications of M-theory, with an exponential potential. For flat and closed cosmologies it is seen that a positive acceleration is always transient for both compactifications. For open cosmologies, both compactifications can give at late times periods of positive acceleration. As a function of proper time this acceleration has a power law decay and can be either positive, negative or oscillatory.Comment: 10 pages, LaTeX, 2 figure

Towards inflation and dark energy cosmologies from modified Gauss-Bonnet theory

We consider a physically viable cosmological model that has a field dependent Gauss-Bonnet coupling in its effective action, in addition to a standard scalar field potential. The presence of such terms in the four dimensional effective action gives rise to several novel effects, such as a four dimensional flat Friedmann-Robertson-Walker universe undergoing a cosmic inflation at early epoch, as well as a cosmic acceleration at late times. The model predicts, during inflation, spectra of both density perturbations and gravitational waves that may fall well within the experimental bounds. Furthermore, this model provides a mechanism for reheating of the early universe, which is similar to a model with some friction terms added to the equation of motion of the scalar field, which can imitate energy transfer from the scalar field to matterComment: 35 pages, 21 eps figs; section 6 expanded improving explanations, refs added, final in JCA

Isolation and characterization of extremely halophilic CO-oxidizing Euryarchaeota from hypersaline cinders, sediments and soils and description of a novel CO oxidizer, Haloferax namakaokahaiae Mke2.3\u3csup\u3eT\u3c/sup\u3e, sp. nov.

© FEMS 2016. All rights reserved. phylogenetic affiliations of organisms responsible for aerobic CO oxidation in hypersaline soils and sediments were assessed using media containing 3.8 M NaCl. CO-oxidizing strains of the euryarchaeotes, Haloarcula, Halorubrum, Haloterrigena and Natronorubrum, were isolated from the Bonneville Salt Flats (UT) and Atacama Desert salterns (Chile). A halophilic euryarchaeote, Haloferax strain Mke2.3T, was isolated from Hawai\u27i Island saline cinders. Haloferax strain Mke2.3T was most closely related to Haloferax larsenii JCM 13917T (97.0% 16S rRNA sequence identity). It grew with a limited range of substrates, and oxidized CO at a headspace concentration of 0.1%. However, it did not grow with CO as a sole carbon and energy source. Its ability to oxidize CO, its polar lipid composition, substrate utilization and numerous other traits distinguished it from H. larsenii JCM 13917T, and supported designation of the novel isolate as Haloferax namakaokahaiae Mke2.3T, sp. nov (= DSM 29988, = LMG 29162). CO oxidation was also documented for \u27Natronorubrum thiooxidans\u27 HG1 (Sorokin, Tourova and Muyzer 2005), N. bangense (Xu, Zhou and Tian 1999) and N. sulfidifaciens AD2T (Cui et al. 2007). Collectively, these results established a previously unsuspected capacity for extremely halophilic aerobic CO oxidation, and indicated that the trait might be widespread among the Halobacteriaceae, and occur in a wide range of hypersaline habitats

Unusual photoemission resonances of oxygen-dopant induced states in Bi2_{2}Sr2_2CaCu2_2O8+x_{8+x}

We have performed an angular-resolved photoemission study of underdoped, optimally doped and overdoped Bi$_{2}$Sr$_2$CaCu$_2$O$_{8+x}$ samples using a wide photon energy range (15 - 100 eV). We report a small and broad non-dispersive A$_{1g}$ peak in the energy distribution curves whose intensity scales with doping. We attribute it to a local impurity state similar to the one observed recently by scanning tunneling spectroscopy and identified as the oxygen dopants. Detailed analysis of the resonance profile and comparison with the single-layered Bi$_{2}$Sr$_2$CuO$_{6+x}$ suggest a mixing of this local state with Cu via the apical oxygens.Comment: 4 pages, 4 figure