Slow-roll inflation in non-geometric flux compactification

By implementing a genetic algorithm we search for stable vacua in Type IIB non-geometric flux compactification on an isotropic torus with orientifold 3-planes. We find that the number of stable dS and AdS vacua are of the same order. Moreover we find that in all dS vacua the multi-field slow-roll inflationary conditions are fulfilled. Specifically we observe that inflation is driven by the axio-dilaton and the Kähler moduli. We also comment on the existence of one stable dS vacuum in the presence of exotic orientifolds.     

Generalized self-similar scalar-tensor theories

In order to analyze how the gravitational constant, G, and the cosmological constant, Λ, may vary we study through symmetry principles the form of the functions in the generalized scalar-tensor theories under the self-similar hypothesis. The results obtained are absolutely general and valid for all the Bianchi models and the flat FRW one. We study the concrete example of the Kantowski–Sachs model finding some new exact self-similar solutions.     

Cosmic no-hair conjecture in scalar-tensor theories

We have shown that, within the context of scalar-tensor theories, the anisotropic Bianchi-type cosmological models evolve towards de Sitter Universe. A similar result holds in the case of cosmology in Lyra manifold. Thus the analogue of cosmic no-hair theorem of Wald [1] hold in both the cases. In fact, during inflation there is no difference between scalar-tensor theories, Lyra’s manifold and general relativity (GR).     

Conformal transformations in scalar-tensor gravitation theories

Conformal transformations in scalar-tensor gravitation theories are investigated in the present paper. It is demonstrated that the field equations of these theories can be expressed in the Vaidya form. A scalar field equation is derived based on the compatibility condition for the field equations. The conformal factor, dilaton, and restrictions on the metric are determined for the diagonal metric of type I in the Bianchi classification. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 2, pp. 10–14, February, 2006.     

Slow-roll inflation in f(T,T ) modified gravity

In this study, we explore the concept of cosmological inflation within the framework of the f(T, T)theory of gravity, where f is a general function of the torsion scalar T and the trace T of the energy-momentum tensor.It is assumed that the conditions of slow-roll inflation are applicable in f(T, T) gravity. To determine different observables related to inflation, such as the tensor-to-scalar ratio r, scalar spectral index ns, spectral index αs, and tensor spectral index nt, the Hubble slow-roll...     

Conformal invariance and scalar-tensor theories

A new generalisation of Einstein’s theory is proposed which is invariant under conformal mappings. Two scalar fields are introduced in addition to the metric tensor field, so that two special choices of gauge are available for physical interpretation, the ‘Einstein gauge’ and the ‘atomic gauge’. The theory is not unique but contains two adjustable parameters ζ anda. Witha=1 the theory viewed from the atomic gauge is Brans-Dicke theory (ω=−3/2+ζ/4). Any other choice ofa leads to a creation-field theory. In particular the theory given by the choicea=−3 possesses a cosmological solution satisfying Dirac’s ‘large numbers’ hypothesis.     

Holographic dark energy model and scalar-tensor theories

We study the holographic dark energy model in a generalized scalar tensor theory. In a universe filled with cold dark matter and dark energy, the effect of potential of the scalar field is investigated in the equation of state parameter. We show that for a various types of potentials, the equation of state parameter is negative and transition from deceleration to acceleration expansion of the universe is possible.     

New inflation in supersymmetric theories

A new inflationary universe scenario is considered based upon phase transitions in supersymmetric unified models where the Higgs potential is of the Witten-O'Raifeartaigh type.     

Some remarks on the genesis of scalar-tensor theories

Between 1941 and 1962, scalar-tensor theories of gravitation were suggested four times by different scientists in four different countries. The earliest originator, the Swiss mathematician W. Scherrer, was virtually unknown until now whereas the chronologically latest pair gave their names to a multitude of publications on Brans–Dicke theory. P. Jordan, one of the pioneers of quantum mechanics and quantum field theory, and Y. Thiry, known by his book on celestial mechanics, a student of the mathematician Lichnerowicz, complete the quartet. Diverse motivations for and conceptual interpretations of their theories will be discussed as well as relations among them. Also, external factors like language, citation habits, or closeness to the mainstream are considered. It will become clear why Brans–Dicke theory, although structurally a déjà-vu, superseded all the other approaches.     

Conditions for equilibrium of perfect fluids in the scalar-tensor theories

We have determined the first and second order radial and time symmetric variations of the mass-energy of spherically symmetric isentropic perfect fluids in the framework of the scalar-tensor theories of gravitation. The results are then used to study the conditions for the total mass-energy to be a minimum under variations which conserve the total number of baryons of the system, and these conditions are interpreted as necessary for the dynamical stability of static configurations subjected to the above perturbations. The post-Newtonian approximation is performed, and we obtain a stability criterion in the Chandrasekhar form, > 4/3 + 2K _st(GM/R) _c, 4/3, where is the pressure averaged adiabatic index,M andR are the total mass and the radius of the configuration respectively, andK _st is a constant that depends on the density distribution, the scalar field,ø, coupling function(ø), andd/dø. The value ofK _st, is compared to that in Einstein's general relativity and in the Jordan-Brans-Dicke theory. This is done for the specific scalar-tensor theories of Barker, Schwinger, and for models with curvature coupling.     

A comment on the geometry of some scalar-tensor theories

We show that the scalar field in scalar-tensor theories such as the Jordan-Brans-Dicke theory has an interpretation as a potential for the torsion in a Riemannian manifold. The relation is similar to that of the metric to the connection.     

Exact solutions for the early Friedmann-Robertson-Walker universe in general scalar-tensor theories

Homogeneous and isotropic cosmologies with trace-free energy-momentum tensors are studied in general scalar-tensor theories. A method is presented which allows one to construct exact solutions for theories with arbitrary coupling functionω(φ). Particular attention is paid to Schwinger's theory.