Some general results in self-creation cosmologies

Raychaudhuri-type equations are written for a cosmological model filled with perfect fluid and obeying the equations of a self-creation theory recently proposed by G. A. Barber. In addition some general results on spatially homogeneous cosmological models are obtained. The Hawking-Penrose energy condition suggests that the singularity can be avoided in this theory.     

Brans-Dicke class of cosmologies

The historical motivation for the Brans-Dicke theory and its connection with Mach’s principle has been discussed. Some examples of actions which can be reduced to the Brans-Dicke type have been given. Further, the recent developments in the theory in the context of inflationary cosmology have been briefly pointed out.     

A comparative study of Brans-Dicke and general relativistic cosmologies in terms of observationally measurable quantities

A comparison between general relativistic and Brans-Dicke cosmologies is made in terms of quantities measurable by an observational astronomer. Numerical integration of the Brans-Dicke field equations was employed to find the relationships of the mean density of cosmic matter, the age, and the time derivative of the gravitational constant to the Hubble constant and deceleration parameter. The difference between general relativistic and Brans-Dicke apparent magnitude-redshift diagrams was found to be negligible even at large redshifts under the assumption of no galactic evolution in absolute magnitude.     

Exact Brans-Dicke cosmologies with a cosmological constant

We present the general Brans-Dicke-Friedmann-Robertson-Walker (k=0) vacuum solutions with a cosmological constant. The space-times considered here are the generalizations of the de Sitter models of the general theory of relativity.     

The general dust solutions in the Brans-Dicke theory

We discuss the Brans-Dicke field equations for the non-flat (k ²=1) Friedmann-Robertson-Walker models filled with dust. It is shown that the problem of constructing the general dust solutions can be reduced to the problem of solving a first-order differential equation.     

Some results in the general scalar tensor theory

Raychaudhuri-type equations are written for a cosmological model filled with a perfect fluid and obeying the equations of general scalar-tensor theory proposed by Nordtvedt. It is found that a non-rotating cosmological model may, in general, have a minimum volume. The Hawking-Penrose energy condition suggests that the singularity can be avoided in this theory.     

Comment on the general vacuum solutions in Brans-Dicke cosmology

We would like to point out that the non-flat FRW-solutions of the Brans-Dicke theory, recently obtained by Chauvet, can be presented in a much simpler form.     

Friedmann cosmology with a cosmological constant in the modified Brans-Dicke theory

Homogenous isotropic cosmologies in the presence of a cosmological constant A are studied in the modified Brans-Dicke theory. A class of exact solutions are obtained in Dicke's revised units for empty space as well as for the models filled with dust or radiation. Behaviour of these models near the singularity are discussed.     

Some Brans-Dicke fields generated from Einstein's vacuum fields,III

In the previous papers (I and II), we have obtained exact solutions of the Brans-Dicke scalar-tensor theory in plane and cylindrically-symmetric space-times. In the present work we have obtained a more general set of solutions from those given in Papers I and II, by Morganstern's unit transformation.     

Nucleosynthesis in bouncing cosmologies

It is shown that certain anomalies connected with the primordial abundances of light nuclei may be resolved if it is assumed that the Universe oscillates between phases of finite densities. Since general relativity does not produce bouncing models of the Universe, such models are obtained through the introduction of a negative energy scalar field of zero rest mass. It is shown that all the relevant parameters of the dynamics of the model and the nucleosynthesis in it are determined by observations and that a self-consistent picture emerges. The model is capable of admitting more than three neutrino flavours without an embarrassingly high primordial helium content. It is also shown that the calculations could be adapted to described production of light nuclei in compact massive bouncing objects.     

Subhaloes in scale-free cosmologies

We explore the dependence of the subhalo mass function on the spectral index n of the linear matter power spectrum using scale-free Einstein-de Sitter simulations with   n =−1  and −2.5. We carefully consider finite volume effects that may call into question previous simulations of   n < −2  power spectra. Subhaloes are found using a 6D friends-of-friends algorithm in all haloes originating from high-σ peaks. For   n =−1  , we find that the cumulative subhalo mass function is independent of the parameters used in the subhalo finding algorithm and is consistent with the subhalo mass function found in Λ cold dark matter (ΛCDM) simulations. In particular, the subhalo mass function is well fit by a power-law with an index of  α=−0.9  , that is the mass function has roughly equal mass in subhaloes per logarithmic interval in subhalo mass. Conversely, for   n =−2.5  , the algorithm parameters affect the subhalo mass function since subhaloes are more triaxial with less well-defined boundaries. We find that the index α is generally larger with  α≳−0.75  . We infer that although the subhalo mass function appears to be independent of n so long as   n ≳−2  , it begins to flatten as   n →−3  . Thus, the common practice of using  α≈−1.0  may greatly overestimate the number of subhaloes at the smallest scales in the CDM hierarchy.     

The general class of Bianchi cosmological models with viscous fluid and particle creation in Brans-Dicke theory

This paper deals with the general class of Bianchi cosmological models with bulk viscosity and particle creation described by full causal thermodynamics in Brans-Dicke theory. We discuss three types of average scale-factor solutions for the general class of Bianchi cosmological models by using a special law for the deceler- ation parameter which is linear in time with a negative slope. The exact solutions to the corresponding field equations are obtained in quadrature form and solutions to the Einstein field equations are obtained for three different physically viable cosmologies. All the physical parameters are calculated and discussed in each model.     

Cylindrically symmetric cosmologies in Lyra geometry

A class of new exact solutions of the Einstein field equations have been investigated for stationary cylindrically symmetric space-time around a local cosmic string in the theory based on Lyra’s geometry in normal gauge in the presence and absence of an electromagnetic field. The cosmological solutions have been analyzed through various physical and geometrical parameters. It has also been shown that the solutions are space-time inhomogeneous and filled with charged dust.     

Density perturbations in the Brans-Dicke theory

We give here the calculation of density perturbations in a gravitation theory with a scalar field non-minimally coupled to gravity, i.e., the Brans-Dicke theory of gravitation. The purpose is to show the influence of this scalar field on the dynamic behaviour of density perturbations along the eras where the equation of state for the matter can be put under the formp=, where is a constant. We analyse the asymptotic behaviour of this perturbations for the cases =0, =–1, =1/3 and =0. In general, we obtain a decaying and growing modes. In the very important case of inflation, =–1, there is no density perturbation, as it is well known. In the vacuum phase the perturbations on the scalar field and the gravitational field present growing modes at the beginning of the expansion and decaying modes at the end of this phase. In the case =0 it is possible, for some negative values of , to have an amplification of the perturbations with a superluminal expansion of the scale factor. We can also obtain strong growing modes for the density contrast for the case where there is a contraction phase which can have physical interest in some primordial era.     

General vacuum-universe solutions in Brans-Dicke cosmology

By a rescalation of the scalar field ? of the Jordan-Brans and Dicke cosmology, the general solutions of the Friedmannian ‘vacuum’ Universe are obtained. Only the flat space solution was previously known. Each solution is caracterized by the sign of the second time derivative of the rescaled field ψ≡?R ³ (R being the scale factor of the Robertson-Walker line-element): \(\ddot \psi\) = 0 (flat space), \(\ddot \psi\) < 0 (closed space), and \(\ddot \psi\) > 0 (open space), so that the solutions are mutually exclusive. Of these, the open space one is damped-oscillatory andR attains its absolute minimum, equal to zero, in only one of the two ‘extreme’ cycles. Otherwise,R min remains positive. If the ?-field is dominant near the singularity, these solutions may have physical significance. Also obtained, by the method mentioned above, is the general flat space solution for a ‘dust’ Universe and from it a closed space ‘dust’ solution. Both were found before by different authors, each one using a different method and, therefore, seemed up to now unrelated.     

Tidal tails in cold dark matter cosmologies

We study the formation of tidal tails in pairs of merging disc galaxies with structural properties motivated by current theories of cold dark matter (CDM) cosmologies. In a recent study, Dubinski, Mihos & Hernquist showed that the formation of prominent tidal tails can be strongly suppressed by massive and extended dark haloes. For the large halo-to-disc mass ratio expected in CDM cosmologies their sequence of models failed to produce strong tails like those observed in many well-known pairs of interacting galaxies. In order to test whether this effect can constrain the viability of CDM cosmologies, we construct N ‐body models of disc galaxies with structural properties derived in analogy to the recent analytical work of Mo, Mao & White. With a series of self-consistent collisionless simulations of galaxy–galaxy mergers we demonstrate that even the discs of very massive dark haloes have no problems developing long tidal tails, provided the halo spin parameter is large enough. For our class of models, the halo-to-disc mass ratio is not a good indicator of the ability to produce tails. Instead, the relative size of disc and halo or, alternatively, the ratio of circular velocity to local escape speed at the half mass radius of the disc is a more useful criterion. This result holds in all CDM models. While tidal tails can provide useful information on the structure of galaxies, it thus appears unlikely that they are able to constrain the values of the cosmological parameters within these models.     

A study of Brans-Dicke theory in FRW-model

In FRW space time Brans-Dicke theory is developed for two cases: (i) the vacuum and (ii) the perfect fluid model. The field equations are transformed into a much simpler form under a change of time co-ordinates and then the solutions are determined for the above cases. An equation of statep =/3 (radiation) is assumed in the case of perfect fluid.     

An anisotropic cosmological model in Brans-Dicke theory

Solutions of Brans-Dicke field equations are obtained when the source of the gravitational field is a perfect fluid with pressure equal to energy density and the metric is cylindrically symmetric of Marder-type. Various physical and geometrical properties of the model have been discussed. Finally the solutions have been transformed to the original form of Brans-Dicke (1961) theory and then through unit transformation to a general form.