Astronomy

We investigate four-dimensional effective theories which are obtained by dimensional reduction of multidimensional cosmological models with factorizable geometry and consider the interaction between conformal excitations of the internal space (geometrical moduli excitations) and Abelian gauge fields. It is assumed that the internal space background can be stabilized by minima of an effective potential. The conformal excitations over such a background have the form of massive scalar fields (gravitational excitons) propagating in the external spacetime. We discuss cosmological and astrophysical implications of the interaction between gravexcitons and four-dimensional photons as well as constraints arising on multidimensional models of the type considered in our paper. In particular, we show that due to the experimental bounds on the variation of the fine structure constant, gravexcitons should decay before nucleosynthesis starts. For a successful nucleosynthesis the masses of the decaying gravexcitons should be m > ∼ 10 4 GeV. Furthermore, we discuss the possible contribution of gravexcitons to UHECR. It is shown that, at energies E ∼ 10 20 eV, the decay length of gravexcitons with masses m > ∼ 10 4 GeV is very small, but that for m < ∼ 10 2 GeV it becomes much larger than the Greisen-Zatsepin-Kuzmin cut-off distance. Finally, we investigate the possibility for gravexciton-photon oscillations in strong magnetic fields of astrophysical objects. The corresponding estimates indicate that even the high magnetic field strengths B of magnetars (special types of pulsars with B > B critical ∼ 4.4 × 10 13 Gauss) are not sufficient for an efficient and copious production of gravexcitons.

In this paper we consider a multidimensional Kaluza-Klein (KK) model with a Ricci-flat internal space, e.g., a Calabi-Yau manifold. We perturb this background metrics by a system of gravitating masses, e.g., astrophysical objects such as our Sun. We suppose that these masses are pressureless in the external space but they have relativistic pressure in the internal space. We show that metric perturbations do not depend on coordinates of the internal space and gravitating masses should be uniformly smeared over the internal space. This means, first, that KK modes corresponding to the metric fluctuations are absent and, second, particles should be only in the ground quantum state with respect to the internal space. In our opinion, these results look very unnatural. According to statistical physics, any nonzero temperature should result in fluctuations, i.e. in KK modes. We also get formulae for the metric correction terms which enable to calculate the gravitational tests: the deflection of light, the time-delay of the radar echoes and the perihelion advance.

In this paper, we consider a system of gravitating bodies in Kaluza-Klein models with toroidal compactification of extra dimensions. To simulate the astrophysical objects (e.g., our Sun and pulsars) with energy density much greater than pressure, we suppose that these bodies are pressureless in the external/our space. At the same time, they may have nonzero parameters ω (ᾱ−3) (ᾱ = 4, . . . , D) of the equations of state in the extra dimensions. We construct the Lagrange function of this many-body system for any value of Σ = ᾱ ω (ᾱ−3) . Moreover, the gravitational tests (PPN parameters, perihelion and periastron advances) require negligible deviation from the latent soliton value Σ = −(D − 3)/2. However, the presence of pressure/tension in the internal space results necessarily in the smearing of the gravitating masses over the internal space and in the absence of the KK modes. This looks very unnatural from the point of quantum physics.

In this paper, we consider dynamical behavior of astrophysical objects such as galaxies and dwarf galaxies taking into account both the gravitational attraction between them and the cosmological expansion of the Universe. First, we obtain the general system of equations and apply them to some abstract systems of galaxies. Then we investigate the collision between the Milky Way and Andromeda in future. We demonstrate that for currently known parameters of this system the collision is hardly possible because of the angular momentum. These galaxies will approach the minimum distance of about 290 Kpc

A multidimensional cosmological model with space-time consisting of n(n ≥ 2) Einstein spaces M i is investigated in the presence of a cosmological constant Λ and a homogeneous minimally coupled scalar field ϕ(t) as a matter source. Classical and quantum wormhole solutions are obtained for Λ < 0 and all M i being Ricci-flat.