We analyse the distribution of Abell clusters of galaxies to study the regularity of the superclu... more We analyse the distribution of Abell clusters of galaxies to study the regularity of the supercluster-void network. We apply a new method sensitive to the geometry of the location of clusters, and measure the goodness of regularity of the network. We find that the superclustervoid network resembles a cubical lattice over the whole space investigated. The distribution of rich superclusters is not isotropic: along the main axis of the network it is periodic with a step of length ≈ 130 h −1 Mpc, whereas along the diagonal of the network the period is larger, as expected for a cubical lattice. This large-scale inhomogeneity is compatible with recent CMB data.
Bronstein's is only the second work, after L. Rosenfeld's 1930 papers [1,2], on the quantization ... more Bronstein's is only the second work, after L. Rosenfeld's 1930 papers [1,2], on the quantization of (linearized) gravity. It was carried out in 1935, at a time when modern quantum field theory was in its infancy-well before Fierz and Pauli's treatment of spin 2, and soon after Dirac's (1931) and Fermi's (1934) formulations of QED. Its birthplace-Leningrad-was also remarkable: here a small group of young theoreticians were forming the nucleus of the future Landau school and many other eminent Russian institutions. Indeed, Bronstein had close ties, both personal and scientific, with Landau and his circle, and the spirit animating this work clearly reflects these affinities of style, recognizable to readers of the Landau & Lifshitz series. Had Bronstein's life not come to a tragic end only a few years later, in Stalin's purges, he would certainly have fulfilled his promise as a most valuable contributor to forefront theoretical physics.
The Future of the Universe and the Future of Our Civilization, 2000
The existence of a positive and possibly varying Lambda-term opens a much wider field of possibil... more The existence of a positive and possibly varying Lambda-term opens a much wider field of possibilities for the future of our Universe than it was usually thought before. Definite predictions may be made for finite (though very large) intervals of time only, as well as in other branches of science. In particular, our Universe will continue to expand as far as the Lambda-term remains positive and does not decay to other forms of matter, even if the Universe is closed. Two new effects due to the presence of a constant Lambda-term are discussed: reversal of a sign of the redshift change with time for sufficiently close objects and inaccessibility of sufficiently distant objects in the Universe for us. A number of more distant and speculative possibilities for the future evolution of the Universe is listed including hitting a space-time singularity during an expansion phase. Finally, in fantastically remote future, a part of our Universe surrounding us can become supercurved and superdense due to various quantum-gravitational effects.
Recent observations of Type 1a supernovae indicating an accelerating universe have once more draw... more Recent observations of Type 1a supernovae indicating an accelerating universe have once more drawn attention to the possible existence, at the present epoch, of a small positive Λ-term (cosmological constant). In this paper we review both observational and theoretical aspects of a small cosmological Λ-term. We discuss the current observational situation focusing on cosmological tests of Λ including the age of the universe, high redshift supernovae, gravitational lensing, galaxy clustering and the cosmic microwave background. We also review the theoretical debate surrounding Λ: the generation of Λ in models with spontaneous symmetry breaking and through quantum vacuum polarization effects -mechanisms which are known to give rise to a large value of Λ hence leading to the 'cosmological constant problem'. More recent attempts to generate a small cosmological constant at the present epoch using either field theoretic techniques, or by modeling a dynamical Λ-term by scalar fields are also extensively discussed. Anthropic arguments favouring a small Λ-term are briefly reviewed. A comprehensive bibliography of recent work on Λ is provided.
This review summarizes recent attempts to reconstruct the expansion history of the Universe and t... more This review summarizes recent attempts to reconstruct the expansion history of the Universe and to probe the nature of dark energy. Reconstruction methods can be broadly classified into parametric and non-parametric approaches. It is encouraging that, even with the limited observational data currently available, different approaches give consistent results for the reconstruction of the Hubble parameter H(z) and the effective equation of state w(z) of dark energy. Model independent reconstruction using current data allows for modest evolution of dark energy density with redshift. However, a cosmological constant (= dark energy with a constant energy density) remains an excellent fit to the data. Some pitfalls to be guarded against during cosmological reconstruction are summarized and future directions for the model independent reconstruction of dark energy are explored.
The structure of the space-time and physical processes inside the black holes
ABSTRACT The existence of the Cauchy horizon in the general case of a real physical body collapse... more ABSTRACT The existence of the Cauchy horizon in the general case of a real physical body collapse is studied. The evolution of physical fields and quantum effects inside the black holes are discussed.
In a recent paper Boyanovsky et al (hep-ph/9608205) studied preheating for a particular theory of... more In a recent paper Boyanovsky et al (hep-ph/9608205) studied preheating for a particular theory of the inflaton field. Although they results essentially confirm those of the original papers on reheating after inflation, they claimed that most of the earlier works on preheating did not take into account backreaction of created particles, misused the Mathieu equation, and were inconclusive about the symmetry restoration after preheating. We explain why we cannot agree with these statements. PACS: 98.80.Cq, 04.62.+v, 05.70.Fh SU-ITP-96-39, UHIFA96-45, hep-ph/9608341
Flat FRW perfect fluid cosmologies can be reproduced as particular solutions of suitable field th... more Flat FRW perfect fluid cosmologies can be reproduced as particular solutions of suitable field theoretical models. Here we investigate the stability of perfect fluid model trajectories with respect to sets of trajectories of the corresponding field models having generic initial conditions. It is shown that the trajectories of barotropic perfect fluid models and those of the Chaplygin gas model are stable. The total probability to reach the Chaplygin gas regime early enough to achieve a matter dominated stage having a realistic duration is calculated for a scalar field model assuming a natural measure in the space of initial conditions taken near a cosmological singularity. An example is presented of a two-fluid cosmological model where the stability is partially absent.
In a recent paper (astro-ph/0311364) Alam et al argued that the SNe data of Tonry et al 2003 and ... more In a recent paper (astro-ph/0311364) Alam et al argued that the SNe data of Tonry et al 2003 and Barris et al 2003 appear to favour DE which evolves in time, provided no other priors are invoked. (The effect of invoking priors such as the age of the Universe, the values of $H_0$ and $\Omega_m$ and CMB/LSS observations could modify
Some recent papers have claimed the existence of static, spherically symmetric wormhole solutions... more Some recent papers have claimed the existence of static, spherically symmetric wormhole solutions to gravitational field equations in the absence of ghost (or phantom) degrees of freedom. We show that in some such cases the solutions in question are actually not of wormhole nature while in cases where a wormhole is obtained, the effective gravitational constant G eff is negative in some region of space, i.e., the graviton becomes a ghost. In particular, it is confirmed that there are no vacuum wormhole solutions of the Brans-Dicke theory with zero potential and the coupling constant ω > −3/2 , except for the case ω = 0 ; in the latter case, G eff < 0 in the region beyond the throat. The same is true for wormhole solutions of F (R) gravity: special wormhole solutions are only possible if F (R) contains an extremum at which G eff changes its sign.
We investigate the Tolman-Oppenheimer-Volkoff equations for the generalized Chaplygin gas with th... more We investigate the Tolman-Oppenheimer-Volkoff equations for the generalized Chaplygin gas with the aim of extending the findings of V. Gorini, U. Moschella, A. Y. Kamenshchik, V. Pasquier, and A. A. Starobinsky [Phys. Rev. D 78, 064064 (2008)]. We study both the standard case, where we reproduce some previous results, and the phantom case. In the phantom case we show that even a superluminal group velocity arising for α > 1 cannot prevent the divergence of the pressure at a finite radial distance. Finally, we investigate how a modification of the generalized Chaplygin gas equation of state, required by causality arguments at densities very close to Λ, affects the results found so far.
Mixed cold-hot dark matter cosmological models (CHDM) with $\Omega_{tot}=1$, approximately flat i... more Mixed cold-hot dark matter cosmological models (CHDM) with $\Omega_{tot}=1$, approximately flat initial spectrum of adiabatic perturbations and 1, 2 or 3 types of massive neutrinos are compared and tested using recent observational data. The models with 2 or 3 neutrino types of equal mass permit as the best fit larger values of both the Hubble constant ($H_0\le 60$ for 2 types, $H_0\le 65$ for 3 types) and the total $\Omega_{\nu}$ (up to 0.3 for 3 types) than the model with 1 massive type. Also, they have less problems with abundances of early compact objects including $Ly-\alpha$ clouds.
We consider scalar Born-Infeld type theories with arbitrary potentials V (T ) of a scalar field T... more We consider scalar Born-Infeld type theories with arbitrary potentials V (T ) of a scalar field T . We find that for models with runaway potentials V (T ) the generic inhomogeneous solutions after a short transient stage can be very well approximated by the solutions of a Hamilton-Jacobi equation that describes free streaming wave front propagation. The analytic solution for this wave propagation shows the formation of caustics with multi-valued regions beyond them. We verified that these caustics appear in numerical solutions of the original scalar BI non-linear equations. Our results include the scalar BI model with an exponential potential, which was recently proposed as an effective action for the string theory tachyon in the approximation where high-order spacetime derivatives of T are truncated. Since the actual string tachyon dynamics contain derivatives of all orders, the tachyon BI model with an exponential potential becomes inadequate when the caustics develop because high order spatial derivatives of T become divergent. BI type tachyon theory with a potential decreasing at large T could have interesting cosmological applications because the tachyon field rolling towards its ground state at infinity acts as pressureless dark matter. We find that inhomogeneous cosmological tachyon fluctuations rapidly grow and develop multiple caustics. Any considerations of the role of the tachyon field in cosmology will have to involve finding a way to predict the behavior of the field at and beyond these caustics.
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Papers by A. Starobinsky