arXiv subjects

Carsten van de Bruck

Publications and source records attributed to Carsten van de Bruck.

98 records · Page 6Linked to original sources

arXiv2001

Galaxy-QSO Correlation induced by Weak Gravitational Lensing arising from Large-Scale Structure

Observational evidence shows that gravitational lensing induces an angular correlation between the distribution of galaxies and much more distant QSOs. We use weak gravitational lensing theory to calculate this angular correlation, updating previous calculations and presenting new results exploring the dependence of the correlation on the large-scale structure. We study the dependence of the predictions on a variety of cosmological models, such as cold dark matter models, mixed dark matter models, and models based on quintessence. We also study the dependence on the assumptions made about the nature of the primordial fluctuation spectrum: adiabatic, isocurvature and power spectra motivated by the cosmic string scenario are investigated. Special attention is paid to the issue of galaxy biasing, which is fully incorporated. We show that different mass power spectra imply distinct predictions for the angular correlation, and therefore the angular correlation provides an extra source of information about cosmological parameters and mechanisms of structure formation. We compare our results with observational data and discuss their potential uses. In particular, it is suggested that the observational determination of the galaxy-QSO correlation may be used to give an independent measurement of the mass power spectrum.

astro-ph↗

arXiv2001

Dynamical Instabilities of the Randall-Sundrum Model

We derive dynamical equations to describe a single 3-brane containing fluid matter and a scalar field coupling to the dilaton and the gravitational field in a five dimensional bulk. First, we show that a scalar field or an arbitrary fluid on the brane cannot evolve to cancel the cosmological constant in the bulk. Then we show that the Randall-Sundrum model is unstable under small deviations from the fine-tuning between the brane tension and the bulk cosmological constant and even under homogeneous gravitational perturbations. Implications for brane world cosmologies are discussed.

hep-th↗

arXiv2000

On Gravity, Holography and the Quantum

The holographic principle states that the number of degrees of freedom describing the physics inside a volume (including gravity) is bounded by the area of the boundary (also called the screen) which encloses this volume. A stronger statement is that these (quantum) degrees of freedom live on the boundary and describe the physics inside the volume completely. The obvious question is, what mechanism is behind the holographic principle. Recently, 't Hooft argued that the quantum degrees of freedom on the boundary are not fundamental. We argue that this interpretation opens up the possibility that the mapping between the theory in the bulk (the holographic theory) and the theory on the screen (the dual theory) is always given by a (generalized) procedure of stochastic quantization. We show that gravity causes differences to the situation in Minkowski/Euclidean spacetime and argue that the fictitious coordinate needed in the stochastic quantization procedure can be spatial. The diffusion coefficient of the stochastic process is in general a function of this coordinate. While a mapping of a bulk theory onto a (quantum) boundary theory can be possible, such a mapping does not make sense in spacetimes in which the area of the screen is growing with time. This is connected to the average process in the formalism of stochastic quantization. We show where the stochastic quantization procedure breaks down and argue, in agreement with `t Hooft, that the quantum degrees of freedom are not fundamental degrees of freedom. They appear as a limit of a more complex process.

gr-qc↗

arXiv1998

Cosmic Strings and Structure Formation

We discuss structure formation in cosmic string theories. Emphasis is laid on the properties of the peculiar velocity field, non-gaussian features and clusters of galaxies. It is found that the predicted peculiar velocity field is quiet ($v<150$ km/s for $L>30 h^{-1}$ Mpc) for models with ($Ω_0 h \leq 0.2$) which is in disagreement with the results from POTENT at large scales ($>30h^{-1}$ Mpc), consistent, however, with the analysis of the peculiar velocity field of spiral galaxies by Giovannelli et al.. Using the Press-Schechter formalism we calculate the abundances of X-ray clusters. It is found that in CDM models a mass fluctuation $σ_8 = 0.6...0.7$ is needed with $Ω_0 h =0.1...0.2$ in order to explain the current data.

astro-ph↗

arXiv1998

The Cosmological Constant $Λ$, the Age of the Universe and Dark Matter: Clues from the Ly$α$-Forest

Evidence for a positive cosmological constant $Λ$, derived from the Ly$α$-forest in high-resolution spectra of quasars, leads to a closed, low-density, $Λ$-dominated universe. The analysis is based on the assumption of a universal shell structure expanding predominantly with the Hubble flow. Supporting evidence comes from two pairs of very wide absorption lines in the spectra of two quasars separated by 8 arcmin on the sky. These results contradict the higher values of the density parameter $Ω_{M,0}$ derived, for example, from clusters of galaxies by the pure gravitational instability theory. Implications thereof and for the amount of non-baryonic dark matter in the universe are discussed with some $Λ$-dominated models, in particular with $Ω_{M,0} = 0.2$ and with pure baryonic models. The evidence points to low-density, closed models with spherical metric, expanding forever.

astro-ph↗

arXiv1997

The Peculiar-Velocity-Field in Structure Formation Theories with Cosmic Strings

We investigate the peculiar velocity field due to long cosmic strings in several cosmological models and analyse the influence of a nonscaling behaviour of the string network, which is expected in open cosmological models or models with a cosmological constant. It is shown that the deviation of the propability distribution of the peculiar velocity field from the normal distribution is only weak in all models. It is further argued that one can not necessarily obtain the parameter $β=Ω_{0}^{0.6}/b$ from density and velocity fields, where $Ω_0$ is the density parameter and $b$ the linear biasing parameter, if cosmic strings are responsible for structure formation in the universe. An explanation for this finding is given.

astro-ph↗

arXiv1997

Cosmic String Network Evolution in arbitrary Friedmann-Lemaitre models

We use the velocity-dependent one-scale model by Martins & Shellard to investigate the evolution of a GUT long cosmic string network in arbitrary Friedmann-Lemaitre models. Four representative models are used to show that in general there is no scaling solution. The implications for structure formation are briefly discussed.

astro-ph↗

arXiv1995

On the Origin of Very Wide Ly$α$-Absorption-Lines in Quasar Spectra

We present a new explanation for the very wide absorption features in quasar spectra. In our model, a very wide absorption feature will originate, when the line of sight crosses a bubble wall tangentially. We demonstrate this on the quasar pair (2138-4427), (2139-4434). Both show two very wide absorption lines in their spectra at the same redshift. The bubble wall model can explain these observations in low density Friedmann-Lemaitre models with spherical metric. It contradicts models with euclidian or hyperbolic metric.

astro-ph↗