Facts and ideas in modern cosmology

In modern astroparticle physics and cosmology, the nature of Dark Matter is one of the central problems. Particle Dark Matter in form of WIMPs is favoured among many proposed candidates. The EDELWEISS direct Dark Matter search uses Germanium bolometers to detect these particles by nuclear recoils. Here, the use of two signal channels on an event-by-event basis, namely the heat and ionisation signal, enables the detectors to discriminate between electron and nuclear recoils. This technique leaves neutrons in the underground laboratory as the main background for the experiment. Besides (α,n) reactions of natural radioactivity, neutrons are produced in electromagnetic and hadronic showers induced by cosmic ray muons in the surrounding rock and shielding material of the Germanium crystals. To reach high sensitivities, the EDELWEISS-II experiment, as well as other direct Dark Matter searches, has to efficiently suppress this neutron background. The present work is devoted to study the muon-induced neutron flux in the underground laboratory LSM and the interaction rate within the Germanium crystals by using the Monte Carlo simulation toolkit Geant4. To ensure reliable results, the implemented physics in the toolkit regarding neutron production is tested in a benchmark geometry and results are compared to experimental data and other simulation codes. Also, the specific energy and angular distribution of the muon flux in the underground laboratory as a consequence of the asymmetric mountain overburden is implemented. A good agreement of the simulated muon flux is shown in a comparison to preliminary experimental data obtained with the EDELWEISS-II muon veto system. Furthermore, within a detailed geometry of the experimental setup, the muon-induced background rate of nuclear recoils in the bolometers is simulated. Coincidences of recoil events in the Germanium with an energy deposit of the muon-induced shower in the plastic scintillators of the veto system are studied to determine the veto efficiency. Finally, the remaining background rate of muon-induced bolometer hits after applying the veto condition is approximately r bg ≤ 10 -5 kg -1 d -1 . The muon detection efficiency in the simulations of the EDELWEISS-II veto system corresponds to (99.94 ± 0.01 +0.06 -0.1 )%. This translates to a potential reduction of the muon-induced background in the order of R ≈O(10 3 ). As a result of this work, the sensitivity of the EDELWEISS-II experiment is in principle not limited by muon-induced background down to a WIMP-nucleon cross section in the range of 10 -10 pb.

Simulations of the muon-induced neutron background of the EDELWEISS-II experiment for dark matter search

A new cosmological model called black hole universe is proposed. According to this model, the universe originated from a hot star-like black hole with several solar masses, and gradually grew up through a supermassive black hole with billion solar masses to the present state with hundred billion-trillion solar masses by accreting ambient materials and merging with other black holes. The entire space is structured with infinite layers hierarchically. The innermost three layers are the universe that we are living, the outside called mother universe, and the inside star-like and supermassive black holes called child universes. The outermost layer is infinite in radius and limits to zero for both the mass density and absolute temperature. The relationships among all layers or universes can be connected by the universe family tree. Mathematically, the entire space can be represented as a set of all universes. A black hole universe is a subset of the entire space or a subspace. The child universes are null sets or empty spaces. All layers or universes are governed by the same physics - the Einstein general theory of relativity with the Robertson-walker metric of spacetime - and tend to expand outward physically. The evolution of the space structure is iterative. When one universe expands out, a new similar universe grows up from its inside. The entire life of a universe begins from the birth as a hot star-like or supermassive black hole, passes through the growth and cools down, and expands to the death with infinite large and zero mass density and absolute temperature. The black hole universe model is consistent with the Mach principle, the observations of the universe, and the Einstein general theory of relativity. Its various aspects can be understood with the well-developed physics without any di culty. The dark energy is not required for the universe to accelerate its expansion. The inflation is not necessary because the black hole universe does not exist the horizon problem.

A New Cosmological Model: Black Hole Universe

This thesis tackles the vast question of generating accelerated periods of expansion of the universe. Models loosely related were developed in the early and late universe. In the early universe, generalizations of the Schwinger effect were developed in curved (de Sitter) space and some backreaction effects were estimated. In the late universe, a fractal model was developed and confronted to supernovae data. This relies on the idea of an accelerated expanding universe being nothing but a mirage due to inhomogeneities disposed in a fractal (in this particular model) way. Finally a model of interacting energy based on an Einstein-Cartan gravitational theory was phenomenologically investigated.

On early and late phases of acceleration of the expansion of the universe

This study focuses on children’s experiences and the creation of “the big picture”, the Universe. It draws data from an age range 6–16 and is based on 270 children’s drawings of how they imagine the Universe to be, and on their answers to a number of short questions about it. Results are discussed using as a base a specially developed systemic network, which is considered to be a formulation broad enough to cover the different ways of experiencing the Universe. The categories of descriptions which have been developed are exemplified by children’s characteristic drawings, and analogies with historical conceptions are discussed. They have also been tested with groups of teachers’ and student–teachers’ descriptions. Moreover, dominant images held during the history of Science, have been explored in terms of their relevance to the categories of the systemic network. It appears that, although there is no analogical evolution of the ideas between these two fields, some historical instances resemble some of the children’s models.

Aleksandr Friedmann

Papers

125. On the Curvature of Space