Branenkosmologie als alternative Erklärung der dunklen Energie

Diese Arbeit beschäftigt sich mit einer Theorie, der Branenkosmologie, die neben den üblichen vier Raumzeitdimensionen noch eine weitere Dimension annimmt. Demnach wäre die uns bekannte Raumzeit eine Hyperebene (die Bran), die in eine fünfdimensionale Mannigfaltigkeit (den Bulk) eingebettet ist. Diese Bran kann von Materiefeldern nicht verlassen werden. Die fünfte Dimension, die entweder raum- oder zeitartig sein kann, hat allerdings Einfluss auf das Expansionsverhalten des Universums und kann somit als dunkle Energie wirken. Neben den in der Standardkosmologie auftretenden Dichteparametern tauchen in der Branenkosmologie nämlich drei zusätzliche Parameter auf, die dafür sorgen, dass sich bestimmte Branenmodelle innerhalb großer Rotverschiebungsintervalle wie andere Modelle dunkler Energie verhalten, dabei allerdings einige Probleme, die in diesen Theorien auftauchen, vermeiden. Schließlich werden noch einige Methoden vorgestellt, mit denen kosmologische Modelle getestet werden können. Abschließend werden dann mit Hilfe dieser kosmologischen Tests Einschränkungen für die Parameterwerte des Branenmodells vorgenommen und die Verträglichkeit dieser Theorie mit den Beobachtungsdaten überprüft

Braneworlds with timelike extra-dimension

In this work, we consider a braneworld model with a timelike extra-dimension. There are strong constraints to the parameter values of such a model resulting from the claim that there must be a physical solution to the Friedmann equation at least between now and the time of recombination. We fitted the model to supernova type Ia data and checked the consistency of the result with other observations. For parameter values that are consistent with observations, the braneworld model is indistinguishable from a LambdaCDM universe as far as the considered cosmological tests are concerned.Comment: 7 pages, 8 figures, matches version accepted by Phys. Rev.

Reconstruction of dark energy and expansion dynamics using Gaussian processes

An important issue in cosmology is reconstructing the effective dark energy equation of state directly from observations. With few physically motivated models, future dark energy studies cannot only be based on constraining a dark energy parameter space, as the errors found depend strongly on the parametrisation considered. We present a new non-parametric approach to reconstructing the history of the expansion rate and dark energy using Gaussian Processes, which is a fully Bayesian approach for smoothing data. We present a pedagogical introduction to Gaussian Processes, and discuss how it can be used to robustly differentiate data in a suitable way. Using this method we show that the Dark Energy Survey - Supernova Survey (DES) can accurately recover a slowly evolving equation of state to w = ±0.05 (95% CL) at z = 0 and ±0.25 at z = 0.7, with a minimum error of ±0.025 at the sweet-spot at z 0.16, provided the other parameters of the model are known. Errors on the expansion history are an order of magnitude smaller, yet make no assumptions about dark energy whatsoever

Null tests of the cosmological constant using supernovae

The standard concordance model of the Universe is based on the cosmological constant as the driver of accelerating expansion. This concordance model is being subjected to a growing range of inter-locking observations. In addition to using generic observational tests, one can also design tests that target the specific properties of the cosmological constant. These null tests do not rely on parametrizations of observables, but focus on quantities that are constant only if dark energy is a cosmological constant. We use supernova data in null tests that are based on the luminosity distance. In order to extract derivatives of the distance in a model-independent way, we use Gaussian Processes. We find that the concordance model is compatible with the Union 2.1 data, but the error bars are fairly large. Simulated datasets are generated for the DES supernova survey and we show that this survey will allow for a sharper null test of the cosmological constant if we assume the Universe is flat. Allowing for spatial curvature degrades the power of the null test.IS

Model- and calibration-independent test of cosmic acceleration

We present a calibration-independent test of the accelerated expansion of the universe using supernova type Ia data. The test is also model-independent in the sense that no assumptions about the content of the universe or about the parameterization of the deceleration parameter are made and that it does not assume any dynamical equations of motion. Yet, the test assumes the universe and the distribution of supernovae to be statistically homogeneous and isotropic. A significant reduction of systematic effects, as compared to our previous, calibration-dependent test, is achieved. Accelerated expansion is detected at significant level (4.3 sigma in the 2007 Gold sample, 7.2 sigma in the 2008 Union sample) if the universe is spatially flat. This result depends, however, crucially on supernovae with a redshift smaller than 0.1, for which the assumption of statistical isotropy and homogeneity is less well established.Comment: 13 pages, 2 figures, major change