arXiv subjects
Publications and source records attributed to Robert H. Brandenberger.
Topological defects are produced during phase transitions in the very early Universe. They arise in most unified theories of strong, weak and electromagnetic interactions. These lectures focus on the role of topological defects in cosmology, with particular emphasis on the models of structure formation based on defects. The role of topological defects in baryogenesis is also reviewed.
Good statistics for measuring large-scale structure in the Universe must be able to distinguish between different models of structure formation. In this paper, two and three dimensional ``counts in cell" statistics and a new ``discrete genus statistic" are applied to toy versions of several popular theories of structure formation: random phase cold dark matter model, cosmic string models, and global texture scenario. All three statistics appear quite promising in terms of differentiating between the models.
A unified theory of all forces should be nonsingular. In such a unified theory, Einstein's general relativity will be a very low curvature effective theory. At larger curvatures, new terms will become important. The question then arises as to whether it is possible to construct an effective action for gravity which leads to a nonsingular theory. In this work, we construct an effective action for gravity in which all homogenerous and isotropic solutions are nonsingular. In particular, there is neither a big bang nor a big crunch. Preliminary work indicates that our construction provides a theory in which anisotropies decrease at large curvatures, and in which the inside of a black hole is singularity free.
We construct an effective action for gravity in which all homogeneous solutions are nonsingular. In particular, there is neither a big bang nor a big crunch. The action is a higher derivative modification of Einstein's theory constructed in analogy to how the action for point particle motion in special relativity is obtained from Newtonian mechanics.
The COBE satellite has discovered anisotropies in the cosmic microwave background (CMB) consistent with a scale invariant spectrum of density perturbations. As reviewed in this lecture, topological defect models of structure formation generically produce such a spectrum. We give a summary of the cosmic string and global texture models, focusing on distinct observational signatures in large-scale structure surveys and in the microwave background. It is shown that the amplitude of the CMB quadrupole detected by COBE is in good agreement with the predictions of the cosmic string model.
If stable electroweak strings are copiously produced during the electroweak phase transition, they may contribute significantly to the presently observed baryon to entropy ratio of the universe. This analysis establishes the feasibility of implementing an electroweak baryogenesis scenario without a first order phase transition.
We calculate the flux of ultra high energy photons from individual ordinary (i.e. non-superconducting) cosmic strings and compare the results with the sensitivity of current and proposed TeV and EeV telescopes. Our calculations give only upper limits for the gamma ray flux, since the source of the photons, jets from particle production at cusps, may be weakened by back reaction effects. For the usual cosmic distribution of strings, the predicted bursts from strings with the value of mass per unit length associated with galaxy formation or light strings may just be detectable. A diffuse gamma ray background from light strings may also be seen by the Fly's Eye detector at above $7 \times 10^{10}$ GeV.
We consider grand unified theories with superconducting cosmic strings and which admit the mechanism for generating primordial magnetic fields recently discussed by Vachaspati. We show that these models are severely constrained by cosmological arguments. Quite generically, either stable springs or vortons will form. Provided the mass per unit length of the strings is sufficiently large, these stable configurations will overclose the Universe.
If stable electroweak strings are copiously produced during the electroweak phase transition, they may contribute significantly to the presently observed baryon to entropy ratio of the Universe. This analysis establishes the feasibility of implementing an electroweak baryogenesis scenario without a first order phase transition.