arXiv subjects

Robert H. Brandenberger

Publications and source records attributed to Robert H. Brandenberger.

At least 127 records · Page 7Linked to original sources

arXiv1998

Anomalous Global Strings and Primordial Magnetic Fields

We propose a new mechanism for the generation of primordial magnetic fields, making use of the magnetic fields which are induced by anomalous global strings which couple to electromagnetism via Wess-Zumino type interactions. This mechanism can be realized in QCD by utilizing pion strings, global vortices which appear in the linear sigma model which describes physics below the QCD confinement scale. During the chiral symmetry breaking phase transition, pion strings can be produced, thus leading to primordial magnetic fields. We calculate the magnitude and coherence length of these fields. They are seen to depend on the string dynamics. With optimistic assumptions, both the magnitude and coherence scale of the induced magnetic fields can be large enough to explain the seed magnetic fields of greater than $10^{-23}$ Gauss necessary to produce the observed galactic magnetic fields via the galactic dynamo mechanism.

hep-ph↗

arXiv1998

Topological Defects and Cosmology

Many particle physics models of matter admit solutions corresponding to stable or long-lived topological defects. In the context of standard cosmology it is then unavoidable that such defects will form during phase transitions in the very early Universe. Certain types of defects lead to disastrous consequences for cosmology, others may play a useful role, as possible seeds for the formation of structure in the Universe, or in mediating baryon number violating processes. In all cases, topological defects lead to a fruitful interplay between particle physics and cosmology.

hep-ph↗

arXiv1998

Astrophysical Constraints on Primordial Black Hole Formation from Collapsing Cosmic Strings

Primordial Black Holes (PBH) may have formed from the collapse of cosmic string loops. The spectral shape of the PBH mass spectrum can be determined by the scaling argument for string networks. Limits on the spectral amplitude derived from extragalactic gamma-ray and galactic gamma-ray and cosmic ray flux observations as well as constraints from the possible formation of stable black holes remnants are reanalyzed. The new constraints are remarkably close to those derived from the normalization of the cosmic string model to the cosmic microwave background anisotropies.

astro-ph↗

arXiv1997

Pion and Eta Strings

In this paper we construct a string-like classical solution, the pion-string, in the linear sigma model. We then study the stability of the pion-string, and find that it is unstable in the parameter space allowed experimentally. We also speculate on the existance of an unstable eta-string, associated with spontaneous breakdown of the anomalous $U_A(1)$ symmetry in QCD at high temperatures. The implications of the pion and eta strings for cosmology and heavy ion collisions are briefly mentioned.

hep-ph↗

arXiv1997

Inflation and the Theory of Cosmological Perturbations

The hypothesis that the Universe underwent a period of exponential expansion at very early times has become the most popular theory of the early Universe. Not only does it solve some of the problems of standard big bang cosmology, but it also provides a causal theory for the origin of primordial density fluctuations which may explain the observed density inhomogeneities and cosmic microwave fluctuations in the Universe. In these lectures, a review of the basic principles of inflationary cosmology is given, focusing on the theory of the origin and growth of cosmological perturbations. The lectures also focus on some recent progress in inflationary cosmology (in particular on an improved theory of reheating), and on several problems with the present inflationary Universe models. A couple of possible approaches to resolve these issues are suggested.

astro-ph↗

arXiv1997

Limits on Black Hole Formation from Cosmic String Loops

In theories with cosmic strings, a small fraction of string loops may collapse to form black holes. In this Letter, various constraints on such models involving black holes are considered. Hawking radiation from black holes, gamma and cosmic ray flux limits and constraints from the possible formation of stable black hole remnants are reanalyzed. The constraints which emerge from these considerations are remarkably close to those derived from the normalization of the cosmic string model to the cosmic microwave background anisotropies.

astro-ph↗

arXiv1997

Inflation, Topological Defects and Baryogenesis: Selected Topics at the Interface between Particles & Fields and Cosmology

Modern cosmology provides a connection between the physics of particles and fields and observational cosmology. Making use of this link, a wealth of new observational data can be utilized to explore and constrain theories of fundamental physics. Inflationary Universe models and topological defect theories are the most popular current paradigms for explaining the origin of structure in the Universe. In these lectures, I discuss various aspects of inflation and topological defects in which there has been interesting recent progress or in which there are outstanding problems. Particular emphasis is given to how baryogenesis scenarios can be influenced by inflation and topological defects.

hep-ph↗

arXiv1997

Particle Physics Aspects of Modern Cosmology

Modern cosmology has created a tight link between particle physics / field theory and a wealth of new observational data on the structure of the Universe. These lecture notes focus on some of the most important aspects concerning the connection between theory and observations. The lectures begin with an overview of some recent progress and problems in inflationary cosmology. In particular, a pedagogical discussion of the theory of reheating is presented. The second topic is a survey of the theory of cosmological perturbations, the cornerstone of modern cosmology. The focus is on the gauge-invariant classical and quantum theory of fluctuations. The third topic concerns the role of topological defects in cosmology. Reviews of the cosmic string theory of galaxy formation and of defect-mediated GUT and electroweak baryogenesis are given.

hep-ph↗

arXiv1996

Some Key Issues Confronting Inflationary Cosmology

Does inflation provide a compelling explanation for why the universe is so large, so flat, and so old, and a predictive theory of density perturbations? In this brief contribution (based on the role of the author as moderator of the discussion session on inflation), a list of some of the key issues confronting inflationary cosmology will be given, with the hope of focusing the debate on inflation and drawing more attention to some of the potential problems of the inflationary theory.

astro-ph↗

arXiv1996

Can Asymptotic Series Resolve the Problems of Inflation?

We discuss a cosmological scenario in which inflation is driven by a potential which is motivated by an effective Lagrangian approach to gravity. We exploit the recent arguments \cite{ARZ} that an effective Lagrangian $L_{eff}$ which, by definition, contains operators of arbitrary dimensionality is in general not a convergent but rather an asymptotic series with factorially growing coefficients. This behavior of the effective Lagrangian might be responsible for the resolution of the cosmological constant problem. We argue that the same behavior of the potential gives a natural realization of the inflationary scenario.

hep-ph↗

arXiv1996

Topological Defects and the Formation of Structure in the Universe

Topological defects, in particular cosmic strings, give rise to an interesting mechanism for generating the primordial perturbations in the early Universe which are required to explain the present structure. An overview of the cosmic string model will be given, focusing on the predictions of the theory for the large-scale structure of the Universe and for cosmic microwave background anisotropies.

astro-ph↗

arXiv1995

Physics of the Very Early Universe

In this report of the workshop on `Physics of the Very Early Universe' held at GR14, the recent status and open problems in a selected number of major areas of interest are reviewed, focusing on attempts to develop a superstring cosmology and on progress in understanding current theories of structure formation.

astro-ph↗

arXiv1995

Implementing Markov's Limiting Curvature Hypothesis

The effective action for gravity at high curvatures is likely to contain higher derivative terms. These corrections may have profound consequences for the singularity structure of space-time and for early Universe cosmology. In this contribution, recent work is reviewed which demonstrates that it is possible to construct a class of effective gravitational actions for which all solutions with sufficient symmetries have limited curvature and are nonsingular. Near the limiting curvature, the coupling between matter and gravity goes to zero and in this sense the theory is asymptotically free.

gr-qc↗

arXiv1995

Formation of Structure in the Universe

An introduction to modern theories for the origin of structure in the Universe is given. After a brief review of the growth of cosmological perturbations in an expanding Universe and a summary of some important observational results, the lectures focus on the inflationary Universe scenario and on topological defect models of structure formation. A summary of the theory and current observational status of cosmic microwave background temperature fluctuations is given. The final chapter is devoted to some speculative ideas concerning the connection between cosmology and fundamental physics, in particular to ways in which the singularity problem of classical cosmology may be resolved.

astro-ph↗

arXiv1995

ACCRETION OF HOT DARK MATTER ONTO SLOWLY MOVING COSMIC STRINGS

Cosmic strings with small-scale structure have a coarse-grained mass per unit length $μ$ which is larger than the string tension. This leads to an effective Newtonian gravitational line source and to a characteristic translational velocity which is smaller than for strings without small-scale structure. Here, the accretion of hot dark matter onto such strings is studied by means of the Zel'dovich approximation. We find that clustering is greatly enhanced by the Newtonian line source. In the limit of vanishing translational velocity, the first nonlinear filaments form at a redshift of greater than 100 for standard values of $μ$.

astro-ph↗

arXiv1995

NONSINGULAR COSMOLOGY AND PLANCK SCALE PHYSICS

New Planck scale physics may solve the singularity problems of classical general relativity and may lead to interesting consequences for very early Universe cosmology. Two approaches to these questions are reviewed in this article. The first is an effective action approach to including the effects of Planck scale physics in the basic framework of general relativity. It is shown that effective actions with improved singularity properties can be constructed. The second approach is based on superstring theory. A scenario which eliminates the big bang singularity and possibly explains the dimensionality of space-time is reviewed.

gr-qc↗

arXiv1994

Modern Cosmology and Structure Formation

An introduction to modern cosmology is given, focusing on theories for the origin of structure in the Universe. After a brief review of the theory of growth of cosmological perturbations and a summary of some important observational results, most of the article is devoted to discussions of the inflationary Universe scenario and of topological defect models of structure formation. The final chapter is a summary of the theory of cosmic microwave background temperature fluctuations and of the current observational status.

astro-ph↗

arXiv1994

Formation of Topological Defects in a Second Order Phase Transition

The classical evolution equations of the Abelian Higgs model are studied at temperatures below the Ginsburg temperature of a phase transition which is assumed to be second order. It is shown that the initial thermal fluctuations provide a domain structure which is stable against late time fluctuations. This result lends support to the Kibble mechanism for the formation of topological defects.

hep-ph↗