arXiv subjects

Robert Brandenberger

Publications and source records attributed to Robert Brandenberger.

212 records · Page 12Linked to original sources

arXiv2002

Noncommutative Spacetime, Stringy Spacetime Uncertainty Principle, and Density Fluctuations

We propose a variation of spacetime noncommutative field theory to realize the stringy spacetime uncertainty relation without breaking any of the global symmetries of the homogeneous isotropic universe. We study the spectrum of metric perturbations in this model for a wide class of accelerating background cosmologies. Spacetime noncommutativity leads to a coupling between the fluctuation modes and the background cosmology which is nonlocal in time. For each mode, there is a critical time at which the spacetime uncertainty relation is saturated. This is the time when the mode is generated. These effects lead to a spectrum of fluctuations whose spectral index is different from what is obtained for commutative spacetime in the infrared region, but is unchanged in the ultraviolet region. In the special case of an exponentially expanding background, we find a scale-invariant spectrum. but with a different magnitude than in the context of commutative spacetime if the Hubble constant is above the string scale.

hep-th↗

arXiv2001

Loitering Phase in Brane Gas Cosmology

Brane Gas Cosmology (BGC) is an approach to M-theory cosmology in which the initial state of the Universe is taken to be small, dense and hot, with all fundamental degrees of freedom near thermal equilibrium. Such a starting point is in close analogy with the Standard Big Bang (SBB) model. The topology of the Universe is assumed to be toroidal in all nine spatial dimensions and is filled with a gas of p-branes. The dynamics of winding modes allow, at most, three spatial dimensions to become large, thus explaining the origin of our macroscopic 3+1-dimensional Universe. Here we conduct a detailed analysis of the loitering phase of BGC. We do so by including into the equations of motion that describe the dilaton gravity background some new equations which determine the annihilation of string winding modes into string loops. Specific solutions are found within the model that exhibit loitering, i.e. the Universe experiences a short phase of slow contraction during which the Hubble radius grows larger than the physical extent of the Universe. As a result the brane problem (generalized domain wall problem) in BGC is solved. The initial singularity and horizon problems of the SBB scenario are solved without relying on an inflationary phase.

hep-th↗

arXiv2001

Termination of the Phase of Quintessence by Gravitational Back-Reaction

We study the effects of gravitational back-reaction in models of Quintessence. The effective energy-momentum tensor with which cosmological fluctuations back-react on the background metric will in some cases lead to a termination of the phase of acceleration. The fluctuations we make use of are the perturbations in our present Universe. Their amplitude is normalized by recent measurements of anisotropies in the cosmic microwave background, their slope is taken to be either scale-invariant, or characterized by a slightly blue tilt. In the latter case, we find that the back-reaction effect of fluctuations whose present wavelength is smaller than the Hubble radius but which are stretched beyond the Hubble radius by the accelerated expansion during the era of Quintessence domination can become large. Since the back-reaction effects of these modes oppose the acceleration, back-reaction will lead to a truncation of the period of Quintessence domination. This result impacts on the recent discussions of the potential incompatibility between string theory and Quintessence.

hep-ph↗

arXiv2001

Non-Commutative Inflation

We show how a radiation dominated universe subject to space-time quantization may give rise to inflation as the radiation temperature exceeds the Planck temperature. We consider dispersion relations with a maximal momentum (i.e. a mimimum Compton wavelength, or quantum of space), noting that some of these lead to a trans-Planckian branch where energy increases with decreasing momenta. This feature translates into negative radiation pressure and, in well-defined circumstances, into an inflationary equation of state. We thus realize the inflationary scenario without the aid of an inflaton field. As the radiation cools down below the Planck temperature, inflation gracefully exits into a standard Big Bang universe, dispensing with a period of reheating. Thermal fluctuations in the radiation bath will in this case generate curvature fluctuations on cosmological scales whose amplitude and spectrum can be tuned to agree with observations.

hep-th↗

arXiv2001

Super-Hubble Nonlinear Perturbations During Inflation

We show that the non-linear evolution of long wavelength perturbations may be important in a wide class of inflationary scenarios. We develop a solution for the evolution of such nonlinear perturbations which is exact to first order in a gradient expansion. As a first application, we demonstrate that in single field models of inflation there can be no parametric amplification of super-Hubble modes during reheating. We consider the implications of the solution for recent discussions of the back-reaction effect of long wavelength perturbations on the background geometry, give a new derivation of the equation of motion of stochastic inflation, and demonstrate that if the (generalized) slow-rolling condition is not satisfied, then inevitably long wavelength vector modes for gravitational fluctuations will be generated.

gr-qc↗

arXiv2000

Attractive Forces Between Global Monopoles and Domain Walls

We study the interaction between stable monopoles and domain walls in a $SO(3) \times Z(2)$ scalar field theory. Numerical simulations reveal that there is an attractive force between the monopole and the wall, but that after the monopole and the wall collide, the monopole does not unwind. We present an analytic explanation for the origin of the attractive force, and conclude that this is a generic feature of monopole-wall interactions which does not depend on the detailed structure of the model. The existence of the attractive force supports the hypothesis of Dvali {\em et al.} (hep-ph/9710301), who proposed that monopoles can be ``swept up'' by domain walls, thereby alleviating or solving the monopole problem associated with phase transitions occurring after or in the absence of inflation.

hep-ph↗

arXiv2000

Cosmic defects and cosmology

We provide a pedagogical overview of defect models of structure formation. We first introduce the concept of topological defect, and describe how to classify them. We then show how defects might be produced in phase transitions in the Early Universe and approach non-pathological scaling solutions. A very heuristic account of structure formation with defects is then provided, following which we introduce the tool box required for high precision calculations of CMB and LSS power spectra in these theories. The decomposition into scalar vector and tensor modes is reviewed, and then we introduce the concept of unequal-time correlator. We use isotropy and causality to constrain the form of these correlators. We finally show how these correlators may be decomposed into eigenmodes, thereby reducing a defect problem to a series of ``inflation'' problems. We conclude with a short description of results in these theories and how they fare against observations. We finally describe yet another application of topological defects in cosmology: baryogenesis.

astro-ph↗

arXiv1999

On the Interaction of Monopoles and Domain Walls

We study the interaction between monopoles and embedded domain walls in a O(3) linear sigma model. We discover that there is an attractive force between the monopole and the wall. We provide evidence that after the monopole and domain wall collide, the monopole unwinds on the wall, and that the winding number spreads out on the surface. These results support the suggestion by Dvali et al. (hep-ph/9710301) that the monopole problem can be solved by monopole-domain wall interactions.

hep-ph↗

arXiv1996

The Impossibility of Baryogenesis at a Second Order Electroweak Phase Transition

We investigate whether baryogenesis is possible at a second order electroweak phase transition. We find that under rather general conditions, the departure from thermal equilibrium is suppressed by the expansion rate of the Universe, and hence baryon production is also suppressed by the expansion rate. We conclude that if no additional sources of departure from thermal equilibrium such as topological defects are present, then electroweak baryogenesis is ruled out if the phase transition is second order or crossover. However, a non-vanishing net baryon to entropy ratio {\it is} generated, and we provide both upper and lower bounds for the result. Our technique is also applicable to other baryogenesis mechanisms taking place during and immediately after a second order phase transition. We estimate the lowest value of the transition energy scale for which the resulting baryon to entropy ratio might be large enough to explain observations.

hep-ph↗

arXiv1995

Dynamical Breaking of CPT Symmetry in Defect Networks and Baryogenesis

Based on a study of {\it charge,} (C), {\it parity} (P) and {\it time reversal} (T) symmetries we show how a CP violating network of defects in the early Universe may bias baryon number production. A static network, even though it violates CP, respects CPT and hence does not bias baryon number production. On the other hand, the {\it ordering dynamics\/} of defects in a network, governed by the interplay of string tension, friction, inertia and the expansion of the Universe, results in the dynamical breakdown of CPT symmetry and may lead to a net baryon number production.

hep-ph↗

arXiv1995

Nucleosynthesis Constraints on Defect-Mediated Electroweak Baryogenesis

In the defect-mediated electroweak baryogenesis scenario, baryons are produced in well separated regions of space. It is shown that between the electroweak phase transition at a temperature of $T \sim 100 {\rm GeV}$ and the end of nucleosynthesis at $T \sim 1 {\rm KeV}$ the baryon inhomogeneities dissipate, and that no constraints on defect-mediated electroweak baryogenesis can be derived from considerations of inhomogeneous nucleosynthesis.

astro-ph↗

arXiv1994

Local and Nonlocal Defect-Mediated Electroweak Baryogenesis

We consider the effects of particle transport in the topological defect-mediated electroweak baryogenesis scenarios of Ref. 1. We analyze the cases of both thin and thick defects and demonstrate an enhancement of the original mechanism in both cases due to an increased effective volume in which baryogenesis occurs. This phenomenon is a result of imperfect cancellation between the baryons and antibaryons produced on opposite faces of the defect.

hep-ph↗

arXiv1994

Signatures of Cosmic Strings on the Microwave Background

ABSRACT: We review recent progress on testing the hypothesis of the existence of cosmic string perturbations in microwave background maps. Using an analytical model for the string network we show that the predicted amplitude and spectrum of MBR fluctuations are consistent with the COBE data for a reasonable value of the single free parameter of the string model (the mass per unit length of the string $μ$). To {\it distinguish} the predictions of cosmic strings from those of Gaussian models it is necessary to apply specific {\it statistical tests} which are sensitive to non-random phases of the MBR temperature maps. We discuss two such statistical tests: First, the probability distribution and the moments of fluctuations ${{δT}\over T}$ and second the corresponding statistical quantities for the {\it gradient} of fluctuations along a fixed direction. We show that the second statistic can detect cosmic string specific signatures on MBR temperature maps with resolution of a few arcminutes or smaller.

astro-ph↗

arXiv1993

Formation of Topological Defects at First Order Phase Transitions

We analyze the evolution of scalar and gauge fields during first order phase transitions and show how the Kibble mechanism for the formation of topological defects emerges from the underlying dynamics, paying particular attention to problems posed by gauge invariance when a local symmetry is spontaneously broken. We discuss also the application of the mechanism to semilocal defects and electroweak strings.

hep-ph↗