arXiv subjects

R. H. Brandenberger

Publications and source records attributed to R. H. Brandenberger.

10 recordsLinked to original sources

arXiv2006

On the Transfer of Metric Fluctuations when Extra Dimensions Bounce or Stabilize

In this report, we study within the context of general relativity with one extra dimension compactified either on a circle or an orbifold, how radion fluctuations interact with metric fluctuations in the three non-compact directions. The background is non-singular and can either describe an extra dimension on its way to stabilization, or immediately before and after a series of non-singular bounces. We find that the metric fluctuations transfer undisturbed through the bounces or through the transients of the pre-stabilization epoch. Our background is obtained by considering the effects of a gas of massless string modes in the context of a consistent 'massless background' (or low energy effective theory) limit of string theory. We discuss applications to various approaches to early universe cosmology, including the ekpyrotic/cyclic universe scenario and string gas cosmology.

hep-th↗

arXiv2004

Single Field Baryogenesis and the Scale of Inflation

In the context of inflationary cosmology, we discuss a minimal baryogenesis scenario in which the resulting baryon to entropy ratio is determined by the amplitude of the anisotropies of the cosmic microwave background. The model involves a new $SU(2)_L$ scalar field which generates a Dirac neutrino mass and which is excited by quantum fluctuations during inflation, yielding a CP-violating phase. During the scalar field decay after inflation, an asymmetry in the left-handed neutrino number is generated, which then converts to a net baryon asymmetry via sphalerons. A lower limit on the expected initial value of the scalar field translates to a lower limit on the baryon to entropy ratio (which also depends on the Dirac neutrino Yukawa coupling). Consistency with the limits on baryonic isocurvature perturbations requires that the spectral index of adiabatic perturbations produced during inflation be very close to unity. In a variant of our scenario in which the scalar is a gauge singlet, the connection between the baryon to entropy ratio and the inflationary scale is lost, although the basic mechanism of baryogenesis remains applicable.

hep-ph↗

arXiv2003

On the Dependence of the Spectra of Fluctuations in Inflationary Cosmology on Trans-Planckian Physics

We calculate the power spectrum of metric fluctuations in inflationary cosmology starting with initial conditions which are imposed mode by mode when the wavelength equals some critical length $\ell_{_{\rm C}}$ corresponding to a new energy scale $M_{_{\rm C}}$ at which trans-Planckian physics becomes important. In this case, the power spectrum can differ from what is calculated in the usual framework (which amounts to choosing the adiabatic vacuum state). The fractional difference in the results depends on the ratio $σ_0$ between the Hubble expansion rate $H_{\rm inf}$ during inflation and the new energy scale $M_{_{\rm C}}$. We show how and why different choices of the initial vacuum state (stemming from different assumptions about trans-Planckian physics) lead to fractional differences which depend on different powers of $σ_0$. As we emphasize, the power in general also depends on whether one is calculating the power spectrum of density fluctuations or of gravitational waves.

hep-th↗

arXiv2000

Cosmological Perturbations in Brane-World Theories: Formalism

We develop a gauge-invariant formalism to describe metric perturbations in five-dimensional brane-world theories. In particular, this formalism applies to models originating from heterotic M-theory. We introduce a generalized longitudinal gauge for scalar perturbations. As an application, we discuss some aspects of the evolution of fluctuations on the brane. Moreover, we show how the five-dimensional formalism can be matched to the known four-dimensional one in the limit where an effective four-dimensional description is appropriate.

hep-th↗

arXiv1999

High Energy Fluxes from a Non-Scaling Cosmic String Network

Topological defects, particularly cosmic strings, can provide a mechanism to produce particles with energies of the order 10^21 eV and higher. Here, we report on order of magnitude calculations of fluxes from a cosmic string network which evolves according to a new scenario according to which the main channel for energy loss is the particle production rather than gravitational radiation. We compare the predicted fluxes for protons (anti-protons) and neutrinos (anti-neutrinos) with observations of extremely high energy cosmic rays.

hep-ph↗

arXiv1998

Group Identification in N-Body Simulations: SKID and DENMAX Versus Friends-of-Friends

Three popular algorithms (FOF, DENMAX, and SKID) to identify halos in cosmological N-body simulations are compared with each other and with the predicted mass function from Press-Schechter theory. It is shown that the resulting distribution of halo masses strongly depends upon the choice of free parameters in the three algorithms, and therefore much care in their choice is needed. For many parameter values, DENMAX and SKID have the tendency to include in the halos particles at large distances from the halo center with low peculiar velocities. FOF does not suffer from this problem, and its mass distribution furthermore is reproduced well by the prediction from Press-Schechter theory.

astro-ph↗

arXiv1997

The Energy-Momentum Tensor for Cosmological Perturbations

We study the effective energy-momentum tensor (EMT) for cosmological perturbations and formulate the gravitational back-reaction problem in a gauge invariant manner. We analyze the explicit expressions for the EMT in the cases of scalar metric fluctuations and of gravitational waves and derive the resulting equations of state. The formalism is applied to investigate the back-reaction effects in chaotic inflation. We find that for long wavelength scalar and tensor perturbations, the effective energy density is negative and thus counteracts any pre-existing cosmological constant. For scalar perturbations during an epoch of inflation, the equation of state is de Sitter-like.

gr-qc↗

arXiv1997

Back Reaction of Gravitational Perturbations

The back reaction of gravitational perturbations in a homogeneous background is determined by an effective energy-momentum tensor quadratic in the perturbations. We show that this nonlinear feedback effect is important in the case of long wavelength scalar perturbations in inflationary universe models. We also show how to solve an old problem concerning the gauge dependence of the effective energy-momentum tensor of perturbations.

gr-qc↗

arXiv1993

Gamma Rays from Small Scale Structures on Long Cosmic Strings

The formation of cusps on long cosmic strings is discussed and the probability of cusp formation is estimated. The energy distribution of the gamma-ray background due to cusp annihilation on long strings is calculated and compared to observations. Under optimistic assumptions about the cusp formation rate, we find that strings with a mass per unit length $μ$ less than ${Gμ} = 10^{-14}$ will have an observable effect. However, it is shown that the gamma-ray bursters can not be attributed to long ordinary strings (or loops).

hep-ph↗

arXiv1993

Gamma Ray Bursts from Ordinary Cosmic Strings

We give an upper estimate for the number of gamma ray bursts from ordinary (non-superconducting) cosmic strings expected to be observed at terrestrial detectors. Assuming that cusp annihilation is the mechanism responsible for the bursts we consider strings arising at a GUT phase transition and compare our estimate with the recent BATSE results. Further we give a lower limit for the effective area of future detectors designed to detect the cosmic string induced flux of gamma ray bursts.

hep-ph↗