170 research outputs found
Do Fermi-LAT observations imply very large Lorentz factors in GRB outflows ?
Recent detections of GeV photons in a few GRBs by Fermi-LAT imply huge bulk
Lorentz factors to avoid a large gamma gamma optical depth at high energy.
Estimates can be as high as Gamma ~ 1000 in the most extreme cases. This puts
severe constraints on models of the central engine and the jet acceleration in
GRBs. These estimates are however obtained from a simplified single zone model.
We present here a more realistic calculation which takes into account the time,
space and direction dependent photon field existing in an outflow with several
relativistically moving emitting zones. The formalism is general and can be
applied to many models of the prompt GRB emission. We present results obtained
for a numerical implementation in the framework of the internal shock model. We
show that (i) the minimum Lorentz factor Gamma_min in bright LAT GRBs is
reduced by a factor ~ 2-3 compared to previous estimates if the GeV and MeV
emission are produced in the same region, and by an additional factor ~ 2-8 if
the GeV emission is produced at larger radii. We provide an improved
approximate formula for Gamma_min which is in good agreement with our numerical
results and can be directly applied to LAT GRB data; (ii) a delayed GeV onset
can be due to the time evolution of the opacity. As an illustration of these
two first results, we present a synthetic GRB that reproduces most features of
GRB 080916C with a mean Lorentz factor of ~ 340, an optically thin regime for
gamma gamma opacity at 3GeV in bin 'b', a variability timescale of 0.5s in the
MeV lightcurve and a delayed onset of ~ 5s of the GeV emission; (iii) the gamma
gamma opacity can smooth the short timescale variability in the GeV lightcurve.
This last result implies that the observed variability at high energy is not
necessarily a good test to distinguish between an internal and an external
origin for the GeV emission in GRBs. [abridged]Comment: 23 pages, 11 figure
Stochastic Effects in Hybrid Inflation
Hybrid inflation is a two field model where inflation ends due to an
instability. In the neighborhood of the instability point, the potential is
very flat and the quantum fluctuations dominate over the classical motion of
the inflaton and waterfall fields. In this article, we study this regime in the
framework of stochastic inflation. We numerically solve the two coupled
Langevin equations controlling the evolution of the fields and compute the
probability distributions of the total number of e-folds and of the inflation
exit point. Then, we discuss the physical consequences of our results, in
particular the question of how the quantum diffusion can affect the observable
prediction of hybrid inflation.Comment: 17 pages, 17 figures, match published versio
Cosmological Inflation and the Quantum Measurement Problem
According to cosmological inflation, the inhomogeneities in our universe are
of quantum mechanical origin. This scenario is phenomenologically very
appealing as it solves the puzzles of the standard hot big bang model and
naturally explains why the spectrum of cosmological perturbations is almost
scale invariant. It is also an ideal playground to discuss deep questions among
which is the quantum measurement problem in a cosmological context. Although
the large squeezing of the quantum state of the perturbations and the
phenomenon of decoherence explain many aspects of the quantum to classical
transition, it remains to understand how a specific outcome can be produced in
the early universe, in the absence of any observer. The Continuous Spontaneous
Localization (CSL) approach to quantum mechanics attempts to solve the quantum
measurement question in a general context. In this framework, the wavefunction
collapse is caused by adding new non linear and stochastic terms to the
Schroedinger equation. In this paper, we apply this theory to inflation, which
amounts to solving the CSL parametric oscillator case. We choose the
wavefunction collapse to occur on an eigenstate of the Mukhanov-Sasaki variable
and discuss the corresponding modified Schroedinger equation. Then, we compute
the power spectrum of the perturbations and show that it acquires a universal
shape with two branches, one which remains scale invariant and one with nS=4, a
spectral index in obvious contradiction with the Cosmic Microwave Background
(CMB) anisotropy observations. The requirement that the non-scale invariant
part be outside the observational window puts stringent constraints on the
parameter controlling the deviations from ordinary quantum mechanics...
(Abridged).Comment: References added, minor corrections, conclusions unchange
The stochastic spectator
We study the stochastic distribution of spectator fields predicted in different slow-roll inflation backgrounds. Spectator fields have a negligible energy density during inflation but may play an important dynamical role later, even giving rise to primordial density perturbations within our observational horizon today. During de-Sitter expansion there is an equilibrium solution for the spectator field which is often used to estimate the stochastic distribution during slow-roll inflation. However slow roll only requires that the Hubble rate varies slowly compared to the Hubble time, while the time taken for the stochastic distribution to evolve to the de-Sitter equilibrium solution can be much longer than a Hubble time. We study both chaotic (monomial) and plateau inflaton potentials, with quadratic, quartic and axionic spectator fields. We give an adiabaticity condition for the spectator field distribution to relax to the de-Sitter equilibrium, and find that the de-Sitter approximation is never a reliable estimate for the typical distribution at the end of inflation for a quadratic spectator during monomial inflation. The existence of an adiabatic regime at early times can erase the dependence on initial conditions of the final distribution of field values. In these cases, spectator fields acquire sub-Planckian expectation values. Otherwise spectator fields may acquire much larger field displacements than suggested by the de-Sitter equilibrium solution. We quantify the information about initial conditions that can be obtained from the final field distribution. Our results may have important consequences for the viability of spectator models for the origin of structure, such as the simplest curvaton models
Can power spectrum observations rule out slow-roll inflation?
The spectral index of scalar perturbations is an important observable that allows us to learn about inflationary physics. In particular, a detection of a significant deviation from a constant spectral index could enable us to rule out the simplest class of inflation models. We investigate whether future observations could rule out canonical single-field slow- roll inflation given the parameters allowed by current observational constraints. We find that future measurements of a constant running (or running of the running) of the spectral index over currently available scales are unlikely to achieve this. However, there remains a large region of parameter space (especially when considering the running of the running) for falsifying the assumed class of slow-roll models if future observations accurately constrain a much wider range of scales
Early precipitated micropyrite in microbialites: A time capsule of microbial sulfur cycling
Microbialites are organosedimentary rocks that have occurred throughout the Earth’s
history. The relationships between diverse microbial metabolic activities and isotopic
signatures in biominerals forming within these microbialites are key to understanding
modern biogeochemical cycles, but also for accurate interpretation of the geologic
record. Here, we performed detailed mineralogical investigations coupled with
NanoSIMS (Nanoscale Secondary Ion Mass Spectrometry) analyses of pyrite S
isotopes in mineralising microbial mats from two different environments, a hypersaline
lagoon (Cayo Coco, Cuba) and a volcanic alkaline crater lake (Atexcac, Mexico).
Both microbialite samples contain two distinct pyrite morphologies: framboids and
euhedral micropyrites, which display distinct ranges of δ34S values1. Considering
the sulfate-sulfur isotopic compositions associated with both environments, micropyrites display a remarkably narrow range
of Δpyr (i.e. Δpyr ≡ δ34SSO4 − δ34Spyr) between 56 and 62‰. These measured Δpyr values agree with sulfate-sulfide equilibrium
fractionation, as observed in natural settings characterised by low microbial sulfate reduction respiration rates. Moreover, the
distribution of S isotope compositions recorded in the studied micropyrites suggests that sulfide oxidation also occurred at
the microbialite scale. These results highlight the potential of micropyrites to capture signatures of microbial sulfur cycling
and show that S isotope composition in pyrites record primarily the local micro-environments induced by the microbialite
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