73 research outputs found
Some conceptual issues in loop quantum cosmology
Loop quantum gravity is a mature theory. To proceed to explicit calculations
in cosmology, it is necessary to make assumptions and simplifications based on
the symmetries of the cosmological setting. Symmetry reduction is especially
critical when dealing with cosmological perturbations. The present article
reviews several approaches to the problem of building a consistent formalism
that describes the dynamics of perturbations on a quantum spacetime and tries
to address their respective strengths and weaknesses. We also review the main
open issues in loop quantum cosmology.Comment: Invited article for an IJMP volume dedicated to loop quantum gravit
Non-Gaussianity in Loop Quantum Cosmology
We extend the phenomenology of loop quantum cosmology (LQC) to second order
in perturbations. Our motivation is twofold. On the one hand, since LQC
predicts a cosmic bounce that takes place at the Planck scale, the second order
contributions could be large enough to jeopardize the validity of the
perturbative expansion on which previous results rest. On the other hand, the
upper bounds on primordial non-Gaussianity obtained by the Planck Collaboration
are expected to play a significant role on explorations of the LQC
phenomenology. We find that the bounce in LQC produces an enhancement of
non-Gaussianity of several orders of magnitude, on length scales that were
larger than the curvature radius at the bounce. Nonetheless, we find that one
can still rely on the perturbative expansion to make predictions about
primordial perturbations. We discuss the consequences of our results for LQC
and its predictions for the cosmic microwave background.Comment: Minor updates: current version matches the accepted PRD manuscrip
Observational Exclusion of a Consistent Quantum Cosmology Scenario
It is often argued that inflation erases all the information about what took
place before it started. Quantum gravity, relevant in the Planck era, seems
therefore mostly impossible to probe with cosmological observations. In
general, only very ad hoc scenarios or hyper fine-tuned initial conditions can
lead to observationally testable theories. Here we consider a well-defined and
well motivated candidate quantum cosmology model that predicts inflation. Using
the most recent observational constraints on the cosmic microwave background B
modes, we show that the model is excluded for all its parameter space, without
any tuning. Some important consequences are drawn for the deformed algebra
approach to loop quantum cosmology. We emphasize that neither loop quantum
cosmology in general nor loop quantum gravity are disfavored by this study but
their falsifiability is established.Comment: 5 pages, 2 figur
Can we neglect relativistic temperature corrections in the Planck thermal SZ analysis?
Measurements of the thermal Sunyaev-Zel'dovich (tSZ) effect have long been
recognized as a powerful cosmological probe. Here we assess the importance of
relativistic temperature corrections to the tSZ signal on the power spectrum
analysis of the Planck Compton- map, developing a novel formalism to account
for the associated effects. The amplitude of the tSZ power spectrum is found to
be sensitive to the effective electron temperature, , of the cluster
sample. Omitting the corresponding modifications leads to an underestimation of
the -power spectrum amplitude. Relativistic corrections thus add to the
error budget of tSZ power spectrum observables such as . This could
help alleviate the tension between various cosmological probes, with the
correction scaling as for Planck. At the current level of
precision, this implies a systematic shift by , which can also
be interpreted as an overestimation of the hydrostatic mass bias by , bringing it into better
agreement with hydrodynamical simulations. It is thus time to consider
relativistic temperature corrections in the processing of current and future
tSZ data.Comment: 6 pages, 4 figures, minor changes, updated to match version accepted
by MNRA
Improved calculations of electron–ion bremsstrahlung Gaunt factors for astrophysical applications
Electron-ion Bremsstrahlung (free-free) emission and absorption occur in many
astrophysical plasmas for a wide range of physical conditions. This classical
problem has been studied multiple times, and many analytical and numerical
approximations exist. However, accurate calculations of the transition from the
non-relativistic to the relativistic regime remain sparse. Here we provide a
comprehensive study of the free-free Gaunt factors for ions with low charge
(Z<=10). We compute the Gaunt factor using the expressions for the differential
cross section given by Elwert & Haug (EH) and compare to various limiting
cases. We develop a new software package, BRpack, for direct numerical
applications. This package uses a combination of pre-computed tables and
analytical approximations to efficiently cover a wide range of electron and
photon energies, providing a representation of the EH Gaunt factor to better
than 0.03% precision for Z<=2. Our results are compared to those of previous
studies highlighting the improvements achieved here. BRpack should be useful in
computations of spectral distortions of the cosmic microwave background,
radiative transfer problems during reionization or inside galaxy clusters, and
the modeling of galactic free-free foregrounds. The developed computational
methods can furthermore be extended to higher energies and ion charge.Comment: 12 pages + 5 pages appendix and references, 17 figures, to be
submitted to MNRAS, BRpack will be made available at www.chluba.de/BRpac
Primordial scalar power spectrum from the Euclidean Big Bounce
In effective models of loop quantum cosmology, the holonomy corrections are
associated with deformations of space-time symmetries. The most evident
manifestation of the deformations is the emergence of an Euclidean phase
accompanying the non-singular bouncing dynamics of the scale factor. In this
article, we compute the power spectrum of scalar perturbations generated in
this model, with a massive scalar field as the matter content. Instantaneous
and adiabatic vacuum-type initial conditions for scalar perturbations are
imposed in the contracting phase. The evolution through the Euclidean region is
calculated based on the extrapolation of the time direction pointed by the
vectors normal to the Cauchy hypersurface in the Lorentzian domains. The
obtained power spectrum is characterized by a suppression in the IR regime and
oscillations in the intermediate energy range. Furthermore, the speculative
extension of the analysis in the UV reveals a specific rise of the power.Comment: 13 pages, 4 figure
- …