386 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
f(R) as a dark energy fluid
We study the equations for the evolution of cosmological perturbations in
and conclude that this modified gravity model can
be expressed as a dark energy fluid at background and linearised perturbation
order. By eliminating the extra scalar degree of freedom known to be present in
such theories, we are able to characterise the evolution of the perturbations
in the scalar sector in terms of equations of state for the entropy
perturbation and anisotropic stress which are written in terms of the density
and velocity perturbations of the dark energy fluid and those in the matter, or
the metric perturbations. We also do the same in the much simpler vector and
tensor sectors. In order to illustrate the simplicity of this formulation, we
numerically evolve perturbations in a small number of cases.Comment: 12 pages, 5 figure
Comparison of primordial tensor power spectra from the deformed algebra and dressed metric approaches in loop quantum cosmology
Loop quantum cosmology tries to capture the main ideas of loop quantum
gravity and to apply them to the Universe as a whole. Two main approaches
within this framework have been considered to date for the study of
cosmological perturbations: the dressed metric approach and the deformed
algebra approach. They both have advantages and drawbacks. In this article, we
accurately compare their predictions. In particular, we compute the associated
primordial tensor power spectra. We show -- numerically and analytically --
that the large scale behavior is similar for both approaches and compatible
with the usual prediction of general relativity. The small scale behavior is,
the other way round, drastically different. Most importantly, we show that in a
range of wavenumbers explicitly calculated, both approaches do agree on
predictions that, in addition, differ from standard general relativity and do
not depend on unknown parameters. These features of the power spectrum at
intermediate scales might constitute a universal loop quantum cosmology
prediction that can hopefully lead to observational tests and constraints. We
also present a complete analytical study of the background evolution for the
bouncing universe that can be used for other purposes.Comment: 15 pages, 7 figure
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