2,718 research outputs found
Loop quantum cosmology, non-Gaussianity, and CMB power asymmetry
We argue that the anomalous power asymmetry observed in the cosmic microwave
background (CMB) may have originated in a cosmic bounce preceding inflation. In
loop quantum cosmology (LQC) the big bang singularity is generically replaced
by a bounce due to quantum gravitational effects. We compute the spectrum of
inflationary non-Gaussianity and show that strong correlation between
observable scales and modes with longer (super-horizon) wavelength arise as a
consequence of the evolution of perturbations across the LQC bounce. These
correlations are strongly scale dependent and induce a dipole-dominated
modulation on large angular scales in the CMB, in agreement with observations.Comment: 7 pages, 3 figur
Loop Quantum Cosmology
This Chapter provides an up to date, pedagogical review of some of the most
relevant advances in loop quantum cosmology. We review the quantization of
homogeneous cosmological models, their singularity resolution and the
formulation of effective equations that incorporate the main quantum
corrections to the dynamics. We also summarize the theory of quantized metric
perturbations propagating in those quantum backgrounds. Finally, we describe
how this framework can be applied to obtain a self-consistent extension of the
inflationary scenario to incorporate quantum aspects of gravity, and to explore
possible phenomenological consequences.Comment: To appear as a Chapter of "The Springer Handbook of Spacetime,"
edited by A. Ashtekar and V. Petkov. (Springer-Verlag, at Press). 52 pages, 5
figure
Unitarity and ultraviolet regularity in cosmology
Quantum field theory in curved space-times is a well developed area in
mathematical physics which has had important phenomenological applications to
the very early universe. However, it is not commonly appreciated that on time
dependent space-times ---including the simplest cosmological models--- dynamics
of quantum fields is not unitary in the standard sense. This issue is first
explained with an explicit example and it is then shown that a generalized
notion of unitarity does hold. The generalized notion allows one to correctly
pass to the Schr\"odinger picture starting from the Heisenberg picture used in
the textbook treatments. Finally, we indicate how these considerations can be
extended from simple cosmological models to general globally hyperbolic
space-timesComment: 30 pages, 0 figures. Version to be published in PR
Detailed analysis of the predictions of loop quantum cosmology for the primordial power spectra
We provide an exhaustive numerical exploration of the predictions of loop
quantum cosmology (LQC) with a post-bounce phase of inflation for the
primordial power spectrum of scalar and tensor perturbations. We extend
previous analysis by characterizing the phenomenologically relevant parameter
space and by constraining it using observations. Furthermore, we characterize
the shape of LQC-corrections to observable quantities across this parameter
space. Our analysis provides a framework to contrast more accurately the theory
with forthcoming polarization data, and it also paves the road for the
computation of other observables beyond the power spectra, such as
non-Gaussianity.Comment: 24 pages, 5 figure
An Extension of the Quantum Theory of Cosmological Perturbations to the Planck Era
Cosmological perturbations are generally described by quantum fields on
(curved but) classical space-times. While this strategy has a large domain of
validity, it can not be justified in the quantum gravity era where curvature
and matter densities are of Planck scale. Using techniques from loop quantum
gravity, the standard theory of cosmological perturbations is extended to
overcome this limitation. The new framework sharpens conceptual issues by
distinguishing between the true and apparent trans-Planckian difficulties and
provides sufficient conditions under which the true difficulties can be
overcome within a quantum gravity theory. In a companion paper, this framework
is applied to the standard inflationary model, with interesting implications to
theory as well as observations.Comment: 50 pages, no figures. This is first of the two detailed papers which
form the basis of Phys. Rev. Lett. 109, 251301 (2012). A few references and
clarifications added. Version to appear in Phys. Rev.
Preferred instantaneous vacuum for linear scalar fields in cosmological space-times
We discuss the problem of defining a preferred vacuum state at a given time
for a quantized scalar field in Friedmann, Lema\^itre, Robertson Walker (FLRW)
space-time. Among the infinitely many homogeneous, isotropic vacua available in
the theory, we show that there exists at most one for which every Fourier mode
makes vanishing contribution to the adiabatically renormalized energy-momentum
tensor at any given instant. For massive fields such a state exists in the most
commonly used backgrounds in cosmology, and provides a natural candidate for
the ground state at that instant of time. The extension to the massless and the
conformally coupled case are also discussed.Comment: 19 pages, 4 figures. Section VI was expanded to include a discussion
on semi-classical gravity. Version to appear in PR
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
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