arXiv subjects

Béatrice Bonga

Publications and source records attributed to Béatrice Bonga.

44 records · Page 3Linked to original sources

arXiv2016

Inflation in the closed FLRW model and the CMB

Recent cosmic microwave background (CMB) observations put strong constraints on the spatial curvature via estimation of the parameter $Ω_k$ assuming an almost scale invariant primordial power spectrum. We study the evolution of the background geometry and gauge-invariant scalar perturbations in an inflationary closed FLRW model and calculate the primordial power spectrum. We find that the inflationary dynamics is modified due to the presence of spatial curvature, leading to corrections to the nearly scale invariant power spectrum at the end of inflation. When evolved to the surface of last scattering, the resulting temperature anisotropy spectrum ($C_{\ell}^{TT}$) shows deficit of power at low multipoles ($\ell<20$). By comparing our results with the recent Planck data we discuss the role of spatial curvature in accounting for CMB anomalies and in the estimation of the parameter $Ω_k$. Since the curvature effects are limited to low multipoles, the Planck estimation of cosmological parameters remains robust under inclusion of positive spatial curvature.

astro-ph.CO↗

arXiv2016

Inflation with the Starobinsky potential in Loop Quantum Cosmology

A self-consistent pre-inflationary extension of the inflationary scenario with the Starobinsky potential, favored by Planck data, is studied using techniques from loop quantum cosmology (LQC). The results are compared with the quadratic potential previously studied. Planck scale completion of the inflationary paradigm and observable signatures of LQC are found to be robust under the change of the inflaton potential. The entire evolution, from the quantum bounce all the way to the end of inflation, is compatible with observations. Occurrence of desired slow-roll phase is almost inevitable and natural initial conditions exist for both the background and perturbations for which the resulting power spectrum agrees with recent observations. There exist initial data for which the quantum gravitational corrections to the power spectrum are potentially observable.

gr-qc↗

arXiv2016

Cosmic variance in inflation with two light scalars

We examine the squeezed limit of the bispectrum when a light scalar with arbitrary non-derivative self-interactions is coupled to the inflaton. We find that when the hidden sector scalar is sufficiently light ($m\lesssim0.1\,H$), the coupling between long and short wavelength modes from the series of higher order correlation functions (from arbitrary order contact diagrams) causes the statistics of the fluctuations to vary in sub-volumes. This means that observations of primordial non-Gaussianity cannot be used to uniquely reconstruct the potential of the hidden field. However, the local bispectrum induced by mode-coupling from these diagrams always has the same squeezed limit, so the field's locally determined mass is not affected by this cosmic variance.

astro-ph.CO↗

arXiv2016

Phenomenological investigation of a quantum gravity extension of inflation with the Starobinsky potential

We investigate the pre-inflationary dynamics of inflation with the Starobinsky potential, favored by recent data from the Planck mission, using techniques developed to study cosmological perturbations on quantum spacetimes in the framework of loop quantum cosmology. We find that for a large part of the initial data, inflation compatible with observations occurs. There exists a subset of this initial data that leads to quantum gravity signatures that are potentially observable. Interestingly, despite the different inflationary dynamics, these quantum gravity corrections to the powerspectra are similar to those obtained for inflation with a quadratic potential, including suppression of power at large scales. Furthermore, for super horizon modes the tensor modes show deviations from the standard inflationary paradigm that are unique to the Starobinsky potential and could be important for non-Gaussian modulation and tensor fossils.

gr-qc↗

arXiv2015

Gravitational waves from isolated systems: Surprising consequences of a positive cosmological constant

There is a deep tension between the well-developed theory of gravitational waves from isolated systems and the presence of a positive cosmological constant $Λ$, however tiny. In particular, even the post-Newtonian quadrupole formula, derived by Einstein in 1918, has not been generalized to include a positive $Λ$. We first explain the principal difficulties and then show that it is possible to overcome them in the weak field limit. These results also provide concrete hints for constructing the $Λ>0$ generalization of the Bondi-Sachs framework for full, non-linear general relativity.

gr-qc↗

arXiv2015

Asymptotics with a positive cosmological constant: III. The quadrupole formula

Almost a century ago, Einstein used a weak field approximation around Minkowski space-time to calculate the energy carried away by gravitational waves emitted by a time changing mass-quadrupole. However, by now there is strong observational evidence for a positive cosmological constant, $Λ$. To incorporate this fact, Einstein's celebrated derivation is generalized by replacing Minkowski space-time with de Sitter space-time. The investigation is motivated by the fact that, because of the significant differences between the asymptotic structures of Minkowski and de Sitter space-times, many of the standard techniques, including the standard $1/r$ expansions, can not be used for $Λ>0$. Furthermore since, e.g., the energy carried by gravitational waves is always positive in Minkowski space-time but can be arbitrarily negative in de Sitter space-time \emph{irrespective of how small $Λ$ is}, the limit $Λ\to 0$ can fail to be continuous. Therefore, a priori it is not clear that a small $Λ$ would introduce only negligible corrections to Einstein's formula. We show that, while even a tiny cosmological constant does introduce qualitatively new features, in the end, corrections to Einstein's formula are negligible for astrophysical sources currently under consideration by gravitational wave observatories.

gr-qc↗

arXiv2015

Asymptotics with a positive cosmological constant: II. Linear fields on de Sitter space-time

Linearized gravitational waves in de Sitter space-time are analyzed in detail to obtain guidance for constructing the theory of gravitational radiation in presence of a positive cosmological constant in full, nonlinear general relativity. Specifically: i) In the exact theory, the intrinsic geometry of $\scri$ is often assumed to be conformally flat in order to reduce the asymptotic symmetry group from $\Diff$ to the de Sitter group. Our {results show explicitly} that this condition is physically unreasonable; ii) We obtain expressions of energy-momentum and angular momentum fluxes carried by gravitational waves in terms of fields defined at $\scrip$; iii) We argue that, although energy of linearized gravitational waves can be arbitrarily negative in general, gravitational waves emitted by physically reasonable sources carry positive energy; and, finally iv) We demonstrate that the flux formulas reduce to the familiar ones in Minkowski space-time in spite of the fact that the limit $Λ\to 0$ is discontinuous (since, in particular, $\scri$ changes its space-like character to null in the limit).

gr-qc↗

arXiv2014

Quantum astrometric observables II: time delay in linearized quantum gravity

A clock synchronization thought experiment is modeled by a diffeomorphism invariant "time delay" observable. In a sense, this observable probes the causal structure of the ambient Lorentzian spacetime. Thus, upon quantization, it is sensitive to the long expected smearing of the light cone by vacuum fluctuations in quantum gravity. After perturbative linearization, its mean and variance are computed in the Minkowski Fock vacuum of linearized gravity. The naïve divergence of the variance is meaningfully regularized by a length scale $μ$, the physical detector resolution. This is the first time vacuum fluctuations have been fully taken into account in a similar calculation. Despite some drawbacks this calculation provides a useful template for the study of a large class of similar observables in quantum gravity. Due to their large volume, intermediate calculations were performed using computer algebra software. The resulting variance scales like $(s \ell_p/μ)^2$, where $\ell_p$ is the Planck length and $s$ is the distance scale separating the ("lab" and "probe") clocks. Additionally, the variance depends on the relative velocity of the lab and the probe, diverging for low velocities. This puzzling behavior may be due to an oversimplified detector resolution model or a neglected second order term in the time delay.

gr-qc↗