181 research outputs found
Gravitational waves from compact binaries to the fourth post-Newtonian order
The precise knowledge of the gravitational phase of compact binaries is
crucial to the data analysis for gravitational waves. Until recently, it was
known analytically (for non-spinning systems) up to the 3.5 post-Newtonian (PN)
order, i.e. up to the correction beyond the leading order. If this
precision is sufficient for the data analysis of the current generation of
detectors, the next ones (notably LISA and ET) may require higher accuracy.
Using a post-Newtonian-multipolar-post-Minkowskian matching algorithm, we have
pushed the accuracy to the next level, namely the 4PN order. This derivation
involved challenging technical issues, due to the appearance of non-physical
divergences, that have to be properly regularized, as well as non-linear
interaction terms.Comment: Contribution to the 2023 Gravitation session of the 57th Rencontres
de Moriond. 4 pages, 1 figur
Where does curvaton reside? Differences between bulk and brane frames
Some classes of inflationary models naturally introduce two distinct
metrics/frames, and their equivalence in terms of observables has often been
put in question. D-brane inflation proposes candidates for an inflaton embedded
in the string theory and possesses descriptions on the brane and bulk
metrics/frames, which are connected by a conformal/disformal transformation
that depends on the inflaton and its derivatives. It has been shown that
curvature perturbations generated by the inflaton are identical in both frames,
meaning that observables such as the spectrum of cosmic microwave background
(CMB) anisotropies are independent of whether matter fields---including those
in the standard model of particle physics---minimally couple to the brane or
the bulk metric/frame. This is true despite the fact that the observables are
eventually measured by the matter fields and that the total action including
the matter fields is different in the two cases. In contrast, in curvaton
scenarios, the observables depend on the frame to which the curvaton minimally
couples. Among all inflationary scenarios, we focus on two models motivated by
the KKLMMT fine-tuning problem: a slow-roll inflation with an inflection-point
potential and a model of a rapidly rolling inflaton that conformally couples to
gravity. In the first model, the difference between the frames in which the
curvaton resides is encoded in the spectral index of the curvature
perturbations, depicting the nature of the frame transformation. In the second
model, the curvaton on the brane induces a spectral index significantly
different from that in the bulk and is even falsified by the observations. This
work thus demonstrates that two frames connected by a conformal/disformal
transformation lead to different physical observables such as CMB anisotropies
in curvaton models.Comment: 16 pages, v2: published versio
Conservative (failed)-tail effects at the fifth post-Newtonian order
This work deals with the tail and ``failed'' tail sectors of the conservative
dynamics for compact binary systems at the 5PN order. We employ the Fokker
Lagrangian method with dimensional regularization, and our results for the tail
sector are perfectly consistent with the previous EFT computations. As for the
``failed'' tail sector, we have good hopes that this new computation will help
solving the current discrepancy in the literature.Comment: 8 pages. v2: local report number added, harmless typo corrected in
Eq. (3.8
Upwind stabilization of Navier-Stokes solvers
We present a study of the effect of upwinding on stabilisation of both advective and pressure terms in a family of primitive-variable Navier-Stokes solvers. We consider two MUSCL schemes, the first one applies to compressible flow, the second one to incompressible flow. We illustrate the fact that both numerical models suffer oscillations if a minimal (but not large) amount of upwinding is not associated with acoustics, while advection can be stabilized by the physical diffusion terms when the mesh Reynolds number is small enough
The Quadrupole Moment of Compact Binaries to the Fourth post-Newtonian Order: Relating the Harmonic and Radiative Metrics
Motivated by the completion of the fourth post-Newtonian (4PN)
gravitational-wave generation from compact binary systems, we analyze and
contrast different constructions of the metric outside an isolated system,
using post-Minkowskian expansions. The metric in "harmonic" coordinates has
been investigated previously, in particular to compute tails and memory
effects. However, it is plagued by powers of the logarithm of the radial
distance when (with const). As a result, the tedious
computation of the "tail-of-memory" effect, which enters the gravitational-wave
flux at 4PN order, is more efficiently performed in the so-called "radiative"
coordinates, which admit a (Bondi-type) expansion at infinity in simple powers
of , without any logarithms. Here we consider a particular
construction, performed order by order in the post-Minkowskian expansion, which
directly yields a metric in radiative coordinates. We relate both
constructions, and prove that they are physically equivalent as soon as a
relation between the "canonical" moments which parametrize the radiative
metric, and those parametrizing the harmonic metric, is verified. We provide
the appropriate relation for the mass quadrupole moment at 4PN order, which
will be crucial when deriving the "tail-of-memory" contribution to the
gravitational flux.Comment: Updated a reference: Blanchet, Faye & Larrouturou 2022 in CQ
How to preserve the mass fractions positivity when computing compressible multi-component flows
Disponible dans les fichiers attachés à ce documen
Remarks on a model of A.Majda for combustion waves
Résumé disponible sur le PD
The equations of one-dimensional unsteady flame propagation : Existence and uniqueness
Disponible dans les fichiers attachés à ce documen
Semi-Implicit Roe-Type Fluxes for Low-Mach Number Flows
Two semi-implicit methods based on the splitting of the Euler equations flux into fluid and acoustic parts applied to low Mach number flows are presented. The first method is based on the splitting of slow and fast eigenvalues of the jacobian matrix of the fluxes and a semi-implicit scheme is constructed by introducing only the fast eigenvalues in the implicit matrices. The second method is based on the splitting of the Euler flux by separating the terms in velocity and the terms in pressure ; this system is solved by a fractional step method. A semi-implicit scheme is obtained by using a linearised implicit scheme for the acoustic step only. These two methods are applied to the convection of a density pulse for Mach numbers equal to 0.1 and 0.01. Accuracy and efficiency of the different schemes are compared
Electromagnetic fields in compact binaries: post-Newtonian wave generation and application to double white dwarfs systems
The aim of this work is twofold: (i) to properly define a wave-generation
formalism for compact-supported sources embedded in Einstein-Maxwell theory,
relying on matched post-Newtonian and multipolar-post-Minkowskian expansions;
(ii) to apply this formalism (which is valid for any type of post-Newtonian
sources) to the case of two stars with constant and aligned magnetic dipoles,
by computing the fluxes of energy and angular momentum to the next-to-leading
order, as well as the gravitational amplitude modes. Assuming eccentric orbits,
we derive the evolution of orbital parameters, as well as the observables of
the system, notably the gravitational phase for quasi-circular orbits. Finally,
we give some numerical estimates for the contribution of the magnetic dipoles
for some realistic systems.Comment: 44 page
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