11 research outputs found
Unequal-mass boson-star binaries: Initial data and merger dynamics
We present a generalization of the curative initial data construction derived
for equal-mass compact binaries in Helfer {\it et al} (2019 Phys. Rev. D 99
044046; 2022 Class. Quantum Grav. 39 074001) to arbitrary mass ratios. We
demonstrate how these improved initial data avoid substantial spurious
artifacts in the collision dynamics of unequal-mass boson-star binaries in the
same way as has previously been achieved with the simpler method restricted to
the equal-mass case. We employ the improved initial data to explore in detail
the impact of phase offsets in the coalescence of equal- and unequal-mass boson
star binaries.Comment: 37 pages, 12 figures, to match published version in CQ
The Gravitational Afterglow of Boson Stars
In this work we study the long-lived post-merger gravitational wave signature
of a boson-star binary coalescence. We use full numerical relativity to
simulate the post-merger and track the gravitational afterglow over an extended
period of time. We implement recent innovations for the binary initial data,
which significantly reduce spurious initial excitations of the scalar field
profiles, as well as a measure for the angular momentum that allows us to track
the total momentum of the spatial volume, including the curvature contribution.
Crucially, we find the afterglow to last much longer than the spin-down
timescale. This prolonged gravitational wave afterglow provides a
characteristic signal that may distinguish it from other astrophysical sources.Comment: Movie: https://youtu.be/JE5FRG7kgvU Data:
https://github.com/ThomasHelfer/BosonStarAfterglo
GRChombo: An adaptable numerical relativity code for fundamental physics
GRChombo is an open-source code for performing Numerical Relativity time
evolutions, built on top of the publicly available Chombo software for the
solution of PDEs. Whilst GRChombo uses standard techniques in NR, it focusses
on applications in theoretical physics where adaptability, both in terms of
grid structure, and in terms of code modification, are key drivers
Black holes, gravitational waves and fundamental physics: a roadmap
The grand challenges of contemporary fundamental physics—dark matter, dark energy, vacuum energy, inflation and early universe cosmology, singularities and the hierarchy problem—all involve gravity as a key component. And of all gravitational phenomena, black holes stand out in their elegant simplicity, while harbouring some of the most remarkable predictions of General Relativity: event horizons, singularities and ergoregions.
The hitherto invisible landscape of the gravitational Universe is being unveiled before our eyes: the historical direct detection of gravitational waves by the LIGO-Virgo collaboration marks the dawn of a new era of scientific exploration. Gravitational-wave astronomy will allow us to test models of black hole formation, growth and evolution, as well as models of gravitational-wave generation and propagation. It will provide evidence for event horizons and ergoregions, test the theory of General Relativity itself, and may reveal the existence of new fundamental fields. The synthesis of these results has the potential to radically reshape our understanding of the cosmos and of the laws of Nature.
The purpose of this work is to present a concise, yet comprehensive overview of the state of the art in the relevant fields of research, summarize important open problems, and lay out a roadmap for future progress. This write-up is an initiative taken within the framework of the European Action on 'Black holes, Gravitational waves and Fundamental Physics'
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Numerical modelling of gravitational wave sources in general relativity
The first direct detection of gravitational waves (GWs) from a black-hole (BH) binary, GW150914, by the advanced Laser Interferometer Gravitational-wave Observatory (aLIGO) detectors in 2015 heralded a new era in GW physics. Since then, over 90 compact binary merger events have been detected by the GW detector network with many more expected in the decades to come. A significant part of the theoretical foundations that underpins this achievement is the modelling of GW sources in General Relativity (GR) using numerical relativity (NR). In this thesis, we discuss the features and capabilities of the NR code GRChombo. Although GRChombo is no longer a new code, its original development and design targeted applications beyond the conventional astrophysical paradigm that other NR codes have focussed on. Here, we describe more recent additions that have allowed GRChombo to model BH binaries and other GW sources with good accuracy. Through direct comparison, we demonstrate that this accuracy is comparable to that of a more mature NR code. One of the key capabilities of GRChombo is its adaptive mesh refinement (AMR). This allows the numerical grid to dynamically adjust itself in order to sufficiently resolve the large range of spatial and temporal scales that characteristically arise in non-trivial solutions of GR as a consequence of the theory’s non-linearity. However, this flexibility requires careful control in order to achieve the desired accuracy and we discuss in detail the lessons learned in order to achieve this with GRChombo. We apply GRChombo and these techniques to the investigation of the effect of orbital eccentricity on the GW emission and the gravitational recoil imparted to the BH merger remnant from the inspiral and merger of unequal-mass non-spinning BH binaries. Finally, we explore the modelling of a more exotic type of compact object: boson stars (BSs) which are comprised of complex scalar field matter. In particular, we investigate the construction of suitable initial data describing BS binaries and its effect on the ensuing evolutions
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Anomalies in the gravitational recoil of eccentric black-hole mergers with unequal mass ratios
The radiation of linear momentum imparts a recoil (or "kick") to the center of mass of a merging black hole binary system. Recent numerical relativity calculations have shown that eccentricity can lead to an approximate 25% increase in recoil velocities for equal-mass, spinning binaries with spins lying in the orbital plane ("superkick" configurations) [U Sperhake et al. Phys. Rev. D 101 (2020) 024044 (arXiv:1910.01598)]. Here we investigate the impact of nonzero eccentricity on the kick magnitude and gravitational-wave emission of nonspinning, unequal-mass black hole binaries. We confirm that nonzero eccentricities at merger can lead to kicks which are larger by up to ~25% relative to the quasicircular case. We also find that the kick velocity has an oscillatory dependence on eccentricity, that we interpret as a consequence of changes in the angle between the infall direction at merger and the apoapsis (or periapsis) direction
Malaise and remedy of binary boson-star initial data
Through numerical simulations of boson-star head-on collisions, we explore
the quality of binary initial data obtained from the superposition of
single-star spacetimes. Our results demonstrate that evolutions starting from a
plain superposition of individual boosted boson-star spacetimes are vulnerable
to significant unphysical artefacts. These difficulties can be overcome with a
simple modification of the initial data suggested in [PRD 99 (2018) 044046] for
collisions of oscillatons. While we specifically consider massive complex
scalar field boson star models up to a 6th-order-polynomial potential, we argue
that this vulnerability is universal and present in other kinds of exotic
compact systems and hence needs to be addressed
The critical layer in quadratic flow boundary layers over acoustic linings
A straight cylindrical duct is considered containing an axial mean flow that is uniform everywhere except within a boundary layer near the wall, which need not be thin. Within this boundary layer the mean flow varies parabolically. The linearized Euler equations are Fourier transformed to give the Pridmore-Brown equation, for which the Green's function is constructed using Frobenius series. The critical layer gives a non-modal contribution from the continuous spectrum branch cut, and dominates the downstream pressure perturbation in certain cases, particularly for thicker boundary layers. The continuous spectrum branch cut is also found to stabilize what are otherwise convectively unstable modes by hiding them behind the branch cut. Overall, the contribution from the critical layer is found to give a neutrally stable non-modal wave when the source is located within the sheared flow region, and to decay algebraically along the duct as O(x−5/2) for a source located with the uniform flow region. The Frobenius expansion, in addition to being numerically accurate close to the critical layer where other numerical methods lose accuracy, is also able to locate modal poles hidden behind the branch cut, which other methods are unable to find; this includes the stabilized hydrodynamic instability. Matlab code is provided to compute the Green's function