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# Publikationen seit 2015

2019 | Compatibility complex for black hole spacetimes | Steffen Aksteiner, Lars Andersson, Thomas Bäckdahl, Igor Khavkine, Bernard Whiting Link zum Preprint

### Compatibility complex for black hole spacetimes

#### Autoren: Steffen Aksteiner, Lars Andersson, Thomas Bäckdahl, Igor Khavkine, Bernard Whiting (2019)

The set of local gauge invariant quantities for linearized gravity on the Kerr spacetime presented by two of the authors (S.A, T.B.) in (arXiv:1803.05341) is shown to be complete. In particular, any gauge invariant quantity for linearized gravity on Kerr that is local and of finite order in derivatives can be expressed in terms of these gauge invariants and derivatives thereof. The proof is carried out by constructing a complete compatibility complex for the Killing operator, and demonstrating the equivalence of the gauge invariants from (arXiv:1803.05341) with the first compatibility operator from that complex.

2019 | Stability for linearized gravity on the Kerr spacetime | Lars Andersson, Thomas Bäckdahl, Pieter Blue, Siyuan Ma Link zum Preprint

### Stability for linearized gravity on the Kerr spacetime

#### Autoren: Lars Andersson, Thomas Bäckdahl, Pieter Blue, Siyuan Ma (2019)

In this paper we prove integrated energy and pointwise decay estimates for solutions of the vacuum linearized Einstein equation on the Kerr black hole exterior. The estimates are valid for the full, subextreme range of Kerr black holes, provided integrated energy estimates for the Teukolsky Master Equation holds. For slowly rotating Kerr backgrounds, such estimates are known to hold, due to the work of one of the authors arXiv:1708.07385. The results in this paper thus provide the first stability results for linearized gravity on the Kerr background, in the slowly rotating case, and reduce the linearized stability problem for the full subextreme range to proving integrated energy estimates for the Teukolsky equation. This constitutes an essential step towards a proof of the black hole stability conjecture, i.e. the statement that the Kerr family is dynamically stable, one of the central open problems in general relativity.
The proof relies on three key steps. First, there are energy decay estimates for the Teukolsky equation, proved by applying weighted multiplier estimates to a system of spin-weighted wave equations derived from the Teukolsky equation, and making use of the pigeonhole principle for the resulting hierarchy of weighted energy estimates. Second, working in the outgoing radiation gauge, the linearized Einstein equations are written as a system of transport equations, driven by one of the Teukolsky scalars. Third, expansions for the relevant curvature, connection, and metric components can be made near null infinity. An analysis of the dynamics on future null infinity, together with the Teukolsky Starobinsky Identity plays an important role in the argument.

2019 | An overview of the gravitational spin Hall effect | Marius A. Oancea, Claudio F. Paganini, Jérémie Joudioux, Lars Andersson Link zum Preprint

### An overview of the gravitational spin Hall effect

#### Autoren: Marius A. Oancea, Claudio F. Paganini, Jérémie Joudioux, Lars Andersson (2019)

In General Relativity, the propagation of electromagnetic waves is usually described by the vacuum Maxwell's equations on a fixed curved background. In the limit of infinitely high frequencies, electromagnetic waves can be localized as point particles, following null geodesics. However, at finite frequencies, electromagnetic waves can no longer be treated as point particles following null geodesics, and the spin angular momentum of light comes into play, via the spin-curvature coupling. We will refer to this effect as the gravitational spin Hall effect of light. Here, we review a series of theoretical results related to the gravitational spin Hall effect of light, and we compare the predictions of different models. The analogy with the spin Hall effect in Optics is also explored, since in this field the effect is well understood, both theoretically and experimentally.

2019 | The Zilch Electromagnetic Conservation Law in Variational Characteristic Form | Sajad Aghapour, Lars Andersson, Kjell Rosquist Link zum Preprint

### The Zilch Electromagnetic Conservation Law in Variational Characteristic Form

In this paper we consider the zilch conservation laws for Maxwell theory and demonstrate that in the duality-symmetric version of Maxwell theory, the zilch arises as a Noether current for a variational symmetry of the duality symmetric Lagrangian which we identify through an application of the reverse of the Noether theorem. A variational symmetry leaves Lagrangian invariant up to a total divergence, without restricting to solutions of the field equations. This fact was previously known only for the so-called chirality current, i.e. the 00-component of zilch.

2019 | On the asymptotic behavior of static perfect fluids | Lars Andersson, Annegret Y. Burtscher Zeitschrift: Annales Henri Poincaré Verlag: Springer Seiten: 813-857 Band: 20, no. 3 Link zur Publikation, Link zum Preprint

### On the asymptotic behavior of static perfect fluids

#### Autoren: Lars Andersson, Annegret Y. Burtscher (2019)

Static spherically symmetric solutions to the Einstein-Euler equations with prescribed central densities are known to exist, be unique and smooth for reasonable equations of state. Some criteria are also available to decide whether solutions have finite extent (stars with a vacuum exterior) or infinite extent. In the latter case, the matter extends globally with the density approaching zero at infinity. The asymptotic behavior largely depends on the equation of state of the fluid and is still poorly understood. While a few such unbounded solutions are known to be asymptotically flat with finite ADM mass, the vast majority are not. We provide a full geometric description of the asymptotic behavior of static spherically symmetric perfect fluid solutions with linear and polytropic-type equations of state. In order to capture the asymptotic behavior we introduce a notion of scaled quasi-asymptotic flatness, which encodes a form of asymptotic conicality. In particular, these spacetimes are asymptotically simple.

Zeitschrift:
Annales Henri Poincaré
Verlag:
Springer
Seiten:
813-857
Band:
20, no. 3

2019 | New identities for linearized gravity on the Kerr spacetime | Steffen Aksteiner, Lars Andersson, Thomas Bäckdahl Zeitschrift: Physical Review D Verlag: American Physical Society Seiten: 044043 Band: 99 Link zur Publikation, Link zum Preprint

### New identities for linearized gravity on the Kerr spacetime

#### Autoren: Steffen Aksteiner, Lars Andersson, Thomas Bäckdahl (2019)

In this paper we derive a differential identity for linearized gravity on the Kerr spacetime and more generally on vacuum spacetimes of Petrov type D. We show that a linear combination of second derivatives of the linearized Weyl tensor can be formed into a complex symmetric 2-tensor Mab which solves the linearized Einstein equations. The identity makes this manifest by relating Mab to two terms solving the linearized Einstein equations by construction. The self-dual Weyl curvature of Mab gives a covariant version of the Teukolsky-Starobinsky identities for linearized gravity which, in addition to the two classical identities for linearized Weyl scalars with extreme spin weights, includes three additional equations. In particular, they are not consequences of the classical Teukolsky-Starobinsky identities, but are additional integrability conditions for linearized gravity. The result has direct application in the construction of symmetry operators and also yields a set of nontrivial gauge invariants for linearized gravity.

Zeitschrift:
Physical Review D
Verlag:
American Physical Society
Seiten:
044043
Band:
99

2018 | On the geometry and topology of initial data sets with horizons | Lars Andersson, Mattias Dahl, Gregory J. Galloway, Daniel Pollack Zeitschrift: Asian J. Math. Verlag: International Press Seiten: 863-881 Band: 22, no. 5 Link zur Publikation, Link zum Preprint

### On the geometry and topology of initial data sets with horizons

#### Autoren: Lars Andersson, Mattias Dahl, Gregory J. Galloway, Daniel Pollack (2018)

We study the relationship between initial data sets with horizons and the existence of metrics of positive scalar curvature. We define a Cauchy Domain of Outer Communications (CDOC) to be an asymptotically flat initial set (M,g,K) such that the boundary ∂M of M is a collection of Marginally Outer (or Inner) Trapped Surfaces (MOTSs and/or MITSs) and such that M\∂M contains no MOTSs or MITSs. This definition is meant to capture, on the level of the initial data sets, the well known notion of the domain of outer communications (DOC) as the region of spacetime outside of all the black holes (and white holes). Our main theorem establishes that in dimensions 3 ≤ n ≤ 7, a CDOC which satisfies the dominant energy condition and has a strictly stable boundary has a positive scalar curvature metric which smoothly compactifies the asymptotically flat end and is a Riemannian product metric near the boundary where the cross sectional metric is conformal to a small perturbation of the initial metric on the boundary ∂M induced by g. This result may be viewed as a generalization of Galloway and Schoen's higher dimensional black hole topology theorem to the exterior of the horizon. We also show how this result leads to a number of topological restrictions on the CDOC, which allows one to also view this as an extension of the initial data topological censorship theorem, established by Eichmair, Galloway, and Pollack in dimension n = 3, to higher dimensions.

Zeitschrift:
Asian J. Math.
Verlag:
International Press
Seiten:
863-881
Band:
22, no. 5

2018 | Helicity and spin conservation in Maxwell theory and Linearized Gravity | Lars Andersson, Sajad Aghapour, Reebhu Bhattacharyya Link zum Preprint

### Helicity and spin conservation in Maxwell theory and Linearized Gravity

A duality symmetric formulation of linearized gravity has been introduced by Barnett and used to show the conservation of helicity. However, the relation between helicity and spin as well as the separate conservation of the spin and orbital parts of angular momentum, which is known to hold in Maxwell theory, was not considered. These conservation laws are known to follow from the conservation of the so-called helicity array, an analog of the zilch tensor, which includes helicity, spin, and spin-flux or infra-zilch. In the present paper we prove the conservation of spin and orbital angular momentum for linearized gravity on Minkowski space, and construct the analog of the helicity array for linearized gravity.

2018 | Hidden symmetries and decay for the Vlasov equation on the Kerr spacetime | Lars Andersson, Pieter Blue, Jeremie Joudioux Zeitschrift: Commun. Part. Diff. Eq. Verlag: Taylor & Francis Seiten: 47-65 Band: 43, no. 1 Link zur Publikation, Link zum Preprint

### Hidden symmetries and decay for the Vlasov equation on the Kerr spacetime

#### Autoren: Lars Andersson, Pieter Blue, Jeremie Joudioux (2018)

This paper proves the existence of a bounded energy and integrated energy decay for solutions of the massless Vlasov equation in the exterior of a very slowly rotating Kerr spacetime. This combines methods previously developed to prove similar results for the wave equation on the exterior of a very slowly rotating Kerr spacetime with recent work applying the vector-field method to the relativistic Vlasov equation.

Zeitschrift:
Commun. Part. Diff. Eq.
Verlag:
Taylor & Francis
Seiten:
47-65
Band:
43, no. 1

2018 | Wave and Dirac equations on manifolds | Lars Andersson, Christian Bär Verlag: de Gruyter Buchtitel: Space - Time - Matter Seiten: 324-348 Link zur Publikation, Link zum Preprint

### Wave and Dirac equations on manifolds

#### Autoren: Lars Andersson, Christian Bär (2018)

We review some recent results on geometric equations on Lorentzian manifolds such as the wave and Dirac equations. This includes well-posedness and stability for various initial value problems, as well as results on the structure of these equations on black-hole spacetimes (in particular, on the Kerr solution), the index theorem for hyperbolic Dirac operators and properties of the class of Green-hyperbolic operators.

Verlag:
de Gruyter
Buchtitel:
Space - Time - Matter
Seiten:
324-348

2018 | Geometry of black hole spacetimes | Lars Andersson, Thomas Bäckdahl, Pieter Blue Reihe: London Mathematical Society Lecture Note Series Verlag: Cambridge University Press Buchtitel: T. Daudé, D. Häfner, J.-P. Nicolas (eds.): Asymptotic Analysis in General Relativity Seiten: 9-85 Band: 443 Link zur Publikation, Link zum Preprint

### Geometry of black hole spacetimes

#### Autoren: Lars Andersson, Thomas Bäckdahl, Pieter Blue (2018)

These notes, based on lectures given at the summer school on Asymptotic Analysis in General Relativity, collect material on the Einstein equations, the geometry of black hole spacetimes, and the analysis of fields on black hole backgrounds. The Kerr model of a rotating black hole in vacuum is expected to be unique and stable. The problem of proving these fundamental facts provides the background for the material presented in these notes.
Among the many topics which are relevant for the uniqueness and stability problems are the theory of fields on black hole spacetimes, in particular for gravitational perturbations of the Kerr black hole, and more generally, the study of nonlinear field equations in the presence of trapping. The study of these questions requires tools from several different fields, including Lorentzian geometry, hyperbolic differential equations and spin geometry, which are all relevant to the black hole stability problem.

Reihe:
London Mathematical Society Lecture Note Series
Verlag:
Cambridge University Press
Buchtitel:
T. Daudé, D. Häfner, J.-P. Nicolas (eds.): Asymptotic Analysis in General Relativity
Seiten:
9-85
Band:
443

2017 | Mode stability on the real axis | Lars Andersson, Siyuan Ma, Claudio Paganini, Bernard F. Whiting Zeitschrift: J. Math. Phys. Verlag: AIP Publishing Seiten: 072501 Band: 58, no. 7 Link zur Publikation, Link zum Preprint

### Mode stability on the real axis

#### Autoren: Lars Andersson, Siyuan Ma, Claudio Paganini, Bernard F. Whiting (2017)

A generalization of the mode stability result of Whiting (1989) for the Teukolsky equation is proved for the case of real frequencies. The main result of the paper states that a separated solution of the Teukolsky equation governing massless test fields on the Kerr spacetime, which is purely outgoing at infinity, and purely ingoing at the horizon, must vanish. This has the consequence, that for real frequencies, there are linearly independent fundamental solutions of the radial Teukolsky equation which are purely ingoing at the horizon, and purely outgoing at infinity, respectively. This fact yields a representation formula for solutions of the inhomogenous Teukolsky equation.

Zeitschrift:
J. Math. Phys.
Verlag:
AIP Publishing
Seiten:
072501
Band:
58, no. 7

2017 | Global Regularity for the 2+1 Dimensional Equivariant Einstein-Wave Map System | Lars Andersson, Nishanth Gudapati, Jeremie Szeftel Zeitschrift: Ann. PDE Verlag: Springer Seiten: Art. 13 Band: 3, no. 2 Link zur Publikation, Link zum Preprint

### Global Regularity for the 2+1 Dimensional Equivariant Einstein-Wave Map System

#### Autoren: Lars Andersson, Nishanth Gudapati, Jeremie Szeftel (2017)

In this paper we consider the equivariant 2+1 dimensional Einstein-wave map system and show that if the target satisfies the so called Grillakis condition, then global existence holds. In view of the fact that the 3+1 vacuum Einstein equations with a spacelike translational Killing field reduce to a 2+1 dimensional Einstein-wave map system with target the hyperbolic plane, which in particular satisfies the Grillakis condition, this work proves global existence for the equivariant class of such spacetimes.

Zeitschrift:
Ann. PDE
Verlag:
Springer
Seiten:
Art. 13
Band:
3, no. 2

2017 | Nonlinear stability of the Milne model with matter | Lars Andersson, David Fajman Link zum Preprint

### Nonlinear stability of the Milne model with matter

#### Autoren: Lars Andersson, David Fajman (2017)

We show that any 3+1-dimensional Milne model is future nonlinearly, asymptotically stable in the set of solutions to the Einstein-Vlasov system. For the analysis of the Einstein equations we use the constant-mean-curvature-spatial-harmonic gauge. For the distribution function the proof makes use of geometric L^2-estimates based on the Sasaki-metric. The resulting estimates on the energy momentum tensor are then upgraded by employing the natural continuity equation for the energy density.

2017 | Morawetz estimate for linearized gravity in Schwarzschild | Lars Andersson, Pieter Blue, Jinhua Wang Link zum Preprint

### Morawetz estimate for linearized gravity in Schwarzschild

#### Autoren: Lars Andersson, Pieter Blue, Jinhua Wang (2017)

The equations governing the perturbations of the Schwarzschild metric satisfy the Regge-Wheeler-Zerilli-Moncrief system. Applying the technique introduced in [2], we prove an integrated local energy decay estimate for both the Regge-Wheeler and Zerilli equations. In these proofs, we use some constants that are computed numerically. Furthermore, we make use of the rp hierarchy estimates [13, 32] to prove that both the Regge-Wheeler and Zerilli variables decay as t-3/2 in fixed regions of r.

2017 | A new tensorial conservation law for Maxwell fields on the Kerr background | Lars Andersson, Thomas Bäckdahl, Pieter Blue Zeitschrift: J. Differential Geom. Verlag: Project Euclid Seiten: 163-176 Band: 105, no. 2 Link zur Publikation, Link zum Preprint

### A new tensorial conservation law for Maxwell fields on the Kerr background

#### Autoren: Lars Andersson, Thomas Bäckdahl, Pieter Blue (2017)

A new, conserved, symmetric tensor field for a source-free Maxwell test field on a four-dimensional spacetime with a conformal Killing–Yano tensor, satisfying a certain compatibility condition, is introduced. In particular, this construction works for the Kerr spacetime.

Zeitschrift:
J. Differential Geom.
Verlag:
Project Euclid
Seiten:
163-176
Band:
105, no. 2

2016 | Dynamical compact elastic bodies in general relativity | Lars Andersson, Todd A. Oliynyk, Bernd G. Schmidt Zeitschrift: Archive for Rational Mechanics and Analysis Verlag: Springer Seiten: 849-887 Band: 220, no. 2 Link zur Publikation, Link zum Preprint

### Dynamical compact elastic bodies in general relativity

#### Autoren: Lars Andersson, Todd A. Oliynyk, Bernd G. Schmidt (2016)

We prove the local existence of solutions to the Einstein-Elastic equations that represent self-gravitating, relativistic elastic bodies with compact support.

Zeitschrift:
Archive for Rational Mechanics and Analysis
Verlag:
Springer
Seiten:
849-887
Band:
220, no. 2

2016 | Decay of solutions to the Maxwell equation on the Schwarzschild background | Lars Andersson, Thomas Bäckdahl, Pieter Blue Zeitschrift: Class. Quantum Grav. Verlag: IOP Publishing Seiten: 085010 Band: 33, no. 8 Link zur Publikation, Link zum Preprint

### Decay of solutions to the Maxwell equation on the Schwarzschild background

#### Autoren: Lars Andersson, Thomas Bäckdahl, Pieter Blue (2016)

A new Morawetz or integrated local energy decay estimate for Maxwell test fields on the exterior of a Schwarzschild black hole spacetime is proved. The proof makes use of a new superenergy tensor $H_{ab}$ defined in terms of the Maxwell field and its first derivatives. The superenergy tensor, although not conserved, yields a conserved higher order energy current $H_{ab}(\partial_t)^ b$. The tensor $H_{ab}$ vanishes for the static Coulomb field, and the Morawetz estimate proved here therefore yields integrated decay for the Maxwell field to the Coulomb solution on the Schwarzschild exterior.

Zeitschrift:
Class. Quantum Grav.
Verlag:
IOP Publishing
Seiten:
085010
Band:
33, no. 8

2016 | Gauge-invariant perturbations of Schwarzschild spacetime | Abhay G. Shaw, Bernard F. Whiting, Steffen Aksteiner, Lars Andersson, Thomas Bäckdahl Link zum Preprint

### Gauge-invariant perturbations of Schwarzschild spacetime

#### Autoren: Abhay G. Shaw, Bernard F. Whiting, Steffen Aksteiner, Lars Andersson, Thomas Bäckdahl (2016)

We study perturbations of Schwarzschild spacetime in a coordinate-free, covariant form. The GHP formulation, having the advantage of not only being covariant but also tetrad-rotation invariant, is used to write down the previously known odd- and even-parity gauge-invariants and the equations they satisfy. Additionally, in the even-parity sector, a new invariant and the second order hyperbolic equation it satisfies are presented. Chandrasekhar's work on transformations of solutions for perturbation equations on Schwarzschild spacetime is translated into the GHP form, i.e., solutions for the equations of the even- and odd-parity invariants are written in terms of one another, and the extreme Weyl scalars; and solutions for the equations of these latter invariants are also written in terms of one another. Recently, further gauge invariants previously used by Steven Detweiler have been described. His method is translated into GHP form and his basic invariants are presented here. We also show how these invariants can be written in terms of curvature invariants.

2015 | Self-gravitating elastic bodies | Lars Andersson Reihe: Fundamental Theories of Physics Verlag: Springer Buchtitel: D. Puetzfeld, C. Lämmerzahl, B. Schutz (eds.): Equations of Motion in Relativistic Gravity Seiten: 543-559 Band: Volume 179 Link zur Publikation, Link zum Preprint

### Self-gravitating elastic bodies

Extended objects in GR are often modelled using distributional solutions of the Einstein equations with point-like sources, or as the limit of infinitesimally small "test" objects. In this note, I will consider models of finite self-gravitating extended objects, which make it possible to give a rigorous treatment of the initial value problem for (finite) extended objects.

Reihe:
Fundamental Theories of Physics
Verlag:
Springer
Buchtitel:
D. Puetzfeld, C. Lämmerzahl, B. Schutz (eds.): Equations of Motion in Relativistic Gravity
Seiten:
543-559
Band:
Volume 179

2015 | Hidden symmetries and decay for the wave equation on the Kerr spacetime | Lars Andersson, Pieter Blue Zeitschrift: Annals of Mathematics Verlag: Princeton University Seiten: 787-853 Band: 182, no. 3 Link zur Publikation, Link zum Preprint

### Hidden symmetries and decay for the wave equation on the Kerr spacetime

#### Autoren: Lars Andersson, Pieter Blue (2015)

Energy and decay estimates for the wave equation on the exterior region of slowly rotating Kerr spacetimes are proved. The method used is a generalisation of the vector-field method that allows the use of higher-order symmetry operators. In particular, our method makes use of the second-order Carter operator, which is a hidden symmetry in the sense that it does not correspond to a Killing symmetry of the spacetime.

Zeitschrift:
Annals of Mathematics
Verlag:
Princeton University
Seiten:
787-853
Band:
182, no. 3

2015 | Spin geometry and conservation laws in the Kerr spacetime | Lars Andersson, Thomas Bäckdahl, Pieter Blue Reihe: Surveys in Differential Geometry Verlag: International Press of Boston Buchtitel: L. Bieri, S.-T. Yau (eds.): One hundred years of general relativity Seiten: 183-226 Band: Volume 20 Link zur Publikation, Link zum Preprint

### Spin geometry and conservation laws in the Kerr spacetime

#### Autoren: Lars Andersson, Thomas Bäckdahl, Pieter Blue (2015)

In this paper we will review some facts, both classical and recent, concerning the geometry and analysis of the Kerr and related black hole spacetimes. This includes the analysis of test fields on these spacetimes. Central to our analysis is the existence of a valence $(2,0)$ Killing spinor, which we use to construct symmetry operators and conserved currents as well as a new energy momentum tensor for the Maxwell test fields on a class of spacetimes containing the Kerr spacetime. We then outline how this new energy momentum tensor can be used to obtain decay estimated for Maxwell test fields. An important motivation for this work is the black hole stability problem, where fields with non-zero spin present interesting new challenges. The main tool in the analysis is the 2-spinor calculus, and for completeness we introduce its main features.

Reihe:
Surveys in Differential Geometry
Verlag:
International Press of Boston
Buchtitel:
L. Bieri, S.-T. Yau (eds.): One hundred years of general relativity
Seiten:
183-226
Band:
Volume 20

2015 | Uniform energy bound and asymptotics for the Maxwell field on a slowly rotating Kerr black hole exterior | Lars Andersson, Pieter Blue Zeitschrift: J. Hyperbolic Differ. Equ. Verlag: World Scientific Seiten: 689-743 Band: 12, no. 4 Link zur Publikation, Link zum Preprint

### Uniform energy bound and asymptotics for the Maxwell field on a slowly rotating Kerr black hole exterior

#### Autoren: Lars Andersson, Pieter Blue (2015)

We consider the Maxwell equation in the exterior of a very slowly rotating Kerr black hole. For this system, we prove the boundedness of a positive definite energy on each hypersurface of constant $t$. We also prove the convergence of each solution to a stationary Coulomb solution. We separate a general solution into the charged, Coulomb part and the uncharged part. Convergence to the Coulomb solutions follows from the fact that the uncharged part satisfies a Morawetz estimate, i.e. that a spatially localised energy density is integrable in time. For the unchanged part, we study both the full Maxwell equation and the Fackerell-Ipser equation for one component. To treat the Fackerell-Ipser equation, we use a Fourier transform in $t$. For the Fackerell-Ipser equation, we prove a refined Morawetz estimate that controls 3/2 derivatives with no loss near the orbiting null geodesics.

Zeitschrift:
J. Hyperbolic Differ. Equ.
Verlag:
World Scientific
Seiten:
689-743
Band:
12, no. 4

2015 | The Merger of Small and Large Black Holes | P. Mösta, L. Andersson, J. Metzger, B. Szilágyi, J. Winicour Zeitschrift: Class. Quantum Grav. Verlag: IOP Publishing Seiten: 235003 Band: 32, no. 23 Link zur Publikation, Link zum Preprint

### The Merger of Small and Large Black Holes

#### Autoren: P. Mösta, L. Andersson, J. Metzger, B. Szilágyi, J. Winicour (2015)

We present simulations of binary black holes mergers in which, after the common outer horizon has formed, the marginally outer trapped surfaces (MOTSs) corresponding to the individual black holes continue to approach and eventually penetrate each other. This has very interesting consequences according to recent results in the theory of MOTSs. Uniqueness and stability theorems imply that two MOTSs which touch with a common outer normal must be identical. This suggests a possible dramatic consequence of the collision between a small and large black hole. If the penetration were to continue to completion then the two MOTSs would have to coalesce, by some combination of the small one growing and the big one shrinking. Here we explore the relationship between theory and numerical simulations, in which a small black hole has halfway penetrated a large one.

Zeitschrift:
Class. Quantum Grav.
Verlag:
IOP Publishing
Seiten:
235003
Band:
32, no. 23

2015 | Is there proof that backreaction of inhomogeneities is irrelevant in cosmology? | T. Buchert, M. Carfora, G.F.R. Ellis, E.W. Kolb, M.A.H. MacCallum, J.J. Ostrowski, S. Räsänen, B.F. Roukema, L. Andersson, A.A. Coley, D.L. Wiltshire Zeitschrift: Class. Quantum Grav. Verlag: IOP Publishing Seiten: 215021 Band: 32, no. 21 Link zur Publikation, Link zum Preprint

### Is there proof that backreaction of inhomogeneities is irrelevant in cosmology?

#### Autoren: T. Buchert, M. Carfora, G.F.R. Ellis, E.W. Kolb, M.A.H. MacCallum, J.J. Ostrowski, S. Räsänen, B.F. Roukema, L. Andersson, A.A. Coley, D.L. Wiltshire (2015)

No. In a number of papers Green and Wald argue that the standard FLRW model approximates our Universe extremely well on all scales, except close to strong field astrophysical objects. In particular, they argue that the effect of inhomogeneities on average properties of the Universe (backreaction) is irrelevant. We show that this latter claim is not valid. Specifically, we demonstrate, referring to their recent review paper, that (i) their two-dimensional example used to illustrate the fitting problem differs from the actual problem in important respects, and it assumes what is to be proven; (ii) the proof of the trace-free property of backreaction is unphysical and the theorem about it fails to be a mathematically general statement; (iii) the scheme that underlies the trace-free theorem does not involve averaging and therefore does not capture crucial non-local effects; (iv) their arguments are to a large extent coordinate-dependent, and (v) many of their criticisms of backreaction frameworks do not apply to the published definitions of these frameworks. It is therefore incorrect to infer that Green and Wald have proven a general result that addresses the essential physical questions of backreaction in cosmology.

Zeitschrift:
Class. Quantum Grav.
Verlag:
IOP Publishing
Seiten:
215021
Band:
32, no. 21