# What does numerical relativity have to do with detecting gravitational waves?

Heather Fong — a PhD candidate in Physics at the University of Toronto, who also loves travelling and gastronomy photography — gives us an insight into her group’s work on using numerical relativity simulations for the detection of gravitational waves.

Heather Fong, a PhD candidate in Physics at the University of Toronto.

Answer: quite a lot! Numerical relativity (NR) provides the most accurate solutions to the binary black hole problem, which is exactly the type of source LIGO wants to detect — and has succeeded at! Most of the time, LIGO’s data streams are overwhelmed with noise, and so we use a technique called matched-filtering to identify gravitational-wave signals. Finding and characterizing signals requires a massive amount of accurate waveforms, and we use semi-analytic waveform models as filters which are built using the results of NR simulations.

Why don’t we use NR alone to identify signals? It certainly would be ideal if the theoretical template waveforms were generated entirely from NR; not only would we be using the most accurate waveforms available, it would also allow us to Continue reading

# Fractals and black hole shadows

Jake Shipley and Dr Sam Dolan work in the Particle Astrophysics and Gravitation group at the University of Sheffield, focusing on general relativity, wave propagation and black hole physics. Here they provide us with an insight into their research.

Jake Shipley is a Ph.D student in the School of Mathematics and Statistics at the University of Sheffield. If Jake were a black hole, you would also see a lensed version of Dr Sam Dolan, standing behind the camera.

This has been a “miracle year” for relativity.

LIGO detected gravitational waves. The LISA Pathfinder mission demonstrated near-perfect freefall in space. And the era of gravitational-wave astronomy began in some style.

A century after black holes and gravitational waves were first predicted, we have learnt something truly mind-boggling: When two black holes collide, they shake the fabric of space-time with more power than is radiated by all the stars in the known universe put together!

The “chirps” from distant black hole collisions will travel for millions of years, at the speed of light, to reach our growing network of gravitational-wave detectors on Earth … and one day, out in space.

Next year, attention will turn to the Event Horizon Telescope (EHT): a global network of radio telescopes linked together to form an Earth-sized virtual telescope, using the technique of Very Long Baseline Interferometry. The EHT will Continue reading

# Wormholes can fix black holes

Diego Rubiera-Garcia (left, Lisbon University) and Gonzalo J. Olmo (right, University of Valencia – CSIC) after crossing a wormhole that connects Europe with the beaches of the Brazilian Northeast.

According to Einstein’s theory of general relativity (GR), black holes are ferocious beasts able to swallow and destroy everything within their reach. Their strong gravitational pull deforms the space-time causal structure in such a way that nothing can get out of them once their event horizon is crossed. The fate of those incautious observers curious enough to cross this border is to suffer a painful spaghettification process due to the strong tidal forces before being destroyed at the center of the black hole.

Antonio Sanchez-Puente (University of Valencia – CSIC) enjoying a sunny day in Valencia after submitting yet another postdoc application.

For a theoretical physicist, the suffering of observers is admissible (one might even consider it part of an experimentalist’s job) but their total destruction is not. The destruction of observers (and light signals) is determined by the fact that the affine parameter of their word-line (its geodesic) stops at the center of the black hole. Their clocks no longer tick and, therefore, there is no way for them to exchange or acquire new information. This implies the breakdown of the predictability of the laws of physics because physical measurements are no longer possible at that point. For this reason, when a space-time has incomplete geodesics — word-lines whose affine parameter does not cover the whole real line — we say that it is singular.

In order to overcome the conceptual problems raised by singularities, a careful analysis of what causes the destruction of observers is necessary. Our intuition may get satisfied by blaming the enormous tidal forces near the center, but the problem is much subtler. This is precisely what we explore in our paper. Continue reading

# Do black holes really have no hair?

Tim Johannsen is a postdoctoral fellow at Perimeter Institute for Theoretical Physics and the University of Waterloo specializing in black-hole astrophysics and tests of general relativity.

Black holes have no hair – so they say. Formally, this statement refers to several famous theorems in general relativity that were established mostly from the late 1960s to the early 1970s and are collectively known as the no-hair theorem. According to this theorem, a black hole only depends on its mass, angular momentum (or spin), and electric charge and is uniquely described by the Kerr-Newman metric. So, just about everyone would expect that astrophysical black holes are indeed the Kerr black holes of general relativity understanding that any net electric charge would quickly Continue reading

# Insights from the Boundary: black holes in a magnetic universe

Hari Kunduri

Following from the seminal work of Dain, a great deal is now known concerning geometric inequalities relating the area, charge, and angular momentum of axisymmetric black hole horizons in (possibly dynamical) spacetimes.  A key feature of these results is that they are quasi-local: they depend on spacetime only near the horizon itself and so are not sensitive to the asymptotic behaviour of the geometry.

For Einstein-Maxwell theory the celebrated uniqueness theorems tell us under certain conditions, that the Kerr-Newman (KN) family of solutions are the only stationary, axisymmetric and asymptotically flat black hole spacetimes. These are the model geometries that originally motivated the inequalities. However if we relax the condition of asymptotic flatness there are many other families of black hole solutions. While in general these will not contain event horizons (whose standard definitions require flat or AdS asymptotics) they still contain singularities and Killing horizons. In this paper we focussed Continue reading

# Black-hole laboratories in the era of gravitational-wave astronomy

Helvi and Paolo visiting Toronto during the International Conference on Black Holes at the Fields Institute last year.
Helvi is Research Fellow in the School of Mathematical Sciences at the University of Nottingham. Paolo is Assistant Professor at Sapienza University of Rome and Research Scientist at the Instituto Superior Técnico in Lisbon.

We are proud to present the completed Focus Issue on “Black holes and Fundamental Fieldsone year after its first contribution has been published online.

This issue appears serendipitously at the same time as LIGO’s historic detection of gravitational waves which, simultaneously, provided us with the first direct observational evidence for the existence of black holes (BHs). We wish to take this opportunity to congratulate the LIGO/VIRGO Scientific Collaboration and everyone involved on  their breakthrough discovery!

The true excitement around this discovery arises from the fact that it marks the beginning of the long-sought-for era of gravitational-wave astronomy. As Kip Thorne recently put it, “Recording a gravitational wave […] has never been a big motivation for LIGO, the motivation has always been to open a new window to the Universe”. The outstanding observation of a BH binary coalescence — and the expectation of Continue reading

# Accreting onto almost Kerr-de Sitter black holes

Bondi-type accretion in the Reissner-Nordström-(anti-)de Sitter spacetime
Filip Ficek 2015 Class. Quantum Grav. 32 235008

arXiv: 1509.07005
*until 30/12/15

Filip Ficek is a graduate student in Theoretical Physics at Jagiellonian University.

In spite of numerous investigations, accretion flows onto the Kerr black hole are still not fully understood, especially for radially dominated flows, where aside from a very specific case of an ultra-hard fluid, general solutions are not known. Some insight may be provided by considering a simpler problem instead, namely spherically symmetric, steady accretion in Reissner-Nordström spacetimes. It is well known that rotating Kerr black holes and charged Reissner-Nordström black holes feature similar horizon and causal structures. In fact, it is common to treat a Reissner-Nordström black hole as a toy model of an astrophysical black hole. If we also take into account the cosmological constant, we may suppose, that accretion solutions in Reissner-Nordström-(anti-)de Sitter spacetime will Continue reading

# The spin limit for cosmological black holes

The area-angular momentum inequality for black holes in cosmological spacetimes
María Eugenia Gabach Clément, Martín Reiris and Walter Simon 2015 Class. Quantum Grav. 32 145006

arXiv:1501.07243

In colloquial terms, the main achievement of our recent CQG article is simple to state: We have proven that the angular momentum $J$ of an axially symmetric black hole (the Noether current) with surface area $A$ satisfies the bound.

Here $\Lambda$ is the cosmological constant –  a standard ingredient in Einstein’s Continue reading

# The curvature on a black hole boundary

On the Bartnik mass of apparent horizons
Christos Mantoulidis and Richard Schoen 2015 Class. Quantum Grav. 32 205002

arXiv:1412.0382
*until 04/11/15

Christos Mantoulidis is a graduate student in Mathematics at Stanford University.

In our latest CQG paper we study the geometry (i.e. curvature) of apparent horizons and its relationship with ADM mass.

We were motivated by the following two foundational results in the theory of black holes in asymptotically flat initial data sets (slices of spacetime) satisfying the dominant energy condition (DEC):

1. Apparent horizons are topologically equivalent to (one or more) two-dimensional spheres.(1)
2. When the initial data set is additionally time symmetric (totally geodesic in spacetime), the apparent horizon’s total area $A$ is bounded from above by the slice’s ADM mass per $A \leq 16\pi m^2$. This is called the Penrose inequality.(2) Equality is only achieved on Schwarzschild data, whose apparent horizon is a single sphere with constant Gauss curvature.

One then naturally wonders: Continue reading

# Black holes in massive gravity

Eugeny Babichev is a CNRS researcher at the Laboratory for Theoretical Physics at Orsay. His work focuses on modified gravity theories, including their theoretical and observational issues, as well as phenomenological aspects, such as cosmology and black holes.

One of the biggest puzzles of modern physics is the present-day accelerated expansion of the Universe. The acceleration is usually attributed to the presence of a mysterious dark energy, a yet unknown substance of the Universe. Although in the framework of conventional General Relativity (GR), a cosmological constant can be added to mimic dark energy, the fine tuning required to adjust its value makes this explanation unsatisfactory. We can then ask whether a modification of Continue reading