Black holes in massive gravity

Eugeny Babichev

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

Generating dynamical bosons from kinematical fermions

Ümit Ertem is a postdoctoral researcher in Ankara University (http://en.ankara.edu.tr/) Department of Physics. He will be visiting The University of Edinburgh for the next six months. He also writes on his own blog (https://ucuiyagi.wordpress.com/) sometimes.

Ümit Ertem is a postdoctoral researcher in Ankara University Department of Physics. He will be visiting The University of Edinburgh for the next six months. He also writes on his own blog, sometimes.

Spinors are mathematical objects used in physics mainly for defining fermions. Fermions are particles/field excitations that have half-integer spins as opposed to bosons that have integer spins. While fermions correspond to elementary constituents of matter, bosons correspond to the fundamental interactions of matter. There is a distinguishing property of fermions that an even number of them can combine to exhibit bosonic behaviour in analogy with the defining algebraic properties of half-integers and integers.

So immediately one can easily grasp the fact that the product of two spinors represent the above mentioned bosonic Continue reading

A quantum kinematics for asymptotically flat gravity

Miguel and Madhavan

Miguel Campiglia, a postdoc at the Raman Research Institute (RRI), enjoying a traditional south Indian dish: masala dosa.
Madhavan Varadarajan (professor at RRI) not enjoying traditional South American drink: mate.

Isolated gravitating systems are modelled by asymptotically
flat space-times with the classical gravitational field subject to intricate and detailed asymptotic behaviour. The question we are interested in is: Is there a notion of an isolated quantum gravitating system? Specifically, can the classical
asymptotic conditions be suitably incorporated in quantum theory? Our work analyses this issue in the broad context of the Loop Quantum Gravity (LQG) approach.

At first it may seem this cannot be possible: The fundamental excitations in LQG are Continue reading

Internal-external-dynamics decoupling in canonical general relativity

Gerhard Schäfer

Gerhard Schäfer is a retired professor at the University of Jena. His main scientific interests are equations of motion in general
relativity and their applications in astronomy and astrophysics.

Research on general-relativistic equations of motion based on Hamiltonian or canonical frameworks is not quite a main-stream doing; likely because of the all-over covariance of the theory and canonical is just not covariant but rather quite the opposite. Covariance under spacetime coordinate transformations makes the theory a spacetime-local one with its local scalars, vectors and tensors, the canonical picture on the other side is at home in the phase space of the dynamics which combines position and momentum variables. Crucial object-changing operations in spacetime are covariant derivatives, crucial ones in phase space are Poisson brackets.

What is the benefit of performing research in general relativity within a canonical framework? Let us concentrate on gravitating systems living in asymptotically flat spacetimes. Then there exist global quantities — energy, linear momentum, angular momentum, Lorentz-boost vector — which are nicely conserved. If those quantities are calculated within Continue reading

Spectral analysis in resonant interferometry: following the traces of thermal deformation

How researchers from the LIGO scientific collaboration use signals generated from higher-order mode resonances to glean crucial information about the thermal state of their interferometers.

Chris Mueller

Chris Mueller received his Ph.D. in physics with Guido Mueller at the University of Florida and has since moved to industry.

Imagine for a moment that you’ve accepted the challenge of trying to make the first direct detection of gravitational waves. To achieve such a daunting task you’ll need to devise an instrument capable of measuring a change in length of just 10-19 m over a distance of several km. At these length scales everything matters; the ground is vibrating, air molecules are buzzing around, and the molecules which make up the test masses of your detector are quivering. This challenge is precisely Continue reading

How wild can a static spacetime get?

Stationary spacetimes—sounds fairly simple, unchanging.  Static—even more boring.  But are they?

Steve (Stacey) Harris

Steve (also known as Stacey) Harris is a mathematical relativist in the Department of Mathematics at Saint Louis University, mostly working on global structures such as causal boundary, and a notorious campus gadfly and rebel, being active in AAUP and fanatical about shared governance. Off campus there’s hiking and playing flute in concert band.

Consider an experiment of emitting a photon along a closed path—closed either due to a constraining light-tube or due to a topological closure in the spacetime—and finding the time till the photon returns to the starting point.  (Our naive expectation is for the time to be the same as the length of the path, if our clocks and measuring rods are in geometric units, set to show speed of light is unity.)  Now turn around and emit a photon along the same path but in the reverse direction—does it take the same time to Continue reading

Can’t solve an equation….bypass it !!!

Radouane Gannouji

Radouane Gannouji is an associate professor at the Department of Physics, PUCV.

In general relativity, to understand how the spacetimes behave in presence of a given form of matter, we have to solve the Einstein field equations, which in general, are a set of 10 very complicated coupled nonlinear second order partial differential equations that describes the fundamental interaction of gravitation as a result of spacetime being curved by matter and energy. Once we solve these set of field equations we get the metric of the spacetime that describes all the general important physical features of the spacetime, for example Continue reading

The return of Newton-Cartan geometry

Jelle Hartong

Jelle Hartong is a postdoctoral researcher at the Université Libre de Bruxelles. His research concerns the foundations and applications of various non-AdS holographies and non-relativistic gravity.

Non-relativistic field theories defined on Newton-Cartan Geometry and its extension called Torsional Newton-Cartan Geometry, have (re-)appeared in recent studies of non-AdS holography and condensed matter physics.

Relativistic, Poincaré invariant, field theories are defined on Minkowski space-time. This flat background can be turned into a curved geometry by coupling the theory to a Lorentzian metric as one does when adding matter to Einstein’s theory of gravity. There are many areas of physics, notably Continue reading

Designing curved blocks of quantum space-time…Or how to build quantum geometry from curved tetrahedra in loop quantum gravity

Etera Livine

Etera Livine is a CNRS researcher and focuses especially on mathematical aspects of loop quantum gravity and spinfoam path integral models. Etera and Christoph both work on quantum gravity at the Laboratoire de Physique de l’Ecole Normale Supérieure de Lyon (LPENSL) in France.

Among the various approaches to the quantum gravity challenge, loop quantum gravity proposes a framework for a canonical quantization of general relativity, describing how the 3d geometry evolves in time. It does not require a priori extra dimensions or supersymmetry. It defines spin network states for the quantum geometry directly at the Planck scale, with a discrete spectra of areas and volumes, and computes their transition amplitudes by path integrals inspired from topological field theory, called spinfoam models. This framework is mathematically rigorous but Continue reading

Black-hole superradiance and the hunt for dark matter

Paulo Pani

Paolo Pani is a Marie Curie Fellow at Sapienza University of Rome and FCT Researcher at Instituto Superior Tecnico in Lisbon. His research interests include black holes, foundations of General Relativity and relativistic astrophysics. He is co-author of the book “Superradiance” (Springer-Verlag), now in press.

Little is known about dark matter, despite the numerous searches for its constituents. Fortunately, everything falls in the same way, so possible imprints of dark matter can be found in gravitational fields. In particular, if ultralight bosons exist in nature, they would make spinning black holes unstable. How does such instability evolve in realistic scenarios? And what can it teach us about the existence of dark matter?

In our recent CQG paper, we take the first step to address these questions by studying how a light scalar field grows near Continue reading