Monthly Archives: August 2015

Black holes in massive gravity

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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

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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

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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

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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 →