Gravity and Unification

by Kirill Krasnov and Roberto Percacci

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The geometric unification of gravity with the other interactions is not currently a popular subject. It is generally believed that a unified theory can only be constructed once a quantum theory of gravity is available. The purpose of this CQG+ contribution is to advocate that it may be fruitful and even necessary to reverse the logic: instead of “quantising in order to unify” it may be necessary to “unify in order to quantise”. If the latter perspective is correct, our current approaches to quantum gravity would be similar to trying to understand the quantum theory of electricity and magnetism separately before they were unified in Maxwell’s theory.

There are several arguments for such a change of priorities.

Kirill Krasnov, Professor of Mathematical Physics, University of Nottingham

Roberto Percacci, Associate Professor of Theoretical Particle Physics at SISSA, Italy

A Study of Time Delay from Different Time Zones

Netta Engelhardt (University of California, Santa Barbara) and Sebastian Fischetti (Imperial College) gave us an insight into their communication methods whilst collaborating for their research paper recently published in CQG.

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On a dark London evening and a sunny California day — January 19, 2016, to be precise — Netta sent Sebastian a Skype message:

So began a new project for this dynamic duo, published recently in CQG. Unlike our previous project, this one presented a new challenge (with which researchers are all too familiar): we were separated by an eight-hour time difference. Thus began a three-way collaboration: Netta, Sebastian, and Skype (with the third member being the least cooperative).

The process began

Insight: Some one loop gravitational interactions in string theory

Anirban Basu is a researcher at Harish-Chandra Research Institute, Allahabad
India

String theory yields ultraviolet finite scattering amplitudes in theories of gravity coupled to matter. While the matter content of the theory is dependent on the compactification, the presence of gravity in the spectrum is universal. Hence, this is drastically different from the high energy behavior of conventional quantum field theories of point like excitations because such amplitudes are generically ultraviolet divergent. While in quantum field theory the ultraviolet divergences arise from short distance effects which manifest themselves as divergences arising from high momentum modes in loop integrals in various Feynman diagrams, these divergences are absent in string theory where analogous loop integrals involve an integration over the fundamental domain of the moduli space of two dimensional Riemann surfaces which is the Euclidean worldsheet of the string propagating in the background spacetime. The fundamental domain precisely excludes the regions of moduli space which yield the ultraviolet divergences in quantum field theory. The ultraviolet finiteness of string theory makes it, among other reasons, particularly attractive in the quest for a theory of quantum gravity. On the other hand, there are infrared divergences that arise from the boundaries of moduli space in calculating string amplitudes which reproduce expectations from quantum field theory, which must be the case as string theory must reproduce field theory at large distances. Hence, their cancellation proceeds as in field theory.

However, calculating these loop amplitudes in perturbative string theory is not an entirely trivial exercise. In the absence of Ramond–Ramond backgrounds, tree level amplitudes have been calculated in superstring theory. The one loop amplitudes, which are more complicated, have also been

No go on spacetime reconstruction inside horizons

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Read the full article for free* in Classical and Quantum Gravity:
Covariant constraints on hole-ograhpy
Netta Engelhardt and Sebastian Fischetti 2015 Class. Quantum Grav. 32 195021
arXiv:1507.00354
*until 28/10/15

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Spacetime reconstruction in holography is limited in the presence of strong gravity.

Netta Engelhardt (left) and Sebastian Fischetti (right) practicing some of their less-developed skills at UCSB. Netta is a graduate student at UCSB. Sebastian was a graduate student at UCSB at the time of writing, and is now a postdoc at Imperial College London.

In recent years, it has become clear that there is a deep connection between quantum entanglement and geometry.  This mysterious connection has the potential to provide profound insights into the inner workings of a complete theory of quantum gravity.  Many concrete hints for how geometry and entanglement are related come from the so-called AdS/CFT duality conjectured by J.Maldacena, which relates certain types of quantum field theories (the “boundary”) to string theory on a negatively-curved spacetime called anti-de Sitter (AdS) space (the “bulk”) of one higher dimension.  In a certain limit, the string theory is

Ambitwistor Strings and Soft Theorems

Arthur Lipstein is a Postdoctoral Research Associate at University of Hamburg/DESY.

Ambitwistor string theories are a family of chiral (holomorphic) string theories whose target space is the space of complexified null geodesics in a general space-time. Like conventional strings, they are critical in 10 dimensions and describe supergravity, but unlike conventional strings, they do not admit a tower of higher massive modes (and are correspondingly not thought to be ultraviolet finite). They provide a natural generalization of the twistor-strings of Witten, Berkovits and Skinner to arbitrary dimension and their correlators give rise to the beautiful formulae for gravitational and Yang-Mills scattering amplitudes in all dimensions recently discovered by Cachazo, He and Yuan (CHY). Ambitwistor strings were also used to obtain new formulae in four dimensions by the authors of this article.

In a recent series of papers, Strominger and

Holographic entanglement obeys strong subadditivity

Aron Wall is a member of the School of Natural Sciences at the Institute for Advanced Study. In his spare time he blogs at Undivided Looking. He was the 2013 recipient of the Bergmann-Wheeler thesis prize, which is sponsored by Classical and Quantum Gravity.

Gauge-gravity duality allows us to calculate properties of certain quantum field theories (QFT) from classical general relativity. One famous piece of this conjecture, due to Ryu and Takayanagi, relates the entanglement entropy in a QFT region to the area of a surface in the gravitational theory. In addition to being a clue about quantum gravity, this proposal is one of the few tools which allow us to calculate entanglement entropy analytically. Since the entanglement entropy is of increasing interest for field theory and condensed matter applications, it is important to check if the conjecture is true.

One important property of the entropy is strong subadditivity (SSA). This quantum inequality says that the sum of the entropies in two regions is always greater than the sum of the entropies of their union and intersection. My article uses proof

Black holes as beads on cosmic strings

Amjad Ashoorioon (left) is a Senior Research Associate at the physics department of Lancaster University in the United Kingdom. Robert B. Mann (right) is a Professor of Physics and Applied Mathematics at the University of Waterloo, Ontario, Canada.

Cosmic strings have been a source of fascination in cosmology since Tom Kibble first proposed their existence 40 years ago. Like an imperfection in a solidifying crystal, a cosmic string is a thread of energy that might have formed in the early universe during a symmetry breaking phase transition. Twenty years ago Ruth Gregory pointed out that a black hole could have a cosmic string as a single “hair”.   Turning this idea around, in this article we have proposed that a

New hair on black rings

Simon Ross is a professor in the department of mathematical sciences at Durham University, and a member of the university’s Centre for Particle Theory.

Relating different charge densities gives black rings with non-trivial profiles with smooth horizons.

There is a rich space of solutions in five-dimensional supergravity, including smooth horizonless supertube solutions and black ring solutions. Supertubes can have arbitrary profiles, and varying charge densities along the profile, but previously-known black ring solutions required a constant charge density along the ring to have a smooth horizon.

Recently, we discovered a new kind of supersymmetric horizonless object which generalizes the supertube, which we dubbed the magnetube. They carry coordinated electric charge densities with

Melvin magnetic cosmologies

Magnetic fields are ubiquitous in the universe – observed on scales ranging from stellar, through galactic and beyond – and are key to the physics of dramatic astrophysical objects such as pulsars and active galactic nuclei. Meanwhile, the origin of large-scale magnetic fields is still a topic of great debate in the cosmological literature.

Our recent CQG article presents a new family of exact solutions to the Einstein-Maxwell equations for cosmological magnetic fields. These solutions are both inhomogeneous and anisotropic, with the magnetic field having nontrivial dependence on