# Current research topics

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1. Dec 30, 2015

### annihilatorM

Hello,
I just wanted to know what some of the current hot topics in physics beyond the standard model are. What are some examples of specific research topics and how do they relate to a broader problem in physics?

2. Dec 30, 2015

### marcus

Here's one way to answer. I'll copy a selection from the 20 recent QG papers (fourth quarter 2015) I plan to put in the next MIP (most important paper) poll. In the next post I'll try to say why some of these are important and point to interesting directions of QG research. These are all from Oct-Dec 2015.
http://arxiv.org/abs/1512.02083
Tests of Quantum Gravity induced non-locality via opto-mechanical quantum oscillators
Alessio Belenchia, Dionigi M. T. Benincasa, Stefano Liberati, Francesco Marin, Francesco Marino, Antonello Ortolan
(Submitted on 7 Dec 2015)
Several quantum gravity scenarios lead to physics below the Planck scale characterised by nonlocal, Lorentz invariant equations of motion. We show that such non-local effective field theories lead to a modified Schrödinger evolution in the nonrelativistic limit. In particular, the nonlocal evolution of opto-mechanical quantum oscillators is characterised by a spontaneous periodic squeezing that cannot be generated by environmental effects. We discuss constraints on the nonlocality obtained by past experiments, and show how future experiments (already under construction) will either see such effects or otherwise cast severe bounds on the non-locality scale (well beyond the current limits set by the Large Hadron Collider). This paves the way for table top, high precision experiments on massive quantum objects as a promising new avenue for testing some quantum gravity phenomenology.
5 pages, 1 figure
Obviously important if you can probe QG with a table-top experiment! Should become "hot" area of research. See comment by Bee Hossenfelder in her "Backreaction" blog.

http://arxiv.org/abs/1512.08346
Quantum black hole without singularity
Claus Kiefer
(Submitted on 28 Dec 2015)
We discuss the quantization of a spherical dust shell in a rigorous manner. Classically, the shell can collapse to form a black hole with a singularity. In the quantum theory, we construct a well-defined self-adjoint extension for the Hamilton operator. As a result, the evolution is unitary and the singularity is avoided. If we represent the shell initially by a narrow wave packet, it will first contract until it reaches the region where classically a black hole would form, but then re-expands to infinity. In a way, the state can be interpreted as a superposition of a black hole with a white hole.
5 pages, invited contribution to the BH6 session at the Marcel Grossmann Conference MG14
Claus Kiefer is a prominent figure in QG research. It's remarkable that he is presenting a QG black hole that bounces---in extreme slow motion because of time dilation during the high density "almost singularity" phase. There is more to do along the line he initiates here and it could become a hot topic.

http://arxiv.org/abs/1512.01589
Asymptotic safety in an interacting system of gravity and scalar matter
Pietro Donà, Astrid Eichhorn, Peter Labus, Roberto Percacci
(Submitted on 4 Dec 2015)
Asymptotic safety is an attractive scenario for the dynamics of quantum spacetime. Here, we work from a phenomenologically motivated point of view and emphasize that a viable dynamics for quantum gravity in our universe must account for the existence of matter. In particular, we explore the scale-dependence of a scalar matter-gravity-vertex, and investigate whether an interacting fixed point exists for the so-defined Newton coupling. We find a viable fixed point in the pure-gravity system, disregarding scalar quantum fluctuations. We explore its extensions to the case with dynamical scalars, and find indications of asymptotic safety in the matter-gravity system. We moreover examine the anomalous dimensions for different components of the metric fluctuations, and find significant differences between the transverse traceless and scalar component.
15 pages, 14 figures
Again, Roberto Percacci is a major figure and Asymptotic Safety approach to QG is an important line of investigation. Adding MATTER to the ASQG picture is an obvious next step. This could become a fruitful research area.

http://arxiv.org/abs/1512.04993
Complexity, Action, and Black Holes
Adam Brown, Daniel A. Roberts, Leonard Susskind, Brian Swingle, Ying Zhao
(Submitted on 15 Dec 2015)
Our earlier paper "Complexity Equals Action" conjectured that the quantum computational complexity of a holographic state is given by the classical action of a region in the bulk (the Wheeler-DeWitt' patch). We provide calculations for the results quoted in that paper, explain how it fits into a broader (tensor) network of ideas, and elaborate on the hypothesis that black holes are fastest computers in nature.
55+14 pages, many figures

http://arxiv.org/abs/1512.03684
Anisotropic loop quantum cosmology with self-dual variables
Edward Wilson-Ewing
(Submitted on 11 Dec 2015)
A loop quantization of the diagonal class A Bianchi models starting from the complex-valued self-dual connection variables is presented in this paper. The basic operators in the quantum theory correspond to areas and generalized holonomies of the Ashtekar connection and the reality conditions are implemented via the choice of the inner product on the kinematical Hilbert space. The action of the Hamiltonian constraint operator is given explicitly for the case when the matter content is a massless scalar field (in which case the scalar field can be used as a relational clock), and it is shown that the big-bang and big-crunch singularities are resolved in the sense that singular and non-singular states decouple under the action of the Hamiltonian constraint operator.
16 pages

http://arxiv.org/abs/1511.05441
3D holography: from discretum to continuum
Valentin Bonzom, Bianca Dittrich
(Submitted on 17 Nov 2015)
We study the one-loop partition function of 3D gravity without cosmological constant on the solid torus with arbitrary metric fluctuations on the boundary. To this end we employ the discrete approach of (quantum) Regge calculus. In contrast with similar calculations performed directly in the continuum, we work with a boundary at finite distance from the torus axis. We show that after taking the continuum limit on the boundary - but still keeping finite distance from the torus axis - the one-loop correction is the same as the one recently found in the continuum in Barnich et al. for an asymptotically flat boundary. The discrete approach taken here allows to identify the boundary degrees of freedom which are responsible for the non-trivial structure of the one-loop correction. We therefore calculate also the Hamilton-Jacobi function to quadratic order in the boundary fluctuations both in the discrete set-up and directly in the continuum theory. We identify a dual boundary field theory with a Liouville type coupling to the boundary metric. The discrete set-up allows again to identify the dual field with degrees of freedom associated to radial bulk edges attached to the boundary. Integrating out this dual field reproduces the (boundary diffeomorphism invariant part of the) quadratic order of the Hamilton-Jacobi functional. The considerations here show that bulk boundary dualities might also emerge at finite boundaries and moreover that discrete approaches are helpful in identifying such dualities.
42 pages

http://arxiv.org/abs/1511.03644
Hessian and graviton propagator of the proper vertex
Atousa Chaharsough Shirazi, Jonathan Engle, Ilya Vilensky
(Submitted on 11 Nov 2015)
The proper spin-foam vertex amplitude is obtained from the EPRL vertex by projecting out all but a single gravitational sector, in order to achieve correct semi-classical behavior. In this paper we calculate the gravitational two-point function predicted by the proper spin-foam vertex to lowest order in the vertex expansion. We find the same answer as in the EPRL case in the continuum spectrum' limit, so that the theory is consistent with the predictions of linearized gravity in the regime of small curvature. The method for calculating the two-point function is similar to that used in prior works: we cast it in terms of an action integral and to use stationary phase methods. Thus, the calculation of the Hessian matrix plays a key role. Once the Hessian is calculated, it is used not only to calculate the two-point function, but also to calculate the coefficient appearing in the semi-classical limit of the proper vertex amplitude itself. This coefficient is the effective discrete "measure factor" encoded in the spin-foam model. Through a non-trivial cancellation of different factors, we find that this coefficient is the same as the coefficient in front of the term in the asymptotics of the EPRL vertex corresponding to the selected gravitational sector.
20 pages

http://arxiv.org/abs/1510.00699
A Perfect Bounce
Steffen Gielen, Neil Turok
(Submitted on 2 Oct 2015)
We study the quantum cosmology of a universe with conformal matter comprising a perfect radiation fluid and a number of conformally coupled scalar fields. For FRW backgrounds, we are able to perform the quantum gravity path integral exactly. We find the evolution to describe a "perfect bounce," in which the universe passes smoothly through the singularity. The Feynman path integral amplitude is precisely that of a relativistic oscillator, for which the scale factor of the universe is the time and the scalar fields are the spatial coordinates. This picture provides natural, unitary quantum mechanical evolution across a bounce. We also study the quantum evolution of anisotropies and of inhomogeneous perturbations, at linear and nonlinear order. We provide evidence for a semiclassical description in which all fields pass "around" the cosmological singularity along complex classical paths.
5 pages.
Neil Turok is director of Perimeter Institute. This looks to me like a significant advance in QG cosmology. Following up on what Gielen and Turok did here might prove to be a hot line of research. Replacing the cosmological singularity by a bounce opens up possibilities of dispensing with inflation and making testable predictions.

Last edited: Dec 30, 2015
3. Dec 30, 2015

### Staff: Mentor

If you are more interested in experiment-driven theory developments:

76 papers triggered by the ATLAS+CMS diphoton excess

LHCb sees some anomalies concerning lepton universality (muons/electrons) and the angular distributions of some decay products (P5'). There are some attempts to combine this with the tension observed for the muon g-2 discrepancy. See Jester's blog article for details.

4. Dec 30, 2015

### Chronos

As noted by marcus, Quantum gravity is the headliner among most beyond the standard model studies in physics. Other subjects of interest include include noncommutative geometry and, of course, supersymmetry.