|09:30 - 10:00||Welcome Coffee|
|Chair: Dong-han Yeom (2 hours)|
|10:00 - 11:00||Antonino Marciano||Cosmology with fermions
Moving from the consideration that matter fields must be treated in terms of their fundamental quantum counterparts, we analyze models for early cosmology that involve fermion fields. We comment on the phenomenological consequences of this choice, and show that cosmological quantum perturbations can actually be accounted for in terms of Dirac fields. We recover a class of states of the Hilbert space that implement the semiclassical limit of the theory, and correspond to non Bunch Davies group coherent states. We then discuss quantum features of these non Bunch Davies states, and derive signatures for CMB observables also deploying path-integral methods.
|11:00 - 11:30||Suddho Brahma||Emergence of non-Riemannian space-time structures
It is a general expectation that classical space-times give way to a more 'fuzzy' picture in quantum gravity theories. In this talk, new mathematical methods shall be invoked to classify inequivalent background structures for canonincal quantum gravity. Particular quantum effects, specific to models of loop quantum gravity, lead to new non-Riemannian geometries once the important condition of quantum gravitational anomaly-freedom is imposed. The main effect of this new feature is to have a deformed notion of general covariance leading to non-singular signature change, with fascinating consequences for black-holes and early universe cosmology.
|11:30 - 12:00||Jibril Ben Achour||Rotating Black Holes States counting in Loop Quantum Gravity
Loop Quantum Gravity (LQG) provides a very elegant proposal for the description of quantum geometry. In the LQG context, the black hole entropy turns out to be obtained by a careful counting of this quantum geometry excitations of the horizon. However, the standard LQG counting is based on the so called real Ashtekar-Barbero variables. In the context of spherically symmetric isolated horizons (IH), one recovers successfully the Bekenstein-Hawking area law, but up to a fine tuning on the Immirzi parameter, which is the only free constant of the theory. This unnatural fine-tuning has raised criticisms concerning the implementation of the LQG counting. Recently, a new proposal has been put forward to solve this problem. The idea is to perform an analytic continuation from the real Immirzi parameter to its purely imaginary value, which corresponds to work with the so called self dual variables. Doing so, one recovers exactly the Bekenstein-Hawking area law without any fine tuning. In the rotating case, the counting based on the real variables turns out to not reproduce the Bekenstein-Hawking area law at leading order. A generalization of the analytic continuation to this rotating case was recently introduced, which successfully solve the problem and allow to recover the area law. Those results points towards the peculiar status of the self dual variables in the LQG quantization of black holes. We will present those recent results, and describe the procedure of analytic continuation and the new directions of research it suggests.
|12:00 - 13:30||Lunch|
|Chair: Fabien Nugier (2 hours)|
|13:30 - 14:00||Frederico Arroja||Mimetic Gravity
I will present my recent work on generalized mimetic theories of gravity. I will show that general scalar-tensor theories are invariant under disformal transformations. However for non-invertible transformations one obtains new theories that are generalizations of the mimetic dark matter scenario. Simple mimetic models can have the same expansion history as LCDM. I will discuss linear scalar perturbations in mimetic Horndeski gravity and show: i) the sound speed is exactly zero, ii) the comoving curvature perturbation is constant on all scales. Furthermore, the system does not have any wave-like scalar degrees of freedom (dof) around any background. Time permitting, I will present a way to introduce propagating dof but show that it suffers from either gradient or ghost instabilities. In the ghost branch of the model, the time scale of the instability is estimated and shown to be phenomenologically viable and thus the mimetic dark matter scenario is a viable alternative to the dark matter paradigm.
|14:00 - 14:30||Dong-han Yeom||Fuzzy Euclidean wormholes in de Sitter space
We investigate Euclidean wormholes in Einstein gravity with a massless scalar field in de Sitter space. Euclidean wormholes are possible due to the analytic continuation of the time as well as complexification of all fields, where we need to impose the classicality after the Wick-rotation to the Lorentzian signatures. For some parameters, wormholes are preferred than Hawking-Moss instantons, and hence wormholes can be more fundamental instantons than Hawking-Moss type instantons. Euclidean wormholes can be interpreted in three ways: (1) classical big bounce, (2) either tunneling from a small to a large universe or a creation of a collapsing and an expanding universe from nothing, and (3) either a quantum big bounce from a contracting to a bouncing phase or a creation of two expanding universes from nothing. These various interpretations shed some lights to the resolution of the singularity and tensions between various kinds of quantum gravity theories.
|14:30 - 15:00||Fabien Nugier||Adapted coordinates, turning past light-cones into double light-cones
Many efforts were done recently towards using adapted coordinates in cosmological applications, including quantitative predictions. The geodesic light-cone coordinates in particular were employed to compute the luminosity distance, number counts, lensing quantities, and time-delay of ultra-relativistic particles. I will show that these coordinates can be modified, replacing their time coordinates with a future light-cone coordinate, to give back the famous double-null coordinates of Brady, Droz, Isreal and Morsink (1995). If time allows it I will comment on possible applications, including black holes.
|15:00 - 15:30||Hsu-Wen Chiang||A Quantization of Spacetime Based on Spin(3,1) Symmetry
We introduce a new type of spacetime quantization based on the spinorial description suggested by loop quantum gravity. Specifically, we build our theory on a string theory inspired Spin(3, 1) worldsheet action. Because of its connection with quantum gravity theories, our proposal may in principle link back to string theory, connect to loop quantum gravity where SU(2) is suggested as the fundamental symmetry, or serve as a Lorentzian spin network. We derive the generalized uncertainty principle and demonstrate the holographic nature of our theory. Due to the quantization of spacetime, geodesics in our theory are fuzzy, but the fuzziness is shown to be much below conceivable astrophysical bounds, which makes our theory safe from deleterious effects.
|15:30 - 16:00||Coffee Break|
|Chair: Frederico Arroja (2 hours)|
|16:00 - 16:30||Jakub Bilski||Quantum Reduced Loop Gravity
Loop Quantum Gravity (LQG) is an attempt to quantize Einsteinian gravity, cast in terms of variable with SU(2) symmetry, deploying tools borrowed from lattice gauge theories. I will present a procedure to select a finite amount of degrees of freedom in LQG, which is called Quantum Reduced Loop Gravity (QRLG), and show that it can be applied to all gravitational models with diagonal metric tensor, including all the solutions of the Einstein equations relevant for cosmology and black holes physics. I will discuss the symplectic structure of the phase space of QRLG and comment on the quantization procedure applied to the Hamiltonian constraint for the Einsteinian theory of gravity and for the Standard Model of matter fields. I will finally discuss results obtained, and show in particular that the Hamiltonian eigenvalues approach the classical Hamiltonian at the leading order, while the next-to-the-leading order corrections can be discussed in conjunction with their possible phenomenological implications.
|16:30 - 17:00||Yu-Hsiang Lin||Initial conditions of inflation and the power suppression of CMB spectrum
There is an apparent power deficit relative to the Lambda-CDM prediction of the cosmic microwave background spectrum at large scales, which, though not yet statistically significant, persists from WMAP to Planck data. Proposals that invoke some form of initial condition for the inflation have been made to address this apparent power suppression, albeit with conflicting conclusions. In this talk, by studying the curvature perturbations of a scalar field in the Friedmann-Lemaitre-Robertson-Walker universe parameterized by the equation of state parameter w, we show that the large-scale spectrum at the end of inflation reflects the superhorizon spectrum of the initial state. The large-scale spectrum is suppressed if the universe begins with the adiabatic vacuum in a superinflation (w < −1) or positive-pressure (w > 0) era. In the latter case, there is however no causal mechanism to establish the initial adiabatic vacuum. On the other hand, as long as the universe begins with the adiabatic vacuum in an era with −1 < w < 0, even if there exists an intermediate positive-pressure era, the large-scale spectrum would be enhanced rather than suppressed. We further calculate the spectrum of a two-stage inflation model with a two-field potential and show that the result agrees with that obtained from the ad hoc single-field analysis.
|17:00 - 17:30||Chien-Ting Chen||Dark Energy Induced Anisotropy in Cosmic Expansion
In order to understand the nature of dark energy, it is important to experimentally discriminate between dynamical dark energy models and the cosmological constant. If dark energy already exists prior to inflation, one expect that a dynamical dark energy would leave some footprint in the anisotropy of the late-time accelerated expansion. We invoke the quintessence field, one of the simplest dynamical dark energy models, to investigate the effects of its quantum fluctuations during inflation and estimate the anisotropy of the cosmic expansion so induced. For that we calculate the perturbed luminosity distance and its power spectrum, which is an estimator of anisotropicity of the late-time accelerated expansion. We find that the smaller the redshift and the multipole, the larger the anisotropy in the cosmic expansion.
|17:30 - 18:00||Che-Yu Chen||Quantum avoidance of the big rip singularity in Eddington-inspired-Born-Infeld theory
The Eddington-inspired-Born-Infeld theory (EiBI) is characterized by its ability to cure the initial big bang singularity of the universe. However, the future big rip singularity, which is driven by phantom dark energy with a constant equation of state, is still unavoidable in the EiBI theory. The bound structure will be ripped apart before the singularity. In this work we apply quantum geometrodynamics to investigate the quantum effects near the classical big rip singularity. The Wheeler-DeWitt equation with physical scale factor and auxiliary scale factor being two independent variables is obtained and solved. We found that the wave function of the universe approaches zero at the region of the configuration space corresponding to the big rip singularity and the singularity is expected to be avoided through quantum effects. This result seems to be robust against the choices of the factor ordering and the quantization method applied.
|18:00 - 18:30||Chih-Hung Wu||Broken bridges：A counter-example of the ER=EPR conjecture
Recently, the ER=EPR conjecture was proposed as a general principle within quantum gravity. It states that for a pair of maximally entangled black holes, there should be an Einstein-Rosen bridge between them. Since both of the ER bridge and the EPR entangled pair do respect the principle of locality, we constructed a counter-example through realizing the breakdown of locality. Due to the vacuum decay of the anti-de Sitter background toward a deeper vacuum, two parts of maximally entangled black holes can be trapped by bubbles. If these bubbles are reasonably large, then within the scrambling time, there should appear an Einstein-Rosen bridge between the two black holes. Now by tracing more details on the bubble dynamics, one can identify parameters such that one of the two bubbles either monotonically shrinks or expands. Because of the change of vacuum energy, one side of the black hole would evaporate completely. Due to the shrinking of the apparent horizon, a signal of one side of the Einstein-Rosen bridge can be viewed from the opposite side. In the talk, I will also discuss recent debates with J. Maldacena regarding whether we can construct a traversable wormhole.