Seminars in 2017

Seminars from April 2017 through March 2018.

QG-Tagen seminar

Title
Wandering set as generalized photon sphere
Speaker
Masaru Siino (Tokyo Institute of Technology)
Date & Time
Monday, November 13, 2017, 2:30 p.m.
Place
ES744 (QG colloquium room)
Abstract
'Wandering null geodesics' and 'wandering set' are defined as a generalization of photon sphere. Several characteristics of the wandering set would be revealed. This talk will shows following results about the wandering set. 1. the exisitence theorem of the wandering set 2. conjugate point of wandering null geodesics 3. formation of the (truncated) wandering set 4. global structure of the congruence of wandering null geodesics 1-3 might be simple applications of a textbook, 'general relativity by Wald'.

QG colloquium

Title
Theoretical consistency of stochastic approach
Speaker
Junsei Tokuda (Kyoto University)
Date & Time
Tuesday, October 24, 2017, 2:30 p.m.
Place
ES744 (QG colloquium room)
Abstract
It is known that stochastic approach allows us to treat the effective dynamics of long-wavelength (IR) modes of inflaton subject to the quantum backreaction of short-wavelength (UV) modes. In a rough approximation, UV modes are treated as independent harmonic oscillators, and then the dynamics of IR modes becomes classical Brownian motion. However, it has not been clarified whether the dynamics of IR modes can be really described as a classical stochastic process once the nonlinearities of UV modes are taken into account. Motivated by above observations, we formulate the way of deriving an effective equation of motion (EOM) for IR modes of massless scalar field with general potential V(φ) on de Sitter background. We also discuss the positivity of the probability distribution function of the stochastic noise which appears in this effective EOM, because this positivity is necessary in order to interpret this effective EOM as a classical stochastic process.

KMI theory-QG-C joint seminar

Title
An astrometric search method for individually resolvable gravitational wave sources with Gaia
Speaker
Christopher Moore (CENTRA, IST, Lisboa)
Date & Time
Tuesday, October 17, 2017, 3:00 p.m.
Place
ES635 (KMI Science Symposia)
Abstract
Gravitational waves cause the apparent position of distant stars to oscillate with a characteristic pattern on the sky. Astrometric measurements (e.g. those made by the Gaia spacecraft) provide a new way to search for GWs. The main difficulty facing such a search is the large size of the data set; Gaia observes more than one billion stars. I will discuss recent progress towards a practicable search for gravitational waves with Gaia and discuss the similarities with pulsar timing arrays.

QG seminar

Title
Testing General Relativity with Gravitational Waves
Speaker
Vitor Cardoso (CENTRA, IST, Lisboa)
Date & Time
Tuesday, September 26, 2017, 5:00 p.m.
Place
ES035
Abstract
Two pivotal breakthroughs in physics have just celebrated their centennial: the discovery of the Schwarzschild solution, describing a non-rotating black hole, and Einstein's prediction of gravitational waves. Gravitational waves offer a unique glimpse into the unseen universe in different ways, and allow us to test the basic tenets of General Relativity, some of which have been taken for granted without observations: are gravitons massless? Are black holes the simplest possible macroscopic objects? do event horizons and black holes really exist, or is their formation halted by some as-yet unknown mechanism? Do singularities arise in our universe as the outcome of violent collisions? Can gravitational waves carry information about the nature of the elusive dark matter? In this talk, I will describe the science encoded in a gravitational wave signal and what the upcoming years might have in store regarding fundamental physics and gravitational waves.

QG seminar

Title
On Static Solutions of the Einstein-Vlasov System
Speaker
Maximilian Thaller (Chalmers University of Technology)
Date & Time
Wednesday, August 2, 2017, 1:30 p.m.
Place
QG colloquium (ES744)
Abstract
The Einstein-Vlasov system describes the motion of an ensemble of collisionless particles in the framework of general relativity. This presentation focuses on static solutions in spherical symmetry. Three different settings will be discussed: Solutions with cosmological constant, massless solutions with matter of compact support, and solutions describing charged particles. Using properties of static solutions in the "simple setting" (i.e. no cosmological constant, no charge) and other techniques existence of solutions in the three cases is shown. Moreover, properties of massless solutions with compactly supported matter are discussed and analogies to the notion of geons are pointed out. In the case of charged particles, limits that saturate an inequality for the critical stability radius will be addressed. Some of the results are a collaboration with H. Andrsson (Chalmers Univ. of Technology) and D. Fajman (Univ. of Vienna).

QG colloquium

Title
Driving Mechanism of Black Hole Jets
Speaker
Kenji Toma (Tohoku Unisersity)
Date & Time
Tuesday, July 11, 2017, 2:30 p.m.
Place
QG colloquium (ES744)
Abstract
Relativistic Jets are collimated plasma outflows with speeds close to the light speed from the vicinities of black holes, and observed in association with some active galaxies and gamma-ray bursts. They could be the emitters of high-energy cosmic-rays, high-energy neutrinos, and gravitational waves. In spite of the significant progress in recent years, there are still many unresolved key issues concerning the energy injection, mass injection, acceleration, collimation, and energy dissipation/emission mechanisms of jets. This talk reviews the current theoretical framework for solving those issues and then focuses on the energy injection mechanism, in particular the Blandford-Znajek process, i.e., steady electromagnetic energy injection by rotating black holes themselves. Physical differences among this process, pulsar winds, and mechanical Penrose process are discussed.

QG colloquium

Title
On the Energetics of Vacuum Entanglement
Speaker
Jason Pye (University of Waterloo)
Date & Time
Monday, July 10, 2017, 3:30 p.m.
Place
QG colloquium (ES744)
Abstract
The relation between vacuum entanglement and energy/geometry has been a central theme of Relativistic Quantum Information and Quantum Gravity, usually studied in the context of black hole thermodynamics or the Unruh effect. In this work we analyze the energetics of the vacuum entanglement of a ball-shaped subregion of a spacelike hypersurface. For this bipartition of the field's Hilbert space, we explicitly find the Schmidt basis for the entangled pure state. We show that the vacuum can be expressed as a tensor product of squeezed vacua between interior and exterior modes, whose mode functions we construct from Jacobson's Modular Hamiltonian flow. Using this explicit state expression, we study contributions to the stress energy tensor for three cases of variations from the ground state. The first two cases consist of varying the Schmidt squeezing parameters, to either amplify or reduce the entanglement unitarily. In the third case, we break the entirety of the interior-exterior entanglement by measuring the interior Schmidt modes' number occupancy. We show that the observable of this measurement is, by construction, the observable in the interior region which provides the maximal amount of information about the state in the exterior region. We explicitly show that in this case of maximal entanglement breaking via measurement, one creates a "firewall" of energy mostly concentrated at the region's boundary, and we provide visualizations of this firewall. In all three cases, we compare the entanglement's spatial profile to the stress-energy tensor expectation values, for both a linear harmonic chain and continuum quantum field theory.

Colloquium

Title
宇宙の大規模構造の理論的記述:標準宇宙モデルを超えて
Speaker
Atsushi Taruya (YITP, Kyoto Univ.)
Date & Time
Tuesday, June 27, 2017, 2:00 p.m.
Place
Science Symposia (ES635)

Special Lecture (for DC)

Title
暗黒物質宇宙における構造形成
Speaker
Atsushi Taruya (YITP, Kyoto Univ.)
Date & Time
Monday, June 26–Wednesday, June 28, 2017
Place
Science Symposia (ES635)

CEHQG joint seminar

Title
Inflationary massive gravity
Speaker
Misao Sasaki (YITP, Kyoto Univ.)
Date & Time
Tuesday, June 6, 2017, 5:00 p.m.
Place
Science Symposia (ES635)
Abstract
Inflation is a natural platform for modified gravity. Here we present a new massive gravity theory in which the gravitational wave (GW) modes become massive during inflation. Then we discuss its observational signatures, which are particularly intriguing in this coming era of GW physics/astronomy.

QG colloquium

Title
Boost Breaking in the EFT of Inflation
Speaker
Toshifumi Noumi (Kobe Univ.)
Date & Time
Monday, May 29, 2017, 3:30 p.m.
Place
QG colloquium (ES744)
Abstract
Primordial fluctuations, quantum fluctuations generated during inflation, provide a seed of the structure in our universe such as the cosmic microwave background (CMB) temperature fluctuations and the large scale structure. In particular, it is known that primordial density fluctuations can be interpreted as the Nambu-Goldstone (NG) boson associated with the spontaneous breaking of time translation symmetry. Correspondingly, the effective field theory (EFT) approach based on such a symmetry breaking structure provides a powerful framework for the study of primordial fluctuations. In this talk, I will first review such an EFT approach for inflation. I will then discuss how the boost symmetry breaking can be accommodated in the EFT framework. In particular I will illustrate that the NG bosons associated with broken boost symmetries acquire the Hubble scale mass and induce observable non-Gaussianities.