Seminars

Winter term 2017/18


 

Is Standard Semiclassical Einstein Gravity Viable?

Abstract: It will be examined whether standard semiclassical Einstein gravity (SSEG), represented by the standard semiclassical Einstein equation, constitutes a self-consistent and empirically adequate theory of matter-gravity coupling. This will be done from two points of view: SSEG as a fundamental proposal, and SSEG as a mean-field approximation to a (putative) quantum theory of gravity. It will be shown that, from either point of view, there are compelling reasons to question the self-consistency and empirical adequacy of SSEG. Finally, the implications will be discussed for one of the most prominent applications of SSEG - Hawking's 1975 argument (and refinements thereof) that black holes evaporate due to semiclassical gravitational back-reaction of emitted Hawking radiation.

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The chronon in M-theory

Abstract: In this seminar we discuss some aspects of chronon physics in an M-theory model. We start considering the possibility that quantum fluctuations are stretched on very large distances allowing a quantum mechanical treatment of physics on certain macroscopic scales. A crucial element of our analysis is the relativity of time. Indeed, the presence of a 5D black hole (with its gravitational field) leads us to a scenario where small quantum fluctuations produced near the black hole become very large for an asymptotic observer in harmony with the relativity of time. In the deep IR region, gravity shows new phenomena (related to an orbifold of time) which cannot be described through a field theory on the brane and, in this sense, these phenomena resemble the action-at-a-distance of Newtonian gravity when interpreted from the standpoint of S-brane physics. If an observer on the brane tries to analyze quantum gravity in his ground state, the gauge fixing procedure shows that all the matter fields and all the interactions of the Standard Model become redundant: the only physical degree of freedom for quantum gravity in this case is the time (namely the chronon) with its two dimensions (parametrized by the dilaton and the radion). Remarkably, the dynamics of the chronon is governed by a modified Schroedinger equation and, in this sense, the Schroedinger equation is the most fundamental equation of physics. A dilatonic signal traveling in the bulk can bring information from the future (of a different dilatonic time dimension) to our present dilatonic time. The selection of the dilatonic time dimension is related to the value of the chameleonic radion which is stabilized by some UV dynamics. Therefore, a modification of the environment produces a small shift in the radionic coordinate and this selects a totally different dilatonic time dimension (this mechanism is reminiscent of a butterfly effect). A number of consequences of this approach will be discussed.

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Is quantum theory exact, or approximate?

Abstract: Why does the wave-function of a quantum system collapse during a measurement? It maybe possible to answer this question, as in the many-worlds interpretation, without modifying the theory. In this case, quantum theory is exact. On the other hand, it maybe that the theory has to be modified, as in the phenomenological model of Spontaneous Collapse. This model proposes that every quantum particle in nature undergoes spontaneous collapse, and that collapse is a fundamental property of nature, along with Schrödinger evolution. The predictions of this model differ from those of quantum theory, and we review the current experimental bounds on these models. We also review work which attempts to provide a theoretical underpinning for this phenomenological model, including the possible role of gravity, and a possible inter-connection between the measurement problem and the problem of time in quantum theory.

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How to make sense of quantum geometrodynamics of higher derivative gravity?

Abstract: Quantum geometrodynamics is, in its traditional formulation, a canonical quantization of General Relativity, which is based on the Einstein-Hilbert action. However, the semiclassical gravity - considered to be an energy regime between the classical and quantum gravity sacales - requires the action to be supplemented with additional terms such as Weyl-tensor squared and R-squared terms. These terms have to do with quantum-mechanical corrections rendering the high energy behavior of the theory finite, but traditional quantum geometrodynamics does not take them into account (except on one occasion, see Mazzitelli, Phys. Rev. D8 (1992), 2814).

It is natural to ask the following question, which is the topic of this talk: if the Einstein-Hilbert action is modified at higher energies by these corection terms, then how to construct a quantum geometrodynamics theory expected at even higher energies, such that these correction terms are taken into account and give the correct semiclassical limit?

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Scale Invariant Two-Field Inflation

Abstract: In this thesis, a two-field inflationary scenario is investigated. In our considerations, the two scalar fields originate from the Higgs field, and the higher derivative R2 term. We study the non-minimal Higgs and Starobinsky inflationary scenarios together, motivated from their success of fitting the inflationary observables when considered separately. Starting from the action in the Jordan frame, the corresponding two field potential is derived in the Einstein frame. We then introduce the covariant formalism, which is required due to the additional complication of having non-canonical kinetic terms for the two scalar fields in the Einstein frame. In this setting, we first constraint our model from the CMB observations, and within that constraint explore the possible generation of isocurvature modes, a feature that is exclusive to multifield scenarios, and cannot be explained by non-minimal Higgs or Starobinsky inflation alone.

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Is the cosmological Lambda-term a new fundamental constant?

Abstract: TBA

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Quantum dynamics of the outermost dust shell in the LTB model

Abstract: An action for the outermost shell in the Lema\^{i}tre-Tolman-Bondi model for spherically symmetric, inhomogeneous dust collapse is derived starting from the Einstein-Hilbert action. The resulting Hamiltonian is then quantized and shown to admit exact solutions. Because the coordinates are fixed (to the ones naturally provided by the dust) one can construct an Hilbert space, and wavefunctions have the usual probability interpretation. Restricting to positive ADM energies, all solutions contained in this Hilbert space avoid the classical singularity: The shell never collapses to a point. Furthermore, a wave packet centered initially around the classical trajectory is shown to bounce, meaning after collapse to some non-zero minimal radius (which can lie below the apparent horizon), it expands again in a time-reversal symmetric fashion. Hence it can be said, in a slight misuse of terminology, that this model features a quantum transition from black hole to white hole. Finally some implications of this bouncing behavior are discussed: the nature of the horizon, and the lifetime of the temporary 'grey' hole from the perspective of a co-moving and exterior observer, respectively.

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Dynamics of the general Bianchi IX model with tilted dust

Abstract: In this talk we will examine the dynamics of the non-diagonal Bianchi IX cosmological model filled with a tilted pressureless fluid (dust). Particular attention shall be given to the dynamics in the regime close to the singularity.

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A correspondence between 1st and 2nd order formalism by a metricity constraint

Abstract: A way to obtain a correspondence between the first order and second order formalism is studied. By introducing a Lagrange multiplier coupled to the covariant derivative of the metric, a metricity constraint is implemented. The new contributions which comes from the variation of the Lagrange multiplier transforms the field equations from the first order to the second order formalism, yet the action is formulated in the first order. In this way all the higher derivatives terms in the second order formalism appear as derivatives of the Lagrange multiplier. Using the same method for breaking metricity condition and building conformal invariant theory is briefly discussed, so the method goes beyond just the study of first order or second formulations of gravity, in fact vast new possible theories of gravity are envisioned this way.

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Waiting for Unruh

Abstract: How long does a uniformly accelerated observer need to interact with a quantum field in order to record thermality in the Unruh temperature? In the limit of large excitation energy, the answer turns out to be sensitive to whether (i) the switch-on and switch-off periods are stretched proportionally to the total interaction time T, or whether (ii) T grows by stretching a plateau in which the interaction remains at constant strength but keeping the switch-on and switch-off intervals of fixed duration. For a pointlike Unruh-DeWitt detector, coupled linearly to a massless scalar field in four spacetime dimensions and treated within first order perturbation theory, we show that letting T grow polynomially in the detector's energy gap E suffices in case (i) but not in case (ii), under mild technical conditions. These results limit the utility of the large E regime as a probe of thermality in time-dependent versions of the Hawking and Unruh effects, such as an observer falling into a radiating black hole. They may also have implications on the design of prospective experimental tests of the Unruh effect.

Based on arXiv:1605.01316 (published in CQG) with Christopher J Fewster and Benito A Juarez-Aubry.

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Date Time Speaker Topic Room
April 10 12:00 Maaneli Derakhshani (Utrecht)
Is Standard Semiclassical Einstein Gravity Viable? Konferenzraum 1 (Neubau)
April 24 12:00 Andrea Zanzi (Ferrara)
The chronon in M-theory Konferenzraum 1 (Neubau)
April 27 16:30 T. P. Singh (Tata Institute, Mumbai)
Is quantum theory exact, or approximate? Room 0.03 new building
May 2 16:00 Branislav Nikolic
Making sense of quantum geometrodynamics of higher derivative gravity TBA
May 15 11:30 Anirudh Gundhi
Scale Invariant Two-Field Inflation (Master Colloquium) Konferenzraum 1 (Neubau)
May 15 12:00 Yurii Dumin (Moscow)
Is the cosmological Lambda-term a new fundamental constant? Konferenzraum 1 (Neubau)
May 29 12:00 Tim Schmitz
Quantum dynamics of the outermost dust shell in the LTB model Konferenzraum 1 (Neubau)
June 5 12:00 Nick Kwidzinski
Dynamics of the general Bianchi IX model with tilted dust Konferenzraum 1 (Neubau)
June 12 12:00 Yi-Fan Wang
Conference report Konferenzraum 1 (Neubau)
June 19 12:00
Presentations by the group Konferenzraum 1 (Neubau)
June 26 12:00
Presentations by the group Konferenzraum 1 (Neubau)
July 3 12:00
Presentations by the group Konferenzraum 1 (Neubau)
July 10 12:00 Sumanta Chakraborty (Kolkata)
TBA Konferenzraum 1 (Neubau)
July 17 12:00 David Chay Benisty (Ben Gurion University)
A correspondence between 1st and 2nd order formalism by a metricity constraint Konferenzraum 1 (Neubau)
August 23 12:00 Jorma Louko (Nottingham)
Waiting for Unruh Konferenzraum 1 (Neubau)

 


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