Date |
Time |
Speaker |
Topic |
Room |
April 10 |
12:00 |
Maaneli Derakhshani (Utrecht)
|
Is Standard Semiclassical Einstein Gravity Viable?
|
Konferenzraum 1 (Neubau) |
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.
Close
April 24 |
12:00 |
Andrea Zanzi (Ferrara)
|
The chronon in M-theory
|
Konferenzraum 1 (Neubau) |
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.
Close
April 27 |
16:30 |
T. P. Singh (Tata Institute, Mumbai)
|
Is quantum theory exact, or approximate?
|
Room 0.03 new building |
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.
Close
May 2 |
16:00 |
Branislav Nikolic
|
Making sense of quantum geometrodynamics of higher derivative gravity
|
TBA |
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?
Close
May 15 |
11:30 |
Anirudh Gundhi
|
Scale Invariant Two-Field Inflation
(Master Colloquium)
|
Konferenzraum 1 (Neubau) |
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.
Close
May 15 |
12:00 |
Yurii Dumin (Moscow)
|
Is the cosmological Lambda-term a new fundamental constant?
|
Konferenzraum 1 (Neubau) |
Is the cosmological Lambda-term a new fundamental constant?
Abstract:
TBA
Close
May 29 |
12:00 |
Tim Schmitz
|
Quantum dynamics of the outermost dust shell in the LTB model
|
Konferenzraum 1 (Neubau) |
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.
Close
June 5 |
12:00 |
Nick Kwidzinski
|
Dynamics of the general Bianchi IX model with tilted dust
|
Konferenzraum 1 (Neubau) |
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.
Close
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)
|
Quantum field theory in some Black hole background and possible
implications
|
Konferenzraum 1 (Neubau) |
Quantum field theory in some Black hole background and possible
implications
Abstract:
Quantum field theory on black hole spacetime has bizarre
consequences. Even though Hawking effect for asymptotic observers has been
well studied, there have been very little effort to understand the behaviour
of quantum fields in the interior of the event horizon. In this talk we will
try to understand the same using the regularized stress-energy tensor for the
quantum fields. Besides, we will try to pose a version of the information loss
paradox in the context of CGHS spacetime and will depict how a plausible
resolution can be arrived at. Another interesting feature associated with this
spacetime will also be demonstrated.
Close
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) |
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.
Close
August 23 |
12:00 |
Jorma Louko (Nottingham)
|
Waiting for Unruh
|
Konferenzraum 1 (Neubau) |
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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