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SUMMARY:Swagat Mishra (Inter-University Centre for Astronomy and Astrophys
ics\, Pune)
DTSTART;VALUE=DATE-TIME:20200616T103000Z
DTEND;VALUE=DATE-TIME:20200616T113000Z
DTSTAMP;VALUE=DATE-TIME:20200812T031059Z
UID:virtualgravity/1
DESCRIPTION:Title: Primordial Black Holes from a tiny bump/dip in the Infl
aton potential\nby Swagat Mishra (Inter-University Centre for Astronomy an
d Astrophysics\, Pune) as part of IACS Gravity Research Seminars\n\n\nAbst
ract\nScalar perturbations during inflation can be substantially amplified
by tiny features in the inflaton\npotential. A bump-like feature behaves
like a local speed-breaker and lowers the speed of the scalar\nfield\, the
reby locally enhancing the scalar power spectrum. A bump-like feature emer
ges naturally\nif the base inflaton potential contains a local correction
term\, which leads to a large peak in the\ncurvature power spectrum and to
an enhanced probability of black hole formation. Remarkably this\ndoes no
t significantly affect the scalar spectral index nS and tensor to scalar r
atio r on CMB scales.\nConsequently\, such models can produce higher mass
primordial black holes (M_{PBH} ≥ 1 M☉ ) in\ncontrast to models with `
near inflection-point potentials' in which generating higher mass black\nh
oles severely affects nS and r. With a suitable choice of the base potenti
al—such as the string\ntheory-based (KKLT) inflation or the α-attractor
models—the amplification of primordial scalar\npower spectrum can be as
large as ten million\, which leads to a significant contribution of\nprim
ordial black holes (PBHs) to the dark matter density today\, Interestingly
\, our results remain\nvalid if the bump is replaced by a dip. In this cas
e\, the base inflaton potential contains a negative\nlocal correction term
which also leads to an enhanced probability of PBH formation. We conclude
\nthat primordial black holes in the mass range 10−17 M_☉ ≤ M_{PBH}
≤ 100 M_☉ can easily form in single-field inflation in the presence of
small bump-like and dip-like features in the inflaton potential.\n
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BEGIN:VEVENT
SUMMARY:Amitabh Virmani (CMI)
DTSTART;VALUE=DATE-TIME:20200623T103000Z
DTEND;VALUE=DATE-TIME:20200623T113000Z
DTSTAMP;VALUE=DATE-TIME:20200812T031059Z
UID:virtualgravity/2
DESCRIPTION:Title: Quasinormal modes of supersymmetric microstate geometri
es from the D1-D5 CFT\nby Amitabh Virmani (CMI) as part of IACS Gravity Re
search Seminars\n\n\nAbstract\nIn this talk\, I will describe the computat
ion of the real and imaginary parts of the quasinormal modes of supersymme
tric microstate geometry and show that in the parameter range when the geo
metries have large AdS region\, the spectrum is precisely reproduced from
a D1-D5 orbifold CFT analysis. The spectrum includes the slow decaying mod
es pointed out by Eperon\, Reall\, and Santos. I will also provide an over
view of the geometries known as "superstrata" and the very recent quasinor
mal modes computations for these geometries by Bena\, Eperon\, Heidmann\,
and Warner.\n
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BEGIN:VEVENT
SUMMARY:Jahanur Hoque (Charles University)
DTSTART;VALUE=DATE-TIME:20200701T103000Z
DTEND;VALUE=DATE-TIME:20200701T113000Z
DTSTAMP;VALUE=DATE-TIME:20200812T031059Z
UID:virtualgravity/3
DESCRIPTION:Title: Peeling or not peeling?\nby Jahanur Hoque (Charles Univ
ersity) as part of IACS Gravity Research Seminars\n\n\nAbstract\nIn an asy
mptotically simple space-time using the conformal completion technique\, t
he peeling theorem shows that Weyl scalar $\\Psi_{n}$ falls off as $r^{n-
5}$ (r is affine parameter along null geodesic). In this talk\, we will di
scuss peeling property with or without a positive cosmological constant. W
e will also comment on the violation of the usual peeling property for a c
lass of space-time. This talk is designed to be accessible for a broad aud
ience.\n
END:VEVENT
BEGIN:VEVENT
SUMMARY:Jyotirmoy Bhattacharya (IIT Kharagpur)
DTSTART;VALUE=DATE-TIME:20200707T103000Z
DTEND;VALUE=DATE-TIME:20200707T113000Z
DTSTAMP;VALUE=DATE-TIME:20200812T031059Z
UID:virtualgravity/4
DESCRIPTION:Title: An entropy current for dynamical black holes in four-de
rivative theories of gravity\nby Jyotirmoy Bhattacharya (IIT Kharagpur) as
part of IACS Gravity Research Seminars\n\n\nAbstract\nI will begin with a
brief overview of black hole thermodynamics\, especially the second law\,
for two-derivative Einstein gravity. I will then briefly discuss Wald's e
ntropy for higher derivative corrections to Einstein's gravity\, pointing
out the difficulty in proving a general second law for it. Subsequently\,
I shall review a recent attempt by Aron Wall at generalizing Wald entropy
in linearized dynamic situations\, which obeys the second law. Through a c
loser examination of this construction\, we shall demonstrate the existenc
e of a local current on the event horizon\, for four-derivative theories o
f gravity. A local version of the second law can be conveniently expressed
in terms of this current. We will finally conclude with a discussion of t
he implications of this current\, with an outlook for future work.\n
END:VEVENT
BEGIN:VEVENT
SUMMARY:Anupam Mazumdar (University of Groningen)
DTSTART;VALUE=DATE-TIME:20200714T103000Z
DTEND;VALUE=DATE-TIME:20200714T113000Z
DTSTAMP;VALUE=DATE-TIME:20200812T031059Z
UID:virtualgravity/5
DESCRIPTION:Title: Spin Entanglement Witness for Quantum Gravity in a Labo
ratory\nby Anupam Mazumdar (University of Groningen) as part of IACS Gravi
ty Research Seminars\n\n\nAbstract\nUnderstanding gravity in the framework
of quantum mechanics and quantum field theory is one of the great challen
ges in modern physics. Along this line\, a prime question is to find wheth
er gravity is a quantum entity subject to the rules of quantum mechanics.
It is fair to say that there are no feasible ideas yet to test the quantum
coherent behaviour of gravity directly in a laboratory experiment. Here\,
I will introduce an idea for such a test based on the principle that two
objects cannot be entangled without a quantum mediator. I will show that d
espite the weakness of gravity\, the phase evolution induced by the gravit
ational interaction of two micron size test masses (diamond/Rydberg atoms
or completely a novel material) in adjacent matter-wave interferometers ca
n detectably entangle them even when they are placed far apart enough to k
eep Casimir-Polder forces at bay. I will provide a prescription for witnes
sing this entanglement\, which certifies gravity as a quantum coherent med
iator\, through simple correlation measurements between two spins: one emb
edded in each test mass. Fundamentally\, the above entanglement is shown t
o certify the presence of non-zero off-diagonal terms in the coherent stat
e basis of the gravitational field modes.\n
END:VEVENT
BEGIN:VEVENT
SUMMARY:Simone Speziale (CPT\, Aix Marseille University)
DTSTART;VALUE=DATE-TIME:20200721T093000Z
DTEND;VALUE=DATE-TIME:20200721T103000Z
DTSTAMP;VALUE=DATE-TIME:20200812T031059Z
UID:virtualgravity/6
DESCRIPTION:Title: Boundary effects in general relativity with tetrads\nby
Simone Speziale (CPT\, Aix Marseille University) as part of IACS Gravity
Research Seminars\n\n\nAbstract\nTBA\n
END:VEVENT
BEGIN:VEVENT
SUMMARY:Chiranjeeb Singha (IISER Kolkata)
DTSTART;VALUE=DATE-TIME:20200728T103000Z
DTEND;VALUE=DATE-TIME:20200728T113000Z
DTSTAMP;VALUE=DATE-TIME:20200812T031059Z
UID:virtualgravity/7
DESCRIPTION:Title: Hamiltonian-based derivation of the Hawking effect\nby
Chiranjeeb Singha (IISER Kolkata) as part of IACS Gravity Research Seminar
s\n\n\nAbstract\nIn order to achieve a Hamiltonian-based canonical derivat
ion of the Hawking effect\, one usually faces multiple hurdles. Firstly\,
spacetime foliation using Schwarzschild time does not lead to hyper-surfac
es which are always spacelike. Secondly\, the null coordinates which are f
requently used in the covariant approach\, do not lead to a true matter Ha
miltonian. In order to overcome these difficulties\, We have introduced a
new set of near-null coordinates which allows one to perform an exact cano
nical derivation of the Hawking effect. However\, there too one faces the
difficulty of having to deal with non-vanishing matter diffeomorphism gene
rator as the spatial decomposition involves a non-zero shift-vector. Then
we have introduced a new set of coordinates that allows one to perform an
exact canonical derivation of the Hawking effect without having to deal wi
th matter diffeomorphism generator. These derivations open up an avenue to
explore the Hawking effect in the framework of different canonical quanti
zation methods such as the polymer quantization. After that\, we have used
polymer quantization and shown that in polymer quantization the Hawking e
ffect is short-lived and it eventually disappears for an asymptotic future
observer.\n
END:VEVENT
BEGIN:VEVENT
SUMMARY:Bibhas Majhi (IIT Guwahati)
DTSTART;VALUE=DATE-TIME:20200804T103000Z
DTEND;VALUE=DATE-TIME:20200804T113000Z
DTSTAMP;VALUE=DATE-TIME:20200812T031059Z
UID:virtualgravity/8
DESCRIPTION:Title: Fluid-gravity correspondence: a new cut-off surface app
roach\nby Bibhas Majhi (IIT Guwahati) as part of IACS Gravity Research Sem
inars\n\n\nAbstract\nFluid-gravity correspondence connects the Navier-Stok
es (NS) equation with the Einstein’s equation on a particular hypersurfa
ce. We propose an alternative to this. We show that the vanishing of the p
rojected acceleration of a suitably chosen velocity field onto the flat ti
melike hypersurface corresponding to a particular constructed (p+2) dimens
ional manifold requires the satisfaction of the incompressible NS equation
in (p+1) dimensions. Additionally\, the incompressibility condition of th
e fluid arises from a vanishing expansion parameter. In this approach Eins
tein’s equations do not play any role and this can be regarded as an off
-shell description of fluid- gravity correspondence. We argue that our met
ric effectively encapsulates the information of forcing terms in the gover
ning equations as if a free fluid is moving on this hypersurface. We final
ly discuss the implications of this interesting observation and its potent
iality in helping us to understand hydrodynamical flows in a probable new
setting.\n
END:VEVENT
BEGIN:VEVENT
SUMMARY:Debajyoti Sarkar (IIT Indore)
DTSTART;VALUE=DATE-TIME:20200811T103000Z
DTEND;VALUE=DATE-TIME:20200811T113000Z
DTSTAMP;VALUE=DATE-TIME:20200812T031059Z
UID:virtualgravity/9
DESCRIPTION:Title: Endpoint contributions to excited-state modular Hamilto
nians\nby Debajyoti Sarkar (IIT Indore) as part of IACS Gravity Research S
eminars\n\n\nAbstract\nWe compute modular Hamiltonians for excited states
obtained by perturbing the vacuum with a unitary operator. We use operator
methods and work to first order in the strength of the perturbation. For
the most part we divide space in half and focus on perturbations generated
by integrating a local operator J over a null plane. Local operators with
weight n>=2 under vacuum modular flow produce an additional endpoint cont
ribution to the modular Hamiltonian. Intuitively this is because operators
with weight n>=2 can move degrees of freedom from a region to its complem
ent. The endpoint contribution is an integral of J over a null plane. We s
how this in detail for stress tensor perturbations in two dimensions\, whe
re the result can be verified by a conformal transformation\, and for scal
ar perturbations in a CFT. This lets us conjecture a general form for the
endpoint contribution that applies to any field theory divided into half-s
paces. This work is based on ArXiv:2006.13317 [hep-th] with Dan Kabat\, Gi
lad Lifschytz and Phuc Nguyen.\n
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