The upcoming seminars will be on the online platform ZOOM. The ZOOM link will be shared with the registered participants by Email.
Jointly Organized by IIT Gandhinagar Physics discipline & IACS Kolkata.
Contacts:
Sumanta Chakraborty
Indian Association for the Cultivation of Science, Kolkata
Email: sumantac.physics@gmail.com
Sudipta Sarkar
IIT Gandhinagar | Email: sudiptas@iitgn.ac.in
Gravitational memory effect is the permanent displacement in the relative separation between freely falling particles resulting from the passage of gravitational wave train. We obtain a closed form expression for the linearized perturbation upto quadrupolar order around de Sitter space-times generated by spatially compact sources. We demonstrate that such a source causes a displacement memory effect close to future infinity. We also discuss a correspondence between memory effect and asymptotic symmetries of de Sitter.ee
Gravitational waves open the possibility to investigate the nature of compact objects and probe the existence of horizons in black holes. This is of particular interest given some quantum-gravity models which predict the presence of horizonless and singularity-free compact objects. Such exotic compact objects can emit a different gravitational-wave signal relative to the black hole case. In this talk, I derive the characteristic oscillation frequencies of horizonless compact objects in the ringdown. Finally, I describe how parametrised tests on general relativity can allow for tests of the black hole paradigm.
If the conventional picture of black hole thermodynamics is correct then the laws of black hole mechanics should be robust against the inclusion of higher derivative effective field theory (EFT) corrections to the equations of motion. For the first law of black hole mechanics this was confirmed by the work of Wald in the early 1990s. Ever since then it has been an open problem to extend this work to obtain a definition of black hole entropy that satisfies a second law of black hole mechanics in gravitational EFTs. In this talk I will present recent work that solves this critical problem.
Effective field theory (EFT) provides a way of parameterising strong-field deviations from General Relativity that might be observable in the gravitational waves emitted in a black hole merger. To perform numerical simulations of mergers in such theories it is necessary that the equations be written in a form that admits a well-posed initial value formulation. Until now it has not been known how to do this for equations involving higher derivative EFT corrections. In this talk I will describe work with Aron Kovacs in which we found a well-posed formulation of the equations of motion for gravity coupled to a scalar field including the leading (4-derivative) EFT corrections. This is based on a new class of ``modified harmonic” gauges. I will explain how this idea also works for 4-derivative corrections to Einstein-Maxwell theory. I will also discuss causality in these theories.
We discuss a family of four-dimensional, asymptotically flat, charged black holes that develop scalar hair as one increases their charge at fixed mass. Surprisingly, the maximum charge for given mass is a nonsingular hairy black hole with nonzero Hawking temperature. The implications for Hawking evaporation are discussed.
When we dive inside the event horizon, these black holes resemble the interior of a holographic superconductor. There are analogs of the Josephson oscillations of the scalar field, and the final Kasner spacelike singularity replaces the would-be Cauchy horizon and depends very sensitively on the black hole parameters near the onset of the instability.
I will review no-hair theorems and black holes with scalar hair. I will then argue that, in most interesting scenarios in which compact object could carry scalar hair, supermassive black holes are still expected to be described to high precision by the Kerr spacetime. I will then present recent results demonstrating that gravitational wave observations from extreme mass ratio inspirals, - systems in which a much smaller compact object orbits around and eventually plunged into a supermassive black hole - can measure or constrain scalar charge with unprecedented accuracy.
Kontsevich and Segal (K-S) have proposed a criterion to determine which complex metrics should be allowed, based on the requirement that quantum field theories may consistently be defined on these metrics, and Witten has recently suggested that their proposal should also apply to gravity. We explore this criterion in the context of gravitational path integrals, in simple minisuperspace models, specifically considering de Sitter (dS), no-boundary and Anti-de Sitter (AdS) examples. These simple examples allow us to gain some understanding of the off-shell structure of gravitational path integrals. In all cases, we find that the saddle points of the integral lie right at the edge of the allowable domain of metrics, even when the saddle points are complex or Euclidean. Moreover the Lefschetz thimbles, in particular the steepest descent contours for the lapse integral, are cut off as they intrude into the domain of non-allowable metrics. In the AdS case, the implied restriction on the integration contour is found to have a simple physical interpretation. In the dS case, the lapse integral is forced to become asymptotically Euclidean. We also point out that the K-S criterion provides a reason, in the context of the no-boundary proposal, for why scalar fields would start their evolution at local extrema of their potential.
The quasinormal-mode spectrum of a horizonless compact object can differ significantly from that of the corresponding classical black hole. However, the time response is initially very similar if the object is sufficiently compact. A generic smoking gun of the absence of a classical horizon is the presence of echoes in the late-time ringdown. The echo delay time and morphology depend crucially on the properties of the object down to its potential well. Most of the echo analyses so far have considered toy or phenomenological models. I will present recent results on the ringdown phenomenology for a class of multicenter geometries describing the microstates of a static BPS black hole in N=2 supergravity, unveiling the whole ringdown phenomenology studied in recent years for exotic compact objects albeit in much more complicated settings in which the ringing object has a complex multipolar structure. The numerical method is based on numerical-relativity simulations of a test scalar field propagating on these geometries and can be applied to any stationary microstate, including non-BPS ones. Our results provide the first numerical evidence for the dynamical linear stability of fuzzballs, and pave the way for an accurate discrimination between fuzzballs and black holes using gravitational-wave spectroscopy.
The Hawking effect of spontaneous emission of thermal radiation by black holes is one of the most remarkable consequences of quantum field theory in curved spacetimes. It provides a deep connection between causal horizons and thermodynamics, whose full range is yet to be understood. It was further noticed by W. Unruh in 1981 that this relationship goes beyond causal horizons generated from gravitational effects—this observation gave birth to the exploration and experimental search of the Hawking effect in other systems, including Bose-Einstein condensates, optical systems, fluids, etc.
The weak intensity of the Hawking radiation makes its direct observation really challenging. Stimulating the process, as we do to generate intense laser beams, could be a promising avenue. However, the stimulated Hawking effect is commonly regarded as a purely classical process, of little value to amplify the quantum aspects of the Hawking effect. In this talk, we will argue otherwise, and describe a protocol to amplify and observe these quantum features, based on stimulating the process with non-classical inputs. Although our ideas are general, we formulated them in the context of optical systems containing the analog of a pair white-black holes. These results open the door to new possibilities of experimental verification of the Hawking effect.
The discovery of the accelerated expansion of the Universe has come relatively late in our study of the cosmos, but in showing that gravity can act repulsively, it has opened up many new questions about the nature of gravity and what the Universe might contain. Is the acceleration being driven by dark energy? Or is general relativity (GR) itself in error, requiring a modification at large scales to account for the late acceleration? Structure formation in our Universe can be different even if the geometry of the homogeneous and isotropic universe is the same in these two classes of models, offering a possibility to distinguish between them observationally. I will discuss cosmological tests of gravity using the combination of latest cosmological observations.
Abstract: Hypothetical ultralight bosonic fields will spontaneously form macroscopic bosonic halos around Kerr black holes, via superradiance, transferring part of the mass and angular momentum of the black hole into the halo. Such a process, however, is only efficient if resonant: when the Compton wavelength of the field approximately matches the gravitational scale of the black hole. For a complex-valued field, the process can form a stationary, bosonic field-black hole equilibrium state - a black hole with synchronised hair. For sufficiently massive black holes, such as the one at the centre of the M87 supergiant elliptical galaxy, the hairy black hole can be robust against its own superradiant instabilities, within a Hubble time. Studying the shadows of such scalar hairy black holes, we can constrain the amount of hair which is compatible with the Event Horizon Telescope (EHT) observations of the M87 supermassive black hole, assuming the hair is a condensate of ultralight scalar particles of mass ∼1E−20 eV, as to be dynamically viable. We show the EHT observations set a weak constraint, in the sense that typical hairy black holes that could develop their hair dynamically, are compatible with the observations, when taking into account the EHT error bars and the black hole mass/distance uncertainty. We will also discuss a recent theorem establishing that an equilibrium Black Hole must admit, under generic conditions, at least one circular Light Ring orbit outside the horizon. The proof relies on a topological argument and makes virtually no assumptions on the matter content or gravity model.
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