Astronomy

Gravitational Collapse of Inhomogenous Perfect Fluid
We study the complete gravitational collapse of a class of inhomogeneousperfect fluid models obtained by introducing small radial perturbations in anotherwise homogeneous matter cloud. The key feature that we assume for theperturbation profile is that of a mass profile that is separable in radial andtemporal coordinates. The known models of dust and homogeneous perfect fluidcollapse can be obta...


The Firewall Paradox
In this MSc. thesis, we have attempted to give an overview of the firewallparadox and various approaches towards its resolution. After an introductorychapter on some basic concepts in quantum field theory in curved spacetimessuch as Hawking radiation, we introduce the paradox. It arises out ofapplication of principles each of which is thought or assumed to be correct: 1)unitary black hole evapo...
Gravitational dispersion in a torsional wave machine
We demonstrate that mechanical waves traveling in a torsional, mechanicalwave machine exhibit dispersion due to gravity and the discreteness of themedium. We also show that although the dispersion due to discreteness isnegligible, the dispersion due to gravity can be easily measured, and can beshown to disappear in a zero-gravity environment.


Non-embeddable relational configurations
Relational configurations are defined by the relationships between systems. Individualist configurations are defined by the properties of individualsystems. Non-embeddable relational configurations are those for which there isno equivalent individualist description. We suggest that situations in physicswhere a spacetime description breaks down may be the result of trying to useindividualist con...
The Loudest Gravitational Wave Events
We derive the universal distribution of signal-to-noise ratios forgravitational wave detection. Because gravitational waves (GWs) are almostimpossible to obscure via dust absorption or other astrophysical processes, thestrength of the detected signal is dictated solely by the emission strength andthe distance to the source. Assuming that the space density of an arbitrarypopulation of GW sources...


Magnetic Effects in Hot Jupiter Atmospheres
We present magnetohydrodynamic (MHD) simulations of the atmospheres of hotJupiters ranging in temperature from 1100-1800K. Magnetic effects arenegligible in atmospheres with temperatures $lesssim$ 1400K. At highertemperatures winds are variable and in many cases, mean equatorial flows canbecome westward, opposite to their hydrodynamic counterparts. Ohmic dissipationpeaks at temperatures $sim$15...
Effects Of The Ionosphere On Ground-Based Detection Of The Global 21 CM Signal From The Cosmic Dawn And The Dark Ages
Detection of global HI 21 cm signal from the Cosmic Dawn and the Epoch ofReionization is the key science driver for several ongoing ground-based andfuture ground/space based experiments. The crucial spectral features in theglobal 21cm signal (turning points) occurs at low radio frequencies < 100 MHz. In addition to the human-generated RFI (Radio Frequency Interference), Earth'sionosphere drasti...

New Constraints on the Faint-end of the UV Luminosity Function at z~7-8 using the Gravitational Lensing of the Hubble Frontier Fields Cluster A2744
Exploiting the power of gravitational lensing, the Hubble Frontier Fields(HFF) program aims at observing six massive galaxy clusters to explore thedistant Universe far beyond the depth limits of blank field surveys. Using thecomplete {em Hubble Space Telescope} observations of the first HFF clusterAbell 2744, we report the detection of 50 galaxy candidates at $z sim 7$ andeight candidates at $z...
Fine Structures and Overlying Loops of Confined Solar Flares
Using the H$alpha$ observations from the New Vacuum Solar Telescope atFuxian Solar Observatory, we focus on the fine structures of three confinedflares and the issue why all the three flares are confined instead of eruptive. All the three confined flares take place successively at the same location andhave similar morphologies, so can be termed homologous confined flares. In thesimultaneous ima...

Neutrino Telescope Array (NTA) - Towards Survey of Astronomical $nu_tau$ Sources
The Earth-skimming $nu_{tau}$ method allows for huge target mass anddetection volume simultaneously. In part motivated by IceCube PeVastro-neutrino events, the planned NTA observatory has three site stationswatching the air mass surrounded by Mauna Loa, Mauna Kea, and Hualalai onHawaii Island, plus a site station at the center watching the lower night sky. Sensitivities equivalent to $> 100$ km...
On tidal capture of primordial black holes by neutron stars
The fraction of primordial black holes (PBHs) of masses $10^{17} - 10^{26}$ gin the total amount of dark matter may be constrained by considering theircapture by neutron stars (NSs), which leads to the rapid destruction of thelatter. The constraints depend crucially on the capture rate which, in turn, isdetermined by the energy loss by a PBH passing through a NS. Two alternativeapproaches to es...

On the reliability of microvariability tests in quasars
Microvariations probe the physics and internal structure of quasars. Unpredictability and small flux variations make this phenomenon elusive anddifficult to detect. Variance based probes such as the C and F tests, or acombination of both, are popular methods to compare the light-curves of thequasar and a comparison star. Recently, detection claims in some studies dependon the agreement of the r...
Evidence for the concurrent growth of thick discs and central mass concentrations from S$^4$G imaging
We have produced $3.6mu{rm m}+4.5mu{rm m}$ vertically integrated radialluminosity profiles of 69 edge-on galaxies from the Spitzer Survey of StellarStructure in Galaxies (S$^4$G). We decomposed the luminosity profiles into adisc and a central mass concentration (CMC). These fits, combined withthin/thick disc decompositions from our previous studies, allow us to estimatethe masses of the CMCs, t...
A Space Weather Mission Concept: Observatories of the Solar Corona and Active Regions (OSCAR)
Coronal Mass Ejections (CMEs) and Corotating Interaction Regions (CIRs) aremajor sources of magnetic storms on Earth and are therefore considered to bethe most dangerous space weather events. The Observatories of Solar Corona andActive Regions (OSCAR) mission is designed to identify the 3D structure ofcoronal loops and to study the trigger mechanisms of CMEs in solar ActiveRegions (ARs) as well...
The role of quantum interference and partial redistribution in the solar Ba II D2 4554 A line
The Ba II D2 line at 4554 A is a good example, where the F-state interferenceeffects due to the odd isotopes produce polarization profiles, which are verydifferent from those of the even isotopes that do not exhibit F-stateinterference. It is therefore necessary to account for the contributions fromthe different isotopes to understand the observed linear polarization profilesof this line. In th...