Research

Past Research

Black hole thermodynamics: Pathbreaking work of Stephan Hawking, published in 1975, had raised an important point that black holes are not really black but radiate just like a blackbody. This discovery along with the works of Jacob Bekenstein, who argued in favor of black hole entropy, provided an invaluable concept of black holes thermodynamics. Understanding thermodynamics of black holes, in particular, and of spacetimes in general has a profound importance in the quest of understanding fundamental building blocks of Nature. Could it be possible that this thermodynamic structure points a statistical description of gravity? This line of thought naturally comes since any thermodynamic system should have an underlying statistical description. If this is true, is that possible that gravity itself is an effective theory of some underlying, yet to be discovered, statistical theory? Are we going in a wrong direction while trying to quantize gravity like the other three fundamental forces of Nature? What are the fundamental degrees of freedom of this effective theory? Understanding black holes as thermodynamic systems is a good starting point and it has been a playground of generating new ideas for decades.

I did Ph.D thesis on black hole thermodynamics. My contribution can be briefly summarized in the following manner - I provided a thermodynamic interpretation of the Smarr mass formula as a thermodynamic identity E = 2ST where E is closely analogous to the total energy of the black hole and S, T are respectively the entropy and temperature of the black hole [PRD 2010, IJTP 2012]; I calculated the leading order correction to Bekenstein-Hawking entropy (which is logarithmic) applicable to all stationary black holes of Einsetin gravity in 3+1 dimensions [JHEP 2009, PLB 2009]; and I provided a new approach based on Ehrenfest's equations, to establish and classify the phase transition phenomena in black holes and made a comaprison with other simple systems [EPJC 2010, JHEP 2012, PRD 2011].

Holographic Superconductivity: The AdS/CFT conjecture by Maldacena provides a duality between the strongly correlated conformal field theory with the weakly coupled String Theory which can safely be assumed as a classical (super) gravity theory. This, after a considerable stretch, was extended by Hartnoll, Herzog and Horrowtiz [PRL, 2008] to explain the exotic superconductivity using a gravitational dual defined in an anti-de Sitter space. On the other hand there is another conjecture by Ryu and Takayanagi where they define entanglement entropy, so called Holographic Entanglement Entropy (HEE), using a gravitational dual [PRL 2006]. I was interested in these topics and wrote a paper on the behavior of HEE for the case of imbalanced superconductors [JHEP 2014].

Present interests

The topics that I have been involved in recent years are the following:

• Quantum Field Theory in Curved Spacetime (QFTCS): pioneering works of Hawking, Unruh, Fulling, Davies, Bunch, Parker and others have generated significant interest in studying QFTCs. Even before considering an interacting field theory things become immensely interesting because of the nontrivial nature of the quantum vacuum of matter fields in a curved space. Vacuum is not unique: different observers may have different definition of vacuum states and interesting things happen when they look at each others vacuums. Also, dyanmics of background spacetime shows nontrivial effects on the excitation of the vacuum states. In some of my recent works, in collaboration with T. Padmanabhan, D. Singleton and others I have made an effort to better understand this in the setting of an expanding universe [PRD 2012, PRD 2013, IJMPD 2012, EPJC 2014]. In addition, recently, I installed a new coordinate system in the radiation dominated phase of early universe and introduced a new vacuum state which was referred as the T-vacuum. A consistent construction of QFT and the existence of Unruh like effect was discovered for the cosmological observers [PRD 2018, IJMPD 2019, JHEP 2020(in press)].

• Quantum foundation and black hole information puzzle: the widely known problem of ''black hole information paradox'' is one of the intriguing, unanimously unresolved problems of modern physics. It apparently violates the unitarity property of quantum mechanics whereby the information about the initial state of the matter (which creats a black hole and the latter then evaporates away due to the Hawking evaporation) appears to be lost. In collaboration with Daniel Sudarsky and other group members I have shown that the paradox can be dissolved by using the so called wavefunction collapse models of quantum mechanics. In fact our point is that this paradox is intricately related with another lingering problem in the foundation of quantum mechanics -- the so called ''measurement problem'' [PRD 2015, GRG 2015, PRD 2016, EPJC 2018].

• Quantum gravity phenomenology: the main problem in testing any proposal on the quantum theory of gravity is its energy scale which is ten to the power nineteen orders of magnitude in GeV scale. Direct probe of quantum gravity may not be available ever. The next best possible approach is to consider any effective theory or an indirect low energy test of a readily available proposal of quantum gravity. Two popular approaches, such as the String Theory and the Loop Quantum Gravity, both, have proposed that the commutator brackets between the position and momentum operator may get modified in a certain way if there is indeed a fundamental discreteness of space. This so called ''minimal length'' is considered to be near or at the Planck value (which is ten to the power minus 35 meters). I have recently shown that by studying dispersion of free molecular wavepacket we may get some hint about the existence of this ''minimal length'' [PRD 2019, CQG 2019].