By now gravitational wave observations have established that black holes are ubiquitous in our universe. But what exactly is a black hole? What exactly is it that forms as a result of a gravitational collapse and evaporates due to quantum radiation? The common answer is of course ‘horizons’. But the notion of horizons is rather enigmatic. For example, an event horizon may be forming and growing in your room in anticipation of a gravitational collapse that may take place a billion years from now! However there are alternative notions that can be used in an effective manner both in the analysis of black hole mergers in classical general relativity as well as the quantum process of black hole evaporation. In this talk I will confine myself to classical considerations where the seminal advances made by Professor A. K. Raychaudhuri play a key role.

In this talk, I will show that there are serious indications of an extended scalar sector in the LHC data, and discuss the rich phenomenology under the assumption of an underlying renormalisable theory. 

GW190521 is the first confident intermediate-mass black hole merger event in the international gravitational wave network. The discovery was unique and posed pertinent questions on black hole formation channels. Observation of these systems carry signatures of eccentricity, precession and thus the formation channels. Detecting these binaries in LIGO-Virgo detectors face several challenges due to its peculiar morphology. The talk will highlight the discovery, detection strategies, and the astrophysical implication.

The standard cosmological model assumes an exactly isotropic & homogeneous (FLRW) space-time. Interpreting astronomical data in this framework leads to the conclusion that the Universe is dominated by a Cosmological Constant, Λ ~ H_0^2, which is driving acceleration of the Hubble expansion rate H_0. The local Universe is however rather inhomogeneous and this is supposedly why there is a prominent dipole anisotropy in the cosmic microwave background - interpreted as due to our `peculiar motion' wrt the cosmic rest frame. There should then be a corresponding dipole anistropy in the sky distribution of distant quasars & radio sources. We find that this expectation is rejected at >5σ significance, thus challenging the FLRW assumption and the consequent inference of Λ.

According to the standard paradigm of primordial magnetogenesis, the seed magnetic fields on cosmological scales are generated during inflation by breaking the conformal invariance of the standard electromagnetic action. This is usually achieved through a non-conformal coupling of the electromagnetic field to the inflaton. Also, a parity violating term is often added to the action to generate helical magnetic fields. In the first part of the talk, I will discuss the effects of deviations from slow roll inflation on the spectra of non-helical as well as helical electromagnetic fields over large and small scales in single field models of inflation. We find that, to generate nearly scale invariant spectra of magnetic fields, even in slow roll inflation, one has to construct non-conformal coupling functions that are dependent on the inflationary model being considered. We show that sharp features in the scalar power spectrum generated due to departures from slow roll inflation inevitably lead to strong features in the power spectra of the electromagnetic fields. Moreover, we show that such effects can also considerably suppress the strengths of the electromagnetic fields generated over the scales of cosmological interest. While it seems possible to undo the strong features that arise in the electromagnetic power spectra in such situations, we point out that it is realized at the cost of severely fine-tuned non-conformal coupling functions. To circumvent the challenges that arise in single field models, in the second part of the talk, I will describe the generation of magnetic fields in two field models of inflation. We will illustrate that, in two field models, with suitably chosen non-conformal coupling functions, we can obtain spectra of magnetic fields of the required strength and shape even in situations involving strong departures from slow roll. I will also discuss the imprints of the primordial magnetic fields generated in certain two field inflationary models on the anisotropies in the cosmic microwave background. In the third and final part, I will describe the evolution of the quantum state associated with the Fourier modes of the electromagnetic field during inflation. We will track the evolution of the state using measures such as the Wigner ellipse, squeezing amplitude and quantum discord. Specifically, we will show that the violation of parity leads to an enhancement of the squeezing amplitude and the quantum discord associated with one of the two states of polarization. I will conclude the talk with a short summary and outlook.