Abstract: I will discuss a new model that resolves the 'impossible early galaxy' problem while complying with supernovae type 1a and baryonic acoustic oscillations data. The problem was noticed in some Hubble Space Telescope observations and confirmed by the James Webb Space Telescope's cosmic dawn observations. The model is a hybrid of two ideas from the first half of the last century: a) Zwicky's tired light and b) Dirac's varying coupling constants. The existence of tired light (TL) reduces the redshift contribution from the expanding Universe, which increases the age of the Universe to 26.7 billion years. Covarying coupling constants (CCC) eliminate the need for the cosmological constant, hence the dark energy. The critical density in the CCC+TL model is only adequate for observable (baryonic) matter with no room for dark matter or dark energy. The accelerated expansion of the Universe is caused by the weakening of the coupling constants rather than by dark energy. I'll present our latest findings, including the model’s success in predicting the baryonic content of galaxies from their observed rotation curves.

Abstract: Internal shocks are one of the leading dissipation mechanisms for powering the prompt phase of Gamma-ray bursts (GRBs). The basic paradigm is that a central engine produces a variable outflow wherein the faster trailing material collides with the slower leading material at a distance from the engine. Each collision produces a pair of shock fronts, a reverse and a forward shock front propagating in the faster and the slower material respectively. In general, the physical conditions in both the shocked regions are very different. In my talk, I will show how, starting from a few basic central engine parameters, a self-consistent hydrodynamic solution of both shocked regions provides ingredients for understanding the spectra and the light pulses of prompt-GRBs. I will also show how internal shocks in structured jets can give a unifying theme for classical GRBs (CGRBs), X-ray riches (XRRs), and  X-ray flashes (XRFs). 

Abstract: In this talk, two different cosmological scenarios associated with gravitational waves produced from binaries of black holes will be discussed. In the first part of the talk, I shall present the investigation of the stochastic gravitational wave (GW) background produced by primordial black hole (PBH)-binaries during their early inspiral stage, while accreting high-density radiation surrounding them in the early radiation-dominated Universe. In this work, we first calculated the correction terms appearing in the GW amplitude generated from such a PBH-binary due to changing PBH masses. Then, we showed that the significance of the correction terms persists for the overall stochastic GW background produced from those PBH-binaries. Also, We investigated the detectability of this stochastic background with present and future gravitational wave detectors. In the second part of the talk, I shall discuss the impact of change of masses of black holes due to spherical accretion of k-essence dilatonic ghost-condensate model of dark energy on the evolution of the binaries formed with those black holes. We found that the average power of the emitted GW from these binaries increases significantly faster than the constant-mass case. Furthermore, we estimated the reduction in coalescence time-intervals of the binaries due to the growth of the black hole masses. This work signifies the effect of accretion of similar scalar-field dark energies on the orbital evolution of binaries of black holes of certain mass-ranges, their coalescence time-scales and as a consequence their merging-rates too.

Abstract: Centres of many galaxies host a dense stellar system called nuclear star cluster with a central massive black hole. Extreme stellar densities make this environment an efficient breeding ground of a variety of astrophysical transients, like tidal disruption events (TDEs) of stars and extreme mass ratio inspirals (EMRIs) of stellar-mass black holes. The upcoming wide field survey instruments like Vera C Rubin Observatory and mHz gravitational wave detectors like LISA and Tianqin will revolutionize the field of nuclear transients. These transients offer a unique probe into the dynamical environment of galactic nuclei and demographic properties of astrophysical black holes. The classical channel of weak two-body scatterings among stellar objects is the standard route to exciting the orbital eccentricities of a star/BH leading to plausible formation of a TDE/EMRI. I will present a semi-analytical framework that identifies the underlying self-similar nature of this standard channel of EMRI formation, and improves upon the previous estimates of transient formation rates. Then, I will discuss the non-classical ways to channel these transients in a gas-rich active galactic nucleus (AGN) hosting a massive gas disk, where higher orbital eccentricities can be excited over much shorter secular timescales. I will show that the TDE formation rates can be enhanced more efficiently for a time-evolving AGN. These findings are in alignment with the observed preference of TDE hosts for recently-faded AGNs. In the end, I will discuss the impact of traditionally ignored strong scatterings on transient formation. In particular, strong scatterings can suppress EMRI rates in highly dense nuclei upto an order of magnitude, and should be mandatorily incorporated in future detection rate estimates for LISA.

Abstract: I will discuss some of today's major issues in galaxy formation. I will discuss observations of galaxies and show that there is a dichotomy in many galaxy properties.  I will describe how hydrodynamic computer simulations are used to study structure formation and in particular galaxy formation.  I will present some of the latest results, which show that there are also dichotomies in galaxy accretion and outflow processes and that these dichotomies might give rise to the observed dichotomies. I will then discuss some outstanding issues.

Abstract: Core-collapse supernovae (CC SNe), the explosive deaths of massive stars, play a pivotal role in galactic chemical evolution, star formation, and the creation of neutron stars or black holes. However, the fate of stars in the ~8–12 solar mass range remains poorly understood. These stars occupy the critical boundary between those that form neutron stars and those that end as white dwarfs. Despite comprising ~50% of massive stars that explode, such events are rarely observed, likely due to their connection with faint, hard-to-detect low-luminosity SNe. I will present results from the Zwicky Transient Facility Census of the Local Universe, the largest volumetric SN survey to date, focusing on the landscape of low-luminosity CC SNe. By examining candidate supernovae in this mass range, I will evaluate whether they can account for the missing SNe population and provide insights into the fate of these stars. I will conclude by discussing how future time-domain surveys will further advance this field.

Abstract: Core-collapse supernovae are recognized as significant contributors to dust formation in both local and high-redshift galaxies. Supernova environments are characterized by exotic phenomena such as shocks, radioactivity, non-equilibrium chemical processes, and rapid cooling. In my presentation, I will provide an overview of the current state-of-the-art in modeling dust formation in supernovae, drawing on the physics and chemistry of these environments. I will discuss the nature and mechanisms of dust production in (a) the pre-supernova progenitor, (b) the ejecta post-explosion, and (c) the interaction region of the forward shock and the circumstellar medium (CSM). This talk will address all the properties of supernova dust, observed in last two years with JWST, as well as its connection with Spitzer observations over the past few decades. A similar formalism is applicable to many stellar outbursts or winds, dominated by heavy or sporadic mass-loss episodes, binary interactions or collisions. I will present the scenario of dust production in evolved stars in this light.

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Abstract: Accretion disks are essential for understanding the dynamics around black holes, particularly in the magnetically arrested disk (MAD) state, where the magnetic flux near the event horizon becomes saturated. This MAD state has garnered significant attention following observations of supermassive black holes in M87 and Sagittarius A* by the Event Horizon Telescope (EHT) collaboration, which suggest that this is the preferred accretion state for such systems. In particular, low-luminosity systems like Sagittarius A* are significantly influenced by radiative cooling processes, which profoundly affect the thermal, magnetic, and dynamical properties of the accretion disk. In this talk, I will describe how radiative cooling impacts the structure and behavior of MADs, especially at sub-Eddington accretion rates. We analytically identify a critical mass accretion rate below which synchrotron radiation becomes a dominant cooling mechanism, altering the disk's thermal equilibrium and the MAD parameter. Using general relativistic magnetohydrodynamic (GRMHD) simulations from our massively parallel code cuHARM, I will explore how these cooling effects influence force balance, magnetic saturation, and jet efficiency for a range of black hole spins and accretion rates.

Abstract: Fast radio bursts (FRBs) are a remarkable class of transient astronomical phenomena detectable over cosmological distances. Their defining characteristics, including high dispersion measures and short pulse width, position them as invaluable tools for advancing our understanding of cosmology. This presentation will begin with a brief overview of FRBs, highlighting their discovery, properties, and observational signatures. Subsequently, the focus will shift to key cosmological insights enabled by their study. By examining a sample of localized FRBs, I will demonstrate how these phenomena provide a novel approach to addressing the Hubble tension, a persistent discrepancy in the determination of the Hubble constant. Furthermore, the implications of alternative cosmological models on the precision of fundamental constants, such as the fine-structure constant, the proton-to-electron mass ratio, and constraints on the fraction of primordial black holes constituting dark matter, will be explored. The presentation will conclude with a discussion of the capabilities of existing and upcoming observational facilities, emphasizing their potential to detect a statistically significant population of FRBs and thereby enhancing our understanding of the Universe.

Abstract: The origins and distribution of chemical elements in the Universe have long been a subject of investigation, with many unresolved questions remaining. The oldest stars in our Milky Way are rare relics from the early Universe, preserving the chemical imprints of the first stars and supernova explosions. These stars are crucial in addressing questions about element formation processes that occurred around 13 billion years ago. I will explain on how I employ “Stellar Archaeology”: the use of observations and analysis of the chemical properties of the oldest stars in Galaxy, to answer outstanding questions about the early Universe and the origins of the chemical elements in the Cosmos. One of the significant unanswered questions in astrophysics is the site of the rapid neutron-capture process (r-process). While the optical counterpart AT 2017gfo of the kilonova GW 170817 did provide evidence of the r-process in neutron star mergers, important details are still unsolved. Neutron star mergers alone seem to be unable to explain r-process enrichment in the Universe, and there are still open questions with respect to their time scale. I will discuss some of the results of r-process stars observed with the Gran Telescopio Canarias (GTC) and the Very Large Telescope (VLT), as well as findings from the HESP-GOMPA survey conducted by our group.

Abstract: Type II-P supernovae (SNe) were canonically established with a typical plateau length of 100 d, which is still a kind of ‘magic number’ for most of these SNe observed in nature. However, time and again, theoretical works have shown a great diversity in plateau lengths ranging from tens of days to more than 150 d. Longer plateau SNe have earlier been studied in several works, yet, the short-plateau SNe were missing from the observational scenario. This talk is based on detailed studies on four rare short-plateau SNe: SN 2018gj, SN 2020jfo, SN 2021wvw, and the decadal SN 2023ixf. The plateau lengths of these SNe vary from 65 d to 75 d. We attempted to constrain various observational and physical properties associated with these events using ground and space-based multiwavelength observations. We further modeled these events by performing 1D hydrodynamical simulations to ascertain their explosion parameters and progenitor properties. The progenitors of short-plateau events were earlier thought to be high-mass red supergiant stars, going through an evolutionary process with standard mass loss prescription. Our study revealed a large diversity in the progenitors of these events, indicating that the short-plateau events could arise from a wide range of red supergiant stars with elevated mass loss. 

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Abstract: Contrary to many stereotypes about massive galaxies, the observed systems are diverse in their star formation rates, kinematic properties, and morphologies.  Studying how they evolve into and express such diverse characteristics is an important piece of the galaxy formation puzzle. Here, we focus on a subset of massive galaxies, the brightest group galaxies (BGGs). We use a high-resolution cosmological suite of simulations based on the Romulus model, and compare simulated central galaxies in group-scale halos at 𝑧 = 0 to observed BGGs. Since most galaxy formation models are calibrated using measures that are strongly influenced by the properties and evolution of “normal”  Milky-Way like galaxies, this exercise is also an opportunity to test the limits of these models. The comparison encompasses the stellar mass-halo mass relation, various kinematic properties and scaling relations, morphologies, and the star formation rates.  We find Romulus BGGs that are early-type S0 and elliptical galaxies as well as late-type disk galaxies; we find Romulus BGGs that are fast-rotators as well as slow-rotators; and we observe galaxies transforming from late-type to early-type following strong dynamical interactions with satellites. In sum, we find that Romulus reproduces the full spectrum of diversity in the properties of the BGGs very well.   However, we also find a tendency towards lower than the observed fraction of quenched BGGs, with increasing halo mass.   The problem appears to be due to decreasing effectiveness of AGN feedback with increasing halo mass.  Examining some of the other galaxy formation models, we find that they too run into trouble on the same scale — but in an opposite way. I will conclude by discussing what we are to make of this.

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Abstract: Deep learning, particularly Convolutional Neural Networks (CNNs), has demonstrated remarkable success in identifying non-variable gravitationally lensed systems using multi-band static images. With the advent of time-domain surveys like the Rubin Observatory’s Legacy Survey of Space and Time (LSST), which will image the sky locations at multiple epochs, there is a unique opportunity to exploit temporal variations alongside spatial features in 2D images to classify lensed supernovae (SNe) among other transient phenomena. To achieve this, we employ a Convolutional Long Short-Term Memory (ConvLSTM) network designed to capture spatial and temporal correlations simultaneously. Our approach incorporates real galaxies from the Hyper Suprime-Cam (HSC) dataset as lenses, onto which we simulate synthetic lensed SN images using a dedicated simulation pipeline. These lensed SN images are overlaid on multi-band HSC galaxy cutouts at different epochs, representing the evolution of the SN over time. Negative samples include a diverse set of transient events, such as variable stars, active galactic nuclei (AGNs), and unlensed SNe, ensuring robust classification. As HSC data matches LSST in depth, resolution, and band throughput, it provides an excellent testbed for this methodology. In this talk, I will present the details of our simulation pipeline, the design of the ConvLSTM network, and the initial results, highlighting the potential of this technique for discovering lensed SNe in the LSST era.

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Abstract: According to the theory of cosmic inflation, all the structures observed in our Universe (clusters of galaxies, CMB anisotropy etc.) are of quantum-mechanical origin. The fact that the inflationary predictions match the astrophysical observations is an indirect justification of this scenario. In this talk, I discuss whether we could obtain a direct proof of the quantum nature of the primordial fluctuations.

Abstract: According to the theory of cosmic inflation, all the structures observed in our Universe (clusters of galaxies, CMB anisotropy etc.) are of quantum-mechanical origin. The fact that the inflationary predictions match the astrophysical observations is an indirect justification of this scenario. In this talk, I discuss whether we could obtain a direct proof of the quantum nature of the primordial fluctuations.

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Abstract: Changing-look AGNs (CLAGNs) switch between type 1 and type 2 state in a timescale of months to years. This rapid variability cannot be explained by unified model of AGNs which has been used to explain all the AGN phenomenon in the last 40 years. Here we discuss our current understanding and future prospects of CLAGNs.

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Abstract: The observations through very long baseline interferometry (VLBI) by the Event Horizon Telescope (EHT) collaboration has provided a new window of probing the strong gravity regime of black holes via black hole imaging. However, practical constraints such as sparse coverage of the observational Fourier domain, scattering effects of Earth’s atmosphere and maximum permissible baseline lengths due to the size of the Earth, have led to proposed ground-based and space-based missions extending the EHT, namely the next generation EHT (ngEHT) and the black hole explorer (BHEX) mission respectively. In this talk, I shall present my work done with collaborators (Daniel Palumbo, Michael Johnson and Shep Doeleman) on the ability of these missions to probe, through polarimetric signatures, the black hole’s photon’s ring, a characteristic consequence of General Relativity on strong lensing. Through geometric modelling of the photon ring’s signature in the Fourier domain, exact expressions will be presented for the point in the Fourier domain where the photon ring signal begins to dominate. These formulae are then compared with the full suite of general relativistic magnetohydrodynamic (GRMHD) simulations of M87* at 230 GHz. The subsequent inferences for spin measurement, discriminating magnetic field morphologies will be discussed along with peculiar signatures in circular polarisation for specific models that always seem to be photon ring dominated. Lastly, the ability of ngEHT and BHEX to observe these signals is quantified by interfacing M87*’s polarimetric best-bet models with instrumentation considerations of thermal noise and antenna diameter requirements for these missions.

Abstract: Magnetic reconnection is a fundamental process in plasmas where magnetic field lines tear and reconnect leading to conversion of magnetic energy to plasma kinetic energy in natural systems like solar flares, coronal mass ejection, and substorms in earth’s magnetosphere. We present laboratory measurements showing the 2-D structure of energy conversion during magnetic reconnection for the general case where the reconnection proceeds in the presence of a finite guide field (GF) such that the magnetic field lines meet at an angle less than 180°. The experiments showed that the guide field reconnection develops different electric field structure than anti-parallel reconnection with various regions developing electric fields either predominantly parallel or perpendicular to the magnetic field. We find that the electrons are energized by the parallel electric field in two regions of the reconnection layer, in the electron diffusion region (EDR) and outside EDR near the low density separatrices. The energy deposition on ions is driven by the perpendicular electric field in the vicinity of the high density separatrices where electrons work against the electric field. The perpendicular electric field is sufficiently strong to ballistically energize the ions. An energy balance calculation shows that about 40% of the magnetic energy is converted into particle energy, 2/3rd of which is transferred to ions and 1/3rd to electrons. Additional measurements showed that part of the energy deposited on electrons and ions causes heating. The results have implications for understanding magnetic reconnection in space plasmas by providing a comprehensive picture of where particle energization occurs within reconnection regions.

Abstract: The missing mass hypothesis is one of the fundamental challenges in physics today. The Lambda-Cold Dark Matter (LCDM) cosmological model advocates that our Universe consists of particles, termed dark matter (DM), which interact only via gravity. Modified Newtonian Dynamics (MOND), proposed by M. Milgrom in 1983 in an attempt to circumvent the dark matter paradigm. It postulates that systems that have acceleration, a, less than the fundamental acceleration constant a0, should exhibit deviations from Newtonian dynamics, thus offering an alternative explanation for the missing mass problem. Galaxies fall in the regime of low accelerations, a < a0, making them ideal testing grounds for this theory. MOND as a paradigm has modified gravity versions (AQUAL and QUMOND) and a modified inertia version. Many numerical solvers have implemented the modified gravity versions of MOND, thus allowing one to test MOND via numerical simulations. In this talk, I will briefly introduce MOND, its basic tenets, predictions, and tensions. Most importantly, I will present results from our work on simulations of disc galaxies, ultra-diffuse galaxies in galaxy clusters in MOND, and few other simulations, which have been used to test MOND. 

Abstract: Magnetic fields permeate the entire universe, extending from the smallest to the largest observable length scales. A popular explanation for the origin of the magnetic fields observed in galaxies, clusters of galaxies, and the intergalactic medium is that seed fields generated due to quantum fluctuations in the primordial universe are amplified later by astrophysical processes. According to the standard paradigm of 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 scalar field that drives inflation. I will begin the talk with a brief introduction to the essential idea of inflation. Thereafter, I will highlight the challenges in generating primordial magnetic fields during inflation in scenarios involving departure from slow roll in single field models. I will also discuss how we circumvent those challenges with suitable construction of coupling functions in different scenarios. Further, I will discuss the scenario of pure ultra slow roll inflation and show that scale invariant magnetic fields can be obtained in such situations with the aid of a non-conformal coupling function that depends on the kinetic energy of the inflaton. Apart from the power spectrum, an important probe of the primordial magnetic fields is the three-point function, specifically, the cross-correlation between the curvature perturbation and the magnetic field. We calculate the three-point cross-correlation between the curvature perturbations and the magnetic fields in pure ultra slow roll inflation for the new choice of the non-conformal coupling function. I will compare the results we obtain for these three point functions in case of slow roll and ultra slow roll scenarios, particularly emphasizing on the squeezed limit.

Abstract: The presence of a strong, large-scale magnetic field in an accretion flow leads to the extraction of the rotational energy of the black hole through the Blandford-Znajek (BZ) process, believed to power relativistic jets in various astrophysical sources. I will present the results of 3D GRMHD simulations of a highly magnetized cold thin disk surrounding a black hole, exploring the extraction of its rotational energy through the BZ process. Our findings reveal a weaker dependence of magnetic flux on black hole spin in a thin cold disk compared to hot accretion flows, and a significant fraction of extracted energy is potentially channelled into winds or disk radiation rather than the jet. I will highlight the implications of our results for understanding X-ray corona formation, black hole spin measurements, and interpreting transient phenomena, and discuss how strong magnetic fields enhance disk radiative efficiency.

Abstract: The formation and evolution of the massive disk galaxies in the nearby universe (z<0.1) represents an important open question in the present-day galactic astrophysics research. One class of such galaxies are the giant low surface brightness galaxies, e.g, Malin1 ,UGC1378, UGC 1382 etc, that are observed to show a complex morphology- a central high surface brightness stellar disk (HSB) surrounded by an extended low surface brightness (LSB) disk. The extended LSB disk could form due to external accretion or by secular evolution processes. In this talk, I will discuss our exploration with IllustrisTNG50 simulation data to identify and study various properties of such double-exponential, massive disk galaxies in the stellar mass range >=1011 solar mass. The structural properties of such galaxies are obtained by 2D GALFIT modeling. The radial scale length of the LSB disks are found to lie in the range of ~10-30 Kpc, in agreement with observations. The specific star-formation properties of the above double-disk galaxies are studied to understand their distribution from blue star-forming to red quenched region. Finally, we study such galaxies in the Baryonic Tully-Fisher relation. Our theoretical exploration will be potentially useful in exploring such galaxies in the upcoming deep observed data such as from LSST.

Abstract: Galaxy formation and evolution is tied to the physical state of gas in the circumgalactic medium (CGM) and its interface with the intergalactic medium (IGM), which is determined by the complex interplay between inflows from the IGM and galaxy feedback. Therefore, a comprehensive understanding of the physical conditions of gas within and surrounding galaxies is of paramount importance to understanding the physical processes that regulate galaxy formation and evolution. Numerous efforts to trace the diffuse gas seen as an absorption line in the background quasar spectra have revealed that intervening metal absorbers arise from multiple pathways, including gas inflows and outflows, the intragroup medium, and cool stripped gas from environmental processes. In particular, MgII absorbers, which trace cool, 10^4K, metal-rich gas, are frequently observed across a wide range of impact parameters, up to 200 kpc. However, the notion that the absorption is caused by galaxies at close impact parameters remains viable because it is highly challenging to find such faint galaxies in the glare of a bright background quasar. I will discuss the possible origin of intervening metal absorbers, the distribution of gas in the circumgalactic medium, and how it relates to the absorber properties in general over a wide range of redshifts and stellar masses.

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Abstract: POLIX is the main scientific payload on XPoSat (X-ray Polarimeter Satellite), a dedicated X-ray polarimetry mission, launched on 01st January 2024. POLIX is sensitive in the X-ray energy range of 8-30 keV. Measuring polarization in celestial X-ray sources is a new frontier in X-ray astronomy, providing crucial insights into emission mechanisms in X-ray sources. POLIX is based on the anisotropic Thomson scattering of polarized X-rays from a low atomic mass scatterer (Beryllium) and their subsequent detection in X-ray proportional counters. During this talk, I will discuss some instrument development aspects for the POLIX payload and some of the key scientific prospects for POLIX. In particular, I will discuss interesting science for the Ultraluminous X-ray sources (ULXs), their X-ray timing and spectral characteristics and possible X-ray polarization measurement for these sources.

Abstract: Standard stellar evolutionary theory explains the evolution of the majority of low-mass stars. However, some stars show anomalies like rapid rotation, infrared excess, and unusual surface chemical composition during their evolution. Our extensive studies made significant progress in understanding anomalous high Lithium and extreme deficiency of carbon in red giants, which were nagging problems in stellar astrophysics for many decades, thanks to asteroseismology and large-scale spectroscopic surveys. In this talk, I will discuss the observational results from the above studies that constrain non-standard processes during the evolution of giants from red giant branch to red clump phase, binary mass transfer, and merger scenarios of star-planets/star-white dwarf systems.

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Abstract: Type I supernovae (SNe) are, in general, deprived of Hydrogen in their early spectral sequence. Type Ia SNe, a subtype of type I SNe, result from thermonuclear explosion of white dwarf. I will talk about type Iax SNe, a rare subclass of type Ia SNe. These are low luminous, less energetic cousins of type Ia SNe and heterogeneous in nature. Because of the low energy budget of type Iax SNe, several progenitor systems have been proposed based on photometric and spectroscopic features. I will discuss a few promising progenitor channels for these events which are supported by observations. Type Ib SNe are another subtype of type I SNe with He signatures in their spectral time series. They are supposed to be originating from explosion of massive stars, stripped off their outer hydrogen envelope. I will also discuss their optical nature and most plausible progenitor channel.

Abstract: The Dark Energy Survey Instrument (DESI) is a Stage-IV large spectroscopic survey aimed to observe more than ~40 million optical spectra of galaxies, stars, and quasars by 2026. In June 2023, the early data release (EDR), comprising 1 percent of the survey data, was released, containing ~2 million spectra. The first major data release (DR1) is scheduled for early next year, which will be the most extensive spectroscopic dataset to date, comprising over ~20 million spectra. In this talk, I will provide an overview of the current status of the DESI survey, including the first cosmological results based on the EDR and DR1, as well as prospects for its future extensions. Additionally, I will also present how DESI can provide deep insights into the properties of metals in the Universe using quasar absorption lines.

Abstract: Fast radio bursts (FRBs) are among the newest tools in observational astrophysicists’ repertoire to study ionized gas. Their unique, millisecond-duration radio signal is subject to propagation effects in the intervening plasma. One such effect is the plasma dispersion of FRB pulses. FRB dispersion measures (DMs) quantify the net free electron column density through the sightline. FRB DMs can be precisely measured (~0.1%) and thus are sensitive to the most diffuse plasma in the intergalactic medium (IGM) that traditional probes have found challenging to illuminate. This ability to provide novel constraints on plasma has motivated studies of the circumgalactic medium (CGM) of galaxies intersecting FRB sightlines and the cosmic web filaments of the IGM. In my talk, I will highlight some of the work done leveraging FRBs and introduce the FLIMFLAM survey. FLIMFLAM is an ongoing spectroscopic endeavor to map foreground matter density along ~30 FRB sightlines. Its ultimate aim is to produce statistical constraints on key parameters describing matter distribution in the universe, including the fractions of ionized baryons residing in the diffuse IGM and the virialized gas of halos. With the first data release already published earlier this year, I shall discuss our results and ongoing work for the second data release. I will end with the prospects for the near future with FRB foreground mapping along a few hundred FRB sightlines in the era of large spectroscopic surveys such as DESI and 4MOST.

Abstract: We use the anti-de Sitter/conformal field theory (AdS/CFT) correspondence to find the least bounce action in an AdS false vacuum state, i.e., the most probable decay process of the metastable AdS vacuum within the Euclidean formalism by Callan and Coleman. It was shown that the O(4) symmetric bounce solution leads to the action minimum in the absence of gravity, but it is non-trivial in the presence of gravity. The AdS/CFT duality is used to evade the difficulties particular to a metastable gravitational system. To this end, we show that the Fubini bounce solution in CFT, corresponding to the Coleman de Luccia (CdL) bounce in AdS, gives the least action among all finite bounce solutions in a conformal scalar field theory. Thus, we prove that the CdL action is the least action among all possible large and thin-wall configurations under certain conditions.

Abstract: Stellar bars are ubiquitous in disc galaxies (including the Milky Way) in the Local Universe, with about two-thirds of them harbouring a stellar bar. Bars are present in high redshift (z ~1) disc galaxies as well. Recent JWST observations further revealed the presence of conspicuous stellar bars even at a higher redshift (z ~ 3). At these high redshifts, the galactic discs are known to be thick, kinematically hot (and turbulent), and more gas rich. A consensus of whether these bars are tidally-induced or formed due to the internal gravitational instability is still largely missing. In this talk, I will present results regarding the bar formation scenario in the presence of (kinematically-hot) thick discs using a suite of N-body models of (kinematically cold) thin and (kinematically hot) thick discs. I will further discuss the physical processes involved behind different bar formation scenarios as well as how the thick disc mass fraction impacts the properties and morphology of the resulting stellar bar. In addition, I will briefly mention the robustness of different bar instability criteria when applied to this suite of simulated barred galaxies.

Abstract: Recent results of the event horizon-scale images of M87* and Sagittarius A* from the Event Horizon Telescope Collaboration show that strong magnetic fields are likely present around the central black holes in these sources. Magnetically arrested disks (MADs), the end stage of magnetic flux saturation around black holes, are especially rich in horizon-scale physics due to the presence of powerful jets and magnetic flux eruptions that provide significant feedback on the accretion mechanism. I will provide an overview of our current knowledge about the magnetic field evolution in numerical simulations of accreting black holes, focusing on relativistic jet launching, black hole-ISM feedback, and black hole imaging of MADs.

Abstract: The dynamics of merging black holes occur in the strong field limits of the general theory of relativity. Gravitational waves emitted during this process offer an unique opportunity to empirically assess whether our understanding of gravity still holds in this extreme regime. A merger leads to the formation of a distorted black hole that "rings" down as it settles into a final stable state. The gravitational waves emitted during this process is a crucial probe for exploring the strong-field gravity dynamics. I will briefly review conventional tests like black hole spectroscopy that probe the linear dynamics predicted using perturbation theory in this regime. Then, I will introduce a novel test—the amplitude-phase consistency test—designed to indirectly probe dynamics in the non-linear regime. Finally, I will explore the prospects and challenges associated with implementing this test using data from current and future gravitational wave detectors.

Abstract: Clusters of galaxies are the largest, most massive gravitationally bound objects in the Universe. They are also the most recent of the cosmic objects to form. According to the currently accepted models of cosmic structure formation, the number density distribution of these systems is very sensitive to the parameters describing the large-scale geometry and the expansion history of the universe. For this reason, galaxy clusters are regarded as important cosmological probes. However, to use clusters as precision probes of the cosmological parameters, we need to be able to "weigh them". To do so, and do so properly is challenging. Here I will describe our effort to get a handle on the various systematics and biases that can influence the outcome, and present our mass measurements for 50 galaxy clusters comprising the "Canadian Cluster Comparison Project" or CCCP sample and the LoCuSS Cluster Sample. Using clusters as cosmological probes, however, requires many more than 50-odd clusters with known masses but this is not feasible at the present. I will discuss our effort to identify and calibrate "easy-to-observe" proxies for the mass, focusing on the clusters' Compton Y-parameter. One remarkable upshot from this work is that cosmology suggested by clusters is in tension with Planck CMB results.

Abstract: The study of astrophysical phenomena within turbulent, multiphase environments presents unique challenges. Initially, the coexistence of turbulence and multiphase media may appear paradoxical, as turbulence tends to homogenize the gas, but recent research has illuminated the role of radiative cooling in addressing this issue. However, a critical yet unexplored aspect is the impact of magnetic fields. Magnetic fields can significantly alter the dynamics of turbulent multiphase media. This has implications for phenomena such as small-scale cold gas evolution, low-density gas surface brightness, and the broader baryon cycle. In this talk, I will introduce and share the journey of the field, while also sharing our latest findings from small-scale idealised simulations aimed at understanding the dynamics of multiphase gas. Such small-scale physics poses a substantial problem for cosmological simulations and similar theoretical studies are necessary for their inclusion into such large-scale simulations as sub-grid models.

Abstract: Supermassive black holes at the centres of merging galaxies are expected to form binary systems whose orbital motion generates gravitational waves. A cosmological population of such systems combine to build up a gravitational wave background (GWB). A significant detection of this GWB will provide the first stringent constraints on the dynamical evolution of supermassive black holes and their host galaxies while also providing a tantalising probe into the properties of the early Universe. Searches for the GWB have typically used sensitive radio telescopes around the world which observe an ensemble of extremely stable millisecond pulsars to probe the characteristics of the GWB signal. In this talk, I will discuss the first compelling evidence of the GWB seen by global pulsar timing array (PTA) collaborations as announced this year, its scientific impact, potential biases and the road ahead. Focussing on future advancements, I will talk about the powerful potential of a gamma-ray PTA and how it can improve our understanding of the astrophysical origins of the GWB.

Abstract: We will discuss interpretation of the nHz stochastic gravitational wave background (SGWB) seen by NANOGrav and other Pulsar Timing Array (PTA) Collaborations in the context of supermassive black hole (SMBH) binaries. The frequency spectrum of this stochastic background is predicted more precisely than its amplitude. We will discuss how Dark Matter friction can suppress the spectrum around nHz frequencies, where it is measured, allowing robust and significant bounds on the Dark Matter density, which, in turn, controls indirect detection signals from galactic centers. Next we will discuss alternative cosmological interpretations including cosmic strings, phase transitions, domain walls, primordial fluctuations and axion-like physics each of which may lead to Primordial Black Hole formation. 

Focussing on primordial black holes (PBHs) production in various cosmological scenarios involving single-field inflation, multiple fields, particularly the Curvaton model, as well as those based on the presence of remnants dominated by the false vacuum and show the PBH formation from these remnants including the contribution from the false vacuum and the bubble walls, during strong first-order phase transition by estimating the collapse using the hoop conjecture. Such PBH formations have associated Gravitational Waves from bubble collisions, the spectral shape of which is distinct from that of scalar-induced GW. Finally we will end by putting these comparative studies to test via We will discuss how well these different hypotheses fit the NANOGrav data, both in isolation and in combination with SMBH binaries, and address the questions: which interpretations fit the data best, and which are disfavoured. Finally we also discuss experimental signatures that can help discriminate between different sources of the PTA GW signals with complementary probes using CMB experiments and searches for light particles in DUNE, IceCUBE-Gen2, neutrinoless double beta decay, and forward physics facilities at the LHC like FASER nu, etc. along with Primordial Black Hole formation and its constraints.

Abstract: A Bayesian method is used in this extensive work to generate a large set of minimally constrained equations of state (EOSs) for matters in neutron stars (NS). These EOSs are analyzed for their correlations with key NS properties, such as the tidal deformability, radius, and maximum mass, within the mass range of 1.2−2 solar mass. The observed connections between the pressure of β-equilibrated matter and the properties of NSs at different densities offer significant insights into the behaviour of NS matter in a nearly model-independent manner. The study also examines the influence of various factors on the correlation of symmetry energy parameters, such as slope and curvature parameters at saturation density (ρ_0 = 0.16 fm^−3 ), with the tidal deformability and radius of NSs. This study investigates the robustness of the observed correlations by considering the distributions and interdependence of symmetry energy parameters. Furthermore, the utilization of Principal Component Analysis (PCA) is employed to unveil the complicated relationship between various nuclear matter parameters and properties of NSs. This analysis highlights the importance of employing multivariate analysis techniques in order to comprehend the variety in tidal deformability and radius observed across distinct masses of NS. This comprehensive study aims to establish a connection between the parameters of nuclear matter and the properties of NSs, providing significant insights into the behaviour of NS matter across different circumstances.

Abstract: In standard cosmology, it is postulated that the Universe underwent an accelerated period in its initial phase. This brief episode of rapid expansion is referred to as inflation. The inflationary paradigm provides a simple and elegant mechanism for the origin of perturbations in the early universe. After inflation, curvature perturbation leads to the inhomogeneities in matter distribution, which are amplified by gravitational instability and become observable structures in the universe like galaxies and clusters of galaxies. The inflationary expansion is usually driven with the aid of one or more scalar fields. While the classical component of the scalar fields is supposed to drive the rapid expansion, it is the quantum fluctuations associated with the scalar fields that are supposed to be responsible for the primordial perturbations. The quantum fluctuations are expected to grow and turn into classical perturbations during the later stages of inflation. In this talk, I will discuss the evolution of the quantum state of the perturbations in single and multi-field models of inflation. I will utilize measures such as squeezing, entanglement entropy or quantum discord to track the evolution of the quantum state. 

Abstract: Fast Radio Bursts (FRBs) are one of the super-energetic radio pulsed signals with a short (< 1 sec) time duration. In recent years, numerous theoretical explanations for the origin of FRBs have been proposed. However, even with exotic physics, models have been unable to universally explain the properties of these events, such as peak flux and pulse width. In this study, we present a novel model that explains the origin of FRBs of GHz frequency radio waves. The model has three ingredients: compact object, progenitor with very strong effective magnetic field strength, and GHz frequency gravitational waves (GWs). Due to the Gertsenshtein-Zel'dovich effect, when GWs pass through the magnetosphere of such compact objects, their energy is converted into electromagnetic waves. This conversion produces bursts of electromagnetic waves in the GHz range, leading to FRBs. Therefore, we infer that millisecond pulsars may be the origin of FRBs. Further, our model offers a novel perspective on the indirect detection of GWs at high-frequency beyond detection capabilities.

Abstract: Gravitational waves are a new observational probe that is bringing new insights about the cosmos. I will discuss how this avenue can explore new frontiers that can play a vital role in mapping the history of the Universe. I will show some latest findings from the current gravitational wave observations and discuss the future scope of gravitational waves in discovering uncharted territories in astrophysics, cosmology, and fundamental physics in synergy with other cosmic messengers. 

Abstract: The solar atmosphere is a complex and dynamic region that consists of several layers, including the photosphere, chromosphere, transition region, and corona. These layers are interconnected and magnetically coupled. MHD waves transfer mass and energy between different layers of the solar atmosphere. These waves are often excited by the turbulent convective motion of the plasma in the photosphere consisting of disordered plasma motions across a wide variety of lengthscales and/or timescales. Vortices or rotational motions are omnipresent in turbulent flows, and the solar surface is no exception. Vortices are known to perturb magnetic flux footprints anchored at the solar surface and excite torsional Alfven waves. These waves travel high in the atmosphere and potentially heat the plasma. Thus, investigating kinetic vortices and associated magnetic perturbations is essential to probe their role in the excitation of Alfven waves. We compare the distribution of vortices for three different magnetic regions, viz., Quiet Sun, Weak Plage and Strong Plage, using the realistic three-dimensional radiation-MHD code, MURaM. The spatial scales of vortices at different heights, their origin at Intergranular lanes and the opposite sense of rotation between velocity and magnetic vortices are verified, which validates the Alfv ́enic nature of chromospheric vortices. By examining power spectra of horizontal velocity at various layers, we conjecture that vortex interaction leads to energy transfer to smaller-scale vortices and contributes to chromospheric turbulence. Although photospheric kinetic vortices show similar properties in all magnetic configurations, the associated kinetic and magnetic vortices in the chromosphere are highly correlated for the Quiet Sun configuration compared to the Plage regions.

Abstract: We investigate the photon analog of Fermi acceleration where a photon scatters with shearing layers of relativistic plasma and produces power-law-shaped spectra at high energies. It is an alternative to existing explanations of power law spectra such as synchrotron process or inverse Comptonization. Among several potential applications of this phenomenon, I will describe two examples in this talk. (i) We explain the high energy spectra of Gamma-ray bursts (GRBs). (ii) we demonstrate that the Compton scattering of photons with shearing plasma leads to a natural explanation for well-observed phenomena of Limb brightening in the blazar jet base.

Abstract: Understanding the intergalactic medium is essential for comprehending galaxy evolution and structure formation. While our theoretical understanding of the high-redshift intergalactic medium (z>2) aligns well with observations, the low-redshift intergalactic medium (z<1) presents significant challenges. Observations reveal that over 30% of the gas predicted by the standard model of the Universe remains unaccounted for, and the distribution of Doppler widths in the low-redshift Lyman alpha forest eludes accurate reproduction in all existing simulations. There are still unexplored periods spanning 5 to 10 billion years of cosmic time where measurements of the UV ionizing background and the thermal state of the intergalactic medium are lacking. Additionally, the impact of galaxy formation feedback on the intergalactic medium, particularly at low redshifts, cannot be ignored. In this talk, the speaker will address these pressing issues, focusing on new measurements of the thermal state of the intergalactic medium that suggests something is missing in either simulations or theoretical understanding of the intergalactic medium.

Abstract: While nuclei lighter than iron are fused over the course of typical stellar evolution, almost half of the elements heavier than iron are created through the rapid neutron capture process (r-process). These nuclei are thought to be produced in magnetized outflows from neutron-rich explosive events including compact mergers and core-collapse supernovae. In this talk, I will discuss the potential of neutrino-driven winds from strongly magnetized and rapidly rotating protomagnetars as plausible sites for r-process nucleosynthesis. As heavy nuclei can eventually produce ultra-high energy cosmic rays, we examine the acceleration and survival conditions for these nuclei. We also explore the propagation of these jets within Wolf-Rayet stars and blue/red supergiants. In particular, we analyze the criteria for a successful jet breakout, maximum energy deposited into the cocoon and structural stability of these magnetized jets. We show that high-energy neutrinos can be produced for extended progenitors like blue/red supergiants and estimate the detectability of these neutrinos with IceCube-Gen2.

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Abstract: Massive O and B stars release a significant portion, approximately 50%, of their original mass into space through stellar winds throughout their lifespan. This continuous mass loss plays a crucial role in shaping their evolution and the turbulence within nearby interstellar gas clouds. However, simulating magnetic massive stars remains challenging due to limitations in the employed simulation tools. The recent development of the Riemann-Geomesh MHD code allows for the handling of high rotation rates and large magnetic tilt angles, facilitating a comprehensive analysis of stellar winds and the X-ray emission from magnetic massive stars.

The initial set of simulations utilizes an isothermal MHD approach to investigate the overall dynamics of magnetically channeled winds under varying conditions, including different magnetic strengths, magnetic tilt angles and high rotation rates. These simulations are run until a quasi-steady state is reached, exhibiting the dynamics of magnetic channeling and centrifugal breakout events in the mass outflow. The mass outflow is confined by the magnetic field loops that form the closed magnetosphere of the star. The cataloged results provide valuable perspective on the variations in the rates of angular momentum loss and mass loss across different configurations of rotation rate, magnetic field strength, and large magnetic tilt angles.

Additionally, a second set of simulations focuses specifically on studying the high-temperature X-ray emission from θ1 Orionis C, a magnetic massive star. These simulations incorporate the energy equation and cooling terms, enabling accurate modeling of the formation and subsequent cooling of dense shock regions. Remarkably, the simulated data consistently reproduces the observed features, such as the peaks and positive slope in the X-ray emission measure distribution of θ1 Orionis C, as obtained from Chandra data. Furthermore, the total X-ray luminosity obtained from the simulated data aligns well with the observed range of θ1 Orionis C.

Abstract: Universe has several poorly constrained periods over its evolution. Formation of first stars and galaxies, followed by reionization of the intergalactic medium is one such epoch. Often referred to as "cosmic dawn", observing this period is extremely challenging due to the faint nature of the signals originating from it. The redshifted 21-cm line from neutral hydrogen offers one such possibility to observe the cosmic dawn and extract information about the nature of first stars and galaxies. In this talk, I will discuss our attempts at detecting the 21-cm signal from cosmic dawn. I will elaborate the challenges involved in detecting faint cosmological signals, and how our in-house designed experiments address those challenges. I will finally discuss the recent results from our observations, and explore how 21-cm signal can also be employed to probe other cosmic mysteries at different redshifts. 

Abstract: This work focuses on the gravitational interactions of astrophysical systems. In particular,  we focus on the triple system dynamics, including mildly hierarchical three body secular dynamics, as well as precession induced resonances of binaries under the perturbation of a third companion. We apply our theoretical investigations of these physical processes to wide-orbit planetary systems and black hole binaries embedded in AGN disks. More specifically, we consider the secular dynamics of a test particle in a mildly-hierarchical configuration. We find the limit within which the secular approximation is reliable, present resonances and chaotic regions using surface of sections, and characterize regions of phase space that allow large eccentricity and inclination variations. Finally, we apply the secular results to the outer solar system. We focus on the distribution of extreme trans-neptunian objects (eTNOs) under the perturbation of a possible outer planet (Planet-9), and find that in addition to a low inclination Planet-9, a polar or a counter-orbiting one could also produce pericenter clustering of eTNOs, while the polar one leads to a wider spread of eTNO inclinations.

Beyond mildly hierarchical triple dynamics, we also propose a novel pathway through which compact binaries could merge due to eccentricity excitation, including in a near coplanar configuration. Mechanisms have been proposed to enhance the merger rate of stellar mass black hole binaries, such as the Von Zeipel-Lidov-Kozai mechanism (vZLK). However, high inclinations are required in order to greatly excite the eccentricity and to reduce the merger time through vZLK. Specifically, a compact binary migrating in an AGN disk could be captured in a precession-induced resonance, when the apsidial and nodal precession rates of the binary are commensurable to the orbital period around the supermassive black hole. We find 8 such resonances upto quardupole order of the Hamiltonian.  We show that if a binary is captured in these resonances and is migrating towards the companion, it can experience large eccentricity and inclination variations. Eccentricity is excited when the binary sweeps through the resonance which happens only when it migrates on a timescale 10-100 times the libration timescale of the resonance. Libration timescale decreases as the mass of the disk increases. The eccentricity excitation of the binary can reduce the merger timescale by a factor up to $10^{3−5}$.

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Abstract: PRATUSH -- Probing ReionizATion of the Universe using Signal from Hydrogen -- is a proposed cosmology experiment to detect the global red-shifted 21-cm signal from the Cosmic Dawn and Epoch of Reionization (CD/EoR). PRATUSH orbiting the Moon will seek to precisely measure the low-frequency radio sky-spectrum over 40 to 200 MHz. The scientific observations would be made in the radio-quiet region when in the farside of the Moon, and the data would be transmitted back to Earth when in the near-side. PRATUSH was proposed to the Indian Space Research Organization (ISRO) during a call for proposals in the announcement of opportunity for science payloads in 2018. PRATUSH is in the pre-project studies phase. In this talk, I will discuss the motivation for PRATUSH, its current status and future plans.

Abstract: Role of magnetic fields (B-fields) and turbulence in star formation is still under debate but considering the magnetised nature of molecular clouds, we expect B-fields to have a significant impact on this process. Observations of molecular clouds in different environments using different astronomical techniques helps in understanding the various important aspects of star formation. I mostly worked towards mapping B-fields in nearby low-mass star forming regions in different environments viz. molecular clouds in isolation and HII regions at their different evolutionary stages using multi band polarization observations. This was done using optical, near-IR, and sub-mm polarization observations to map B-fields from pc to sub-pc scales. In this talk, I will briefly present my work on B-fields in various Galactic regions i.e. cores, filaments, and HII regions.  We know that grain alignment is the key to understand interstellar dust polarization. Therefore, I will mostly emphasise on my recently accomplished work on investigation of grain alignment efficiency in PDRs using dust polarization measurements.

Abstract: The “quasar mode” (or radiative mode) of AGN feedback, operated through outflows, plays an essential role in the evolution of galaxies. Quasar outflows are detected as blue-shifted broad absorption lines in the UV/optical spectra of quasars. Thanks to the Sloan digital sky survey, ~100,000 broad absorption line quasars are available now for ensemble statistical studies. This rich dataset has also enabled us to identify some peculiar cases of these sources. In this talk, I will review the current understanding of the quasar outflows and present our recent efforts to understand (i) the nature of these sources and (ii) the primary driver for the variability in the absorption lines.

Abstract: Recent coordinated observations and interpretations of disparate "messenger" signals from GW170817-GRB170817A-EM170817 have inaugurated the era of multi-messenger transient gravitational wave (GW) astronomy. I will argue that the bright blazar OJ 287 should allow us to pursue persistent multi-messenger GW astronomy during the era of Square Kilometer Array. This is mainly due to the several successful multi-wavelength observational campaigns that allowed us to argue for the presence of a spinning supermassive black hole binary that spirals in due to the emission of nano-Hertz GWs in the central engine of a unique blazar OJ287. Our on-going efforts, relevant to both the Event Horizon Telescope consortium and the International Pulsar Timing Array consortium which aims to detect GWs from such massive BH systems in the coming years, will be also listed.

Abstract: Coronal Mass Ejections (CMEs) are the most energetic and episodic expulsions of magnetized plasma from the Sun. In the talk, I will begin with the current status of the problem and specific challenges holding back progress in better understanding the evolution of CMEs in the heliosphere. I will highlight our efforts in implementing the three-dimensional reconstruction techniques, primarily using wide field-of-view imager's observations combined with in situ observations and modeling. Our study shows that the kinematics of CMEs can change significantly in the interplanetary medium due to the interaction of multiple CMEs and the substantial aerodynamic drag they experience. The talk will also briefly cover my recent attempts to use the estimated kinematics of the CMEs as inputs to an analytical model to probe the thermodynamic evolution of CMEs at distances much closer to the Sun that have yet to be accessed by in situ spacecraft. The talk's end segment will show a glimpse of solar variability due to episodic CMEs and the ubiquitous quasi-study solar wind over the last two decades. Finally, I will conclude with the lesson learned from our studies and further improvements required in CME research, which have much wider application across astrophysics and the search for habitable exoplanets.

Abstract: Stars are born deeply ensconced in dense clouds of interstellar medium. Protostellar jets launched in the early evolutionary phase of these young stellar objects are targets of intense scrutiny, to understand the properties of the protostellar systems. We have investigated a few jets from massive embedded protostars through their interaction with the ambient medium, in radio and infrared wavebands and we trace emission from shocked regions where the jet impacts the medium. Our results from GMRT, India, at low radio frequencies have confirmed the presence of non-thermal synchrotron emission from these jets, unlike measurements at higher radio frequencies that usually trace thermal free-free emission. We assimilate the observed emission measurements within a numerical model that we have developed, which incorporate the effects of both thermal and non-thermal emission, to understand the properties of the jets and ambient medium in the vicinity.

Abstract: A central aspect of Astrobiology research concerns the delineation of life’s origin and its early evolution on our planet. This intriguing and complex story that is thought to have started several billions of years ago, continues to be a very fascinating scientific mystery. It involves characterising how the transition from chemistry to biology would have occurred on the early Earth. In this context, I will give an overview of the current understanding prevalent in the field while also sharing some of the contributions that the COoL lab@IISER Pune has made towards these attempts. Pertinently, discerning the transition from non-life to life also has important implications for understanding life’s presence/distribution in the Universe.

Abstract: 'Slow-roll' of the inflaton field during inflationary phase is an essential feature of the inflationary dynamics, as the inflationary phase can make a 'graceful exit' when the inflaton field ceases to slow-roll. The slow-rolling of the inflaton field demands the inflaton potential to be nearly flat. However, when the potential becomes extremely flat, such as near any inflection point, the dynamics of the inflaton deviates from slow-roll and enters a phase of so-called Ultra slow-roll. Having a phase of Ultra slow-roll during standard inflationary phase often turns out to be useful, because such an Ultra slow-roll phase can enhance the cosmological perturbations enough which can later collapse and give rise to Primordial Black Holes. Presently, Primordial Black Holes are considered as favoured candidates for the Dark Matter. 

  In this talk, we will ask the question: "What happens in Warm Inflation if the dynamics enters an extremely flat part of the potential?" Warm inflation is a variant inflationary scenario where a concurrent radiation bath is maintained throughout inflation due to the dissipation of the inflaton energy densities to the radiation bath. The system is studied under the assumption of near thermal equilibrium which is one of the essential features of Warm Inflation. We will discuss in this talk the challenges one faces in maintaining the thermal equilibrium of the system when it undergoes an evolution through an extremely flat region of the potential. Thus, realising an Ultra slow-roll phase while keeping the thermal equilibrium in Warm Inflation is a daunting task. We will then show that only a subclass of Warm Inflationary models can undergo Ultra slow-roll while maintaining the thermal equilibrium of the Warm Inflationary system. 

Abstract: Merging stars and compact objects offer extraordinary laboratories to study a wide array of physical phenomena like binary stellar evolution, dust formation, gravitational waves and the origins of the heaviest elements in the universe. Yet, many gaps persist in our understanding of the wide spectrum of outcomes resulting from these mergers, that range from peculiar, low-luminosity explosions to large-amplitude variable stars. In this talk, I will demonstrate how the landscape of mergers can be explored systematically using time-domain surveys. I will describe results from searches for merging massive stars, white dwarfs and neutron stars using the Zwicky Transient Facility (ZTF), Palomar Gattini IR (PGIR) and the newly commissioned Wide-Field Infrared Transient Explorer (WINTER) surveyors at the Palomar Observatory in California. For each class of mergers, I will discuss insights that these systematic samples provide about the formation, evolution and final fates of their progenitor binary systems. In closing, I will describe the exciting potential of the upcoming time-domain surveys in significantly improving our view of the merger landscape.

Abstract: Geodesy is the science of measuring Earth’s shape, size, orientation, and gravity in a 4-D (space-time) coordinate system. This field is at least 2500 years old and is an ally of astronomy. However, the major advances in Geodesy have been made in the last 4 decades after the advent of the satellite era. The most influential development occurred with the launch of GRACE satellite mission in 2002. The concept of this satellite mission is unique and distinctly different from any satellite mission. It provides changes in the gravitational field of the Earth at monthly scale, which has been used to study the mass transport in the Earth system. In this talk, I will shed light on GRACE satellite mission, its working principle, major breakthroughs, and its limitations. Towards the end, I will introduce a new emerging field, Relativistic Geodesy, and talk about opportunities and challenges.

Abstract: The Galactic Warm Ionized Medium (WIM) is one of the four atomic phases of the Galaxy, containing most of the ionized gas (and not HII regions).  The consensus view is that the WIM is powered by ionizing radiation leaking from HII regions. However, we lack clear understanding of how the ionizing radiation diffuses to great distances.  The traditional probes are H-alpha (degree scale; Fabry-Perot) and pulsar dispersion measure (point measurement). Fabry-Perot observations have been conducted in a few optical nebular and auroral lines of metals.  Here, I discuss the diagnostic power of  IFUs on large space- (JWST) & ground-based (Keck) optical telescopes that allow us to study the WIM on scales of arseconds. I will fortify this conclusion by presenting detections of the Galactic diffuse medium via mid-IR fine structure lines with the MIRI-MRS IFU on JWST. I will end the talk reviewing an old but important topic: the ionization fraction of H and He in the WIM.

Abstract: To understand black hole growth, we need to understand the physical processes that drive the accretion of the gas on to the black hole and how they evolve over time. Stellar-mass black holes in X-ray Binaries (XRBs) exhibit extreme spectral state transitions that occur on observable timescales and as a function of accretion rate. A comparison of spectral state changes between stellar and supermassive black holes can inform our understanding of AGN accretion. However, observable timescales for state transitions of AGN are typically not attainable, but can be explored with a large sample of AGN. Here, I will present our analysis of a sample of 3500+ AGN with simultaneous UV and X-ray observations from the XMM-Newton and Neil Gehrels Swift satellites, complemented with radio, optical and infrared data. Our results establish that AGN and XRBs display analogous spectral states, most notably linking the radio emission from the relativistic jet with the energetic emission from the X-ray corona and thermal emission from an accretion disc. I will highlight our unique result demonstrating how the AGN radio morphology correlates with the accretion state change, analogous to the presence of radio jets during a typical XRB outburst. Further, I will present the salient results of our analysis of how different AGN properties, viz. nuclear obscuration, Seyfert type, optical classification are correlated with their observed accretion states, along with their relation to the star formation and stellar masses of their host-galaxies, hinting at the co-evolution of AGN and host galaxy with feeding and feedback.

Abstract: The search for planets beyond our solar system is one of the biggest scientific quests at present. The first exoplanet detection by the radial velocity (RV) method nearly three decades back was awarded the Nobel prize in Physics. Since then, the number of exoplanets found has increased exponentially with nearly 5000 exoplanet detections till date. We have learnt that diverse planetary systems form with orbits, densities, and planetary atmospheres that are very different from the planets in our solar system. In my talk, I will discuss planet demographics across the stellar spectral range focusing on large planets around the solar-type stars and smaller planets around the late spectral type stars. Despite the advent of large telescopes and precise spectrographs, the detection of smaller planets is largely limited by the intrinsic stellar jitter. I will talk about the ways that we devise to disentangle this stellar jitter from the planet signature. I will finally highlight the role of small to medium-size telescopes for exoplanet characterization in the era of current and future space missions like TESS and PLATO.

Abstract: Energy transport across a wide range of dynamical scales in the intracluster medium (and generally in gaseous halos) is one of the most interesting topics in current research and future interest. Hot baryons, visible in the X-rays, need to be stably sustained against radiative cooling over a large inner fraction of the cluster virial radius. A historical motivation has been the lack of sufficient observed cold gas in the cluster cores that is expected in the absence of efficient heating. Quantitatively, there is enough energy from active galactic nuclei to solve the problem at the simplest level, but the complexity of how that energy flows around is not well understood. Multiple transport mechanisms are being actively discussed including long wavelength, nearly isotropic sound waves, anisotropic heat conduction only along local magnetic fields (depending on the local temperature gradient), generation and dissipation of volume-filling turbulence, etc. While sound waves and turbulence have been strong contenders, thermal conduction has been claimed to be further suppressed by gyro-scale whistlers that scatter thermal electrons efficiently in the weakly magnetised ICM. In the latter scenario, thermal instability domain may be enhanced leading to excess and/or smaller scale cold gas. This further implies that observations may need to account for excess cold/mixed phase gas. In my talk, I will discuss these topics of energy transport in the ICM and the consequences. 

Abstract: It is estimated that the solar surface is covered by millions of (almost ballistic) plasma jets at any given time that are believed to be conduits for transfer of mass and momentum especially to the fast solar wind in the coronal holes. About 2 decades ago it was conjectured that these jets are powered by the energy of the bubbling solar convection beneath. However, in later years the narrative has taken a hyperbole to instead include complex microphysics of the solar chromosphere in order explain a certain energetic variety of these spicules (a.k.a type-II) which are believed to be more important than their lethargic counterparts (type I). In our recent work, we combine solar observations, 2D numerical simulations and laboratory experiments to show that observed features of spicule jets can be clearly explained by the inclusion of realistic solar convection in presence of magnetic fields. We further find that our results contradict the existing paradigm of bi-modality (types I vs II) of solar spicules. Our 3D magneto-convection simulations also bring forth a surprising topological possibility- that spicules may not be conical 1D plasma needles dotting the solar surface but, rather like folds in the 2D plasma drapery embedded in the 3D solar atmosphere.

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Abstract: To understand the structuring and dynamics of the upper photosphere and the chromosphere of the Sun, we need to improve and extend the existing numerical radiation-magnetohydrodynamical (MHD) simulations. In the solar chromosphere, radiative energy transport is dominated by only the strongest spectral lines. For these lines, the approximation of local thermo-dynamic equilibrium (LTE) is known to be very inaccurate, and a state of equilibrium cannot be assumed in general. To calculate the radiative energy transport under these conditions, the population evolution equation must be evaluated including all time dependent terms. To this end, we have developed a non-LTE non-equilibrium radiative transfer (RT) module to the well-known MHD code MURaM. In this module we have developed a numerical method to solve the evolution equation for the atomic level populations in a time-implicit way, keeping all time dependent terms to first order. Our main non-equilibrium treatment is of the Hydrogen bound and free states. For the equation of state, to determine kinetic temperature, we treat the Hydrogen molecular evolution also in non-equilibrium. The other elements comprising the gas are treated in LTE. Finally, the pressure and the radiative flux divergence from the RT module are provided to the MHD equations, to evolve the MHD and the radiative quantities self-consistently and iteratively. The module is developed for one, two and three dimensions (1d,2d, 3d) but currently tested in 1d. In this talk, I will describe the method, discuss equilibrium solutions and show some results of the dynamic evolution.

Abstract: The energy produced by supernovae or supermassive black holes plays a major role in dictating the evolution of galaxies. The produced energy is directly responsible for heating the star-forming gas in the interstellar medium or stopping the gas to reach the galaxy thus suppressing overall star formation. The interaction between the produced energy and the interstellar medium is often mediated by bubbles and winds, such as the gamma-ray bubbles (known as the Fermi Bubbles) in our Galaxy. Understanding such processes require multi-wavelength observations and a detailed theoretical understanding of these bubbles and winds. In the talk, I will present some of my recent works to understand how bubbles and winds produced by supernovae and supermassive black holes interact with the interstellar medium and their observational signatures. I will also present my works on the state-of-the-art numerical simulations that have been successful to discover so far unknown properties in these systems and are bringing us much closer to bridging the gap between theory and observations in the interstellar medium.