2025

Abstract: Very little is known about the cosmological history from after the end of inflation until Big Bang Nucleosynthesis. Various well-motivated models predict that the universe could have undergone a period of matter domination in this early epoch. We demonstrate that if the particles causing matter domination have self-interactions, they can form halos that undergo a gravothermal collapse. We thus propose a novel scenario for the formation of primordial black holes, which in particular can lie within the asteroid-mass range. We also find that it is not only black holes that can form in the aftermath of a gravothermal evolution. We show that number-changing annihilations of the particles can create sufficient heat to halt the gravothermal evolution, thus forming a ``cannibal star''. Likewise, the pressure from the particle's repulsive self-interactions can form a boson star during a gravothermal evolution. Thus, our study highlights that structure formation in the early universe can have a rich phenomenology.

Abstract: Curvaton scenario provides an alternative mechanism to explain observed scalar perturbations in CMB. Analytic computations of observables from generic curvaton scenarios are often not complete and give rise to incomplete results. In this talk I will introduce a novel semi-analytic method to compute the perturbations from these scenarios. Additionally I will also talk about the prospect of producing primordial black holes and observed matter anti-matter asymmetry from curvaton scenarios.

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Abstract: Recent polarimetric images by Event Horizon Telescope (EHT) reveals the presence of strong magnetic fields around supermassive black holes (SMBHs). However, the origin of such fields in the vicinity of BHs remains unclear. Existing research suggests that these large-scale fields are predominantly sourced either from companion stars which are tidally locked or coming from the interstellar medium. Nevertheless, the mechanism by which such large-scale magnetic fields are advected near BHs remains an unresolved scientific question. In this talk, I will delve into the physics of advection-dominated disks, which elucidate the accumulation of magnetic fields and their possible structures around these cosmic entities. To elucidate the physics and dynamics of magnetized disks, I will also discuss the general relativistic magnetohydrodynamics (GRMHD) simulations as well as analytic frameworks, which prove to be invaluable tools in this field.

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Abstract: The Sun exhibits a rich variety of periodic phenomena. The most prominent among them is the 11-year sunspot cycle. However, reconstructions of solar activity over centuries reveal that the amplitude of this cycle itself varies on longer, often irregular, timescales. These long-term variations, referred to as supradecadal modulation, remain poorly understood, with several competing hypotheses under debate. In this talk, I will explore the role of stochastic forcing as a potential driver of such modulation. I will also discuss periodic flows on shorter timescales — on the order of months — recently discovered in solar surface velocity fields. These periodic flows, known as solar inertial modes, exhibit evident variability over the solar cycle and are sensitive to internal solar properties that also influence the solar dynamo. Understanding these modes offers a new window into the solar interior. I will present the current state of their theoretical modelling and highlight how assumptions in the continuity equation can significantly affect their interpretation. Together, these perspectives offer valuable insights into the dynamics underlying solar variability.

Abstract: With increasing sensitivities of the current ground-based gravitational wave (GW) detectors, the prospects of detecting a gravitationally lensed GW signal are going to improve in the coming years. Although the lensing of GWs shares similarities with that of electromagnetic waves, their observed effects can exhibit striking differences. When a GW encounters compact objects, such as stars and stellar remnants, there is a possibility of the emergence of wave-optics effects, leading to frequency-dependent modulations of the signal, which we refer to as `microlensing’. In such cases, the geometrical optics approximation breaks down, necessitating the consideration of the wave nature of propagation. Consequently, these frequency-dependent modulations affect the GW strain, which can introduce biases in the inferred parameters if these lensing effects are not accounted for. In this talk, I will discuss broad aspects of the potential impact of microlensing on GWs. I will first explain why understanding microlensing effects is important, addressing how they can bias fundamental tests using GWs, such as tests of General Relativity. Moreover, I will explore the interplay between microlensing and eccentricity, demonstrating how the presence of eccentricity can bias microlensing searches. Finally, I will discuss the challenges of detecting microlensing in more realistic and complex lensing scenarios.

Abstract: The early universe can admit a rich variety of non-equilibrium phenomena, especially when we try to construct theories beyond the Standard Model of particles. For example, first order phase transitions can occur during the exponentially expanding era known as inflation. Another example is preheating, where there is an explosive generation of Standard Model particles after inflation ends.

In this talk, I will present general methods to estimate the non-Gaussianity in temperature and galaxy distribution observable today arising from vacuum transition during inflation and preheating after inflation. The calculation for vacuum transition proceeds by finding the instanton solution in de Sitter space, while for preheating it involves lattice simulations. I will also present preliminary results for the gravitational wave signal expected from collision of big bubbles in a sparse vacuum transition. 

Abstract: The Solar Wind is a continuous stream of charged particles emanating from the Sun that fills the entire heliosphere. With rapidly fluctuating characteristics, it is the primary driver of space weather and, therefore, maintains the Sun-Earth connection. The aim of this scientific exploration is to test one of the leading hypotheses, the role of the Interchange Magnetic reconnection (IMR) process in the formation of highly dynamic solar wind. Here, we present a state-of-the-art computational model, which includes several layers of the solar atmosphere, starting from the cool chromosphere region to the million Kelvin hot solar corona. We evolve this model while considering several critical components of thermodynamics, e.g., volumetric coronal heating, anisotropic heat conduction and radiative cooling. Next, we discuss the synthetic solar wind characteristics and emphasize the role of the IMR in the generation of a bursty wind profile. Finally, we present coronal plasma properties in the context of the recent in situ observations from the Parker Solar Probe mission.

Abstract: Mechanisms that govern the complex gas flows within the galactic disk and across the disk-halo interface are a crucial ingredient for understanding the evolution of galaxies and their star-formation histories. An important diagnostic for such flows is the spatially resolved gas-phase metallicity distribution. What can we then learn about the galaxy’s history based on its current metallicity map? Do positive radial gradients imply close passage of a neighbour or metal recycling from the CGM? I will attempt to answer these questions using both FOGGIE cosmological zoom-in simulations as well as JWST-PASSAGE observations, to bridge that gap between simulations and observations. Comparing the spatially resolved metallicity and SFR maps from the JWST observations, and correlating them with integrated stellar masses, we find a clear trend at z~2. The metallicity-SFR slope---which likely hints at timescales for metal-mixing due to stellar feedback---increases with increasing stellar mass.

Abstract: Identification and characterisation of (un)lensed kilonovae (KNe) can be instrumental in improving our understanding of various aspects of cosmology and astrophysics, such as - measuring the Hubble constant, understanding the physics of the binary neutron star (BNS) merger, and studying the abundances of heavy nuclei elements. However, detecting (un)lensed KNe poses unique challenges due to their rarity and low brightness. Upcoming telescopes, such as Rubin-LSST -- with its deep imaging capabilities and wide field-of-view - will provide a unique opportunity to observe these rare and faint transient events. Rubin-LSST will generate a deluge of data, making it essential to develop fast and efficient methods for identifying genuine (un)lensed events while minimizing false positives. To address this, we realistically simulate both unlensed and lensed KNe and test various strategies for efficiently detecting these events in the simulated light curve and image data. I will present the results of our latest simulations and detection methods.

Abstract: Sub-Neptunes are the most common type of exoplanets in the Galaxy, yet our own solar system does not have one. These worlds sit between Earth and Neptune in size, and their diversity makes them prime targets for understanding planetary habitability with upcoming missions such as the Habitable Worlds Observatory (HWO). Two competing formation pathways have been proposed. In the gas-dwarf scenario, sub-Neptunes form in-situ, accumulating puffy H/He atmospheres that subsequently evolve through intense mass loss, cooling, and contraction. Alternatively, they could form farther out as volatile-rich worlds that migrate inward. Distinguishing between these scenarios requires answering several fundamental questions: What is the atmospheric composition of sub-Neptunes? How do young and mature sub-Neptune atmospheres compare with each other? How diverse are sub-Neptunes immediately after formation? What physical processes govern early evolution and and on what timescales? For the first time, JWST allows us to unravel the atmospheric composition of these mysterious sub-Neptunes with unprecedented precision. In this talk, I will present new JWST results for both young (<100 Myr) and mature (~Gyr) sub-Neptunes, compare their atmospheric compositions across age, temperature, and stellar irradiation, and discuss emerging patterns that hint at their origins. I will connect these insights to formation pathways and early evolutionary mechanisms, and conclude with the key open questions that will define the next decade of observations and modelling as we work toward understanding the most common planets in our Galaxy.

Abstract: Neutron stars offer a unique opportunity to study matter under some of the most extreme conditions in the universe—densities exceeding those in atomic nuclei and temperatures spanning a wide dynamic range. Understanding their internal structure depends critically on the equation of state (EOS), which describes how pressure relates to density and temperature. In this talk, I will present recent progress in constraining the cold EOS using multimessenger observations, including gravitational waves from binary neutron star mergers, electromagnetic observations, and theoretical inputs from nuclear physics. These diverse observations are beginning to narrow down the possible models of dense matter.

In the second part, I will turn to the role of the thermal EOS in simulations of neutron star mergers. While the cold EOS governs equilibrium structure, the merger dynamics are sensitive to thermal effects that arise in the high-temperature, rapidly evolving post-merger environment. I will show how different treatments of thermal physics influence the evolution of the remnant and the associated gravitational wave signal. Together, these observational and computational perspectives are helping to reveal the microphysics of dense matter, with implications for nuclear physics, astrophysics, and gravitational wave astronomy.

Abstract: A significant fraction of stars experience close interactions, including collisions resulting from gravitational encounters and mergers within close binary systems. These processes can produce more massive stars that may give rise to relatively rare objects such as blue stragglers. Distinguishing the outcomes of collisions and mergers is challenging yet essential for interpreting observations. This study utilizes the magnetohydrodynamics code \texttt{AREPO} to simulate collisions and mergers of $5$ to $10 \Msun$ main-sequence stars, systematically comparing the properties of the resulting products. Both collisions and mergers yield more massive, strongly magnetized, rapidly and differentially rotating stars with cores enriched in hydrogen, but notable quantitative differences emerge. Merger products exhibit core hydrogen fractions up to $10\%$ higher than those of collision products. In both scenarios, turbulent mixing amplifies magnetic field energies by $9$ to $12$ orders of magnitude. However, magnetic fields in small-impact-parameter collision products display small-scale reversals that may dissipate over time, whereas merger products and large-impact-parameter collision products develop large-scale ordered, potentially long-lived magnetic fields. Additionally, only merger products display magnetically driven, bipolar outflows with radial velocities exceeding $300$ to $400 \kmsinv$. These distinctions may result in divergent long-term evolutionary outcomes, which warrant further investigation in future studies.

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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: It is now a well established fact that stellar bars, an m=2 asymmetry in the stellar density distribution,  are abundant in disk galaxies in the local Universe as well as in the high redshift disk galaxies. In such barred galaxies, it is observationally found that a preferential light deficit along the bar minor axis exists, commonly coined as ‘Dark gap’.  The properties of dark gaps are thought to be associated with the properties of bars, and their spatial locations are often associated with bar resonances. However, a systematic study, testing the robustness and universality of these assumptions, is still largely missing. In this talk, using a suite of isolated N-body models forming prominent bars, I will discuss in detail how the bar and the dark gap co-evolve and how their properties are correlated. Further, I will talk about the robustness of these dark gaps as a reliable tracer for the resonances induced by bars in disk galaxies. Finally, I will present results from an ongoing study of evolution of dark gaps with redshift, using the SDSS and the JWST data of barred galaxies at different redshift ranges.

Abstract: Protostellar outflows are ubiquitous phenomena at the earliest stages of star formation. Due to active accretion in the central protostar, bipolar jets originate and generate shock in the ambient medium they travel. These bipolar outflows play a crucial role in the accretion process by dispersing the excess angular momentum of the central protostar. Powerful feedback from outflows can provide enough turbulence to support it against gravitational collapse. In this talk, I shall discuss different prospects of protostellar outflows that have been found by our group using the data from Atacama Large Millimeter/submillimeter Array (ALMA).