Posters Selected for Workshop-2
Title	First name		Last name	Affiliation	Abstract
Dr.	SANJAY		OLI	Dayananda Sagar College of Engineering, Bangalore	In this paper we study a two-fluid cosmological model in five-dimensional Kaluza-Klein spacetime. In the two-fluid model, one fluid represent the barotropic fluid of the universe and another fluid is chosen to model the dark energy. The Einstein’s field equations have been solved with the interacting as well as non-interacting two fluid scenario. The physical consequences concerning the evolution of the dark energy paramter in both the cases are compared in view of current observation results.
Dr.	Sachin		Kaothekar	Mahakal Institute of Technology and Management Ujjain	The linear self-gravitational instability of magnetized, finitely electrically and thermally conducting, strongly coupled fluid with radiative effects is investigated using the modified generalized hydrodynamic (GH) model. A general dispersion relation is derived with the help of linearized perturbation equations using the normal mode analysis method and it is discussed for longitudinal and transverse mode of propagation. In longitudinal mode of propagation, we find that Alfven mode is uncoupled with the gravitating mode. The fundamental Jeans criterion of gravitational instability is determined which depends up on thermal conductivity, radiative heat-loss function, shear viscosity and bulk viscosity while it is independent of magnetic field. The viscoelastic effect modifies the fundamental Jeans criterion of gravitational instability. Numerical calculations have been performed to see the effect of different parameters on the growth rate of the Jeans instability. Results are helpful for understanding the formation of white dwarf star.
Dr.	Pradyumn	Kumar	Sahoo	BITS-Pilani, Hyderabad Campus	Title: Viability of the $e^T$ cosmology, \n\nAbstract: We propose a new theoretical approach for a cosmological model, which starts from an exponential of the trace of the energy-momentum tensor-dependence on the gravitational action, to be summed to the Ricci scalar. We derive the referred field equations and Friedmann-like equations. We derive the scale factor, Hubble parameter and deceleration parameter, in terms of both time and redshift. In possession of those parameters in terms of the redshift, we confront their predictions with observational Hubble dataset and the outcomes are pretty satisfactory, so that the model can be seen as a new alternative to the cosmological constant problem. We also present the statefinder diagnostic and discuss profoundly the dynamical behaviour of the model.
Mr.	ASHISH	KUMAR	MEENA	IISER MOHALI	The number of strong lens systems is expected to increase significantly in ongoing and upcoming surveys. With an increase in the total number of such systems we expect to discover many configurations that correspond to unstable caustics. In such cases, the instability can be used to our advantage for constraining the lens model. We have implemented algorithms for detection of different types of singularities in gravitational lensing. We validate our approach on a variety of lens models and then go on to apply it to some inferred mass distribution of real lenses. We propose to represent lenses using these singularities in the image plane. We propose this as a compact representation of complex lens systems that can capture all the details in a single snapshot.
Dr.	Mamta		Gulati	Thapar Institute of Engineering and Technelogy, Patiala.	Title: Slow mode instabilities in nearly Keplerian galactic discs due to presence of gas and dust. \n\nAbstract: Eccentric modes in nearly Keplerian discs have been used by many authors in the literature to explain the observed asymmetric distribution of stars at the centers of the galaxies. The galactic discs at the centers are only assumed to be a single component stellar discs. None of the authors have considered the effect of gas on such eccentric orbits. Gas or dust is an intrinsic part galaxies and hence can not be ignored. The aim of the present work is to study the nature of eccentric modes in a galactic disc in presence of gas. The presence of gas in the disc gives rise to instabilities which we show by means of stability analysis. We also discuss the nature of eignespectra obtained. The analysis is also applicable to non-Keplerian discs and our analysis is not restricted to galactic centers.
Prof.	Munawar		Karim	St John Fisher College	We solve the long-standing problem of infinities in QED by including the effects of general relativity to explain the stability of the electron. We obtain from first principles the radius of the electron as well as the GUT energy.
Ms.	Promila		Biswas	The University of Burdwan	Title: Constraining the Free Parameters of a new Redshift Parameterization of Dark Energy\n\nSince the Ia supernovae observations of late 1990’s, it has been predicted that our universe is experiencing a late time cosmic acceleration. To build a theoretical support to this observation, the existence of hypothetical fluid inside the universe is assumed which exerts negative pressure. Several candidates of\nsuch an exotic fluid have been prescribed so far. A popular method in this alley is to parametrize the equation of state parameter ω = Ï_x0081_ p as a function of redshift. Again some common families of such redshift parametrizations are constructed of which different members have given different properties of universe. Mainly, these were model dependent studies where free parameters are to be constrained by different\nobservations. In the present article, we have considered a new expression for redshift parametrization\nand have constrained its free parameters for two Hubble parameter vs redshift data sets. These two data\nsets are obtained depending on two basic methodologies known as differential ages method and baryonic\nacoustic oscillation method. We locate different confidence contours for our model under the constraints\nof these two data sets. Besides, we analyse different thermodynamic parameters related to the evolution\nof our universe. It is noticed that our model indicates towards a delayed dark matter decay model which\nmimics EoS=-1 phenomena at the present epoch. We study the deceleration parameters behaviors. We\ncompare the outcomes for both the data sets.
Ms.	Purba		Mukherjee	IISER Kolkata	Exact solutions are studied in the context of modified Brans-Dicke theory. The non-linearity of the modified Brans-Dicke field equations is treated with the Euler-Duarte-Moreira method of integrability of anharmonic oscillator equation. While some solutions show a forever accelerating nature, in some cases there is a signature flip in the evolution of deceleration parameter in recent past. Importance of these latter models is studied in the context of late-time acceleration of the universe. Constraints on the model parameters are obtained from Markov Chain Monte Carlo (MCMC) analysis using the Supernova distance modulus data, observational measurements of Hubble parameter, Baryon acoustic oscillation data, and the CMB Shift parameter data.
Mr.	Snehasish		Bhattacharjee	Osmania University	In this work gravitational baryogenesis is studied by considering the simplest non-minimal matter geometry coupled f(R,T) gravity theory where f(R,T) = R+ζRT. Here, R represents Ricci scalar and T denote trace of the stress-energy-momentum tensor. We studied the viability of our model for different baryogenesis interactions proportional to ∂µR, ∂µT and ∂µf(R,T). Further, we obtained baryon to entropy ratio in each case and put constraints on parameters spaces of our model.
Mr.	Parth	Mukeshbhai	Shah	BITS Pilani, KK Birla Goa Campus	Title of abstract: Stability of Cosmological models in modified gravity. Modified Gravity theories have received increased attention lately to understand the late time acceleration of the universe. Among numerous extensions to Einstein's theory of gravity, $f(R) $ theories have received several acknowledgements. In our current work, we try to understand the acceleration of the universe in the modified geometric space using dynamical system analysis. This technique also allows understanding the behaviour of the universe and its stability analysis which could then be compared with observational data.
Mr.	Jerin		Mohan N D	Research Scholar, Department of Physics, CUSAT	Title: Exploring bulk viscous matter dominated universe - Causal approach\nAuthors: Jerin Mohan N D and Titus K Mathew\nAbstract: We have explored the evolution of the bulk viscous matter dominated universe\nusing the full causal Israel-Stewart theory, in the context of recent acceleration of the universe. The bulk viscosity of the form $xi=alpha{rho}^s$ (where $xi$ is the bulk viscosity, $rho$ is the density of matter, $alpha$ and $s$ are constants) has considered in this analysis. For $s=1/2,$ we have analytically obtained the Hubble parameter evolution equation, the behaviour of the model has been analysed and compared with the standard $Lambda$CDM model. The best fit model parameter values have extracted using the type Ia Supernovae data. The model predicts the current accelerating phase of the universe with an early decelerated epoch and a never ending phase of late acceleration with quintessence behaviour. The nature of stability of the solutions indicate that the early decelerated phase represents an unstable equilibrium point and the late phase will be a stable equilibrium point. The evolution of entropy shows that bulk viscous universe behaves like an ordinary macroscopic system and attains a stable equilibrium state through the maximisation of entropy. While, for $sneq1/2$ it is difficult to obtain the analytical solution for the Hubble parameter, hence we have performed the dynamical system analysis. The dynamical system analysis shows that, when $s>1/2,$ the equation of state and the nature of Hubble parameter at the equilibrium points indicate the evolution bulk viscous universe with a prior decelerated phase and a late de Sitter phase. However, the analysis of the dynamical and thermodynamic stability conditions at the equilibrium points doesn
Dr.	Moncy	Vilavinal	John	School of Pure and Applied Physics, Mahatma Gandhi University, Kottayam, Kerala	A cosmological model named `$R_h=ct$ model' was proposed and studied by F. Melia and collaborators. Observational and theoretical aspects of this model, which claims to be a new one, are discussed in several papers. In a recent paper [M.V. John, MNRASL, 484, L35-L37 (2019)], it was pointed out that this model with $\rho + 3p=0$ is identical to a pre-existing `eternal coasting cosmological model', which is a non-empty (non-Milne) one. The latter was studied at length by J.V. Narlikar, K. Babu Joseph and the present author in several publications. This eternal coasting model with matter creation and $H_0t_0=1$ was published even before the discovery of the accelerated expansion of the universe and was shown to have none of the commonly discussed cosmological problems. The $R_h=ct$ model is only the special (flat) case of the eternal coasting model. An additional feature in the eternal coasting model is that $\Omega_m/\Omega_{dark \; energy}$ = some constant of the order of unity, so that also the cosmic coincidence problem is avoided. Here it is argued that in any eternal coasting model with $\rho + 3p =0$, all cosmic energy densities must vary as the inverse square of scale factor to avoid the coincidence problem.
Ms.	TANIMA		DUARY	Indian Institute of Science Education and Research Kolkata (IISER Kolkata)	Thawing and freezing quintessence models are compared thermodynamically. Both of them are found to disobey the Generalized Second Law of Thermodynamics. However, for freezing models, there is still a scope as this breakdown occurs in the past, deep inside the radiation dominated era, when a standard scalar field model with a pressureless matter is not a correct description of the matter content. The thawing model has a pathological breakdown in terms of thermodynamics in a finite future.
Ms.	CHAYANIKA		RABHA	GAUHATI UNIVERSITY	Cosmic expansion history beyond the ΛCDM paradigm: The cases for quiessence, ω(z) and dark radiation\n\n Chayanika Rabha1,2, Sanjeev Kalita1,3\n\n 1Department of Physics, Gauhati University, Guwahati-781014\n 2 chayanika.rabha@gmail.com, 3sanjeevkalita1@ymail.com\n\n\n \nABSTRACT: The recent measurements of the Hubble parameter from the Cosmic microwave background, (H0=67.4 ± 0.5 km s-1 Mpc – 1) and the model independent distance ladder method (H0=73.24 ± 1.74 km s-1Mpc-1) show a noticeable tension. The CMB measurement is done in the framework of the otherwise successful ΛCDM concordance model and the low value of the Hubble parameter is difficult to understand within this model. The Hubble constant is closely related to the properties of dark energy through the equation of state (ω) parameter. Studies beyond the standard model give a scope to investigate the modification of expansion rate in the early and late universe. One of the scopes is studying the expansion history in the regime of modified gravity. However, in this work we study the Hubble function H(z),deceleration function q(z) and the cosmic scale factor a(t) with the help of quiessence, dynamical dark energy and dark radiation for flat as well as curved relativistic FLRW models. The statefinder parameters r(z) and s(z) have also been studied to see the deviation (if any) of the expansion history from that of the ΛCDM.
Mr.	Soumya		Roy	Research Scholar at IUCAA	Title : Characterization of X-ray emission from Cosmological Simulation of Galaxy Group and Clusters\nAuthor : Soumya Roy, Rudrani Kar Chowdhury, Suchetana Chatterjee, Nishikanta Khandai, Tiziana Di Matteo\n\nThe effect of central supermassive black hole (SMBH) on large scale gas and stars in galaxies, through a self-regulatory feedback mechanism influences the growth and evolution of black hole host galaxies and halos. We use a cosmological simulation output from two simulations where SMBH feedback is present and absent respectively, to simulate the X-ray environments of black holes at high redshift (z=1) in the 0.5 – 2 keV energy band. We then process our theoretical maps through Chandra’s ray tracing simulation to create synthetic observations. We then construct stacked X-ray maps using these synthetic observations and carry out an analysis of the X-ray surface brightness profiles for galaxies with and without SMBH feedback.\nOur results are in agreement with the findings of Chatterjee et al. (2015) and Mukherjee et al. (2019), as both studies found a difference in the X-ray emission at the scales of 1-10 arcsecond in the surface brightness profiles of AGN host galaxies and a contrled sample of all galaxies. We discuss how this might be useful for quantifying SMBH feedback through X-ray observations and propose a novel method that can be used to study X-ray source population in high redshift galaxies.
Mr.	Avinash		Singh	Indian Institute of Science Education and Research, Mohali, India.	The tachyon scalar field is a viable candidate for dark energy.\nOn the one hand, this model explains the present-day accelerated expansion successfully, and on the other, it alleviates the fine-tuning problem significantly, which Lambda-CDM model suffers from. We use an exponential potential and an inverse square potential and find constraints on the parameters of this model in light of new data sets. We find that the latest cosmological observations are compatible with this model. We also study perturbations in tachyon dark energy. The dark energy perturbations although less significant at smaller scales, becomes significant at Hubble and super-Hubble scales. We study the growth of structure when tachyon dark energy work as smooth background as well as when it allowed to be perturbed. We compare the growth rate of matter for this model with redshift space distortion (RSD) data and find it compatible. We constrain rms matter fluctuation (sigma_8) at present using this data. It is found that there is a tension of more than three sigma for the value of sigma_8(z=0) between RSD data and Plank data for the Lambda-CDM model. We show that this tension decreases significantly for tachyon models if the equation of state parameter deviates from -1 and dark energy get perturbed.
Mr.	Devendra	Raj	Upadhyay	Amrit Campus, Tribhuvan University, Kathmandu, Nepal	DUST PROPERTIES AROUND ASYMPTOTIC GIANT BRANCH STAR: IRAS 01142+6306 IN IRIS and AKARI MAP\nD. R. Upadhyay^1, 2*, M. Khanal^1, B. Aryal^2\n1 Amrit Campus, Tribhuvan University, Thamel, Kathmandu Nepal.\n2 Central Department of Physics, Tribhuvan University, Kirtipur, Kathmandu Nepal.\n*mnadphy03@gmail.com\n\nAsymptotic giant branch stars are the main dust distributors of dust into interstellar medium due to their high mass loss rates in combination with an effective dust condensation. We have studied dust around IRAS 01142+6306 i.e C-rich AGB star using IRIS and AKARI survey and obtained physical properties like dust color temperature, dust mass and variation of spectral density along extension and contraction direction of cavity like structure in 60 micron and 100 micron in IRIS and 90 micron and 140 micron in AKARI survey. The size of the structure is found to be 41.65 pc ×22.25 pc and 20.01 pc ×8.40 pc in IRIS and AKARI survey respectively. We have obtained the average dust color temperature 23.46 K and 25.31 K in these surveys. The average value of dust mass is found to be 3.02×10^-3M_ʘ and 2.61×10^-3 M_ʘ in that surveys respectively. Temperature is higher near the central region of cavity in both survey and mass is distributed along the outer arm. Contour plot of dust color temperature and dust mass shows inverse relation between them. The core region is found to be edge-on having inclination angle 77.60^o in IRIS survey and 84.56^o in AKARI survey.
Mr.	Darshan		Kumar	Ph.D. Student Department of Physics and Astrophysics, University of Delhi, Delhi-110007	TITLE: Testing the cosmic distance duality relation by using strongly lensed quasar systems.\nAbstract: Time-delay measurements of strongly lensed quasar systems provide “Time-delay distancesâ€_x009d_, which is a combination of three angular diameter distances. Therefore, strongly lensed quasar systems are a powerful tool to determine the different type of cosmological parameters including Hubble Constant (Ho). \nIn this work we use a method, which based on distance sum rule, to constrain the distance duality parameter (η(z)). We combine the time-delay distances which are measured from strongly lensed quasar systems and luminosity distance that are obtained from a parametric form. We check the consistency of this cosmic distance duality relation (CDDR) using 12 time-delay measurements of strongly lensed quasar systems. Assuming the flat ΛCDM model of the universe, we generate the 2000 simulated data points of time-delay distance to constrain the distance duality parameter. We found no violation in the cosmic distance duality relation.
Mr.	Sovan		Sau	Ganesh Chandra Sau	Neutrinos rotating around Abelian Higgs strings will generate a neutral current close to the string. As the string moves through the cosmic plasma, the velocity kick generated by the motion of the string will enhance the neutrino current in the wake region. The neutrino current density depends on its distance from the string and is oscillatory in nature. This leads to neutrino density gradients in the plasma. Such a flux of neutrinos with periodic density fluctuations will lead to electron currents in the plasma. The current will act like a cross-perturbation across the cosmic string wake. The perturbation, as well as the high Reynolds number of the plasma, will result in the generation of magnetic fields in the wake of the cosmic string.
Mr.	Divya		Rana	IUCAA	The comparison between the dynamical mass and the mass determined from gravitational lensing can be used to tests modifications to Einsteins general relativity. We use the Hyper Suprime Cam (HSC) survey to map the matter distribution in the Universe using the weak gravitational lensing technique. We use the first year HSC survey shape catalogue to compute weak lensing signals around galaxy groups from the GAMA survey,binned according to the velocity dispersion of group members. We model the matter distribution in GAMA galaxy groups using the halo model and compare with our measurements to infer their masses. We obtain the relation between the weak lensing mass of the group M and its velocity dispersion, σ and compare it with theoretical expectations. Our results provide a consistency test for general relativity.
Mr.	AMIT		KUMAR	IUCAA	The evolution of satellite galaxies orbiting a galaxy cluster is determined by a number of processes. The dark matter in the outskirts of its subhalo gets stripped due to tidal forces. If dark matter has self interactions it could further lead to the evaporation of the dark matter within the subhalo. Weak gravitational lensing is a direct probe of the matter distribution surrounding the satellite galaxy. We measure the lensing signal induced on the shapes of background galaxies around satellite galaxies from the SDSS redMaPPer cluster catalogue in redshift range, $0.1leq zleq 0.33$. We use the galaxy shape catalogue obtained from the high quality imaging data from the Subaru Hyper Suprime Cam (HSC) survey. We explore the effect of tidal forces on subhalo profile by measuring the lensing signal of satellites binned by their distance from the cluster center. We also comment on the implications of our results on the self interaction cross-section of dark matter.
Ms.	Swati	Satyawan	Gavas	IISER Mohali	Over the years various analytical mass function theories like Press-Schechter, Sheth -Tormen have been developed to model the halo mass function. One of the most important characteristics of these theories is that they are independent of cosmological parameters and power spectrum. Various studies over the past two decades, using N-body simulations, have been able to match the theoretical mass function approximately, and they show a weak dependence on power spectrum and cosmology. We will reinforce this with our suite of simulations ran for scale-free power spectrum in EdS cosmology. Selecting such conditions is advantageous due to self-similar evolution.
Mr.	Manvendra	Pratap	Rajvanshi	Indian Institute of Science Education and Research (IISER), Mohali	There are a number of theoretical alternatives to Lambda for explaining accelerated expansion of universe. These models of dark energy can be tuned so as to produce similar background cosmology. Investigations beyond background cosmology like perturbations are required to obtain observable distinctions between models. In this work address the question: whether two models of different classes, e.g., quintessence and tachyon, tuned to have same background cosmology, show significant deviations at nonlinear perturbations. We present results from numerical studies of spherically symmetric voids/overdense regions simulated using nonlinear relativistic equations. We find that if the background cosmologies are similar, matter perturbations show no significant differences in two cases while for dark energy perturbations, there are small differences. We are exploring whether these differences can be probed using existing or upcoming observations.
Mr.	VIPIN	KUMAR	SHARMA	UNIVERSITY OF LUCKNOW	An attempt has been made by us for the study of galactic dynamics via the $f(R)=alpha R^(1+delta)$ gravity model. It is completely inspired from the geometric relation obtained for such power law gravity\nmodel in vacuum. Here, we analyse the action having\nthe Lagrangian with a small deviation from the Einstein-Hilbert action Lagrangian in vacuum and obtain the contribution of $f(R)$ geometric background for exhibiting the galactic dynamics. Now, we consider the massive spherically-symmetric source in $f(R)$ background and obtain the equation for the rotational velocity of a test star. Then, we test the viability of such model by tracing the motion of test star outside the typical spiral galactic visible ends without any dark\nmatter. We obtain a good agreement of our result in the outer region (upto few tens of\nkpc beyond the visible end) of the spiral galaxy by taking the standard typical galactic data.
Mr.	PRALOY		DAS	Indian Statistical Institute, Kolkata	A new form Time Crystal has been proposed and some of its consequences have been studied. The model is\na generalization of the Friedmann-Robertson-Walker (FRW) cosmology endowed with noncommutative geometry corrections. In the mini-superspace approach the scale factor undergoes the time periodic behavior, or Sisyphus dynamics, which allows us to interpret this Cosmological Time Crystal as a physically motivated toy model to simulate cyclic universe. Analyzing our model purely from Time Crystal perspective reveals many novelties such as a complex singularity structure (more complicated than the previously encountered swallowtail catastrophe) and a richer form of\nSisyphus dynamics. In the context of cosmology, the system can serve as a toy model in which, apart from inducing a form of cyclic universe feature, it is possible to generate an arbitrarily small positive effective Cosmological Constant. We stress that the model is purely geometrical without introduction of matter degrees of freedom.
Mr.	JOSEPH	P	J	IIT Bombay	A large number of dark energy and modified gravity models lead to the same expansion history\nof the Universe, hence, making it difficult to distinguish them from observations. To make the\ncalculations transparent, we consider f (R) gravity with a pressureless matter without making any\nassumption about the form of f (R). Using the late-time expansion history realizations constructed\nby Shafieloo et al , we explicitly show for any f (R) model that the Bardeen potentials $Psi$ and\n$Phi$ evolve differently. For an arbitrary f (R) model that leads to late-time accelerated expansion,\nwe explicitly show that $|Psi + Phi|$ and its time-derivative evolves differently than the $Lambda$CDM model\nat lower redshifts. We show that the $Psi / Phi$ has significant deviation from unity for larger wave-\nnumbers. We discuss the implications of the results for the cosmological observations.
Ms.	NIRMALI		DAS	STUDENT	Title: Possibility of massive black hole formation at z~8 in different dark energy cosmologies- test from simple accretion model.\n\nAuthors: Nirmali Das(1,2), Tanushree Bezbaruah(1,3) and Sanjeev Kalita(1,4)\n\n1. Department of Physics, Gauhati University, Guwahati-781014, Assam\n2.email: nirmalidas995@gmail.com\n3.email: tanushree.bez21@gmail.com\n4.email: sanjeevkalita1@ymail.com\n\n Formation of supermassive black holes in the early universe is a matter of intense research in cosmology. Formation of the massive black holes is essential for onset of quasar activities. Possibility of formation of these black holes can be examined in simple accretion model through the consideration of time scales of accretion and age of the background universe around the epoch of reionization. In this work, the time required for formation of SMBHs and the age of the universe at quasar and reionization epochs are calculated by using different cosmological models and then compared. The value of optical depth (tau = 0.058) has been taken from the Planck 2018 data. It is found that cosmological models-E-dS, ΛCDM and ωCDM allow the formation of a non-rotating SMBH from different initial masses (10^2 M_sun, 10^4 M_sun, 10^5 M_sun). However, rotating black holes are essential for quasar activities. It is found that a rotating SMBH can form from an initial seed 10^5 M_sun, in ΛCDM and ωCDM models. Different values of equation of state, have been explored to calculate time scales in ωCDM model. The time required for formation of a rotating SMBH from 10^5 M_sun seed is found to be (5-6 x 10^8 year). Dynamical dark energy models (where,ω(a)=ω_0+ ω_a (1-a)) have also been tested with pivoting redshift. Using observationally constraint parameters of dark energy (ω_0=-1.104 and ω_a=-0.25) for pivoting redshift z_p=0.5, it has been found that at redshift, z<8.5 non-rotating black holes can form from the above considered seed masses. But, for the same limit of redshift, rotating black holes can form from an initial seed mass of 10^5 M_sun.\n \n\n\n\n\nReferences:\n\n[1] Rees, M., Perspectives in Astrophysical Cosmology, Cambridge University Press, 1995. \n[2] Serjeant, S., Observational cosmology, Cambridge University Press, 2010.\n[3] Aghanim, N. et al., arXiv:1807.06209v1 [astro-ph.CO] (2018).\n[4] Yang et al., arXiv:1811.06932v1 [astro-ph.CO] (2018)\n[5] Ostriker, J. P., arXiv:astro-ph/9912548v2, 2000.\n[6] Madau, P. and Rees, M., The Astrophysical Journal, 551, L27-L30, 2001.
Mr.	ANKIT		SINGH	Indian Institute of Science Education and Research (IISER), Mohali	The environment of galaxies is crucial to its evolution. Small-scale as well as the large-scale environment is known to influence galaxy observables like colours, the rate of formation of stars, etc. Large-scale filaments provide potential wells to trap gas inside them. A significant fraction of this gas cools down and ends up in halos residing in this environment (Liao et. al. 2019). Recent studies (Mahajan et. al. 2018, Cybulski et. al. 2014) using multi-wavelength data have shown that large-scale filament environment quenches star formation of galaxies residing in them.\n\nIn this work, we use EAGLE (Evolution and Assembly of GaLaxies in their Environment) cosmological simulation to study properties of galaxies residing in large-scale filament environment. We do a stacked analysis of various mock observations generated from the snapshot at z=0.1. We compare and contrast our results with the observations.
Mr.	SHIBENDU		GUPTA CHOUDHURY	INDIAN INSTITUTE OF SCIENCE EDUCATION AND RESEARCH KOLKATA	A new strategy for the reconstruction of f(R) gravity models has been attempted using the Raychaudhuri equation. Two examples, one for an eternally accelerating universe and the other for one that mimics a lambda cold dark matter expansion history, have been worked out. For both the cases, the relevant f=f(R) can be found out analytically. In the first case, f(R) is found to be a combination of power-law terms and the expression for the second case involves hypergeometric functions. The evolution history of an universe, given as specific values of the kinematical quantities like the jerk or the deceleration parameter, serves as the input. It is found that the corresponding f(R) gravity models, in both the examples, are not viable options.
Dr.	Parbati		Sahoo	National Institute of Techno logy, Calicut	Wormholes (WHs) are considered as hypothetical shortcuts or tunnels in spacetime. In general relativity (GR), the fundamental ingredient of WH geometry is the presence of exotic matter at the throat, which is responsible for the violation of null energy condition (NEC). However, the modified gravity theories have shown to be able to provide WH solutions satisfying energy conditions (ECs). In this paper, we study the static spherically symmetric WH solutions in modified f(R, T) gravity for a phantom fluid case. The exact solutions of this model are obtained through the equation of state (EoS), p=ωρ, associated with phantom dark energy (DE) ω<−1. We find the existence of a spherically symmetric WH solution supported by phantom energy distribution. The shape function of the WH is obtained in this model obeys all the WH metr ic conditions. In the modified gravity scenario the phantom fluid WH violates the NEC in radial case, unlike in the tangential case. Furthermore, using the "volume integral quantifier" (VIQ) method, the total amount of EC violating matter in spacetime is discussed briefly.
Ms.	NASHIBA		PARBIN	DIBRUGARH UNIVERSITY	TITLE : Scalarons mimicking Dark Matter (DM) in Hu-Sawicki model of F(R) gravity.\nABSTRACT : We conducted a study on the scalar field obtianed from f(R) gravity via Weyl transformation using the Hu-Sawicki model. Our study shows that the scalar field (also named as scalaron) obtained from this model has chameleonic property, i.e. the scalaron becomes light in the low-density region while it becomes heavy in the high-density region of matter, i.e. the scalaron can be regarded as dark matter because the mass of the scalaron in galaxies is comparable with the mass of different DM candidates. The time evolution of the scalaron mass is also investigated in this study.
Dr.	Laur		Jarv	University of Tartu	Title: "From inflation to dark energy in scalar-tensor cosmology". Abstract: The methods of dynamical systems are usually applied in cosmology for a model given before, to find the fixed points and from these to deduce the generic cosmological evolution. In this work we turn the typical reasoning around. Assuming that all the main stages of cosmological evolution are described by the corresponding fixed points, we determine the conditions on the model functions and parameters that allow to realize the full cosmic history. In particular, we focus upon scalar-tensor gravities with radiation and dust matter, and demand the eras of inflation, radiation, matter, and dark energy domination to arise from a sequence of fixed points (three saddles and finally an attractor). We formulate the conditions that the scalar potential and nonminimal coupling must satisfy, for the existence of all required fixed points with suitable properties. We illustrate the construction by a scalar field with quartic potential, with and without quadratic nonminimal coupling to curvature. [The work is done in collaboration with J. Dutta and W. Khyllep in NEHU, Shillong, India.]
Mr.	Akshay		Rana	University of Delhi	In General Relativity (GR), the graviton is massless. However, a common feature in several theoretical alternatives of GR is a non-zero mass for the graviton. These theories can be described as massive gravity theories. Despite many theoretical complexities in these theories, on phenomenological grounds, the implications of massive gravity have been widely used to put bounds on graviton mass. One of the generic implications of giving a mass to the graviton is that the gravitational potential will follow a Yukawa-like fall off. We use this feature of massive gravity theories to probe the mass of graviton by using the largest gravitationally bound objects, namely galaxy clusters. In this work, we use the mass estimates of galaxy clusters measured at various cosmologically defined radial distances measured via weak lensing (WL) and Sunyaev-Zel
Dr.	PRATIK	PREMADARSHI	RAY	BITS-Pilani Hyderabad Campus	The role of anisotropic components on the dark energy and the dynamics of the accelerating universe, starting from early deceleration epoch to late time acceleration epoch, are investigated in this work in the frame work of general relativity. The considered matter _x000c_field is of two non-interacting fluids, the usual string fluid and dark energy (DE) fluid. The anisotropic dark energy fluid with different pressures along different spatial directions is assumed to incorporate the effect of anisotropy. One dimensional cosmic strings aligned along x-direction supplement some kind of anisotropy. A time-varying hybrid scale factor is being incorporated to achieve a physically realistic solution to the field equations that simulates a cosmic transition from early deceleration to late time acceleration. Anisotropy in the dark energy pressure is found to evolve with cosmic expansion at least at late times. At an early phase, the anisotropic effect due to the cosmic strings dominates and substantially affects the dynamics of the accelerating universe. Finally, we analyzed the role of two fluids in the transitional phases of universe with respect to time which depicts the reason behind the cosmic expansion and DE. The role of DE with variable equation of state parameter and skewness parameters are also observed along with physical and geometrical properties. The interesting results and open research problems, found from this investigation, are discussed at the end which solve many queries regarding the nature and anisotropic behavior of dark energy.
Dr.	Selvaganapathy		J	Physical Research Laboratory, Ahmedabad	The natural inflation fails to explain low tensor-to-scalar ratio with current observational data and the inflaton potential is not valid at large-N limit Yang-Mills theory. We address a simple quintessence axionic inflationary model in which the axionic inflaton couples to a large-N limit pure Yang-Mills theory. This pure natural inflation satisfies the current Planck data as well as connecting the smallness of the vacuum energy (Dark energy) at a certain field value of quintessence axion. Further, we show that the quintessence pure natural inflation obeys the strong scalar weak gravity conjecture which follows from the de Sitter distance conjecture for any quasi de Sitter background.
Dr.	Jibitesh		Dutta	NEHU, Shillong	The theory of dynamical system (DS) provides powerful mathematical techniques with applications covering a wide range of fields. It offers a geometrical way of analyzing a com- plicated network of equations. In a cosmological context, it is extremely useful when we are trying to establish whether a given model with complicated governing equations can reproduce the observed dynamics of the Universe. In this work, in order to recover the important features of the braneworld model from a more global perspective, we investigate the global cosmological dynamics of the braneworld model using dynamical system techniques. We first analyze the case where there is just a normal matter on the brane and then extend the analysis to the case with an extra scalar field also trapped on the brane. In the pres- ence of a scalar field, potentials belonging to different classes are considered. The stability behavior of critical points is examined using linear stability analysis and center manifold theory. To understand the global dynamics of a dynamical system, we utilized the Poincar ́e compactification method to capture the properties of all possible critical points. Applying dynamical system analysis, we found that brane gravity is consistent with observed actions of the Universe. In particular, our analysis shows that important cosmological behaviors like the long-lasting matter-dominated era, late time acceleration as well as the avoidance of Big-Rip singularity can be realized in brane gravity for a wide range of initial conditions. Even though this model mimics the ΛCDM well at background level, it was shown that there is a possible deviation at the perturbation level. This might lead to further interesting signatures for the present and future observations. It would, therefore, be worthwhile to further analyze the evolution of cosmological perturbations and the behavior of matter den- sity perturbations by using dynamical system techniques. Furthermore, we have observed a unusual behavior of the matter energy density parameter, i.e. Ωmat > 1 during matter domination era, which is a generic phenomenon of this brane gravity model, with or without an extra scalar field. This can imply interesting observational signatures and perhaps could have something to do with the puzzle of the EDGES signal for the 21 cm hyperfine transition line of neutral hydrogen.
Dr.	Samarjit		Chakraborty	Department of Physics, St. Xavier	In this paper we study the evolution of the FRW universe filled with variable modified Chaplygin gas (VMCG). We begin with a thermodynamical treatment of VMCG described by the equation of state $P = Arho-Brho^{-alpha}$, and obtain its temperature as a function of redshift $z$. We show that the results are consistent with similar works on other types of Chaplygin gas models. In addition to deriving the exact expression of temperature of the fluid in terms of the boundary conditions and redshift, we also used observational data to determine the redshift at the epoch of transition from the decelerated to the accelerated phase of expansion of the universe. The values of other relevant parameters like the Hubble parameter, the equation-of-state parameter and the speed of sound are obtained in terms of the redshift parameter, and these values are compared with the results obtained from previous works on MCG and other Chaplygin gas models for the various values of $n$ permitted by thermodynamic stability. We assume the present value of temperature of the microwave background radiation to be given by $ T_0 = 2.7 K $, and the parameter $ A $ in the equation of state is taken as $ 1/3 $ since it corresponds to the radiation-dominated phase of the universe. The value of the parameter $Omega_x$ has been assumed to be $0.7$ in our calculation. Since it is known that the redshift of photon decoupling is $ zsimeq 1100 $, we used this value to calculate the temperature of decoupling. [arXiv:1906.12185v1 [gr-qc]]
Ms.	Richa		Arya	Physical Research Laboratory, Ahmedabad	Title: Primordial Black Holes Generation from Warm Inflation\n\nAbstract: Warm inflation is a description of inflation in which the inflaton dissipates into the radiation fields both during and after the inflationary phase. It is a more natural and plausible mechanism for inflation as compared to the standard cold\ninflationary paradigm. In this study, we discuss the consequences of the inflaton coupling and dissipation to the other fields during inflation. To this end, we study the formation of Primordial Black Holes (PBHs) by the collapse of overdensities generated in a model of warm inflation. In our analysis, we find that for some range of model parameters, we can produce an interesting abundance of PBHs for our warm inflation model.
Dr.	Debottam		Nandi	Postdoctoral fellow	Distinguishing conformally coupled frames from the tree-level perturbative observables is challenging in cosmology as they are nearly identical. However, since the background evolution in these two frames differs significantly, we can look for potential signatures in the reheating constraints to discriminate these frames. In this work, we study the reheating phase in these frames and find that the difference in the inflationary energy scales in these frames contributes to a significant difference in the reheating e-folding number and hence, different reheating temperature. This difference will eventually lead to a contrasting thermal history in the two frames, which may have a potential observational signature in future observations. This study will open up an avenue for distinguishing various conformally connected otherwise indistinguishable frames and may finally lead us to the correct theory of gravity for our Universe.1\n\n1. D. Nandi and P. Saha, Einstein or Jordan: seeking answers from the reheating constraints, [arXiv:1907.10295]
Mr.	Sambo		Sarkar	INDIAN INSTITUTE OF TECHNOLOGY, KHARAGPUR	In a recent work we plan to study the multi-partite nature of dark matter in a paradigm in which dark matter is considered to undergo interactions with itself. Considering certain motivated interactions among the dark matter candidates, we thereby try to constrain them from astrophysical observations. In the hope of addressing the small scale issues of structure formation of the Lamda Cold dark matter dominated universe, with a special focus on the long standing core vs cusp problem.
Ms.	Heena		Dua	Sant Longowal Institute of Engineering and Technology, Deemed-to-be-University, under MHRD.	Cosmological models with variable deceleration parameter in Brans-Dicke theory of gravity\nHeena Dua, R.K. Mishra\nSant Longowal Institute of Engineering and Technology\nDeemed-to-be-University, Under MHRD, Govt. of India\nLongowal-148106, Punjab, India\nEmail: heena.dua29@gmail.com\nEmail: ravkmishra@yahoo.co.in\n\nIt is well known that alternate or modified theories of gravitation provide a natural generalization of the general theory of relativity. In the present study, late-time acceleration of the universe in Brans-Dicke theory of gravity with variable cosmological constant has been investigated. In order to solve gravitational field equations, deceleration parameter is assumed as a time variable quantity. The choice of deceleration parameter yields a class of cosmological models which shows phase conversion in the expansion of the cosmos i.e. from early decelerated expansion phase to present accelerated expansion phase. Such behavior of the models of the universe is in good agreement with the type Ia supernova observations. The nature of various cosmological parameters is examined with the help of their pictorial representations. In order to distinguish the proposed models with other dark energy models, the detailed behavior of stateinder pair is observed. It is noticed that our proposed models behave like lambda CDM model in the late time evolution of the universe.
Ms.	Shivani		Sharma	Birla Institue of Technology and Science Pilani, Pilani campus	We consider the anisotropic extension of the Chevallier-Polarski-Linder model (CPL), via Bianchi type-I spacetime. This modifies the Friedmann equation by rendering the expansion rate $H(a)$ of the Universe and including a new term $Omega_{sigma 0}$ (mimicking the stiff fluid). We constrain the stiff-fluid-like effect of the expansion anisotropy in the absence of anisotropic sources using Hubble, Pantheon, baryonic acoustic oscillations and cosmic microwave background datasets. In all the cases of the dataset combinations, the Bayesian evidence of the anisotropic CPL model in comparison with the CPL model is calculated as per the Jeffrey’s scale. We further discuss our results in the context of matter-radiation equality, big bang nucleosynthesis and CMB quadrupole problem.
Dr.	Jaffino	Stargen	D	None	Title: Quantum-to-classical transition of primordial cosmological perturbations in bouncing universes\n\nAbstract: The perturbations in the early universe are generated as a result of the interplay between quantum field theory and gravitation. Since these primordial perturbations lead to the anisotropies in the cosmic microwave background and eventually to the inhomogeneities in the Large Scale Structure (LSS), they provide a unique opportunity to probe issues which are fundamental to our understanding of quantum physics and gravitation. One such fundamental issue that remains to be satisfactorily addressed is the transition of the primordial perturbations from their quantum origins to the LSS which can be characterized completely in terms of classical quantities. Bouncing universes provide an alternative to the more conventional inflationary paradigm as they can help overcome the horizon problem in a fashion very similar to inflation. While the problem of the quantum-to-classical transition of the primordial perturbations has been investigated extensively in the context of inflation, we find that there has been a rather limited effort towards studying the issue in bouncing universes. In this work, we analyze certain aspects of this problem with the example of tensor perturbations produced in bouncing universes. We investigate the issue mainly from two perspectives. Firstly, we approach the problem by examining the extent of squeezing of a quantum state associated with the tensor perturbations with the help of the Wigner function. Secondly, we study the effects of wave function collapse, using a phenomenological model known as continuous spontaneous localization, on the tensor power spectra. We conclude with a discussion of results.
Dr.	Alexey		Toporensky	Sternberg Astronomical Institute	In this presentation we address two important issues which could affect reaching the exponential and Kasner asymptotes in Einstein-Gauss-Bonnet cosmologies -- spatial curvature and anisotropy in both three- and extra-dimensional subspaces. In the first part of the presentation we consider cosmological evolution of spaces being the product of two isotropic and spatially curved subspaces. It is demonstrated that the dynamics in D=2 (the number of extra dimensions) and D⩾3 is different. It was already known that for the Λ-term case there is a regime with \stabilization\" of extra dimensions where the expansion rate of the three-dimensional subspace as well as the scale factor (the \"size\") associated with extra dimensions reach constant value. This regime is achieved if the curvature of the extra dimensions is negative. We demonstrate that it take place only if the number of extra dimensions is D⩾3. In the second part of the presentation we study the influence of initial anisotropy. Our study reveals that the transition from Gauss-Bonnet Kasner regime to anisotropic exponential expansion (with expanding three and contracting extra dimensions) is stable with respect to breaking the symmetry within both three- and extra-dimensional subspaces. However the details of the dynamics in D=2 and D⩾3 are different. Combining the two described affects allows us to construct a scenario in D⩾3 where isotropisation of outer and inner subspaces is reached dynamically from rather general anisotropic initial conditions"
Dr.	Kalyani		Desikan	Department of Mathematics\nSchool of Advanced Sciences\nVellore Institute of Technology, Chennai	In most cosmological studies the universe is assumed to be filled with a perfect fluid. But viscosity is expected to play a significant role during some stages of the evolution of the Universe. There are many theories of viscosity. Eckart’s first-order theory permitted superluminal signals and equilibrium states were found to be unstable. Israel-Stewart proposed a second-order theory to overcome these problems. Recently, a new first-order theory that provides a solution to these problems has been put forth by Disconzi. In this paper we attempt to find homogeneous and isotropic solutions for Cosmological models based on Disconzi's theory of viscosity. We find solutions by considering different assumptions for specific enthalpy that impacts the viscous pressure and discuss the behaviouur of the models.
Dr.	Chanchal		Chawla	Punjabi University, Patiala	To study the dynamics of the universe concerning the late time acceleration, we have constructed cosmo- logical models of the universe with Friedmann-Lemaitre-Robertson-Walker (FLRW) space-time in f(R, T) gravity theory, with particular functional of the form f(R, T) = λR + λT, where R and T are respectively Ricci scalar and trace of energy momentum tensor and λ is a constant. In order to obtain the determinis- tic solutions to Einstein’s field equations (EFE), we have chosen a general parametrization for the Hubble parameter. Moreover, analytical and graphical behavior of involved physical parameters with time (t) and redshift (z), has been discussed. Further, the energy conditions of the model are studied and statefinder diagnostic pair has been investigated. It is demonstrated that the model is in good agreement with various observations.
Dr.	Kumar		Das	S. N. Bose national Centre For Basic Sciences	We have considered the F -term Hybrid inflationary model in the presence of a modulus sector. Previously it has been claimed (W. Buchmiller et. al.) that while the spectral index for the standard F -term Hybrid inflation model is in tension with the current observational bounds, the model may be possible to make consistent with the current PLANCK bound with an additional light scalar field (i.e modulus) in the theory. The\nmodulus in this model is stabilized with a mass at least of the order of the Hubble scale during inflation. When this stabilization is achieved in conjunction with the inflationary\nsector, the minimum of the modulus potential undergoes small displacement which has now become a function of the inflaton field. We first neglect the quantum corrections, but account for this shift in the potential minimum. We have computed the full field dynamics of the model and the spectral index turns out to be blue-tilted. Moreover, one loop correction does not happen to have any further improvement in making the scenario\nfavoured by observation.
Mr.	Dipayan		Mukherjee		We study $f(R)$ gravity model which either leads to or are equivalent to Einstein gravity with a Quintessence field action in a conformally connected spacetime (in Einstein frame). The potential of the Quintessence field corresponds to the choice of the $f(R)$ model. We constrain our $f(R)$ models using this correspondence.\n\nIn Jordan frame, modified $f(R)$ gravity can be treated as a correction to Einstein gravity, such that the correction itself is responsible for the accelerating universe. Here we have considered an effective Energy Momentum tensor that does not rely on non-minimal gravitational coupling. \n\nWe study the class of $f(R)$ theories which can successfully lead to quintessence field potential, consistent with observations. We argue that the existence of dark energy implemented by a viable Quintessence field is equivalent to particular $f(R)$ actions in these two models.
Dr.	Daniel		Muller	Instituto de Física - Universidade de Brasília	We will present Starobinsky inflation in Jordan frame. We will also compare our results with the much more explored case done in Einstein frame. It is shown the inflationary solution basin of attraction for zero spatial curvature case, taking into account constraints form recent CMBR Planck.
Dr.	Mahavir		Sharma	ICRAR Curtin	Galaxies are the building blocks of the Universe and it is of paramount importance to understand how do they form, and how do they impact the ionization state of the Universe. We present a model for the formation of galaxies that builds on an analogy with the evolution of main sequence stars; that enables us to reproduce galactic properties such as the stellar mass function, and mass metallicity relation. Galaxies are also the candidates that produce ionizing photons to reionize the Universe after the dark ages. We find that the bursty brighter galaxies at high redshift were the main drivers of reionization; majority of those galaxies within the detection limit of Hubble.