Jasjeet Singh Bagla

Research Summary:

My main research interest for the last few years has been Cosmology and the physics of the high redshift Universe. Most of these studies require use of N-Body simulations and developing algorithms for these is also one of my interests. Suryadeep Ray and I have developed a highly optimised parallel version of the TreePM code that can be used to run simulations with more than 10⁷ particles. With this code, we can run such a large simulation through more than 10³ time steps in less than two days. This allows us to model gravitational clustering and galaxy formation in a more realistic manner.

Studies of clustering of galaxies in the non-linear regime are plagued by the issue of bias. It has long been recognised that measurement of higher order correlations and moments allow determination of bias. However in absence of clean analytical formulation in the non-linear regime, there is no clear method of making use of higher order moments for this purpose. Numerical studies carried out with Suryadeep Ray suggest that it is nearly impossible to differentiate models using higher order moments if we are looking the distribution of over-dense regions.

With Jayanti Prasad and Suryadeep Ray, we studied the interplay of clumping at small scales with the collapse and relaxation of large perturbations. Substructure can play an important role in the relaxation process. It can induce mixing in the phase space, or change density profiles by introducing transverse motions, and, gravitational interactions between small clumps can make the relaxation effectively collisional even for a collisionless fluid. Thus it is important to understand the role played by substructure in gravitational collapse and relaxation in the context of an expanding background. In our preliminary studies we modelled the large scale collapse as that of a plane wave. We find that in absence of substructure, collapse of a plane wave perturbation leads to formation of a pancake with multi-stream regions. A comparison with the adhesion approximation suggests that the preferred value of effective viscosity depends primarily on the number of streams in a region. The presence of sub-structure speeds relaxation of plane wave and makes the collapsed region thinner. In turn, the collapse of plane wave leads to larger clumps. We are studying the role played by substructure in gravitational collapse by considering more generic situations.

Recent observations have shown that the universe is accelerating and it is also nearly flat, i.e., there is no spatial curvature. The accelerated expansion of the universe requires either a cosmological constant or some form of dark energy to drive the acceleration, with w &equiv p/&rho < -1/3. Although a cosmological constant is the simplest solution from a phenomenological point of view, requiring just one fine tuned parameter, there is no natural explanation of the small observed value. This has led theorists to develop models in which a field, typically a scalar field, provides the source of dark energy. In the absence of significant spatial variation in the dark energy, the key difference between such models and the one with the cosmological constant is that, in general, w is a function of redshift in the former. With H.K.Jassal and T.Padmanabhan, we are carrying out a detailed study of available observations to see if the variation of w with redshift can be detected or ruled out.

Publications:

• Bagla J.S. and Ray S. : Performance characteristics of the TreePM method, New Astronomy 8, 665, 2003.

• Bagla J.S. : A parallel TreePM code, Numerical Simulations in Astronomy, ed. K.Tomisaka and T.Hanawa, 2003, p.32.

• Bagla J.S. : Cluster Computing at HRI, Proceedings of DAE meeting on Parallel computing and its applications in science and engineering, BARC, p.219, 2003.

• Bagla J.S. and White M. : Prospects for Detecting Neutral Hydrogen Using 21-cm Radiation from Large Scale Structure at High Redshifts, the IAU 8th Asian-Pacific Regional Meeting, Proceedings volume I, Eds. Satoru Ikeuchi, John Hearnshaw, and Tomoyuki Hanawa; ASP conference series, vol.289, p.251, 2003.

Conference/Workshops Attended:

• The Provocative Universe, a conference held in honour of Prof.J.V.Narlikar, IUCAA, Pune, June 30 -- July 2, 2003.

• Parallel computing and its applications in science and engineering, BARC, Mumbai, Nov.27--28, 2003.

• International Conference on Gravitation and Cosmology, Cochin, Jan.5--10, 2003.

Visits to other Institutes:

• Inter-University Centre for Astronomy and Astrophysics, Pune, June 29 -- July 28, 2003.

• Inter-University Centre for Astronomy and Astrophysics, Pune, Jan.12--25, 2004.

• Raman Research Institute, Bangalore, Jan.27 -- Feb.12 and Feb.16--20, 2004.

• Insitute of Mathematical Sciences, Chennai, Feb.13--15, 2004.

Invited Lectures/Seminars:

• The galaxy correlation function and bias, an invited talk in the Provocative Universe, a conference held in honour of Prof.J.V.Narlikar, IUCAA, Pune, June 30 -- July 2, 2003.

• Cluster computing at HRI, a talk in the meeting on Parallel computing and its applications in science and engineering, BARC Mumbai, Nov.27--28, 2003.

• Cluster computing at HRI, seminar at IUCAA, Pune, Jan.21, 2004.

• A series of lectures on N-Body simulations of gravitating systems, Raman Research Institute, Bangalore, Jan.29 -- Feb.6, 2004.

• Studying galaxy clustering on Linux clusters, colloquium at the Dept. of Physics, Indian Institute of Science, Bangalore, Feb.10, 2004.

• Cluster computing at HRI, a presentation at Centre for Development of Advanced Computing (CDAC), Bangalore, Feb.11, 2004.

• Cluster computing at HRI, colloquium at IMSc, Chennai, Feb.13, 2004.

• Gravitational Clustering , colloquium at Indian Institute of Astrophysics (IIA), Bangalore, Feb.17, 2004.

• Gravitational Lensing and the Cosmic Microwave Background Radiation, an invited talk in a discussion meeting on Cosmic Microwave Background Radiation, Coorg, Feb.21--26, 2004.

Academic Recognition and Awards:

• I have been nominated a member of the International Astronomical Union (IAU). Members from each country are nominated by a national coordination committee.

Other Activities:

• I managed the procurement and setting up of a Linux cluster at HRI. The 42 node cluster, called Kabir, was set up during the summer of 2003. Several other facilities, such as large data storage and data backup have also been set up for use with the cluster. Kabir is one of the fastest super-computing facilities in India. Its utilisation in the first four months after commissioning has been more than 50%. System management of this cluster is done by a group of users led by me. More information on this is available at http://cluster.mri.ernet.in/.

• I organised an advanced school on the physics of galaxy formation. A total of 20 participants attended courses offered by more than 10 experts, a large fraction of experts and many participants were from outside India. Presentations made by speakers in this school are available online.

• I gave a series of six lectures on basics of galaxy formation and on N-Body simulations in the school on physics of galaxy formation.

• I gave a lecture on astrophysics in the rajbhasha programme here at HRI.

• Two students did projects with me over the summer of 2003. These students were Nishikanta Khandai (IIT Mumbai) and Manu Awasthi (IT-BHU, Varanasi).

• I gave a one semester course on Astrophysics for second year graduate students.