Acknowledgements

The work presented in this thesis was initiated by my unofficial supervisor T. Pad- manabhan. Paddy's constant encouragement and valuable guidance has left a lasting impression on my subsequent work. He always encouraged me for inde pendence work in my research. I wish to take this opportunity to thank him for his friendship and guidance.

It is indeed a pleasure to thank my official supervisor Jayant Narlikar. I have benefited greatly from fruitful interactions with him. These interactions helped me understand, among others, some aspects of the subject I was working on.

The academic members of IUCAA have all had great impacts on me in different ways. I would like to name and acknowledge some of them:

Varun Sahni, from whom I have benefited immensely through many lively discussions. S. Sridhar, a good teacher who taught me to extract physics from astrophysics. Somak Raychaudhury, Ajit Kembhavi and R. Srianand have been my keen well-wishers from whom I always received valuable advice and useful suggestions.

As the first international research scholar of IUCAA, I think, I have to thank

i speciallySnehalata Shankar, Jayant Narlikar, Naresh Dadhich and V. Chellathurai for helping me to obtain the very difficult obtainable research visa from Indian

Embassy in Tehran as well as registering in University of Pune and overcoming its very bureaucratic red tape.

I have also benefited from useful interactions with Jasjeet Bagla, Sunu En gineer, Nissim Kanekar, K. Harikrishna and S. Banerjee.

I would like the IUCAA library and computer centre staffs, particularly Nirupama Bawdekar and Sarah Ponrathnam. My special thanks to Senith Samuel, V. Samak, S. Tarpe, Manjiri Mahabal and Susan Mathew for their valuable helps. In brief, I would like to thank all the IUCAA members whose active supports and encouragements made life much easier for me and my family.

The research work described in this thesis was supported by the University Grant Commission, India through a Research Fellowship at IUCAA.