|Born||10 March 1957|
|Died||17 September 2021 (aged 64)|
|Alma mater||Kerala University,|
Tata Institute of Fundamental Research
|Institutions||Inter-University Centre for Astronomy and Astrophysics|
|Doctoral advisor||Jayant Narlikar|
Thanu Padmanabhan (10 March 1957 – 17 September 2021) was an Indian theoretical physicist and cosmologist whose research spanned a wide variety of topics in gravitation, structure formation in the universe and quantum gravity. He published nearly 300 papers and reviews in international journals and ten books in these areas. He made several contributions related to the analysis and modelling of dark energy in the universe and the interpretation of gravity as an emergent phenomenon. He was a Distinguished Professor at the Inter-University Centre for Astronomy and Astrophysics (IUCAA) at Pune, India.
Born to Thanu Iyer and Lakshmi on 10 March 1957 in Thiruvananthapuram (then Trivandrum), Padmanabhan attended school there. He earned his B.Sc. (1977) and M.Sc. (1979) in Physics from the University College Thiruvananthapuram, part of Kerala University. He published his first research paper (on general relativity) when he was still a B.Sc. student, at the age of 20. He joined the Tata Institute of Fundamental Research (TIFR) in Mumbai in 1979 for his Ph.D. and became a faculty member there in 1980. He held various faculty positions at TIFR from 1980 to 1992 and also spent a year (in 1986–87) at the Institute of Astronomy, Cambridge. He moved to Pune to work at the Inter-University Centre for Astronomy and Astrophysics in 1992 and served as its Dean, Core Academic Programmes, for 18 years (1997–2015).
Padmanabhan served as adjunct faculty of TIFR, the Harish-Chandra Research Institute (Allahabad), the Raman Research Institute (Bangalore) and the Indian Institute of Science, Education and Research (IISER, Pune) at different periods in his career. He was adjunct faculty of IISER, Mohali.
Padmanabhan served as chairman (2006–09) of the Time Allocation Committee of the Giant Meterwave Radio Telescope of NCRA. He was chairman (2008–11) of the Indian National Science Academy's National Committee which interfaces with the activities of the International Astronomical Union. In addition to advising the Government on policy issues, this also required him to coordinate the International Year of Astronomy 2009 activities in the country.
He was elected president of the Cosmology Commission (2009–2012) of the International Astronomical Union (IAU) and provided advice to IAU activities in this field. He was elected in 2011 as chairman of the Astrophysics Commission (2011–2014) of the International Union of Pure and Applied Physics (IUPAP) and co-ordinated the activities of IUPAP in this area. He was also a visiting faculty at many institutes including the California Institute of Technology, Princeton University, and a Sackler Distinguished Astronomer of the Institute of Astronomy, Cambridge. He was an elected fellow of The World Academy of Sciences and of all three National Academies of Science in India (the Indian National Science Academy, Indian Academy of Sciences, and the National Academy of Sciences, India).
In addition to his scientific research, Padmanabhan worked actively to popularize science and gave over 300 popular science lectures and authored more than 100 popular science articles. He did a comic strip serial The Story of Physics aimed at school children. Published by Vigyan Prasar (New Delhi), it was translated into half a dozen regional Indian languages and made available at an affordable price at Indian schools. To commemorate the International Year of Astronomy (IYA) in 2009, he published (with J. V. Narlikar and Samir Dhurde) the IYA Astronomical Diary 2009, which comprises 53 illustrated pages of astronomical information. In 2019, he co-authored with Vasanthi Padmanabhan The Dawn of Science (published by Springer).
He was married to Vasanthi Padmanabhan, who has a Ph.D. in astrophysics from TIFR, Mumbai, and they had one daughter, Hamsa Padmanabhan, who herself has a Ph.D. in astrophysics from IUCAA, Pune.
He died on 17 September 2021 at the age of 64 after a heart attack at his residence in Pune. A tribute article written by his former graduate students and post-doctoral fellows and physics friends from college days has been posted on the Physics arXiv.
Padmanabhan received several national and international awards including:
Padmanabhan's research was on the fields of gravitation and cosmology which includes quantum gravity and nature of dark energy. During 2002–2015, he provided a clear interpretation of gravity as an emergent phenomenon (like elasticity or fluid dynamics) and showed that this paradigm extends to a wide class of theories of gravitation including, but not limited to, general relativity. Padmanabhan could show that several peculiar aspects of classical gravitational theories find natural interpretations in this approach. Such an interpretation also provides a novel solution to the cosmological constant problem. He gave two lectures at the Oxford–Cambridge collaborative conference on "Cosmology and the Constants of Nature" about this.
Popular (non-technical) descriptions of Padmanabhan's research have been published in Scientific American (India), and a more technical description is available in an article from the Gravity Research Foundation in 2008, that describes his First Prize work. Another popular article about his work which appeared in a German science magazine along with the English translation is available on his home page. An interview of Padmanabhan by George Musser about his work can be found here.
In the earlier part of Padmanabhan's career (1980–2001), he made important contributions to quantum cosmology, structure formation in the universe and statistical mechanics of gravitating systems. In the 1980s, he came up with an interpretation of the Planck length as the 'zero-point length' of the spacetime based on very general considerations. This result, established by theoretical considerations and well-chosen thought experiments, finds an echo in more recent results in several other candidate models for quantum gravity. He developed the complex path method (in 1998) to study black hole thermodynamics which was a precursor to the 'tunneling paradigm' that became quite popular later on. He was a recognized authority in the subject of the statistical mechanics of gravitating systems and was a pioneer in the systematic application of these concepts to study the gravitational clustering in an expanding universe. He was invited to lecture twice at the Les Houches Schools (in 2002 and 2008) to a broader community about this subject.
In November 2016, Padmanabhan published research studies advocating a new paradigm shift in understanding gravity. A key question in quantum gravity lies in understanding the primordial, pre-geometric phase of the universe, from which the classical, geometric phase described by Einstein's equations emerges along with the notions of space and time themselves. Padmanabhan introduced the notion of Cosmic Information (called 'CosmIn'), which allows these two phases to be connected in a fascinating manner. CosmIn, which is a conserved quantity, measures the total information transferred from the quantum gravitational phase to the classical phase of the universe. Quantum gravitational considerations advocate an astonishingly simple value for CosmIn: 4π, the number of information 'bits' on the surface of a sphere of unit radius. Using these considerations, CosmIn was able to relate the numerical value of the cosmological constant – possibly the deepest unsolved problem in theoretical physics today – to the energy scale at which the universe made the quantum-to-classical transition. This is the first time that a model with no adjustable parameters is able to provide a holistic explanation for both these observations, which has far-reaching implications for the quantum structure of spacetime. A non-technical account covering this latest development in Padmanabhan's research was published in the magazine Nautilus.
Padmanabhan authored several advanced level textbooks. In addition, he authored several popular-level science books.