Ashtekar is the originator of the Ashtekar variables, a reformulation of general relativity that led to the development of loop quantum gravity. He has contributed extensively to loop quantum gravity and its cosmological application, loop quantum cosmology.[3] His research has addressed the asymptotic structure of spacetime, gravitational waves in nonlinear general relativity, quantum geometry at the Planck scale, black hole physics, and the quantum description of the Big Bang.[4]
He has authored or co-authored approximately 300 scientific papers and written or edited nine scholarly books. He has delivered more than 200 plenary lectures at international conferences and workshops.[2]
Ashtekar was born in Shirpur, India, on July 5, 1949. His father was a senior civil servant, and the family relocated frequently. He lived in six towns in Maharashtra before completing high school in Kolhapur in 1965. He ranked among the top 15 students in the Maharashtra State Examination and received a National Merit Scholarship.[8]
Ashtekar developed an interest in general relativity and cosmology and applied to the University of Texas at Austin. Despite not holding a master’s degree, he was admitted to the Ph.D. program in physics in 1969.[10] He worked under the supervision of Robert Geroch and later moved with him to the University of Chicago, where the relativity group was led by Subrahmanyan Chandrasekhar. He completed his Ph.D. in 1974.[11]
He subsequently held postdoctoral positions at the Mathematical Institute, University of Oxford, working with Roger Penrose, and at the Enrico Fermi Institute at the University of Chicago.[5] At Oxford, he developed an algebraic approach to quantum field theory in curved spacetime and, with Richard Hansen, analyzed the asymptotic structure of the gravitational field.[12]
Academic appointments
Ashtekar joined the University of Clermont-Ferrand in France as Professeur Associé, where he worked on gravitational waves and introduced elements of asymptotic quantization.[13] In 1980, he was appointed Assistant Professor at Syracuse University under a National Science Foundation initiative. He was promoted to Associate Professor in 1982, Professor in 1984, Distinguished Professor in 1988, and Erastus Franklin Holden Professor in 1992.[14]
In 1983, he accepted an appointment to the Chaire de Gravitation, at Pierre et Marie Curie Université, in Paris, but chose to return to Syracuse University in 1986.[15]
In 1993, Ashtekar joined Pennsylvania State University as Eberly Chair in Physics and established a new Center for Gravitation. In 1999, he declined an offer of a Directorship at the Max Planck Institute for Gravitational Physics in Germany. He has remained at Penn State since 1993.[16]
Institute for Gravitation and the Cosmos
In 2001, Ashtekar chaired the executive committee of a second Center at Penn State focused on Gravitational Wave Physics. It was funded by the National Science Foundation in the very first year of their Physics Frontier Centers initiative.[17]
The two centers became part of the Institute for Gravitation and the Cosmos (IGC), which expanded to include cosmology and multimessenger astronomy and astrophysics. Ashtekar served as director of the IGC and its predecessor centers from 1993 until stepping down in 2021.[18][19]
Research
Classical general relativity
Ashtekar’s work in classical general relativity includes studies of the asymptotic structure of spacetime, gravitational radiation, and black hole dynamics. With collaborators including Richard Hansen and Anne Magnon, he identified the asymptotic symmetry group at spatial infinity, known as the Spi group, and analyzed associated conserved quantities, especially the subtleties in the notion of angular momentum of space-times with gravitational waves.[20][21]
His research on gravitational waves demonstrated that their physical content is encoded in the curvature of a connection defined on Penrose’s conformal boundary of spacetime.[22] These ideas provided the foundation for later work on infrared effects, gravitational memory and celestial holography and recent efforts to improve gravitational waveform modeling for LIGO–Virgo data analysis.[23]
Ashtekar and collaborators, including Badri Krishnan introduced quasi-local horizons, including isolated and dynamical horizons, which are now widely used in numerical simulations of black hole formation and mergers, and in investigations of the black hole evaporation due to emission of quantum radiation discovered by Stephen Hawking.[24] He has also studied asymptotic properties of gravitational fields in spacetimes with nonzero cosmological constant.[25]
Quantum gravity and Ashtekar variables
Ashtekar’s research on quantum gravity began with work on quantum field theory in curved spacetime, including algebraic and Kähler-geometric methods. In the 1980s, he developed a non-perturbative quantization of the radiative modes of gravity and identified connections between infrared issues and asymptotic symmetries.[26]
In 1986, he reformulated general relativity using self-dual connections, now known as Ashtekar variables. This reformulation simplified Einstein’s equations and cast general relativity in a gauge-theoretic form, enabling the development of loop quantum gravity.[27]
Loop quantum gravity and cosmology
Loop quantum gravity predicts a quantum geometry in which geometric operators corresponding to area and volume have discrete spectra.[28][29] The theory was further developed by numerous researchers, including Ivan Agullo, Jerzy Lewandowski, Carlo Rovelli, Lee Smolin, and Thomas Thiemann, many of whom are former members of Ashtekar’s research group, and by their own students.[30]
In loop quantum cosmology, Ashtekar and collaborators showed that quantum geometric effects resolve the classical Big Bang singularity, replacing it with a cosmological bounce.[31] They also showed that these quantum geometric effects can modify the evolution of primordial perturbations and may leave observable imprints in the cosmic microwave background, potentially accounting for certain large-scale anomalies.[32]
In black hole physics, loop quantum gravity provides a microscopic account of black hole entropy.[33][34] It has also been applied to the study of black hole evaporation through frameworks based on quantum geometry and quasi-local or isolated horizons.[35]
Outreach, service, and mentoring
Ashtekar founded the Frontiers of Science public lecture series at Penn State, which has been held annually for more than three decades and was renamed the Ashtekar Lectures on Frontiers of Science in 2020.[36]
The American Institute of Physics conducted an oral history interview with Ashtekar for the Niels Bohr Library in 2021.[11] He has mentored over 100 postdocs and graduate students including[39];
Postdoctoral Researchers
Laurent Freidel — Professor, Perimeter Institute, Waterloo, Canada
Viqar Husain — Professor, Department of Mathematics and Statistics, University of New Brunswick, Canada
Jerzy Lewandowski — Chair of Gravitation, University of Warsaw, Poland
Renate Loll — Professor of Theoretical Physics, Radboud University, The Netherlands
Jorma Louko — Professor of Mathematical Physics, University of Nottingham, UK
Donald Marolf — Professor, Physics Department, University of California, Santa Barbara, USA
José Mourão — Professor and Chair, Mathematics Department, University of Lisbon, Portugal
Jorge Pullin — Professor and Hearne Chair, Louisiana State University, USA
Thomas Thiemann — Professor, Chair of Theoretical Physics, Friedrich-Alexander-Universität, Germany
Ph.D. Students
Alejandro Corichi — Professor, National Autonomous University of Mexico (UNAM)
Jonathan Engle — Professor, Florida Atlantic University, USA
Stephen Fairhurst — Professor, Cardiff University, UK; Spokesperson of the LIGO Scientific Collaboration
Kirill Krasnov — Professor of Mathematical Physics, University of Nottingham, UK
Badri Krishnan — Professor and Chair, Fundamental Physics from Strong Gravity, Radboud University, The Netherlands
David Sloan — Chief Scientific Officer, Fundamental Questions Institute
Chris Van Den Broeck — Professor, Utrecht University, The Netherlands
Edward Wilson‑Ewing — Professor, Department of Mathematics and Statistics, University of New Brunswick, Canada
Ashtekar and A. Magnon, Translation from French of Elie Cartan's work, "Sur les Varietes a Connexion Affine et la Relativite Generale" with a Commentary and Foreword by A. Trautman. Bibliopolis, Naples, 1986, 199 pages.
Ashtekar and J. Stachel, (Editors); Conceptual Problems of Quantum Gravity. Proceedings of the 1988 Osgood Hill Conference (Birkhauser, N. Y., 1991), 602 pages.
Ashtekar, Lectures on Non-perturbative Canonical Gravity, (Notes prepared in collaboration with R.S. Tate), (World Scientific Singapore, 1991), 334 pages.
Ashtekar (Editor), 100 Years of Relativity – Spacetime structure: Einstein and Beyond (World Scientific, Singapore, 2005).
Ashtekar (Editor in Chief), B. Berger, J, Isenberg and M.A.H. MacCallum (editors), General Relativity and Gravitation: A Centenary Perspective, commissioned by the International Society on General Relativity and Gravitation to celebrate the 100th anniversary of Einstein’s discovery of general relativity (Cambridge University Press, Cambridge, 2015).
Books
A. Magnon and A. Ashtekar, Translation from French of Élie Cartan's work, "Sur les variétés à connexion affine et la théorie de la relativité généralisée" with a commentary and foreword by A. Trautman, Bibliopolis, Naples, 1986, 199 pages.
A. Ashtekar, Asymptotic Quantization. Bibliopolis, Naples, 1987, 107 pages.
A. Ashtekar, (with invited contributions) New Perspectives in Canonical Gravity. Bibliopolis, Naples, 1988, 324 pages.
A. Ashtekar and J. Stachel, Editors; Conceptual Problems of Quantum Gravity. Proceedings of the 1988 Osgood Hill Conference (Birkhauser, N. Y., 1991), 602 pages.
A. Ashtekar, Lectures on Non-perturbative Canonical Gravity, (Notes prepared in collaboration with R.S. Tate), (World Scientific Singapore, 1991), 334 pages.
A. Ashtekar, R.C. Cohen, D. Howard, J. Renn, S. Sarkar and A. Shimony (Editors), Revisiting the Foundations of Relativistic Physics, Festschrift in honor of John Stachel, Boston Studies in Philosophy of Science, Volume 234, (Kluwer Academic, 2003).
↑Rovelli, Carlo; Smolin, Lee (1995). "Discreteness of area and volume in quantum gravity". Nuclear Physics B. 442 (3): 593–619. doi:10.1016/0550-3213(95)00150-Q.
↑Ashtekar, Abhay; Lewandowski, Jerzy (1997). "Quantum theory of geometry I: Area operators". Classical and Quantum Gravity. 14: A55–A82. doi:10.1088/0264-9381/14/1A/006.
↑Ashtekar, Abhay; Lewandowski, Jerzy (2004). "Background independent quantum gravity: A status report". Classical and Quantum Gravity. 21 (15): R53–R152. doi:10.1088/0264-9381/21/15/R01.
↑Ashtekar, Abhay; Pawlowski, Tomasz; Singh, Parampreet (2006). "Quantum Nature of the Big Bang". Physical Review Letters. 96 (14). doi:10.1103/PhysRevLett.96.141301.
↑Agullo, Ivan; Ashtekar, Abhay; Nelson, William (2012). "Quantum Gravity Extension of the Inflationary Scenario". Physical Review Letters. 109 (25): 251301. doi:10.1103/PhysRevLett.109.251301.{{cite journal}}: CS1 maint: article number as page number (link)
↑Rovelli, Carlo (1996). "Black hole entropy from loop quantum gravity". Physical Review Letters. 77 (16): 3288–3291. doi:10.1103/PhysRevLett.77.3288.
↑Ashtekar, Abhay; Baez, John; Corichi, Alejandro; Krasnov, Krzysztof (1998). "Quantum geometry and black hole entropy". Physical Review Letters. 80 (5): 904–907. doi:10.1103/PhysRevLett.80.904.
↑Ashtekar, Abhay; Krishnan, Badri (2004). "Isolated and dynamical horizons and their applications". Living Reviews in Relativity. 7: 10. doi:10.12942/lrr-2004-10.{{cite journal}}: CS1 maint: unflagged free DOI (link)
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