Bharat Ratra

Bharat Vishnu Ratra

Born|  (1960-01-26) 26 January 1960 (age 65)

Bombay, India (now Mumbai, India)

Alma mater| Indian Institute of Technology Delhi (MS)
Stanford University (PhD)
Known for| Quintessence (physics)
Dark energy
inflation
Scientific career
Fields| Theoretical physics
Physical cosmology
Astroparticle physics
Institutions| Kansas State University
Massachusetts Institute of Technology
California Institute of Technology
Princeton University
Stanford University
Doctoral advisor| Leonard Susskind
Michael Peskin

Bharat Vishnu Ratra (born 26 January 1960) is an Indian-American physicist, theoretical cosmologist and astroparticle physicist who is currently a university distinguished professor of physics at Kansas State University.[1]

He is known for his work on dynamical dark energy and on the quantum-mechanical generation of energy density and magnetic field fluctuations during inflation.

## Biography

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Ratra was born in Bombay (Mumbai). He graduated with a Master of Science in physics from the Indian Institute of Technology Delhi in 1982 and completed his doctorate in physics at Stanford University in 1986 under the supervision of Leonard Susskind and Michael Peskin.

Ratra was a postdoctoral fellow at the Stanford Linear Accelerator Center, Princeton University, the California Institute of Technology and the Massachusetts Institute of Technology. He joined Kansas State University in 1996 as an assistant professor of physics. He was promoted to associate professor in 2001 and professor in 2004.

## Academics and research

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Ratra has worked in a number of areas of cosmology and astroparticle and early universe physics.

In 1988, Ratra and Jim Peebles of Princeton University proposed the first dynamical dark energy scalar field, or quintessence, model.[2][3] Dark energy is the leading candidate for the mechanism that is responsible for causing the observed accelerated cosmological expansion.[4][5]

Ratra and his students and collaborators have pioneered measurements of the redshift of the transition between an earlier epoch when cosmological expansion decelerated because dark and baryonic (ordinary) matter dominated the cosmological energy budget and the current epoch where the cosmological expansion accelerates because dark energy dominates the current cosmological energy budget.[6][7]

Ratra and his students and collaborators have developed new cosmological probes and used these in conjunction with better-established ones to measure the Hubble constant (Hubble's law), the geometry of space (Shape of the universe), and dark energy dynamics. [8][9][10]

Ratra's early universe research includes the first consistent semi-classical computation of the spectrum of energy density perturbations from inflation. He collaborated with Willy Fischler of the University of Texas at Austin and Leonard Susskind of Stanford University on this computation.[11][12] He has also computed the power spectrum of energy density perturbations in non-spatially-flat inflation models.[13] [14] [15]

Ratra also proposed the first inflation model that can generate, from quantum fluctuations, a large-enough primordial cosmological magnetic field to be able to explain observed galactic magnetic fields.[16]

## Honours

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* National Science Foundation CAREER award (1999)
* Fellow of the American Physical Society (2002)
* Fellow of the American Association for the Advancement of Science (2005)
* Olin Petefish Award in Basic Science (2017)
* Fellow of the American Astronomical Society (2023)[17]
* American Physical Society Julius Edgar Lilienfeld Prize (2025)[18]

## References

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1. ^ "Department of Physics". ksu.edu.
2. ^ P. J. E. Peebles and B. Ratra, "Cosmology with a time-variable cosmological 'constant'", Astrophys. J. 325, L17 (1988) (PDF)
3. ^ B. Ratra and P. J. E. Peebles, "Cosmological consequences of a rolling homogeneous scalar field", Phys. Rev. D 37, 3406 (1988)
4. ^ P. J. E. Peebles and B. Ratra, "The cosmological constant and dark energy", Rev. Mod. Phys. 75, 559 (2003)
5. ^ * B. Ratra and M. S. Vogeley, "The beginning and evolution of the universe", Pub. Astron. Soc. Pacific 120, 235 (2008)
6. ^ O. Farooq and B. Ratra, "Hubble parameter measurement constraints on the cosmological deceleration-acceleration transition redshift", Astrophys. J. 766, L7 (2013)
7. ^ O. Farooq, F. R. Madiyar, S. Crandall and B. Ratra, Astrophys. J. 835, 26 (2017)
8. ^ S. Cao and B. Ratra, "Using lower redshift, non-CMB, data to constrain the Hubble constant and other cosmological parameters", Mon. Not. Roy. Astron. Soc. 513, 5686 (2022)
9. ^ J. de Cruz Perez, C.-G. Park and B. Ratra, Phys. Rev. D 107, 063522 (2023)
10. ^ J. de Cruz Perez, C.-G. Park and B. Ratra, Phys. Rev. D 110, 023506 (2024)
11. ^ W. Fischler, B. Ratra and L. Susskind, "Quantum mechanics of inflation", Nucl. Phys. B 259, 730 (1985)
12. ^ B. Ratra, "Restoration of spontaneously broken continuous symmetries in de Sitter spacetime", Phys. Rev. D 31, 1931 (1985) (PDF)
13. ^ B. Ratra and P. J. E. Peebles, "Inflation in an open universe", Phys. Rev. D 52, 1837 (1995) (PDF)
14. ^ B. Ratra, "Inflation in a closed universe", Phys. Rev. D 96, 103534 (2017) (PDF)
15. ^ B. Ratra, "Tilted spatially nonflat inflation", Phys. Rev. D 106, 123524 (2022) (PDF)
16. ^ B. Ratra, "Cosmological `seed' magnetic field from inflation", Astrophys. J. 391, L1 (1992) (PDF)
17. ^ AAS Names 22 New Fellows for 2023
18. ^ Bharat Ratra, Winner of APS Lilienfeld Prize, on What’s Next for Cosmology

## External links

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Wikiquote has quotations related to Bharat Ratra.

* 13 February 2017 University of Kansas Colloquium: Dark Energy: constant or time variable? (... and other open questions)
* 5 November 2019 Kansas State University Colloquium: The Accelerating Expanding Universe: Dark Matter, Dark Energy, and Einstein's Cosmological Constant, or Why Jim Peebles was Awarded Half of the 2019 Physics Nobel Prize
* 2 February 2023 Canadian Institute for Theoretical Astrophysics Seminar: Spatial Curvature, Dark Energy Dynamics, Neither, or Both?
* 7 May 2024 EPFL Bernoulli Center Generation, evolution, and observations of cosmological magnetic fields conference lecture: Cosmological Seed Magnetic Field from Inflation
* 8 May 2024 Ústav teoretické fyziky a astrofyziky PřF MU: Conversations about the Cosmos with Bharat Ratra about the Dark Energy (in English)
* 31 January 2025 American Physical Society Medal and Society Prizes Documentary

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