L. Sriramkumar


Research Summary:

The enormous red-shifting of the modes during the inflationary epoch suggests that physics at the very high energy scales (at the Planck scale and beyond?) may manifest itself at much lower energies thereby possibly providing us with an opportunity to utilize cosmological observations to test fundamental physics. In particular, the measurement of anisotropies in the Cosmic Microwave Background (CMB) to a higher and higher precision has led to the hope that, maybe, we are on the threshold of observing Planck scale effects in the CMB. Most of my research work during the last year was focused on understanding the effects of Planck scale physics on the inflationary density perturbation spectrum and the CMB. Being a closely related issue, I have also been analyzing the effects of very high energy modes on entropy bounds in the early universe.

In addition, I have also been working on understanding the thermodynamical properties of black hole and cosmological horizons in brane-world scenarios.

Planck scale corrections to the primordial spectrum : Over the last few years, considerable amount of attention has been devoted to evaluating the Planckian corrections to the inflationary perturbation spectrum, often using high energy models that violate Lorentz invariance locally. However, certain astrophysical observations seem to indicate that Lorentz invariance may be preserved to extremely high energies. In such a situation, to study the Planck scale effects, it becomes important to consider models that preserve Lorentz invariance even as they contain a fundamental scale. In this work, we construct one such model and evaluate the resulting spectrum of density perturbations in the power-law inflationary scenario. While our model reproduces the standard spectrum on small scales, it naturally predicts a suppression of power on large scales--a feature that seems to be necessary to explain lower power in the quadrupole and the octopole moments of the CMB. We find that the spectrum we obtain is very similar in form to the one that has recently been obtained from non-commutative inflation. Interestingly, with a suitable choice of initial conditions, our approach can lead to corrections at the infra-red as well as at the ultra-violet ends of the spectrum.

High energy modes and entropy bounds : Very high energy effects may, in addition to modifying the energy associated with each mode of a quantum field (such as in models involving high frequency dispersion), alter the density of states of these modes. These effects, in turn, will modify the entropy associated with the matter fields. We are currently investigating the generic effects of the high energy modes on different entropy bounds, such as, for e.g., the holographic and the Hubble bounds, in the early universe.

Entropy of horizons in brane-world scenarios : Recently, exact solutions that describe black holes as well as cosmological models that are bound to a brane in a higher dimensional bulk have been constructed. These brane-world models provide interesting situations to examine the thermodynamical properties of horizons. We are presently involved in evaluating the entropy of black hole and cosmological horizons in brane-world scenarios through the brick wall approach.


Publications:

  • L. Sriramkumar, Interpolating between the Bose-Einstein and the Fermi-Dirac distributions in odd dimensions, Gen. Rel. Grav. 35, 1699--1705 (2003).


Preprints:

  • S. Shankaranarayanan and L. Sriramkumar, Trans-Planckian corrections to the primordial spectrum in the infra-red and the ultra-violet, hep-th/0403236.


Conference/Workshops Attended:

  • Advanced School on the Physics of Galaxy Formation, Harish-Chandra Research Institute, Allahabad, December 16--26, 2003.

  • International Conference on Gravitation and Cosmology, Cochin University for Science and Technology, Kochi, January 5--10, 2004.


Visits to other Institutes:

  • Inter-University Centre for Astronomy and Astrophysics, Pune, July 20--28, 2003.

  • High Energy Section, The Abdus Salam International Centre for Theoretical Physics, Trieste, Italy, February 26--March~25, 2004.


Invited Lectures/Seminars:

  • On trans-Planckian signatures in the inflationary density perturbation spectrum, Invited presentation in the Cosmology Session at the International Conference on Gravitation and Cosmology, Cochin University for Science and Technology, Kochi, January 5--10, 2004.

  • Guest Lecturer for Prof. T. Padmanabhan's course on Introduction to Standard Model of Cosmology at the SERC Main School in Theoretical High Energy Physics, Department of Physics, University of Rajasthan, Jaipur, February 9--17, 2004.


Other Activities:

  • Was involved in conducting (the Physics part of) the HRI Science Talent Test 2003.

  • Taught a course on Classical Mechanics to first year physics graduate students at HRI during August--December, 2003.

  • Guided the following M.Sc. Physics students on projects under the Visiting Students' Program:

    • Prabhanshu Shekhar, Indian Institute of Technology, Kharagpur,
    • Anwesha Tapadar, Indian Institute of Technology, Kanpur and
    • P. C. Bharadwaj, Indian Institute of Technology, Chennai.

 



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