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Resume of Tirthankar Roy Choudhury

Resume of Tirthankar Roy Choudhury September 15, 2013 Educational Qualifications Degree B.Sc.

University Visva Bharati, Santiniketan, India

Year 1996

M.Sc.

Visva Bharati, Santiniketan, India University of Pune, Pune, India

1998

Ph.D

2005 submitted in 2003

Subjects Physics (Hons), Mathematics, Chemistry Physics

Grades 81.1%

82.0%

Thesis Title: Physics of Structure Formation in the Universe; Supervisor: T. Padmanabhan

Research Experience Position Research Fellow Post-doctoral Fellow Visiting Scientist/ Project Consultant Post-doctoral Fellow Reader ‘F’ Reader ‘F’

Place Inter-University Centre for Astronomy and Astrophysics, Pune, India SISSA/International School for Advanced Studies, Trieste, Italy Centre for Theoretical Studies, Indian Institute of Technology, Kharagpur, India Institute of Astronomy, University of Cambridge, Cambridge, UK Harish-Chandra Research Institute, Allahabad, India National Centre for Radio Astrophysics, TIFR, Pune, India

Period Aug 1998 Oct 2003 Nov 2003 Dec 2005 Jan 2006 Oct 2006 Nov 2006 Aug 2008 Sep 2008 Mar 2012 Mar 2012 present


Computational Skills and Experience • Operating Systems: Linux, Unix, Solaris, Windows • Programming Languages: Fortran, Mathematica, Shell, Perl, Octave • Parallel Computing: MPI, OpenMP • High Performance Computing Experience: Darwin High Performance Computing Service (Cambridge University, UK), COSMOS Supercomputer (Cambridge University, UK), High Performance Computing (Harish-Chandra Research Institute, India).

Teaching Experience • Astrophysics (∼ 35 lectures 1.5 hours each), HRI Graduate School, Allahabad, India, 2011. • Mathematical Physics (∼ 35 lectures 1.5 hours each), HRI Graduate School, Allahabad, India, 2009 and 2010. • Cosmology (full-semester reading course), HRI Graduate School, Allahabad, India, 2009. • Cosmological Large-scale Structure Formation (full-semester reading course), HRI Graduate School, Allahabad, India, 2009. • Perturbed Universe (6 lectures 1.5 hours each), Summer School on Gravitation & Cosmology, Allahabad, India, 2010. • Cosmological Reionization (4 lectures 1 hour each), IUCAA-NBU School on “Recent Advances in Cosmology”, Siliguri, India, 2011. • Cosmological Reionization (4 lectures 1.5 hour each), IUCAA Resource Centre, University of Delhi, India, 2012.


Supervision and Training of Researchers • Ph.D. Thesis: Sourav Mitra (HRI, Allahabad), 2010-present. • Master Thesis: Abhinav Agrawal (BITS, Pilani), 2011. • Bachelor Internship: Vaibhav Sharma (IIIT, Allahabad), 2011. • Ph.D. Thesis (informal association): Simona Gallerani (SISSA, Trieste, Supervisor: Andrea Ferrara), 2003-2005. • Ph.D. Thesis (informal association): Kanan Datta (IIT, Kharagpur, Supervisor: Somnath Bharadwaj), 2005-2008. • Ph.D. Thesis (informal association): Suman Majumdar (IIT, Kharagpur, Supervisor: Somnath Bharadwaj), 2007-present. • Ph.D. Thesis (informal association): Girish Kulkarni (HRI, Allahabad, Supervisor: Jasjeet Singh Bagla), 2009-present.

Academic Distinctions • Regular Associate of the Abdus Salam International Centre for Theoretical Physics, Trieste, Italy for the period 2010-2015. • Late Deblina Choudhari Award of Indian Physics Association for best oral presentation by a research scholar (1999) • CSIR-UGC (India) National Eligibility Test (1998): Qualified for Junior Research Fellowship and Lectureship • Graduate Aptitude Test in Engineering, Government of India (1998): 99.87 percentile (all-India rank 2nd) • Masters in Science (1998): Ranked 2nd in Physics in the University • Bachelor of Science (1996): Ranked 1st in Physics in the University • High school leaving examination (1993): Ranked 1st in the board • School leaving examination (1991): Ranked 2nd in the board


Agency Funded Research Projects • Fast Track Young Scientist Project funded by Department of Science and Technology, Government of India 2006 (could not take it up because I moved to Cambridge).

Professional and Organizational Experience • Co-organized an international meeting on “Cosmological Reionization” at Harish-Chandra Research Institute, Allahabad, India (February 2010). • Co-organized the “Summer School on Gravitation & Cosmology” at HarishChandra Research Institute, Allahabad, India (May 2010). • Co-organized the 26th Meeting of the Indian Association for General Relativity and Gravitation (IAGRG) “Sangam: Confluence of Gravitation and Cosmology” at Harish-Chandra Research Institute, Allahabad, India (January 2011). • Member of the International Astronomical Union (IAU). • Member of the Astronomical Society of India (ASI). • Member of the coordination committee for preparing the question paper of Joint Entrance Screening Test (JEST) 2011. • Member of the Computer Committee in Harish-Chandra Research Institute, Allahabad (2009–2012). • Member of the Graduate Studies & Admission (JEST) Committee in HarishChandra Research Institute, Allahabad (2009–2011). • Regularly give popular lectures for school children (2009–present). • Refereeing scientific articles in journals Monthly Notices of the Royal Astronomical Society, Astronomy & Astrophysics, Classical & Quantum Gravity


Publications (in reverse chronological order): Refereed Journals: 39. The effect of peculiar velocities on the epoch of reionization (EoR) 21-cm signal S. Majumdar, T. Roy Choudhury & S. Bharadwaj Mon. Not. R. Astron. Soc. 434 1978 (2013), arXiv:1209.4762 38. Constraining thawing dark energy using galaxy cluster number counts N. Chandrachani Devi, T. Roy Choudhury & A. A. Sen Mon. Not. R. Astron. Soc. 432 1513 (2013), arXiv:1112.0728 37. The escape fraction of ionizing photons from high-redshift galaxies from data-constrained reionization models S. Mitra, T. Roy Choudhury & A. Ferrara Mon. Not. R. Astron. Soc. 428 L1 (2013), arXiv:1207.3803 36. Constraining quasar and intergalactic medium properties through bubble detection in redshifted 21-cm maps S. Majumdar, S. Bharadwaj & T. Roy Choudhury Mon. Not. R. Astron. Soc. 426 3178 (2012), arXiv:1111.6354 35. Constraining large scale HI bias using redshifted 21-cm signal from the post-reionization epoch T. Guha Sarkar, S. Mitra, S. Majumdar & T. Roy Choudhury Mon. Not. R. Astron. Soc. 421 3570 (2012), arXiv:1109.5552 34. Joint quasar-cosmic microwave background constraints on reionization history S. Mitra, T. Roy Choudhury & A. Ferrara Mon. Not. R. Astron. Soc. 419 1480 (2012), arXiv:1106.4034 33. Data-constrained reionization and its effects on cosmological parameters S. Pandolfi, A. Ferrara, T. Roy Choudhury, A. Melchiorri & S. Mitra Phys. Rev. D 84 123522 (2011), arXiv:1111.3570 32. Reply to “Comment on ‘Quasi normal modes in Schwarzschild-DeSitter spacetime: A simple derivation of the level spacing of the frequencies”’ T. Roy Choudhury & T. Padmanabhan Phys. Rev. D 83 108502 (2011), arXiv:1105.6192 31. Reionization constraints using Principal Component Analysis S. Mitra, T. Roy Choudhury & A. Ferrara Mon. Not. R. Astron. Soc. 413 1569 (2011), arXiv:1011.2213


30. The impact of anisotropy from finite light travel time on detecting ionized bubbles in redshifted 21-cm maps S. Majumdar, S. Bharadwaj, K. Datta & T. Roy Choudhury Mon. Not. R. Astron. Soc. 413 1409 (2011), arXiv:1006.0430 29. Reionization and feedback in overdense regions at high redshift G. Kulkarni & T. Roy Choudhury Mon. Not. R. Astron. Soc. 412 2781 (2011), arXiv:1008.2509 28. Cross-correlation of the Lyman-α Forest and HI 21 cm: A Probe of Cosmology T. Guha Sarkar, S. Bharadwaj, T. Roy Choudhury & K. Datta Mon. Not. R. Astron. Soc. 410 1130 (2011), arXiv:1002.1368 27. Probing intergalactic radiation fields during cosmic reionization through gamma-ray absorption S. Inoue, R. Salvaterra, T. Roy Choudhury, A. Ferrara, B. Ciardi & R. Schneider Mon. Not. R. Astron. Soc. 404 1938 (2010), arXiv:0906.2495 26. The optimal redshift for detecting ionized bubbles in HI 21-cm maps Kanan K. Datta, Somnath Bharadwaj & T. Roy Choudhury Mon. Not. R. Astron. Soc. 399 L132 (2009), arXiv:0906.0360 25. Inside-out or Outside-in: The topology of reionization in the photonstarved regime suggested by Lyman-alpha forest data T. Roy Choudhury, M. G. Haehnelt & J. Regan Mon. Not. R. Astron. Soc. 394 960 (2009), arXiv:0806.1524 24. Simulating the impact of HI fluctuations on matched filter search for ionized bubbles in redshifted 21 cm maps Kanan K. Datta, Suman Majumdar, Somnath Bharadwaj & T. Roy Choudhury Mon. Not. R. Astron. Soc. 391 1900 (2008), arXiv:0805.1734 23. Testing Reionization with Gamma Ray Burst Absorption Spectra S. Gallerani, R. Salvaterra, A. Ferrara & T. Roy Choudhury Mon. Not. R. Astron. Soc. 388 L84 (2008) arXiv:0710.1303 22. Glimpsing through the high redshift neutral hydrogen fog S. Gallerani, A. Ferrara, X. Fan & T. Roy Choudhury Mon. Not. R. Astron. Soc. 386 359 (2008), arXiv:0706.1053 21. On the minimum mass of reionization sources T. Roy Choudhury, A. Ferrara & S. Gallerani Mon. Not. R. Astron. Soc. 385 L58 (2008), arXiv:0712.0738


20. Cosmic microwave background polarization constraints on radiative feedback C. Burigana, L. A. Popa, R. Salvaterra, R. Schneider, T. Roy Choudhury & A. Ferrara Mon. Not. R. Astron. Soc. 385 404 (2008), arXiv:0712.1913 19. Detectable signatures of cosmic radiative feedback R. Schneider, R. Salvaterra, T. Roy Choudhury, A. Ferrara, C. Burigana & L. A. Popa Mon. Not. R. Astron. Soc. 384 1525 (2008), arXiv:0712.0538 18. Concept of temperature in multi-horizon spacetimes: Analysis of SchwarzschildDe Sitter metric T. Roy Choudhury & T. Padmanabhan Gen. Rel. Grav. 39 1789 (2007), gr-qc/0404091 17. Detecting ionized bubbles in redshifted 21 cm maps Kanan K. Datta, Somnath Bharadwaj & T. Roy Choudhury Mon. Not. R. Astron. Soc. 382 809 (2007), astro-ph/0703677 16. Searching for the reionization sources T. Roy Choudhury & A. Ferrara Mon. Not. R. Astron. Soc. 380 L6 (2007), astro-ph/0703771 15. The multi-frequency angular power spectrum of the epoch of reionization 21 cm signal Kanan K. Datta, T. Roy Choudhury & Somnath Bharadwaj Mon. Not. R. Astron. Soc. 378 119 (2007), astro-ph/0605546 14. On the Size of HII Regions around High Redshift Quasars A. Maselli, S. Gallerani, A. Ferrara & T. Roy Choudhury Mon. Not. R. Astron. Soc. 376 L34 (2007), astro-ph/0608209 13. Updating reionization scenarios after recent data T. Roy Choudhury & A. Ferrara Mon. Not. R. Astron. Soc. 371 L55 (2006), astro-ph/0603617 12. Constraining the reionization history with QSO absorption spectra S. Gallerani, T. Roy Choudhury & A. Ferrara Mon. Not. R. Astron. Soc. 370 1401 (2006), astro-ph/0512129 11. A very extended reionization epoch? A. Melchiorri, T. Roy Choudhury, P. Serra & A. Ferrara Mon. Not. R. Astron. Soc. 364 873 (2005), astro-ph/0506486 10. Experimental Constraints on Self-consistent Reionization Models T. Roy Choudhury & A. Ferrara Mon. Not. R. Astron. Soc. 361 577 (2005), astro-ph/0411027


9. Cosmological parameters from supernova observations: A critical comparison of three data sets T. Roy Choudhury & T. Padmanabhan Astron. Astrophys. 429 807 (2005), astro-ph/0311622 • Determined as “The Fast Breaking Paper” for October, 2005 (Space Science) by Thomson-ISI Essential Science Indicators http://www.esi-topics.com/fbp/2005/october05-Choudhury− Padmanabhan.html

8. Quasi normal modes in Schwarzschild-DeSitter spacetime: A simple derivation of the level spacing of the frequencies T. Roy Choudhury & T. Padmanabhan Phys. Rev. D 69 064033 (2004), gr-qc/0311064 7. A theoretician’s analysis of the supernova data and the limitations in determining the nature of dark energy T. Padmanabhan & T. Roy Choudhury Mon. Not. R. Astron. Soc. 344 823 (2003), astro-ph/0212573 6. Can the clustered dark matter and the smooth dark energy arise from the same scalar field? T. Padmanabhan & T. Roy Choudhury Phys. Rev. D 66 081301 (2002), hep-th/0205055 5. Probing the dark ages with redshift distribution of gamma ray bursts T. Roy Choudhury & R. Srianand Mon. Not. R. Astron. Soc. 336 L27 (2002), astro-ph/0205446 4. A simple analytical model for the abundance of damped Lyα absorbers T. Roy Choudhury & T. Padmanabhan Astrophys. J. 574 59 (2002), astro-ph/0110359 3. Semianalytic approach to understanding the distribution of neutral hydrogen in the Universe: comparison of simulations with observations T. Roy Choudhury, R. Srianand & T. Padmanabhan Astrophys. J. 559 29 (2001), astro-ph/0012498 2. Semi-analytic approach to understanding the distribution of neutral hydrogen in the Universe T. Roy Choudhury, T. Padmanabhan & R. Srianand Mon. Not. R. Astron. Soc. 322 561 (2001), astro-ph/0005252 1. The issue of choosing nothing: what determines the low energy vacuum state of nature? T. Padmanabhan & T. Roy Choudhury Mod. Phys. Lett. A 15 1813 (2000), gr-qc/0006018


Review Articles: 4. Dark ages and cosmic reionization T. Roy Choudhury Vignettes in Gravitation and Cosmology. Edited by L.Sriramkumar & T.R.Seshadri. Published by World Scientific Publishing Co. Pte. Ltd. ISBN #9789814322072, pp. 15-49 (2009) 3. Analytical Models of the Intergalactic Medium and Reionization T. Roy Choudhury Invited Review, Current Science 97 841 (2009), arXiv:0904.4596 2. Physics of Cosmic Reionization T. Roy Choudhury & A. Ferrara Published in Cosmic Polarization, ed. R. Fabbri (Research Signpost; 2006), astro-ph/0603149 1. Physics of Structure Formation in the Universe T. Roy Choudhury Thesis presentation at the 22nd meeting of Astronomical Society of India (2003) Bull. Astr. Soc. Ind. 31 281 (2003), astro-ph/0305033 Citation Details: • According to SAO/NASA ADS, publications listed above have more than 1400 citations (including self-citations) as on 07 February, 2013. Corresponding h-index is 19. • There is one paper with citation above 250, two papers with citations between 100–249 and four papers with citations between 50-99.


Selected Talks • Probing the Universe through Neutral Hydrogen Distribution Theoretical Physics Colloquium Tata Institute of Fundamental Research, Mumbai, India, September 2011 • Invited Talk: Constraining Reionization and First Stars Indo-UK Meeting on Confronting particle-cosmology with Planck and LHC Inter-University Centre for Astronomy and Astrophysics, Pune, India, August 2011 • Physics of Cosmological Reionization Institute Colloquium National Centre for Radio Astrophysics, Pune, India, July 2011 • Invited Talk: Dark Ages & Reionization: “Final Frontier” of Observational Cosmology Emerging Trends in Gravitation and Cosmology Department of Physics, Jadavpur University, Kolkata, India, March 2011 • Invited Review: Cosmological Reionization 29th Scientific Meeting of the Astronomical Society of India (ASI) Pt. Ravishankar Shukla University, Raipur, India, February 2011 • Invited Talk: Constraints on reionization physics from CMBR and other observations Cosmology Rapid Response Meeting International Centre for Theoretical Sciences, Tata Institute of Fundamental Research, Mumbai, India, April 2010 • Invited Talk: Observational Constraints on Reionization History Cosmological Reionization Harish-Chandra Research Institute, Allahabad, India, February 2010 • Invited Review: Cosmological Reionization From Black Holes to the Universe: Gravity at Work, 25th Meeting of the Indian Association for General Relativity & Gravitation (IAGRG) Saha Institute of Nuclear Physics, Kolkata, India, January 2009 • Invited Talk: Probing the topology of reionization with 21 cm emission in the “photon-starved” scenario Cosmological evolution in diffuse baryons: Reionization epoch to the present day Orange County, Coorg, India, December 2008


• Reionization: Detectability of various aspects with current and future experiments Astrophysics Seminar Imperial College, London, UK, February 2008 • Detectability of various aspects related to reionization with present and near future experiments Astrophysics Seminar Harish-Chandra Research Institute, Allahabad, India, January 2008 • Observational constraints on dark ages and cosmic reionization Astrophysics Seminar SISSA/ISAS, Trieste, Italy, June 2007 • Invited Talk: Observational Constraints on Reionization History HI Survival through Cosmic Times Abbazia di Spineto, Sarteano, Italy, June 2007 • Observational constraints on dark ages and cosmic reionization Oxford Astrophysics Theoretical Astrophysics “Brown Bag” Seminars Oxford, UK, May 2007 • Invited Talk: Dark ages and cosmic reionization RAS National Astronomy Meeting 2007 Centre for Astrophysics, University of Central Lancashire, Preston, UK, April 2007 • Limitations in determining the nature of dark energy from supernova data Astrophysics Colloquium Academia Sinica, National Taiwan University, Taipei, February 2006 • Invited Talk: Reionization of the Universe Cosmology Winter School and Reionization Workshop Theoretical Institute for Advanced Research in Astrophysics, Taiwan, February 2006 • Invited Review: Reionization of the Universe: Before and After WMAP IX Workshop on High Energy Physics Phenomenology (WHEPP-9) Institute of Physics, Bhubaneswar, India, January 2006


Statement of Research: Past and Present My research interests lie in various aspects of theoretical astrophysics and cosmology, in particular, reionization, intergalactic medium, neutral hydrogen at high redshifts and dark energy. A brief summary of each of these components is given below:

1

Reionization

In the framework of the hot big bang model, the baryonic matter in the Universe is expected to become almost neutral after the recombination epoch at z ∼ 1100. Given the fact (known from observations of quasar absorption spectra) that the Universe is highly ionized at z < 6, it is crucial to understand as to when and how did the Universe reionize. Recently, the interest in cosmic reionization has received a big boost, thanks both to the availability of QSOs located at z & 6 which allow to probe the physical state of the neutral part of the intergalactic medium (IGM) via absorption line experiments, and to the determination of the Thomson electron scattering optical depth from the WMAP data. These observations suggest that the reionization occurred somewhere between z ∼ 6 − 15, and that the nature of this process is quite complex. Furthermore, it is also quite clear that the reionization process is very closely coupled to the formation of galaxies and other structures (Choudhury 2009).

1.1 Semi-analytical Models of Reionization I have been involved in developing detailed semi-analytical models of reionization and compare the predictions with observational data. In fact, given such a wide variety in the data, it is challenging to construct theoretical models which are able to match all the data sets. Clearly such an exercise is beyond the scope of current numerical simulations as one has to deal with a wide range of spatial and temporal scales. Hence, the best way of approaching this problem is through semi-analytical calculations, with the hope that the reionization models will be more and more constrained as more data sets are available. Hence I have been studying self-consistent models that can, at the same time, satisfy a larger number of experimental requirements concerning the quasar absorption spectra, WMAP observations, source counts at z ≈ 10, IGM temperature evolution, helium reionization, the number of Lyman-limit systems and the cosmic star formation history. Although this might seem, at first, a very ambitious aim, it turns out that once the relevant physics is included, these results allow a very clear scenario to emerge. The model we have developed (Choudhury & Ferrara 2005; Choudhury & Ferrara 2006) implements most of the relevant physics governing reionization, such as the inhomogeneous IGM density distribution, three different classes of ionizing


photon sources (massive PopIII stars, PopII stars and QSOs), chemical feedback for PopIII → PopII transition and radiative feedback suppressing star formation in low-mass galaxies. The best-fit model is found to be consistent with available data on redshift evolution of Lyman-limit absorption systems, Lyα and Lyβ transmitted flux, electron scattering optical depth, source counts at z ≈ 10, temperature of the IGM and cosmic star formation history. According to our model, hydrogen reionization starts around z = 15, driven by the metal-free stars (with normal Salpeter-like IMF),and is 90% complete by z ≈ 10. The photoionizing power of PopIII stars fades for z . 10 because of the concomitant action of radiative and chemical feedbacks, which causes the reionization process to stretch considerably and to end only by z ≈ 6.

1.2 Principal Component Analysis of Reionization Using the model described above, we study the observational constraints on reionization via a principal component analysis (PCA). The advantage of this approach is that it provides constraints on reionization in a model-independent manner. Assuming that reionization at z > 6 is primarily driven by stellar sources, we decompose the unknown function Nion (z), representing the number of photons in the IGM per baryon in collapsed objects, into its principal components and constrain the latter using the photoionization rate obtained from Ly-alpha forest GunnPeterson optical depth, the WMAP7 electron scattering optical depth and the redshift distribution of Lyman-limit systems at z ∼ 3.5 (Mitra, Choudhury, & Ferrara 2011). The main findings of our analysis are: (i) It is sufficient to model Nion (z) over the redshift range 2 < z < 14 using 5 parameters to extract the maximum information contained within the data. (ii) All quantities related to reionization can be severely constrained for z < 6 because of a large number of data points whereas constraints at z > 6 are relatively loose. (iii) The weak constraints on Nion (z) at z > 6 do not allow to disentangle different feedback models with present data. There is a clear indication that Nion (z) must increase at z > 6, thus ruling out reionization by a single stellar population with non-evolving IMF, and/or star-forming efficiency, and/or photon escape fraction. The data allows for non-monotonic Nion (z) which may contain sharp features around z ∼ 7. (iv) The PCA implies that reionization must be 99% completed between 5.8 < z < 10.3 (95% confidence level) and is expected to be 50% complete at z ≈ 9.5 − 12. With future data sets, like those obtained by Planck, the z > 6 constraints will be significantly improved.

1.3 Other Applications of the Semi-analytical Model Our semi-analytical model described above is found to be extremely useful for studying various other aspects related to reionization. We describe some of the applications of the model below: (i) The Lyα absorption spectra of QSOs at z ≈ 6 show wide regions of zero transmission which have been interpreted as signatures of large neutral hydrogen


fractions. We have used our semi-analytical models to simulate realizations of absorption spectra at z > 6 with the aim of constraining the cosmic reionization history. We found that the reionization history can be well-constrained by the distribution of dark gap widths in the absorption spectra at z > 6 (Gallerani, Choudhury, & Ferrara 2006). By comparing the statistics of these spectral features with our model, we conclude that the neutral hydrogen fraction xHI evolves smoothly from 10−4.4 at z = 5.3 to 10−4.2 at z = 5.6, with a robust upper limit xHI < 0.36 at z = 6.3 (Gallerani et al. 2008a). In addition, we apply our model to the spectra of high redshift GRBs and show that the observation of GRB 050904 strongly favours reionization models which predict a highly ionized intergalactic medium at z ∼ 6, with an estimated xHI = (6.4 ± 0.3) × 10−5 (Gallerani et al. 2008b). (ii) We have investigated in detail the nature and properties of reionization sources using our model (Choudhury & Ferrara 2007). We find that more than 80% of the ionizing power at z ≥ 7 comes from haloes of mass M < 109 M predominantly harbouring metal-free (PopIII) stars; a turnover to a PopII dominated phase occurs shortly after, with this population, residing in M > 109 M haloes, yielding 60% of ionizing power at z = 6. We conclude that z > 7 sources tentatively identified in broad-band surveys are relatively massive (M ∼ 109 M ) and rare objects which are only marginally (∼ 1%) adding to the reionization photon budget. Sources which contribute most significantly to reionization would be potentially detected in future telescopes like the JWST. (iii) We have used our model to estimate the minimum mass of star-forming haloes required to match the current data (Choudhury, Ferrara, & Gallerani 2008). Models which do not include haloes of total mass M < 109 M fail at reproducing the Gunn-Peterson and electron scattering optical depths simultaneously, as they contribute too few (many) photons at high (low, z ∼ 6) redshift. Marginally acceptable solutions require haloes with M ∼ 5 × 107 M at z ∼ 10, corresponding to virial temperatures (∼ 104 K) for which cooling can be ensured by atomic transitions. However, a much better match to the data is obtained if minihaloes (M ∼ 106 M ) are included in the analysis. We have critically examined the assumptions made in our model and conclude that reionization in the large-galaxiesonly scenario can remain viable only if metal-free stars and/or some other exotic sources at z > 6 are included. (iv) We have applied our model to constrain the effects of feedback using 21cm signal at high redshifts (Schneider et al. 2008) and CMBR polarization data (Burigana et al. 2008). Also, our model predictions for the intergalactic radiation fields at very high redshifts can be constrained through the absorption of high-energy gamma-rays, which will leave signatures in spectra of blazars or GRBs (Inoue et al. 2010).

2

Intergalactic Medium

At redshifts 1 < z < 5, a fair fraction of the baryons are found in the form of a ionized intergalactic medium (IGM), which is usually probed through the absorption


spectra of distant QSOs. The IGM is very important in understanding the baryonic structure formation in the universe.

2.1 Quasar Absorption Lines and the Lyman-α Forest: There have been various attempts towards modelling the IGM – both analytically and numerically. We have developed a semi-analytical model for the quasi-linear regime of the IGM, where phenomena like shock heating or star formation are not very effective. These systems manifest themeselves as a series of narrow absorption lines usually known as the “Lyα forest” in the QSO spectra. For such regions, one can tackle the problem of non-linear density perturbations by assuming that they are related to the linear perturbations through some simple function. One of the first attempts in this direction has been to use the lognormal approximation to relate the linear and non-linear density fields. It turns out that this approach can be used effectively to constrain various parameters related to the IGM by comparing with observations. We have studied this model both analytically (Choudhury, Padmanabhan, & Srianand 2001) and numerically (Choudhury, Srianand, & Padmanabhan 2001). Given a cosmological model, we have obtained constraints on (i) the mean temperature, (ii) the power law index of the density-temperature relation and (iii) a combination of the mean baryonic density of the universe and the local ionizing radiation field. The best-fit model is able to match various statistical quantities related to the Lyα forest data at z ∼ 2.5.

3

Neutral Hydrogen at High Redshifts

Perhaps the most promising prospect of detecting the fluctuations in the neutral hydrogen (HI) density during the (pre-)reionization era is through the 21 cm emission experiments like GMRT, MWA and LOFAR. I have worked on different aspects of theoretical modelling the HI distribution, both analytically and numerically.

3.1 The Topology of Reionization We use a set of semi-numerical simulations based on the Zel’dovich approximation, the friends-of-friends algorithm and excursion set formalism to generate reionization maps of high dynamic range with a range of assumptions regarding the distribution and luminosity of ionizing sources and the spatial distribution of sinks for the ionizing radiation (Choudhury, Haehnelt, & Regan 2009). We find that ignoring the inhomogeneous spatial distribution of regions of high gas density where recombinations are important - as is often done in studies of this kind can lead to misleading conclusions regarding the topology of reionization, especially if reionization occurs in the photon-starved regime suggested by Lyα forest data. The inhomogeneous spatial distribution of recombinations significantly reduces the mean-free path of ionizing photons and the typical size of coherently ionized regions. Reionization proceeds then much more as an outside-in process.


Low-density regions far from ionizing sources become ionized before regions of high gas density not hosting sources of ionizing radiation. The spatial distribution of sinks of ionization radiation also significantly affects the shape and amplitude of the power spectrum of fluctuations of 21cm emission. The slope of the 21cm power spectrum as measured by upcoming 21cm experiments should be able to distinguish to what extent the topology of reionization proceeds outside-in or inside-out, while the evolution of the amplitude of the power spectrum with increasing ionized mass fraction should be sensitive to the spatial distribution and the luminosity of ionizing sources.

3.2 The Multi-frequency Angular Power Spectrum of the Epoch of Reionization 21 cm Signal In the standard scenarios, where reionization is completed by the UV sources, the neutral hydrogen distribution before reionization is expected to be inhomogeneous and quite patchy (depending on the clustering of ionizing sources and the topology of the ionized regions). These inhomogeneities in HI distribution can be well detected in the angular power spectrum of the 21 cm signal. We have calculated this quantity using a model where the effects of patchy reionization are incorporated through two parameters which are related to the ionized bubble size and the clustering of the bubble centers relative to the dark matter (bias) respectively. We find that the signal peaks at the angular scales of the individual bubbles where it is Poisson fluctuations dominated, and the signal is considerably enhanced for large bubble size (Datta, Choudhury, & Bharadwaj 2007).

3.3 Detection of Ionized Bubbles The reionization of the Universe, it is believed, occurred by the growth of ionized regions (bubbles) in the neutral intergalactic medium. We study the possibility of detecting these bubbles in radio-interferometric observations of redshifted neutral hydrogen (HI) 21-cm radiation. The signal (< 1mJy) will be buried in noise and foregrounds, the latter being at least a few orders of magnitude stronger than the signal. We develop a visibility based formalism that uses a filter to optimally combine the entire signal from a bubble while minimizing the noise and foreground contributions. We find that there is a fundamental limitation on the smallest bubble that can be detected arising from the statistical fluctuations in the HI distribution (Datta, Bharadwaj, & Choudhury 2007; Datta et al. 2008). We estimate that the redshift range 8.1 ± 1.1 and 9.8 ± 1 are optimum for detecting ionized bubbles with the telescopes GMRT and MWA, respectively (Datta, Bharadwaj, & Choudhury 2009). More recently, we have also studied he impact of anisotropy from finite light travel time on detecting ionized bubbles in the 21-cm maps (Majumdar et al. 2011).


3.4 Cross-correlation of the Lyman-α Forest and 21 cm Signal Separating the cosmological redshifted 21-cm signal from foregrounds is a major challenge. We have proposed the cross-correlation of the redshifted 21-cm emission from neutral hydrogen (HI) in the post-reionization era with the Lyα forest as a new probe of the large-scale matter distribution in the redshift range z = 2−3 without the problem of foreground contamination (Guha Sarkar et al. 2011). Though the 21-cm and the Lyα forest signals originate from different astrophysical systems, they are both expected to trace the underlying dark matter distribution on large scales. The multi-frequency angular cross-correlation power spectrum estimator is found to be unaffected by the discrete quasar sampling, which only affects the noise in the estimate. Assuming parameters for upcoming observations and surveys, we find that a 5σ detection of the cross-correlation signal is possible at angular multipoles 600 ≤ l ≤ 2000. The cross-correlation signal will be a new, independent probe of the astrophysics of the diffuse IGM, the growth of structure and the expansion history of the Universe.

4

Dark Energy

I have also been involved in understanding the nature of dark energy using various cosmological observations which seems to have caught the attention of the community.

4.1 Evidence of Dark Energy from Supernova Observations: I have been involved in careful analysis of the cosmological supernova Type Ia data to verify the evidence of accelerating Universe and dark energy. Our analysis concentrates mainly on some key theoretical issues. We have developed a modelindependent (kinetic) analysis to show that current data sets strongly rule out nonaccelerating models, thus supporting the presence of dark energy (Padmanabhan & Choudhury 2003; Choudhury & Padmanabhan 2005).

4.2 A model for dark matter and dark energy: In general, analyses suggest the existence of two different kinds of energy densities dominating at small (. 500 Mpc) and large (& 1000 Mpc) scales. The dark matter component, which dominates at small scales, contributes Ωm ≈ 0.35 and has an equation of state p = 0 while the dark energy component, which dominates at large scales, contributes ΩΛ ≈ 0.65 and has an equation of state p ' −ρ. It is usual to postulate weakly interacting massive particles (WIMPs) for the first component and some form of scalar field or cosmological constant for the second component. p We explore the possibility of a scalar field with a Lagrangian L = −V (φ) 1 − ∂ i φ∂i φ acting as both clustered dark matter and smoother dark energy and having a scale dependent equation of state. This model predicts a


relation between the ratio r = ρΛ /ρDM of the energy densities of the two dark components and expansion rate n of the universe (with a(t) ∝ tn ) in the form n = (2/3)(1 + r). For r ≈ 2, we get n ≈ 2 which is consistent with observations (Padmanabhan & Choudhury 2002).

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