Imperial College London

Dr Jonathan R. Pritchard

Faculty of Natural SciencesDepartment of Physics

Senior Lecturer
 
 
 
//

Contact

 

+44 (0)20 7594 7557j.pritchard Website CV

 
 
//

Location

 

1018CBlackett LaboratorySouth Kensington Campus

//

Summary

 

Publications

Publication Type
Year
to

45 results found

Majumdar S, Pritchard JR, Mondal R, Watkinson CA, Bharadwaj S, Mellema Get al., Quantifying the non-Gaussianity in the EoR 21-cm signal through bispectrum

The epoch of reionization (EoR) 21-cm signal is expected to be highlynon-Gaussian in nature and this non-Gaussianity is also expected to evolve withthe progressing state of reionization. Therefore the signal will be correlatedbetween different Fourier modes ($k$). The power spectrum will not be ablecapture this correlation in the signal. We use a higher-order estimator -- thebispectrum -- to quantify this evolving non-Gaussianity. We study thebispectrum using an ensemble of simulated 21-cm signal and with a large varietyof $k$ triangles. We observe two competing sources driving the non-Gaussianityin the signal: fluctuations in the neutral fraction ($x_{HI}$) field andfluctuations in the matter density field. We find that the non-Gaussiancontribution from these two sources vary, depending on the stage ofreionization and on which $k$ modes are being studied. We show that the sign ofthe bispectrum works as a unique marker to identify which among these twocomponents is driving the non-Gaussianity. We propose that the sign change inthe bispectrum, when plotted as a function of triangle configuration$\cos{\theta}$ and at a certain stage of the EoR can be used as a confirmativetest for the detection of the 21-cm signal. We also propose that the trajectoryof the signal, in a $P(k)-B(k, k, k)$ space, with evolving $x_{HI}$ can be usedas a joint estimator of the signal for the future radio interferometricobservations with the SKA, to put robust constrains on the reionizationparameters.

JOURNAL ARTICLE

Schmit CJ, Pritchard JR, Emulation of reionization simulations for Bayesian inference of astrophysics parameters using neural networks

Next generation radio experiments such as LOFAR, HERA and SKA are expected toprobe the Epoch of Reionization and claim a first direct detection of thecosmic 21cm signal within the next decade. Data volumes will be enormous andcan thus potentially revolutionize our understanding of the early Universe andgalaxy formation. However, numerical modelling of the Epoch of Reionization canbe prohibitively expensive for Bayesian parameter inference and how tooptimally extract information from incoming data is currently unclear.Emulation techniques for fast model evaluations have recently been proposed asa way to bypass costly simulations. We consider the use of artificial neuralnetworks as a blind emulation technique. We study the impact of trainingduration and training set size on the quality of the network prediction and theresulting best fit values of a parameter search. A direct comparison is drawnbetween our emulation technique and an equivalent analysis using 21CMMC. Wefind good predictive capabilities of our network using training sets of as lowas 100 model evaluations, which is within the capabilities of fully numericalradiative transfer codes.

JOURNAL ARTICLE

Burns JO, Bradley R, Tauscher K, Furlanetto S, Mirocha J, Monsalve R, Rapetti D, Purcell W, Newell D, Draper D, MacDowall R, Bowman J, Nhan B, Wollack EJ, Fialkov A, Jones D, Kasper JC, Loeb A, Datta A, Pritchard J, Switzer E, Bicay Met al., 2017, A Space-based Observational Strategy for Characterizing the First Stars and Galaxies Using the Redshifted 21cm Global Spectrum, ASTROPHYSICAL JOURNAL, Vol: 844, ISSN: 0004-637X

JOURNAL ARTICLE

Watkinson CA, Majumdar S, Pritchard JR, Mondal Ret al., 2017, A fast estimator for the bispectrum and beyond – a practical method for measuring non-Gaussianity in 21-cm maps, Monthly Notices of the Royal Astronomical Society, Vol: 472, Pages: 2436-2446, ISSN: 0035-8711

JOURNAL ARTICLE

Harker GJA, Mirocha J, Burns JO, Pritchard JRet al., 2016, Parametrizations of the 21-cm global signal and parameter estimation from single-dipole experiments, MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Vol: 455, Pages: 3829-3840, ISSN: 0035-8711

JOURNAL ARTICLE

Liu A, Pritchard JR, Allison R, Parsons AR, Seljak U, Sherwin BDet al., 2016, Eliminating the optical depth nuisance from the CMB with 21 cm cosmology, PHYSICAL REVIEW D, Vol: 93, ISSN: 2470-0010

JOURNAL ARTICLE

Pritchard J, 2015, Reionization and the SKA, ASTRONOMY & GEOPHYSICS, Vol: 56, ISSN: 1366-8781

JOURNAL ARTICLE

Watkinson CA, Mesinger A, Pritchard JR, Sobacchi Eet al., 2015, 21-cm signatures of residual H I inside cosmic H II regions during reionization, MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Vol: 449, Pages: 3202-3211, ISSN: 0035-8711

JOURNAL ARTICLE

Watkinson CA, Pritchard JR, 2015, The impact of spin-temperature fluctuations on the 21-cm moments, MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Vol: 454, Pages: 1416-1431, ISSN: 0035-8711

JOURNAL ARTICLE

Glover SCO, Chluba J, Furlanetto SR, Pritchard JR, Savin DWet al., 2014, Atomic, Molecular, and Optical Physics in the Early Universe: From Recombination to Reionization, Editors: Arimondo, Berman, Lin, Publisher: ELSEVIER ACADEMIC PRESS INC, Pages: 135-270, ISBN: 978-0-12-800129-5

BOOK CHAPTER

Koopmans LVE, Pritchard J, Mellema G, Abdalla F, Aguirre J, Ahn K, Barkana R, Van Bemmel I, Bernardi G, Bonaldi A, Briggs F, De Bruyn AG, Chang TC, Chapman E, Chen X, Ciardi B, Datta KK, Dayal P, Ferrara A, Fialkov A, Fiore F, Ichiki K, Illiev IT, Inoue S, Jelić V, Jones M, Lazio J, Maio U, Majumdar S, Mack KJ, Mesinger A, Morales MF, Parsons A, Pen UL, Santos M, Schneider R, Semelin B, De Souza RS, Subrahmanyan R, Takeuchi T, Trott C, Vedantham H, Wagg J, Webster R, Wyithe Set al., 2014, The cosmic dawn and epoch of reionization with the square kilometre array

© Copyright owned by the author(s) under the terms of the Creative Commons Attribution-NonCommercial-ShareAlike Licence. Concerted effort is currently ongoing to open up the Epoch of Reionization (z ∼15-6) for studies with IR and radio telescopes. Whereas IR detections have been made of sources (Lyman-a emitters, quasars and drop-outs) in this redshift regime in relatively small fields of view, no direct detection of neutral hydrogen, via the redshifted 21-cm line, has yet been established. Such a direct detection is expected in the coming years, with ongoing surveys, and could open up the entire universe from z ∼6-200 for astrophysical and cosmological studies, opening not only the Epoch of Reionization, but also its preceding Cosmic Dawn (z ∼30-15) and possibly even the later phases of the Dark Ages (z ∼200-30). All currently ongoing experiments attempt statistical detections of the 21-cm signal during the Epoch of Reionization, with limited signal-to-noise. Direct imaging, except maybe on the largest (degree) scales at lower redshifts, as well as higher redshifts will remain out of reach. The Square Kilometre Array (SKA) will revolutionize the field, allowing direct imaging of neutral hydrogen from scales of arc-minutes to degrees over most of the redshift range z ∼6-28 with SKA1-LOW, and possibly even higher redshifts with the SKA2-LOW. In this SKA will be unique, and in parallel provide enormous potential of synergy with other upcoming facilities (e.g. JWST). In this chapter we summarize the physics of 21-cm emission, the different phases the universe is thought to go through, and the observables that the SKA can probe, referring where needed to detailed chapters in this volume. This is done within the framework of the current SKA1 baseline design and a nominal CD/EoR straw-man survey, consisting of a shallow, medium-deep and deep survey, the latter probing down to ∼1mK brightness temperature on arc-minute scales at the end of reion

CONFERENCE PAPER

Pritchard J, Masters K, Allen J, Contenta F, Huckvale L, Wilkins S, Zocchi Aet al., 2014, Asking gender questions, Astronomy and Geophysics, Vol: 55, Pages: 6.8-6.12, ISSN: 1366-8781

JOURNAL ARTICLE

Pritchard J, Masters K, Allen J, Contenta F, Huckvale L, Wilkins S, Zocchi Aet al., 2014, Asking gender questions, ASTRONOMY & GEOPHYSICS, Vol: 55, Pages: 8-12, ISSN: 1366-8781

JOURNAL ARTICLE

Watkinson CA, Pritchard JR, 2014, Distinguishing models of reionization using future radio observations of 21-cm 1-point statistics, MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Vol: 443, Pages: 3090-3106, ISSN: 0035-8711

JOURNAL ARTICLE

Liu A, Pritchard JR, Tegmark M, Loeb Aet al., 2013, Global 21 cm signal experiments: A designer's guide, PHYSICAL REVIEW D, Vol: 87, ISSN: 1550-7998

JOURNAL ARTICLE

Mellema G, Koopmans LVE, Abdalla FA, Bernardi G, Ciardi B, Daiboo S, de Bruyn AG, Datta KK, Falcke H, Ferrara A, Iliev IT, Iocco F, Jelic V, Jensen H, Joseph R, Labroupoulos P, Meiksin A, Mesinger A, Offringa AR, Pandey VN, Pritchard JR, Santos MG, Schwarz DJ, Semelin B, Vedantham H, Yatawatta S, Zaroubi Set al., 2013, Reionization and the Cosmic Dawn with the Square Kilometre Array, EXPERIMENTAL ASTRONOMY, Vol: 36, Pages: 235-318, ISSN: 0922-6435

JOURNAL ARTICLE

Pritchard JR, Loeb A, 2013, Evolution of the 21cm signal throughout cosmic history, Physical Review D: Particles, Fields, Gravitation and Cosmology, Vol: 78, ISSN: 1089-4918

The potential use of the redshifted 21 cm line from neutral hydrogen for probing the epoch of reionization is motivating the construction of several low-frequency interferometers. There is also much interest in the possibility of constraining the initial conditions from inflation and the nature of the dark matter and dark energy by probing the power spectrum of density perturbations in three dimensions and on smaller scales than probed by the microwave background anisotropies. Theoretical understanding of the 21 cm signal has been fragmented into different regimes of physical interest. In this paper, we make the first attempt to describe the full redshift evolution of the 21 cm signal between 0<z<300. We include contributions to the 21 cm signal from fluctuations in the gas density, temperature, and neutral fraction, as well as the Lyα flux, and allow for a post-reionization signal from damped Lyα systems. Our comprehensive analysis provides a useful foundation for optimizing the design of future arrays whose goal is to separate the particle physics from the astrophysics, either by probing the peculiar velocity distortion of the 21 cm power spectrum, or by extending the 21 cm horizon to z≳25 before the first galaxies had formed, or to z≲6 when the residual pockets of hydrogen trace large-scale structure.

JOURNAL ARTICLE

Burns JO, Lazio J, Bale S, Bowman J, Bradley R, Carilli C, Furlanetto S, Harker G, Loeb A, Pritchard Jet al., 2012, Probing the first stars and black holes in the early Universe with the Dark Ages Radio Explorer (DARE), ADVANCES IN SPACE RESEARCH, Vol: 49, Pages: 433-450, ISSN: 0273-1177

JOURNAL ARTICLE

Harker GJA, Pritchard JR, Burns JO, Bowman JDet al., 2012, An MCMC approach to extracting the global 21-cm signal during the cosmic dawn from sky-averaged radio observations, MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Vol: 419, Pages: 1070-1084, ISSN: 0035-8711

JOURNAL ARTICLE

Loeb A, Pritchard J, 2012, The missing reel, NEW SCIENTIST, Vol: 216, Pages: 30-33, ISSN: 0262-4079

JOURNAL ARTICLE

Pritchard JR, Loeb A, 2012, 21 cm cosmology in the 21st century, REPORTS ON PROGRESS IN PHYSICS, Vol: 75, ISSN: 0034-4885

JOURNAL ARTICLE

Silva MB, Santos MG, Pritchard JR, Cen R, Cooray Aet al., 2012, Probing the very first galaxies with the SKA, Pages: 101-116, ISSN: 1570-6591

We describe the use of the high redshift 21 cm signal to probe the very first galaxies to appear in the Universe. Using fast large volume simulations of the pre-Reionization epoch we have shown that the Lyman alpha radiation emitted from these young galaxies makes a strong contribution to the 21cm signal on large scales at z∼20. With the current setup the Square Kilometre Array (SKA) should be able to measure this signal, therefore making it probably the only telescope capable of giving us detailed information about the radiation emitted by the first stars and characterizing their host galaxies. SKA-pathfinders with ∼10%of the full collecting area should be capable of making a statistical detection of the 21cm power spectrum at redshifts z < 20.We then discuss the use of the redshift space distortions as a way to further constrain the Lyman alpha signal and demonstrate that they can be used as a model independent way to extract this signature with the SKA. © Springer-Verlag Berlin Heidelberg 2012.

CONFERENCE PAPER

Silva MB, Santos MG, Pritchard JR, Cen R, Cooray Aet al., 2012, Probing the Very First Galaxies with the SKA, Joint European and National Astronomy Meeting (JENAM)/18th Annual Meeting of the European-Astronomical-Society/20th Annual Portuguese Meeting of Astronomy and Astrophysics/Symposium 7 - Square Kilometre Array (SKA), Publisher: SPRINGER, Pages: 101-116, ISSN: 1570-6591

CONFERENCE PAPER

Abazajian KN, Calabrese E, Cooray A, De Bernardis F, Dodelson S, Friedland A, Fuller GM, Hannestad S, Keating BG, Linder EV, Lunardini C, Melchiorri A, Miquel R, Pierpaoli E, Pritchard J, Serra P, Takada M, Wong YYYet al., 2011, Cosmological and astrophysical neutrino mass measurements, ASTROPARTICLE PHYSICS, Vol: 35, Pages: 177-184, ISSN: 0927-6505

JOURNAL ARTICLE

Adshead P, Easther R, Pritchard J, Loeb Aet al., 2011, Inflation and the scale dependent spectral index: prospects and strategies, JOURNAL OF COSMOLOGY AND ASTROPARTICLE PHYSICS, ISSN: 1475-7516

JOURNAL ARTICLE

Lidz A, Furlanetto SR, Oh SP, Aguirre J, Chang T-C, Dore O, Pritchard JRet al., 2011, INTENSITY MAPPING WITH CARBON MONOXIDE EMISSION LINES AND THE REDSHIFTED 21 cm LINE, ASTROPHYSICAL JOURNAL, Vol: 741, ISSN: 0004-637X

JOURNAL ARTICLE

Mirabel IF, Dijkstra M, Laurent P, Loeb A, Pritchard JRet al., 2011, Stellar black holes at the dawn of the universe, ASTRONOMY & ASTROPHYSICS, Vol: 528, ISSN: 0004-6361

JOURNAL ARTICLE

Pritchard JR, Loeb A, 2011, 21-cm cosmology, Rep. Prog. Phys., Vol: 75

Imaging the Universe during the first hundreds of millions of years remainsone of the exciting challenges facing modern cosmology. Observations of theredshifted 21 cm line of atomic hydrogen offer the potential of opening a newwindow into this epoch. This would transform our understanding of the formationof the first stars and galaxies and of the thermal history of the Universe. Anew generation of radio telescopes is being constructed for this purpose withthe first results starting to trickle in. In this review, we detail the physicsthat governs the 21 cm signal and describe what might be learnt from upcomingobservations. We also generalize our discussion to intensity mapping of otheratomic and molecular lines.

JOURNAL ARTICLE

Santos MG, Silva MB, Pritchard JR, Cen R, Cooray Aet al., 2011, Probing the first galaxies with the Square Kilometer Array, ASTRONOMY & ASTROPHYSICS, Vol: 527, ISSN: 0004-6361

JOURNAL ARTICLE

Pritchard J, Loeb A, 2010, COSMOLOGY Hydrogen was not ionized abruptly, NATURE, Vol: 468, Pages: 772-773, ISSN: 0028-0836

JOURNAL ARTICLE

This data is extracted from the Web of Science and reproduced under a licence from Thomson Reuters. You may not copy or re-distribute this data in whole or in part without the written consent of the Science business of Thomson Reuters.

Request URL: http://wlsprd.imperial.ac.uk:80/respub/WEB-INF/jsp/search-html.jsp Request URI: /respub/WEB-INF/jsp/search-html.jsp Query String: respub-action=search.html&id=00713803&limit=30&person=true