Institute of Astronomy & Clare College University of Cambridge June 2020 GALAXY-SCALE SIGNATURES OF SCREENED MODIFIED GRAVITIES 3v '8~~~~~~~~~~~~~~~~~~~~~V .ADVENTIST',,, ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ 9. A23 ANEESH P.NAIK A: This thesis is submitted for the degree of Doctor of Philosophy. A, A Illustration: Lord Rosse’s > ?, hand-drawing of M51, reproduced from Rosse (1850). This content downloaded from 81.156.200.236 on Wed, 04 Mar 2020 15:33:27 UTC All use subject to https://about.jstor.org/terms To the trees and birds of Cambridge and East Anglia, particularly: the copper beech overhanging the river at Clare, the avocets at Welney, the giant redwood by the Observatory, and the bluetits in my garden, for whom this thesis is unlikely to be of the remotest consequence. iii Declaration This thesis is the result of my own work and includes nothing which is the outcome of work done in collaboration except as declared in the preface and specified in the text. It is not substantially the same as any work that has already been submitted before for any degree or other qualification except as declared in the preface and specified in the text. The use of ‘we’ instead of ‘I’ throughout various parts of this thesis merely reflects a stylistic choice. Parts of this thesis are based on articles that have been published or have been submitted for publication. These are: • Chapter 2: A. P.Naik, E. Puchwein, A.-C. Davis, C. Arnold Imprints of chameleon f(R) gravity on galaxy rotation curves MNRAS, Volume 480, Issue 4, November 2018, Pages 5211–5225 • Chapter 3: A. P.Naik, E. Puchwein, A.-C. Davis, D. Sijacki, H. Desmond Constraints on chameleon f(R) gravity from galaxy rotation curves of the SPARC sample MNRAS, Volume 489, Issue 1, October 2019, Pages 771–787 • Chapter 4: A. P.Naik, N. W. Evans, E. Puchwein, H. Zhao, A.-C. Davis Stellar Streams in Chameleon Gravity Submitted to PRD, preprint available at arXiv:2002.05738 I undertook the majority of the research presented in each of these chapters, and the dec- laration at the beginning of each chapter explicitly outlines the contributions made by the co-authors of the respective articles. The Introduction (Chapter 1) does not present any original research. The length of this thesis does not exceed the limit of 60,000 words specified by the Degree Committee of Physics and Chemistry. iv Galaxy-Scale Signatures of Screened Modified Gravities Aneesh Naik Summary In recent years, theories of gravity incorporating a scalar field coupled to gravity—‘scalar- tensor’ theories—have been subject to increased attention. In these theories, the scalar field mediates gravitational-strength ‘fifth forces’. For such scalar fields to retain cosmological relevance while also evading stringent con- straints from high-precision post-Newtonian tests of gravity, ‘screening mechanisms’ are invoked, in which the fifth force is suppressed in regions of high density or deep gravita- tional potential. One example of a screening mechanism is the ‘chameleon’ mechanism, in which the scalar has a density-dependent mass, such that the mass becomes very large in regions of high density, and the fifth force is exponentially suppressed as a consequence. While the primary effect of screening mechanisms is to mask the effects of modified gravity in the Solar System, they can nevertheless give rise to interesting astrophysical signatures elsewhere, searches for which can serve as tests of screened modified gravity. These signa- tures are the subject of this thesis. The Introduction of this thesis in Chapter 1 presents some historical background and scientific context, particularly in the fields of cosmology, the astrophysics of galaxies, and screened modified gravity theories. Subsequently, Chapters 2, 3, and 4 present original research regarding two galaxy-scale signatures of screened modified gravity: ‘upturns’ in galaxy rotation curves and asymmetries in stellar streams. If a galaxy is partially screened, it will have a ‘screening radius’, within which the fifth force is suppressed. Outside the screening radius, the fifth force on a test particle will be proportional to the mass enclosed in the shell between the test particle and the screening radius. Thus, the fifth force will contribute to the galaxy’s rotation curve, but only outside the screening radius. At the screening radius itself, there will be an upturn in the curve. In Chapter 2, based on an article published in the Monthly Notices of the Royal Astronomical Society (Naik et al., 2018), I give the first prediction of this effect, specifically in the con- text of Hu-Sawicki f (R) gravity, a widely-studied example of a chameleon theory. By post- processing simulated galaxies of the Auriga Project using the f (R) gravity code MG-Gadget, I produce mock rotation curves for a range of galaxy masses and values of the key theory parameter fR0, forecasting competitive constraints on fR0. In Chapter 3, also based on an article published in the Monthly Notices (Naik et al., 2019), I turn to observational data. Analysing the high-quality rotation curves of the SPARC sample, I find that in certain f (R) parameter regimes there is a strong signal, but it is better explained with standard grav- ity plus a ‘cored’ dark matter halo profile than with modified gravity plus a theoretically- predicted ‘cuspy’ halo. I am thus able to place competitive constraints on f (R) gravity, with v the caveat that if cored haloes can not ultimately be motivated under the standard ΛCDM cosmological paradigm, then screened modified gravity could feasibly ease the tension be- tween observed cores and predicted cusps. In Chapter 4, I consider the observable imprints of screening on stellar streams around the Milky Way. For reasonable parameter regimes in chameleon theories, main sequence stars will be screened, and thus neither source nor couple to the fifth force. Thus, a situation can arise in which a dark matter dominated dwarf galaxy is unscreened, but the stars within it are screened. If such a galaxy were to be tidally disrupted by the Milky Way, its stars would be preferentially stripped into the trailing stellar stream rather than the leading stream. The streams would therefore be asymmetric about their progenitor. Using a restricted N-body method, I explore this effect for a variety of satellite orbits and modified gravity regimes. Taking f (R) gravity as a fiducial theory, I forecast some of the strongest constraints to date from future data releases of the Gaia satellite. This chapter is based on an article submitted to Physical Review D (Naik et al., 2020). Finally, Chapter 5 gives some concluding remarks and a discussion of future prospects in this field. vi Acknowledgments This thesis is the culmination of nearly four years of work undertaken in Cambridge, at the Institute of Astronomy. Two pages from here, I make the University-mandated decla- ration that this work is primarily my own. However, it is certainly not the case that it was performed in isolation, and debts of gratitude are owed to the various people, groups, and institutions who helped me along the way, both directly and indirectly. The first such debt is due of course to my supervisor, Ewald Puchwein. Throughout my PhD, he has been a profoundly patient and knowledgeable guide. The invaluable role he played in supervising my research and helping me to mature as a scientist has necessitated a considerable investment of his time over these years, and I am enormously grateful. This gratitude extends to my other supervisors, Anne-Christine Davis and Debora Sijacki, both of whom were also very generous with their time and insights. Discussions with them have greatly broadened my knowledge and improved my research immeasurably. I wish to also acknowledge the various contributions made by my other research collaborators: Christian Arnold, Harry Desmond, Wyn Evans, and Hongsheng Zhao, as well as the many helpful dis- cussions I’ve had with a large number of people in Cambridge and elsewhere. Of particular note are Claudio Llinares, Baojiu Li, Philippe Brax, Harley Katz, Federico Lelli, Cameron Lemon (see below), Girish Kulkarni, Martin Haehnelt, Matt Auger, Vasily Belokurov, Denis Erkal, Sergey Koposov, Jason Sanders, and Paul Hewett. The research presented in Chapter 2 was facilitated by the Auriga Project team, who al- lowed me access to their simulations. Similarly, Chapter 3 was made possible by the SPARC team, who have made their rotation curve data publicly available. Acknowledgment is also due to the Science and Technology Facilities Council (STFC), who provided funding for this research. Furthermore, the STFC supercomputing service DiRAC was used extensively in all of my research projects. By extension, this gratitude should lie ultimately with the citi- zens and tax-payers of the United Kingdom. Even in these times of economic uncertainty and political unrest, a small share of the nation’s resources goes towards the funding of ab- stract scientific research without any apparent industrial or commercial agenda. I don’t know the extent to which the proverbial man on the Clapham omnibus is aware of this fact or would necessarily approve of it, but I sincerely hope he is and would. All of the research presented in this thesis was conducted from a desk in room 35 of the Observatory. This is one of the largest offices in the Institute, and throughout my four years there, it has played host to an ever-changing cast of delightful characters, who were con- sistently supportive and helpful in a myriad of small ways: Sid, Adam, Laura, Andy, Holly, vii and the young’uns. Particular acknowledgment goes to Cameron, with whom I discov- ered the joy of trees. More generally, the Institute provided a welcoming, collegial working environment, with green spaces and afternoon tea.