DOCSLIB.ORG

University of Southampton Faculty of Physical Sciences and Engineering Department of Physics and Astronomy

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
University of Southampton Faculty of Physical Sciences and Engineering Department of Physics and Astronomy University of Southampton Faculty of Physical Sciences and Engineering Department of Physics and Astronomy Diffeomorphism-invariant averaging in quantum gravity and cosmology Anthony William Henry Preston Submitted for the degree of DOCTOR OF PHILOSOPHY arXiv:1612.06872v1 [gr-qc] 20 Dec 2016 September 2016 UNIVERSITY OF SOUTHAMPTON ABSTRACT FACULTY OF PHYSICAL SCIENCES AND ENGINEERING Department of Physics and Astronomy Doctor of Philosophy DIFFEOMORPHISM-INVARIANT AVERAGING IN QUANTUM GRAVITY AND COSMOLOGY Anthony William Henry Preston This thesis concerns research undertaken in two related topics concerning high-energy gravitational physics. The first is the construction of a manifestly diffeomorphism- invariant Exact Renormalization Group (ERG). This is a procedure that constructs effective theories of gravity by integrating out high-energy modes down to an ultraviolet cutoff scale without gauge-fixing. The manifest diffeomorphism invariance enables us to construct a fully background-independent formulation. This thesis will explore both the fixed-background and background-independent forms of the manifestly diffeomorphism- invariant ERG. The second topic is cosmological backreaction, which concerns the effect of averaging over high-frequency metric perturbations to the gravitational field equa- tions describing the universe at large scales. This has been much studied the context of the unmodified form of General Relativity, but has been much less studied in the con- text of higher-derivative effective theories obtained by integrating out the high-energy modes of some more fundamental (quantum) theory of gravity. The effective stress- energy tensor for backreaction can be used directly as a diffeomorphism-invariant ef- fective stress-energy tensor for gravitational waves without specifying the background metric. This thesis will construct the manifestly diffeomorphism-invariant ERG and compute the effective action at the classical level in two different schemes. We will then turn to cosmological backreaction in higher-derivative gravity, deriving the general form of the effective stress-energy tensor due to inhomogeneity for local diffeomorphism-invariant effective theories of gravity. This an exciting research direction, as it begins the con- struction of a quantum theory of gravity as well as investigating possible implications for cosmology. Contents 1 Introduction1 1.1 Physics at many scales of length . .1 1.2 Statistical mechanics and the Renormalization Group . .4 1.2.1 The Ising model . .4 1.2.2 Phase transitions . .5 1.2.3 Kadanoff blocking and RG transformations . .6 1.2.4 RG flow . .8 1.2.5 Statistical field theory . .9 1.2.6 Parisi-Sourlas supersymmetry . 11 1.3 Overview of General Relativity . 13 1.3.1 Conceptual background . 14 1.3.2 Geometry of curved spacetime . 15 1.3.3 Derivation of field equations from an action . 18 1.3.4 The stress-energy tensor . 20 1.3.5 Metric perturbations . 22 1.3.6 The diffeomorphism symmetry . 23 1.4 Observational status of modern cosmology . 24 1.4.1 Evidence for GR . 24 1.4.2 The big bang theory . 26 1.4.3 Homogeneity and isotropy . 28 1.4.4 The dark side of the universe . 32 1.4.5 Inflation . 36 2 Review of the manifestly gauge-invariant ERG 43 2.1 Kadanoff blocking continuous fields . 43 2.2 Classical ERG for massless scalar fields . 47 2.3 Gauge-invariant ERG for Yang-Mills theories . 50 v vi CONTENTS 2.4 Loopwise expansion . 55 2.5 Additional regularization . 56 3 Manifestly diffeomorphism-invariant ERG 59 3.1 Background-independent gravity flow equation . 59 3.2 Background-independent calculations . 64 3.2.1 Effective action in Weyl scheme . 69 3.2.2 Running away from the fixed point with dimensionful couplings . 73 3.2.3 Effective action in Einstein scheme . 75 3.3 Fixed-background formalism . 79 3.4 Ward identities . 81 3.4.1 Ward identities for an action . 82 3.4.2 Ward identities for the kernel . 85 3.4.3 Ward identity consistency in the flow equation . 88 3.5 Metric determinant expansion in the metric perturbation . 90 3.6 Functional derivatives of the covariantized kernel . 91 3.7 Transverse 2-point functions . 92 3.8 Classical 2-point functions at fixed-points . 95 3.8.1 Classical 2-point function in Weyl scheme . 95 3.8.2 Classical 2-point function in Einstein scheme . 96 3.9 Further regularization via supermanifolds . 97 3.10 Outlook . 99 4 Review of cosmological backreaction 101 4.1 Conceptual overview . 101 4.2 Notation . 103 4.3 Isaacson's shortwave approximation . 104 4.4 Burnett's weak-limit averaging formalism . 107 4.5 Green and Wald weak-limit framework . 110 5 Backreaction in modified gravity 115 5.1 Incorporating additional scales of length . 118 5.2 Zero tensor forms for modified gravity . 120 5.3 Stress-energy tensor in modified gravity . 121 5.4 Alternative derivation in scalar-tensor formulation . 122 5.5 Diffeomorphism invariance . 126 CONTENTS vii 5.6 Dark energy and new physics in the ultraviolet . 127 6 Generalized cosmological backreaction 133 6.1 General scaling properties . 133 6.2 Contributing action terms . 134 6.2.1 Riemann tensor expansions . 135 6.2.2 Covariant derivative expansions . 136 6.3 Zero tensors . 138 6.4 Stress-energy tensor . 139 6.5 Effective stress-energy tensor trace . 141 6.5.1 General consistency of the trace . 141 6.5.2 Evaluation of the generalized trace . 142 6.6 Diffeomorphism invariance . 143 6.7 Generalized backreaction discussion . 144 7 Summary 149 Bibliography 152 viii CONTENTS List of Figures 2.1.1 Example of a smooth infrared cutoff function in position representation 45 2.2.1 General flow equation for an action with a single scalar field at some n-point level . 48 2.3.1 Gauge-invariant n-point flow equation with a covariantized kernel . 54 ix Declaration of Authorship I, Anthony Preston, declare that this thesis, entitled “Diffeomorphism-invariant averag- ing in quantum gravity and cosmology", and the work presented in it, are my own and have been generated by me as the result of my own original research. I confirm that: 1. This work was done wholly or mainly while in candidature for a research degree at this University; 2. No part of this thesis has previously been submitted for a degree or any other qualification at this University or any other institution; 3. Where I have consulted the published work of others, this is always clearly at- tributed; 4. Where I have quoted from the work of others, the source is always given. With the exception of such quotations, this thesis is entirely my own work; 5. I have acknowledged all main sources of help; 6. Chapters1,2 and4 are reviews, written by me, describing the previously existing literature. Chapters3 and5 report on original research conducted in collaboration with my supervisor, Tim Morris [1,2]. Chapter6 reports on original research I conducted alone [3]. Chapter7 is a summary of the whole thesis. Signed: ........................ Date: .......................... Acknowledgements First of all, I would like to thank my supervisor, Tim Morris, for guiding me on this path and entertaining my wilder speculations while also teaching me how to formulate this research rigorously. I thank the University of Southampton for funding my PhD with a Mayflower Scholarship. I thank my parents and my brother for patiently supporting and encouraging me throughout my long education. During my time in Southampton, I have met many brilliant people who have made PhD life colourful and rewarding. In particular, I would like to thank my fellow PhD students in the Southampton High Energy Physics Group for exchange of ideas and friendship. Working clockwise through the weird and wonderful world of my office, the amazing 46/4007, I thank Thomas Neder, the Big Friendly German, for lively discussions on physics and the meaning of life; I thank Hamzaan Bridle for regularly surprising us with edible treats and unexplained machinery; I thank Marc Scott for his artistry in decorating the office and for lending a sympathetic ear throughout the highs and lows of the past four years; I thank the visiting students, David Garner, Diana Rojas and Rafael Delgado for the completely different ways they each enriched our office culture and mythology; I thank Andrew Meadowcroft for his surgical wit and unexpected pranks; I thank Tadeusz Janowski for outstanding contributions to 4007's physics knowledge, creative chaos and reliquary; I thank Alex Titterton for an endless stream of puns; I thank Jeff Choi for his helpfulness and stoicism; I thank Simon King for keeping us all on our toes; I thank Matt Spraggs for his entertainingly heated monologues; I thank Ronnie Rodgers for his remarkable enthusiasm for formal physics; I thank Maien Binjonaid for being the office’s wise elder statesman; I thank Azaria Coupe for injecting some optimism amid the madness; I thank Edwin Lizarazo for keeping the peace during the frequent office dramas; I thank Andrew Lawson for sound programming advice. Beyond the walls of 4007, I thank Jason Hammett for helping me to get settled into the office and maintaining a cheerful disposition amid the slings and arrows of outrageous leptoquarks. I thank the double-doctor Marc Thomas for inspirational conversations. I thank the Asymptotic Safety students, J¨urgenDietz and Zo¨eSlade, for interesting discussions and good humour in the face of my surreal tomfoolery. Many other people in Southampton and beyond have had a positive impact on my time here, and I thank you all for helping me along this adventure. Chapter 1 Introduction The layout of this introduction is as follows. Section 1.1 will give a broad discussion of how varying scales of length are studied in different physical contexts. Section 1.2 will discuss how problems with many degrees of freedom are tackled in statistical mechanics.
Load more

Recommended publications
  • Large-Scale Structure Under Λ-CDM Paradigm
    Large-Scale Structure under Λ-CDM Paradigm Timothy Faerber Astrophysical Tests of Physical Theories - May 2020 1 Introduction We live in a universe dominated by matter and energy. On the smallest scale, all matter and energy is composed of \elementary particles", as predicted by the standard model of particle physics, seen in Figure 1. Figure 1: The Standard Model of Particle Physics All of these particles interact or are influenced via one of the four funda- mental forces of nature: the electromagnetic force (responsible for every- day forces, i.e. friction and tension), the nuclear weak force (responsible 1 for beta decay), the nuclear strong force (responsible for binding of neu- trons and electrons to form more atoms), and the gravitational force (responsible for the gravitational attraction between two massive bod- ies) [4]. In this paper we will focus on the gravitational force and the effect that it has on carving out the Large-Scale Structure (LSS) that we see when observing our universe on the scale of superclusters of galaxies (≈ 110 − 130h−1Mpc) [5]. The LSS in our universe is composed of many gravitationally-bound smaller-scale structures such as groups of galax- ies (less than ≈ 30 − 50 giant galaxies, individual galaxies, gas clouds, stars and planets. On scales the size of superclusters, gravity dominates the other three fundamental forces, however without them, small-scale structures could not exist to form the LSS observed. Responsible for holding together every atom in the universe, and thus making it possi- ble for matter to exist as we know it, is the electromagnetic force.
    [Show full text]
  • I–Visibility Correlation Somnath Bharadwaj & Shiv K. Sethi
    J. Astrophys. Astr. (2001) 22, 293–307 HI Fluctuations at Large Redshifts: I–Visibility correlation Somnath Bharadwaj1 & Shiv K. Sethi2∗ 1Department of Physics and Meteorology & Center for Theoretical Studies, I.I.T. Kharagpur, 721 302, India email: [email protected] 2Raman Research Institute, C. V. Raman Avenue, Sadashivnagar, Bangalore 560 080, India email: [email protected] Received 2002 March 15; accepted 2002 May 31 Abstract. We investigate the possibility of probing the large scale structure in the universe at large redshifts by studying fluctuations in the redshifted 1420 MHz emission from the neutral hydrogen (HI) at early epochs. The neutral hydrogen content of the universe is known from absorp- < tion studies for z ∼ 4.5. The HI distribution is expected to be inhomoge- neous in the gravitational instability picture and this inhomogeneity leads to anisotropy in the redshifted HI emission. The best hope of detecting this anisotropy is by using a large low-frequency interferometric instru- ment like the Giant Meter-Wave Radio Telescope (GMRT). We calculate the visibility correlation function hVν(U)Vν0 (U)i at two frequencies ν and ν0 of the redshifted HI emission for an interferometric observation. In par- ticular we give numerical results for the two GMRT channels centered around ν = 325 MHz and ν = 610 MHz from density inhomogeneity and peculiar velocity of the HI distribution. The visibility correlation is ' 10−10–10−9 Jy2. We calculate the signal-to-noise for detecting the cor- relation signal in the presence of system noise and show that the GMRT might detect the signal for integration times ' 100 hrs.
    [Show full text]
  • Prospects for Detecting the 326.5 Mhz Redshifted 21-Cm HI Signal with the Ooty Radio Telescope (ORT)
    J. Astrophys. Astr. (2014) 35, 157–182 c Indian Academy of Sciences Prospects for Detecting the 326.5 MHz Redshifted 21-cm HI Signal with the Ooty Radio Telescope (ORT) Sk. Saiyad Ali1,∗ & Somnath Bharadwaj2 1Department of Physics, Jadavpur University, Kolkata 700 032, India. 2Department of Physics and Meteorology & Centre for Theoretical Studies, IIT Kharagpur, Kharagpur 721 302, India. ∗e-mail: [email protected] Received 7 October 2013; accepted 13 February 2014 Abstract. Observations of the redshifted 21-cm HI fluctuations promise to be an important probe of the post-reionization era (z ≤ 6). In this paper we calculate the expected signal and foregrounds for the upgraded Ooty Radio Telescope (ORT) which operates at frequency νo = 326.5MHz which corresponds to redshift z = 3.35. Assuming that the visibilities contain only the HI signal and system noise, we show that a 3σ detec- tion of the HI signal (∼ 1 mK) is possible at angular scales 11 to 3◦ with ≈1000 h of observation. Foreground removal is one of the major chal- lenges for a statistical detection of the redshifted 21 cm HI signal. We assess the contribution of different foregrounds and find that the 326.5 MHz sky is dominated by the extragalactic point sources at the angular scales of our interest. The expected total foregrounds are 104−105 times higher than the HI signal. Key words. Cosmology: large scale structure of Universe—intergalactic medium—diffuse radiation. 1. Introduction The study of the evolution of cosmic structure has been an important subject in cos- mology. In the post reionization era (z < 6) the 21-cm emission originates from dense pockets of self-shielded hydrogen.
    [Show full text]
  • Small-Scale Secondary Anisotropics in the Cosmic Microwave Background
    Small-Scale Secondary Anisotropics in the Cosmic Microwave Background by Jonathan Dudley M.Sc. Department of Physics McGill University Montreal, Quebec August 2007 A thesis submitted to McGill University in partial fulfilment of the requirements of the degree of Master of Science in Physics © Jonathan Dudley (2007) Library and Bibliotheque et 1*1 Archives Canada Archives Canada Published Heritage Direction du Branch Patrimoine de I'edition 395 Wellington Street 395, rue Wellington Ottawa ON K1A0N4 Ottawa ON K1A0N4 Canada Canada Your file Votre reference ISBN: 978-0-494-51263-0 Our file Notre reference ISBN: 978-0-494-51263-0 NOTICE: AVIS: The author has granted a non­ L'auteur a accorde une licence non exclusive exclusive license allowing Library permettant a la Bibliotheque et Archives and Archives Canada to reproduce, Canada de reproduire, publier, archiver, publish, archive, preserve, conserve, sauvegarder, conserver, transmettre au public communicate to the public by par telecommunication ou par Plntemet, prefer, telecommunication or on the Internet, distribuer et vendre des theses partout dans loan, distribute and sell theses le monde, a des fins commerciales ou autres, worldwide, for commercial or non­ sur support microforme, papier, electronique commercial purposes, in microform, et/ou autres formats. paper, electronic and/or any other formats. The author retains copyright L'auteur conserve la propriete du droit d'auteur ownership and moral rights in et des droits moraux qui protege cette these. this thesis. Neither the thesis Ni la these ni des extraits substantiels de nor substantial extracts from it celle-ci ne doivent etre imprimes ou autrement may be printed or otherwise reproduits sans son autorisation.
    [Show full text]
  • Suman Majumdar
    Suman Majumdar Contact Suman Majumdar E-mail: [email protected], Information Department of Astronomy, Astrophysics and Space Engineering [email protected] Indian Institute of Technology Indore, Voice: +91 6296136775 Indore-453552, M.P., India http://www.iiti.ac.in/people/~sumanm/ Research Cosmic Dawn and Epoch of Reionization, 21-cm Cosmology, Square Kilometre Array, Simulations of CD-EoR Interests and Large Scale Structures, N-body Simulations, Statistical Inference, Low Frequency Radio Interferometry. Education Indian Institute of Technology Kharagpur, West Bengal, India Ph.D., April, 2013 Probing the Epoch of Reionization through radio-interferometric observations of neutral hydrogen. Supervisors: Somnath Bharadwaj and Sugata Pratik Khastgir. M.Sc. in Physics, August, 2007, Performance: 1st Class. Maulana Azad College, University of Calcutta, Kolkata, West Bengal, India. B.Sc. (Honours) in Physics, August, 2005, Performance: 1st Class. Academic Assistant Professor (Tenured) May, 2018 - present Positions Department of Astronomy, Astrophysics and Space Engineering, Indore, India Indian Institute of Technology Indore Research Associate December, 2015 - April, 2018 Department of Physics, Imperial College London, UK Postdoctoral Fellow December, 2012 - November, 2015 Department of Astronomy, Stockholm University Stockholm, Sweden CSIR Senior Research Fellow May, 2011 - September, 2012 Indian Institute of Technology Kharagpur Kharagpur, India Senior Research Fellow July, 2009 - April, 2011 Indian Institute of Technology Kharagpur
    [Show full text]
  • Large Scale Structure
    PRAMANA ­c Indian Academy of Sciences Vol. 53, No. 6 — journal of December 1999 physics pp. 977–987 Large scale structure SOMNATH BHARADWAJ Department of Physics and Meteorology and Centre for Theoretical Studies, Indian Institute of Technology, Kharagpur 721 302, India Email: [email protected] Abstract. We briefly discuss some aspects of the problem of forming large scale structures in the Universe. The basic picture that initially small perturbations generated by inflation grow by the process of gravitational instability to give the observed structures is largely consistent with the observations. The growth of the perturbations depends crucially on the contents of the Universe, and we discuss a few variants of the cold dark matter model. Many of these models are consistent with present observations. Future observations hold the possibility of deciding amongst these models. Keywords. Cosmology; theory; large scale structure. PACS Nos 98.65.Dx 1. Introduction Our present understanding of the Universe is based on two important assumptions 1. the Universe is homogeneous and isotropic on large scales 2. gravitation governs the dynamics of the Universe on large scales. The time-evolution of such a Universe can be described by just one function of time – the ´Øµ scale factor a which tells us about the expansion or contraction of the Universe (these ´Øµ being the only two possibilities which preserve assumption (1). The evolution of a is governed by the laws of gravitation eq. (4), and the observation that all the distant galax- ies are moving away from us leads us to the conclusion that the Universe is expanding [1].
    [Show full text]
  • Advancing Precision Cosmology with 21 Cm Intensity Mapping by Kiyoshi
    Advancing precision cosmology with 21 cm intensity mapping by Kiyoshi Wesley Masui A thesis submitted in conformity with the requirements for the degree of Doctor of Philosophy Graduate Department of Physics University of Toronto c Copyright 2013 by Kiyoshi Wesley Masui Abstract Advancing precision cosmology with 21 cm intensity mapping Kiyoshi Wesley Masui Doctor of Philosophy Graduate Department of Physics University of Toronto 2013 In this thesis we make progress toward establishing the observational method of 21 cm intensity mapping as a sensitive and efficient method for mapping the large-scale struc- ture of the Universe. In Part I we undertake theoretical studies to better understand the potential of intensity mapping. This includes forecasting the ability of intensity mapping experiments to constrain alternative explanations to dark energy for the Universe's accel- erated expansion. We also consider how 21 cm observations of the neutral gas in the early Universe (after recombination but before reionization) could be used to detect primordial gravity waves, thus providing a window into cosmological inflation. Finally we show that scientifically interesting measurements could in principle be performed using intensity mapping in the near term, using existing telescopes in pilot surveys or prototypes for larger dedicated surveys. Part II describes observational efforts to perform some of the first measurements using 21 cm intensity mapping. We develop a general data analysis pipeline for analyzing intensity mapping data from single dish radio telescopes. We then apply the pipeline to observations using the Green Bank Telescope. By cross-correlating the intensity mapping survey with a traditional galaxy redshift survey we put a lower bound on the amplitude of the 21 cm signal.
    [Show full text]
  • The Stabilizing Effect of Spacetime Expansion on Relativistic Fluids with Sharp Results for the Radiation Equation of State
    THE STABILIZING EFFECT OF SPACETIME EXPANSION ON RELATIVISTIC FLUIDS WITH SHARP RESULTS FOR THE RADIATION EQUATION OF STATE JARED SPECK∗ Abstract. In this article, we study the 1 + 3 dimensional relativistic Euler equations on a pre-specified conformally flat expanding spacetime background with spatial slices that are diffeomorphic to R3. We assume that the fluid ver- 2 ifies the equation of state p = csρ, where 0 ≤ cs ≤ p1/3 is the speed of sound. We also assume that the inverse of the scale factor associated to the expanding spacetime metric verifies a cs−dependent time-integrability condi- tion. Under these assumptions, we use the vectorfield energy method to prove that an explicit family of physically motivated, spatially homogeneous, and spatially isotropic fluid solutions is globally future-stable under small pertur- bations of their initial conditions. The explicit solutions corresponding to each scale factor are analogs of the well-known spatially flat Friedmann-Lemaˆıtre- Robertson-Walker family. Our nonlinear analysis, which exploits dissipative terms generated by the expansion, shows that the perturbed solutions exist for all future times and remain close to the explicit solutions. This work is an extension of previous results, which showed that an analogous stability result holds when the spacetime is exponentially expanding. In the case of the radi- ation equation of state p = (1/3)ρ, we also show that if the time-integrability condition for the inverse of the scale factor fails to hold, then the explicit fluid solutions are unstable. More precisely, we show that arbitrarily small smooth perturbations of the explicit solutions’ initial conditions can launch perturbed solutions that form shocks in finite time.
    [Show full text]
  • The Effects of the Small-Scale DM Power on the Cosmological Neutral Hydrogen (HI ) Distribution at High Redshifts
    PREPARED FOR SUBMISSION TO JCAP The effects of the small-scale DM power on the cosmological neutral hydrogen (HI ) distribution at high redshifts Abir Sarkar,1,2 Rajesh Mondal,3,4 Subinoy Das,5 Shiv.K.Sethi,1 Somnath Bharadwaj,3,4 David J. E. Marsh6 1Raman Research Institute, Bangalore, India 2Indian Institute of Science, Bangalore, India 3Department of Physics, Indian Institute of Technology Kharagpur, Kharagpur - 721302, India 4Centre for Theoretical Studies, Indian Institute of Technology Kharagpur, Kharagpur - 721302, India 5Indian Institute of Astrophysics, Bangalore,India 6Department of Physics, King’s College London, Strand, London, WC2R 2LS, United Kingdom E-mail: [email protected], [email protected] Abstract. The particle nature of dark matter remains a mystery. In this paper, we consider two dark matter models—Late Forming Dark Matter (LFDM) and Ultra-Light Axion (ULA) models—where the matter power spectra show novel effects on small scales. The high redshift universe offers a powerful probe of their pa- rameters. In particular, we study two cosmological observables: the neutral hydrogen (HI) redshifted 21-cm signal from the epoch of reionization, and the evolution of the collapsed fraction of HI in the redshift range 2 <z < 5. We model the theoretical predictions of the models using CDM-like N-body simulations with modified initial conditions, and generate reionization fields using an excursion-set model. The N-body approx- imation is valid on the length and halo mass scales studied. We show that LFDM and ULA models predict an increase in the HI power spectrum from the epoch of reionization by a factor between 2–10 for a range of −1 scales 0.1 <k< 4Mpc .
    [Show full text]
  • Jayanta Final Version-2.Pdf
    Título: A fragmentação durante a formação primordial estrela Resumo: Compreender os mecanismos físicos que são responsáveis pela formação e evolução das primeiras estrelas do universo, conhecidas como estrelas de população III (ou Pop II) é fundamental para compreendermos como evoluiu o universo até hoje. No modelo padrão da formação de estrelas de Pop III, a matéria bariónica é constituída principalmente por hidrogénio atómico na forma gasosa, e colapsa gravitacionalmente em mini-halos (pequenos halos) de matéria escura, dando origem à formação das estrelas. No entanto, muito pouco se sabe como a evolução dinâmica e química do gás primordial são afetadas pelas condições iniciais dos mini- halos, em particular no que diz respeito ao efeito da rotação nos aglomerados estelares instáveis que se formam dentro dos mini-halos, ao impacto da turbulência, à formação de hidrogénio molecular, e ao impacto das variações cósmicas entre mini-halos. Neste trabalho usamos uma versão modificada do código Gadget-2, um programa de simulação hidrodinâmica baseado num algoritmo numérico conhecido por SPH (“smoothed particle hydrodynamics”), que permite seguir a evolução do gás durante o colapso, tanto no caso de mini-halos idealizados como em casos de mino-halos mais realistas. Em contraste com algumas simulações numéricas mais antigas, a implementação das partículas coletoras (“sink particles”) permite seguir a evolução do disco de acreção que se forma no centro dos fragmentos e dos mini-halos. Descobrimos que o processo de fragmentação depende do valor adotado para a taxa de formação (“three-body H2 formation rate”) de hidrogénio molecular (H2). Verificamos que o aumento da taxa de arrefecimento durante o período em que o hidrogénio atómico é convertido em hidrogénio molecular é compensado pelo aquecimento causado pela contração do gás.
    [Show full text]
  • Issues in Gravitational Clustering and Cosmology
    Issues in Gravitational Clustering and Cosmology A Thesis submitted to the UNIVERSITY OF PUNE for the degree of Doctor of Philosophy in Physics by JATUSH V. SHETH arXiv:astro-ph/0602433 18 Feb 2006 INTER-UNIVERSITY CENTRE FOR ASTRONOMY AND ASTROPHYSICS PUNE July 2004 Declaration CERTIFIED that the work incorporated in the thesis Issues in Gravitational Clus- tering and Cosmology submitted by Jatush V. Sheth was carried out by the cadidate under my supervision. Such material as has been obtained from other sources has been duly acknowledged in the thesis. Place: IUCAA, Pune Professor Varun Sahni Date : July, 2004 Thesis Supervisor ii To my parents..... Acknowledgments Carrying out a thesis-work is such an elaborate process ! Over years of hard work, one evolves both as a researcher as well as a person. Many a times, it is a journey in solitude; a journey into uncharted territories. It is a funful adventure to seek goals amidst uncertainties. To me, the experience of carrying out this thesis-work has been this, and much more. It is a pleasure to have reached this destination successfully. There are numerous people who have directly or indirectly helped me find my way, have nurtured my personal and/or academic growth. You all have been my fellow-travellers, friends and at times, my guiding light! Today I want to thank you all for being with me in this journey. It was during my course work that I developed a sense of fascination for large-scale structure. Through SURFGEN, I could give an objective meaning to this nameless, somewhat poetic sense of fascination.
    [Show full text]
  • Cosmological Backreaction in Higher-Derivative Gravity Expansions Arxiv:1605.06121V2
    Cosmological backreaction in higher-derivative gravity expansions Anthony W. H. Preston Department of Physics and Astronomy, University of Southampton Highfield, Southampton, SO17 1BJ, U.K. [email protected] Abstract We calculate a general effective stress-energy tensor induced by cosmological inhomogeneity in effective theories of gravity where the action is Taylor-expandable in the Riemann tensor and covariant derivatives of the Riemann tensor. This is of interest as an effective fluid that might provide an alternative to the cosmological constant, but it also applies to gravitational waves. We use an adaptation of Green and Wald's weak-averaging framework, which averages over perturbations in the field equation where the perturbation length scales are small compared to the averaging scale. In this adaptation, the length scale of the effective theory, 1=M, is also taken to be small compared with the averaging scale. This ensures that the perturbation length scales remain in fixed proportion to the length scale of the effective theory as the cosmological averaging scale is taken to be large. We find that backreaction from higher-derivative terms in the effective action can continue to be important in the late universe, given a source of sufficiently high-frequency metric perturbations. This backreaction might also provide a window on exotic particle physics in the far ultraviolet. arXiv:1605.06121v2 [gr-qc] 1 Aug 2016 1 Introduction The standard cosmological concordance model is the ΛCDM model. Assumed by this is the Cos- mological Principle, that the universe is homogeneous and isotropic at sufficiently large distance scales and can thus the metric at large scales is accurately approximated by a Friedmann-Lema^ıtre- Robertson-Walker (FLRW) metric.
    [Show full text]