An investigation of relativistic gravitational effects of use for understanding and interpreting dark energy

Lead Research Organisation: University of Oxford
Department Name: Oxford Physics

Abstract

Normal matter, such as the electrons, protons and neutrons that make up the everyday things we are familiar with, have the property of attracting each other through gravity. A universe that were filled with only matter of this type, together with radiation, neutrinos and a few other ingredients, would either slow down in its expansion as time goes by, or settle into a steady rate of constant expansion. Instead, astronomers see a Universe that appears to be accelerating in its expansion. This is a highly unexpected result, and one that requires the introduction of a highly unusual substance that must permeate the whole of the Universe and be responsible for pushing everything apart. This substance has been given the name Dark Energy, and to explain the observations it must make up approximately 2/3 of the total energy budget of the Universe. There are lots of ideas as to what Dark Energy might be, but at the moment no-one knows for sure. The research I am proposing to perform is an investigation of some of these ideas. In particular, I am planning to focus on three specific areas, which I will outline below. The first area is modifications to general relativity. General relativity is the theory that governs the behaviour of space-time, and it has been suggested by some that the apparent need for Dark Energy may be a sign that we have not understood how gravity, and hence general relativity, behave on the largest scales. A number of modifications to general relativity have been proposed, but it is not always clear what the full implications of these modifications are, and if they are compatible with all of the astrophysical and cosmological observations that we have available to us. I intend to study these modified theories, and to produce a frame-work through which they can be straightforwardly compared to observational data, in order to determine their validity. The second area is inhomogeneous models of the Universe. The usual model for the Universe is based on small deviations from a background that, at every point, looks the same in every direction. This has been very successful for understanding and explaining observations of the Universe, but it has been suggested that it may not be entirely sufficient, and that Dark Energy may be the result of interpreting data within an incorrect model. I aim to critically investigate this suggestion, either to bolster the validity of the usual model, or to provide a credible alternative. The third area is Dark Energy from string theory. Many physicists now agree that string theory does not have a single unique vacuum, but very many different possible vacua. Each of these will have different properties, including different types of Dark Energy. If the Universe expanded very rapidly early on, then these different types of vacuum will be realised in different parts of the Universe. If there are enough vacua, and the Universe is big enough, there should be at least one part with a Dark Energy that looks like ours. While this is a promising idea, there are some fundamental difficulties with understanding how likely it is for our type of Dark Energy to be realised. I intend to investigate this area to improve our understanding of these problems, and how they can be resolved. Understanding Dark Energy is of critical importance in our understanding of the Universe, and the research I aim to perform will contribute substantially towards achieving this goal. The existence of Dark Energy provides excellent motivation for improving our understanding of the foundations of our models of the Universe, and, with many new and exciting observational missions planned in the near future, now is the perfect time to perform the research programme I have outlined above.

Publications


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Clifton T (2013) A gravitational entropy proposal in Classical and Quantum Gravity
Clifton T (2014) Piecewise silence in discrete cosmological models in Classical and Quantum Gravity
Clifton T (2014) The method of images in cosmology in Classical and Quantum Gravity
Clifton T (2017) The magnetic part of the Weyl tensor, and the expansion of discrete universes in General Relativity and Gravitation
CLIFTON T (2013) BACK-REACTION IN RELATIVISTIC COSMOLOGY in International Journal of Modern Physics D
Clifton T (2013) Exact evolution of discrete relativistic cosmological models in Journal of Cosmology and Astroparticle Physics
Clifton T (2012) Observational cosmology in macroscopic gravity in Journal of Cosmology and Astroparticle Physics
Sanghai V (2015) Post-Newtonian cosmological modelling in Physical Review D
Clarkson C (2012) Observational constraints on the averaged universe in Physical Review D

Related Projects

Project Reference Relationship Related To Start End Award Value
ST/H005048/1 31/03/2011 01/04/2012 £457,825
ST/H005048/2 Transfer ST/H005048/1 02/04/2012 01/04/2016 £371,980
 
Description I have discovered that a single large under-density is not sufficient to explain the observations attributed to the accelerating expansion of the universe, that cosmological models that contain arbitrarily large density contrasts can approach the behaviour of smooth cosmological models (in appropriate limits), that interaction energies in cosmological models can potentially affect the large-scale expansion of the Universe, that entropy in gravitational fields can potentially be understood in terms of the super-energy-momentum tensor of free gravitational fields, that fourth-order theories of gravity that are used to explain accelerating expansion generically have problems with embedding weak-field systems into cosmology, that the spatial curvature term that is introduced by smoothing over small-scale structure in the Universe can be constrained with observations, that collapse of structure can produce effects that mimic accelerating expansion of the universe, that the microwave background radiation can in principle be used to constrain the geometry of the universe without using the Copernican Principle, and that inhomogeneities can affect luminosity distances and redshifts of distant bodies in the universe in a non-trivial way.
Exploitation Route I have pioneered various different methods to explore the effects of inhomogeneity and modified gravity on the expansion of the universe. All of these methods can be explored and extended in future studies.
Sectors Other
 
Description My findings have been used to further the understanding of inhomogeneity and modifications to gravity on the large-scale expansion of the Universe.
First Year Of Impact 2011
Sector Other
Impact Types Societal
 
Description Alan Coley 
Organisation Dalhousie University
Department Department of Mathematics and Statistics
Country Canada 
Sector Academic/University 
PI Contribution I have performed research in the areas on which we have published papers.
Collaborator Contribution He has performed research in the areas on which we have published papers.
Impact This collaboration has resulted in two publications in peer reviewed journals.
Start Year 2011
 
Description Anne Marie Nzioki 
Organisation University of Cape Town
Country South Africa, Republic of 
Sector Academic/University 
PI Contribution Calculations
Collaborator Contribution Calculations
Impact DOI: 10.1103/PhysRevD.87.063517
Start Year 2013
 
Description Antonio Padilla 
Organisation University of Nottingham
Department School of Physics and Astronomy Nottingham
Country United Kingdom of Great Britain & Northern Ireland (UK) 
Sector Academic/University 
PI Contribution I have performed research in the areas on which we have published.
Collaborator Contribution He has performed research in the areas on which we have published.
Impact We published a paper in a peer reviewed journal.
Start Year 2010
 
Description Chris Clarkson 
Organisation University of Cape Town
Department Department of Mathematics and Applied Mathematics
Country South Africa, Republic of 
Sector Academic/University 
PI Contribution I have performed research in the areas on which we have published.
Collaborator Contribution He has performed research in the areas on which we have published.
Impact We have published three papers in peer reviewed journals.
Start Year 2008
 
Description Constantinos Skordis 
Organisation University of Nottingham
Department School of Physics and Astronomy Nottingham
Country United Kingdom of Great Britain & Northern Ireland (UK) 
Sector Academic/University 
PI Contribution I have performed research in the areas on which we have published.
Collaborator Contribution He has performed research in the areas on which we have published.
Impact We published a paper in a peer reviewed journal.
Start Year 2010
 
Description Daniele Gregoris 
Organisation University of Stockholm
Country Sweden, Kingdom of 
Sector Academic/University 
PI Contribution Calculations
Collaborator Contribution Calculations
Impact DOI: 10.1088/1475-7516/2013/11/010
Start Year 2013
 
Description George Ellis 
Organisation University of Cape Town
Country South Africa, Republic of 
Sector Academic/University 
PI Contribution Calculations
Collaborator Contribution Calculations
Impact DOI: 10.1088/0264-9381/30/12/125009
Start Year 2013
 
Description Kane O'Donnell 
Organisation University of Cambridge
Department Department of Applied Mathematics and Theoretical Physics (DAMTP)
Country United Kingdom of Great Britain & Northern Ireland (UK) 
Sector Academic/University 
PI Contribution I have performed research in the areas on which we have published.
Collaborator Contribution He has performed research in the areas on which we have published.
Impact We published a paper in a peer reviewed journal.
Start Year 2011
 
Description Kjell Rosquist 
Organisation University of Stockholm
Department Department of Physics
Country Sweden, Kingdom of 
Sector Academic/University 
PI Contribution I have performed research in the areas on which we have published.
Collaborator Contribution He has performed research in the areas on which we have published.
Impact We have published a paper in a peer reviewed journal.
Start Year 2011
 
Description Pedro Ferreira 
Organisation Mathematical, Physical & Life Sciences Division
Department Department of Physics; University of Oxford
Country United Kingdom of Great Britain & Northern Ireland (UK) 
Sector Academic/University 
PI Contribution I have performed research in the areas on which we have published.
Collaborator Contribution He has performed research in the areas on which we have published.
Impact We have published nine papers in peer reviewed journals
Start Year 2008
 
Description Peter Dunsby 
Organisation University of Cape Town
Country South Africa, Republic of 
Sector Academic/University 
PI Contribution Calculations
Collaborator Contribution Calculations
Impact DOI: 10.1103/PhysRevD.87.063517
Start Year 2012
 
Description Philip Bull 
Organisation Mathematical, Physical & Life Sciences Division
Department Department of Physics; University of Oxford
Country United Kingdom of Great Britain & Northern Ireland (UK) 
Sector Academic/University 
PI Contribution I have performed research in the areas on which we have published.
Collaborator Contribution He has performed research in the areas on which we have published.
Impact We have published three papers in peer reviewed journals.
Start Year 2011
 
Description Reza Tavakol 
Organisation Queen Mary University of London (QMUL)
Department School of Physics and Astronomy
Country United Kingdom of Great Britain & Northern Ireland (UK) 
Sector Academic/University 
PI Contribution I have performed research in the areas on which we have published.
Collaborator Contribution He has performed research in the areas on which we have published.
Impact We have published a paper in a peer reviewed journal.
Start Year 2011
 
Description Rituparno Goswami 
Organisation University of Cape Town
Country South Africa, Republic of 
Sector Academic/University 
PI Contribution Calculations
Collaborator Contribution Calculations
Impact DOI: 10.1103/PhysRevD.87.063517
Start Year 2013
 
Description Robert van den Hoogen 
Organisation Mathematics and Statistics; St Francis Xavier University
Country Canada 
Sector Academic/University 
PI Contribution I have performed research in the areas on which we have published.
Collaborator Contribution He has performed research in the areas on which we have published.
Impact We published a paper in a peer reviewed journal.
Start Year 2011
 
Description Rockhee Sung 
Organisation University of Cape Town
Department Department of Mathematics and Applied Mathematics
Country South Africa, Republic of 
Sector Academic/University 
PI Contribution I have performed research in the areas on which we have published.
Collaborator Contribution She has performed research in the areas on which we have published.
Impact We published a paper in a peer reviewed journal.
Start Year 2011
 
Description Viraj Sanghai 
Organisation Queen Mary University of London (QMUL)
Department Queen Mary College
Country United Kingdom of Great Britain & Northern Ireland (UK) 
Sector Academic/University 
PI Contribution I have performed research in the areas on which we have published.
Collaborator Contribution He has performed research in the areas on which we have published.
Impact We have published one paper in a peer reviewed journal.
Start Year 2013