The University of Portsmouth is a university in the city of Portsmouth, England. It was previously known as Portsmouth Polytechnic until 1992, when it was granted university status through the Further and Higher Education Act 1992.The University is a member of the University Alliance and The Channel Islands Universities Consortium. Wikipedia.
Clark T.,Friedrich - Alexander - University, Erlangen - Nuremberg |
Clark T.,University of Portsmouth
Wiley Interdisciplinary Reviews: Computational Molecular Science | Year: 2013
σ-Holes are regions of positive molecular electrostatic potential collinear with and opposite to covalent bonds to atoms of Groups IV-VII. They are responsible for many noncovalent bonding interactions, such as halogen bonding. σ-Holes make 'negatively charged' atoms act as if they were 'positively charged'. The existence of σ-hole bonding emphasizes what has been called 'the fallacy of net atomic charges', which means that many covalently bonded atoms cannot be represented adequately by a single charge because they look negative from some directions and positive from others. Hydrogen bonding can also be regarded as a special case of σ-hole bonding, although in this case the origin of the σ-hole is rationalized differently than in the heavier elements. Phenomena such as the directionality of hydrogen bonds and 'blue-shifted' hydrogen bonds can be explained very simply using the σ-hole concept. © 2012 John Wiley & Sons, Ltd.
Koyama K.,University of Portsmouth
Reports on Progress in Physics | Year: 2016
We review recent progress in the construction of modified gravity models as alternatives to dark energy as well as the development of cosmological tests of gravity. Einstein's theory of general relativity (GR) has been tested accurately within the local universe i.e. the Solar System, but this leaves the possibility open that it is not a good description of gravity at the largest scales in the Universe. This being said, the standard model of cosmology assumes GR on all scales. In 1998, astronomers made the surprising discovery that the expansion of the Universe is accelerating, not slowing down. This late-time acceleration of the Universe has become the most challenging problem in theoretical physics. Within the framework of GR, the acceleration would originate from an unknown dark energy. Alternatively, it could be that there is no dark energy and GR itself is in error on cosmological scales. In this review, we first give an overview of recent developments in modified gravity theories including f(R) gravity, braneworld gravity, Horndeski theory and massive/bigravity theory. We then focus on common properties these models share, such as screening mechanisms they use to evade the stringent Solar System tests. Once armed with a theoretical knowledge of modified gravity models, we move on to discuss how we can test modifications of gravity on cosmological scales. We present tests of gravity using linear cosmological perturbations and review the latest constraints on deviations from the standard CDM model. Since screening mechanisms leave distinct signatures in the non-linear structure formation, we also review novel astrophysical tests of gravity using clusters, dwarf galaxies and stars. The last decade has seen a number of new constraints placed on gravity from astrophysical to cosmological scales. Thanks to on-going and future surveys, cosmological tests of gravity will enjoy another, possibly even more, exciting ten years. © 2016 IOP Publishing Ltd.
Sakstein J.,University of Portsmouth
Physical Review Letters | Year: 2015
The most general scalar-tensor theories of gravity predict a weakening of the gravitational force inside astrophysical bodies. There is a minimum mass for hydrogen burning in stars that is set by the interplay of plasma physics and the theory of gravity. We calculate this for alternative theories of gravity and find that it is always significantly larger than the general relativity prediction. The observation of several low mass red dwarf stars therefore rules out a large class of scalar-tensor gravity theories and places strong constraints on the cosmological parameters appearing in the effective field theory of dark energy. © 2015 American Physical Society.
Tasinato G.,University of Portsmouth
Journal of High Energy Physics | Year: 2014
We discuss a consistent theory for a self-interacting vector field, breaking an Abelian symmetry in such a way to obtain an interesting behavior for its longitudinal polarization. In an appropriate decoupling limit, the dynamics of the longitudinal mode is controlled by Galileon interactions. The full theory away from the decoupling limit does not propagate ghost modes, and can be investigated in regimes where non-linearities become important. When coupled to gravity, this theory provides a candidate for dark energy, since it admits de Sitter cosmological solutions characterized by a technically natural value for the Hubble parameter. We also consider the homogeneous evolution when, besides the vector, additional matter in the form of perfect fluids is included. We find that the vector can have an important role in characterizing the universe expansion. © 2014 The Author(s).
Rozas-Fernandez A.,University of Portsmouth
Physics Letters, Section B: Nuclear, Elementary Particle and High-Energy Physics | Year: 2012
A ghost dark energy model has been recently put forward to explain the current accelerated expansion of the Universe. In this model, the energy density of ghost dark energy, which comes from the Veneziano ghost of QCD, is proportional to the Hubble parameter, ρ D=αH. Here α is a constant of order ΛQCD3 where Λ QCD~100 MeV is the QCD mass scale. We consider a connection between ghost dark energy with/without interaction between the components of the dark sector and the kinetic k-essence field. It is shown that the cosmological evolution of the ghost dark energy dominated Universe can be completely described a kinetic k-essence scalar field. We reconstruct the kinetic k-essence function F(X) in a flat Friedmann-Robertson-Walker Universe according to the evolution of ghost dark energy density. © 2012 Elsevier B.V.