Through our theoretical cosmology research – much of which takes place within the University's Institute of Cosmology and Gravitation – we're exploring the inflation of the very early Universe, monitoring the expansion of the Universe and investigating the impact of dark energy on its geometry.
Very early Universe
Our research into the very early Universe explores inflation – the period of rapid expansion believed to have happened when the Universe we observe was much smaller than today. This expansion explains why the Universe looks so uniform at large scales and explains the formation of present day cosmic structures.
We’re researching the mechanisms for inflation using the latest theories in particle physics and quantum gravity. The goal is to connect very early Universe cosmology with what we know about physics at the very high energy scales where inflation occurs.
We study the dynamics of the inhomogeneities produced during inflation and how they can leave specific imprints, such as cosmic microwave background (CMB) radiation and high-redshift galaxy surveys. The aim of our research is to provide tools to distinguish between different realisations of inflation and alternative models, such as ekpyrotic cyclic universe theory or pre-big bang scenarios.
Recent research goes beyond standard calculations to uncover more information about the very early Universe. For example, we have developed a fast and accurate numerical code to model fluctuations in cosmic microwave background radiation, including non-linear evolution.
We've known the Universe is expanding since Slipher and Hubble’s work in the 1920’s, which formed the basis of the modern model of the expanding Universe.
The expansion of the Universe explains why distant galaxies are seen to be moving away from us. It was assumed that gravity would cause the rate of expansion to slow down – perhaps resulting in a ‘big crunch’ in which the Universe collapses in on itself. But astronomers discovered in the 1990’s that the speed of expansion is actually accelerating with time – a discovery which earned Saul Perlumutter, Brian P. Schmidt and Adam G. Riess the 2011 Nobel Prize in Physics.
This acceleration means we need to radically change our models of cosmology. One possibility is that the Universe is filled with a gravitationally repulsive substance – dark energy. The existence of dark energy may also account for the total amount of matter in the universe. The universe appears to be close to flat, and in order for it to be this shape, the mass-energy density of the universe must be equal to the critical density.
The total amount of matter in the universe as measured from the cosmic microwave background (CMB) spectrum, accounts for only about 30% of the critical density. This implies the existence of an additional form of energy – dark energy – to account for the remaining 70%. We need to research dark energy to understand it better, particularly how it behaves, whether it interacts with ordinary matter, and whether it is a cosmological constant or varies in time and space.
We study dark energy's impact on the Universe and how it affects the way large structures collapse under gravity. We can measure background geometry by using supernovae as standard candles (astronomical objects with a known absolute magnitude), and features in the way galaxies are distributed as standard rulers (astronomical objects for which the actual physical size is known). We also use weak gravitational lensing and the velocity of galaxies to study the distribution of dark matter and uncover how it evolves.
The presence of dark energy leads to the decay of gravitational potentials on super-galactic scales. This can change the energy of photons as they pass through and in particular leaves an impression on the map of CMB photons across the sky. We can study this effect, by searching for correlations between the galaxy distribution and maps of the CMB.
Our discovery of this correlation between the CMB and surveys such as the Sloan Digital Sky Survey (SDSS) and the National Radio Astronomy Observatory VLA Sky Survey (NVSS) provided independent evidence for the existence of dark energy and an important constraint on models of modified gravity.
Cosmological tests of gravity
Gravity is an essential force in the Universe, governing the dynamics of galaxies, stars and planets. While gravity has been tested accurately in laboratories and the solar system, it has not been tested on small and large scales. The Universe is massive, but was tiny once – cosmology is the means to test the nature of gravity on these scales.
The standard model of cosmology assumes Einstein’s General Relativity as a theory to describe gravity. According to General Relativity, the expansion of the Universe must be slowing down. But in the 1990’s, it was discovered that the expansion of the Universe is actually accelerating. The discovery of the expansion of the Universe presented the most challenging problem in theoretical physics.
We're developing cosmological tests of gravity, seeking answers to the question of why the expansion of the Universe is accelerating, and challenging conventional General Relativity.
We're one of only a few institutes in the world with access to multiple international astronomical surveys of the galaxy distribution in the cosmos. This makes our Institute of Cosmology and Gravitation (ICG) a global centre for research and knowledge in cosmological tests of gravity.
Portsmouth is a member of the Sloan Digital Sky Survey (SDSS-IV), the Dark Energy Survey, and the Large Synoptic Survey Telescope (LSST). We're also involved in the Dark Energy Spectroscopic Instrument (DESI) and major international collaborations, including ESA’s Euclid satellite mission and the Square Kilometre Array (SKA).
These surveys will dramatically transform our measurements of the cosmic expansion and the large scale structure of the Universe. It will provide a new opportunity to test gravity on astrophysical and cosmological scales.
Emanuela Dimastrogiovanni, Matteo Fasiello, Robert J. Hardwick, Hooshyar Assadullahi, Kazuya Koyama, David Wands JCAP 1811 (2018) no.11, 029
Chris Pattison, Vincent Vennin, Hooshyar Assadullahi, David Wands JCAP 1710 (2017) no.10, 046
Vincent Vennin, Kazuya Koyama, David Wands, JCAP 1603 (2016) no.03, 024
Tommaso Giannantonio, Robert Crittenden, Robert Nichol, Ashley J. Ross, MNRAS 426, 2581 (2012)
Mark Linton, Alkistis Pourtsidou, Robert Crittenden and Roy Maartens, JCAP 1804 (2018) no.04, 043
Gong-Bo Zhao et al. Nat.Astron. 1 (2017) no.9, 627-632
Simone Peirone, Kazuya Koyama, Levon Pogosian, Marco Raveri, Alessandra Silvestri, Phys.Rev. D97 (2018) no.4, 043519
Thomas E. Collett et.al. Science 360 (2018) 1342
Kazuya Koyama, Rept.Prog.Phys. 79 (2016) no.4, 046902
Discover our areas of expertise
We're studying supernovae and the appearance of distance between Earth and galaxies, and measuring the positions of large-scale structures in the Universe.
We're researching the evolution of galaxies, from the most local to the most distant, and using their light to model stellar radiation and probe the formation and development of the Universe.
Interested in a PhD in Cosmology & Astrophysics?
Browse our postgraduate research degrees – including PhDs and MPhils – at our Cosmology & Astrophysics postgraduate research degrees page.