Probing Extra Dimensions with images of Distant Galaxies

1. Probing Extra Dimensions with images of Distant Galaxies Shaun Thomas, Department of Physics and Astronomy Supervisor: Dr, Jochen Weller Modified Gravity and Extra Dimensions An alternative theory of gravity with a simple physical interpretation is known as DGP braneworld gravity [3]. This model is based on the idea that there exists an infinitely large extra dimension, in addition to our familiar three of space and one of time. The idea is that all particles are confined to the usual four space-time dimensions, whereas gravity is allowed to flow through all dimensions (see Figure 3). In this way gravity leaks into the fifth dimension and over large, cosmological, distances it becomes weaker. The extra weakness of gravity makes it incapable of halting the expansion of the Universe, thus explaining the acceleration effect. Introduction Cosmology is currently gripped by the most tantalising problem: distant galaxies appear to be accelerating away from us. This phenomenon has been demonstrated by a multitude of cosmological probes such as observations of distances to exploding stars, or supernova [1]. One should expect the recession of the distant galaxies to be decelerating, given that all the matter in the Universe attracts gravitationally. This radical new notion could, potentially, be met with a radical explanation. Perhaps General Relativity, Einstein’s powerful theory of gravity that explains how our Universe evolves, is incomplete. If so, we need a new modified theory of gravity. In this work we examine a new theory that contains a large extra spatial dimension. We investigate how this can be detected from future planned surveys of ‘Weak Lensing’. Figure 3 – This represents the extra dimensional scenario. All normal matter is confined to the four-dimensions of our Universe (shown as a sheet) but gravity is allowed to ‘leak’ out through all dimensions. This is inspired by String Theory. Results and Conclusions We find, by comparing the predicted statistical quantities-the Power Spectra-for General Relativity and the five dimensional model, that the proposed DUNE mission will indeed have the ability to distinguish between them. This result is shown in Figure 4 below. The key quantity in the figure is α on the y-axis, a parameter which interpolates between General Relativity when zero and the full five dimensional modified theory when one. The x-axis represents the amount of deflecting mass in all the galaxies and clusters in the Universe. The contours, increasing from 68% to 99% significance levels, do not reach α = 1 (an extra dimension). Thus, by observing the light from distant galaxies, we are able to distinguish between General Relativity and a model with an extra dimension. Courtesy of hubblesite.org Figure 1 – Light paths deflected by the presence of mass underlie the principle of ‘Weak Lensing’ The Deflection of light by mass General Relativity, and the majority of other theories of gravity, state that the very fabric of space and time become curved and distorted by the presence of mass (See Figure 1). Paths of light, attempting to travel in a straight line in space-time, will be distorted too. We find, therefore, that images of distant galaxies, having passed enormous clumps of matter in clusters of galaxies, are stretched. An extreme example of this galaxy stretching is shown in Figure 2. Courtesy of Hubble Figure 2 – The thin arcs shown above are extreme examples of how images are distorted by the presence of vast quantities of mass. The Experiment A promising future survey, which we choose to examine, is the Dark UNiverse Explorer (DUNE) [4]. This space bound 1.2m telescope, set to perform an all-sky survey observing some 3000 million galaxies, is ideal for such a Weak Lensing experiment. Moreover, many members of the Physics and Astronomy department at UCL are heavily involved in this collaboration. Figure 4 – This contour plot with three significance levels demonstrates the ability of the DUNE Weak Lensing survey to distinguish between General Relativity and DGP-like theories with extra dimensions Weak Gravitational Lensing Weak Lensing is the process whereby the stretching effect on a particular galaxy is small. However, by taking a large sample of galaxies it is possible to examine it statistically. The statistical signature is generally referred to as a ‘Power Spectrum’. Different theories of gravity are likely to give rise to different distributions of deflecting matter (galaxies and clusters) and different space-time distortions. In this way different theories of gravity are predicted to have different statistical Power Spectra. We compare these spectra for a given Weak Lensing experiment to find whether it will be able to ‘observe’ an extra dimension. • References • [1] A. G. Riess, et al., Astron. J. 116, 1009-1038 (1998); S. Perlmutter, et al., Astrophys. J. 517, 565-586 • [2] G.R. Dvali, G. Gabadadze, and M. Porrati, Phys. Lett. B485, 208 (2000), hep-th/0005016 • [3] A. Refregier & the DUNE collaboration, astro-ph/0802.2522 • For the interested party or potential collaborator please contact the author: sat@star.ucl.ac.uk