antimatter from the same location were deflected to different places, then it would indicate a difference in the forces that they experienced on their journey. This is difficult because gravity has a far weaker influence on these particles than the other forces they experience so a special type of particle is needed, one that has no charge and so is not affected by the electromagnetic force. Fortuitously there is such a particle, the neutrino, which is found in abundance throughout the universe. Unfortunately the same lack of charge that makes them suitable also makes them very difficult to detect, as they have to be identified by their interactions with other particles. Despite the difficulty, this particular experiment has been carried out on neutrinos emitted from an exploding star, or supernova, as it cooled 23 which concluded that neutrinos and antineutrinos are affected in the same way by gravity. The results of this experiment were far from conclusive, not only were very few neutrinos and antineutrinos detected (around 20 in total) but due to the indirect nature of their detection, it is possible that none were actually observed. To complicate matters further neutrinos may be their own antiparticle. As they have no charge the distinction between the two is based on an alternate property known as spin (Appendix 1), however it is not clear if all neutrinos do in fact have the same spin or if those that spin the other way are very rare. If this were the case then they would both be affected in the same way by gravity, irrelevant of the gravitational interaction between matter and antimatter. Another proposed experiment is being prepared at CERN where they plan to fire cold, neutral antihydrogen (an antiproton orbited by a positron) at a target and investigate how it is deflected by the earth’s gravity (Appendix 2). This experiment is known as AEgIS, and aims to build on past research that has only managed to exclude a repulsive gravity ten times stronger than conventional gravity. The difficulty in this experiment is the production of antihydrogen which annihilates with the surrounding atoms, even in the near perfect vacuum surrounding the experiment. Furthermore, it requires incredibly sensitive instruments to detect minute variations in the antihydrogens’ flight path. Dark energy is the name given to the observation that the universe appears to contain a larger amount of energy than can be accounted for by the visible matter. The need for this energy is based on two principle observations, firstly that the speed at which certain supernovae are moving away from the earth appears to be increasing. 4 Secondly the flat shape of the universe requires some additional energy to account for the discrepancy between the observed energy content of the universe and the energy required to prevent any form of curvature 5 .
Dark Energy http://hyperphysics.phy-astr.gsu.edu/hbase/astro/dareng.html
2 A.K. Mann (1997) Shadow of a star: The neutrino story of Supernova 1987A W.H Freeman p. 122 3 S. Pakvasa, W. A. Simmons, and T. J. Weiler (1989), Test of equivalence principle for neutrinos and antineutrinos, Physical Review Letters 4 A. G. Riess et al. Observational Evidence from Supernovae for an Acceleration Universe and a Cosmological Constant 15/5/1998 Astronomical Journal arxiv.org/abs/astro-ph/9805201 27/08/14 5 D. N. Spergel et al. (WMAP collaboration) March 2006 Wilkinson Microwave Anisotropy Probe (WMAP) three year results: implications for cosmology
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