Conventionally dark energy is explained in one of two ways. The first of these is that the universe has a certain energy in a given volume of space, known as the cosmological constant. This means that the amount of energy the universe contains increases as it gets bigger 6 this is sufficient to fuel the universe’s expansion despite the gravitational attraction which would otherwise cause the universe’s expansion to slow down. The second is known as quintessence 7 , a force that accelerates the expansion of the universe. Both of these proposals have problems, not least that the predicted strength of the cosmological constant is 10 120 times stronger than would be required by the theory which would have prevented the formation of galaxies. 8 Therefore, some alternative is required. Antigravity provides one such alternative. This is because the repulsion between matter and antimatter galaxies in the universe could be greater than the attraction between matter galaxies, forcing them apart at an accelerating rate. These antimatter galaxies could either exist in intergalactic voids 9 (large volumes of space where there appears to be no matter), or they could be hidden amongst the matter galaxies. Both of these proposals have problems, the first because antimatter galaxies should look identical to matter galaxies and so would not be hidden in galactic voids. The problem with the other is that the neighbouring matter and antimatter galaxies should annihilate, causing the release of lots of energetic gamma rays that we would detect. However, antigravity could also solve this problem as it could keep the matter and antimatter apart and prevent its annihilation.
Galaxy rotation curve http://en.citizendium.org/wiki/galaxy_rotation_curve
Furthermore, the lack of antimatter in cosmic rays is commonly interpreted as indicating that there is little antimatter in the universe 10 . However, it is possible that this conclusion is wrong. Any antimatter traveling from an antimatter galaxy would probably have annihilated with the matter in its path. Indeed, heavier, slower moving particles would be more likely to annihilate and are correspondingly rarer. Furthermore, more positrons are detected at higher energies 11 which fits with such an explanation as higher energy particles would be less likely to annihilate. Dark matter 12 describes the observation that the amount of matter we see in the universe is apparently not enough. This is based on the fact the speed at which galaxies rotate does not fit the predicted curve (below), indicating the presence of more mass than we can see. Another key tool is gravitational lensing 13 which allows us to observe the gravitational field by looking at other galaxies through the field and observing how the light from these galaxies is bent by it. The most convincing observation 6 Cosmological Constant, http://www.scholarpedia.org/article/Cosmological_constant 26/04/14 7 S. Tsujikawa (2013), Quintessence: A Review, Cornel University Library Online 8 S. E. Rugh and H. Zinkernagel 2001 The Quantum Vacuum and the Cosmological Constant Problem philsci- archive.pitt.edu/398/ 27/08/14 9 Astronomy & Cosmology – Large Scale Structure of the Universe, http://www.whillyard.com/science-pages/voids.html 27/04/14 10 Cosmic Rays, http://hyperphysics.phy-astr.gsu.edu/hbase/astro/cosmic.html 27/04/13 11 First Result from the Alpha Magnetic Spectrometer on the International Space Station: Precision Measurement of the Positron Fraction in Primary Cosmic Rays of 0.5–350 GeV, http://hdl.handle.net/1721.1/81241, 04/2013, American Physical Society 12 Dark matter, http://home.web.cern.ch/about/physics/dark-matter 27/04/14 13 What is gravitational lensing? http://www.cfhtlens.org/public/what-gravitational-lensing 27/04/14
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