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Dark Energy
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why dark energy speed up expansion? i use cosmological constant as an example. it is indeed sort of anti-gravitate. however, this is not a very understandable explanation a better argument is the following the expansion of the universe is adiabatic. this means, cut out a volume in the universe. as universe expand, this volume also expand. however, the energy within this volume is conserved. now, if the energy density is a constant (such as a cosmological constant). As volume increase, there is a contribution from background energy density that grows with volume. in order for the energy within a volume to be conserved, there must be a pressure push outward, doing work, decreasing energy. This is sometimes called negative pressure. the effect of a negative pressure is of course not just balancing energy within a comoving volume. it also pushes. it makes the expansion faster. =============================================================== for phantom energy. it is an extreme form of dark enery whose effect is even stronger than the cosmological constant and it could be increasing... it is could be from a negative kinetic term, and so on. One should be able to find the discussion of it from the citation of the paper astro-ph/9908168 http://www.slac.stanford.edu/spires/find/hep/www?rawcmd=eprint+astro-ph2F9908168&FORMAT=WWW&SEQUENCE= I found the discussion in astro-ph/0104112 pretty interesting. personally, I am not a big fan of it. mainly because I think it is experimentally too early to tell. tthe real mytery is not the form of dark energy, but its size. |