2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/53129A tachyon is considered to be sick in the context of particle mechanics, but in field theory just indicates instability of a background. We consider a similar possibility that a ghost in field theory might be just an indication of instability of a background and that it can condense to form a different background around which there is no ghost. We construct a low energy effective field theory based on the derivative expansion around the stable background. Possible applications are discussed, including dark energy, dark matter, inflation and black hole.4 pages. Talk given at JGRG 14. This complements the final version of hep-th/0502189High Energy Physics - TheoryAstrophysicsGeneral Relativity and Quantum CosmologyHigh Energy Physics - Phenomenologytext