Or is there some incontrovertible obstruction that

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Unformatted text preview: e Ã3 . If this is true, it is important that the Ã3 theory is ghost free beyond the decoupling limit. It should also be noted that massive gravitons already exist in nature, in the form of tensor mesons which carry spin 2. There is a nonet of them, which at low energies can be described in chiral perturbation theory as a multiplet of massive gravitons (Chow and Rey, 1998). Here we know that these states find a UV completion in QCD, where they are simply excited states of bound quarks. In this review we focused on theories with a vacuum that preserves Lorentz invariance, but there is a whole new world that opens up when one allows for Lorentz violation. There exist theories that explicitly break Lorentz invariance, and theories such as the ghost condensate (Arkani-Hamed et al., 2004) which have Lorentz invariance spontaneously broken (Arkani-Hamed et al., 2005) by some non-Lorentz invariant background. In the former case, a systematic study of the possible mass terms and their degrees of freedom, generalizing the Fierz-Pauli analysis to the case where the mass term preserves only rotation invariance, is performed by Dubovsky (2004). For examples of the latter case, see Berezhiani et al. (2007) and Blas, Deffayet, and Garriga (2007). See also Rubakov (2004), Gabadadze and Grisa (2005), Rubakov and Tinyakov (2008), Bebronne (2009b), and Mironov et al. (2010) for reviews. There is still much to be learned about massive gravitons on curved spaces and cosmologies. For instance, does a generalization exist of the higher cutoff Ã3 theory around curved backgrounds? Is there a consistent fully interacting theory of the partially massless theories on de Sitter space? Are there consistent theories with cosmological backgrounds, and, in particular, can they nonlinearly realize the screening of a large bare cosmological constant while maintaining consistency with Solar System constraints? Finally, a topic worthy of a separate review is the observable signatures that would be characteristic of a massive graviton. What would be the signatures of a cosmological constant screened by a graviton mass? For some examples of various proposed signatures, see Dubovsky, Tinyakov, and Rev. Mod. Phys., Vol. 84, No. 2, April–June 2012 707 Tkachev (2005), Bebronne (2009a), Bessada and Miranda (2009a, 2009b), Dubovsky et al. (2010), and Wyman (2011). We end this review by quoting the tantalizing current experimental limits on the mass of the graviton (under some hypotheses, of course) m & 7 Â 10À32 eV (Goldhaber and Nieto, 2010; Nakamura et al., 2010), about an order of magnitude above the Hubble scale, the value needed to theoretically explain the cosmological constant naturalness problem. ACKNOWLEDGMENTS The author would like to thank Claudia de Rham, Gregory Gabadadze, Lam Hui, Justin Khoury, Janna Levin, Alberto Nicolis, and Claire Zukowski for discussions and comments on the manuscript, and Mark Trodden for discussions, comments, and for encouraging the writing of this...
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This document was uploaded on 09/28/2013.

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