Talk detail

Participant	Pilo, Luigi
Institution	Dept. of Physical and Chemical Sciences, Univ. L'Aquila - Via Vetoio (Coppito) - Italy - L'Aquila - Italy

Session	DE1	Accepted	Yes	Order	6	Time	17:40	20'
Talk	Oral abstract	Title	Cosmology of Lambda-media
Coauthors	Celoria, Marco; Comelli, Denis
Abstract	We address the question whether a medium featuring p+ρ=0, dubbed Λ-medium, has to be necessarily a cosmological constant. By using effective field theory, we show that this is not the case for a class of media comprising perfect fluids, solids and special super solids, providing an explicit construction. The low energy excitations are non trivial and lensing and the growth of large scale structures can be used to clearly distinguish Λ-media from a cosmological constant. Based on arXiv:1712.04827
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Session	AT4	Accepted	Yes	Order	2	Time	15:45	20'
Talk	Oral abstract	Title	On the 6th Mode in Massive Gravity
Coauthors	Marco, Celoria; Denis, Comelli
Abstract	Generic massive gravity models in the unitary gauge correspond to a self-gravitating medium with six degrees of freedom. It is widely believed that massive gravity models with six degrees of freedom have an unavoidable ghost-like instability; however, the corresponding medium has stable phonon-like excitations. The apparent contradiction is solved by the presence of a non-vanishing background pressure and energy density of the medium that opens up a stability window. The result is confirmed by looking at linear stability on an expanding Universe, recovering the flat space stability conditions in the small wavelength limit. Moreover, one can show that under rather mild conditions, no ghost-like instability is present for any wavelength. As a result, exploiting the medium interpretation, a generic massive gravity model with six degrees of freedom is perfectly viable.
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