Enrique Álvarez, Mario Herrero-Valea
The only allowed source of the gravitational field in the unimodular theory, invariant under area-preserving (transverse) diffeomorphisms as well as Weyl transformations, is just the traceless piece of the energy-momentum tensor. This fact notwithstanding, the free energy produced by arbitrary sources (not only static ones) is identical to the one predicted by general relativity. This encompasses all weak field tests of gravitation. There is an important difference with general relativity though. A pure trace source, such as the one produced by a pure vacuum energy with frozen scalar field does not weigh at all. This sheds new light on the direct cosmological constant problem.
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http://arxiv.org/abs/1209.6223
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