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NIRG Report Summary

Project ID: 307934
Funded under: FP7-IDEAS-ERC
Country: France

Mid-Term Report Summary - NIRG (New paradigms for InfraRed modifications of Gravity)

Our project addresses theoretical and phenomenological properties of large distance (“Infra-Red”, IR in the following) modifications of the gravitational interaction. Such modifications are motivated by two main reasons: firstly, to find alternative explanations to the presence of dark matter or dark energy in cosmology; secondly, to better understand the currently well accepted cosmological model, disentangling there what does from what does not depend on the large distance dynamics of gravity and extracting as much as possible new information on gravity from the latest cosmological observations. For the second goal, it matters to have at hand alternatives to the standard cosmological model based on general relativity, to serve as benchmarks. Recently, new ideas have been proposed to modified gravity in the IR. First, a large class of scalar-tensor theories featuring the “k-mouflaging” of the scalar has been proposed and partly classified, under the generic name of Galileons and their extensions. Second, new kinds of massive gravities which might be devoid of the standard pathologies of those models have been discovered. Third, models of non local gravity have been proposed with many interesting features. Some of these models, interesting to produce the MOND law, have been studied in the first part of this project. Besides, the current outcomes of this project include the first ever obtained consistent (as far as number of degree of freedom is concerned) quadratic theory for a massive graviton propagating on a totally arbitrary background metric, which can then be used in particular to study the issue of partial masslessness, an issue which has also been investigated in this project, so far in link with bi-gravity theories. The project has also lead to the proof of a no-go theorem for vector Galileons as well as to the discovery of new theories of this type for p-forms. The Hamiltonian of the simplest model of covariantized Galileon has also been computed and studied for the first time in conjunction with the study of the crucial question of the counting of degrees of freedom of this model. Last, an EFT approach has been implemented to test variousIR modifications of gravity against cosmological observations.

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