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Title: A Well-Tempered Cosmology

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Abstract:

The cosmological constant problem (CCP) has remained a persistent thorn in the side of contemporary theoretical physics. Currently lacking a reasonable resolution from the field theory sector, in recent years, much attention has been placed in finding solutions within the gravitational sector. A particular approach that has been of interest lately, is to consider self-tuning solutions to the CCP, in which one introduces a scalar field in the that is able to screen the net vacuum energy density from the spacetime curvature, thus de-gravitating it. An initially promising model adopting this approach, was the Fab-Four, however, this has all but been ruled out by gravitational wave constraints. Recently, a new so-called Well-Tempered self-tuning solution has been proposed. This model is not only consistent with the gravitational wave constraints, but also admits a standard cosmological history, with the de Sitter solution (independent of the vacuum energy) being a stable attractor. In this talk, I will discuss recent work in which I have generalised the original construction of this model, whilst preserving its desirable properties. In particular, I will show how this generalisation forces a particular form for the Horndeski function G_3, if the theory is to permit self-tuning.