Last week's idea was to get dark matter and dark energy from the excitations and the VEV of the scalar field in a scalar-tensor theory of gravity. However, we could return to the WDM idea of dark matter as keV-mass neutrinos, and still have a use for this scalar field.
In fact, let me list some of the uses of scalars. They can provide mass to fermions (Higgs mechanism). They can provide dark matter (scalar particles). They can provide dark energy (VEV). They can drive inflation (inflaton field).
An ultimate minimalism might be a scalar-tensor gravity in which the scalar field is simultaneously the Higgs field, the inflaton field, and the source of dark matter and dark energy. At the other extreme we could have a separate scalar for each of the following: scalar component of gravity, providing mass to SM fermions, providing mass to sterile neutrinos, inflation, dark matter, dark energy. And in between are multi-scalar theories in which some or all of the scalars perform multiple functions.
So what I'm thinking is that there could be a whole spectrum of conservative extensions of (standard model + gravity), defined by number of sterile neutrinos, number of scalars, and assignment of roles to these new fields.
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