Let us start to remind how the standard general relativistic framework can be extended to make dynamical on the example of scalar-tensor theories, in which gravity is mediated not only by a massless spin-2 graviton but also by a spin-0 scalar field that couples universally to matter fields (this ensures the universality of free fall). In the Jordan frame, the action of the theory takes the form
The variation of this action gives the following field equations, we have an equation, which reduces to the standard Einstein equation when is constant and a new equation to describe the dynamics of the new degree of freedom while the conservation equation of the matter fields is unchanged, as expected from the weak equivalence principle.
It is useful to define an Einstein frame action through a conformal transformation of the metric . In the following all quantities labeled by a star (*) will refer to Einstein frame. Defining the field and the two functions and (see, e.g., ) by
The action (157) defines an effective gravitational constant . This constant does not correspond to the gravitational constant effectively measured in a Cavendish experiment. The Newton constant measured in this experiment is
This illustrates the main features that will appear in any such models: (i) new dynamical fields appear (here a scalar field), (ii) some constant will depend on the value of this scalar field (here is a function of the scalar field). It follows that the Einstein equations will be modified and that there will exist a new equation dictating the propagation of the new degree of freedom.
In this particular example, the coupling of the scalar field is universal so that no violation of the universality of free fall is expected. The deviation from general relativity can be quantified in terms of the post-Newtonian parameters, which can be expressed in terms of the values of and today as. The solar system constraints imply to be very small, typically while can still be large. Binary pulsar observations [125, 189] impose that . The time variation of is then related to , and the time variation of the scalar field today
The example of scalar-tensor theories is also very illustrative to show how deviation from general relativity can be fairly large in the early universe while still being compatible with solar system constraints. It relies on the attraction mechanism toward general relativity [130, 131].
Consider the simplest model of a massless dilaton () with quadratic coupling (). Note that the linear case correspond to a Brans–Dicke theory with a fixed deviation from general relativity. It follows that and . As long as , the Klein–Gordon equation can be rewritten in terms of the variable as, this is the equation of motion of a point particle with a velocity dependent inertial mass, evolving in a potential and subject to a damping force, . During the cosmological evolution the field is driven toward the minimum of the coupling function. If , it drives toward 0, that is , so that the scalar-tensor theory becomes closer and closer to general relativity. When , the theory is driven way from general relativity and is likely to be incompatible with local tests unless was initially arbitrarily close from 0.
It follows that the deviation from general relativity remains constant during the radiation era (up to threshold effects in the early universe [108, 134] and quantum effects ) and the theory is then attracted toward general relativity during the matter era. Note that it implies that postulating a linear or inverse variation of with cosmic time is actually not realistic in this class of models. Since the theory is fully defined, one can easily compute various cosmological observables (late time dynamics , CMB anisotropy , weak lensing , BBN [105, 106, 134]) in a consistent way and confront them with data.
Given this example, it seems a priori simple to cook up a theory that will describe a varying fine-structure constant by coupling a scalar field to the electromagnetic Faraday tensor as
Let us also note that such a simple coupling cannot be eliminated by a conformal rescaling since
This example shows that we cannot couple a field blindly to, e.g., the Faraday tensor to make the fine-structure constant dynamics and that some mechanism for reconciling this variation with local constraints, and in particular the university of free fall, will be needed.
Living Rev. Relativity 14, (2011), 2
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