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The general theory developed in the above sections is valid for any metric
theory of gravity. Let us now examine the implications of Eqs.(65) and (66)
within the scalar-tensor theories of gravity.
The class of theories that we consider here is described by the action
where R is the Ricci scalar curvature (), is
the scalar gravitational field, g is the determinant of the metric components
, is an arbitrary function of the scalar field , and
is the matter action. We assume that is a
functional of the metric and of the matter fields only. This means
that does not depend explicitly upon the scalar field (it is the assumption of universal coupling between matter and metric).
We consider here the weak-field approximation only. So we assume that the
scalar field is of the form
| |
(69) |
where is a first order perturbation of an averaged constant value
. Consequently, the field equations deduced from
(74) reduce to the following system:
| |
(70) |
| |
(71) |
where is the energy-momentum tensor of the matter fields
at the lowest order, and denotes the
d'Alembertian operator on Minkowski spacetime: .
It is easily seen that any solution to the field equations
(76) is given by
| |
(72) |
where is a solution to the equations
| |
(73) |
which are simply the linearized Einstein equations with an effective
gravitational constant . Indeed, inserting (78) in
(67) yields the following expression for the curvature tensor
from which one deduces the Ricci tensor
| |
(74) |
Then substituting for from its expression
(81) into the field equations (76) gives Eqs.(79), thus proving the proposition.
The decomposition (78) of the gravitational perturbation implies that each term contributing to can be split
into a part built from the Einsteinian perturbation only
and into an other part built from the scalar field alone. In what
follows, we use the superscript ST for a functional of a solution to the field equations (76)-(77) and the superscript E for the same
kind of functional evaluated only with the corresponding solution .
In order to perform the calculation of the integrals (65) and (66), we note
that is the usual total derivative of the quantity F along
the unperturbed ray path, which implies that
| |
(75) |
The 4-vector (supposed here to be future oriented, i.e
such that K0 > 0) gives the direction of propagation of the light coming from
the observed source. For a given observer moving with the 4-velocity ,
let us put
| |
(76) |
We have . Since is orthogonal to
by construction, can be identified to the unit 3-vector
giving the direction of propagation of the light ray in the usual
3-space of the observer.
Using (76), (77), (80) and the assumption , it is
easily seen that (65) and (66) can be respectively written as
| |
(77) |
and
where
| |
(78) |
and
| |
(79) |
As a consequence, the rate of variation in the photon flux as received by
the observer is given by the general formula
| |
(80) |
In a vacuum , the metric satisfies the linearized Einstein field equations (71) and Eq.(84) reduces to
| |
(81) |
In Eq.(88), is reduced to the term
given by Eq.(73), where is constructed with
.
It follows from (89) that contrary to general relativity, the scalar-tensor
theories (defined by (74)) predict the existence of a first-order geometrical
scintillation effect produced by gravitational waves. This effect is
proportional to the amplitude of the scalar perturbation. It should be noted
that an effect of the same order of magnitude is also due to the change in the
spectral shift.
To finish, let us briefly examine the case where the scalar wave is
locally plane (it is a reasonable assumption if the source of gravitational
wave is far from the observer). Thus we can put in the vicinity of the observer
located at the point xobs
| |
(82) |
where u is a phase function which admits the expansion
| |
(83) |
with
| |
(84) |
It follows from Eq.(77) with T(0) = 0 that is a null vector
of Minkowski spacetime.
Replacing by in (83) defines the
spacelike vector , which can be identified with the unit 3-vector
giving the direction of propagation of the scalar wave in the
3-space of the observer. Then introducing the angle between
and , a simple calculation yields
| |
(85) |
This formula shows that the contribution of the scalar wave to the
scintillation cannot be zero, whatever be the direction of observation of the
distant light source.
Next: Are observational tests possible?
Up: Scintillation in scalar-tensor theories
Previous: Weak-field approximation
10/9/1998