Below we discuss cosmologically-motivated mechanisms used to explain the Pioneer anomaly.
An inverse time dependence for the gravitational constant produces effects similar to that of an
expanding universe. So does a length or momentum scale-dependent cosmological term in the gravitational
action functional [231, 317
]. It was claimed that the anomalous acceleration could be explained in the
frame of a quasi-metric theory of relativity [276
]. The possible influence of the cosmological constant on the
motion of inertial systems leading to an additional acceleration has been discussed [317]. Gravitational
coupling resulting in an increase of the constant
with scale is analyzed by Bertolami and
García-Bellido [52]. A 5-dimensional cosmological model with a variable extra dimensional
scale factor in a static external space [43, 44] was also proposed. It was suggested that the
coupling of a cosmological “constant” to matter [197] may provide a connection with the Pioneer
anomaly.
Kagramanova et al. [163] (see also [168, 332]) have studied the effect of the cosmological constant on
the outcome of the various gravitational experiments in the solar system by taking the metric of the
Schwarzschild–de Sitter spacetime:
The authors of [27] have shown that if the cosmological expansion would be at the origin of the Pioneer
anomaly, such a mechanism would produce an opposite sign for the effect. Taken at face value, the anomaly
would imply a negative cosmological constant of
, which contradicts both the
solar system data and the data on cosmological expansion. Indeed, the highest limit on
allowable by the solar system tests is set by the data on the perihelion advance, which limit the
value of the cosmological constant to
[332]. However, the data on the
cosmological accelerated expansion yields the value of
, leading Kerr et al. [168] to
conclude that the cosmological effects are too small to be measured in the solar system dynamical
experiments.
Hackmann and Lämmerzahl [141, 142] developed the analytic solution of the geodesic equation in Schwarzschild–(anti-)de Sitter spacetimes and show that the influence of the cosmological constant on the orbits of test masses is negligible. They concluded that the cosmological constant cannot be held responsible for the Pioneer anomaly.
Thus, there is now a consensus that the Pioneer anomaly cannot be of a cosmological origin and,
specifically, cannot be responsible for the observed anomalous acceleration of the Pioneer 10 and 11
spacecraft. However, the discussion is still ongoing.
The effect of cosmological expansion on local systems had been studied by a number of
authors [21, 82, 118, 137, 202], (for reviews, see [73
, 124
, 177
, 422
]). To study the behavior of
small isolated mass in expanding universe, one starts with the weak field ansatz [73
, 177
]:
Oliveira [273] conjectured that the solar system has escaped the gravity of the Galaxy, as evidenced by its orbital speed and radial distance and by the visible mass within the solar system radius. Spacecraft unbound to the solar system would also be unbound to the galaxy and subject to the Hubble law. However, this hypothesis produces practically unnoticeable effects.
The cosmological effects on the planetary orbits has been addressed in many papers recently (for instance, [124]). To study the effect of cosmological modification of planetary orbit, one considers the action
In the case of weak time dependence of
The question of whether or not the cosmic expansion has an influence on the size of the Solar system was addressed in conjunction to the study of the Pioneer anomaly. In particular, is there a difference between locally bound and escape orbits? If the former are proven to be practically immune to cosmic expansion, what about the latter? In fact, the properties of bound (either electrically or gravitationally) systems in an expanding universe have been discussed controversially in many papers, notably by [21, 203].
Effects of cosmological expansion on local systems were addressed by a number of authors (for reviews
see [73, 176
]). Gautreu [137] studied the behavior of a spherical mass with the energy-stress tensor
taken in the form of an ideal fluid. The obtained results show that the outer planets would
tend to out-spiral away from the solar system. Anderson [21] has studied the behavior of local
systems in the cosmologically-curved background. He obtained cosmological modifications of
local gravitational fields with an additional drag term for escape orbits and demonstrated that
for bound orbits. Cooperstock et al. [82] used the Einstein–de Sitter universe
and derived a geodesic deviation equation in Fermi normal
coordinates
. Clearly, the additional terms are too small be observed in the solar
system.
As far as the Pioneer anomaly is concerned, the papers above are consistent in saying that planetary
orbits in the solar system should see the effects of the anomaly and may not be of gravitational origin.
However, Anderson [21] found an interesting result that suggests that the expansion couples to escape
orbits, while it does not couple to bound orbits.
To study this possibility Lämmerzahl [176] used a PPN-inspired spacetime metric:
The idea that the cosmic acceleration of the Universe may be caused by a modification of gravity at very
large distances, and not by a dark energy source, has recently received a great deal of attention
(see [319, 351]). Such a modification could be triggered by extra space dimensions, to which gravity spreads
over cosmic distances. Recently, models involving inverse powers of the curvature or other invariants have
been proposed as an alternative to dark energy. Although such theories can lead to late-time acceleration,
they typically result in one of two problems: either they are in conflict with tests of general relativity
in the solar system, due to the existence of additional dynamical degrees of freedom [77], or
they contain ghost-like degrees of freedom that seem difficult to reconcile with fundamental
theories.
Gravity theories that supplement the Einstein–Hilbert Lagrangian with a nonlinear function of
the curvature scalar have attracted interest in the context of inflationary cosmological scenarios, and
because these theories may explain the accelerated expansion of the universe without the need to postulate
scalar fields or a cosmological constant (for a review, see [351]).
As an upshot of these efforts, Bertolami et al. [50] explored a particular family of Lagrangians
that gives rise to an extra acceleration in the form
A similar result was obtained by Saffari and Rahvar [320] who used a metric formalism in
modified gravity to study the dynamics of various systems from the solar system to
the cosmological scales. Replacing
with
, the authors postulated the
ansatz
where
is a dimensionless constant and
. They
obtained an anomalous acceleration term of
, which agrees with the Pioneer anomaly for
.
On the other hand, Exirifard [122] demonstrated that a correction to the Einstein–Hilbert
Lagrangian in the form of cannot offer a covariant resolution to the Pioneer
anomaly.
Capozziello et al., [72] developed a general analytic procedure to investigate the Newtonian limit of
gravity. The authors discussed the Newtonian and the post-Newtonian limits of these
gravity models, including the investigation of their possible relevance to the Pioneer anomaly.
Capozziello et al. [69] mention that, by treating the Pioneer anomaly as a correction to the
Newtonian potential, it could be studied in the general theoretical scheme of
gravity
theories.
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