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 . The possible influence of the cosmological constant on the motion of inertial systems leading to an additional acceleration has been discussed . Gravitational coupling resulting in an increase of the constant with scale is analyzed by Bertolami and García-Bellido . 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  may provide a connection with the Pioneer anomaly.
Kagramanova et al.  (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  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 . However, the data on the cosmological accelerated expansion yields the value of , leading Kerr et al.  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]: observed that the additional acceleration towards the gravitating body is of the second order in the Hubble constant . As such, this potential practically does not participate in the cosmic expansion; thus, there is no support for the cosmological origin of .
Oliveira  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, ). To study the effect of cosmological modification of planetary orbit, one considers the action noted that in this scenario and stay nearly constant. In addition, and of the planetary orbits practically do not participate in cosmic expansion. Remember that the stability of adiabatic invariants is governed by the factor of , where is the characteristics time of change of external parameter (here: ) and is the characteristic time of the periodic motion (here: = periods of planets). In the the case of periodic bound orbits , thus, any change of the adiabatic invariants is truly negligible.
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  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  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.  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  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  used a PPN-inspired spacetime metric: concludes that in cosmological context the behavior of bound orbits is different from that of unbound ones. However, cosmological expansion results in a decelerating drag term, which is a factor too small to account for the Pioneer effect.
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 , 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 ).
As an upshot of these efforts, Bertolami et al.  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  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  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.,  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.  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.
This work is licensed under a Creative Commons License.