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Projects: Pioneer Anomaly

Alternative Gravities

August 10, 2006

Recent progress in observational astronomy, astrophysics, and cosmology has raised important questions related to fundamental laws of nature. In fact, this progress confronted us with a very challenging reality -- our universe is filled up with some sort of mysterious stuff that we cannot see and do not know the nature of, which we conveniently call "dark matter". In addition, we find that the universe suddenly and, in cosmic terms, very recently started to accelerate its expansion, as if it were subjected to some sort of a hidden force, which we call "dark energy". In fact, in the standard model of cosmology, visible matter, such as stars, planets, and physics textbooks accounts for just 4 percent of the total universe, leaving nearly 24 percent for dark matter and almost 72 percent for dark energy to complete the inventory of the universe.

Faced with a lack of direct evidence for dark matter -- which remains elusive despite years of dedicated searches -- and absence of any understanding of dark energy, some physicists started to inspect every piece of our understanding of the fundamental laws of nature. One of the possibilities is that our description of gravity is incomplete and, despite the empirical success of Einstein's general theory of relativity, it may need an alternative. Discussions of the possibility of alternative gravities were the main focus of a recent workshop on dark matter and alternative gravities recently held at the Royal Observatory in Edinburgh, UK. Pioneer Anomaly project leader Slava G. Turyshev, who was invited to review the current status of the tests of gravity in the solar system, represented The Planetary Society at the meeting.

Over the years, our solar system provided unique conditions to test general relativity to ever increasing accuracy; this progress ultimately made it the standard gravity model for astronomy, astrophysics, cosmology, and spacecraft navigation. Surprisingly, navigating the Pioneer 10 and 11 spacecraft in deep space left us with another interesting puzzle, the Pioneer anomaly. The inability to explain the anomalous behavior of the Pioneers with conventional physics has resulted in a growing discussion about the origin of the detected signal. In the unlikely case that this effect is of a gravitational origin, the corresponding "dark force" that act on the Pioneers may be yet another piece of the same mysterious puzzle. But in this case, instead of ripping the universe apart or accelerating galactic rotations, this force slows-down the probes as they move farther from our Sun. Is this effect a different facet of the same phenomena? This was exactly the question that the meeting participants discussed.

In fact, many of the presently available alternative gravity models -- in addition to explaining galaxy rotation curves -- predict a small anomalous acceleration of the same order as the one observed with the Pioneers. One such theory is called modified Newtonian dynamics, or MOND, in which the modified behavior kicks in below a certain acceleration -- rather than distance -- scale. Remarkably, the value of this universal acceleration parameter was chosen to allow MOND to describe the dynamics of galaxies, while preserving Newtonian gravity elsewhere. It is intriguing that this small parameter is very close to the anomalous acceleration observed by the Pioneers.

A relativistic extension to MOND, called TeVeS (for Tensor Vector Scalar structure of the model), has all the desirable features of an alternative theory of gravity: it reduces Einstein's theory for high speeds and large accelerations (thereby accounting for gravitational lensing); to Newtonian gravity for low speeds and small accelerations (such as those on Earth); and to MOND when accelerations are smaller still (thereby predicting the observed galaxy-rotation curves). Being fully relativistic, TeVeS can also make predictions about the universe on the largest scales. In fact, with TeVeS one could match cosmological observations without dark matter by replacing general relativity with an alternative gravitational theory. It can also provide theoretical footing for the Pioneer effect.

What is most important is that any theories that attempt to explain galactic rotational curves via gravity modification necessarily predict a small acceleration of the size observed by the Pioneer 10 and 11 deep space probes. However, presently there are some limitations to further explore this possibility, and a new investigation of the Pioneer anomaly is called for. In fact, the limited size of the previously analyzed data set also limits our current knowledge of the anomaly. Therefore, in order to determine the origin of the effect and especially before any serious discussion of new physics can take place, one must analyze the entire set of radiometric Doppler data received from the Pioneers.

As of summer 2006, an effort to recover this critical information, initiated in June 2005, has been completed; we now have almost 30 years of Pioneer 10 and 20 years of Pioneer 11 Doppler data, most of which was never used in the investigation of the anomaly. A unique feature of the upcoming effort is the use of telemetry files documenting the thermal and electrical state of the craft. This unique information was not available previously; however, the telemetry files for the entire durations of both missions were recovered, pre-processed, and are ready for the upcoming study. Both of the newly assembled data sets are pivotal to establishing the origin of the detected signal; we are ready to analyze it.

Only after concluding that analysis will we be in a position to tell if the Pioneer anomaly contributes to darkening the physics at our stellar backyard or if it is an engineering artifact, the explanation that would leave the solar system brightly illuminated for us to enjoy.