How can an object in a vacuum slow down at a rate not explainable by gravity?
A few years ago I read about the Pioneer Anomaly, which is the observed deviation from predicted trajectories and velocities of various unmanned spacecraft visiting the outer solar system, most notably Pioneer 10 and Pioneer 11. Yes, they are still experiencing some sunward acceleration due to the sun's gravity outside the heliopause, but they are both slowing down slightly more than expected. As I continued reading, I found out that these are not the only mystery — there is also the flyby anomaly, wherein spacecraft that transit over Earth's polar regions (rotational AND magnetic) experience an acceleration rather than a deceleration...this is very similar to the Pioneer spacecraft's deceleration relative to the Sun. But then I noticed something in the first reference above: "The magnitude of the Pioneer effect is numerically quite close to the product of the speed of light and the Hubble constant", and the writer notes that the significance of this is not known.
So now we have three observations: the Pioneer anomaly, the flyby anomaly, and the interesting degree of the Pioneer effect. To these I added another anomaly, the galaxy rotation problem, which notes that the arms of spiral galaxies, from just outside the central bulge to the edge of the galaxy, spin at an almost constant speed...and according to Newtonian physics, such masses traveling at such speeds would tend to fly away and escape from the galaxy.
I realized that there must be some kind of spatial boundary just outside the heliopause, and close to masses such as Earth...and wondered if perhaps there was such a boundary outside spiral galaxies preventing the ejection of stellar masses orbiting the central bulge so fast that that the centripetal force should eject them from the galaxy. What would possibly explain such boundaries? At first I considered that perhaps the consistency of the fabric of space itself was not constant, that space itself was 'lighter' in proximity to masses...which would have explained to some extent the anomalies above, but left a nagging question: what would happen to the speed of light from extrastellar sources? That, and what would this mean to the current search for Dark Matter and Dark Energy?
So I spent several months ruminating over that issue, and last night (in the shower, of all places), it came to me: what if the fabric of space was stretched in proximity to masses? What if a kilometer of space in near-earth orbit did not match with a kilometer of space in interstellar regions? If this were true, then when the Pioneer craft exited the Solar System, they did not experience additional sunward acceleration (or, in more understandable terms, 'slow down'). No, instead the Pioneer craft were traveling precisely as many kilometers per hour as they should have been...but the kilometers they traversed were shortened. To observers here on Earth, the Pioneer craft appeared to slow down more than they should have, but if I'm correct, their velocity did not change more than it should have due to stellar gravity!