Measurements of Voyager I and II showed an anomalous reduction in deceleration as they left the solar system. Because the Sun’s gravity acts upon them, they are expected to slowly decelerate as they leave the solar system, and they are but not quite as quickly as expected.
The mysterious force that caused this was attributed to yet another cosmological fudge factor, dark energy.
I suspect there may be a much more mundane explanation. To understand how the velocity of these craft is measured, one has to be familiar with Doppler shift. When a train blows it’s whistle as it approaches you and continues as it passes then heads away, at first you hear the whistle at a higher pitch, then as it moves past you and away, a lower pitch. The same is true for electromagnetic radiation.
The velocity of these craft is measured by measuring the Doppler shift of their transmitters. The transmitters are designed to be extremely stable to allow Doppler shift to be measured precisely.
In order to measure the Doppler shift precisely, you have to measure the frequency of the received signal precisely. This means you have to measure how many cycles occur in a specific period of time. This in turn requires that you can very accurately measure out a segment of time. That in turn requires a highly accurate clock.
The United States maintains a time standard and provides the reference to the world from the National Institute of Science and Technology, formerly the National Bureau of Standards. There, they have highly accurate atomic clocks clicking off the nanoseconds.
NASA, in order measure frequencies accurately as well as time launches and other time critical operations precisely, has to have accurate clocks, they have their own time reference.
On the other side of the country, NewNet, an IRC network I founded in 1995, was trying to get a couple of servers to link up. IRC uses a protocol known as Time Stamp Protocol, to determine after a server split, who owns a particular nick or channel if there is a conflict.
In other words, let’s say you have two servers, A and B on a network and normally they communicate in real time. User “MyNick” connects to server A and is talking to people. Then a split occurs (server A temporarily looses communications with server B) and during that time another user logs into server B with the nick “MyNick”. Now, the split is resolved, both servers resume communications and there are two people on the network with the same nick. The servers use Time Stamp Protocol to determine which user had the Nick first and force the other user to change his nick.
In order for Time Stamp Protocol, both servers much agree on what time it is. This requires that the servers be synchronized to some accurate external time source. If for some reason the clocks on two servers do not agree, they will not link.
So here I am working with another site trying to link their server to mine and they will not connect. The admin of the other server says to me, “Your clock must be off because mine is sync’d to NASA”. I say to him, “Mine is sync’d to NIST so it must be yours that is off.”
We both test our servers and find that indeed our servers clocks do agree with the sources we sync’d them to. Then I checked NASA’s time server against NIST’s, there was five seconds difference.
Now getting back to Voyager I and II… If the clock you are using to measure the received frequencies of their transmitters is in accurate, then those received frequency measurements will be inaccurate. If the received measurements are accurate, then the velocities calculated to be causing the received Doppler shift is inaccurate.
Maybe it’s just me but before I would introduce a whole new fudge factor into cosmology, I’d check the clocks. Apparently that hasn’t occurred to NASA however, and I guess this explains why shuttles explode, less than half the Mars missions actually make it to Mars, etc.