This observation is not a cast iron proof, so to speak, but it does involve an incredible set of coincidences to explain away.
Here goes:
As you probably know the Moon is tidally locked to the Earth. It's rotational period and orbital periods are the same. Thus it always keeps the same face towards the Earth. This is caused by the tidal forces in the Earth-Moon system. Since the Moon is only 1/81 times the mass of the Earth it long since became locked. The Earth's rotation rate is slowing down but is nowhere near being tidally locked with the Moon.
Now this effect also occurs in binary star systems. I should mention here that as this trend towards locking occurs there is a commensurate circularisation of the orbits. Initially elliptical orbits become more circular as time goes on.
So far so good, this is just basic Newtonian dynamics applied to masses with finite size - no idealised point masses here.
Now stellar evolution theory makes predictions as to the age of stars of an observed mass, spectral type etc.
When we observe a cluster of stars we see definite evidence of a common birth date for the stars in the cluster. We don't see a random conglomeration of ages - it all fits into the basic interpretation according to stellar evolution theory. Of course someone might trump up and say well they could all be very young, agreeing on the common birth time, but not accepting the astrophysicists dating of the stars via stellar theory.
Now many of these stars in these clusters are members of binary systems. We can measure the orbital properties of these systems. These parameters include, size of the orbit & eccentricity & masses of the component stars.
Now the important point is:
From Newtonian dynamics we can calculate the time it takes an orbit of a given size and star masses to circularise.
We also have our guess as to the age of the stars based upon stellar theory.
When we plot the average separation of the binaries that have become circular versus the age of the cluster we get a direct correlation.
Thus cluster stars we say are young are only in circularised binaries that are very close. The binaries of greater separation have not had time to do this. As the clusters get older more and more binaries have become circularised out to greater and greater separations.
NOT ONLY IS THE OBSERVED TREND CORRECT, THE THEORETICAL TIME SCALES OPERATING AS CALCULATED USING BASIC CLASSICAL MECHANICS ARE PRESENT IN THE DATA.
This phenomena is observed with clusters up to some 5 billion years old. And the fraction of binaries circularised for given separations is concordant with the known dynamical time scales operating.
This needs an incredible set of coincidences in stellar physics, Newtonian dynamics and observational biases to explain away as anything but evidence for an old Universe.
(There is related data on older globular cluster systems based upon core collapse and dynamical relaxation and the binary system properties that takes this time frame up to 11-12 billion years ago.)
Sorry for the length of this post, but I tried to be as precise as possible given the limited space and no place to put a graph.
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